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Factors influencing the growth of microorganisms - Preservation of Foods

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(3)  By control of one or more unfavorable environmental conditions such as
nutrients, moisture, pH, or temperature. The greater the number of conditions that are
unfavorable means the longer the delay of the beginning of cell growth.
(4)
By damage to organisms through processing methods such as heat or
irradiation.
1-17. FACTORS INFLUENCING GROWTH OF MICROORGANISM
There are several factors that influence the growth of microorganisms: nutrition,
oxygen, pH, temperature, and moisture. Lack of food retards bacterial growth, and
growth is favored by a sufficient quantity of the proper kind of food. Moisture is required
to carry foods in solution into the cell, to carry wastes in solution away from the cell, and
to maintain the moisture content of the cytoplasm. Temperature has a profound
influence on the growth rate of microorganisms. Microorganisms subjected to adverse
temperatures are either destroyed or are not able to multiply. The optimum temperature
of a microorganism is the temperature that provides for the most rapid growth of that
microorganism. The pH of the medium in which microorganisms grow exerts a
considerable influence on their rate of growth. All microorganisms have an optimum pH
at which they grow best. Most species of bacteria have an optimum pH between 6.0
and 8.5. Molds will grow in a pH range between 2.0 to 8.5. Yeasts have an optimum
pH range from 5.5 to 6.5.
1-18. NUTRITION
Nutrients or foods are substances which are outside the cell and which, upon
entering a cell after passing across the cell membrane, can be used by the cell for
building material or for obtaining energy. Food requirements of bacteria show great
variations from species to species. Some organisms can obtain all their food
requirements from inorganic matter while others need many complex organic
compounds. Although any one species may be able to use only a small number of
materials as sources of food, bacteria as a group are able to utilize all the naturally
occurring organic compounds as well as many inorganic substances.
a. Requirements. Bacteria require foods for the same purposes, as do other
forms of life, namely, as sources of material for cellular synthesis and for energy in
order to perform these synthetic processes. Requirements include carbohydrates
(sugar, starches, and celluloses), a source of nitrogen, vitamins, water, and a source of
energy.
b. Sources. The majority of bacteria species use naturally occurring organic
materials such as carbohydrates, proteins, fats, and so forth, not only as sources of carbon,
hydrogen, oxygen, and nitrogen but also for the energy needed to synthesize these
materials into protoplasm (that material referred to as the physical basis of life and
which is common to all living cells). The requirements of most bacteria for inorganic
materials can be satisfied by salts containing sodium, potassium, calcium, magnesium,
MD0703
1-16



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