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A complete guide for preserving food for a very long time!
Food are prepared for consumption by taking materials of plant, animal or microbial origin. The most important food items from plant origin are vegetable and vegetable products, fruits and fruit products, cereals and cereal products and sugar and sugar products. The important food from animal origin includes milk and milk products, meat and meat products, egg and poultry products and fish and other seafood.
Microorganisms are also serving as food such as single cells proteins, mushroom, or makers of food such as bread, cake and other fermented food. Due to activity of microorganisms, the nutritive value of food becomes enriched in terms of more digestibility, increased mineral and vitamin content, taste, flavour and keeping quality.
Most of the plant based foods such as grains are available in dried form for long term preservation and consumption. The preservation and keeping quality of food materials depend mainly on their composition and water content. Long term preservation or storage of food materials brings changes in their chemical composition and nutritive value.
These changes could be due to physical, chemical or microbiological activities occurring in food or food materials. Microbiological changes occurring in food or food materials mainly depend on water content, water activity (aw) and temperature of storage.
Principles Underlying Preservation of Food:
Food preservation is the process of physical, chemical or microbiological treatments of all kinds of food items to prevent food spoilage and food borne diseases. Preservation from microbial, chemical and physical contamination, as well as enzymatic activity, is necessary for preserving and extending the shelf life of food.
Adequate packaging is important in preserving food preservation and processing of foods make food last, look good and taste good. Food preservation should also take the type of food into consideration before applying the preservation method. There could be perishable food, semi-perishable food and non-perishable or stable food.
There are so many intrinsic and extrinsic factors of food that should be considered before application of a preservation method. Intrinsic parameters involve the natural state of food and which includes acidity, alkalinity, total dissociated and non- dissociated inorganic and organic acids and buffer capacity of the food.
The other intrinsic factors are moisture, nutrients and redox potential. The extrinsic factors that should be considered are production conditions, harvesting methods and period, processing conditions, etc. The production conditions could be temperature, humidity, aeration, etc.
The harvesting methods could be manual, mechanized and the period of harvest may be in dry season or in rainy season, etc. The processing conditions include grinding, mixing, fermenting, cooking, baking and several other methods of food preparation.
Preservation of food is necessary for their seasonal availability to the consumer. It could be short duration or long term preservation during which food or food materials should not lose their nature in terms of composition, nutritive value and microbiological quality.
To protect these values, varieties of methods are practiced depending on necessity and practicability. The most important principles of food preservation are based on self-decomposition of food or decomposition by microorganisms.
The major principles behind the food preservation are described below:
1. Prevention of Self Decomposition of Food:
Prevention of self-decomposition of food is achieved by various methods such as:
(i) Inactivation, inhibition or destruction of food enzymes (blanching).
(ii) Prevention or delay of chemical reactions (prevention of oxidation by use of anti-oxidants).
Several food materials have different types of enzymes in them which are responsible for some changes in food items. Some of these enzymes such as cellulose, amylases, proteases, lipase, etc., present in food in inactive form under dry conditions are activated by absorption of water from the environment.
These enzymes degrade the substrates like cellulose, starch, triglycerides and proteins present in food items making them easily decomposable by various factors including microbial attack and spoilage. Such enzymes are inactivated by drying (dehydration) or other physical or chemical methods.
2. Prevention of Microbial Decomposition of Food:
Prevention of microbial decomposition of food is achieved by various methods such as:
(i) Keeping out microorganisms (asepsis).
(ii) Removal of microorganisms (filtration).
(iii) Arresting the growth of microorganisms (use of chemicals, low temperature treatment, drying, food additives, anaerobic conditions).
(iv) Killing the microorganisms (heat treatment and radiation treatment).
Growth and multiplication of microorganisms in food changes their texture, aroma, flavor, consistency, quality and content. Presence of pathogenic microorganisms in food leads to food poisoning and other types of food borne outbreaks. Therefore, microorganisms are kept away from food by practicing hygienic methods at various stages of production or processing food.
Microorganisms present in food are removed by different methods. Method of removing microorganisms from food depends on their type like solid, semisolid or liquid food. Microorganisms present in liquid food are removed by physical separation using filtration methods. Various types of filters and filtration methods are practiced depending on the size of microorganisms to be removed, thickness and consistency of liquid food.
Growth of microorganisms in food can be arrested by several means. Microbial growth is affected by various physical factors like temperature, water content, aeration, etc. Growth of microorganisms in food is prevented by altering one or more of such physical factors. Low temperature hampers growth of microorganisms in food by reducing enzyme activities so; preservation of most of the perishable food is practiced by keeping them at refrigerators or cold storage facilities in large quantities.
Preservation of food or food materials at low temperature also protects them for physical quality and nutritional value for longer durations. Foods are also preserved by the addition of antimicrobial agents as food preservatives. Different chemicals like organic acids, inorganic salts are normally used as food preservatives which prevent growth of microorganisms directly, inhibiting them or creating a condition in food which becomes inhibitory to their growth.
