Fundamental Food Microbiology, Third Edition - Fuad Fathir

FACTORS INFLUENCING MICROBIAL GROWTH IN FOOD 77
A. Temperature and Growth
1. Principle
Microbial growth is accomplished through enzymatic reactions. It is well known
that within a certain range, with every 10�C rise in temperature, the catalytic rate
of an enzyme doubles. Similarly, the enzymatic reaction rate is reduced to half by
decreasing the temperature by 10�C. This relationship changes beyond the growth
range. Because temperature influences enzyme reactions, it has an important role in
microbial growth in food.
2. Food and Temperature
Foods are exposed to different temperatures from the time of production until
consumption. Depending on processing conditions, a food can be exposed to high
heat, from 65�C (roasting of meat) to more than 100�C (in ultrahigh temperature
processing). For long-term storage, a food can be kept at 5�C (refrigeration) to
–20�C or below (freezing). Some relatively stable foods are also kept between 10
and 35�C (cold to ambient temperature). Some ready-to-eat foods are kept at warm
temperature (50� to 60�C) for several hours (e.g., in the supermarket deli). Different
temperatures are also used to stimulate desirable microbial growth in food
fermentation. 11
3. Microbial Growth and Viability
Microorganisms important in foods are divided into three groups on the basis of
their temperature of growth, each group having an optimum temperature and a
temperature range of growth: (1) thermophiles (grow at relatively high temperature),
with optimum ca. 55�C and range 45 to 70�C; (2) mesophiles (grow at ambient
temperature), with optimum at 35�C and range 10 to 45�C; and (3) psychrophiles
(grow at cold temperature), with optimum at 15�C and range –5 to 20�C. However,
these divisions are not clear-cut and overlap each other.
Two other terms used in food microbiology are very important with respect to
microbial growth at refrigerated temperature and survival of microorganisms to low
heat treatment or pasteurization, because both methods are widely used in the storage
and processing of foods. Psychrotrophs are microorganisms that grow at refrigerated
temperature (0 to 5�C), irrespective of their optimum range of growth temperature.
They usually grow rapidly between 10 and 30�C. Molds; yeasts; many Gramnegative
bacteria from genera Pseudomonas, Achromobacter, Yersinia, Serratia, and
Aeromonas; and Gram-positive bacteria from genera Leuconostoc, Lactobacillus,
Bacillus, Clostridium, and Listeria are included in this group (see Chapter 20).
Microorganisms that survive pasteurization temperature are designated as thermodurics.
They include species from genera Micrococcus, Bacillus, Clostridium, Lactobacillus,
Pediococcus, and Enterococcus. Bacterial spores are also included in this
group. They have different growth temperatures and many can grow at refrigerated
temperature as well as thermophilic temperature.
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