Food Preservation Course Outline PDF

‫‪SSP‬‬ ‫لكية الزراعة سااب ابشا‬ ‫الربامج اخلاصة‬ ‫مقرر تكنلوجيا حفظ األغذية وااللبان‬ ‫الدكتور‪ /‬محمد أحمد السيد جمعة‬ ‫كلية الزراعة سابا باشا‬ ‫جامعة اإلسكندرية‬ ‫قسم علوم األغذية‬ 1 Principles food preservation _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 2 Traditional methods of preservation Traditional methods of food preservation began from the essential need to store supplies when they were plentiful and to keep the food fresh for as long as possible to last through the winter months. Although food preservation has been in use for thousands of years, it is only in the last two centuries that many of the ‘new’ food processing techniques have been developed. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 3 Principles of food preservation The principles underlying methods of preservation used in the past are still the same as today. The aim of preservation is to prevent food spoilage as a result of growth of micro-organisms and breakdown of food by enzymes. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 4 Food spoilage As soon as food is harvested, slaughtered or manufactured into a product it starts to change. This is caused by two main processes: autolysis – self destruction, caused by enzymes present in the food; microbial spoilage – caused by the growth of bacteria, yeasts and moulds. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 5 Factors that promote enzymes and microbial activity Micro-organisms and enzymes need certain conditions to survive and reproduce. These include: temperature; oxygen; food; time; moisture; pH level. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 6 Principles of food preservation Some of the factors affecting the growth of micro - organisms can be manipulated in different ways to prolong the life of the food product. Temperature Chilling or freezing the food to retard growth of micro-organisms and inhibit enzyme activity. Alternatively, heating the food to destroy micro-organisms and prevent enzyme activity. Oxygen Food kept in an airtight container will deprive micro-organisms of oxygen and prevent contamination. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 7 Principles of food preservation Moisture Reducing the moisture content of the food to make water, (which is essential for growth), unavailable to micro-organisms. Alternatively, placing food in a sugary solution will make water unavailable for the growth of micro-organisms. pH level Placing food in an acidic or alkaline solution will inhibit the growth of micro-organisms. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 8 Methods of food preservation - Chilling Over the past 50 years chilling and freezing has become the most popular domestic method of preserving food. This is mainly due to wider ownership of domestic refrigerators and freezers and developments in technology, rather than the discovery of new preservation principles. Chilling reduces the temperature to between 1ºC -4ºC. Chilling food cannot preserve a food indefinitely, but can reduce spoilage caused by micro-organisms and enzymes. Moulds can still grow in cold temperatures. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 9 Methods of food preservation - Freezing Reducing the temperature of the food to below – 18ºC reduces the activity of the micro-organisms and enzymes. Freezing also reduces the availability of water because ice crystals are formed. In China, freezing has been a method of preservation for hundreds of years, dating back to 1800 BC. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 10 Disadvantages of freezing Most food contains large amounts of water. When water is frozen, ice is formed. Large ice crystals are formed when food is slowly frozen, this can damage the cell structure of the food. When the food defrosts, the water enclosed within the cells is released, e.g. cell damage in soft fruits (strawberries) and the collapse of some colloidal systems in food products, e.g. cream. Freezing food quickly can reduce the size of ice crystals. When frozen, micro-organisms do not die, they simply become dormant, retarding their growth. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 11 Methods of food preservation - Sugar preserves The initial boiling of the fruit will destroy the enzymes and micro- organisms (but not spores), preventing spoilage later on. The high concentration of sugar added during the jam making process makes the water unavailable thus reducing the microbial activity through dehydration effect. Jam jars are normally heated before the jam is added which destroys the micro-organisms found in the jars. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 12 Methods of food preservation - Salting Coating food in salt or placing it in a salt solution (brine) reduces the moisture content of the food, i.e. reduces the availability of water. With little moisture, micro-organism growth is retarded. However, the taste of the food may change considerably. