TECHNICAL REPORT ON FOOD PRESERVATION

Chapter 1

INTRODUCTION

        Food is considered contaminated when unwanted microorganisms are present. Most of the time the contamination is natural, but sometimes it is artificial. Natural contamination occurs when microorganisms attach themselves to foods while the foods are in their growing stages. For instance, fruits are often contaminated with yeasts because yeasts ferment the carbohydrates in fruits. Artificial contamination occurs when food is handled or processed, such as when bacteria enter food through improper handling procedures.

Food spoilage is a disagreeable change or departure from the food's normal state. Such a change can be detected with the senses of smell, taste, touch, or vision. Changes occurring in food depend upon the composition of food and the microorganisms present in it and result from chemical reactions relating to the metabolic activities of microorganisms as they grow in the food.

Types of spoilage: Various physical, chemical, and biological factors play contributing roles in spoilage. For instance, microorganisms that break down fats grow in sweet butter and cause a type of spoilage called rancidity. Certain types of fungi and bacteria fall into this category. Species of the Gram-negative bacterial rod Pseudomonas are major causes of rancidity. The microorganisms break down the fats in butter to produce glycerol and acids, both of which are responsible for the smell and taste of rancid butter.

Another example occurs in meat, which is primarily protein. Bacteria able to digest protein and break down the protein in meat and release odoriferous products such as putrescent and cadaverine. Chemical products such as these result from the incomplete utilization of the amino acids in the protein.

Food spoilage can also result in a sour taste. If milk is kept too long, for example, it will sour. In this case, bacteria that have survived pasteurization grow in the milk and produce acid from the carbohydrate lactose in it. The spoilage will occur more rapidly if the milk is held at room temperature than if refrigerated. The sour taste is due to the presence of lactic acid, acetic acid, butyric acid, and other food acids.

1.1 Sources of microorganisms: 

The general sources of food spoilage microorganisms are the air, soil, sewage, and animal wastes. Microorganisms clinging to foods grown in the ground are potential spoilers of the food. Meats and fish products are contaminated by bacteria from the animal's internal organs, skin, and feet. Meat is rapidly contaminated when it is ground for hamburger or sausage because the bacteria normally present on the outside of the meat move into the chopped meat where there are many air pockets and a rich supply of moisture. Fish tissues are contaminated more readily than meat because they are of a looser consistency and are easily penetrated.

Canned foods are sterilized before being placed on the grocery shelf, but if the sterilization has been unsuccessful, contamination or food spoilage may occur. Swollen cans usually contain gas produced by members of the genus Clostridium. Sour spoilage without gas is commonly due to members of the genus Bacillus. This type of spoilage is called flat-sour spoilage. Lactobacilli are responsible for acid spoilage when they break down the carbohydrates in foods and produce detectable amounts of acid.

Among the important criteria determining the type of spoilage are the nature of the food preserved, the length of time before it is consumed, and the handling methods needed to process the foods. Various criteria determine which preservation methods are used.

1.2 Food Preservation

Food preservation is the process of treating and handling food to stop or slow down  (loss of quality, edibility or nutritional value) and thus allow for longer storage.

Preservation usually involves preventing the growth of bacteria, yeasts, fungi, and other micro-organisms (although some methods work by introducing benign bacteria, or fungi to the food), as well as retarding the oxidation of fats which cause rancidity. Food preservation can also include processes which inhibit visual deterioration that can occur during food preparation; such as the enzymatic browning reaction in apples after they are cut.

Many processes designed to preserve food will involve a number of food preservation methods. Preserving fruit, by turning it into jam, for example, involves boiling (to reduce the fruit’s moisture content and to kill bacteria, yeasts, etc.), sugaring (to prevent their re-growth) and sealing within an airtight jar (to prevent recontamination). There are many traditional methods of preserving food that limit the energy inputs and reduce carbon footprint.

Maintaining or creating nutritional value, texture and flavor is an important aspect of food preservation, although, historically, some methods drastically altered the character of the food being preserved. In many cases these changes have now come to be seen as desirable qualities cheese, yoghurt and pickled onions being common examples.

