(1) Freezing and undesirable changes. Freezing of many foods will cause undesirable changes, such as the destruction of emulsions and texture. Emulsified products, such as salad dressing and mustard, contain a fat/oil and water mixture which does not combine without special processing or additives. If these types of products are frozen, the emulsion will be destroyed and the fat and water will separate into distinct layers. Fruits and vegetables that are allowed to freeze and then thaw will have their texture disrupted. Skins will crack, leaving the food susceptible to attack by microorganisms. The texture of canned fruits and vegetables becomes softened and mushy due to uncontrolled freezing.
(2) Cold damage. Cold damage to foods does not necessarily require the extreme of freezing. Many fruits and vegetables, like other living systems, have optimum temperature requirements after harvest. Held at common refrigeration temperatures of about 41 °F (5°C), several fruits and vegetables are weakened or killed and deteriorative processes follow. The deteriorations include off-color development, surface pitting, and various forms of decay. Bananas, lemons, squash, and tomatoes are examples of products that should be held at temperatures no lower than 50°F (10°C) for maximum quality retention. This provides an exception to the inaccurate generalization that cold storage preserves all foods, and the colder the better.
b. High Temperature. There is a moderate temperature range over which much food is handled, such as 50°-100°F (10°-38°C). Within this range, for every 18°F (10°C) rise in temperature, the rate of chemical reaction is approximately doubled. This includes the rates of many enzymatic as well as nonenzymatic reactions. Excessive heat can denature proteins, break emulsions, dry out foods by removing moisture, and destroy vitamins.
(1) Effect on vegetables. Excessive heat in green vegetables causes cell walls and membranes to lose their integrity and acids and enzymes to be released. All of these result in the development of a soft texture as well as the development of off-colors and off-flavors.
(2) Effect on muscle tissue. The consequences of excessive heat on muscle tissue are that proteins are denatured, the proteins clump together, and enzymes are inactivated. This results in a toughening of the texture, loss of waterholding capacity, cooked or caramel flavors, and development of off-colors.
c. Dehydration. Dehydration, another form of physical change that causes food deterioration, can be simply defined as the loss of water from the food product. Foods, especially fresh, chilled, and frozen, are subject to dehydration.
(1) Amount of water in food. Foods contain a substantial amount of water. Meat products contain from 70 to 75 percent water, whereas fresh fruits and vegetables contain from 80 to 95 percent water. Since water vapor is continually seeking to go from an area of high concentration to an area of low concentration, improper storage conditions and improper packaging will result in dehydration. When the humidity is too low in a storage area, dehydration results.
(2) Signs of dehydration. The signs of dehydration include dryness or shriveling on the surface of the food item. The development of off-colors, usually a darkening effect, will also be observed.
(3) Terms used. In frozen foods, the dehydration is known as freezer burn. In fresh fruits and vegetables, it is known as wit.
d. Excessive Moisture. Another form of physical change that causes food deterioration is excessive moisture.
(1) Foods that take in moisture. The gross changes in foods from excessive moisture are part of everyday experience. Dried, dehydrated, and freeze-dried foods are especially susceptible to this form of deterioration. These types of food are very hygroscopic (readily taking up and retaining moisture); if not properly packaged, the product will become lumpy or caked if excessive moisture is present. This condition can possibly lead to other forms of deterioration, such as bacterial growth and chemical reactions such as oxidation.
(2) Effect of surface moisture. Moisture need not be present throughout the food to exert major effects. Surface moisture resulting from slight changes in relative humidity can be a major cause of lumping and caking, as well as surface defects such as mottling, crystallization, and stickiness. The slightest amount of condensation on the surface of the food can become a virtual pool for the multiplication of bacteria or the growth of mold.
(3) Condensation from the food product. This condensation need not come from the outside. In a moisture-proof package, food materials such as fruits and vegetables can give off moisture from respiration and transpiration. This moisture is then trapped within the package and can support the growth of microorganisms.
e. Mechanical Damage. The fifth form of physical change is mechanical damage.
(1) Entering point for microorganisms. When an item receives mechanical damage, not only is the appearance of the item affected but the damaged food tissue also becomes more susceptible to other forms of deterioration. Mechanically damaged foods are more susceptible to invasion by microorganisms, for the damaged area serves as a port of entry.
(2) Starting point for enzyme activity. The cell walls of foods are also destroyed by mechanical abrasion, and the inherent enzymes in the food product are liberated from the cells. Once liberated, the enzymes begin the process of deterioration or, more specifically, autolysis. The changes noted would be a softening in the texture, development of off-colors, and development of off-flavors.
f. Light. Light, another form of physical change that causes food deterioration, can cause fading of color in many food items. Some vitamins are destroyed by light, notably riboflavin, vitamin A, and vitamin C. Milk in bottles exposed to the sun develops "sunlight" flavor due to light induced fat oxidation and changes in the protein.
(1) Importance of type of light. Not all wavelengths making up natural or artificial light are equally absorbed by food constituents, nor are they equally destructive. Surface discolorations of sausages and meat pigments are different, depending upon the natural light and fluorescent light that may be encountered in display cases.
(2) Control techniques. Sensitive foods often can be protected from light by impervious packaging or by incorporating compounds into glass and transparent films that screen out specific wavelengths. A related problem has to do with maintaining the true color of inks used to depict products in food advertising and on food product labels.
(3) Greening of potatoes. Another problem due to light is the greening of potatoes. This condition is also referred to as sunburn. It is the result of the exposure of the potatoes to sunlight during growth or after digging, and to artificial light during display for sale. After exposing for two days or longer to either natural or artificial light, a green pigment develops. The skin and the flesh are affected. Chlorophyll and an alkaloid called solanine are produced due to exposure to light. The green tubers acquire a bitter, pungent taste. If eaten in quantity, they may be poisonous. Solanine is the bitter and poisonous component. Chlorophyll is tasteless and harmless.
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