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 13 Methods of food preservation - Pickling The initial boiling of the ingredients will destroy enzymes and micro-organisms (but not spores), preventing spoilage later on. Vegetables and fruits are covered in vinegar and other ingredients, often including spices. The high concentration of acid inhibits bacterial growth and multiplication. The acidic nature of the solution prevents growth of micro-organisms. Pickle/or chutney jars are normally heated before the product is added to destroy micro-organisms found in the jars. www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 14 Principles of home food preservation For further information, go to: www.foodafactoflife.org.uk _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 15 Modern Methods of Home Food Preservation – Canning – Freezing – Drying – Pickling – Sugar concentrates – jams, jellies, butters, preserves, etc. – Curing, smoking www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ 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_____________________________________________________________________________________ _____________________________________________________________________________________ __________ 17 600 million disease 420 000 Unsafe 110 billion US$ death food Global economic, trade, and tourism _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 18 Food contamination Biological Chemical Physical www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 19 Technologies used for microbial decontamination of the food and food contact surfaces a)- Physical technologies 1) Ultraviolet light (UV) → (surface) 2) Pulsed light (PL) → (surface) 3) Microwave (MW) → (food) 4) Ionising radiation (IR) → (food) 5) High-power ultrasound (HPUS) → (food + surface) 6) Pulsed electric field (PEF) → (food) 7) High hydrostatic pressure (HHP) → (food) b)- Chemical technologies 1) Ozone → (food + surface) 2) Chlorine dioxide → (food + surface) 3) Essential oils → (food + surface) 4) Organic acids → (food + surface) 5) High-pressure carbon dioxide (HPCD) → (food) 6) Hydrogen peroxide → (food + surface) c)- Hurdle technology www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 20 1- UV light _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 21 Applications UV Advantages Limitations www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 22 – 2- Pulsed light (PL) – (for food + surface) ❑ The modified form of UV-C. ❑ It has many advantages over continuous UV light. ❑ Disinfection mechanism has two steps. – _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 23 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 24 3- microwave (MW) (for food) Overview Theory Applications www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 25 – Microwave ❑ Mode of microbial inactivation ❑ Advantages ❑ Disadvantages ❑ Improvements _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 26 4- Ionising radiation (for foods) ❑ Overview ❑ Advantages ❑ Mechanism of microbial inactivation ❑ Applications and doses _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 27 Ionising radiation www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 28 – 5- High-power ultrasound (US) – (for foods and surfaces) Mechanism Overview of microbial Applications inactivation _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 29 ultrasound (US) _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 30 – 6- Pulsed electric field (PEF) (for foods) Mechanism Overview of microbial inactivation _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 31 Pulsed electric field (PEF) _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 32 – 7- High hydrostatic pressure (HHP) (for foods) ❑ Overview ❑ Disinfection mechanisms ❑ Factors influencing efficacy _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 33 – High hydrostatic pressure (HHP) ✓ Applications and advantages ✓ Limitations ✓ Improvements – _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 34 – b)- Chemical technologies – 1- Ozone (for foods + surfaces) Ozone Factors Disinfection Overview influencing mechanisms efficacy _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 35 Applications Advantages Ozone Limitations Improvements _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 36 – 2- Chlorine dioxide (for foods + surfaces) ❖ Overview ❖ Disinfection mechanisms ❖ Applications ❖ Factors influencing efficacy _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 37 – Chlorine dioxide ❑ Advantages ❑ Limitations ❑ Improvements _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 38 – 3- Essential oils (EOs) (for foods + surfaces) Applications Disinfection Overview mechanisms and advantages _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 39 Essential oils Factors influencing efficacy Limitations Improvements _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 40 – 4- Organic acids (OA) (for foods + surfaces) ❑ Overview ❑ Disinfection mechanisms ❑ Applications ❑ Limitations ❑ Improvements _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 41 5- High-pressure carbon dioxide (HPCD) (for foods) Disinfection Overview mechanisms Applications _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 42 High-pressure carbon dioxide Advantages Factors influencing efficacy Limitations www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 43 6- Hydrogen peroxide (H2O2) (for foods + surfaces) ❑ Overview ❑ Advantages ❑ Limitations www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ 44 c)- Hurdle technology ❑ Definition ❑ Mechanism ❑ Importance ❑ Examples www.foodafactoflife.org.uk © Food – a fact of life 2019 _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________ __________ Part I Preservation and Protection 1 Methods of Food Preservation TSVETKO PROKOPOV1 AND STOYAN TANCHEV2 1. Introduction Virtually all foods are derived from living cells from animals and plant origin and in some cases from some microorganisms by biotechnology methods. Thus, foods are for the most part composed of “edible biochemicals”. One of the most important goals of the food scientist is to make foods as safe as possible whether they are used fresh or processed. The judicious application of food processing, storage and preservation methods helps prevent outbreaks of foodborne illness, that is the occurrence of disease or illness resulting from the consumption of contaminated food. The processed food industry has an out- standing record preventing such cases when it is considered that billions of cans,jars, packets and pouches of processed and fresh food products are consumed annually. Occasionally, however, this excellent record has been broken by limited outbreaks in which persons do succumb to the effects of toxic foods. Food preservation is an action or method of designed to maintain foods at a desired level of quality. A number of new preservation techniques are being developed to satisfy current demands of economic preservation and consumer sat- isfaction in safety, nutritional and sensory aspects (Potter and Hotchkiss, 1995). 