Food Preservation Lecture Notes PDF

Dept. of Agricultural Microbiology Principles of Food Preservation: 1. Prevention or delay of microbial contamination. 1. By keeping out microorganisms (sterilization) 2. By removal of microorganisms, e.g., by filtration 3. By inhibiting the growth and activity of microorganisms, e...

Dept. of Agricultural Microbiology Principles of Food Preservation: 1. Prevention or delay of microbial contamination. 1. By keeping out microorganisms (sterilization) 2. By removal of microorganisms, e.g., by filtration 3. By inhibiting the growth and activity of microorganisms, e.g., by low temperature, drying, anaerobic conditions, or chemicals. 4. By killing the microorganisms, e.g., by high heat or radiation Principles of Food Preservation: 2. Prevention or delay of self-decomposition of the food. 1. By destruction or inactivation of food enzymes, e.g., by blanching 2. By prevention or delay of purely chemical reactions, e.g., prevention of oxidation using an antioxidant. 3. Prevention of damage because by insects, animals, mechanical causes, etc., For Application Food Preservation : Especially important in food preservation is the lengthening, as much as possible, of the lag phase and the phase of positive acceleration. 1. By introducing as few spoilage organisms as possible, i.e., by reducing the amount of contamination; the fewer organisms present and longer the lag phase. 2. By avoiding the addition of actively growing organisms (from the logarithmic phase of growth). Such organisms might be growing on unclean containers, equipment, or utensils that come in contact with foods. For Application Food Preservation: 3. By one or more unfavorable environmental conditions, unfavorable food, moisture, temperature, pH, O-R potential, or presence of inhibitors. The more unfavorable the conditions, the longer or the delay of the initiation of growth. 4. By actual damage to organisms by processing methods such as heating or irradiation. Thus, for example, bacteria or their spores subjected to sublethal heat treatments have been found to require a better culture medium for growth than do unheated organisms. Often a combination of methods for delaying the initiation of growth is enough to give a food the desired storage life. Methods of Food Preservation: A. Preservation by use of High temperatures: THE MOST COMMEN METHOD IN FOOD PERSERVATION – The main objective of heat treatment of food is to kill the microbes that cause food spoilage or that pose a threat to the health of the consumer, – heat kills the microbes through the accumulation or clotting of microbial protein, coagulation, or denaturation so that it becomes unable to perform its vital functions, high temperature inhibits the enzymes of metabolism and also terminate the gene responsible for reproduction as well as destroying the outer wall of the cell. Factors influence the thermal resistance of microbial cells or spores: 1. The relationship between temperature and time: the time of killing cells or spores under a certain group conditions decrease with increasing temperature. 2. Initial concentration of spores (or cells): The higher the number of spores or cells, the greater the number of spores Heat treatment is necessary to kill them all.  Factors influence the thermal resistance of microbial cells or spores: 3. The Previous history of the vegetative cells or spores: the conditions in which the cells live, they have grown and produced spores and their processing will then affect their heat resistance. 4. Phases of growth or age: Bacterial cells know their greatest resistance during the late lag phase but almost as great resistance during their maximum stationary phase, followed by a decline in resistance. Heat treatments employed in processing foods: The various degrees of heating used on foods might be classified as A- Heating below 100°C (Pasteurization). B- Heating at about 100°C (boiling). C- Heating above 100°C (sterilization).  1. Pasteurization: is a heat treatment that kills part (PATHOGENS) but not all of the microorganisms present in foods and the temperature applied is below 100°C. Heating may be by steam, hot water, dry heat, or electric currents. Products are cooled immediately after the heat treatment (heat shock). Pasteurization is used: 1. When more rigorous heat treatments might harm the quality of the product. 2. To kill pathogens in milk. 3. When the main spoilage organisms are not very heat resistant (ex. Yeasts in fruit juices). 4. When competing microorganisms are to be killed, allowing the desired fermentation. Preservative methods used to supplement pasteurization include 1. Refrigeration Ex: Milk 2. Keeping out microorganisms Ex: Packaging 3. Addition of high concentrations of sugar. Ex: Sweetened condensed milk 4. Presence or addition of chemical preservatives. Ex: Organic acids on pickles.  