Vegetative Stages and Spore Formation

Although vegetative bacteria may survive low—even freezing— temperatures, they can be killed by high temperatures. For example, pathogenic bacteria can be killed during proper cooking. Some types of bacteria, however, have the ability to change into a different form, called a spore. The spore’s thick wall protects the bacteria against unfavorable conditions, such as high or low temperature, low moisture, and high acidity.

While a spore cannot reproduce, it is capable of turning back into a vegetative organism when conditions again become favorable. For example, bacteria in food may form a spore when exposed to freezer temperatures, allowing the bacteria to survive. As the food thaws and conditions improve, the spore can turn back into a vegetative cell and begin to grow in the food.

Since spores are so difficult to destroy, it is important to cook, cool, and reheat food properly.

Conditions that Support the Growth of Microorganisms

All microorganisms, except viruses, are able to grow in food if the conditions are right. To grow, microorganisms need food, water, appropriate temperatures, oxygen, and proper pH. Therefore, you must control these necessities to control their growth of microorganisms.

FAT TOM: What Microorganisms Need to Grow

The conditions that favor the growth of most foodborne microorganisms can be remembered by the acronym FAT TOM. (F ood, A cidity, T ime, T emperature, O xygen, M oisture ) Each of these conditions for growth will be explained in more detail in the next several paragraphs.

Food

To grow, foodborne microorganisms need nutrients, specifically proteins and carbohydrates. These substances are commonly found in potentially hazardous food items such as meat, poultry, dairy products, and eggs.

Acidity

Foodborne microorganisms typically do not grow in highly acidic or highly alkaline food. They grow best in food with a neutral to slightly acidic pH. The pH of a substance tells how acidic or alkaline it is. The pH scale ranges from 0 to 14.0. Food with a pH between 0 and 7.0 is acidic, while food with a pH between 7.0 and 14.0 is alkaline. A pH of 7.0 is neutral.

Some microorganisms grow better than others in food with a specific pH. Yeasts and molds tend to grow in acidic food (with a pH below 4.6) such as fruit, fruit juice, and jam. Pathogenic bacteria grow well in food with a pH between 4.6 and 7.5 (slightly acidic to neutral), which includes most of the food we eat. A few bacteria have been known to grow in food with a pH below 4.6 and above 7.5. For example, E. coli O157:H7 has been found to reproduce in unpasteurized apple juice, which has a pH below 4.0.

Temperature

Most foodborne microorganisms grow well between the temperatures of 4l°F and 14O°F (5°C and 60°C).

This range is known as the temperature danger zone. However, exposing microorganisms to temperatures outside the danger zone does not necessarily kill them. Refrigeration temperatures, for example, may only slow their growth. Some bacteria—such as Listeria monocytogenes and Yersinia enterocolitica —are able to grow at refrigeration temperatures. Bacterial spores can often survive extreme heat and cold. Food must be handled very carefully when it is thawed, cooked, cooled, and reheated since it can be exposed to the temperature danger zone during these times.

Time

Foodborne microorganisms need sufficient time to grow. This means that even under favorable conditions, microorganisms need enough time to move from the lag phase (slow growth) to the log phase (rapid growth). Keep in mind that some bacteria can double their population every twenty minutes.

If contaminated food remains in the temperature danger zone for four hours or more, pathogenic microorganisms can grow to levels high enough to make someone ill. Therefore, it is important to control the amount of time potentially hazardous food remains in the temperature danger zone.

Oxygen

Different microorganisms have different oxygen requirements for growth. They can be categorized as aerobic, anaerobic, or facultative. Aerobic microorganisms require oxygen to grow. Anaerobic microorganisms can grow only when oxygen is absent. Growth of anaerobic microorganisms has been known to occur in thick, heat-treated plant food that has been temperature abused, untreated garlic-in-oil mixtures, and foil-wrapped baked potatoes that have been temperature abused. Facultative microorganisms can grow either with or without the presence of oxygen. Most microorganisms that cause foodborne illness are facultative. Manufacturers use different types of packaging to control the oxygen requirements of the most likely type of microorganism found on the food. For example, vacuum packaging is used to control the growth of aerobic microorganisms that can be found on bacon.

Moisture

Because most foodborne microorganisms require water to grow, they grow well in moist food. The amount of moisture available in a food for microorganisms to grow is called its Water activity (a_w). It is measured on a scale of 0 through 1.0, with water having a water activity of 1.0. Most microorganisms that cause foodborne illness grow best in foods with water activities between.85 and.97, although some can grow in foods with lower water-activity levels. Potentially hazardous food items typically have a water activity of.85 or higher.

Дата добавления: 2015-10-31