2. Why Do We Need to Preserve? The preservation, processing and storage of the food are vital for the continuous supply of foods during seasons and off-seasons. One very important considera- tion that differentiates the agricultural from all other industrial processes is their 1 Tsvetko Prokopov, University of Food Technologies, 26 Maritsa Blvd., 4002 Plovdiv, Bulgaria, e- mail: [email protected] 2 Stoyan Tanchev, University of Food Technologies, 26 Maritsa Blvd., 4002 Plovdiv, Bulgaria, e-mail [email protected] 3 4 Tsvetko Prokopov and Stoyan Tanchev seasonal nature. The main reasons for food processing and preservation are: to overcome seasonal production in agriculture; to produce value-added products; and to provide variety in diets. People like to eat wide varieties of foods, having different tastes, flavours, nutritional, dietetic and other characteristics. Unfortunately it has been estimated that as many as 2 billion people do not have enough to eat and that perhaps as many as 40 000 die every day from diseases related to inadequate diets, including the lack of sufficient food, protein or specific nutrients. Inadequate nutrition in extreme cases can produce in children an advanced state of protein deficiency known as kwashiorkor or the more widespread protein. Major processes of food deterioration are caused by environmental factors such as temperature, humidity, oxygen and light which can be reason for several reaction mechanisms that may lead to food deterioration to such an extent that they are either rejected by or harmful to the consumer. Microbial effects are the leading cause of food deterioration and spoilage (Desai, 2000). 2.1. The necessity to preserve Foods are perishable or deteriorative by nature. Based on the mode of action, major food preservation techniques can be categorised as: slowing down or inhibiting chemical deterioration and microbial growth; directly inactivating bac- teria, yeast, moulds and enzymes and avoiding recontamination before and after processing. A number of techniques or methods from these categories are pre- sented in Figure 1. 3. Conventional Food Preservation Methods 3.1. Food preservation by heat treatment Heat is by far the most commonly used method of food preservation. There are various degrees of preservation by heating that ultimately dictate the type of final product manufactured, the terms used are pasteurisation and sterilisation. However, to be effective, these processes must be carried out under a combina- tion of strict temperature and time control to ensure the killing of pathogenic andnon-pathogenic microorganisms. These same factors also cause thermal inactiva- tion of food enzymes and some destruction of food constituents (Heldman and Lund, 1992). 3.1.1. Heat resistance of microorganisms Heat resistance of microorganisms is a basic topic of thermobacteriology, which is a very important part of microbiology including food microbiology. The most heat resistant pathogen found in foods, especially those that are canned and held under anaerobic conditions is Clostridium botulinum. It is spore forming, prote- olytic anaerobe, which is able to produce the most harmful known toxin since 1. Methods of Food Preservation 5 Food preservation methods Inhibition of chemical, Inactivation Recontamination microbiological, enzymatic and non-enzymatic deterioration and/or spoilage Low temperatures Pasteurization Packaging Freezing temperatures Sterilization Cleaning Reduced water activity Radiation Sanitary Decrease oxygen Electrifying treatment Increase CO2 High pressure Acidification Chemical Fermentation preservation Chemical preservatives Antioxidants Surface coating Structure modification Gas removal FIGURE 1. Major foodChemical modification, etc. preservation methods. amount of about 10−6 – 10−8g is able to kill one person. However, there are non- pathogenic, spore forming food spoilage bacteria, such as the putrefactive anaer-obe Clostridium sporogenes 3679 (PA3679) and Bacillus stearothermophilus (FS1518) which are more heat resistant than spores of Cl. botulinum. This means that if a heat treatment inactivates spores of these spoilage microorganisms, the spores of Cl. botulinum and all others pathogens will be also killed (Bell and Kyrakides, 2000). 3.1.2. Kinetics of heat destruction of microorganisms Thermal death time The thermal death time is the time of heating required to kill all vegetative cells of microorganisms. Theoretically this is not possible but this expression is used in thermobacteriology for practical purposes. Microorganisms are killed by heat at a rate that is very nearly proportional to the number of cells of a specified organism (expressed on a logarithmic basis) present in the system (food, laboratory nutritive medium, water, etc.) being heated. This is referred as a logarithmic order of death. A typical thermal death rate curve is shown in Figure 2. It provides data on the rate of destruction of spe- cific microorganisms in specific media or food at specific constant temperature, 6 Tsvetko Prokopov and Stoyan Tanchev 10000 1000 Survivors 100 log a - log b = 1.00 10 D 1 Time at a Constant Temperature FIGURE 2. Bacterial destruction rate curve showing logarithmic order of death. D, decimal reduction time. which is able to kill the corresponding microorganism – pathogenic, toxicogenic, or spoilage organism of the specific food. Figure 2 shows the logarithmic dependence between the time () and number