The chief methods of food preservation are as follows;

        i.            Asepsis, or keeping out microorganisms.

      ii.            Removal of microorganisms.

    iii.            Maintenance of anaerobic condition.

    iv.            Use of high temperature.

      v.            Use of low temperature.

    vi.            Drying.

  vii.            Use of chemical preservatives.

viii.            Irradiation.

    ix.            Mechanical destruction of microorganisms. 

1.3 Principles of food preservation:

In accomplishing the preservation of foods by the various methods, the following principles are involved:

1.      Preservation or delay of microbial decomposition

                                i.            By keeping out microorganisms(asepsis)

                              ii.            By removal of microorganisms, example, by filtration

                            iii.            By hindering the growth and activity of microorganisms. Example, by low temperature, drying, anaerobic condition, etc.

                            iv.            By killing the microorganisms- heat and radiation.

2.      Preservation or delay of self decomposition of the food.

                                i.            By destruction or inactivation of food enzymes.

                              ii.            By destruction or delay of purely chemical reactions- antioxidants

3.      Preservation of damage because of insects, animals, mechanical causes.

1.4 Delay of microbial decomposition:

Many common methods of food preservation depend not on the destruction or removal of microorganisms but on delay in the initiation of growth and hindrance to growth once it has begin. A summary of the major preservation factors and their mode of action are explained by using microbial growth curve. The different phases of microbial growth are,

1.  Lag phase:  growth rate null;

2.  Acceleration phase:  growth rate increases;

3.  Exponential phase:  growth rate constant;

4.  Deceleration phase:  growth rate decreases;

5.  Stationary phase:  growth rate null;

6.  Death phase:  growth rate negative

Growth of microbial cultures:

Figure 1.1 growth curve of microbial cultures

During lag phase, bacteria adapt themselves to growth conditions. It is the period where the individual bacteria are maturing and not yet able to divide. During the lag phase of the bacterial growth cycle, synthesis of RNA, enzymes and other molecules occurs.

1.     Exponential phase (sometimes called the log phase or the logarithmic phase) is a period characterized by cell doubling. The number of new bacteria appearing per unit time is proportional to the present population. If growth is not limited, doubling will continue at a constant rate so both the number of cells and the rate of population increase doubles                                                                         with each consecutive time period. For this type of exponential growth, plotting the natural logarithm of cell number against time produces a straight line. The slope of this line is the specific growth rate of the organism, which is a measure of the number of divisions per cell per unit time. The actual rate of this growth (i.e. the slope of the line in the figure) depends upon the growth conditions, which affect the frequency of cell division events and the probability of both daughter cells surviving. Under controlled conditions, cyanobacteria can double their population four times a day. Exponential growth cannot continue indefinitely, however, because the medium is soon depleted of nutrients and enriched with wastes.

2.     During stationary phase, the growth rate slows as a result of nutrient depletion and accumulation of toxic products. This phase is reached as the bacteria begin to exhaust the resources that are available to them. This phase is a constant value as the rate of bacterial growth is equal to the rate of bacterial death.

3.     At death phase, bacteria run out of nutrients and die.

This basic batch culture growth model draws out and emphasizes aspects of bacterial growth which may differ from the growth of macrofauna. It emphasizes clonality, asexual binary division, the short development time relative to replication itself, the seemingly low death rate, the need to move from a dormant state to a reproductive state or to condition the media, and finally, the tendency of lab adapted strains to exhaust their nutrients.

In reality, even in batch culture, the four phases are not well defined. The cells do not reproduce in synchrony without explicit and continual prompting (as in experiments with stalked bacteria) and their exponential phase growth is often not ever a constant rate, but instead a slowly decaying rate, a constant stochastic response to pressures both to reproduce and to go dormant in the face of declining nutrient concentrations and increasing waste concentrations.

Batch culture is the most common laboratory growth method in which bacterial growth is studied, but it is only one of many. It is ideally spatially unstructured and temporally structured. The bacterial culture is incubated in a closed vessel with a single batch of medium. In some experimental regimes, some of the bacterial culture is periodically removed and added to fresh sterile medium. In the extreme case, this leads to the continual renewal of the nutrients. This is a chemostat, also known as continuous culture. It is ideally spatially unstructured and temporally unstructured, in a steady state defined by the rates of nutrient supply and bacterial growth. In comparison to batch culture, bacteria are maintained in exponential growth phase, and the growth rate of the bacteria is known.

