Food preservation notes PDF

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Food preservation Introduction All the food produced in the farm or by animals cannot be eaten immediately when harvested. Moreover it is important to have food throughout the seasons; in the dry seasons, wet seasons, summer, and winter and throughout the year. Mother Nature intends that food begins to spoil immediately it is harvested or collected. Food spoilage causes loss of very essential resources; the work force, time and money spent to produce the food, the energy used, and the nutrients. Just to mention a few. Food preservation aims at preventing food deterioration or food spoilage and increasing the time duration before spoilage (shelf life). It contributes to food stability by ensuring availability of food supply throughout the year. Food preservation also ensures that people who have no immediate access (have not produced) to a particular food are able to get it in its fresh state.

FOOD PRESERVATION Food preservation is a process of physical, chemical or microbiological treatment of food and beverages to prevent food spoilage by slowing down or preventing the action of the agents of spoilage. Food preservation aims at; 1. Removal of micro-organisms or inactivating them: Microorganisms are one of the agents of food spoilage. The destruction of microorganisms will therefore enhance the keeping quality of food. The control of microorganisms can be done by; keeping them out of food, then can be removed from the food, their growth can also be delayed and the can be killed and their spores destroyed too. This is achieved by removing air, water (moisture), lowering or increasing temperature, increasing the concentration of salt or sugar or acid in foods. If you want to preserve a given food, you can remove all/some of the water so as to ensure that microorganisms cannot survive or their growth has been delayed. You do this by drying. There are other methods discussed below that make the environment unfavorable for microorganism to survive 2. Inactivating enzymes: Enzymes in foods contribute to food deterioration. Enzyme activity is affected mainly by temperature and moisture. Vegetables are blanched by putting them for a few

minutes in boiling water for preservations purposes. The principle behind this is the inactivation of enzymes. Factors that are important in controlling enzyme activity responsible for food spoilage include the following: i.

ii.

iii.

iv.

Substrate concentration: It is experimentally shown that if the enzyme concentration is fixed and the substrate concentration is gradually increased, the reaction velocity increases until it reaches a maximum. After this point, any more increase in the substrate concentration will not increase the velocity. Temperature: Most enzymes are adversely affected by a temperature above 40° C. They are rapidly denatured, i.e. lose catalytic activities. Enzymes lose their activity at extremely low temperatures as well. This may account for storing enzymes at 5° C or below without affecting the enzymatic activity permanently. PH: Enzymes are affected by changes in the pH. The most favourable pH, at which an enzyme exhibits its maximum activity, is known as the optimum pH for the enzyme. Extremely high or low pH values generally result in complete loss of enzymatic activity. The optimum pH value varies greatly from enzyme to enzyme. Inhibitors and activators: Many molecules affect the rates of enzyme catalyzed reactions. Some molecules bind to the enzyme or the substrate or the enzyme-substrate complex and lower the reaction rate. These are known as inhibitor molecules. Similarly, some bind to the enzyme molecule and consequently increase the reaction rate. These are known as activator molecules.

3. Removal of insects, worms, rodents and other physical causes of food deterioration: this is another major cause of food spoilage. The removal of these physical contaminants can increase the shelf life of food. Preservation of food should be able to eliminate potential harm to the consumer, maintain the food’s quality attributes such as taste smell color e.t.c and also maintain the food’s nutritional value CLASSIFICATION OF FOODS Perishable foods: these are foods that are minimally processed and have a shelf life of 6 MONTHS. They include foods such as; cereal grains, frozen food, canned food, dehydrated vegetables. METHODS OF FOOD PRESERVATION Heat preservation. This is one of the most effective methods of killing microorganisms and denaturing enzymes. Most microorganisms grow best with temperature range of about 16-38 degrees. Most bacteria are killed at 82-930C degrees. Not all foods require the same amount of heat since the types of microorganisms found in different foods is different. Additionally different microorganisms require different heat treatment (temperature and duration) for them to be destroyed. The heat treatment methods can be divided into two; 

Mild

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Severe

Mild heat treatment Include 

Pasteurization

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Blanching

Pasteurization Pasteurization is named after Pasteur, who demonstrated that wine spoiled because of the presence of microorganisms and that a mild heat treatment could be used to inactivate the microorganisms extended its shelf life. It is mostly known for its application to milk, which is strictly regulated through the FDA’s Pasteurized Milk Ordinance.it Is a mild heat treatment used for mainly for liquids such as milk, fruit juices and beer. The heating to temperature is usually high enough but < 100oC to destroy most of pathogens The main aim of pasteurization includes; 

Destroy pathogenic organisms that may be associated with the food (mycobacteria, brucella, salmonella, staphylococcus spp., Campylobacter,Yersinia and Vibrio spp.,etc.) thermophilic bacteria and bacterial spores may survive.

