Basis of Spoilage and Preservation :
Bacteria

General characteristics: Bacteria are rod-shaped, round, or spiral in form. They grow and increase in number by splitting in two. Bacteria that cause diseases are known as pathogens. Many non-pathogenic bacteria cause spoilage if allowed to grow on food. A number of bacteria are also "good guys" and desirable in food; for example the bacteria involved in fermentation processes to produce cheese, pickles, and sauerkraut.

Effect of temperature on growth rate of bacteria: Bacteria can increase in number very quickly; a few can even double in number every 10 minutes. The following shows how quickly one bacterium can multiply to dangerous numbers, given ideal conditions and doubling every 15 minutes.

Amount of Time Number of Bacteria  start 1 30 minutes  4 1 hour 16 1.5 hours 64 2 hours  256 2.5 hours  1024 3 hours 4096 3.5 hours  16384 4 hours 65536

Spoilage is normally detected-food develops a sour or putrid odor and/or a slimy texture-when populations reach from 1 to 10 million organisms per gram of food. ( An ounce equals about 28 grams of food. ) If bacteria grow and multiply very rapidly on food (which requires ideal conditions of warm temperatures, moisture and so forth), they are capable of spoiling the food or causing a foodborne illness in less than four hours.

Food temperature is very important in controlling bacterial growth. Most bacteria multiply most rapidly between 90 to 110°F. Let's examine a food, which initially contained 10,000 bacterial cells per gram, after two days at different storage temperatures.

If the food storage temperature is:		Then the number of bacterial cells after two days could be:
33°F		30,000 cells per gram of food
40°F		160,000
50°F		692,000,000
60°F		21,400,000,000

Temperatures below 40°F and above 135°F prevent the growth of most foodborne pathogens, but allow slow growth of some spoilage-causing organisms.

Different types of bacteria require different temperatures for maximum growth. Some bacteria will grow at refrigerator temperature (called psychrophiles). Some will only grow at moderate temperatures (called mesophiles). Warmth-loving bacteria grow at temperatures above 135°F (thermophiles). At temperatures above and below the optimum, bacteria grow and reproduce at a slower rate. Food spoilage bacteria grow best at environmental temperatures of 70° to 100°F. A few pathogens can grow at refrigerator temperatures (for example, Listeria has been shown to grow at temperatures as low as 29°F).

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Other factors which affect the growth rate of bacteria. Moisture content, type of nutrients present, presence or absence of oxygen and acidity of food are also important in determining the type of bacteria that can grow in food and rate at which they grow.

Water. Bacteria thrive on moist foods. The nutrients needed for growth must be dissolved in water and absorbed through their cell walls. Within the cell, water enters directly into many chemical reactions that generate energy and synthesize new cells. Water also helps remove wastes from cells.

In foods, water activity is a term that describes the amount of water available to microorganisms. It differs somewhat from water content, because water in food can be tied up by food components, such as sugar or salt, and be unavailable to microorganisms.

Various bacteria, yeasts and molds have different water activity requirements. This chart illustrates the types of microorganisms that will grow at various water activities:

Salt and sugar decrease the water activity in foods by causing free water to bind to the sugar or salt, thus becoming unavailable for microorganisms. Use of sugar in jams and jellies and use of a concentrated salt solution in brining are effective methods of food preservation because they lower the activity of water enough so that it can no longer support growth of many microorganisms which cause food spoilage. Salt lowers the activity of water more effectively than does sugar.

Nutrients. Some vegetative cells need only simple salts, sugars, water, and minerals for growth. Others have additional requirements, like amino acids, vitamins, and more complex carbohydrates.

Oxygen. Growth of microorganisms is also affected by oxygen. Most bacteria require oxygen, but a few, such as the Clostridia species, require very low environmental oxygen content.

Bacteria can be classified by their oxygen requirements. Anaerobic microorganisms, such as Clostridia species grow only when oxygen is 5 percent or less in their environment (atmospheric oxygen content is about 20 percent). Aerobic organisms must have an oxygen content of 16 to 21 percent to grow. Microaerophilic organisms fall between the aerobic and anaerobic as they require from 6 to 15 percent oxygen content for growth.

Controlling oxygen content is not a good way for home food preservers to control bacterial growth; if you make the oxygen content unfavorable for one group of bacteria, you may be encouraging the growth of another group. (Controlling oxygen content is useful for controlling mold growth and food deterioration due to fat oxidation.) Control of oxygen content is used by the food processing industry to control microbial growth in products such as pre-packaged sandwiches or washed salad mixes.

Acidity. Bacterial growth is also affected by the acidity or alkalinity in food. Use of the term pH is preferred when talking about this effect, since it is an accurate measure of acidity or alkalinity.

The pH scale ranges from 0 to 14. A pH value of 7 is neutral, neither acid nor alkaline. Below 7, as the numbers decrease, the scale becomes increasingly more acidic. Above 7, increasing values reflect increasing alkalinity. pH values are expressed on a logarithmic scale, so the simple rules of arithmetic do not apply to pH. The pH value of an equal mixture of two foods cannot be predicted by adding the two pH values and dividing by two.

The acidity of a food is very important in food preservation. Pathogenic bacteria, particularly Clostridium botulinum, do not grow when the pH is below 4.6. Foods that have a pH of 4.6 or lower are called high-acid foods and include fruits, tomatoes and pickles. Low-acid foods have a pH between 4.6 and 7.0. They include meats, poultry, fish, and vegetables. The pH values of some common food products are given in the Supplemental Information at the end of this Lesson. Consumers cannot measure pH of foods at home.

Most bacteria grow best in foods with pH values of 6.0 to 8.0. However, a few bacteria prefer, and many others tolerate, the acidity of foods having pH values from 4.0 to 6.0. Bacteria that cause human diseases (pathogenic bacteria) will not grow in highly acid foods (pH below 4.6). However, some, such as E.coli:O157:H7 , can survive in acidic foods in levels sufficient to cause illness.

Inhibitors. Many chemical substances prevent or inhibit the growth of bacteria. Examples include sanitizers, antibiotics, naturally contained preservatives, and added preservatives. The effect of an inhibitor varies with pH, food moisture, and bacterial species.

Initial level of bacteria. Keeping initial bacteria levels low is important. 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 food may be shortened to 7 days.

Preventing Growth of Bacteria - controlling one or more of their growth requirements prevents the growth of bacteria. Normal cooking practices kill vegetative cells on fresh foods. Heat is also used to pasteurize or sterilize foods for short- or long-term preservation. Cold-storage temperatures are used to prevent or reduce bacterial growth. Some foods are intentionally dried, acidified, prepared with preservatives, or vacuum packaged to prevent or reduce bacterial growth.