Chapter 2

FOOD PRESERVATION TECHNIQUES

2.1 Asepsis

To keep microorganisms out of food, contamination is minimized during the entire food preparation process by sterilizing equipment, sanitizing it, and sealing products in wrapping materials. Microorganisms may be removed from liquid foods by filtering and sedimenting them or by washing and trimming them. Washing is particularly valuable for vegetables and fruits, and trimming is useful for meats and poultry products.

Packing of foods is a widely used application of asepsis. It requires protection, tampering resistance, and special physical, chemical, or biological needs. It also shows the product that is labeled to show any nutrition information on the food being consumed. The food enclosed in the package may require protection from, among other things, shock, vibration, compression, temperature, A barrier from oxygen, water vapor, dust is often required. Permeation is a critical factor in design. Some packages contain desiccants or oxygen absorbers to help extend shelf life. Modified atmosphere or controlled atmospheres are also maintained in some food packages. Keeping the contents clean, fresh, and safe for the intended shelf life is a primary function.

2.2 Preservation by Use of High Temperatures

Preservation of food by the use of heat finds very wide applications compared to other methods. Heat may be used either for processing or conversion of foods or simply as a means or preserving the food. In heat processing or conversion the application of heat is used primarily to effect chemical changes in food. Cooking, frying and baking involve both processing and preservation operations. Cooking makes food tender and also destroys a large proportion of microorganisms and natural enzymes. Cooked foods can be stored for several days provided they are protected from recontamination. Refrigeration of cooked food is a normal household practice to prolong the storage time. However, cooking will not sterilize a product. Cooking also destroys the toxin formed by Clostridium botulinum during a ten minute exposure of the food to moist heat at 100 degree C. Thus cooking provides a final measure of protection for consumer form food borne diseases. The killing of microorganisms by heat is due to thermal denaturation of protein and enzymes of the microorganism required for its metabolic activity and growth. The heat treatment necessary to kill the organisms or spores varies with the kind of organism, its state and the environment during heating.
            The type of heat treatment will depend on the kind of organism to be killed, other preservative methods to be employed and the effect of heat on the food. The use of heat also affects the food adversely and hence it is necessary to use only mild heat treatment that ensures freedom from pathogens and enzyme activity and enhance the self life of the food.
2.2.1Pasteurization

Pasteurization is a process of heating a food, usually a liquid, to a specific temperature for a definite length of time and then cooling it immediately. This process slows spoilage due to microbial growth in the food.

Unlike sterilization, pasteurization is not intended to kill all micro-organisms in the food. Instead, it aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurized product is stored as indicated and is consumed before its expiration date). Commercial-scale sterilization of food is not common because it adversely affects the taste and quality of the product. Certain foods, such as dairy products, may be superheated to ensure pathogenic microbes are destroyed.

The process of pasteurization is applied to most milk today. Pasteurization of cream to increase the keeping qualities of butter was practiced in England before 1773 and was introduced to Boston by 1773, although it was not widely practiced in the United States for the next 20 years. It was still being referred to as a "new" process in American newspapers as late as 1802.

Pasteurization of milk was suggested by Franz von Soxhlet in 1886. It is the main reason for milk's extended shelf life. High-temperature, short-time pasteurized milk typically has a refrigerated shelf life of two to three weeks, whereas ultra-pasteurized milk can last much longer, sometimes two to three months. When ultra-heat treatment is combined with sterile handling and container technology(such as aseptic packaging), it can even be stored unrefrigerated for 6 to 9 months.

2.2.1.1 Effectiveness of pasteurization

Milk pasteurization has been scientifically proven to be at least 90% effective in eliminating harmful bacteria in milk. While some few pathogens are heat resistant, modern equipment is readily able to test and identify bacteria in milk being processed. Pasteurization is the only effective means of eliminating 90% or more of harmful organisms in milk.

Nonpasteurized, raw milk, according to the Centers for Disease Control (CDC), was responsible for 86 reported food poisoning outbreaks between 1998 and 2008, resulting in 1,676 illnesses, 191 hospitalizations, and two deaths. Improperly handled raw milk is responsible for nearly three times more hospitalizations than any other food borne disease outbreak.