of the killed cells (C) at constant temperature, or lgC = f () at t˚C = constant. The logarithmic order of thermal killing is valid for all spores and vegetative cells but latter are killed faster. It is valid also for yeast and moulds. The D-value is real kinetic constant determined at t˚C = constant. That is why it is normally written as Dt. For example, if t = 100˚C then it should be D100 which means that the valueof D is determined at 100˚C and it is valid only for this temperature and product in which the cells have been suspended (Ray, 2000). This “D-value”, or decimal reduction time, is defined as the time, in minutes, at specified temperature required to destroy 90% of the cells at the respective microbial population. During each time interval (1 min or 3 min or 6 min) num- bers of the cells is reduced 10 times, let say from 1000 to 100 or from 100 to 10, etc. This means that 90% of the cells are killed during each interval. In each case 1000/100 = 100/10 = 10 which is the reason it is called the decimal reduc- tion time. In other words, the D-value represents the time for the number of cells to be reduced by one logarithmic cycle, for example from 10 6 to 105 cells per 1 g. 1. Methods of Food Preservation 7 TABLE 1. Effective time-temperature relationships for destruction of Clostridium botulinum spores Temperature (˚C) Time (min) 100 330 104 150 110 36 116 10 118 5.27 121 2.78 124 1.45 127 0.78 Dependence between the thermal death time and temperature For example, if the time-temperature combinations required for destruction of Clostridium botulinum spores in low-acid media (i.e pH >4.5) are taken from this type of relationship. The time-temperature relationships that will be equally effective are shown in table 1. From such data, the dependence of thermal death time on the temperature can be presented graphically in semi-logarithmic co-ordinates called an Arrhenius plot where the heating time is plotted in a logarithmic scale. This shows (Figure 3) that for any one initial concentration of the cells, time-temperature relationship is linear and can be described by the equation: lg  = f (t˚C), at C = constant This figure illustrates two terms or kinetic constants, the “Z-value” and the “F-value”. The “Z- value” is the number of degrees required to pass through one log cycle, which means that the thermal death time is changed by factor of 10, let say from 100 to 10 min or from 10 to 1 min. The “F-value” is defined as the num-ber of minutes at a specific temperature, required to destroy the desired number of cells of any microorganism. The “F-value” is a measure of the capacity of anyheat treatment applied to a specific food product in order to sterilise it. When it is written as F0 it means that the tested microorganism is spores of Cl. botulinumwhen they are treated at 121.1˚C (Ray, 2000). The dependence of thermal death times on the temperature has been deter- mined for many important pathogens and food spoilage microorganisms. Such curves for putrefactive anaerobe Clostridium sporogenes (PA3679) and Bacillus stearothermophilus (FS1518) are shown in Figure 4. This figure shows the length of time it takes to kill these microorganisms at a chosen temperature. For exam- ple it would take about 60 min at 105˚C to kill the specified number of spores of PA 3679. At 121.1˚C, the same numbers of spores are killed in a little over 1 min (Shapton and Shapton, 1991). It has been shown that the criterion for commercial sterilisation is that the pop- ulation of spores of Cl. botulinum, should be reduced by 12 log cycles or 12D. This means that if one can contains 106 spores before heating, which is unusually high, then after a 12D heat treatment, out of 1 million cans, 999 999 cans will be 8 Tsvetko Prokopov and Stoyan Tanchev 1000 thermal death time curve 100 HEATINGTIME, MIN one logcycle spores 10 Z-value 1 F-value vegetative cells 0.1 100 105 110 115 120 TEMPERATURE 8C FIGURE 3. Typical thermal death time curves for bacterial spores and vegetative cells. 100 10 TIME, MINUTES FS 1518 z = 18.12°C D121,1 = 1.92 1 PA 3679 z = 16.8°C D121,1 = 1.06 0.1 105 110 115 120 125 130 135 TEMPERATURE 8C FIGURE 4. Thermal death time curves for test microorganisms PA 3679 and FS 1518. 1. Methods of Food Preservation 9 sterile. For spores of Cl. sporogenes PA 3679 and Bacillus stearothermophilus FS1518, in low acid foods, a 5D heat treatment is equivalent to 12D values against Cl. botulinum. For food with pH < 4.6 (higher acid foods) requirement 12D is not valid since Cl. botulinum does not grow in these foods (although spores may survive). In container sterilisation time required to sterilise food is influenced by: heat resistance of microorganisms and/or the enzymes in the food, when pH < 4.6; heating method – steam, water, flame, etc.; pH of the food; the size of the container; chemical composition of the food; physical state of the product; mechanism of heat exchange (convection or conduction); initial product temperature; temperature of sterilisation; state of the containers during sterilisation – static, shaking, rotating, etc. The mechanisms of heat inactivation and injury of microorganisms is not very well identified since heat will bring about so many changes in biological mate- rial, such as microbial cells, that is why identification of the event that causes death or injury of the cells is difficult to predict (Larousse and Brown, 1997). Thermal death time depends on the microbial cell concentration (Figure 5). 10000 1000 spores killed KILLING TIME (min) 100 C = 100 000/ml 10 spores survive C = 10 000/mlC = 1000/ml 1 100 105 110 115 120 TEMPERATURE8C FIGURE 5. Thermal death curves for bacterial spore suspensions of different initial con- centrations. 