Methods of pasteurization: 1- HTST Method: (High-temperature short time) In this method, high temperature is employed for a short time. The temperature is 71.7°C and time is 15 sec. 2- LTLT Method: (Low-temperature long time) In this method, low temperature is employed for a longer time. The temperature is 62.8°C and the time is 30 minutes. 3- Ultra pasteurization: In this method, the temperature applied is 137.8°C for at least 2 seconds. Earlier pasteurization temperature was set based on pathogenic organism present in milk Mycobacterium tuberculosis. This bacterium killed at 61.7°C.  The phosphatase test – is applied to determine whether milk has been properly pasteurized. – Milk contains the enzyme alkaline phosphatase which is inactivated by the time/temperature combinations applied during pasteurization. – To determine whether a milk sample has been satisfactorily pasteurized and is free from contaminating raw milk, a chromogenic substrate (p-nitrophenyl phosphate) is added. If active phosphatase is still present then it will hydrolyse the substrate producing a colour.  2- Boiling: Heating at about 100°C This treatment was sufficient to kill everything but not bacterial spores in the food and it is sufficient to preserve even low and medium acid foods. Many acid foods can be processed successfully at 100°C or less. 100°C temperature is obtained by boiling a liquid food or by immersion the container of food in boiling water or exposure to steam. It is the incipient or gentle boiling with a temperature about 100°C.  3- Sterilization: ❑ Temperatures above 100°C usually are obtained using steam under pressure in steam pressure sterilizers or retorts (autoclaves). The temperature in the retort increases with rising steam pressures. ❑ Ex: Milk can be heated to temperatures up to 150°C by use of steam injection or steam infusion followed by flash evaporation of the condensed steam and rapid cooling. ❑ Processes such as this for milk have been referred to as Ultra Heat Temperature (UHT) processes. ❑ To detection of the efficiency of the food sterilization process, the presence of Clostridium botulinum is detected because it has great resistance to high heat its spore resist 120˚ for 20 min.  Thermal Destruction of Microorganisms: 1- Thermal death time (TDT): is the time necessary to kill a given number of microorganisms at a specified temperature (condition: temperature is fixed and time is varied). 2-The thermal death point (TDP): which is the temperature necessary to kill a given number of microorganisms in a fixed time (10 minutes). 3- D Value (the decimal reduction time): the time required to destroy 90% of the microorganisms.  B. Preservation by use of Low temperatures Low temperatures are used to retard chemical reactions and action of food enzymes and to slow down or stop the growth and activity of microorganisms in food. Growth of Microorganisms at low temperatures: – Freezing prevents the growth of most food borne microorganisms and refrigeration temperature slow growth rates. – Clostridium botulinum type E has a minimum temperature for growth of about 3.3°C. – Yersinia enterocolitica grow at temperatures 0-3°C. Temperatures employed in low temperature storage: 1) Common or cellar storage: ▪ The temperature in common or cellar storage usually lower than 15°C. ▪ Root crops, potatoes, cabbage, celery, apples stored for limited periods. ▪ The deterioration of such fruits and vegetables by their own enzymes and by microorganisms is not prevented but is slower than at atmospheric temperatures. ▪ Low humidity in the storage cellar results in losses of moisture from the stored food and too high humidity favors spoilage by microorganisms.  Temperatures employed in low temperature storage: 2- Chilling or cold storage: ▪ Chilling storage is at temperatures not far above freezing (ranged from -1 to 8˚C) and usually involves cooling by ice or mechanical refrigeration. ▪ Eggs, dairy products, meats, seafood, vegetables and fruits may be held in chilling storage for a limited time. ▪ Enzymatic and microbial changes in the foods are not prevented but are slowed considerably. ▪ Chilling temperature is selected on the basis of the kind of food and the time and conditions of storage  2- Chilling or cold storage: Ex: Banana stored and keeps best at 13.3 – 16.7°C and Sweet potatoes keep best at 10-12.8°C. – Changes in humidity as well as in temperature during storage may cause “sweating” or precipitation of moisture on the food. A moist surface favors microbial spoilage. – Ventilation or control of air velocities of the storage room is important in maintaining a uniform relative humidity throughout the room, removing odors and preventing the development of stable odors and flavors. – The amounts and proportions of gases in the storage atmosphere influence preservation by chilling.  