2.2.2 Canning

Canning is a method of preserving food in which the food contents are processed and sealed in an airtight container. Canning provides a typical shelf life ranging from one to five years, although under specific circumstances a freeze-dried canned product, such as canned, dried lentils, can last as long as 30 years in an edible state. In 1795 the French military offered a cash price of 12,000 francs for a new method to preserve food. Nicolas Appert suggested canning and the process was first proven in 1806 in test with the French navy and the prize awarded in 1809 or 1810. The packaging prevents microorganisms from entering and proliferating inside.

To prevent the food from being spoiled before and during containment, a number of methods are used: 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 recipe of the foods being preserved, a sufficient dose of ionizing radiation, submersion in a strong saline solution, acid, base, osmotically extreme or other microbially-challenging environments.

Other than sterilization, no method is perfectly dependable as a preservative. For example, the microorganism Clostridium botulinum can only be eliminated at temperatures above the boiling point. From a public safety point of view, foods with low acidity need sterilization under high temperature (116-130 °C). 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.

2.2.3 Sterilization

Sterilization  is a term referring to any process that eliminates (removes) or kills all forms of microbial life, including transmissible agents (such as fungi, bacteria, viruses, spore forms, etc.) present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media. Sterilization can be achieved by applying the proper combinations of heat, chemicals, irradiation and high pressure.

2.2.3.1 Heat Sterilization

A widely-used method for heat sterilization is the autoclave, sometimes called a converter. Autoclaves commonly use steam heated to 121–134 °C (250–273 °F). To achieve sterility, a holding time of at least 15 minutes at 121 °C (250 °F) or 3 minutes at134 °C (273 °F) is required. Additional sterilizing time is usually required for liquids and instruments packed in layers of cloth, as they may take longer to reach the required temperature (unnecessary in machines that grind the contents prior to sterilization). Following sterilization, liquids in a pressurized autoclave must be cooled slowly to avoid boiling over when the pressure is released. Modern converters operate around this problem by gradually depressing the sterilization chamber and allowing liquids to evaporate under a negative pressure, while cooling the contents. Proper autoclave treatment will inactivate all fungi, bacteria, viruses and also bacterial spores, which can be quite resistant..

To ensure the autoclaving process was able to cause sterilization, most autoclaves have meters and charts that record or display pertinent information such as temperature and pressure as a function of time. Indicator tape is often placed on packages of products prior to autoclaving. A chemical in the tape will change color when the appropriate conditions have been met. Some types of packaging have built-in indicators on them.

Biological indicators (bioindicators) can also be used to independently confirm autoclave performance. Simple bioindicator devices are commercially available based on microbial spores. Most contain spores of the heat resistant microbe Geobacillus among the toughest organisms for an autoclave to destroy. Typically these devices have a self-contained liquid growth medium and a growth indicator. After autoclaving an internal glass ampule is shattered, releasing the spores into the growth medium. The vial is then incubated (typically at 56 °C (133 °F)) for 24 hours. If the autoclave destroyed the spores, the medium will retain its original color. If autoclaving was unsuccessful the B.sterothermophilus will metabolize during incubation, causing a color change during the incubation.

For effective sterilization, steam needs to penetrate the autoclave load uniformly, so an autoclave must not be overcrowded, and the lids of bottles and containers must be left ajar. Alternatively steam penetration can be achieved by shredding the waste in some Autoclave models that also render the end product unrecognizable. During the initial heating of the chamber, residual air must be removed. Indicators should be placed in the most difficult places for the steam to reach to ensure that steam actually penetrates there.

For autoclaving, as for all disinfection or sterilization methods, cleaning is critical. Extraneous biological matter or grime may shield organisms from the property intended to kill them, whether it physical or chemical. Cleaning can also remove a large number of organisms. Proper cleaning can be achieved by physical scrubbing. This should be done with detergent and warm water to get the best results. Cleaning instruments or utensils with organic matter, cool water must be used because warm or hot water may cause organic debris to coagulate. Treatment with ultrasound or pulsed air can also be used to remove debris.