10 Tsvetko Prokopov and Stoyan Tanchev 3.2. Preservation by low water activity (aw) Water activity can be reduced by partial removal of the water (drying, reverse osmosis, concentration) or by adding substances which increase the osmotic pres- sure of the food or media such as sugars, ethanol, glycerol, salts, etc. (Booth, 1998). The majority of microorganisms are sensitive to the water status in their immediate environment and they can remain metabolically active only in a nar- row range of high water activities. There is a lot of information about low water activity limits for the growth of microorganisms. However it is typical that those organisms that are tolerant to low aw be also tolerant of very high osmotic pres- sures. The lowest aw limits for growth recorded up to now illustrate the enormous range of tolerances that exist. The most aw tolerant species are able to grow when osmotic pressure is as high as about 800 MPa. They can grow slowly below aw 0.62. The nature of the solute exerts additional affect on potential for growth. Ionic solutes such as NaCl and KCl are more inhibitors than non-ionic solutes such as sugars. Solutes such as glycerol, unlike the salts and sugars, rapidly per- meate most bacteria but not yeast, e.g. Saccharomyces ronxii and Debaryomyceshanseni. However, for the more low aw-tolerant species this simple relationship is no longer valid. Staphylococcus aureus, for example, is extremely salt- tolerant and more sensitive at a higher aw in glycerol than in sodium chloride (Shapton and Shapton, 1991). Lowering the aw by various means may also influence the rate of enzymatic and chemical changes in foods. Whilst all microbiological growth is completely stopped below about aw = 0.6, some enzymatic reactions that cause food spoilage continue and some reactions, such as lipid oxidation, may even be accelerated at very low aw values (Shapton and Shapton, 1991). 3.3. Preservation by low pH and organic acid 3.3.1. Preservation by low pH Foods are classified according to their acidity as follows: non-acid – 7.0-5.3; low or medium acid – 5.3-4.6; acid I – 4.6-3.7 and acid II – 3.7 and lower. Microorganisms have a characteristic range of pH values within which they can grow. Most bacteria have an optimum pH near 6.8 and may grow at pH val- ues ranging from 4.0 to 8.0. A small number bacterial species can multiply when pH < 4.0 or pH > 8.0. Yeast and moulds can sometimes grow at pH less than 2.0. Usually the growth rate decreases as the pH drops below the optimum value. Approaching the lower limiting pH for growth, cells are first inhibited and eventually killed. The degree of inhibition increases as pH decreases and this relationship is linear. The differences in inhibition and/or lethal effects of organic acids used for pH reduction having different pK values are well known. The pK values of some acids are as follows: citric – 3.08; malic – 3.4; tartaric – 2.98; acetic – 4.75 (it is usually used as an effective preservative). The theory of food canning accepts a pH of 4.5 or 4.6 (for USA) as the bor- 1. Methods of Food Preservation 11 derline between acid and low acid foods, which respectively do not need and do need the minimum botulinum cook known to be 12D. However, this assumption does not take into account the ability of Cl. botulinum to grow at pH levels near 4.0 and in very specialised environmental conditions as well as the ability of Staphylococcus aureus and several Salmonella strains which would also be of significance if container leakage occurred when these organ- isms are present in water used for cooling of the sterilised containers (Shapton and Shapton, 1991). The pH limits of growth differ widely among microorganisms. In general het- erotrophic bacteria tend to be least acid tolerant among common food microor- ganisms. Approximate pH range for bacteria is 4.0-9.0; for yeast is 1.5-8.0; for moulds is 1.5-11.0. Bacteria that grow outside of these ranges are well known but are rarely significant food spoilage organisms. The pH limits for growth in labo-ratory media are often much wider than those observed in the foods. The exact details of how microorganisms interact with pH are not fully under-stood. As with other physiological parameters, pH is not also an absolute deter- mining factor in potential for spoilage of food. The type of microorganisms and acids presented into the product also will affect the outcome, as will others environmental factors, which reduce microbiological activity. Yeast and moulds are very acid-tolerant and frequently the pH range for growth extends well below pH values commonly encountered in foods. For example the pH range for Saccharomyces cerevisiae growth is 2.35-8.6, for Acetobacterium spp. is 2.8-4.3, for E. coli is 4.4-8.7, for Bacillus acidocaldurius is 2.0-5.0, etc. Lowering of cytoplasmic pH is probably the major cause of inhibition of growth by weak acid used as food preservatives. However, mechanistic basis of inhibi- tion of pH homeostasis is still not clear. 3.3.2. Preservation by organic acids Some organic acids and their esters are found naturally in many foods or as a product of microbial metabolism in fermented foods. Many foods are preserved by the addition of relatively low concentrations of such compounds, all of which show marked pH-dependant activity as preservatives. These compounds are pri- marily active against yeast and moulds at low concentration but bacteria are affected also. Lowering the pH increases the proportion of undissociated acid molecules, which increases the antimicrobial effectiveness of all such organic acids. It has therefore been generally assumed that the antimicrobial activity of these acids is directly related to the concentration of their undissociated mole- cules. The sensitivity of microorganisms to weak organic acids is a significant species-dependant parameter. Organic acids and esters cover a large group of sub- stances but only a limited number are used as food. Acetic acid has only a limited action as a preservative. Its main action is linked to its pH-reducing capacity. Inhibitory action is effective when concentration is from 0.04% and/or pH = 4.9 for Salmonella anthracis to 2.4% and pH = 4.5 for Saccharomyces ellipsoideus. 