Food Preservation - Principles & Methods PDF

[FOOD PRESERVATION] Is a process through which physical and/or chemical agents are used to prevent microbial spoilage of foods, thus allowing the food to be stored in a fit condition for future use. [AIM]: at treating food in a manner to prolong its storage life. [Foods are classified into three...

[FOOD PRESERVATION] Is a process through which physical and/or chemical agents are used to prevent microbial spoilage of foods, thus allowing the food to be stored in a fit condition for future use. [AIM]: at treating food in a manner to prolong its storage life. [Foods are classified into three categories] - Perishable foods: These are the foods which deteriorate quickly after harvesting such as tomatoes, mangoes, papaya, peaches, plums and other juicy fruits. Meat, fish and poultry also fall in this category. These foods have a high level of moisture content and are highly susceptible to spoilage. 2- Semi - perishable food: These foods have less moisture content e.g. beetroots, carrots, peas, green beans, pumpkins and apples. Eggs, pasteurized milk, highly smoked fish, and pickled vegetables belong to this group. 3- Non - perishable foods: These foods have very low moisture content e.g. mature food grain cereals, pulses and nuts. These foods are not easily susceptible to spoilage by micro- organisms and enzymes. [FOOD PRESERVATION PRINCIPLES Two general principles are employed in food preservation] \(1) Inhibition principle \(2) Killing principle \(1) Inhibition principle: In this principle, food preservation is achieved by inhibition of growth and multiplication of microorganisms. [The inhibition principle may be achieved by any of the following;-] \(a) Reduction of water activity e.g. by drying or salting. \(b) Reduction in pH e.g. by fermentation and \(c) Use of preservatives e.g. sodium benzoate \(d) Use if low temperature e.g. chilling or freezing. \(e) Smoking - This has a drying and preservative effect. Preservation of foods by inhibition methods does not necessarily imply destruction of organisms. On removal of the inhibiting influence, the food will undergo spoilage as the microorganisms present will grow and multiply to cause spoilage. [1- Food preservation by lowering pH] : Many food products can be preserved by lowering pH so that the growth of spoilage and pathogenic bacteria is prevented. The lowering of pH can be achieved by addition of acids or fermentation. [Fermentation] : is the break- down of carbohydrates under anaerobic conditions into alcohol or lactic acid and carbon dioxide. [2- Food preservation by lowering water activity] may be achieved by: - 1- Addition of high content of salt eg sodium chloride, and sometimes nitrates and nitrites. 2- Addition of high content of sugar. 3- Drying:- sun/air drying, electrical drying or freeze drying. [3- The salting procedure] : may be performed in four ways:- 1- Dry cure in which the meat or fish is rubbed with salt. 2- Pickling The products are immersed in pickle of brine, usually containing about 15% salt. 3- The injection cure concentrated salt solution is injected in muscle tissues. 4- Direct salt addition method [4-Food preservation by use of low temperatures :] Two methods are used in application of low temperatures to arrest microbial growth and multiplication 1- Chilling is keeping of food at temperatures between 0 15°C. The common chilling temperatures range between 4 - 5°C. 2- Freezing is keeping food at temperatures between -1°C -35°C. Effect of low temperatures: Low temperatures are used to retard chemical reactions and actions of food enzyme and to slow down or stop the growth and activity of microorganisms in the food [5-Food preservation by use of preservatives] A preservative: is defined as any substance which is capable of inhibiting, retarding or arresting The growth of microorganisms, their enzymes or their genetic mechanisms. Chemical preservatives are generally added after the foods are processed. [Preservatives are divided into two types] Class one (1) preservatives : includes: Common salt Sugar, Dextrose Glucose (syrup) , Vinegar or acetic acid. Class two (2) preservatives : There include - Benzoic acid and Sodium benzoate Methyl and propyl p , Nitrites, Chlorine ,Carbon dioxide. [6- Preservation of food by drying] This is one of the oldest methods of preserving foods. All the cereal grains are preserved by drying. Sun- drying remains the greatest food preservation action. Milk is also preserved through drying in form of milk powder. Meats are also preserved through drying. [2- KILLING PRINCIPLE] In this principle, spoilage microorganisms in the food are destroyed (killed), and the food protected against contamination by being enclosed in an airtight container. [Methods employed to achieve the killing principle] Heat treatment through 1- pasteurization or sterilization. 2- Irradiation with either ionizing or electromagnetic radiation e.g. Gamma rays, cobalt 60 radioactive particles. Radiation kills microorganisms by destruction of DNA and by creating toxic reactive compounds in a medium and in the microbial cells. 3- Use of gases: by use of ethylene oxide or ozone. The gases destroy both vegetative cells and spores [PASTEURIZATION] This is the process of heat treatment, mainly of milk at specific temperatures and times. Pasteurization is aimed at destroying all pathogenic microorganisms without affecting the nutritive value of the food Pasteurization is commonly use in milk processing. [There are three methods of pasteurization;-] 1- Low temperature long time (LTLT) : 63°C for 30mins. 2- High temperature short time (HTST) : 72°C for 15 sec. Flash Method : 80°C for 1-2 sec 3- [STERILIZATION] : This is the use of physical or chemical means to destroy all microorganisms including spores that are present in the food. [Sterilization may be achieved by;-] 1- Heating at high temperature e.g. 100°C - 140°C. 2- Irradiation : This kills bacteria, spores, and insects and also inactivates enzymes. NOTE: a combination of inhibition and killing principles and the various methods are used in food preservation [depending on the food type]. eg.1- Use of pasteurization and chilling of milk. 2 -Use of preservatives and low temperature storage e.g. in fruit juices [Important Terminologies used in Heat Preservation] 1- Decimal reduction time (D - Value) :- This is time required at any temperature to destroy 90% of the spores or vegetative cells of a given organism. The higher the temperature, the faster is the rate of destruction and the shorter it takes to kill 90% of the cells. For example, [D - Value of Clostridium sporogenes] in a given food at 120°C is 1 minute, at 115°C is 4 minutes and at 110°C is 10 minutes.