Although imperfect, cooking and canning are the most common applications of heat sterilization. Boiling water kills the vegetative stage of all common microbes. Roasting meat until it is well done typically completely sterilizes the surface. Since the surface is also the part of food most likely to be contaminated by microbes, roasting usually prevents food poisoning. Note that the common methods of cooking food do not sterilize food - they simply reduce the number of disease-causing micro-organisms to a level that is not dangerous for people with normal digestive and immune systems.

Pressure cooking is analogous to autoclaving and when performed correctly renders food sterile. However, some foods are notoriously difficult to sterilize with home canning equipment, so expert recommendations should be followed for home processing to avoid food poisoning.

2.2.3.2 Chemical sterilization

Chemicals are also used for sterilization. Although heating provides the most reliable way to rid objects of all transmissible agents, it is not always appropriate, because it will damage heat-sensitive materials such as biological materials, fiber optics, electronics, and many plastics. Low temperature gas sterilizers function by exposing the articles to be sterilized to high concentrations (typically 5 - 10% v/v) of very reactive gases (alkylating agents such as ethylene oxide, and oxidizing agents such as hydrogen peroxide and ozone). Liquid sterilants and high disinfectants typically include oxidizing agents such as hydrogen peroxide and peracetic acid and aldehydes such as glutaraldehyde and more recently o-phthalaldehyde. While the use of gas and liquid chemical sterilants/high level disinfectants avoids the problem of heat damage, users must ensure that article to be sterilized is chemically compatible with the sterilant being used. The manufacturer of the article can provide specific information regarding compatible sterilants. In addition, the use of chemical sterilants poses new challenges for workplace safety. The chemicals used as sterilants are designed to destroy a wide range of pathogens and typically the same properties that make them good sterilants makes them harmful to humans. Employers have a duty to ensure a safe work environment (Occupational Safety and Health Act of 1970, section 5 for United States) and work practices, engineering controls and monitoring should be employed appropriately.

2.2.3.3 Radiation sterilization

Methods of sterilization exist using radiation such as electron beams, X-rays and gamma rays.

Gamma rays are very penetrating and are commonly used for sterilization of disposable medical equipment, such as syringes, needles, cannulas and IV sets. Gamma radiation requires bulky shielding for the safety of the operators; they also require storage of a radioisotope (usually Cobalt-60), which continuously emits gamma rays (it cannot be turned off, and therefore always presents a hazard in the area of the facility).

Electron beam processing is also commonly used for medical device sterilization. Electron beams use an on-off technology and provide a much higher dosing rate than gamma or x-rays. Due to the higher dose rate, less exposure time is needed and thereby any potential degradation to polymers is reduced. A limitation is that electron beams are less penetrating than either gamma or x-rays.

X-rays, High-energy X-rays (bremsstrahlung) are a form of ionizing energy allowing to irradiate large packages and pallet loads of medical devices. Their penetration is sufficient to treat multiple pallet loads of low-density packages with very good dose uniformity ratios. X-ray sterilization is an electricity based process not requiring chemical nor radio-active material. High energy and high power X-rays are generated by an X-ray machine that can be turned off for servicing and when not in use.

Ultraviolet light irradiation (UV, from a germicidal lamp) is useful only for sterilization of surfaces and some transparent objects. Many objects that are transparent to visible light absorb UV, glass for example completely absorbs all UV light. UV irradiation is routinely used to sterilize the interiors of biological safety cabinets between uses, but is ineffective in shaded areas, including areas under dirt (which may become polymerized after prolonged irradiation, so that it is very difficult to remove). It also damages some plastics, such aspolystyrene foam if exposed for prolonged periods of time.

2.2.4 Drying

Drying is a method of food preservation that works by removing water from the food, which inhibits the growth of microorganisms. Open air drying using sun and wind has been practiced since ancient times to preserve food. A solar or electric food dehydrator can greatly speed the drying process and ensure more-consistent results. Water is usually removed by evaporation (air drying, sun drying, smoking or wind drying) but, in the case of freeze-drying, food is first frozen and then the water is removed by sublimation. Bacteria, yeasts and molds need the water in the food to grow, and drying effectively prevents them from surviving in the food

Freeze dried vegetables are often found in backpackers food, hunters, military, etc. The exception to this rule is bulbs, such as garlic and onion, which are often dried. Edible and mushrooms, as well as other fungi, are also sometimes dried for preservation purposes, to affect the potency of chemical components, or so they can be used as seasonings.

