Food Spoilage and Preservation - L36 - PDF

**Food Spoilage and Preservation I** **Introduction** Food is defined as any material consisting essentially of protein, carbohydrate and fat used in the body of an organism to sustain growth, repair and vital processes and to furnish energy. As food contains various essential nutrients they are p...

**Food Spoilage and Preservation I** **Introduction** Food is defined as any material consisting essentially of protein, carbohydrate and fat used in the body of an organism to sustain growth, repair and vital processes and to furnish energy. As food contains various essential nutrients they are prone to contamination by various microorganisms. Contamination may be -- 1. Natural contamination -- natural contamination occurs when microorganisms attached themselves to foods. For instance, fruits are often contaminated with yeast because yeast ferments the carbohydrates in fruits. 2. Artificial contamination -- artificial contamination occurs when food is handled or processed such as when fecal bacteria enters food through improper handling procedures. **Food spoilage** In food, several kinds of chemical changes occur once the food is harvested. Some changes may be considered desirable such as meat tenderizing whereas other such as putrefaction render food unfit for consumption. The major classes of food change include -- 1. **Enzymatic processes** -- the flesh of the animal undergoes proteolysis by its own enzymes following its death but plants after harvest undergo other type of change. E.g. in harvested corn the sugar rapidly converts to starch. 2. **Chemical reactions with the environments** -- the most common abiotic chemical reactions involve oxidation by air. E.g. lipid autoxidation generates rancid odours. 3. **Microbiological processes** -- microbes when contaminated the food generates a wide range of chemical products. Food spoilage refers to the microbial changes that render a product unfit or unpalatable for consumption. Most foodstuffs serve as good media for the growth of many different microorganisms. Once the microorganism grows, they will produce changes in appearance, flavour, odour and other qualities of food. The degradation process may be -- 1. Putrefaction : Protein foods + proteolytic microbes amino acids + amines + ammonia + hydrogen sulphide 2. Fermentation : Carbohydrate foods + carbohydrate fermenting microbes acids + alcohol + gases 3. Rancidity : Fatty foods + lipolytic microbes fatty acids + glycerol Different foods spoils in different ways depending on their nutrient content, microbial species and environmental factors such as temperature. **Dairy products** -- although milk by fermentation lead to the production of yogurt, cheese etc, however under certain conditions, bitter off flavours are produced because of the bacterial degradation of proteins. Protein degradation is commonly caused by the psychrophilic organisms (organisms that can grow at cold temperature) while cheese are less susceptible than milk to spoilage because of their solid structure and lower water activity. **Meat and poultry** -- meat is easily contaminated with bacteria in the slaughterhouse from their hides, hooves and intestinal contents. Also, muscle tissue has high water content as well as rich nutrients including glycogen, peptides and amino acids which supports the microbial growth. Therefore, the breakdown of this peptides and amino acids produces the undesirable odorants which is an indication of spoilage. Meat also contains fats but the lipids are largely unavailable to microbial action because they consist of insoluble fats. Meats lipids are commonly spoiled abiotically by autoxidation of unsaturated fatty acids independent of microbial activity. **Seafoods** -- fish spoils more rapidly than meat and poultry. This is because fish do not thermoregulate and they inhabit relatively low temperature environments and also since they grow in low temperature environments, their surface microorganism tends to be more psychrotrophic and thus grow well under refrigeration. In addition, marine fish contain high level of osmoprotectant trimethyamine oxide in which bacteria reduce it to trimethyamine. Trimethylamine is a volatile amine responsible for the fishy smell of seafoods. Finally, the rapid microbial breakdown of proteins and amino acids leads to foul smelling amines and sulphur compounds such as hydrogen sulphide and dimethylsulfide. **Plant foods** -- fruits, vegetables and grains spoils differently than animals food because of their high carbohydrate content and their relatively low water content. And hence because of the low water content of the plant foods, they have considerably longer shelf life than the animal based foods. Further, carbohydrates favour microbial fermentation to acids or alcohols that limit further decomposition and this microbial action can be managed to produce fermented foods. **Microbial flora of fresh foods** The inner tissues of