The chances of an egg being internally contaminated are relatively low, less than one in 10,000 commercial eggs. It is more common for contamination to occur during handling and preparation after the egg has been removed from its shell. Even so, eggs are an excellent breeding ground for microbial activity, and can become internally contaminated through a hen with a Salmonella enteritidis infection in her ovary or oviduct, or from absorbing bacteria through the pores. The latter can occur if the eggs are boiled and then cooled in the presence of infected water or an infected food handler. Externally, the eggs may also be exposed to Salmonella enteritidis by fecal contamination during egg laying. The Centers for Disease Control implicated eggs as the source for a high percentage of Salmonella enteritidis outbreaks, and there is an increasing possibility that Listeria monocytogenes, which can grow at refrigerator temperatures and has already been observed on whole eggs, may also contribute to future outbreaks. There are many precautions that can be taken to prevent foodborne illness from eggs.

• Use an egg separator rather than passing the yolk back and forth between the two shell halves.
• Always store eggs in the refrigerator, never at room temperature. Meringue-covered pies and other egg-containing foods should be refrigerated until served.
• Raw eggs should never be consumed as this is especially the case for the very young, elderly, or immune-compromised.
• Use only pasteurized eggs for food items in which eggs are only lightly cooked or not at all, such as Caesar salad, uncooked hollandaise or bearnaise sauce, and homemade mayonnaise, eggnog, ice cream, etc.
• Do not add raw egg to already scrambled eggs, a practice sometimes used in food service operations to increase the moisture content of dried scrambled eggs.
• Cook eggs until no visible liquid egg remains, especially when preparing French toast, scrambled eggs, poached eggs, and omelets.
• A knife inserted into baked egg dishes such as quiches, baked custards, and most casseroles should come out clean.
• Scrambled eggs should be held on cafeteria and buffet lines at appropriate temperatures.
• Be extra cautious when preparing lightly cooked egg dishes such as mousse, meringue, and other similar dishes, because they may not be sufficiently cooked to eliminate possible bacteria.
• All egg dishes should be heated to 145^oF (63^oC).

Unlike chemical changes, physical changes in foods do not result in the formation of new compounds. A few of the common physical changes occurring in foods as they spoil are evaporation, drip loss, and separation. Water evaporates out of improperly stored foods, creating an unattractive, dried-out appearance and possible undesirable flavor changes. Water can also be lost (syneresis) out of foods such as gelatins, yoghurt, and sour cream as they age. Separation of water and oil occurs in such foods as non-homogenized milk, mayonnaise, salad dressings, and high-moisture cheeses when they are stored too long or are frozen and then thawed. This separation is the reason why sandwiches spread with mayonnaise or made with high-moisture cheeses do not freeze well.

Chemical changes, or reactions, can also contribute to the deterioration of foods. Enzymes play a significant role in catalyzing these reactions and can be categorized depending on the substance they act on (substrate) or their mode of action. Proteases, also called proteolytic enzymes, split proteins into smaller compounds. Fish have many more active proteases than meats, which is one of the reasons they deteriorate so quickly. Lobsters are also prone to proteolytic breakdown, which begins to occur the minute they expire, unless lobsters are kept alive to the very last second. The proteases cause the lower abdomen to partially liquefy and the tail meat to become crumbly when cooked.

Lipids are broken down by enzymes called lipases, which degrade the triglycerides of fat into glycerol and fatty acids. Further degradation leads to rancidity, or off-odors and tastes. Enzymes that decompose carbohydrates are carbohydrases, each named after the particular sugar on which it acts. For example, sucrase breaks sucrose down into glucose and frutose. Yet another group of enzymes serves to oxidize compounds. Some of the more common oxidases include ascorbic acid oxidase, peroxidase, tyrosinase, and polyphenolase. The latter two enzymes are involved in enzymatic browning, which leads to unappetizing brown discoloration in some fruits and vegetables. Hydrolysis may also contribute to the deterioration of foods.

Microorganisms, including bacteria, yeasts, and molds, are the prime biological factor involved in food spoilage. These tiny organisms need food to survive just as people do, so food is a natural target. The most common foods spoiled by bacteria include meat, eggs, milk, and opened canned goods.