12 Tsvetko Prokopov and Stoyan Tanchev Propionic acid: only the sodium and calcium salts are used as food preserva- tives. They are mainly used against moulds in cheese and bakery products. Effective concentrations are from 440 to 850 mM. Lactic acid is generally viewed as being less effective than other organic acids. It is excellent inhibitor of spore-forming bacteria at pH = 5,0 although totally ineffective against yeast and moulds. It is found that aflatoxin and sterigmatocytin formation by fungi are prevented by lactic acid. Sorbic acid is used either as such or as the sodium, potassium and calcium salts but most commonly as the potassium salt. It is more effective against moulds and yeast than bacteria. Growth inhibition of bacteria occurs at concentration of between 50 and 10 000 ppm, for yeast between 25 and 500 ppm and for moulds between 100 and 1000 ppm. Generally has been assumed to possess antimicro- bial activity in the undissociated state only. Benzoic acid is used as such or as its sodium salt, commonly against yeast (20 to 7000 ppm), moulds (20 to 10 000 ppm) and bacteria (50 to 1800 ppm) Bacteria are more variable in their sensitivity. Parabens are esters of p-hydroxybenzoic acid. The most common are methyl, ethyl, propyl and butyl parabens. For bacteria the minimum inhibitory concentra- tion (in ppm) decreases as follows: methyl > ethyl > propyl > butyl. This means that effectiveness increases in the opposite direction. For B. cereus effective con- centrations of these preservatives are respectively 2000, 1000, 125 and 63 ppm. The same rule is valid for yeast and moulds. Compared with the weak acids, parabens as preservatives are effective at significantly lower concentrations. Their activity is practically pH- independent. Some gram-negative bacteria are resistantto parabens with longer side chains (Russel and Gould, 1991). The production of organic acids by food fermentation plays a significant part in preservation of foods. Many dairy products rely upon the metabolic activities of lactobacilli to prevent the growth of spoilage microorganisms. This is believed to be due to the production of lactic and acetic acids but the production of hydrogen peroxide may also be an important factor. Concerning meat, it is believed that the reduction of pH, and not the production of lactic acid, is pri- marily responsible for the preservative action. In dairy fermentation, flavour pro- duction is very important. It has been noted that different rates of acid production may be modulated by temperature, salt concentration and starting pH. The presence of glucose in meat has been suggested to be a major factor in the rate of spoilage. 3.4. Preservation by carbon dioxide, sulphite, nitriteand nitrate 3.4.1. Carbon dioxide (CO2) It is recognised that CO2 has a major role in modifying microbial growth.Modified atmospheres enriched with CO2 are a widespread natural means of extending the shelf life of a variety of non-sterile refrigerated foods. 1. Methods of Food Preservation 13 Concentration of CO2 in normal air is 0.03% but when is more than 5%, it is particularly effective against the psychrotrophic microorganisms which cause spoilage of chilled foods (Gould, 1995). Significant preservative effects have been demonstrated with fresh fermented meats and fish and also fruits and milk. Mechanisms of inhibition of microorganisms by CO 2 are not fully understood. The most likely mode of action is the inhibition of the decarboxylation reaction in living cells. 3.4.2. Sulphur dioxide (SO2) 2− − 2− Sulphur dioxide, sulphite ([SO3] ), bisulfite ([HSO3] ) and metabisulphite ([S2O4] ) are used as preservatives in wine, fruit juices, sausages and other foods (Tapia de Daza et al., 1996). As antioxidants they are used to inhibit various enzyme-catalysed reactions notably enzymatic and non-enzymatic browning. The precise mechanisms of action are not known. It has been suggested that the undis- sociated sulphurous acid is the active molecular species since the inhibitory effect is enhanced at low pH. Bisulfite has been shown to accumulate in yeast at con- centrations 50 fold greater at pH = 3.6 than at higher pH. The bisulfite ion has greater inhibitory activity towards bacteria and fungi than the sulphite ion. 3.4.3. Nitrite and nitrate Nitrite and nitrate, as their sodium and potassium salts, are widely used in fer- mentation of meat products and the curing of pork during ham producing and bacon. Originally added together with sodium chloride these compounds are important because they stabilise the red meat colour and inhibit the growth of pathogenic and spoilage microorganisms. Many bacteria reduce nitrate to nitrite and it is the latter that helps to prevent microbial spoilage. The antibacterial effectiveness of nitrite increases as pH is lowered. Nitrite inhibits the growth of Cl. botulinum, which would otherwise present an unacceptable risk in such products. Nitrite also helps to prevent rancidity in cured meats (Rozum, 1995). 3.5. Preservation by modified and controlled atmospheres The effect of the food’s the gaseous environment on microorganisms is less well understood by microbiologists and food technologists than other factors that influenced microbial growth like pH, aw, etc. The maintenance of a constant gas phase is difficult to achieve but modification of the atmosphere is used mainly for larger storage of fresh and partly processed food including meat, fish, fruits, veg- etables, etc. though individual packs are often gas-flushed. Deliberate attempts to modify the atmosphere in order to aid food preservation occur at three levels of sophistication: 1) Controlled atmospheres. They are mainly used for bulk storage or trans- portation. The gas composition, humidity and temperature can be controlled to provide optimal conditions for long-term storage of fruit, meat and other foods. 