Preserving food in anaerobic conditions inhibits growth of aerobic microorganisms like fungi and bacteria. Microorganisms present in food are killed by high temperature treatment or radiation. Cooking food at high temperatures and pasteurization are the most commonly used method of high temperature treatment for liquid foods and milk. Most of the processed foods under packing are subjected to radiation treatment using ionizing and non-ionizing radiation to kill microorganisms present in them.
3. Prevention of Damage by Animals, Insects and Mechanical Means:
Principle Methods of Food Preservation:
Food preservation is the process of treating and handling food to stop or slow down spoilage (loss of quality, edibility or nutritive value) caused or accelerated by microorganisms. Maintaining or creating nutritional value, texture and flavor is important in preserving its value as food.
The most popular methods used for the preservation all kinds of food in food technology are:
(a) Asepsis – Keeping out of microorganisms.
(b) Removal of microorganisms.
(c) Maintenance of anaerobic conditions, e.g., in a sealed, evacuated container, etc.
(d) Use of high temperature – Pasteurisation, sterilization, UHT (ultrahigh temperature), smoking, baking, frying and canning.
(e) Use of low temperatures – Refrigeration, cold storage, freezing- slow, quick, cryogenic, etc.
(f) Drying – solar drying, drying by mechanical drier, freeze drying, etc.
(g) Use of chemical preservatives – Developed preservatives or added chemicals.
(h) Irradiation – Non ionising- UV, IR, Ionising – X-ray, beta rays, cathode rays and microwaves.
(i) Mechanical destruction of microorganisms – by grinding, high pressure, etc.
(j) Combinations of any two or more above methods.
Food Preservation Using Food Additives and Chemical Preservatives:
A food additive is a substance or mixture of substances that is intentionally added to food for preservation, processing, storage or packaging. The food additives which are specifically added to prevent deterioration or decomposition of food have been referred to as chemical preservatives.
The deteriorations in food may be caused by microorganisms by food enzymes, or by purely chemical reactions. The inhibition of the growth and activity of microorganism in food is the main objectives of chemical preservatives. Preservatives may inhibit variety microorganisms by interfering with their cell membranes, their enzyme activity or their genetic mechanisms.
Other preservatives may be used as antioxidants to hinder the oxidation of unsaturated fats, as neutralizers of acidity, as stabilizers to prevent physical changes, as firming agents and as coatings or wrappers to keep out microorganisms, prevent loss of water, or hinder undesirable microbial enzymatic and chemical reactions.
Characteristic of Ideal Antimicrobial Preservatives:
The most important characteristic features fulfilled by chemical preservatives are listed below:
(i) Chemical preservatives should have a wide range of antimicrobial activity.
(ii) Should be non-toxic to human or animals.
(iii) Should be economical.
(iv) Should not have an effect on flavor, taste or aroma of the original food.
(v) Should not be inactivated by food or any other substances in food.
(vi) Should not influence the growth of resistant strain.
(vii) Should have high microbicidal (ability to kill) activity than microbistatic (ability to inhibit) activity.
The major factors that influence the effectiveness of chemical preservatives in killing microorganisms or inhibiting their growth and activity are:
(i) Concentration of the chemical.
(ii) Kind, number, age and previous history of the organism.
(iii) Temperature.
(iv) Time.
(v) Physical and chemical characteristics of the substrate in which the organism is found (moisture content, pH, kinds and amounts of solutes, surface tension, colloids and other protective substances).
Classification of Chemical Preservative:
Antimicrobial preservatives added to food can be grouped as follows:
(i) Those added preservatives not defined by law – Natural organic acids (citric acid, lactic acid, malic acid, etc.) and their salts, vinegars (acetic is a natural acid), sodium chloride, sugars, spices and their oils, wood smoke, carbon dioxide and nitrogen.
(ii) GRAS (generally recognized as safe) substances food – Propionic acid and sodium and calcium propionates, caprylic acid, sorbic acid and potassium, sodium and calcium sorbates, benzoic acid and benzoates, derivatives of benzoic acid such as methylparaben and propylparaben, sodium diacetate, sulfur dioxide and sulfites, potassium and sodium bisulfite and metabisulfite and sodium nitrite.
(iii) The chemicals other than first two categories. They can be used only when proved safe for humans or animals and they then fall into group 4.
(iv) Chemicals proved safe and approved by FDA (Food and Drug Administration).
Preservatives added to inhibit or kill microorganisms may be classified on various other criteria, such as their chemical composition, mode of action, specificity, effectiveness and legality.
Organic Acids and their Salts:
Lactic acid, propionic acid, citric acid or their salts may be added to food for preservation. Citric acid is used in syrups, drinks, jams and jellies as a substitute for fruit flavors and for preservation. Lactic acid and acetic acid is added to brines of various kinds, green olives, etc.
1. Propionates:
Sodium or calcium propionate is used most extensively in the prevention of mould growth and rope development in baked goods and for mould inhibition in many cheese food and spreads. Propionic acid is a short-chain fatty acid (CH3CH2COOH) and affects the cell-membrane permeability, although its precise mode of fungistatic action is not known. Propionates are effective against moulds, but little action against bacteria and yeast. Their effectiveness decreases with an increase in pH, with an optimal upper limit 5.0-6.0.