2.2.4.1 Solar drying

Hundreds of millions of tonnes of wheat, corn, soybean, rice and other grains as sorghum, sunflower seeds, rapeseed/canola, barley, oats, etc., are dried in grain dryers. In the main agricultural countries, drying comprises the reduction of moisture from about 17-30%w/w to values between 8 and 15%w/w, depending on the grain. The final moisture content for drying must be adequate for storage. The more oil the grain has, the lower its storage moisture content will be (though its initial moisture for drying will also be lower). Cereals are often dried to 14% w/w, while oilseeds, to 12.5% (soybeans), 8% (sunflower) and 9% (peanuts). Drying is carried out as a requisite for safe storage, in order to inhibit microbial growth. However, low temperatures in storage are also highly recommended to avoid degradative reactions and, especially, the growth of insects and mites. A good maximum storage temperature is about 18°C.

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun, particularly infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.

When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off of other objects, it is experienced as diffused light.

The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120watts per square meter. Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer. Sunlight takes about 8.3 minutes to reach the Earth.

2.2.4.2 Drying by mechanical dryers

There are many different driers for drying, each with their own advantages for particular applications. Some of them are

§  Drum dryer

§  Tray drier

§  Rotary drier

§  Spray drier

§  Vacuum drier

§  Solar drier

§  Commercial food dehydrator

§  Household oven, etc.

2.2.4.3 Freeze drying

Freeze-drying (also known as lyophilisation or cryodesiccation) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze-drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase.

Freeze-drying is used to preserve food, the resulting product being very lightweight. The process has been popularized in the forms of freeze-dried ice cream, an example of astronaut food. It is also widely used to produce flavorings to add to food. Because of its light weight per volume of reconstituted food, freeze dried product is also popular. More dried food can be carried per the same weight of wet food, and has the benefit of "long life" compared to wet food that tends to spoil quickly.

The largest dryers are normally used "Off-farm", in elevators, and are of the continuous type: Mixed-flow dryers are preferred in Europe, while Cross-flow dryers in the USA. In Argentina, both types are commonly found. Continuous flow dryers may produce up to 100 metric tonnes of dried grain per hour. The depth of grain the air must traverse in continuous dryers range from some 0.15 m in Mixed flow dryers to some 0.30 m in Cross-Flow. Batch dryers are mainly used "On-Farm", particularly in the USA and Europe. They normally consist of a bin, with heated air flowing horizontally from an internal cylinder through an inner perforated metal sheet, then through an annular grain bed, some 0.50 m thick (coaxial with the internal cylinder) in radial direction, and finally across the outer perforated metal sheet, before being discharged to the atmosphere. The usual drying times range from 1 h to 4 h depending on how much water must be removed, type of grain, air temperature and the grain depth. In the USA, continuous counter flow dryers may be found on-farm, adapting a bin to slowly drying grain fed at the top and removed at the bottom of the bin by a sweeping auger. Grain drying is an active area of manufacturing and research. The performance of a dryer can be simulated with computer programs based on mathematical models that represent the phenomena involved in drying: physics, physical chemistry, thermodynamics and heat and mass transfer. Most recently computer models have been used to predict product quality by achieving a compromise between drying rate, energy consumption, and grain quality. A typical quality parameter in wheat drying is bread making quality and germination percentage whose reductions in \ are somewhat related.

2.2.5 Preservation by Use of low Temperatures

2.2.5.1 Common or Cellar storage

Refrigeration preserves food by slowing down the growth and reproduction of micro-organisms and the action of enzymes which cause food to rot. The introduction of commercial and domestic refrigerators drastically improved the diets of many in the Western world by allowing foods such as fresh fruit, and dairy products to be stored safely for longer periods, particularly during warm weather.

2.2.5.2 Chilling or Cold storage

Chilling is a method of cooling food quickly to a low temperature that is relatively safe from bacterial growth. Bacteria multiply fastest between +8 °C (46 °F) and +68 °C (154 °F). By reducing the temperature of cooked food from +70 °C (158 °F) to +3 °C (37 °F) or below within 90 minutes, the food is rendered safe for storage and later consumption. This method of preserving food is commonly used in food catering and, recently, in the preparation of 'instant' foods, as it ensures the safety and the quality of the food product.

