Food Microbiology - Chapter 6


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Food Microbiology - Chapter 6

  1. 1. CHAPTER 6 FOOD PRESERVATION A. METHODS OF FOOD PRESERVATION. 1. Asepsis- keeping out microorganisms. 2. Removal of microorganisms. 3. Maintenance of anaerobic condition. 4. Use of high temperatures. 5. Use of low temperatures. 6. Drying and smoking. 7. Use of chemical preservatives. 8. Irradiation. 9. Mechanical destruction of microorganisms.10. Combinations of two or more of the above methods.
  2. 2. B. PRINCIPLES OF FOOD PRESERVATION. 1. Prevention or delay or microbial decomposition. • By keeping out microorganisms (asepsis) • By removal of microorganisms (filtration) • By hindering the growth and activity of microorganisms (drying, anaerobic condition, lowering the temperature, etc) • By killing the microorganisms (heat or radiation) 2. Prevention or delay of self-decomposition of the food. • By destruction or inactivation of food enzymes • By prevention or delay of purely chemical reactions • E.g. prevention of oxidation 3. Prevention of damage because of insects, animals, mechanical causes etc.
  3. 3. C. APPLICATIONS OF MICROBIAL GROWTH CURVE TO FOOD PRESERVATION. • It is important in food preservation (prevention of spoilage) to lengthen as much as possible the log and the log phase. • This can be done in different ways: 1. Introducing as few spoilage organisms as possible 2. Avoiding the addition of actively growing organisms 3. One or more unfavorable environmental conditions are introduced. • Microbial decomposition of foods will be prevented if all spoilage organisms are killed and recontamination is prevented. • Hindering the growth of microorganisms by merely stopping the multiplication does not necessarily prevent decomposition because there is possibility that viable organisms or their enzymes may continue to be active.
  4. 4. D. FOOD PRESERVATION (METHOD)1. ASEPSIS• Natural protection - outer layer of animal and plant tissue protects the inner layer free from microorganisms. This protective covering will delay / prevent microbial decomposition e.g. shells of nuts, skins of fruits and vegetables etc.• Packaging of foods - such as wrapping, hermetically sealed containers. This methods will prevents primarily contamination during handling.• Sanitary methods of handling and processing foods e.g. in the dairy industry, contamination with microorganisms is avoided as much as possible in the production and handling of milk.• Food industries - attention is given to prevention of the contamination of foods (from raw material finished products) concerning the "bioburden" of microorganisms on or in a food. BIOBURDEN Kinds of microorganisms Number of microorganisms. present whether any spoilage or Spoilage organisms pathogenic microorganisms. food spoilage Difficult to preserve
  5. 5. 2. REMOVAL OF MICROORGANISMS(a) Filtration The only successful method fort he complete removal by using a pre-sterilized filters e.g. in fruit juices, soft drinks and water.(b) Centrifugation Not very effective because not all microorganisms are removed, examples: Treatment of drinking water.(c) Washing Especially helpful in removing soil microorganisms from fresh fruits and vegetables that may be resistant to heat process during canning.(d) Trimming Trim away spoiled portions of a food.3. MAINTENANCE OF ANAEROBIC CONDITIONS• Anaerobic conditions can be achieved by a complete fill, evacuation of the unfilled space, replacement of air by C02 or N2 and others.• Spores are resistant to heat and may survive in canned food but they unable to germinate in the absence of oxygen.
  6. 6. 4. USE OF HIGH TEMPERATURES• Temperature and time used in heat processing will depend on:(a) The effect of heat on the food(b) Other preservative methods employed Heat Treatment ↑ Microorganisms ↓• Classification of heat treatments used on foods:(a) Pasteurization(b) Heat at 100°C(c) Heat > 100°Ca) Pasteurization• Heat treatments that kills most but not all microorganisms.• Example: milk 63ºC, 30 mins 72ºC, 15 mins Juice 77ºC, 30 mins 88ºC, 30 secs• The pasteurized products are cooled promptly after the heat treatment.