The sodium salt of benzoic acid has been used extensively as an antimicrobial agent in food. It has been incorporated into jams, jellies, margarine, carbonated beverage, fruit salads, pickles, relishes, fruit juices, etc. The mechanism of action of the benzoates is not clear; the effectiveness of the benzoic acid esters increases with an increase in the chain length of the ester group. Sodium benzoate is relatively effective at acidic pH (2.5-4.0). Two important esters of p-hydroxy benzoic acid are methyl parabens, propyl paraben which are effective preservatives.
Sorbic acid is used as a direct anti-microbial additive in food and as a spray, dip, or coating on packaging materials. It is widely used in cheeses, cheese products, baked goods, beverages, syrups, fruit juices, jellies, jams, fruit cocktails, dried fruits, pickles and margain. Sorbates inhibit yeast and mould but not effective against bacteria. It is also effective at acidic pH (4.0)
4. Acetates:
Acetic acid derivates such as monochloroacetic acid, peracetic acid, dehydroacetic acid and sodium diacetate have been recommended as preservatives. Acetic acid and vinegar are more effective against yeast and bacteria than against mould. Acetic acid in the form of vinegar is used in mayonnaise, pickles, catsup, pickled sausages and pig’s feet.
5. Nitrites and Nitrates:
Sodium nitrite, potassium nitrite, sodium nitrate and potassium nitrate are recommended as food preservative. Nitrites decompose to nitric acid, which forms nitrosomyoglobin when reacts with the pigments in meats and thereby forms a stable red color. Nitrate probably only acts as a reservoir for nitrite.
Nitrites can react with secondary and tertiary amines to form nitrosamines, which are known to be carcinogenic. Recent work has emphasized the inhibitory property of nitrites towards Clostridium botulinum in meat products, particularly in bacon and canned or processed hams. Nitrates have a limited effect on limited number of organisms and would not be considered as good chemical preservative.
6. Sulfur Dioxide and Sulfites:
In aqueous solutions, sulfur dioxide and various sulfites, including sodium sulfite, potassium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite and potassium metabisulfite, form sulfurous acid, the active antimicrobial compound.
Sodium sulphite, potassium sulphite, sodium bisulfite, potassium bisulfite, sodium metabisulfite and potassium metabisulfite are effective antimicrobial agents. The inhibitory action is due to the property that it can reduce the disulfide linkages, form various carbonyl compounds, reaction with ketone group and inhibit respiratory mechanism.
7. Ethylene and Propylene Oxide:
Ethylene oxide kills all microorganisms, propylene oxide, although it kills many microorganisms, are not as effective as that of ethylene oxide. They are thought to act strong alkylating agents attacking labile hydrogen. They have also been used successfully in dried fruits, dries eggs, gelatin, cereals, dried yeast and spices.
These are strong alkylating agents and attacks hydrogen bonds. But FAD restricts the use of ethylene oxide, but use of propylene oxide is permitted.
Ethanol is germicidal in concentration between 70-90%, these are lipid solvents and protein dehydrating agent in bacteria. Liquors and distilled liquors usually contain enough alcoholic content to prevent microbial attack. Methanol is poisonous and should not be added to food.
Sodium chloride is used in brines and curing solutions or is applied directly to the food.
Salt has been reported to have its action like:
(a) Causes high osmotic pressure and hence plasmolysis of cells, the percentage of salt necessary to inhibit growth or harm the cells is varying with the microorganism,
(b) It dehydrates food by drawing out and tying up moisture as it dehydrates microbial cells,
(c) It ionizes to yield the chlorine ion, which is harmful to organisms,
(d) It reduces the solubility of oxygen in the moisture,
(e) It sensitizes the cell against carbon dioxide, and
(f) It interferes with the action of proteolytic enzymes.
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 food preserved by high sugar concentrations are sweetened condensed milk, fruits in syrups, jellies, candies, etc.
The addition of formaldehyde to food is not permitted, except as a minor constituent of wood smoke, but this compound is effective against moulds, bacteria and viruses and can be used where its poisonous nature and irritating properties are not objectionable.
Wood smoke contains a large number of volatile compounds that may have bacteriostatic and bactericidal effect. Formaldehyde is considered the most effective of these compounds with phenols and cresols next in importance.
Other compounds in the smoke are aliphatic acids from formic through caproic; primary and secondary alcohols, ketones, acetaldehyde and other aldehydes; waxes; resins; guaiacol and its methyl and propyl isomers; catechol, methyl catechol and pyrogallol and its methyl ester. Wood smoke is more effective against vegetative cells than against bacterial spores and the temperature varies with the kind of wood employed.
12. Spices and Other Essential Oils:
The inhibitory effect of spices differs with the kind of spice and the microorganism being tested. Mustard flour and the volatile oil of mustard are very effective against Saccharomyces cerevisiae but are not as potent as cinnamon and cloves against most bacteria. The essential oils of spices are inhibitory than the corresponding ground spices. Cinnamon and clove, containing cinnamic aldehyde and eugenol are more bacteriostatic than are other spices.
