The University of Georgia College of Agricultural & Environmental Sciences
Cooperative Extension Service

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Food Spoilage and You

George Schuler, William Hurst, Estes Reynolds and James Christian; revised by P.T. Tybor
Extension Food Science

What Causes Microbial Spoilage?
What Causes Non-Microbial Food Spoilage?
Enzymatic Spoilage
Handling Leftovers
Sanitation
Good Manufacturing Practices (GMPs)
How to Preserve Food to Retard Spoilage
When the Electrical Power is Interrupted
Summary

What is food spoilage? Although the question sounds simple, it's hard to answer. Is an egg spoiled if it contains a speck of blood on the yolk? Is chocolate spoiled if it has white streaks on the outside? Are peanuts spoiled if they taste stale? Is cornmetal spoiled if it contains weevils or weevil eggs? Is butter spoiled if it tastes strong? Is meat spoiled if it is darker than normal or if small pock marks are present on the surface of frozen meat? Is a can of peaches spoiled if, after you open them, you find a washer in the can? Slimy, smelly or moldy food is more easily recognized as spoiled food. Spoilage is a natural phenomenon; it occurs at varying rates depending on the storage temperature, kind of food involved, kind of microorganisms present, packaging materials used, food additives used and method of preservation.

Food is considered spoiled when an undesirable change in the color, flavor, odor or texture has occurred. Foreign substances in food products make foods undesirable. The onset of food spoilage is rather indefinite. It is a gradual process occurring because of poor sanitation, enzymatic or chemical reactions, improper temperature controls, microbial growth or physical abuse.

Shelf life refers to the length of time a food remains palatable -- from the time it is harvested or processed until it is eaten.

Types of food spoilage fall into two major categories, according to the cause of the spoilage: microbial spoilage is caused by microorganisms and their products; non-microbial spoilage can be caused by foreign material in the foodstuff or by enzymes that occur in the foodstuff naturally.

What Causes Microbial Spoilage?

Growth of Microorganisms

Microorganisms grow rapidly; we call it logarithmic growth. Table 1 shows "How to Become a Millionaire" by starting with a penny and doubling it each day; 1 becomes 2, 2 becomes 4, 4 becomes 8, 8 becomes 16 and so on. This is exactly the way microorganisms reproduce.

The time it takes a bacterial cell to reproduce is called the generation time. This table also shows how important initial numbers of bacteria are. If we start with th 20th generation containing 524,288 bacteria, it takes only one generation to reach 1,000,000 bacteria, which is a large enough population to cause spoilage to begin in food products. If the equipment, personnel and product are clean, the initial numbers of bacteria will be lower and we may begin with the 15th generation of 16,384 . . .; in this instance, the shelf life will be five times longer than before.

Figure 1 shows a typical growth curve. Four distinct phases occur in the growth curve: lag; log or growth phase; stationary phase; and death phase.

Bacteria need about four hours to adapt to a new environment before they begin rapid growth. In handling food, this means we have less than four hours to make a decision to either cool the food, heat it, or eat it. For example, when chickens arrive at the dock of a fast food outlet, or at a restaurant or at your home, you must decide whether to heat and eat them, to refrigerate them at a low temperature (chickens freeze at 28 degrees F) for a short period of time, or whether to wrap and freeze the chicken for a longer period of time. If you don't decide, the bacteria will enter the log phase of growth where bacteria grow rapidly and cause food to spoil. Bacteria produce the slime, toxins, off colors and odors associated with food spoilage in the log phase of growth. Remember, the four hours bacteria remain in the log phase is approximate and cumulative.

As microorganisms grow, they tend to form colonies. These colonies are made up of millions of individual cells. Once a colony forms, the food available to each cell is limited and excretions from these millions of cells become toxic to a microbe. This is the stationary phase. Some of the cells now begin to die.

If we can control bacterial growth, we can control the major cause of food spoilage. Microbiological growth is discussed at length in What Are Bacteria, Yeasts, and Molds, Georgia Extension Bulletin #817.