  7. 7. • Pasteurization is important when:(a) Heat treatment will not harm the quality of product(b) Main spoilage microbes are not very heat resistant e.g. yeast in fruit juices(c) Kill pathogens(d) Any surviving organisms will be treated with other preservative methods(e) Competing organisms are to be killed, allowing a desired fermentation• Preservative methods used to supplement pasteurization:(a) Refrigeration.(b) Keeping out microorganisms by packaging.(c) Maintenance of anaerobic conditions.(d) Addition of high concentration of sugar.(e) Presence of chemical preservatives
  8. 8. b) Heating at about 100ºC• Sufficient to kill all microbes but not spores,• Many acid foods are successfully preserved at100°C.• Methods:©Boiled ©Immersion ©Baking ©Simmering © Roasting© Frying © Blanching © Exposure to flowing steamc) Heating above 100ºC• Obtained by means of steam under pressure Steam pressure if ↑ Temp. ↑ 121°C. 1 atm.• Commercial sterility: Preserving foods using minimum amount of heat necessary to kill spoilage and pathogenic microorganisms. Also include heating foods at high temperature for a short time e.g. ultra heat treatment.• All commercially sterile foods should be stored in cool, dry, place to prevent any viable thermophilic spores from germinating and cause, spoilage to the foods.• Ultra Heat Treatment: Treatment of milk by heating at 150°C by steam injection followed by flash evaporation of the condensed steam and rapid coding.
  9. 9. • Objective of heating foods:(a) To destroy pathogens and spoilage microorganisms(b) To destroy toxin present in foods(c) To destroy the vegetative cells and spores of yeast, bacteria and moulds(d) To destroy undesirable enzymes this can affect the quality of foods.(e) To control the growth of surviving microorganisms(f) To retain the acceptance and nutritional quality of foods(g) To reduce competitionCanning ProcessPreservation of foods in sealed containers followed by application ofheat treatment.Canning (also known as hermetically sealed containers) is done in tincans, glass containers, aluminum and plastic pouches. Canning Process ↓ Raw food harvested ↓ Prepare, inspect, sorting, grading ↓ Washing ↓ Filled in the can ↓
  10. 10. Brine added ↓ Evacuated by heating the headspace (vacuuming) ↓ Sealing ↓ Heat processing• Spoilage of canned food can be divided into 3 types: (a) Microbial spoilage (b) Chemical spoilage (c) Enzymatic spoilage• In general, microbial spoilage can occur due to under processing and/or leakage after processing.• Types of spoilage in canned food depends on the type of microorganisms involved:
  11. 11. 1. Thermophilic bacteria and sporesThese bacteria can cause 3 types of spoilage especially when cans are keptat > 43°C. (a) "Flat-sour" spoilage • Aerobic and facultative anaerobic bacteria and spore forming ones. • Products becomes acidic • Sealing not- properly done causing entrance of oxygen and the microorganisms • Facultative anaerobic : Bacillus stearothermophilus • They use carbohydrate and produce acid without gas: can do not swell. (b) Thermophilic Anaerobe Spoilage • Cause by thermophilic anaerobic bacteria and/or spores. • Produce acid and gas such as N2 and C02, but no H2S production. • The can swell and food turned sour. • E.g. Closthdium thermosaccarolyticum (c) Sulphide stinker spoilage
  12. 12. • Caused by gram negative, anaerobic, spore formers.• Thermophilic bacteria: Clostridium nigrificans• Produce H2S which is soluble, the can look normal and will not swell.• Product become dark in color and has the odor of rotten eggs.2. Mesophilic bacteria(a) Bacillus spp.• Spoilage caused by these bacteria will turn food sour. Acid-arid gas are produced e.g. Bacillus subtilis(b) Clostridium spp.• Butyric / saccharolytic type will ferment carbohydrate and produce butyric acid, N2 and CO2. e.g: C. butyricum, C. pasteurianum• Proteolytic / putrefactive type has proteinase enzyme which can degrade protein in anaerobic condition and produce H2S, NH3, CO2 and H2, also some mercaptans. The can will swell and produce foul smelling, e.g.: C. sporogenes.