Halogens are added to water for washing food or equipment, for cooling and for addition to some products, i.e., washing butter; water for drinking may be chlorinated by the direct addition of chlorine, or hypochlorites or chloramines may be used. Idoine- impregnated wrappers have been employed to lengthen the keeping time of fruits.
Iodophors, which are combinations of iodine with non-ionic wetting agents and acid, are being used in the sanitization of dairy utensils. Halogens kill organisms by oxidation, injury to cell membranes, or direct combination with cell proteins.
Antibiotics are also used as food additives because of its inhibitory action against many microorganisms. Most of the antibiotics have been tested on raw food like meat, fish and poultry. Aureomycin (chlortetracycline) has been found superior to other antibiotics tested because of its broad spectrum of activity. Terramycin (oxytetracycline) is almost as good for lengthening the time of preservation of food.
Preservatives could be produced in food by microbes; e.g., lactic acid, alcohol, bacteriocin, etc. Their preservative effect is mostly supplemented by one or more additional preservative methods like low temperature, high temperature, anaerobic conditions, sodium chloride, sugar, etc. Developed preservatives play a part in preservation of sauerkraut, pickles, green olives, fermented milk and cheese.
Food Preservations Using Irradiation:
Radiation is the emission and propagation of energy through space or through a medium. Electromagnetic radiations are the choice of radiation for food preservation. Among radiation shorter wavelength radiations are more useful and toxic to microorganisms. The most important types of radiation for the preservation of food are ionizing and non ionizing types. Ionizing radiation is the radiation that have wavelength of 2000 Å or less.
Most important types of such radiations are alpha particle, beta rays, gamma rays, X-rays and cosmic rays. The ionizing radiation ionizes the molecules in their path so they destroy microorganisms without raising the temperature. This process is called cold sterilization.
Features of Radiation Used for the Preservation of Food:
Both ionizing and non-ionizing radiations are important for food preservation.
The important characteristics of radiations are:
Ultraviolet irradiation has been the most widely used in the food industry. It is a powerful bactericidal agent, effective wavelength of about 2600 Å. It is non-ionizing and is absorbed by protein and nucleic acid causing photochemical changes and mutation in the nucleic acid composition (formation of thymine dimer in nucleotide sequences in DNA) leading to death. The poor penetrative capacities of UV light limit its food use to surface applications, where it may catalyze oxidative changes that lead to rancidity, discolorations and other reactions.
Small quantities of ozone may also be produced when UV light is used for the surface treatment of certain foods. The usual source of UV radiation in food industry is from quartz-mercury vapor lamps that emit radiations at 254nm. The lambs are available in various sizes, shapes and power.
There are so many factors influences the activity of UV light in food products these are given below:
The resistance to ultraviolet irradiation by microorganism can vary with the phase of growth and the physiological state of the cell. UV radiation results in damage to the DNA of the cells and death.
(ii) Destruction Intensity:
The intensity of the rays reaching an object will depend on the power of the lamp, the distance from the lamp to the object and the kind and amount of interfering material in the path of the rays. The intensity of destruction of microorganisms falls proportional to the square of the distance from the lamp.
(iii) Penetration Capacity:
The effectiveness of radiation process will depended on the nature of the object or material being irradiated. Penetration is reduced by clear water, which exerts a protective effect on microorganisms. Dissolved mineral salts, especially of iron and cloudiness greatly reduce the effectiveness of the rays.
The treatment would be more effective if the time of exposure is too long to a given concentration.
Beta rays may be defined as streams of electrons emitted from radioactive substances. Beta rays are emitted from cathode in an evacuated tube It has poor penetration power. The important commercial sources of cathode rays are Van de Graff generators and linear accelerators.
Gamma rays are electromagnetic radiations emitted from excited nucleus of 60Co and 137Cs. This is the cheapest form of radiation for food preservation because the source elements are either by- products of nuclear fission or atomic waste products. Gamma rays have excellent penetration power, 60Co has a half-life of about 5 years and the half-life for 137Cs is about 30 years.
X rays are electromagnetic radiation with high penetrating power. These are produced by bombardment of a heavy metal target with cathode rays within a vacuum tube. The demerit of X- rays are cost of production is high. All other properties are same as that of gamma rays.
Microwaves are electromagnetic waves between IR rays and radio waves. The energy or heat produced by microwaves as they are passed through the food is a result of extremely rapid oscillations of the food molecules in an attempt to align them with the electromagnetic field. These rapid oscillations generate heat.
The preservative effect of microwaves or the bactericidal effect is due the heat generated by the oscillated molecules. Microwaves themselves do not produce any inactivation of food borne microbes, but the excitation of food molecules produces heat that results in the microbial destruction.
Factors Influencing the Destruction of Micro Organisms by Irradiation:
Several factors are considered to determine the effect of radiation on microorganisms:
Dried food products have more resistance against radiation than moist food. Similarly frozen foods have high resistance than non frozen food. If the temperature is very low the resistance is increases In the case of ground beef, it is irradiated at -196°C shows high resistance compared to 0°C.