healthy plants and animals are free of microorganisms. However the surfaces of raw vegetables and meats are contaminated by a number of microbes. Hence it is desirable to maintain a very low microbial level of contamination on raw foods. **Meats** -- among the most common bacteria occurring on fresh meats are the pseudomonas, staphylococci, micrococci, enterococci and the coliforms. **Poultry** -- freshly dressed poultry have a bacterial flora on their surface that originates from the bacteria normally present on the live birds and that are contaminated during killing, defeathering etc. Under good sanitary conditions the bacterial count has been reported to be from 100-1000 bacteria per square centimetre of skin surface. **Fruits and vegetables** -- fruits and vegetables are normally susceptible to infection by bacteria, fungi and viruses. In fruits since their pH is acidic ranging from 2.3 for lemons to 5.0 for bananas, it restricts the bacterial growth but does not retard the fungal growth. In case of vegetables since their pH is slightly higher ranging from5.0-7.0, they are more susceptible than fruits to attack by bacteria. **Food preservation** Food preservation is the process of treating and handling food to stop or slow down spoilage caused or accelerated by microorganisms. Preservation usually involved preventing the growth of bacteria, fungi and other microorganisms as well as retarding the oxidation of fats which cause rancidity. **Principles of food preservation** Principles of food preservation includes -- 1. Prevention or delay of microbial decomposition - By keeping out microorganisms (asepsis) - By removal of microorganisms - By hindering the growth and activity of microorganisms (e.g. by low temperature, drying, anaerobic conditions, or chemicals) - By killing the microorganisms (e.g. by heat or radiation) 2. Prevention or delay of self decomposition of the food - By destruction or inactivation of food enzymes (by blanching) - By prevention or delay of chemical reactions (by using antioxidant) **Methods of food preservation** Preservation of food can be achieved by application of physical or chemical methods. Physical means of preservation includes -- - Dehydration and freeze-drying -- as we all know that removal of water prevents microbial growth. So it is removed either by application of heat or by freezing under vaccum known as freeze-drying or lyophilisation. One of the disadvantage of drying is that some nutrients are broken down. - Refrigeration and freezing -- refrigeration temperature which is usually between -2°C to 16°C slows microbial growth. Freezing halts the growth of most microbes but pre-existing contaminant strains often survive to grow again when the food is thawed. - Controlled or modified atmosphere -- food can also be packed or stored under vacuum with decreased oxygen or increased carbon dioxide. Controlled atmosphere limit abiotic oxidation as well as microbial growth. - Pasteurization -- pasteurization is usually carried out at 63°C for 30 minutes followed by quick cooling to 4°C. Pasteurization in effective in extending the shelf life of liquid foods. - Canning -- canning is carried out to for the long term storage of food. For this food is cooked under under pressure to attain a temperature high enough to destroy endospores. Canning effectively eliminates microbial contaminants. One of the disadvantage of canning is that it incurs some loss of food value as well as loss of desirable food texture and taste. - Ionizing radiation -- food irradiation effectively sterilizes many kinds of food for long term storage. Irradiation has also proved highly effective at eliminating pathogens that would cause serious illness. **Chemical methods of food preservation** Many kinds of chemicals are used to preserve foods. The major classes of chemical preservatives include -- - **Acids** -- although by microbial fermentation foods can be preserved by acidification, an alternative approach is to add acids directly to the food. Organic acids that are commonly used to preserve foods are the benzoic acid, sorbic acid and propionic acid. These acids are usually added to the food as salts i.e. sodium benzoate, potassium sorbate and sodium propionate. This acids acts by crossing the cell membranes in the protonated form thereby releasing their protons at a higher intracellular pH and it is for this reason, they work best in foods that already have moderate acidity. - **Esters** -- esters of organic acids shows antimicrobial activity. E.g. fatty acid esters and benzoic acid esters. They are used to preserve processed cheese and vegetables. - **Other organic compounds** -- numerous organic compounds (e.g. eugenol from cinnamon and cloves) shows potent antimicrobial activity. - **Inorganic compounds** -- salts such as phosphates, nitrites and sulphites are some of the inorganic food preservative. Nitrites and sulphites inhibit aerobic respiration of bacteria and their effectiveness is