Food for tomorrow

Factors that spoils the food.
Describe why food spoils.
Food spoilage can be defined as a disagreeable change in a food's normal state. Such changes can be detected by smell, taste, touch, or sight. These changes are due to a number of reasons -- air and oxygen, moisture, light, microbial growth, and temperature. Each is briefly explained below.

Air and Oxygen
One important cause of food spoilage and spoilage is air and oxygen. Because air is colorless, odorless, and tasteless, it is often taken for granted and sometimes forgotten as a means to cause food to spoil.

Air consists of 78% nitrogen, 21% oxygen, and a 1% mixture of other gases. While oxygen is essential for life, it can have deteriorative effects on fats, food colors, vitamins, flavors, and other food constituents. Basically, oxygen can cause food spoilage in several ways. It can provide conditions that will enhance the growth of microorganisms; it can cause damage to foods with the help of enzymes; and it can cause oxidation.

Microorganisms. Oxygen can provide conditions that enhance the growth of microorganisms. Some bacteria require oxygen for growth (aerobes) while others can grow only in the absence of oxygen (anaerobes). Many bacteria can grow under either condition and are called facultative anaerobes. Molds and most yeast that cause food to spoil require oxygen to grow. They can often be found growing on the surface of foods when air is present.

Enzymes. Certain enzymes that are naturally present in food are known as oxidizing enzymes. These enzymes catalyze (speed up) chemical reactions between oxygen and food components, and this leads to food spoilage. Although there are many oxidizing enzymes, two that can cause darkening in diced and sliced vegetables are catalase and peroxidase. The browning of vegetables caused by these enzymes is often accompanied by off-flavors and odors. A simple heat treatment (blanching) is used to inactivate these enzymes.

Oxygen. Oxygen can also cause spoilage of foods spontaneously, by itself. Oxidative spoilage is the chief cause of quality loss in fats and fatty portions of foods. When lipids oxidize, short chain carbon compounds are formed; these compounds have very strong odors and flavors and are very undesirable and unacceptable. The off-odors resulting from this type of spoilage are sharp and acrid and have been described as linseed oil-like, tallowy, fishy or perfume-like.

Moisture
Water is one of the most common substances on earth. It is an essential component of all foods. The amount of water in a food (known as percent water) influences the appearance, texture, and flavor of the food. All living organisms as well as food contain water. Water makes up about 70% or more of the weight of most fresh (unprocessed) foods. Even "dry" foods like beans, flour and cereals contain some water. Fresh fruits and vegetables contain the most water -- between 90% and 95% water. The amount of water in some common foods is shown below.

Food % Food %
Apples 84 Lettuce 96
Bread 32-37 Mayonnaise 15
Butter 16 Milk 88
Corn Flakes 4 Peaches 89
Green Beans 92 Peanut Butter 2
Flour 12 Raisins 18
Jams, Jellies, Preserves 29 Strawberries 90
Sugar 0.5

Although the water content of a food is expressed as a percent, this number does not reflect how the water exists in the food. Water in food is classified according to its availability, or biological activity and is either "free" or "bound." Free water is not bound to any components in a food; it can be used for microbial growth and is also available for chemical reactions. Bound water is physically bound to large (molecules) components in the food. It is not available to microorganisms for their growth and it cannot participate in chemical reactions.

Water or moisture greatly affects the keeping qualities of food. Excessive moisture pickup can result in product spoilage and spoilage by:

Microorganisms -- microorganisms need water to dissolve the food they use. Water allows the food to get into bacterial, yeast and mold cells where it is used for energy and growth. Water also allows waste products to escape from the cells.
Chemical Reactions -- the moisture in food also functions to allow chemical reactions to occur between components in the product.
Food spoilage and spoilage can occur when there are slight changes in relative humidity. Moisture can condense on the surface of a product and this can result in many common food defects. The molding of grain, soggy cereals, and the caking and lumping of dry products like powders and cake mixes can result from excessive moisture. Other defects such as mottling, crystallization and stickiness have also been observed. Moisture condensing on the surface of a food can also provide an environment for bacteria and molds to grow and multiply. Physical defects such as cracking, splitting and crumbling occur when excessive moisture is lost from foods.