(ii) Age of Microorganisms:
Bacteria shown to be more resistant to radiation in the lag phase just prior to active cell division. But, if the cell reaches the log phases it become more radiation sensitive and reach their minimum at the end of this phase.
(iii) Type and Nature of Microorganisms:
Gram-positive bacteria are more resistant to irradiation than gram-negatives and spore formers are more resistant than non- spore-formers. Among spore formers, Bacillus seems to possess a higher degree of resistance than most other aerobic spore formers. Spores of Clostridium botulinum type A appear to be the most resistant of all Clostridial spores.
Similarly Streptococcus, Micrococcus, Lactobacillus are resistant among non spore forming bacteria. But Pseudomonas, Flurobacterium and other gram-negative microbes are sensitive to radiations. Yeast is more sensitive to radiation than moulds but both of them are less sensitive than gram-positive bacteria. The fungal yeast Candida is resistant to radiation.
(iv) Composition of Food Materials:
Microorganisms are more sensitive to radiation when it is suspended in buffer solutions than in nutritional culture media. But if it is a protein containing food, it shows resistance against radiation as proteins exert protective effect against radiation. The presence of nitrite tends to make bacterial spore more sensitive to radiation.
If numbers of microbial flora in food items are more a given dose of radiation is less effective. The numbers of organisms have the same effect on the efficacy of radiations as is the case for heat, chemical disinfection and other method of preservation.
(vi) Presence or Absence of Oxygen:
The radiation resistance capacity of microorganisms increases in the absence of oxygen. Generally the addition of benzyl hydroxyl compounds favors the resistance mechanisms.
(vii) Processing Steps Required for Food before Radiation Procedure:
Prior to the processing of food material for irradiation several processing steps are required.
The important steps are:
(a) Selection Criteria of Food:
The freshness and overall desirable quality of food must be checked before radiation processing. The previous spoilage of food before the processing of irradiation should be avoided.
(b) Cleaning of Food for Radiation:
The food should be properly cleaned before radiation. All visible debris and dirt should be removed. This will reduce the number of microorganisms to be destroyed by the radiation treatment.
Prior to radiation treatment the food should be packed in containers that will be protected against post irradiation contamination. Certain glass containers undergo color change when exposed to radiation around 10 kilo gray (KGy). This color may be undesirable.
(d) Heat Treatment or Blanching:
In order to avoid the undesirable post irradiation it is necessary to destroy the natural enzymes present in the food. Blanching is a process of destructing or inactivating food enzyme by mild heat treatment. This method is excellent to destruct some enzymes present in vegetables, fruits and meats prior to irradiation.
Legal Status of Food Irradiation:
The method of food preservation by irradiation is widely accepted and approved by US Food and Drug Administration (FDA) for almost 40 countries in mid-1989. At least 20 different food-packaging materials have been approved by FDA at levels of 10 or 60 kgy. In 1983, the FDA permitted spices and vegetable seasonings to be irradiated up to 10 kgy.
In 1985 the FDA granted permission for the irradiation of pork at up to 1 kgy to control Trichinella spiralis (a nematode). Subsequently, in May 1990, the USDA approved the irradiation of poultry up to 3.0 kgy. In 1985-1989 USDA approved the use of irradiated pappaya at 0.41 kgy and strawberries of 2.0 kgy.
The irradiated foods were well received by consumer. Sprout inhibition and insect disinfection are the most important applications of food irradiation. Similarly, WHO has also given approval for the radiation dosage up to 7 kgy for various foods. Radappertization, radurization and radicidation are reliable irradiation process for various fish and fish products and some of them were capable of destroying the Clostridium botulinum and other spore forming organisms.
Food Preservation with High Temperature:
The killing of microorganisms by heat is due to the denaturation of the protein especially by the inactivation of enzymes required for the metabolism. The heat treatment necessary to kill organisms or their spores varies with the kinds of organisms, its states and the environment during heating.
Depending on the heat treatment employed, only some of the vegetative cells, or most/all vegetative cells, parts/all spores of organisms may be killed. So high temperature preservation is one of the best among other methods for food preservation.
The heat treatment selected depends on kind of organisms to be killed, other preservative methods to be employed and the effect of heat on food.
Certain factors affect the heat resistance of cells or spores like:
(a) Initial concentration of cells/spores in food to be heated,
(b) Previous history of cells/spores that is the conditions under which they have been grown like culture medium, temperature, growth phage, desiccation, etc.
Factors Affecting Heat Resistance of Microorganisms and High Temperature Preservation:
1. The Temperature Time Relationship:
The time for killing cells or spores under a given sets of conditions decreases as the temperature is increased.
The most important concepts of time-temperature relationship are given below:
Thermal death time is a concept used to determine the time takes to kill specific bacteria at a specific temperature. It was developed for food canning and has found applications in cosmetics and pharmaceuticals, i.e., thermal death time (TDT) is the minimum time required to kill all microbes keeping temperature (or bactericidal agent) constant.