enhanced at low pH. However, these substances may show harmful effects on humans because nitrites can be converted to toxic nitrosamines and sulphites can cause allergic reactions to some peoples. **Conclusion** Any chemical change that renders food unfit for consumption is termed as food spoilage. Food is spoiled through the degradation of enzymes within the food, through spontaneous chemical reactions and through microbial metabolism. Food borne pathogens are spread during harvesting, processing and share consumption of food. Hence food needs to be preserved by the application of various methods to prevent from certain microbial contaminants. **GLOSSARY** **Contamination --** non intended or accidental introduction of infectious microbes **Rancidity --** degradation of fats and oils **Putrefaction --** degradation of proteinaceous foods **Abiotic --** not associated or derived from living organisms **Psychrophilic organism --** organisms that can live in cold environments **Osmoprotectant -** Any osmolyte that helps an organism to survive osmotic stress. **Shelf life -** the time period during which a stored commodity remains effective, useful, or suitable for consumption **Fermentation --** An energy yielding process in which organic molecules serves as both electron donors and acceptors **Pasteurization --** the process of heating milk and other liquids to destroy microorganisms that can spoilage or disease. **Pathogens --** microbes that can cause disease **FAQS (FREQUENTLY ASKED QUESTIONS)** 1. **Define food.** **Answer:** Food is defined as any material consisting essentially of protein, carbohydrate and fat used in the body of an organism to sustain growth, repair and vital processes and to furnish energy. 2. **Differentiate between natural contamination and artificial contamination.** **Answer:** natural contamination occurs when microorganisms attached themselves to foods whereas artificial contamination occurs when food is handled or processed such as when fecal bacteria enters food through improper handling procedures. 3. **Give a detailed account on the major classes of food change.** **Answer:** The major classes of food change are -- **Enzymatic processes** -- the flesh of the animal undergoes proteolysis by its own enzymes following its death but plants after harvest undergo other type of change. E.g. in harvested corn the sugar rapidly converts to starch. **Chemical reactions with the environments** -- the most common abiotic chemical reactions involve oxidation by air. E.g. lipid autoxidation generates rancid odours. **Microbiological processes** -- microbes when contaminated the food generates a wide range of chemical products. 4. What is spoilage? Answer: Food spoilage is the microbial changes that render a product unfit or unpalatable for consumption. 5. Give reason why cheese are less susceptible than milk to spoilage. Answer: Cheese are less susceptible than milk to spoilage because of their solid structure and lower water activity. 6. Write a note on spoilage of meat and poultry. Answer: Meat is easily contaminated with bacteria in the slaughterhouse from their hides, hooves and intestinal contents. Also, muscle tissue has high water content as well as rich nutrients including glycogen, peptides and amino acids which supports the microbial growth. Therefore, the breakdown of this peptides and amino acids produces the undesirable odorants which is an indication of spoilage. Meat also contains fats but the lipids are largely unavailable to microbial action because they consist of insoluble fats. Meats lipids are commonly spoiled abiotically by autoxidation of unsaturated fatty acids independent of microbial activity. 7. Name the different microflora present in meats. Answer: Bacteria occurring on fresh meats are the pseudomonas, staphylococci, micrococci, enterococci and the coliforms. 8. State the principles of food preservation. Answer: Principles of food preservation includes -- 3. Prevention or delay of microbial decomposition - By keeping out microorganisms (asepsis) - By removal of microorganisms - By hindering the growth and activity of microorganisms (e.g. by low temperature, drying, anaerobic conditions, or chemicals) - By killing the microorganisms (e.g. by heat or radiation) 4. Prevention or delay of self decomposition of the food - By destruction or inactivation of food enzymes (by blanching) - By prevention or delay of chemical reactions (by using antioxidant) 9. Name some chemicals that are used in food preservation. Answer: The major classes of chemical preservatives includes acids, esters, organic compounds such as eugenol and inorganic compounds such as phosphates, nitrites and sulphites. 10. How does dehydration and freeze-drying preserved food? Answer: Removal of water prevents microbial growth. Removal of water from food either by application of heat or by freezing under vaccum known as freeze-drying or lyophilisation preserved foods for longer period of time.