Conditions Necessary for Microbial Spoilage

Microbial spoilage is the major cause of food spoilage. It occurs as a result of contamination of food by microorganisms, provision of a suitable environment for their growth, and degradation of the foodstuffs.

To control microbial buildup, you must control the following:

• The source of microorganisms - people, raw materials, equipment, air currents, dust and pests.
• Food residues, which are required for bacterial growth.
• Moisture, which is required for growth; relative humidity should be monitored.
• Time, during which food product is exposed to a given set of conditions that promote bacterial growth.
• Temperature since, in general, temperature determines the generation time of microorganisms.

SOURCE

Foodstuffs are naturally contaminated with microorganisms. To keep numbers of microorganisms as low as possible, wash fresh foods such as fruits and vegetables to physically remove as many microorganisms as possible. Processors of foods must keep their plants and equipment as clean as possible to provide clean work surfaces that come in contact with food. Make every effort to keep the initial numbers of microorganisms as low as possible.

Figure 2 shows why we need to keep initial numbers low. A food product that starts with 100 microorganisms per gram may have a shelf life of 12 days before it develops off odors, slime and spoilage. When the initial number is 5,000 per gram, the shelf life of that same foodstuff may be shortened to seven days. Since so much depends on the initial number of bacteria, temperatures and handling practices, a specific shelf life for a category of food products is difficult to determine.

Initial numbers can be kept low by practicing good personal hygiene, by sanitizing equipment and controlling temperature, and by using chlorinated water where possible.

FOOD

Like every other living thing, bacteria require food to live. They need only very small quantities. Some protein or fat left on the wall of a processing plant, grease on the blade of a knife or saw, or food residues on the wheel of a can opener or on a cutting board are a feast for microorganisms as well as for larger pests. Any equipment that may contact food (food contact surfaces) must be cleaned thoroughly.

MOISTURE

Every living thing requires moisture, and bacteria are no exception. Food that requires refrigeration is usually very high in moisture content. Moist food left over for long periods of time provides adequate moisture for growth.

Figure 3 shows a typical bacterial cell. Its surface is rough, similar to a sponge's surface; its only means of obtaining food is by absorption similar to that of a sponge. Enzymes manufactured inside the cell move out onto foodstuffs, combine with the food, and return to the cell. This process cannot be accomplished without moisture. This is why foods such as dried milk, dried soups and cereals do not spoil microbiologically. The bacteria are present . . . but they can't eat.

TIME

Some organisms grow faster than others. Under ideal conditions, certain bacterial populations can double in 5 to 7 minutes; others require hours. Bacteria that reproduce most quickly will dominate.

TEMPERATURE

Probably the single most important factor in preventing microbial food spoilage is temperature. Generally speaking, the cooler the food can be kept, the longer shelf life it will have. A good "rule of thumb" to use: To double the shelf life of a food that needs refrigeration, lower the temperature 18 degrees F, or for every 18degrees F decrease in storage temperature, food will last twice as long.

A thermometer in the refrigerator is a necessity. Maintain the temperature at 34 to 38 degrees F. Remember, however, that some foods such as tomatoes and lettuce will freeze or damage at 32 to 33 degrees F.

Different bacteria require different temperatures for maximum growth. Some bacteria, called psychrophiles, will grow at refrigerated temperatures. Others, called mesophiles, will only grow at moderate temperatures. Warm-loving bacteria, called thermophiles, grow at temperatures above 140 degrees F. At temperatures above and below the optimum, they grow and reproduce at a slower rate. Food spoilage bacteria grow best at environmental temperatures of 70 to 100 degrees F. A generation time is the amount of time it takes for a bacterium to reproduce itself. The shorter the generation time, the faster food spoilage will occur.

The data in Table 2 indicates that this particular bacterium has less than one-half the generation time at 86 degrees F (29 minutes) as it does at 68 degrees F (60 minutes). This means they reproduce more than twice as fast at 86 degrees F than at 68 degrees F. The optimum temperature for growth for this bacterium indicated by the shortest generation time is approximately 98 degrees F. At temperatures above and below this temperature, the generation time is increased. All types of bacteria do not have the same generation time under identical conditions.

