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The following points highlight the top three methods used for preservation of food. The methods are: 1. Physical Preservation Method 2. Chemical Preservation Method 3. Wood Smoke Method.
1. Physical Preservation Method:
(a) Asepsis:
Asepsis is the process of prevention of growth of microorganisms and their contaminants or both. The covering or wrapping, prevents primarily contamination during handling, protect the process foods from microbial contamination.
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(b) High Temperature:
The killing of microorganisms at high temperature is supposed to be caused by the denaturation of the proteins and especially by the inactivation of enzymes required for metabolism. The heat treatment also kills microorganisms or their spores.
The heat treatment selected will depend on the kinds of organisms to be killed. Certain factors are known to affect the heat resistance of cells of spores and must be kept in mind when microorganisms for the destruction of an organisms considered.
These are:-
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The temperature-time relationship, initial concentration of spores (or cells), previous history of the vegetative cells or spores, composition of substrate in which cells or spores are heated. Moisture (moist heat is a much more effective killing agent than dry heat), and Hydrogen-ion concentration (pH) are the other factors which play role in killing of microbes.
At neutral pH many spores or cells are more heat resistant. An increase in acidity or alkalinity hastens killing by heat, but change towards the acid side is more effective than a corresponding increase in alkalinity.
The heat resistance of microorganisms usually is expressed in terms of their thermal death time (TDT) which is defined as the time it takes at a certain temperature to kill a stated number of cells or spores under specific conditions. This sometimes referred to the absolute thermal death time. Whereas, the thermal death point is the temperature necessary to kill all the organisms in 10 min.
Pasteurization is another method in which microorganisms are killed by heat treatment usually involves the application of temperature below 100° C. The heating may be generated by hot water, dry heat, or electric currents and products are cooled promptly after the heat treatment.
The high- temperature for a short time (HTST), whereas the low-temperature-long-time (LTH) are given in pasteurization. For e.g. the minimal heat of market milk is at 62.8°C for 30 min in the holding method; at 71.7°C for at least 15 sec. in the HT5T methods; and at 137.8°C for at least 2 sec in the unpasteurized methods.
(c) Canning:
Canning is defined as the preservation of foods in sealed containers and usually implies heat treatment as the major factor in the preservation of spoilage. Spallanzani (1765) preserved food by heating in a sealed container. Nicholas Appert, who has been called the “father of canning”, performed heating of foods in sealed containers and also published direction of canning.
Heat Process for Canning:
The heat processes necessary for the preservation of canned foods depend on the factors that influence the heat resistance of the most resistant spoilage organism and those which affect heat penetration.
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With higher retort temperature the times would be shortened, and processes would vary with the varieties of foods canned, the sources used, the can size and shape, the initial temperature of the food and other factors. HTST heat processes, now used for some fluid foods, require special equipment for sterilizing the food in bulk, sterilizing the containers and lids and filling and sealing the sterile containers, under aseptic conditions.
The dole process is an example of the HCF, or heat-cool-fill method. In the Martin HTST Sterile Canning System, mixed liquid and solid pieces are heated directly by contact with high-temperature steam before aseptic canning. When a particular heat resistant spoilage organism is feared, an HTST treatment may be given to a liquid food before canning, followed by a milder heat treatment of the food in the can.
In the SC or sterilizing and canning process, sterilization of food is accomplished before the can is sealed. In the PFC or pressure-filter-cooker method, the food is sterilized by high pressure steam and filled in to the can, then the can is sealed and the heat processing is continued as long as necessary before cooling. Heat is also being combined with other preservative agencies e.g. antibiotic, irradiation or chemicals e.g. hydrogen peroxide.
The Cooling Process:
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Following the application of heat, the containers of food are cooled as rapidly as is practicable. The can may be cooled in the retort or in tank by immersion in cold water or by a spray of water. Glass containers and large cans are cooled more gradually to avoid undue strain or even breakage.
This tempering process involves the use of warm water (or spray). The temperature of water is lowered as cooling processors. Final cooling of containers usually is by means of air currents. Heating process carries out by Canning. In this case, the food is sterilized before filling and sealing the sterile containers.
Heat is also combined with other preservative agencies such as antibiotics, irradiation or chemicals e.g. hydrogen peroxide. On the other hand, following the application of heat, the containers of food are cooled rapidly as possible by immersing in cold water. Final cooling of containers usually is by means of air currents. Low temperatures slow down the growth and metabolic activities of microorganism.
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Desiccation:
Foods contain moisture, hence drying of the food is necessary by removal of water. Various methods of drying such as sun- drying, artificially produced heat etc. are used for desiccation.