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Destroy majority of the spoilage organisms (≥99%) and reduce the bacterial count so as to extend shelf life

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Inactivate enzymes.

Pasteurization products will still contain many living organisms capable of growth but the load will have been reduced. Pasteurization can be achieved by; 

High temperature short time (HTST)s e. g 72-80 degrees for 2-15 seconds

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Low temperature long time (LTLT) e, g 65-70 degrees for over 30 mins

Pasteurised products should always be chilled since they still contain some microorganisms. Blanching Blanching involves dipping the vegetables/fruits in boiling or near boiling water for 1-3 minutes. Mainly used for preservation of vegetables and some fruits prior to freezing. The main aim of blanching is to; inactivate enzymes that would cause deterioration of foods. It helps in retaining the colour, flavor and texture of vegetables and fruits. Some microorganisms are also destroyed during blanching Severe heat treatment Canning This is a method which is used to inactivate microorganisms that cause food-borne disease such as botulism. This process also inactivates microorganisms that cause food spoilage. Temperatures in the range 116-1200 C for several minutes are used for canning. Canned food is not absolutely sterile (absence of all viable microorganisms) but rather it is commercially sterile (absence of all viable microorganisms that could grow under normal storage conditions). There are 2 major methods of canning: 1) placing food inside a cylinder or body of a can then the lid is sealed in place and the can is then heated in a large commercial pressure cooker (this ids referred to as canning).2) Sterilizing the food, then putting it in a sterile container within a sterile environment and sealing the container (referred to as aseptic processing). These processes can be adjusted accordingly in order to retain nutrients and other quality attributes such as taste, flavor, and color. Canned fruits and vegetables are a source of vitamin C especially in seasons where

fruits and vegetables are not available. Heating times temperature vary, but the heat treatment must be sufficient to sterilize every particle present in the food. Main purpose of canning is to achieve commercial sterility Sterilization Sterilization refers to complete destruction of all microorganisms and spores. It requires a treatment of at least 1210 C for 15 minutes or its equivalent. Every particle of food must receive the heat treatment. During sterilization, there may be many changes in the food which reduce its quality therefore many foods need not to be completely sterilized to be safe and have keeping quality. The sterilization process in the canned product can be subdivided into three phases: i.

By means of a heating medium (steam or water), the product temperature is increased from ambient to the required sterilization temperature (Phase 1-heating phase)

ii. iii.

This temperature is maintained for a defined time (Phase 2-holding phase) The temperature in the can is decreased by introduction of cold water into autoclave (phase 3-cooling phase)

Commercial sterility This is the degree of sterilization at which all pathogenic and toxic organisms have been destroyed as well as all other types of microorganisms which if present will ruin the product and produce spoilage under normal handling and storage conditions. Commercially sterilized foods may contain a small no. of heat resistant bacterial spores that are unable to grow under conditions. Most commercially sterilized foods have a shelf life of 2 years or more.

COLD PRESERVATION The principle behind cold preservation is that the growth of most microorganisms is slowed down by low temperature; enzyme activity is also slowed down by the same. Cold preservation does not completely destroy microorganisms, some microorganisms known as psychrotrophs grow at even zero degrees. At temperature below 100 C the growth of many microorganisms is slow and becomes slower with decrease in temperature. Since pasteurization does not completely

destroy microorganisms, some foods need to be frozen or chilled after pasteurisation. When the water in food is completely frozen, there is no multiplication of microorganisms. The slowing of microbial activities with decreased temperature is principle behind refrigeration and freezing preservation. Refrigeration is storage above freezing from about 160 C to -20 C. Household refrigerators operate at 4.5 to 70 C. Freezing Freezing is storing food at temperatures that maintain food in frozen conditions, that is...