(ii) Thermal Death Point (TDP):
TDP is the lowest temp required to kill all microbes in liquid suspension in 10 minutes, i.e. the lowest temperature that kills microorganism under standard conditions in a given time is known as thermal death point.
2. Initial Concentrations of Vegetative Cells or Spores:
If the spores are more, greater the heat treatment necessary to kill of them.
3. Previous History of the Vegetative Cells or Spores:
The conditions under which the cells have been grown and spores have been produced.
So the important factors influencing this process are:
(i) Culture medium,
(ii) Temperature of incubation,
(iii) Phase of growth or age, and
(iv) Desiccation.
Composition of the substrate in which cells or spores are heated.
The material in which the spores or cells are heated is so important that it must be stated.
The important factors related with these concepts are:
(i) Moisture.
(ii) Hydrogen-ion concentration.
(iii) Other constituents of the substrate.
Methods employed for high temperature preservation.
The most important methods employed for the preservation of food items by high temperature are:
(i) Heating below 100°C:
Pasteurization:
The process of pasteurization was named after Louis Pasteur who discovered that spoilage organisms could be inactivated in wine by applying heat at temperatures below its boiling point. The process was later applied to milk and remains the most important operation in the processing of milk, i.e., the heating of every particle of milk or milk product to a specific temperature for a specified period of time without allowing recontamination of that milk or milk product during the heat treatment process.
The most important methods for pasteurization are:
(a) High Temperature Short Time Method (HTST) or Flash Process:
Flash pasteurization, also called “High Temperature Short Time” processing, is a method of heat pasteurization of perishable beverages like fruit and vegetable juices, beer and dairy products. Compared to other pasteurization processes, it maintains color and flavor better. It is done prior to filling into containers in order to kill spoilage microorganisms, to make the products safer and extend their shelf life. The liquid moves in a controlled, continuous flow while subjected to temperatures of 71.7°C for about 15 seconds.
(b) Long Temperature Long Time Method (LTH) or Holding Method:
In holding method the time is increased and temperature is lowered compared to the flash process. The required conditions for holding methods are, temperature is 62.8°C and the time is 30 minutes.
This method is sufficient to kill all vegetative cells of bacterium but the spores may be remained in food.
The important methods are:
Baking – During baking the internal temperature of bread, cake, or other bakery products approaches about 100°C.
Simmering (Gentle Boiling) –
Roasting – In the case of meat while roasting the temperature 60-85°C.
Frying – Frying gets outside of the food very hot, but the center does not reach 100°C.
Warming – Warming up a food means anything from a small increase in temperature up to heating to 100°C.
Temperature above 100°C is obtained by means of steam under pressure in steam pressure sterilizers.
The important high temperature treatments are:
(a) UHT (ultrahigh temperature).
(b) Canning.
(a) Ultra-High-Temperature (UTH) Processing:
Ultra-high temperature processing or (less often) ultra-heat treatment (both abbreviated as UHT) is the partial sterilization of food by heating it for a short time, around 1-2 seconds, at a temperature exceeding 135°C (275°F), which is the temperature required to kill spores in milk. The most common UHT product is milk, but the process is also used for fruit juices, cream, yogurt, wine, soups and stews. High heat during the UHT process can cause Maillard browning and change the taste and smell of dairy products.
Canning is a method of preserving food in which the food is processed and sealed in an airtight container by the application of high temperature. The process was first developed by a French scientist Nicolas Appert. He was regarded as the father of canning. So this method is otherwise known as Appertization.
The packaging prevents microorganisms from entering and proliferating inside. There are so many steps to be carried out prior to the food is being preserved by canning. The important steps includes – pasteurization, boiling (and other applications of high temperature over a period of time), refrigeration, freezing, drying, vacuum treatment, antimicrobial agents that are natural to the foodstuff being preserved, a sufficient dose of ionizing radiation, submersion in a strongly saline, acid, base, osmotically extreme or other microbe-challenging environments.
To achieve temperatures above the boiling point requires the use of a pressure canner. Foods that must be pressure canned include most vegetables, meat, seafood, poultry and dairy products. The only foods that may be safely canned in an ordinary boiling water bath are highly acidic ones with a pH below 4.6, such as fruits, pickled vegetables, or other foods to which acidic additives have been added.
Canning processes require so many equipments and utensils.
They are listed follows:
1. (a) Water bath canners (b) pressure canners (c) jars and (d) other canning utensils helpful items for home canning and preserving.
2. Jar lifter – Essential for easy removal of hot jars
3. Jar funnel – Helps in pouring and packing of liquid and small food items into canning jars
4. Lid wand – Magnetized wand for removing treated jar lids from hot water
5. Clean cloths – Handy to have for wiping jar rims, spills and general cleanup
6. Knives – for preparing food
7. Narrow, flat rubber spatula – for removing trapped air bubbles before sealing jars
8. Timer or clock – for accurate food processing time
9. Hot pads, and
10. Cutting board.
Significance of Canning Process:
Canning is a way of processing food to extend its shelf life. The idea is to make food available and edible long after the processing time. Although canned foods are often assumed to be of low-nutritional value (due to heating processes or the addition of preservatives), some canned foods are nutritionally superior to their natural form. Canned tomatoes have higher available lycopene content.
