Processing of Horticultural Crops

Principles of food preservation

Keeping in view the various causes of deterioration of foods, various methods of food preservation (Table 3.1) have been devised on the basis of following principles:-

1. Prevention or delay of microbial decomposition of food

• By keeping out micro-organisms (asepsis)
• By removal of micro-organisms (filtration)
• By hindering the growth or activity of micro-organisms (use of low temperature, drying, creating anaerobic conditions or using chemicals).
• By killing the micro-organisms (using heat or irradiation).

2. Prevention or delay of self decomposition of food

• By destruction or inactivation of food enzymes (blanching or boiling)
• By prevention or delay of purely chemical reactions (use of antioxidants to prevent oxidation).

3. Prevention of damage by insects, animals, mechanical causes etc (use of fumigants, cushioning, packaging etc).

1. Prevention or delay of microbial decomposition

i) By keeping out micro-organisms (Asepsis): Asepsis refers to keeping out the micro-organisms from the food by making use of either natural covering or providing artificial covering around the food. Natural barrier in foods include outer shell of the nuts (almond, walnut, pecan nut) skin/peel of fruit and vegetables (banana, mango, citrus, ash gourd etc), shells on eggs, skin or fat in meat, husk of ear corn etc. Similarly packaging prevents entry of micro-organisms in the food.
For example peach or mushroom sealed in tin can, clean vessels under hygienic surroundings helps in preventing spoilage of milk during collection and processing by keeping out the micro-organisms.

ii) By removal of micro-organisms (Filtration): Filtration of liquid foods through bacteria proof filters is a common method for complete removal of micro-organisms from the foods. Liquid foods are passed through the filters made of suitable material like asbestos pad, diatomaceous earth, unglazed porcelain etc and allowed to percolate through either with or without nano-filtration etc works on this principle. Centrifugation, sedimentation, trimming and washing etc can also be used but are not very effective.

iii) By hindering the growth and activity of micro-organisms

a. By using low temperature: Microbial growth and enzyme activity is retarded in foods by storing them at low temperatures. The food commodities can be stored under cellar storage (15oC) like root crops, potato, onion refrigerator or chilling temperatures (0-50C) like most fruits and vegetables, meat, poultry, fresh milk and milk products and under freezing temperature (-18oC to -40oC) like frozen peas, mushrooms etc.
b. By drying of food commodity: Removal of water from the food to a level at which micro-organisms fails to grow is an important method of preservation. Moisture can be removed by the application of heat as in sun drying and in mechanical drying or by binding the moisture with addition of sugar (as in jams, jellies) or salt (high salt in raw mangoes) and making it unavailable to the micro-organisms. Examples include osmotic dehydration, dried grapes (raisins), apricots, onion, cauliflower etc.
c. By creating anaerobic conditions: Anaerobic condition can be created by removal or evacuation of air/oxygen from the package, replacement of air by carbon dioxide or inert gas like nitrogen.

• Lack of oxygen prevents growth of any surviving bacteria and their spores under such conditions.
• Production of carbon dioxide during fermentation and its accumulation at the surface makes the conditions anaerobic to prevent the growth of aerobes.
• Carbonation of drinks and storing fresh food under controlled atmospheres serves the same purpose.
• Canned food in which the food is sealed after removal of air (exhausting) illustrates this principle.
• Anaerobic bacteria and their spores present however, need to be killed to prevent the food from being spoiled.
• A layer of oil on top of any food prevents growth of microbes like moulds and yeasts by preventing exposure to air.

d. By use of chemicals: Appropriate quantity of certain chemicals added to the food can hinder the undesirable spoilage in the food by

• Interfering with the cell membrane of the micro-organisms, their enzyme activity or their genetic mechanism
• By acting as an anti-oxidant.
• The optimum quantity of preservative as per approved regulation need to be used as higher concentrations can be a health hazard.
• Chemical preservatives are benzoic acid and its sodium salt, sorbic acid, potassium meta-bi-sulphite, calcium propionates etc.
• Common antioxidants to check off flavour (rancidity) in edible oils include butyl hydroxy anisole (BHA), butyl hydroxy toluene (BHT), tertiary butyl hydroxy quinone (TBHQ), lecithin etc.
• Addition of organic acids like citric, acetic and lactic acid in the food inhibits the growth of many organisms.

iv) By killing the micro-organisms
a) Use of heat: Coagulation of proteins and inactivation of their metabolic enzymes by application of heat leads to destruction of micro-organisms present in foods. Exposure of food to high temperature also inactivates the enzymes present in the food. Foods can be heated either at temperature below 100oC (pasteurization) at 100oC (boiling) or at temperature above 1000C (sterilization).

i) Pasteurization (heating below 100oC): It is a mild heat treatment given to the food to kill most pathogenic micro-organisms and is used in the food where drastic heat treatment cause undesirable changes in the food. It is usually supplemented by other methods to prolong shelf life. Pasteurization is most commonly used in treatment of milk and other dairy products either as low temperature long time (LTLT) or high temperature short time (HTST) process.