  13. 13. 3. Non-spore forming bacteria• E.g. Streptococcus, micrococcus etc. which will produce acid and gas.4. Moulds and Yeasts• Its presence due to pin-hole which can be killed by mild heat;5. USE OF LOW TEMPERATURES• Low temperature reduces the activity of microorganisms by reducing the chemical reaction and action of enzymes.• Hence, less microbial growth and spoilage is delayed / prevented.A. Chilling temperature• Temperature of 6°C can prevent the growth of food poisoning microorganisms except the Clostridium botulinum type E and retard the growth of spoilage microorganisms.• Chilling temperature is the main method for temporary preservation of food.• Psychrotroph are microorganisms which can grow at low temp. e.g. Flavobacterium spp. & Pseud. alcaligenes but they have a low growth rate.
  14. 14. B. Freezing temperature• Cause reduction in number of viable microorganism but does not sterilize the food.• The percentage of microorganisms killed during freezing and storage varies depending on: (a) Substrate (kind of food) (b) Type of freezing• Substrate: protein, fat and other substance can have protective effect; therefore killing rate is reduced. High moisture and low pH can increase killing rate.• Type of freezing : slow and fast freezing Slow ~ temp. lowered to -20°C (3 - 72 hrs) Fast ~ temp. lowered to -20°C (30 mins)• Advantages of fast freezing(a) Smaller ice crystal form - less mechanical destruction to food(b) Short period of solidification - sudden death to microorganisms and quick inactivation of the enzymes.(c) Food quality after thawing is better
  15. 15. 6. DRYING AND SMOKINGA. Drying• Methods which lower the water content of food to a point where the activities of enzymes and food spoilage and food poisoning microorganisms are destroyed / inhibited.• The lower the water activity of food, the greater is the inhibition.• If Aw is between 0.75 - 0.70, the spoilage is delayed. If Aw is 0.65, the spoilage is most unlikely to occur up to 2 years.• Molds and yeasts are more important in spoilage dried foods sine bacteria require higher water content for growth.e.g. Streptomyces rouxii Aw 0.65 Aspergillus glaucus Aw 0.60
  16. 16. • Types of drying: (a) Sun drying (b) Spray drying (c) Freeze drying (d) Smoking• Treatment before drying (to reduce number of microorganisms):(a) Washing(b) Dipping food in alkaline solution such as sodium carbonate for prunes.(c) Treatment with S02 (1000-3000 ppm). This will maintain the foods color as well as conserve the vitamin content. It will also prevent storage changes such as browning of vegetables and fruits.(d) Blanching / scalding. The heat kills most of the microorganisms and destroys the enzymes that may become active and bring about undesirable changes in the finished products.• Addition of sugar before evaporation e.g. sweetened condensed milk. The sugar will tie up the water and make them invailabel for microbial growth.• In any drying method, heat is used to retard the growth or kill the microorganisms. Some microorganisms are resistart e.g. the spores but they stay dormant.• Goals of drying are to produce dried food with total count of not more then 100,000 / gm food. But the total count of coliforms and food poisoning microorganisms should be zero.
  17. 17. • Effects of drying upon foods. Desiccated foods are subjected to certain chemical changes which can cause undesirable product: (a) Dried food that contains fat and oxygen can cause oxidative rancidity to occur. (b) Dried food which contains reducing sugar can undergo a color change called Maillard reaction. Carbonyl groups of reducing sugar react with amino groups of protein and amino acids followed by a series of complicated reaction. The browning is undesirable because of the unnatural color and bitter taste imparted to the food. (c) Loss of vitamin C (d) Discoloration (e) Toughness • Methods of minimizing the chemical changes in dried food:(a) Keep moisture content as low as possible(b) Reduce the level of reducing sugar as low as possible(c) Reduce serial blanching in the same water(d) Use S02 to retain vitamin C and avoid browning
  18. 18. B. Smoking• Heating foods using smoke from various types of wood to preserve foods.• The smoke produces heat which kills some microorganisms on the surface• Heat also reduces the Aw.• It also has an antimicrobial compounds e.g. formaldehyde which can inhibit the growth of some microorganisms.• The presence of aromatic compounds will also give a distinctive flavor and around to the food.• This will make the foods taste better and more tender e.g. smoked fish.• Woodsmoke is more effective against vegetative cells than against bacterial spores.