What Causes Non-Microbial Food Spoilage?

Food may spoil as a result of chemical changes within the food itself or by a reaction between the food and the packaging material.

A general statement concerning how long food will last is difficult to make since so much depends on the type and constituents of food. Beef, chicken and turkey may be kept frozen up to a year, while sausage and bacon have a much shorter three-month shelf life. This is due, in part, to the high fat content of the sausage and bacon. Fats, like butter and oils, are subject to rancidity.

Rancidity is caused by a chemical reaction that breaks down the fatty acids in fat to smaller molecular weight fatty acids and, at the same time, releases certain odiferous products. So as butter becomes older, it tastes stronger. We say peanuts "stale" with age. When restaurants forget or do not change their frying oils, discriminating customers refuse to put up with the repulsive odor and take their business elsewhere.

These are examples of non-microbial spoilage. Another example is inclusion of foreign material in foods. Washers, bolts, nuts and various other items have been found in canned foods. This generally occurs when a maintenance person makes a repair on the line and uses a can for holding parts. The can stays on the line and is filled with the product. Although the product is retorted and sterile, it is aesthetically undesirable to find metallic parts in canned foods.

Hair, flies, roaches, and other pests or parts of pests have been found in food products. A wing and leg of a fly were found on a candy bar received from a vending machine. Many folks have had the undesirable experience of drinking a soft drink at a football game and finding a fly in the remaining ice. All of a sudden the drink didn't taste as good as you expected.

From time to time, grocery and department stores and warehouses have problems with weevils in cornmeal and similar products. Other pests can drill through a plastic bag, an aluminum foil wrap, a paper cup or through chocolate and lay their eggs in the center of a peanut butter candy bar.

From these examples you should learn to look at food before you eat it. Adequate temperature controls and sanitation will solve these problems.

Enzymatic Spoilage

Enzymes are chemicals produced by all living things. They help speed up or slow down chemical reactions, act as transports for foods, and are a normal constituent of foods. For instance, as a banana matures, the color changes from green to yellow to brown to black. The change is caused by the enzymes (chemicals) in the banana. The ripening, then softening, of other fruits such as apples, peaches and tomatoes is another example of enzymatic action. Enzymes can be inactivated by heat, which is the reason for blanching vegetables; or they can be inactivated by cold temperatures below 40 degrees F, which is the reason for placing vegetables under refrigeration. Think of green tomatoes in the refrigerator compared to tomatoes sitting on the window sill.

Bacteria also produce enzymes that break down food and allow them to obtain nutrients through their cell walls. Therefore, lowering the temperature reduces the rate of enzyme action as well as the rate at which bacteria can multiply. Refrigeration increases the time required to spoil food.

As the number of bacteria increases, the amount of enzymes produced increases. Higher temperatures can cause increased enzymatic activity. With large numbers of bacteria and high temperatures, a food will spoil very rapidly. When bacterial contamination is high and the storage temperature is low, a food will keep for a moderate period of time; when the bacterial contamination is low and the storage temperature high, food will keep for a moderate period of time. However, if the contamination of bacteria is low and the storage temperature kept low, the food product will have the longest possible shelf life.

Handling Leftovers

Some leftover foods in the refrigerator will develop an off taste when reheated. This is typical of beef and chicken and is referred to as warmed-over flavor. This is caused by a change in the chemical compounds in the meat responsible for flavor and is not related to bacterial spoilage. Warmed-over flavor may be masked by spices and the food used in barbecued items. Some foods held for long periods of time in the refrigerator after cooking may develop off odors or flavors as a result of the waste products produced by psychrophilic bacteria.