14 Tsvetko Prokopov and Stoyan Tanchev 2) Gas packaging. This method is used for bulk storage and retail packs. Gas mixtures are used. During storage, gas content of CO 2, O2 and N2 may subse- quently change as a consequence of pack permeability, biological activities of packed product, chemical reaction, for example, of oxygen with some compo- nents of the foods like vitamin C. 3) Vacuum packaging. This method is predominantly used for retail packs. The original air atmosphere is evacuated and the atmosphere, which develops during storage, is mainly the result of biological activities of the products itself. Anaerobic growth rates of different bacteria are reduced from 8% (Lactobacillus 173) to 67% (Bacillus cereus) when 100% CO2 is used in com- parison with atmosphere containing 5% CO 2 and 95% N2. Carbon dioxide alone or in mixture with N2 and/or oxygen is most important for food preservation. A reduction in respiratory activity in presence of CO2 is observed for five species of meat spoilage bacteria but Enterobacter and B. thermosphacta are notaffected under aerobic conditions. In current commercial practice, N2, O2 and CO2 in various combinations are the only gases widely used for food preservation (Gould, 1995). By combination of ultra low level oxygen (0,5-1%), 2-3% CO2 and 1-2˚C, Elstar apples can be stored almost a whole year without unacceptable quality loss. In the case of dynamic controlled atmosphere packaging, gas levels are not con- trolled at pre-set levels but are continuously adapted to the physiological response of the stored product. In this way an optimal match is made between the physio- logical demand and tolerance of the product from one side and storage condition to the other side (Rooney, 1995). 3.6. Irradiation preservation of the foods The effects of ionising radiation on biological materials are direct and indirect. In direct action, the chemical events occur as a result of energy deposition by the radi- ation in the target molecule. The indirect effects occur as a consequence of reac- tive diffusible free radical forms from the radiolysis of water, such as, the hydroxyl radical (OH−), a hydrated electron, hydrogen atom, hydrogen peroxide and hydro- gen. Hydrogen peroxide is a strong oxidising agent and a poison to biological sys- tems, while the hydroxyl radical is a strong reducing agent. These two radicals can cause several changes in the molecule structure of organic matter, including foods. Irradiation is used mainly for: Disinfection using low radiation dose of 0.15-0.50 kGy, for damage insects at various stages of development that might be present in some food likes grain; Self-life extension by inhibiting sprouting of potatoes, onions and garlic at 0.2-0.15 kGy; Delaying ripening and senescence of some tropical fruits such as bananas, avo- cado, papayas and mango at 0.12-0.75 kGy; Extending storage of beef, poultry and seafood by destroying spoiling microor- ganisms; 1. Methods of Food Preservation 15 Delaying microbiological spoilage of fruits and vegetables; Pasteurisation of seafood, poultry and beef using low dose (1.0-2.0 kGy); Sterilisation of poultry, spices and seasoning using higher dose 93.0-20 kGy); Product quality improvement for example decreasing gas producing factors in soya beans by using dose of 7.5 kGy; Reduction in the need for nitrate during production of some meat products. Ionisation irradiation affects bacteria, yeast and moulds by causing lesions in the genetic material of the cell. Factors that affect the susceptibility of microor- ganisms to irradiation are dose level, temperature, atmosphere composition,medium including foods and type of organism. In general the higher the dose applied the lower number of survivors. At lower temperatures the rate of chemi- cal reactions, such as the formation of radicals from water molecules is lower. If the product is frozen, radical formation is practically inhibited. The D-value increases from 0.16 kGy at 5˚C to 0.32 kGy at 30˚C when Campylobacter jejuni is inoculated into ground beef. This D-value means the dose by which concen- tration of microorganisms is reduced 10 times, say from 1000 to 100 or from 500 to 50, etc. Bacteria become more resistant to ionisation radiation in frozen state as well in the dry state. The composition of irradiating product will affect the sur- vival of microorganisms. As a rule, the simpler the life form, the more resistant it is to the effect of radiation. For example viruses are more resistant than bacteria, which are more resistant than moulds, which are more resistant than human beings. Also some genera of bacteria are more resistant and bacterial spores are more resistant than their corresponding vegetative cells by a factor of about 5-15. The effectiveness of irradiation to control foodborne parasite depends on the type of organism. Minimum effective doses (kGy) for representative protozoa: Toxoplasma gondii 0.09-0.7 and Entamoeba histolytica 0.251. Killing cyst stages for Trematodes: Fasciola hepatica 0.03; Clonorchis senensis 0.15-0.20;Opisthorchis viverrini 0.10; Paragonium westermani 0.10. For Cestodes: Taenia >3.0, for complete inactivation of larvae; 0,40 to prevent development in humans; 0.3 to eliminate infectivity of Taenia solium; 0.2-0,.7 to eliminate infectivity of Echinococcus granulosus. For Nematodes, the doses are: Trichinella spiralis, 0.10-0.66 for elimination of infectivity; 0.11 for sterilisation of female Angiostrongylus cantonensis; 2.0-4.0 for decreasing infectivity of Gnathostoma spinigirum; 7.0 for reducing larval penetration (Potter and Hotchkiss, 1995). D-values (kGy) for various food borne pathogens are 0.4-0.6 for Listeria; 0.4-0.5 for Salmonella; 0,.25-0.35 for E. coli 0157:H75; 0,14-0,32 for Campylobacter; 0.14-0.21 for Yersinia; 0,14-0,19 for Aeromonas. 