Food Preservation Methods PDF

3. Food preservation (methods 1- 4) Food preservation is the prevention or delay of changes in a food rendering it inedible or objectionable to the consumer Food preservation is important because: 1. Food production is generally seasonal 2. Food sources are often distant from site of consumption 3...

3. Food preservation (methods 1- 4) Food preservation is the prevention or delay of changes in a food rendering it inedible or objectionable to the consumer Food preservation is important because: 1. Food production is generally seasonal 2. Food sources are often distant from site of consumption 3. Spoilage results in economic losses 4. Presence of certain microorganisms may cause illness Involves the prevention or delay of (a) microbial, (b) self-decomposition or (c) mechanical damage (A) Prevention or delay of microbial decomposition by: - Keeping microorganisms out (e.g. asepsis) - Removal of microorganisms (e.g. Filtration) - Hindering the growth and activity of microorganisms e.g. low temp, drying, anaerobic envir. - Killing microorganisms (e.g. heat or radiation) (B) Prevention or delay of self-decomposition - Destruction/inactivation of food enzymes (e.g. heat treatments) - Prevention or delay of chemical reactions (e.g. addition of antioxidant to prevent oxidation) (C) Prevention of damage by insects, animals or mechanical damage - Appropriate packaging, stacking and storage The preservatives should not affect the original nature of the food. In order to achieve these goals 1. the microbial load should be as low as possible & processing should minimise risk of introducing organisms to the food 2. The environmental conditions should be unfavourable for growth 3. Limit/prevent the food from being physically damaged The generation time of organisms will vary depending on the organism and the intrinsic/environmental conditions e.g. pH, temperature, O/R potential, presence of inhibitors etc It is important to keep the growth rate as low as possible to delay the onset of spoilage Methods used to preserve foods: 1. Asepsis 2. Removal- including filtration 3. Anaerobic or gas storage 4. Low temperature 5. High temperature 6. Drying 7. Chemical preservatives 8. Irradiation 9. High hydrostatic pressure 10.Electric field effects 11.Magnetic field effects 12.Ohmic heating 13.Ultrasound Methods of food preservation 1. Asepsis The prevention of introduction of organisms to the food This occurs in nature through skins, shells and coverings on meat, fruit, vegetable and seeds The microbial load of a food is the no. of microorganisms naturally present on an unprocessed food combined with those added through contamination or a failure to control the growth of the initial population Microbial load may be influenced by: - Quality of raw material - Conditions during harvest, storage, transport etc - Conditions of storage at processing plant - Adoption of aseptic approach - Cleaning and sanitisation - Quality of work force – training, awareness, buy-in Industrial packaging act as a protective covering e.g. can, carton etc. Methods of food preservation 2. Filtration & removal of microorganisms Filtration is limited to relatively clear liquids e.g. fruit juices, beer, soft drinks, water, wine - only successful method to completely remove microorganisms Filtration often used in combination with positive pressure Washing of raw foods can reduce microbial load However, water purity must be tested and defined! Washing & sanitation of processing equipment Trimming spoiled portions of food e.g. outer leaves of cabbage Methods of food preservation 3. Anaerobic conditions & gas storage Requires a complete evacuation of the unfilled space (headspace) of the container or the replacement of air by CO2 or by an inert gas such as Nitrogen Spores of some aerobic spore-formers are especially resistant to heat and may survive in canned food but may be unable to germinate in anaerobic conditions. Vacuum packaging, permeable & impermeable packaging, CO 2 and ozone storage are used with various foods Methods of food preservation Vacuum packaging- all air is evacuated Modified atmosphere packaging (MAP)- the gas atmosphere of the package changes as a result of microbial respiration, dissolution of CO2 & gas exchange across the package membrane Controlled atmosphere packaging (CAP)- The product environment remains constant throughout storage https://www.youtube.com/watch?v=Rav6uV9G844 Methods of food preservation Sous-vide Processing Meal components are packaged under vacuum in a barrier bag or container from which air is evacuated without replacement by neutral or bacteriostatic gases It is cooked in a high moisture environment to a temperature that is sufficient to cook and pasteurise but not sterilise the product Some spores may survive this process- Clostridium botulinum type E is particularly problematic Bacteria that produce heat stable toxins are also a concern e.g. Staphylococcus aureus This process improves the nutritional quality and palatability of the food Methods of food preservation 4. Use of low temperatures in food preservation Metabolic activity of food-borne microorganisms is reduced at temperatures above freezing & stopped below freezing Enzyme activity is temperature dependent A rise in temperature is associated with a rise in enzyme reaction rate. This is termed the temperature coefficient (Q10) and for most biological systems, for every 10˚ increase in temperature, there is a 1.5 – 2.5 fold increase in rate of enzyme reactions. Methods of food preservation 4. Use of low temperatures in food preservation While growth of mesophiles & thermophiles will be impaired in cold storage, psychrophiles growth is not affected Most psychrotrophic bacteria of importance in foods belong to the genera: Pseudomonas, Acinetobacter, Alcaligenes, Flavobacterium, Streptococcus, Micrococcus & Bacillus Moulds of the genera Penicillium, Mucor, Cladosporium, Botrytis & Geotrichium Yeasts of the genera Debaromyces, Torulopsis, Candida & Rhodotorula Methods of food preservation 4. Use of low temperatures in food preservation Types of low temperature storage Common or Cellar storage - Approx 10-15 ˚C - Storage of root crops, potatoes, cabbage, apples, cucumbers for limited periods - Where refrigeration is not available, cellar storage is the rule Chilling or Refrigeration Storage - Temperatures just above freezing - Usually involves cooling by ice or mechanical refrigeration - Short term - Eggs, meat, seafood, vegetables, fruit - Consider: temperature, relative humidity, air velocity, atmosphere, UV Methods of food preservation 4. Use of low temperatures in food preservation Types of low temperature storage Freezing or Cold Storage - Usually at -18 ˚C or below - Most microbial growth is completely prevented - Rate of freezing depends on the method, temperature, air circulation, type of food Slow freezing refers to freezing in the air. Temperature range -15 to -29˚C. May take from 3 to 72 hours to achieve freezing. Fast freezing : temperature reaches -20 ˚C within 30 mins. Usually small packages or units of food. Can be achieved through direct immersion in refrigerant (fish in brine), or by indirect contact with the refrigerant or by air Methods of food preservation 4. Use of low temperatures in food preservation The effectiveness of freezing in killing/inactivating microorganisms depends on: - The microorganism and its physiological state - The temperature during storage - Time of storage in the frozen state - Fast vs slow freezing - Composition of the food- higher water content? High sugar, salt? Low pH? - All foods should be sorted, washed, blanched and/or packaged before freezing to minimise microbial load