Raw foods such as meat, poultry, vegetables and other foods may get slimy or develop an off odor that indicates the food is near the point of spoilage. When the food is cooked or brought to an internal temperature of 165 degrees F, both enzymes and bacteria are destroyed. This food, although edible, may have an off flavor or odor. The off flavor or odor developed as a result of the amount of bacteria on the food before it was cooked. Keep leftover foods covered to prevent contamination and put them under refrigeration so the temperature of the food is reduced below 40 degrees F as soon as possible. A metal-stemmed thermometer is handy to make quick checks on temperatures of foods. When food is left over, cool it as quickly as possible. When soups, stews, or meats such as turkey are to be stored in large quantities overnight, they may be cooled by utilizing two steam table pans. Place ice in one pan and the soup, stew or other food in another pan on top of the ice. Once the "flash heat" is removed from the food. it may be placed in four-inch deep covered pans and placed under refrigeration.

If foods like five to 10 gallons of soup or stew must be cooled in bulk, place the food container in a large container of ice. Stir the soup until the temperature reaches 40 degrees F, at which time it may be safely placed under refrigeration. When food is held for serving, it should be maintained at 140 degrees F or above. In other words, keep hot foods hot -- and cold foods cold.

Sanitation

Next to good temperature control of your product, sanitation is the most important factor determining how long your product will be marketable.

The product -- whether it is animal or animal products (eggs, milk), vegetables, nuts or fruits -- should be free from excessive amounts of adhering dirt before it is processed.

Water used should be free of viable micro-organisms. This is accomplished routinely by city water systems. Plants using wells should chlorinate their water to two to five parts per million (ppm) free chlorine.

Processing and auxiliary euipment -- knives, gloves, saws,tables and tubing as well as permanently installed equipment -- should be checked periodically to determine its level of sanitation. This can be done by conducting a microbiological survey in which "contact" plates containing agar (a growth medium for bacteria) are touched to the equipment surfaces. Some people prefer to use swabs. The moistened swab is rubbed over the surface area, transferred to the agar, incubated, then counted. Two often-overlooked places in restaurants and in homes that need cleaning periodically are the wheel on the can opener and the storage drawer for silverware.

Good Manufacturing Practices (GMPs)

Plant facilities should conform to GMPs. They may also be checked for cleanliness by use of a microbiological survey.

Air currents in a food facility should travel from the cleanest areas of the facility (generally the packaging area) to the dirtiest area. This is checked very easily by holding a lighted match or candle to determine which way the air is moving. With the re-introduction of ceiling fans, it is important to remove accumulations of dust and grease from the blades to prevent contamination of food. Although this probably will not cause food spoilage, it is aesthetically undesirable to customers. You should also check air filters on a regular basis for the accumulation of dust.

People must be trained and constantly reminded of their key role in maintaining a good sanitary environ-ment. We are carriers of microorganisms and must stay aware of how our activities can spread them. We can hold down contamination of foodstuffs in several ways: use hair restraints to keep hair out of food, put on clean uniforms in the place of employment, sneeze into the shoulder, keep sneeze guards in place, clean hands frequently, particularly after using the rest-room. Rules for personal hygiene are discussed at length in Food, Hands, and Bacteria, Georgia Extension Bulletin #693.

A strong program of pest control is essential to prevent the introduction and growth of bacteria in plant operations. Good trash disposal, bait programs, screening of windows and doors, fly control by air curtains, electrocuters or aerosols combined with a regular spraying program for roaches and other insects are among the effective methods of dealing with pests.

How to Preserve Food to Retard Spoilage

Packaging Material

Packaging materials serve a multitude of purposes. They act as advertisements for the product, as cushions against breakage (eggs), as protection against tampering (drugs), as a preservation method (vacuum-packaged luncheon meats), as a preparation method (boilable or microwavable pouches), as a means of inventory control with UPC (universal product code), as protection against freezer burn (moisture-vapor-proof wrap).

The list could go on; do not underestimate the importance of good packaging material. The type of packaging material used should be considered carefully. For example, poultry or meat purchased fresh from a store may need to be rewrapped in moisture-vapor-proof paper if you want to freeze it.

The packaging material used will depend on the method of preservation and cooking. If the shelf life of a product is expected to be 12 to 18 days as in poultry, a polyethylene bag is satisfactory. If a longer shelf life is desired, a less oxygen-permeable wrap along with vacuum and back flush with carbon dioxide or nitrogen will give a 26 to 28-day shelf life.

