Preharvest Biological Decay
Before harvest and slaughter, plant and animal foods are subject to a myriad of microbiological diseases including viruses, molds, yeasts, and bacteria. In addition, some foods before harvest can be attacked and diseased or eaten by insects, birds, and rodents. For plants, competition by weeds can result in poor yield. To prevent or control this, one form of processing is the use of chemical or physical means of control or prevention. The use of pesticides, and herbicides are examples of chemical control. Another method of chemical control is the use of drugs such as antibiotics to prevent disease in animals prior to slaughter. Weeding by machine is a physical means of control. These modes of deterioration are not generally considered about shelf life or open dating. However, if the food is subjected to damage, its initial quality will be less. Processing does not make low quality foods better, and overall shelf life will be less after slaughter or harvest than with undamaged foods.
Senescence
Once a fruit, vegetable, cereal grain, or animal product is slaughtered, it is separated from its source of nutrients and water. However, since it is still a viable living system, the enzymes present continue to operate and utilize the available carbohydrate and nutrient stores. For fruits, this process can be of benefit because they can repair post harvest damages, and more important, fruits can be picked prior to optimum maturity, transported long distances to the marketplace and home, and then develop into a high quality product.
For all foods, however, eventually enzymatic biochemical processes occurring post harvest lead to degradation, including loss of color, flavor, nutrients, and texture. In addition, the breakdown products produced damage the tissue so that the foods become subject to microbiological attack with subsequent more rapid decay. To prevent this mode of deterioration, three major processing methods can be used: (1) lowering of temperature slows the reaction; (2) raising the temperature denatures the enzymes and makes them inactive; and (3) removal or binding of water reduce availability (or water activity), which reduces the ability of the enzymes to operate.
Microbiological Disease
Microorganisms constitute a major mechanism by which many foods, especially fresh ones, lose their quality. This is because microbes are ubiquitous in the environment, and can grow rapidly. After a food is harvested or slaughtered, it loses some degree of its ability to fight off microbial attack. If it is physically damaged, then it becomes more susceptible to attack. Bruising, cutting, and trimming constitute such damage. Microbes can grow rapidly on foods (starting with one microbe which divides every 10 min, with sufficient available nutrients, in five hours there would be over one billion microbes present). The basic principle of preservation is to control or destroy them. Much the same controls, are employed as those used for enzymes:
(1) Lower temperature to slow growth.
(2) Raise temperature to kill them.
(3) Remove or bind water to slow or prevent their growth.
(4) Lower pH to slow or stop their growth by adding-acid or fermentation.
(5) Control O2 or CO2 level to control population.
(6) Manipulate food composition to remove nutrients needed by the microbes.
Because in some cases the above methods change the food into a form not desirable by the consumer, chemical means of preservation can be used. These chemicals are used to slow growth or kill the organisms. At the level of use, the chemicals should have no ill effect on the consumer. Examples are the use of calcium propionate in bread and sodium benzoate in some soft drinks.
Knowledge of the rate of growth of microbes as a function of the environmental conditions of the food is very important in the prediction of shelf-life and, thus, the open dating of some foods such as: fresh and ground meats and fresh poultry; fresh fish; dairy products such as milk. Cheese and yogurt; cured meats such as hot dogs, bacon, and bologna: pasteurized fruit drinks; fruits and vegetables; and whole grains.
A second and more serious problem with microorganisms is the fact that some are pathogenic to humans, i.e., they either cause infection when ingested or result in the production of chemicals in the food, which are toxic to humans. Most food processes are designed to guard against contamination with pathogens and the subsequent growth of these pathogens after processing, or as important, to treat the food in such n way as to destroy any potentially harmful microbes that might be inadvertently present in the food. For example, fermentation of foods with useful microbes results in alcohol or acid production. The acid and alcohol prevent the growth of pathogens.
Chemical Deterioration
During the processing of foods, tissue damage occurs which causes the release of various food chemical constituents into the cellular fluid environment. These chemicals can then react with each other or with external factors to lead to deterioration of the food and result in a shortening of the shelf life. Many different reactions can occur which lead to quality and nutrient loss. The major ones are classified below.
Enzymatic
The normal post-harvest enzymatic reactions can lead to a loss in food quality and shelf life. In addition, destruction of cell tissues releases enzymes, which can lead to further deterioration. This reaction is usually very rapid at room temperature, but is controlled in the natural state. Once the food is handled, deterioration starts. Enzymatic decay also occurs in the frozen state unless the enzymes are previously denatured by blanching. Enzymatic reactions also lead to the major mode of deterioration of many refrigerated dough. Unfortunately, heat treatment to denature the enzymes results in loss results in loss of dough functionality. Control of the enzymatic reactions is the same as was discussed for senescence. Knowledge of the rate of these reactions as a function of environmental conditions is very important in prediction of shelf life for open dating. The major environmental factors are oxygen, water, pH, and temperature. Other enzymatic degradations include color losses and vitamin tosses such as for vitamin C.
