Micro Organisms and Their Symptoms

The Hazardous Nature of Food

While it is true that most any food can be contaminated, there are certain foods that are considered more hazardous than others and these are labeled potentially hazardous foods (now called Time/temperature control for safety foods). Hamburger, chicken, fish, seafood and eggs quickly fall into this category. Other additions might include milk, yogurt, soft cheese and cottage cheese.

The key to the hazardous nature of these foods lies in their molecular composition. A brief digression into elementary nutrition science may prove helpful. All foods are composed of carbohydrate, protein or fat, or a combination of any of these. (Another possible component, alcohol, is not considered a food).

Carbohydrate, fat and protein all provide the elements of carbon, hydrogen and oxygen. However, protein is the only food source that also provides nitrogen, which enables rapid growth of bacteria. Notice how fertilizers list their nitrogen content? Nitrogen encourages growth.

The Food Code provides a term for potentially hazardous food. These foods are now called “Time/Temperature Control for Safety Food” or TCS Food. This is now the term used to identify foods that require cold holding or hot holding during storage and display. We will address this further as we progress through this section. Interestingly, hard-boiled eggs with shell intact and pasteurized eggs with shell intact are NOT considered a TCS food. Raw shell eggs may be treated to destroy all viable salmonella organisms and are then safe to be used raw or under cooked in a highly susceptible population (HSP) group. These eggs are usually marked with a red “P” on the shell for easy identification and are known as pasteurized eggs. Another marvel of science.

In addition, there are several other foods to be considered in this list. Some are quite interesting, and ones which we wouldn’t ordinarily suspect.

First, sliced fruit. The problem here involves pathogens on the surface of the fruit. When a knife passes through the rind, the pathogens are carried into the flesh of the fruit. Common examples include cantaloupe and pineapple. Once cut, fruits should be temperature controlled. In 2007, cut tomatoes were added due to Salmonella outbreaks.

The Food Code includes cut leafy greens to this list. This consists of iceberg lettuce, leaf lettuce, escarole, endive, spinach and cabbage. The FDA definition of “cut” means cut, shredded, sliced, chopped or torn. Thus, these foods MUST now be held at 41° or less. This will have quite an impact on the operations of many facilities. The definition does not include herbs, such as cilantro or parsley. Also included are any plant foods that have been heat treated and any cooked plant food (green beans, rice, potatoes, etc.). Basically any food of plant origin that you would expect to see on a steam table in a buffet or cafeteria.

Others include sprouts and garlic in oil mixtures. Sprouts involve the presence of STEC contamination (statistically, a person is more likely to get STEC from sprouts than from ground meat.).

Garlic and oil mixtures involve the possibility of Botulism. Commercial products utilize a preservative to prevent this problem. Stories usually originate in situations where cooks have prepared their own mixture from chopped garlic and olive oil. (Botulism is a fascinating organism and will appear often in our discussion). These foods are listed in the FDA Food Code.

So, while bacteria will settle for any food, those high in nitrogen (protein) will allow rapid growth of microorganisms.

The Acidic Nature of the Food

Acidity is usually expressed as pH and is a measurement of the number of hydrogen ions present in a food. The lower the pH number, the more sour the food tastes.
Generally, the more acidic a food, the less favorable it is for bacterial growth. Foods with a pH of 7.0 is considered neutral. Foods above this number (such as egg whites and olives) are considered alkaline, foods below 7.0 are called acidic. Foods with a pH of less than 4.6, such as citrus fruits and tomatoes, generally do not support the growth of disease producing bacteria.

Most bacteria are capable of growing in a pH range of 4.6 to 9.0. An interesting note: commercially prepared mayonnaise, so often thought of as a hazardous food, has a pH below 4.6 and actually acts as a deterrent to bacterial growth.

Time

Bacteria need time to reproduce. In general, bacteria can double in number every 15 to 30 min. To get a perspective of this, consider your bank account. With a rough estimate of the available balance, visualize its amount doubling 15 minutes from now. Then consider that balance doubling 15 minutes after that, and so on. One bacterial cell can grow to over 1 million in less than 6 hours!

It is generally recognized in the food service industry that bacteria can grow to dangerous levels in 2 hours. This time is cumulative, however, and it must be remembered that part of this growth may have already occurred before the food was received at the preparation or serving facility. An explanation of the phases of bacterial growth may help to elucidate this.

As shown below, bacterial growth takes place in four phases: lag, log, stationary and decline. The lag phase usually lasts for 2 hours, but may vary depending on conditions. During this phase growth is relatively slow. After this, the bacteria move into the log phase, a period of rapid growth where doubling occurs frequently, as described above. When the food source is depleted or the temperature becomes unfavorable, the bacteria move into the stationary phase. Here, they are not dead, they are waiting. As the conditions become more hostile, the number of bacteria begins to decline, hence the term decline phase.

One point of importance: the lag phase, which lasts about 2 hours, occurs once in a pathogen growth curve. It would be foolish to assume that a TCS food can be ignored for 2 hours at dangerous temperatures. Food that arrives contaminated has most likely already moved into the log phase.

Temperature

Temperature is the greatest key to control bacteria. Of the factors mentioned above time is the only variable which the food handler influences. Time, coupled with temperature, are the key control factors in food safety.