Antioxidants

Antioxidants may be placed on wrapping paper, as in certain candy bars. They can also be incorporated into products such as tartar sauce, sausage and margarine or added to liquids such as drinks or boiled egg brines. These chemicals retard spoilage and include BHA, BHT, Propyl Gallate, EDTA, Citric Acid, Sodium Benzoate and Phosphoric Acid, to name just a few.

Vacuum Packaging

Vacuum packaging is superior to conventional tray-pack or over-wrapping of meat and poultry, but costs are higher. Certain foods that respire, such as vegetables, do not lend themselves to vacuum packaging or any other tight wrap. The flavor of processed foods like potato chips, coffee, nuts, hams or hot dogs is protected by vacuum packaging. Bacteria that grow in this vacuum environment can pro-duce gas and may cause ballooning in vacuum- packed products.

Overwrapped or Tray-packed Products

Overwrap or tray-packed packaging provides short-term protection to foods that will be used rather quickly. These wraps are designed for use in coolers and may or may not be impermeable to moisture and vapor. If the method of preservation is changed, such as when purchasing a fresh product with the intent of freezing, you may want to consider rewrapping it.

Freezing Products

Frozen products are packaged in moisture- vapor-proof packaging materials. Care must be taken to avoid rips in the package since this can cause freezer burn. Moisture from the food is drawn out of the package onto the coil of the freezer, leaving small pock marks on the food. Although this does not make the food inedible, the flavor and texture of the foodstuff are decreased.

Drying

Probably the best methods of preservation are the oldest -- drying and smoking. These methods have limited uses, but when drying is coupled with good packaging, foods generally have a very long shelf life. Think of sugar, flour, dried milk, egg powder, dried soups, country ham and jerky. Since bacteria need moisture to grow, this method of preservation is very effective. Store dry foods in airtight containers to prevent moisture take-up. Small amounts of moisture promote bacterial growth and sub-sequent spoilage. Certain dry foods, such as instant coffee, keep their flavor better if they're refrigerated after they're opened. Some foods, such as cornmeal, should be refrigerated, especially in hot summer weather, to prevent the hatching of weevil eggs in the meal.

Canning

Canning, like drying, greatly prolongs the life of food. Canned foods are considered semi-perishable due to the breakdown of the lining of the can and the possibility of chemical reaction between the foodstuffs and the lining. Although refrigeration is not necessary for these semi-perishable foods, keep them in a cool place -- not in a hot garage or warehouse. Quality of canned foods will deteriorate more rapidly if held at high temperatures.

When the Electrical Power Is Interrupted

If the electricity is off for only a short period, DO NOT open your freezer. It will hold the food for eight to 12 hours or more without thawing. If the outage is predicted to be longer, insulating with newspapers is helpful. If dry ice (CO₂) is available, spread several slices throughout the freezer.

If the plug is inadvertently pulled or the circuit breaker flips, correct the problem. If the compressor fails, ask for a neighbor's help or use a frozen food locker until you can get a new freezer. Remove the food to a freezer as soon as possible. Rule of thumb: If the food has some ice crystals or is 40 to 50 degrees F, it's okay to refreeze. If the temperature is higher, the quality and safety of the food is questionable. Depending on the temperature and the foodstuff, dispose of it or cook and use it.

Summary

Food spoilage is caused by microbial growth in foodstuffs, chemical reactions within the food itself or between the food and its environment, or the presence of foreign material in the food. Spoilage can be slowed by practicing the following:

• Package the freshest possible product.
• Use good sanitation and personal hygiene habits when processing and packaging food.
• Use the best possible packaging material for the length of time the food remains in the market channel.
• Cool processed or cooked foods as quickly as possible to below 40 degrees F.
• Keep foods covered.
• Keep food on a steam table hot, above 140 degrees F.

Remember: Use a thermometer to determine the temperature of foods; don't guess! Within certain temperature limitations, for every 18 degrees F decrease in storage temperature, the shelf life of food will double.

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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating.

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