The naturally occurring yeasts in the air are found almost everywhere, but normally pose no threat. The moisture in foods, however, can encourage their growth to a point that becomes unacceptable. Yeasts prefer high-sugar foods such as fruits, vegetables, and fruit preserves, and can cause unwanted fermentation of fruits and fruit juices in the presence of the proper amounts of exygen, moisture, and acidity (PH). Any method that keeps the moisture content low will be successful in their control.

These microorganisms, like yeasts, prefer high-sugar foods, but are particularly drawn to cheese and bread. The appearance of their bloom on foods indicates that spoilage has begun. Molds are easily spread through the air, are very resistant to drying, and can be difficult to control by the means used for bacteria and yeasts. Commercial food enterprises sometimes use vacuum pumps to remove oxygen from containers, because molds cannot grow in its absence.

There are many weapons in the arsenal with which to fight food spoilage resulting from microbial action, including boiling, refrigeration, drying, and curing with high concentrations of sugar or salt.

Foods may be classified as perishable, semiperishable, or non perishable, depending on how susceptible they are to spoiling. The most perishable foods are those with large concentrations of protein and/or water, which accelerate the microbial and chemical processes of decomposition. For example, fish, seafood, meat, eggs, and dairy products, which all have a high protein and water content, are very perishable. Watery fruits and vegetables such as tomatoes, peaches, berries, and leafy vegetables are also highly perishable. Semi-perishable foods contain less water and include those, such as potatoes, carrots, beets, turnips, onions, and apples, that can keep for several days or even months under proper conditions. Processed nuts, cereals, dried tea leaves, pastas, and dried beans and peas are classified as nonperishable because they contain very little water and will keep for months with little loss of quality.

Thawing. The package of meat slowly thawing on the kitchen counter used to be a familiar sight, but now thawing frozen meat at room temperature is considered by many food experts to be an unsafe practice. For safe thawing, one of the following methods should be used -

• Refrigerator, on the bottom shelf to avoid contaminating other foods with any dripping.
• Microwave oven followed by immediate cooking.
• As part of the cooking process.

Running cold water over the meat wrapped in protective plastic or placing it in a bath of ice water and frequently replacing the water, are not as safe as defrosting in a refrigerator.

Cross-Contamination. To prevent cross-contamination, all hands, utensils, and surfaces touching raw food should be thoroughly washed and sanitized before being used again for either raw food or cooked food that will not be further heated. Cross-contamination can occur via tabletops, cutting boards, knives, forks, and slicers, as well as aprons, cleaning cloths, and sponges. Cross-contamination commonly occurs when the uninformed person places raw meat on a plate to be carried out to the barbecue, cooks the meat, and then puts it back on the same plate to take it to the table. Dust and soil should be washed off the tops of cans before they are opened. Raw meats should never be stored in the refrigerator above cooked or ready-to-eat foods where they may drip onto the food below.

Heating. One of the most important factors in controlling bacterial growth is temperature, especially during preparation. In fact, most foodborne outbreaks are a result of improper temperature control. Heating foods to a certain temperature and maintaining that heat for a given time, depending on the food, will destroy most microorganisms. The general rule is to keep cold foods below 40^oF (4^oC) and hot foods above 140^oF (60^oC). Foods such as meat and poultry, however, must reach a minimum temperature during cooking in order to ensure safety.

Temperature guide needed to destroy microorganisms in different foods -

 

The density of the food plays a key role, because heat must penetrate the entire food mass. Since microwaves only penetrate one-half to 2 inches into food, this limits the size and thickness of meats that can be safely cooked in a microwave oven. It is unsafe to cook turkeys in microwave oven, even if they are unstuffed. The short heating time is another concern about the safety of microwave-processed foods.

Holding. When holding foods for extended periods, they must be kept either above or below the temperature danger zone. Food handlers use thermometers to ensure that the food on the serving table meets these guidelines. To put this in perspective, room temperature is usually around 70^oF (21^oC) and in the kitchen it can increase up to 90^oF (32^oC). Critical to controlling bacterial growth is the fact that temperatures under 40^oF (4^oC) retard but do not kill bacteria, while those above 165^oF (74^oC) destroy most microorganisms.

Serving. Serving is an important step of the food production process and another point of vulnerability to contamination. Good personal hygiene on the part of food service employees is essential to the safety of the foods and beverages being served. Even when serving, the 140^oF (60^oC) and 40^oF (4^oC) boundaries must be observed.