To begin, let’s start at the bottom of the temperature range: freezing temperatures. Does freezing kill bacteria? Simply — “No.” Consider the temperature of liquid nitrogen is minus 321°F (-196°C) — far colder than any food freezer — and bacteria are stored in liquid nitrogen for later study. They don’t die there, but remain alive in a dormant state. As the temperature of the food that contains them rises, they begin to respond. Where (on the temperature scale) they respond is very important.

Effects of Temperature on Bacteria

• Freezing does not kill bacteria.
• Bacteria grow rapidly between 41°F (5°C) and 135°F (58°C) — THE DANGER ZONE
• Rapid death of most bacteria occurs at 165°F (74°C). However, this does not destroy toxins that may be present in food.

Generally it is accepted that bacteria grow very slowly at temperatures below 41°F (5°C). Some bacteria (such as Yersinia and C. botulinum) do grow at lower temperatures, between 33° and 41°F (1°and 5°C), but for the most part, 41°F (5°C) begins what is commonly known as the “Danger Zone.” Here is where the log phase of bacterial growth begins — bacteria grow slowly, but eventually will reach dangerous levels.

Growth continues until the temperature reaches 135°F (58°C). Thus, temperatures between 41° and 135°F (5°C and 58°C) define the Danger Zone — where bacteria multiply rapidly. Thus, all TCS foods must be stored below 41°. Above 135°F (58°C), pathogens will begin to die (at their own respective temperatures — for instance, STEC is killed at 155°F (68°C). Rapid death of most bacteria occurs at 165°F (74°C), as shown in the chart below. (Note: one reviewer pointed out that bacteria do not really look like “bugs” — we thank her for that, and remind you that the “bugs” are just for fun). Bacteria grow most quickly between 70 and 110°F. This is an interesting point whose significance will be seen later.

How temperature and time interact in affecting bacterial growth can be seen below. Bacteria grows most quickly between 70° and 110°F (21° and 43°C).

Given this information, it may be logical to conclude that any food heated to a temperature of more than 165°F (74°C) and eaten immediately would be safe. However, there is another factor in the picture which makes food safety far more challenging — the presence of toxins.

As the name implies, toxins are poisonous substances which may be present in food. They come from a variety of sources. Some pathogens — Staphylococcus aureus (Staph) for example — produce toxins as a by product of their growth. In a sense, the food becomes a toxic waste dump. When these poisons are consumed, the person becomes very ill. In food safety terminology, this person is said to have a food intoxication.

The difficulty comes with the fact that most toxins are not destroyed by high temperatures. So, if a TCS food is allowed to remain in the danger zone long enough for the bacteria to produce toxins, there is nothing that can be done to guarantee its safety.

Clostridium botulinum is another pathogen that produces a toxin. Attacking the central nervous system, the disease, botulism, is fatal in approximately 65% of the cases. To show the intensity of its threat, consider that this was one of the chemicals cultivated by Saddam Hussein in his biological warfare program.

Important Points Regarding Toxins

• Toxins are poisonous
• Toxins are not destroyed by high temperatures
• Toxins are produced as a by-product of some bacteria
• Staphylococcus aureus and Botulism are examples of bacteria that produce toxins
• Consumption results in INTOXICATION

A distinction needs to be made between a food infection and a food intoxication. A food infection is caused by a pathogen (this makes sense, if a person has a cold, etc., they say they have an infection). A food intoxication is caused by a toxin that was produced by a pathogen. (This may be remembered easily by relating this to the consumption of a quart of vodka. We could say that the person has been poisoned and that he is intoxicated). There is a third classification: a toxin-mediated infection, where a pathogen present in food produces toxins after ingestion, while in the large intestine. An example of this type of pathogen is Clostridium perfringens.

Oxygen

Bacteria are classified into three types regarding oxygen requirements: those that require oxygen, those that cannot grow in the presence of oxygen and those that can adapt to their environment. Unfortunately, most foodborne bacteria fall into this last group.

Bacteria that require oxygen are called aerobic (like aerobic exercise that requires heavier breathing). Those that cannot survive in the presence of oxygen are called anaerobes. An example of this type of bacteria is Botulism, commonly associated with canned foods, an airless environment.

Bacteria that can grow either with or without oxygen are called facultative anaerobes. Interestingly, most food pathogens are facultative.

Obviously controlling oxygen does not offer a final protection in food safety, and this is what is most important to know. Just because a food is packaged with the oxygen content reduced (such as Reduced Oxygen Packaging foods), these foods still must be temperature controlled just as carefully as those packed fresh. We will discuss this again later. Some foods such as deli meats are received in packaging with oxygen reduced to extend shelf life, and these do require refrigeration. If, however, the food has been heat treated and the oxygen controlled (such as “shelf stable” commercially sterile foods in unopened hermetically sealed containers) then these foods do not require refrigeration.

• Aerobes: require oxygen
• Anaerobes: cannot grow in the presence of oxygen
• Facultative anaerobes: will grow with or without oxygen

Moisture

Bacteria need water to survive — dehydrated food is not likely to be contaminated. Actually, backpacking may be the safest place to eat! Water content of foods is usually expressed as water activity or by the symbol Aw, and represents the amount of water available in a food to support bacterial growth. The range of likely Aw values is shown below.

A water activity of less than 0.85 (85%) usually inhibits the growth of microorganisms. The same effect may be achieved with the presence of large amounts of sugar or salt. This is why beef jerky may be sold without refrigeration — it doesn’t contain enough water to support bacterial growth — and why salting has been a common method of food preservation — salt draws the water from the food.