2.2.5.3 Freezing or Frozen storage

Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. Freezer temperature should be maintained at 0°F and below. Food should never be thawed at room temperature , this increases the risk of bacteria and virus growth and the risk of food poisoning. Once thawed, food should be used and never refrozen. Frozen food should be thawed using the following methods

§  Microwave oven

§  During cooking

§  In cold water (place food in watertight, plastic bag; change water every 30 minutes)

§  In the refrigerator

Throughout foods that have been warmer than 40 °F for more than 2 hours. If there is any doubt at all about the length of time the food has been defrosted at room temperature, it should be thrown out. Freezing does not destroy microbes present in food. Freezing at 0 °F does inactivate microbes (bacteria, yeasts and molds). However, once food has been thawed, these microbes can again become active. Microbes in thawed food can multiply to levels that can lead to foodborne illness. Thawed food should be handled according to the same guidelines as perishable fresh food.

Food frozen at 0°F and below is preserved indefinitely. However, the quality of the food will deteriorate if it is frozen over a lengthy period. The United States Department of Agriculture, Food Safety and Inspection Service publishes a chart showing the suggested freezer storage time for common foods.

2.2.6 Food preservation by Radiation

Food irradiation is the process of exposing food to ionizing radiation to destroy microorganisms, bacteria,viruses , or insects that might be present in the food. Further applications include sprout inhibition, delay of ripening, increase of juice yield, and improvement of re-hydration. Irradiated food does not become radioactive, but in some cases there may be subtle chemical changes.

Irradiation is a more general term of the exposure of materials to radiation to achieve a technical goal (in this context "ionizing radiation" is implied). As such it is also used on non-food items, such as medical devices, plastics, tubes for gas pipelines, hoses for floor heating, shrink-foils for food packaging, automobile parts, wires and cables (isolation), tires, and even gemstones.

Food irradiation acts by damaging the target organism's DNA beyond its ability to repair. Microorganisms can no longer proliferate and continue their malignant or pathogenic activities. Spoilage-causing microorganisms cannot continue their activities. Insects do not survive, or become incapable of reproduction. Plants cannot continue their natural ripening processes.

The energy density per atomic transition of ionizing radiation is very high; it can break apart molecules and induce ionization, which is not achieved by mere heating. This is the reason for both new effects and new concerns. The treatment of solid food by ionizing radiation can provide an effect similar to heat pasteurization of liquids, such as milk. The use of the term "cold pasteurization" to describe irradiated foods is controversial, since pasteurization and irradiation are fundamentally different processes.

By irradiating food, depending on the dose, some or all of the harmful bacteria and other pathogens present are killed. This prolongs the shelf-life of the food in cases where microbial spoilage is the limiting factor. Some foods, e.g., herbs and spices, are irradiated at sufficient doses (five kilograms or more) to reduce the microbial counts by several orders of magnitude; such ingredients do not carry over spoilage or pathogen microorganisms into the final product. It has also been shown that irradiation can delay the ripening of fruits or the sprouting of vegetables.

Insect pests can be sterilized (be made incapable of proliferation) using irradiation at relatively low doses. In consequence, the United States Department of Agriculture (USDA) has approved the use of low-level irradiation as an alternative treatment to pesticides for fruits and vegetables that are considered hosts to a number of insect pests, including fruit flies and seed weevils; the U.S. Food and Drug Administration (FDA) has cleared among a number of other applications the treatment of hamburger patties to eliminate the residual risk of a contamination by a virulent E. coli. The United Nations Food and Agricultural Organization (FAO) has passed a motion to commit member states to implement irradiation technology for their national phytosanitary programs; the General assembly of the International Atomic Energy Agency (IAEA) has urged wider use of the irradiation technology. Additionally, the USDA has made a number of bi-lateral agreements with developing countries to facilitate the importation of exotic fruits and to simplify the quarantine procedures.