  19. 19. 6. CHEMICAL PRESERVATIVES• Food additives / preservatives: "A substance or a mixture of substance which are specifically added to prevent deterioration or decomposition of a food"• Deterioration may be caused by: (a) Microorganisms (b) Food enzymes (c) Chemical reactions• Chemical preservatives are used mainly to inhibit the growth and activity of microorganisms by: (a) Interfering with their cell membranes (b) Their enzymes activity (c) Their genetic mechanisms• They can also be used as: (a) Antioxidants to prevent oxidation of unsaturated fats. (b) Neutralizers of ‘acidity. (c) Stabilizers to prevent physical changes. (d) Firming agents. (e) Coating to keep out microorganisms. (f) Prevent loss of water.
  20. 20. • An ideal chemical preservative should have:(a) A wide range of antimicrobial activity(b) Non toxic to human beings and animals(c) Economical(d) No effect on the flavour, taste or aroma of the original food(e) Should not be inactivated by the food(f) Should not encourage the development of resistant strains(g) Should rather kill than inhibit microorganisms(a) Propionates• Used in bakery, cheese food & spreads• Sodium or calcium propionates used in the prevention of mold growth and rope development(b) Benzoates• Used in jam, jellies, margarine, carbonated drinks, fruits etc.(c) Sorbates• Sorbic acids and sorbate salts are used as a direct antimicrobial additive in foods as a spray, dip or coating on packaging materials.• Used in cheese, dried fruit, bakery etc.
  21. 21. (d) Acetates• Dehydroacetic acid has been used to impregnate wrappers fordisease to inhibit mold growth.• Acetic acid (vinegar) is used in mayonnaise, ketchup, sausagesetc.(e) Nitrites and nitrates• Used in meats because nitrites can give a stable red color.• The use of nitrates is being restricted because it can react to other substance e.g. amines and form nitrosomines which is carcinogenic.• Nitrites are shown to have an inhibitory effect toward C. botulinum.(f) Sulfur dioxide and sulfites• Used in the wine industry to sanitize equipment and to reduce normal flora of the grape must.• In aqueous solution, they form sulfurous acid, the active antimicrobial compound.(g) Ethylene and propylene oxide• Powerful sterilants which kills almost all microorganisms• Useful for fumigation, packaging materials, dried fruits, eggs, cereals and spices.
  22. 22. (h) Alcohol• Ethanol can be germicidal effectively at 70 - 95% concentration.• Flavoring extracts e.g. vanilla & lemon are preserved by alcohol.• Methanol is poisonous and should not be added to foods.(i) Formaldehyde• Not permitted to be added to food except, as a minor constituted of wood smoke.• Very effective against molds, bacterial and viruses.(j) Others• Halogen (chlorine) are added to water for sanitation• Iodine - impregnated wrappers have been employed to lengthen the keeping time of fruits.8. OTHER PRESERVATIVE METHODS(a) Filtration• Although fruit juices can be preserved by chemical preservatives, now we can get product with no preservatives added.• Fruit juices are subjected to filters with steam sterilize methods for 10-20 minutes. The sterile products are filled aseptically in sterile bottles or cartons.
  23. 23. (b) Radiation• Gamma ray is the cheapest form of radiation for food preservation.• X-rays essentially has the same character like gamma rays but produced differently.• Factors affecting radiation: (i) Types and species of microbes -Spores are generally radioresistant (ii) Number of microbes -The more cell present, the less effective a given dose of radiation (iii) Composition of medium -Cells in protein medium are more resistant -Protein exerts protective effect against radiation (iv) Presence or absence of oxygen -Resistance is reportedly increase when oxygen is absent (v) Physical state of food -Dried cells are more resistant than moist cells (vi) Age of cells -Cells in lag phase are more resistant than in other phase
  24. 24. (c) Antibiotics• Antibiotics such as aureomycin, terramysin and chloromycetin were found to be effective in lengthen the storage time of raw food especially meats, fish and poultry at chilling temperature.• Niasin has been used to suppress anaerobes in cheese and cheese products.• Natamycin has been tested in orange juice, fresh fruits, sausage and cheese.• Some problems in the use of antibiotics: (a) Effect of antibiotic on microorganisms vary with the species. (b) Organisms may be adapted to increasing concentrations of an antibiotic so that resistant strains finally develop (c) Other organism which is not a significant food spoiler but has acquired resistant will eventually important in food spoilage (d) Effect of antibiotic to consumer.