Water is controlled in foods by:

Microorganisms -- microorganisms need water to dissolve the food they use. Water allows the food to get into bacterial, yeast and mold cells where it is used for energy and growth. Water also allows waste products to escape from the cells.
Chemical Reactions -- the moisture in food also functions to allow chemical reactions to occur between components in the product.
Food spoilage and spoilage can occur when there are slight changes in relative humidity. Moisture can condense on the surface of a product and this can result in many common food defects. The molding of grain, soggy cereals, and the caking and lumping of dry products like powders and cake mixes can result from excessive moisture. Other defects such as mottling, crystallization and stickiness have also been observed. Moisture condensing on the surface of a food can also provide an environment for bacteria and molds to grow and multiply. Physical defects such as cracking, splitting and crumbling occur when excessive moisture is lost from foods.

Water is controlled in foods by:

Drying (dehydration), concentration and evaporation -- by removing water to a certain level, deteriorative reactions can be reduced or prevented. Examples of products preserved by these drying techniques include dry milk, potato flakes, drink mixes, evaporated milk, and orange juice concentrate.
Freezing - the freezing of foods changes water from liquid to solid form and renders it unavailable to microorganisms and chemical reactions.
Food Additives -- salt and sugar are used in many products to bind water and thereby making it less available for microbial growth and biochemical reactions. Jams, jellies, and cured hams are examples.
Light
Almost all foods are exposed to light from natural and/or artificial sources. The exposure of foods to light can result in the photodegradation (or spoilage) of these products. This photodegradation usually occurs in food constituents, such as pigments, fats, proteins, and vitamins and results in discoloration, off-flavor development, and vitamin losses.

The light that is absorbed by the food can cause deteriorative reactions of the food constituents. In most solid foods, the light only penetrates the outer layer of the product and photodegradation occurs in this surface layer. Discoloration on the surface of foods can certainly affect consumer acceptance of these products.

In liquid foods, light penetration can be greater and with mixing of the products due to agitation, larger portions of food constituents may be deteriorated. The light sensitivity of a food depends on many factors including the: light source strength and type of light that it emits; distance of the light source form the food; length of exposure optical properties of the packaging materials; oxygen concentration of the food; and the temperature.

Microbial Growth
Microorganisms, specifically bacteria, molds, and yeasts, can cause food to spoil. For example, microorganisms that break down fats in unsalted butter can cause it to become rancid. Bacteria that breaks down protein in meat (proteolytic bacteria) and results in foul odors. Also, if milk is kept too long or at improper temperatures, it will sour. Bacteria that have survived pasteurization grow in the milk and produce acid from the lactose that is naturally present in the milk.

The general sources of food spoilage microorganisms are the air, soil, sewage, and animal wastes. Some microorganisms that are naturally present on the surface of foods grown in the ground can also cause food spoilage. Bacteria from the animal's internal organs, skin, and/or feet can contaminate meat and fish. 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 rich supply of moisture. Fish tissues are contaminated more readily than meat because they are more delicate and so more easily penetrated.

Canned foods undergo a sterilization process to make them shelf-stable. If canned foods are not properly processed, food spoilage (or food safety concerns) may occur. Swollen cans usually contain gas produced by the bacteria Clostridium. Sour spoilage without gas is commonly due to 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.

Temperature
When temperatures are not properly controlled, food can spoil. For example, for every 18°F rise in temperature within the moderate temperature range where most food is handled (50°F to 100°F), the rate of chemical reaction is approximately doubled. As a result, excessive heat will increase the rate of natural food enzyme reactions and the reactions of other food constituents. As a result, protein will breakdown or denature; emulsions will break; some vitamins will be destroyed; moisture will be lost and foods will dry out; and the color, flavor and odor of some products may be affected

Exposing foods to uncontrolled cold temperatures will also cause physical spoilage. Fruits and vegetables that accidentally freeze and thaw have their texture and appearance affected. Skins and surfaces of these products will often crack, leaving them more susceptible to microbial contamination. Some foods that become frozen may also be adversely affected. For example, if mayonnaise freezes, the emulsion will break and the components will separate.

Spoilage is caused in many foods by temperatures that are not extreme. Cold damage of several fruits and vegetables can occur at common refrigerator temperatures (35-40°F). Defects in produce exposed to cold temperatures include the development of off-colors, surface pitting and a variety of decays. Uncut, fresh fruits and vegetables such as bananas, lemons, squash and tomatoes are products that should be held at temperatures no colder than 50°F for best quality.

SOURCE: Adapted from a fact sheet developed by: Dr. Robert B. Gravani Cornell University, Ithaca, NY July 1983