Cooling / Reheating. Inappropriately cooled foods are a major cause of food-borne illnesses. Foods should be cooled to below 40^oF (4^oC) within four hours of removal from cooking or they pose a danger to diners. Liquid foods should be placed in shallow pans less than 3 inches deep to cool, and thicker foods in pans less than 2 inches deep. All hot foods must be reheated to at least 165^oF (74^oC) within two hours before serving. In a food service establishment, untouched leftovers are sometimes discarded, because they are a potential source of microbial contamination.

In addition to the precautions that are taken when purchasing any foods, some foods should always be treated with special care. The foods that are best able to support the growth of bacteria are those containing protein and water, and include the following -

• Meat – beef, pork, lamb
• Poultry
• Fish and shellfish
• Dairy
• Eggs
• Broth, stocks
• Gravies/sauces (meat, milk or egg based)
• Tofu and other soy foods
• Stuffings (when exposed to poultry cavity)

These foods are in the high-risk category for bacterial contamination. Proper refrigeration or freezing is a must. COmbination-type foods, such as pasta or tuna salads, Chinese dishes, and Mexican-style meals featuring refried beans, tacos, and enchiladas, are also frequently implicated in foodborne illnesses. Also vulnerable are products made with the high-risk foods and exposed to more handling, such as meatloaf, hamburger, salads using egg-containing mayonnaise (coleslaw, and chicken, egg, and tuna salads), some baked goods, and cream fillings. Egg dishes likely to become contaminated include baked or soft custard, French toast, quiches, Hollandaise sauce, meringues, eggnog, and mayonnaise. Damaged eggs are good vectors for organisms that cause foodborne illnesses and should be discarded. Food establishments have the safer option of using pasteurized eggs.

In recent years, foodborne illnesses have been associated with foods that do not fit into the high-protein/high-water risk category. Fresh fruits and vegetables, cooked rice, sliced fruits, sauteed onions, potatoes, garlic-in-oil combinations, and apple cider have all been implicated as well and care should also be taken.

Incoming raw foods may be contaminated in several ways. The digestive tracts of people and animals naturally contain bacteria. During the rendering of animals at the slaughterhouse, the digestive tract may be accidently cut open or nicked, releasing bacteria that may then come in contact with meat. Other possible sources of contamination include any cuts, skin, feet, hair, hide, or feathers that can carry bacteria. Fruits and vegetables may be contaminated by microorganisms in the soil, or by manure used to fertilize crops.

Contamination may also occur when pathogen-killing processes, such as temperature gauges, heaters, seals, and refrigeration units, fail to work properly. And finally, foods may become contaminated after sanitation. Microorganisms by their nature are ubiquitous and lodge themselves in air filters, drains, equipment, floor cracks, food scraps, and even dust.

Chemical contamination may occur when certain metals contact foods and are then consumed. Potentially toxic metals include lead, brass, copper, zinc, animony, and cadmium. Food safety measures regarding toxic metals allow only approved materials to come in contact with foods. For example, paint brushes should never replace basting brushes, and refrigerator shelves, which may contain cadmium, are not suitable grill substitutes. Certain kinds of containers are not recommended. Galvanized tin containers should never be used to store acidic juices, lemonade, tea, or salad dressing because the acidity can dissolve the zinc coating. Although the American canning industry has long since phased out lead solder in favor of seamless welding, some imported canned foods are still lead soldered. The FDA advises against storing acidic foods such as fruits and tomato products in their opened cans, because acidity can increase the level of lead in food. Some lead-based products and enamelware are not recommended, because they may chip and expose the underlying metal. The glaze on some pottery used for heating and/or serving food was, not so long ago, a source of lead. Now, pottery made in or imported into the United States is required to be lead-free, but imported ceramic coffee cups, older pottery, or pottery purchased abroad may still contain lead. Finally, wine should not be allowed to remain in lead crystal decanters, if at all, except for short periods of time in serving.

Physical Hazards

Any nonfood material found in food is considered to be a physical hazard under the HACCP system. Dangerous, and sometimes bizarre, foreign objects like sharp pieces of metal, wood, plastic, stones, false fingernails, toothpicks, watches, jewelry, insects, staples from food boxes, and many other items have been known to find they way into food. The most commonly reported foreign object found in foods is glass.