The canning process was developed to preserve food safely and for long periods of time. Once a food is packed into a can, the can is heated to a temperature extreme which kills all known microorganisms. In addition, most processed foods are closely monitored, using a system called hazard analysis and critical control point, or HACCP.
HACCP system identifies areas of potential contamination within the food process and builds check points to ensure that the highest possible safety standards are maintained at all times. Modern processors maintain close watch on the heating process, ensuring that the canned food that reaches the market is the safest possible product for the consumer.
Food Preservation with Low Temperature:
Low temperature preservation is used to retard chemical reaction and action of food enzymes and to slow down or stop the activity of microorganisms in food. The use of low temperature to preserve food is based on the fact that the activity of food borne microorganism can be reduced at low temperature because all metabolic reactions of microorganism are enzymes catalyzed process and that is based on temperature.
There are three distinct temperature ranges for low temperature stored food:
(i) Chilling temperature: temperature between 5-7°C or 10-15°C
(ii) Refrigeration temperature: 0-7°C
(iii) Freezer temperature: -18°C
In general, freezing prevents growth of most of the food borne microorganism and refrigeration temperature slows growth rate of microorganism. But some exceptions are – Clostridium botulism type E which can grow at 3-30°C, Yersinia enterocolitica can survive and grow at 0-3°C, Salmonella (5°C), Bacillus cereus (3°C), Staphylococcus aureus (7°C), etc., are bacterial food pathogens they can grow below 7.2°C. In the case of fungus Cladospoprium and Sporotrichum have been growing in food at – 6.7°C and Penicillium grows at – 4°C.
Low Temperature Preservation Methods:
1. Common (Cellular) Storage:
The temperature employed for cellular storage is 15°C. Root crops, potatoes, cabbage, apple and similar types of foods can be stored by cellular storage for limited time. It is not an effective method because the deterioration of fruits and vegetables by enzymes and microorganism is not prevented but better than atmospheric temperature. The most important factor is humidity, low humidity in cellar storage results the loss of moisture from stored food and high temperature results the spoilage of food by microorganisms.
Chilling is a process involving low temperature and involves cooling by ice or by mechanical refrigeration. Most perishable foods like egg, dairy products, meat, seafood, fruits and vegetables are held in chilling temperature for limited amount of time. Chilling temperature will not prevent the complete enzymatic and changes in food but, it will reduce the chance of spoilage.
The important factors influencing chilling storage are:
(i) Temperature of utility.
(ii) Relative humidity.
(iii) Air velocity.
(iv) Composition of the atmosphere in the store room.
(v) Use of UV or other radiation.
3. Freezing (Frozen Storage):
The storage of food in the frozen condition is an important method for food preservation. This type storage reduces the microbial growth and action of food enzymes, if the temperature is very low the chemical and enzymatic, microbial activity is also slow. The rate of freezing of food depends on number of factors such as the method employed, temperature, circulation of air or refrigerant, size and shape of package and type of food.
(i) Sharp Freezing (Slow Freezing):
It refers to freezing in air with only natural air circulation or electric fans. The temperature is usually -23.3°C or (-15 to-29°C) and freezing time around 3-72 hours.
(ii) Quick Freezing (Fast Freezing):
It is the process in which the temperature of food is lowered about -20°C within 30 min.
(iii) Methods of Quick Freezing:
There are three main methods for fast freezing:
(1) Direct immersion of food or packaged food in refrigerant.
(2) Indirect contact with the refrigerant where the food or packages is in contact with the passage through which the refrigerant at -17.8°C to -45.6 flows.
(3) Air blast freezing when frigid air at -17.8 to -34.4°C is flown across the material frozen.
Advantages of Quick Freezing over Slow Freezing:
(i) Smaller crystals are used so less mechanical destruction of intact cells of food.
(ii) These are shortest method of solidification so less time for diffusion of soluble materials and separation of ice.
(iii) More prevention against microbial growth and suppression of food enzymes.
(iv) Quick freezing accompanied changes in properties such as pH, titratable acidity, ionic strength, viscosity, osmotic pressure, vapor pressure, freezing point, surface and the interfacial tension and OR potential.
Fruits and vegetables are subjected to dehydrating process hence half the moisture is removed before freezing.
Cryogenic freezing is a type of freezing which requires extremely low temperatures, generally -150°C. This process is part of a branch of the sciences known as cryogenics, which focuses on the production of very cold temperatures and the study of what happens to objects subjected to these temperatures. Freezing techniques for preserving shrimp and other foods that are high- priced or have low moisture content by spraying them with liquid nitrogen as they pass through a tunnel on a conveyor belt.
The prime factors that influence are given below:
(i) State and Nature of Microorganisms:
The resistance of microorganisms varies with the kind of microorganism, its phase of growth and whether it is a vegetative cell or a spore.