The European Union has regulated processing of food by ionizing radiation in specific directives since 1999; the relevant documents and reports are accessible online. The "implementing" directive contains a "positive list" permitting irradiation of only dried aromatic herbs, spices, and vegetable seasonings. However, any Member State is permitted to maintain previously granted clearances or to add new clearance as granted in other Member States, in the case the EC's Scientific Committee on Food (SCF) has given a positive vote for the respective application. Presently, six Member States (Belgium, France, Italy, Netherlands, Poland, United Kingdom) have adopted such provisions.

Because of the "Single Market" of the EC, any food – even if irradiated – must be allowed to be marketed in any other Member State even if a general ban of food irradiation prevails, under the condition that the food has been irradiated legally in the state of origin. Furthermore, imports into the EC are possible from third countries if the irradiation facility had been inspected and licensed by the EC and the treatment is legal within the EC or some Member state.

The Scientific Committee on Food (SCF) of the EC has given a positive vote on eight categories of food to be irradiated. However, in a compromise between the European Parliament and the European Commission, only dried aromatic herbs, spices, and vegetable seasonings can be found in the positive list. The European Commission was due to provide a final draft for the positive list by the end of 2000; however, this failed because of a veto from Germany and a few other Member States. In 1992, and in 1998 the SCF voted "positive" on a number of irradiation applications that had been allowed in some member states before the EC Directives came into force, to enable those member states to maintain their national authorizations.

In 2003, when Codex Alimentarius was about to remove any upper dose limit for food irradiation, the SCF adopted a "revised opinion", which, in fact, was a re-confirmation and endorsement of the 1986 opinion. The opinion denied cancellation of the upper dose limit, and required that before the actual list of individual items or food classes (as in the opinions expressed in 1986, 1992 and 1998) can be expanded, new individual studies into the toxicology of each of such food and for each of the proposed dose ranges are requested. The SCF has subsequently been replaced by the new European Food Safety Authority (EFSA), which has not yet ruled on the processing of food by ionizing radiation.

2.2.6.1 Electron irradiation

Electron irradiation uses electrons accelerated in an electric field to a velocity close to the speed of light. Electrons are particulate radiation and, hence, have cross section many times larger than photons, so that they do not penetrate the product beyond a few inches, depending on product density. Electron facilities rely on substantial concrete shields to protect workers and the environment from radiation exposure.

2.2.6.2 Gamma irradiation

Gamma radiation is radiation of photons in the gamma part of the electromagnetic spectrum. The radiation is obtained through the use ofradioisotopes, generally cobalt-60 or, in theory, caesium-137. Cobalt-60 is bred from cobalt-59 using neutron irradiation in specifically designed nuclear reactors. Caesium-137 is recovered during the processing of spent nuclear fuel. Because this technology – except for military applications – is not commercially available, insufficient quantities of it are available on the global isotope markets for use in large scale, commercial irradiators. Presently, caesium-137 is used only in small hospital units to treat blood before transfusion to prevent Graft-versus-host disease.

Food irradiation using cobalt-60 is the preferred method by most processors, because the deeper penetration enables administering treatment to entire industrial pallets or totes, reducing the need for material handling. A pallet or tote is typically exposed for several minutes to hours depending on dose. Radioactive material must be monitored and carefully stored to shield workers and the environment from its gamma rays. During operation this is achieved by substantial concrete shields. With most designs the radioisotope can be lowered into a water-filled source storage pool to allow maintenance personnel to enter the radiation shield. In this mode the water in the pool absorbs the radiation. Other uncommonly used designs feature dry storage by providing movable shields that reduce radiation levels in areas of the irradiation chamber.

2.2.6.3  X-ray irradiation

Similar to gamma radiation, X-rays are photon radiation of a wide energy spectrum and an alternative to isotope based irradiation systems. X-rays are generated by colliding accelerated electrons with a dense material (target) such as tantalum or tungsten in a process known as bremsstrahlung-conversion. X-ray irradiators are scalable and have deep penetration comparable to Co-60, with the added benefit that the electronic source stops radiating when switched off. They also permit dose uniformity, but these systems generally have low energetic efficiency during the conversion of electron energy to photon radiation requiring much more electrical energy than other systems. Like most other types of facilities, X-ray systems rely on concrete shields to protect the environment and workers from radiation.