Anaerobiosis:
It is a process in which the replacement of air by CO2 or by an inert gas may bring anaerobic conditions. For example spores of some of the aerobic spore forming organisms are specially resistant to heat and may survive in canned food but are unable to germinate due to lack of oxygen.
2. Chemical Preservation Method:
Food additives are specially added to prevent the deterioration or decomposition of food, has been referred to as chemical preservatives. The food deterioration may occurs due to microorganisms, by food enzymes, or by purely chemical reactions.
The inhibition of the growth and activity of microorganisms may inhibit microbes by interfering with their cell membrane, enzyme activity, or their genetic mechanism. Lactic, acetic, propionic and citric acids or their salts are used as preservatives.
Citric acid used in syrup, drinks, jams, and jellies as a substitute for fruit flavours and for preservation. Lactic and acetic acids are added to drinks of various kinds, green olives etc. Sodium or calcium propionate is used extensively in the prevention of mold growth and rope development in baked foods, cheese foods etc. These are effective against molds, yeast and bacteria.
Propionic acid is a short chain fatty acid (CH3 CH2– COOH) and like some other fatty acids, perhaps affects the cell membrane permeability. The sodium salt of benzoic acid has been used extensively as an antimicrobial agent in foods such as jam, jellies, and margarine, carbonate beverages, fruit salads, pickles, fruit juices etc. Sorbic acid is used as a direct antimicrobial additive in foods and as a spray, dip or coating on packaging materials.
It is widely used in cheese, cheese products, baked goods, beverages, syrups, fruit juices, jellies, fruit cocktails, dried fruits, pickles and margarine. It is most effective against yeast and molds but are less effective against bacteria. Mono-chloroacetic acid, per-acetic acid, di-hydro-acetic acid and sodium diacetate, have been recommended as preservatives but not all are approved.
Di-hydro-acetic acid has been used to impregnate wrappers for cheese to inhibit the growth of molds and as a temporary preservative for squash. Acetic acid in the form of vinegar is used in pickles, pickled sausages, and pig’s feet. Acetic acid is most effective against yeast and bacteria. Combination of various salts of nitrites and nitrates used in curing solutions and mixtures for meats.
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Nitrates decompose nitric acid, which forms nitrosomyoglobin when it reacts with the heme pigment in meats and thereby forms a stable red colour. Sulphur dioxide and sulfite are used in the wine industries to sanitize equipment and to reduce the normal flora of the grape must.
The fumes of burning sulfur are used to treat most light-coloured dehydrated fruits; while dehydrated vegetables are exposed to spray of natural sulfites before drying. Ethylene and propylene oxide are used as sterilants. Ethylene oxide is most effective than propylene oxide. It kills all type of microorganisms. They are thought to act as strong alkalyting agents attacking labile hydrogen.
The primary use have been as sterilants for packaging materials fumigation of ware-house, and cold sterilization of numerous plastics, chemicals, pharmaceuticals, syringes, and hospitals supplies, fully dried fruits, dried eggs, gelatin, cereals, dried yeast. Sugar and salts lower the moisture content and thus, have an adverse effect on microorganism.
Sodium chloride is used in brines that prevents or inhibits the growth of microorganisms. Sugars such as glucose or sucrose, owe their effectiveness as preservatives to their ability to make water unavailable to organisms and to their osmotic effect. Examples of foods preserved by higher sugar concentrations are sweetened condensed milk, fruits in syrups, jellies, and candies.
3. Wood Smoke Method:
Smoking of foods usually adds desired flavours and Acts as preservatives besides, improvement in the colour of the inside of meat. Wood smoke contains a large number of volatile compounds that may have bacteriostatic or bactericidal effect. Formaldehyde is considered as most effective of these compounds, with phenols and cresols next in importance.
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Other compounds in smoke are aliphatic acids from formic through caproic acid. Primary and secondary alcohols, ketones, and acetaldehydes and the aldehydes, waxes, resins and catechol, methyl catechol and pyrogallol and methyl ester. Wood smoke is more effective against vegetative cell than against spores. The woods oak, apple, maple, beech, birch, walnut, mahogany and hardwood such as chickory are used.
Many antibiotics have been tested on the raw foods chiefly proteinaceous like meats, fish and poultry. Aureomycin (chlorotetracycline) has been found superior to other antibiotics tested because of its broad-spectrum activity.
Terramycin (oxytetracycline) is almost as good for lengthening the time of preservation of foods. Antibiotics have been combined with heat in attempts to reduce the thermal treatment necessary for the preservation of low and medium acid canned foods. It has been suggested that a botulinum cooking is enough to inactive all spores of Clostridium botulinum.