3.7. Preservation by low temperatures Food preservation by cooling and freezing are the oldest methods using natural low temperatures. In 1875 the ammonia refrigeration system, that was capable of supporting commercial for foods refrigeration and freezing, was invented. Starting from 1920, the modern frozen food industry grew rapidly. Refrigeration 16 Tsvetko Prokopov and Stoyan Tanchev today markedly influences the practices of marketing and food industry and sets the economic climate in agro-food industry (Gould, 1995). Chilling is used to reduce the rate of biochemical and microbiological changes and hence to extend the shelf life of fresh and processed foods. Chilled foods are grouped into three categories according to their storage temperature range as follow: 1) From −1˚C to +1˚C, fresh fish, meats, sausages, ground meats, etc. 2) From 0˚C to +5˚C, pasteurised milk, cream, yoghurt, prepared salads, sand- wiches, baked goods, fresh pasta, fresh soup and sausages, pizzas, etc. 3) From 0˚C to +8˚C, fully cooked meats, fish, pies, cooked and uncooked cured meats, butter, margarine, cheeses, fruits and vegetables, etc. The rate of biochemical changes of foods, caused by microorganisms or natu- rally occurring enzymes increase logarithmically with temperature increasing. Chilling therefore reduces the rate of enzymatic and microbiological changes and retards respiration of fresh foods. The factors that control the self-life of fresh crops during chilling storage include: Type of food and variety; The part of crop; the fastest growing parts have the highest metabolic rates and the shortest storage life. For example, the relative respiration rate of asparagus is 40, of mushrooms is 21, of spinach is 11, of carrots is 5, of potatoes and gar- lic is 2, of onions is 1, etc. When relative rate of respiration is higher than 17, at 2˚C storage time is a maximum of 4 days, when the rate is 2 to 1, storage time is 25-50 weeks, etc.; Conditions of food at harvest, for example degree of microbial contamination, degree of maturity, etc.; Temperature of harvest, storage distribution, retail display, etc.; The relative humidity of the storage atmosphere which influence dehydration losses. The rate of respiration of fresh fruits is not constant at constant storage tem- perature. For example fruits which undergo “climacteric” ripening show a short but abrupt increase in the rate of respiration that occurs near to the point of opti- mum ripeness. Examples of climacteric fruits are apple, apricot, avocado, etc. and non-climacterics fruits are cherry, cucumber, etc. Temperature has a strong influ- ence on the rate pf respiratory activity, for example for apples at 0˚C it is about 4- 6 time less than at 10˚C Undesirable changes to some fruits and vegetable occur when the temperature is reduced below a specific critical level. These changes are called chilling injury, for example internal or external browning, failure to ripen, etc. The reasons for these changes are not fully understood. For example, for apples such a tempera- ture is less than 2-3˚C, for avocado it is less than 13˚C. In animal tissues aerobic respiration rapidly declines when the supply of oxy- genated blood is stopped after the animal is slaughtered. Anaerobic respiration of glycogen to lactic acid then causes the pH of the meat to fall and the onset of rigor 1. Methods of Food Preservation 17 mortis in which the muscle tissue becomes firm and inextensible. Cooling during anaerobic respiration is necessary to produce the required texture and colour of meat and to reduce bacterial contamination. However, cooling must not be too rapid otherwise cold shortening can occur, which results in tough meat (Potter and Hotchkiss, 1995). A reduction in temperature below the minimum necessary for microbial growth extends the generation time of microorganisms and prevents or retards reproduc-tion. There are four broad categories of microorganisms based on the temperature range for growth, as follows: Thermophilic (minimum 30-40˚C, optimum 55-65˚C); Mezophilic (minimum 5-10˚C, optimum 30-40˚C); Psychrotrophic (minimum 0.82) when aflatoxin- producing fungi grew. Premature splitting, during har- vest or de-husking, resulted in contamination prior to drying. Uniform drying within 48 hours of splitting the nut was found to be key to con- trol the aflatoxins. Also, smoke drying was correlated with low-aflatoxin copra. Sun-dried copra had very high concentrations, and was discouraged. Premiums to farmers were increased to encourage them to produce dry copra. The HACCP steps are provided in Table 9 (see also Coker, 1999). 6.3. Apple juice––South America Apple juice in South America was at risk of exceeding a 50 g kg−1 target level. An HACCP team was formed to address this issue involving equivalent special- ists to the copra example above. A product description and the intended use were 2. The Challenge of Mycotoxins 45 TABLE 9. HACCP Strategy for reducing aflatoxin in coconuts HACCP steps Step 1: Harvesting and dehusking – Critical Control Point (CCP) 1. Eliminate split nuts to isolate any aflatoxin already present by the use of trained harvesters or de-huskers. Validate by determining the aflatoxin concentration of batches of accepted nuts Step 2: Splitting nuts – GAP. It is Good Agricultural Practice (GAP) to ensure that the coconut meat is protected from contact with soil, which is a rich source of inoculum Step 3: Drying – CCP2. Dry to a safe moisture content within 48 hours to prevent growth of fungi and production of afla

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