Lipid Oxidation
Many foods contain unsaturated fats, which are important in the nutrition of humans. Unfortunately, these fats are subject to direct attack by oxygen through an auto catalytic free radical mechanism. This results in the production of rancid off-flavors, which make the food undesirable to consume. Very little fat has to oxidize for the consumer to detect rancidity and reject the food. Unfortunately, the food will still be very nutritious. The free radicals and peroxides produced in this process can react and blench pigments, such as occurs in dried vegetables, and can destroy vitamins C, E, and A. They can also result in protein degradation, making it of poorer quality, as can happen in whole dry milk; can cause darkening of the fat as happens in deep fat frying; and produce toxic substances which have been implicated in some animal studies as potential carcinogens.
Knowledge of the rate of lipid oxidation is important in foods where it be the principal mode of deterioration, for example:
(1) Fried snacks,
(2) Nuts,
(3) Dried meats/vegetables/fish/poultry,
(4) Cereals,
(5) Wheat germ,
(6) Frozen vegetables/meats/fish/poultry,
(7) Some dairy products,
(8) Semi-moist meat products,
(9) Pre cooked refrigerated meats and fish,
(10) Cured meat and fish, coffee, cooking and salad oils, margarine, spices, and dried vegetables such as potatoes and carrots.
Non Enzymatic Browning (NEB)
Non Enzymatic Browning (NEB) is another major chemical reaction leading to a loss of quality and nutritional value. This reaction is the result of reactions between reducing compounds (such as glucose, fructose and lactose) and proteins or amino acids. Browning can also occur as the result of heating sugars to very high temperatures or through the oxidation of vitamin C. In certain cases the reaction is desirable, such as in the toasting of the bread, the crust formed in roasting meats, malting of barley for beer and spirits manufacture, and the production of syrups, molasses, and caramel candies.
Other Chemical Reactions
Other chemical reactions that can lead to food degradation include the thermal destruction of vitamins such as A, B, and C, the effect of light an pigments such as occurs in the browning of meat and bleaching of chlorophyll, the direct oxidation of vitamin C, the effect of light on riboflavin, and the direct oxidation of carotenoid pigments and the loss of flavor through some mechanism. In ever case the effect of temperature, oxygen level, moisture content, and light must be known for the rate of the reaction to be predicted and the time to reach end of shelf life measured. Of importance in all these reactions is a decision as to what extent of degradation is considered to be the end of shelf life.
Physical Degradation
Physical damage can also lead to loss of shelf life. The types of physical damage can be classified into the following categories.
Physical Bruising/Crushing
This mode of deterioration is related to physical abuse of in food harvest, processing, and distribution. It is particularly important to fruits and vegetables, since physical abuse leads to microbial attack and decay. Packaging to prevent abuse is key to long shelf life. With dry materials such as chips, crushing can lead to unacceptability based on consumer desires.
Wilting
Fresh leafy and tuber vegetables can deteriorate if subjected to low relative humidity, losing moisture to their surroundings. This results in loss of crispness and an increased rate of senescence reactions with subsequent quality and nutrient losses. Proper knowledge of the rate of moisture loss for various packaging materials and the maximum allowable moisture loss can be used as one means of setting open dates for fresh produce.
Moisture Loss/Gain
With some food products such as candy, semi moist pet foods, cakes and bread, moisture loss leads to an increase in hardening. If a limit of hardness is known to be unacceptable then predictions to reach this level based on equations which describe the moisture change with time, can be used as one method of prediction of end of shelf life. From this, the open date for best quality can be set. These same equations can be used to predict the moisture loss of flour, pasta, and similar dry products for which a natural loss of moisture occurs and a net weight limit is set for sale. Similarly, some products that gain moisture have a textural limit at which they become too soft, such as potato chips, other dried or fried snacks, and crackers.
Temperature Induced Texture Changes
Temperature fluctuations per se can affect physical modes of deterioration. For example, the continuous rise and fall of temperature around a phase change point leads to melting of fat and the subsequent degradation of quality of some candies and formulated foods.
Staling
Staling is a mode of deterioration important in processed wheat flour products such as bread and cakes. The reaction is a crystallization ion of amylopectin, one of the major starches present in wheat flour. The rate is increased as temperature decreases, which is opposite to the chemical, enzymatic, and microbial reactions discussed earlier. Thus, to prevent staling, the food must not be refrigerated. However, this can result in other reactions causing loss of shelf life.
Induced Textural Changes
Both lipid oxidation and non-enzymatic browning result in degradation of proteins, which leads to toughening and loss of shelf life.