If the freezing is faster the rate of destruction is slow.
(iii) Freezing Temperature:
High freezing temperatures are more lethal than lower one. More organisms are inactivated at -4 to -100°C than at -15 to -30°C.
The initial killing rate in freezing is rapid, then there is gradual reduction of microorganisms and this is called storage death. Storage of frozen food in the critical range of temperatures results in more rapid reduction than at higher or lower freezing temperatures.
Composition of the food influences rate of death of organisms during freezing and storage. Sugar, salt, proteins, colloids, fat and other substances may be protective, whereas high moisture and low pH may influences killing.
(vi) Alternate Freezing and Thawing:
Alternate freezing and thawing is reported to hasten the killing of microorganisms but does not always do so.
Preparation of Food for Freezing:
The preparation of food for freezing includes selection, sorting, washing, blanching and packaging. Food in any state of detectable spoilage should be avoided for freezing meat, poultry, egg, sea food and other food should be fresh as possible.
Blanching:
It is a heat treatment process in which a brief immersion of food into hot water or use of steam.
Its primary functions are given below:
(i) Inactivation of enzymes that causes undesirable changes during freezing of food.
(ii) Enhancement or fixing of green colors of vegetables.
(iii) Reduction of number of microorganism in food.
(iv) Facilitates the package of leafy vegetables by including wilting.
(v) Displacement of entrapped air in the plant tissues.
Preservation of Food by Dehydration or Drying:
Dehydration is accomplished by removing of water or reduction of moisture content from food. It is based on the fact that the microorganism and enzymes need water in order to be active. Moisture can be removed from food by many methods.
The most important types of dehydrated foods are:
1. Sun Dried Foods:
In this type of food, moisture was removed by exposure to sun rays without any artificially produced heat under controlled condition of temperature relative humidity and air flow.
2. Dehydrated or Desiccated Foods:
The food has been dried by artificially produced heat under controlled condition of temperature relative humidity, air flow, etc.
3. Condensed foods:
The moisture has been removed from condensed milk.
Methods of Drying:
The important methods for drying the food items are explained below:
Solar drying is a process of drying under hot sun and dry atmosphere to certain fruits such as apricots, pears, peaches, etc. The fruits are spread out on tray and may be turned during drying fish rice; other spices are dried by this method.
(ii) Drying by Mechanical Dryers:
Most method of artificial dryers involves the passage of heated air with the controlled relative humidity over the food or passage of through that air. Number of devices is used for such type of drying, e.g., evaporator. Another method is conveyor belt in which food items are moved through the heated air.
Liquid foods such as milk, juices, soups and other items may be evaporated by the use of vacuum pan at low temperature.
1. Drum dried – by the passage over heated drum without vacuum.
2. Spray dried – by spraying the liquid in the current of dry heated air.
(iii) Freeze Drying (Lyophilization or Cryophilization):
Lyophilization is a process involves the sublimation of water by frozen food by use of vacuum plus heat applied by the dryer shell. This method is used for number of food such as meat, poultry, sea food, fruits and vegetables.
The rate at which food materials freeze is influenced by the following factors:
i. Temperature difference between the product and cooling or heating machine.
ii. The transferring of heat energy through the product (conduction, convection and evaporation).
iii. The type, size, shape of the product.
iv. The type, size, shape and the chemical properties of the product.
It is one of the excellent preservation methods and the preservative effect of smoking of food is due to the drying of food during smoking. Smoking helps to add desired flavor and aroma to the food. The chemicals present in the smoke during the drying the drying process inhibit microbial growth during storage.
It is used to remove more moisture from the food that is already dried by electric current.
In this method liquid food is whipped to foam dried with warm and crushed to the powder form.
Factors in the Control of Drying:
A proper control of dehydration includes the following factors:
1. Temperature Employed:
It varies with the food and the method of drying taking into consideration the sensitivity of food to temperature.
2. Relative Humidity of the Air:
It influences drying and is varied with the food, the method of drying and also with the stage of drying. It usually higher at the start of drying than at later stage of drying.
It also influences drying by removal of moisture at faster rate if velocity is more and slower rate if velocity is less.
Time of drying determines the extent of dehydration. More the time, more is the drying, but it again depends on other three factors.
Many of the pre treatment of food are carried before drying.
The important steps to be carried out before drying process are:
(a) Selection and sorting for maturity and soundness
(b) Washing
(c) Peeling of fruits and vegetables by hand or machine
(d) Subdivision into halves, slices or cubes
(e) Alkali dipping- sodium carbonate
(f) Blanching, and
(g) Sulfuring of light color red fruits and vegetables (treatment with SO2 gas).
i. Sweating:
It is process of storage of dried item in boxes or bins with the addition of moisture to a desired level, e.g., some fruits and vegetables.
Most foods are packaged soon after drying for protection against moisture contamination of microbes and attack of insects.
It is used to kill pathogenic microbes if present. In this method the temperature employed is 65.6-85°C for 30-70 at 70-100 % of relative humidity.