2- Chilling or cold storage: – In “Gas storage” of foods, where the composition of the atmosphere has been controlled by the introduction of CO2, Ozone or other gas. – Gas storage ordinarily is combined with chilling storage. In the presence of optimal concentrations of carbon dioxide or ozone the following advantages are present A food will remain unspoiled for a longer period. Higher relative humidity can be maintained without harm to the keeping quality of certain foods. – Higher storage temperature can be used without shortening the keeping time of the food. – 2.5% CO2 to be best for eggs – 10.0% CO2 for chilled beef - 100% CO2 for Bacon.  2- Chilling or cold storage: – Combination of U.V. irradiation with chilling storage helps preserve some foods and may permit the use of a higher humidity or storage temperature than with chilling along. U.V. lamps have been installed in rooms for the storage of meat and cheese.  Temperatures employed in low temperature storage: 3- Freezing or frozen storage: – The storage of foods in the frozen condition has been an important preservative method for centuries. Under frozen storage, microbial growth is prevented entirely and the action of food enzymes is greatly retarded. The lower the storage temperature, the slower will be any chemical or enzymatic reactions. – Fruits and vegetables are selected on the basis of their suitability for freezing and their maturity and are washed, trimmed, cut or otherwise pretreated as desired. Vegetables are scalded or blanched. Fruits are packed in syrup. Most foods are packaged before freezing but strawberries are frozen before packaging. Temperatures employed in low temperature storage: 3- Freezing or frozen storage: ❖ Scalding or blanching of vegetables is done with hot water or steam and has the following advantages: 1. Inactivation of most of the plant enzymes which cause toughness, change in colour, mustiness, loss in flavour, softening and loss in nutritive valve. 2. Reduction in the numbers of microorganisms on the food. 3. Enhancement of the green color of vegetables such as spinach.  Types of freezing: Quick Freezing: the temperature of foods is Slow Freezing: temperature is achieved lowered to about −20◦C within 30 minutes within 3–72 hours Small ice crystals formed (intracellular ice Large ice crystals formed (extracellular crystals) crystals) Breakdown of metabolic rapport Blocks or suppresses metabolism Longer exposure to adverse or injurious factors Brief exposure to concentration of Gradual adaptation adverse No shock effect No adaptation to low temperatures Accumulation of concentrated solutes with Thermal shock beneficial effects No protective effect Microorganisms frozen into crystals?  Advantages of Quick Freezing over slow freezing: 1. Smaller ice crystals are formed; hence there is less mechanical destruction of infact cells of the food. 2. There is a shorter period of solidification of ice 3. There is more prompt prevention of microbial growth 4. There is more rapid slowing of enzyme action. 5. Quick frozen foods are supposed to bring to the room temperature before cooking. consumption. This process is called thawing. Ex: Vegetables, meat.  Effect of Freezing on Microorganisms: ❖ Bacteria differ in their capacity to survive during freezing, with cocci being generally more resistant than Gram-negative rods. The food-poisoning bacteria, salmonellae are less resistant than Staphylococcus aureus or vegetative cells of Clostridium sp. ❖ Endospores and food-poisoning toxins are unaffected by low temperatures. ❖ Occurrence of low-temperature-induced changes in fatty acid composition, it is known that an increase in the degree of unsaturation of fatty acids in lipids leads to a decrease in the lipid melting point that increased the synthesis of unsaturated fatty acids at low temperatures which led to highly resistant to low temperature.  Changes during preparation for Freezing: – The rate and kind of deterioration of foods before freezing will depend on the condition of the food at harvesting or slaughter and the methods of handling. The temperatures at which the food is held and other environmental conditions will determine the kinds of microorganisms to grow and the changes to be produced. – The condition of the food at the time of freezing will determine the potential quality of the frozen food. thank you