• Heat treatment of milk at 62.2oC for 30 minutes refers to LTLT process.
• Heating at 72oC for 15 seconds is termed as HTST process.
• Grape wine is pasteurized at 82-85oC for 1 minute and beer is pasteurized at 600C.
• Pasteurization of juices depends upon their acidity and method of packing whether in bulk or in bottle or can.
• Bottled grape juice is pasteurized at 76.70C for 30 minutes while in bulk the juice is heated to 80-85oC for few seconds by flash treatment.
• Carbonated juice is heated at 65.6oC for 30 minutes in bottles and vinegar in bulk is held at 60-65oC for 30 minutes.

ii) Boiling (heating at 100oC): Cooking of food including vegetables, meat etc by boiling with water involves a temperature around 1000C. Boiling of food at 100oC kills all the vegetative cells and spores of yeast and moulds and vegetative cells of bacteria.

• Many foods can be preserved by boiling (e.g. milk).
• Canning of acid fruit and vegetables (tomatoes, pineapple, peaches cherries etc) is carried by boiling at about 100oC.
• Various terms used for heating of food are baking (in bread), simmering (incipient or gentle boiling), roasting (in meat) frying (shallow or deep fat frying) and warming up (small increase in temperature up to 100oC).

iii) Heating above 1000C: Heating by steam under pressure is used to obtain temperature above 1000C by using steam sterilizer or retort. The temperature in the retort increases with increase in steam pressure. The temperature in retort at mean sea level is 100oC; with 5psi pressure at 1090C; with 10psi pressure at 115.5oC and with 1 kg/cm2 (100 Pa) pressure at 121.5oC.

• For canning of mushrooms and other non-acid vegetables the processing temperature of 121.10C at 15 psi pressure are used.
• For sterilization of milk and other liquid foods like juices, ultra high temperature (UHT) process is used.
• In UHT process, the food is heated to very high temperature (1500C) for only few seconds by use of steam injection or steam infusion followed by flash evaporation of the condensed steam and rapid cooling. The process is also used for bulk processing of many foods.

b) Use of radiation: Irradiation consists of exposing the food to either electromagnetic or ionizing radiations to destroy the micro-organisms present in the food. Examples of irradiation include use of ultraviolet lamps in sterilizing slicing knives in bakeries. Gamma (?) radiation from cobalt -60 or cesium 137 source have been used for irradiation of many fruits like papaya, mango and onion, spices, fish etc. They are also used for inhibition of sprouting in onion and potatoes.

2. Prevention or delay of self decomposition of food
i) By destruction or inactivation of food enzymes (blanching or boiling): Blanching is a mild heat treatment given to vegetables before canning, freezing or drying to prevent self decomposition of food by destroying enzymes. Blanching is carried out by dipping the food commodity either in boiling water or by exposing than to steam for few minutes followed by immediate cooling.

ii) By prevention or delay of purely chemical reactions (use of antioxidants to prevent oxidation): Foods containing oils and fat turn rancid and become unfit for consumption due to oxidation. Addition of appropriate quantity of antioxidants like butyl hydroxy anisole (BHA), butyl hydroxyl toluene (BHT), tertiary butyl hydroxy quinone (TBHQ), lecithin etc prevents oxidation and preserves the food.

iii) Prevention of damage by insects, animals, rodents and mechanical causes: Use of fumigants in dried fruits, cereals etc checks the damage caused by insects and rodents. Wrapping of fruits, providing cushioning trays, using light pack and good packaging material checks the damage to fresh food commodities during handling and transportation.

Table 3.1: Methods of food preservation on the basis of food preservation principles.

Physical method	Method
a)By removal of heat (Preservation by low temperature)	Refrigeration, Freezing preservation, dehydro-freezing, carbonation
b. By addition of heat (preservation by high temperature	Pasteurization (LTLT, HTST), sterilization, UHT Processing, microwave.
c. By removal of water	Drying (open sun, solar/poly tunnel solar), Dehydration (mechanical drying), Evaporation/concentration, Freeze concentration, reverse osmosis, freeze drying, foam mat drying and puff drying
d. By Irradiation	UV rays and gamma radiations
e. By non-thermal methods	High pressure processing, pulsed electric fields
Chemical methods
a. By addition of acid (acetic or lactic)	Pickling (vegetable, olive, cucumber, fish, meat)
b. By addition of salt/brine	Salted mango/vegetable slices, salted and cured fish and meat i. Dry salting ii. Brining
c. By addition of sugar along with heating	Confectionary products like jams, jellies, preserves, candies, marmalades etc.
d. By addition of chemical preservatives.	i) Use of class II preservatives like Potassium meta-bi- sulphite, sodium benzoate, sorbic acid in food products. ii) Use of permitted and harmless substances of microbial origin like tyrosine, resin, niacin as in dairy products.
iii. By fermentation	i. Alcoholic fermentation (wine, beer) ii. Acetic acid fermentation (vinegar) iii. Lactic acid fermentation (curd, cheese, pickling of vegetables).
iv. By combination method	i. Combination of one or more methods for synergistic preservation. ii. Pasteurization combined with low temperature preservation. iii. Canning: heating combined with packing in sealed container. iv. Hurdle technology like low pH, salting, addition of acid, use of sugar, humectant and heating.