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Unformatted text preview: red in certain animals and humans, and poor sanitation practices such as keeping processed
foods in the same area as the uncooked ones and being careless about handwashing can cause the contamination of food products.
When contamination occurs, the microorganisms start to adapt to the
new environmental conditions. This initial slow or no-growth period is
called the lag phase, and the shelf life of a food item is directly proportional to the length of this phase (Fig. 4–31). The adaptation period is followed by an exponential growth period during which the population of
microorganisms can double two or more times every hour under favorable
conditions unless drastic sanitation measures are taken. The depletion of
nutrients and the accumulation of toxins slow down the growth and start
the death period.
The rate of growth of microorganisms in a food item depends on the
characteristics of the food itself such as the chemical structure, pH level,
presence of inhibitors and competing microorganisms, and water activity as
well as the environmental conditions such as the temperature and relative
humidity of the environment and the air motion (Fig. 4–32).
Microorganisms need food to grow and multiply, and their nutritional
needs are readily provided by the carbohydrates, proteins, minerals, and
vitamins in a food. Different types of microorganisms have different nutritional needs, and the types of nutrients in a food determine the types of
microorganisms that may dwell on them. The preservatives added to the
*This section can be skipped without a loss of continuity. Microorganism
population Lag Exponential
Time FIGURE 4–31
Typical growth curve of
% Temperature Oxygen
humidity Air motion
The use of inhibitors
pH level FIGURE 4–32
The factors that affect the rate of
growth of microorganisms. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 240 240
HEAT TRANSFER Rate of
growth Temperature FIGURE 4–33
The rate of growth of microorganisms
in a food product increases
exponentially with increasing
environmental temperature. food may also inhibit the growth of certain microorganisms. Different
kinds of microorganisms that exist compete for the same food supply, and
thus the composition of microorganisms in a food at any time depends on
the initial make-up of the microorganisms.
All living organisms need water to grow, and microorganisms cannot
grow in foods that are not sufficiently moist. Microbiological growth in
refrigerated foods such as fresh fruits, vegetables, and meats starts at the
exposed surfaces where contamination is most likely to occur. Fresh meat
in a package left in a room will spoil quickly, as you may have noticed.
A meat carcass hung in a controlled environment, on the other hand, will
age healthily as a result of dehydration on the outer surface, which inhibits
microbiological growth there and protects the carcass.
Microorganism growth in a food item is governed by the combined effects of the characteristics of the food and the environmental factors. We
cannot do much about the characteristics of the food, but we certainly can
alter the environmental conditions to more desirable levels through heating, cooling, ventilating, humidification, dehumidification, and control of
the oxygen levels. The growth rate of microorganisms in foods is a strong
function of temperature, and temperature control is the single most effective mechanism for controlling the growth rate.
Microorganisms grow best at “warm” temperatures, usually between
20 and 60°C. The growth rate declines at high temperatures, and death
occurs at still higher temperatures, usually above 70°C for most microorganisms. Cooling is an effective and practical way of reducing the
growth rate of microorganisms and thus extending the shelf life of perishable foods. A temperature of 4°C or lower is considered to be a safe refrigeration temperature. Sometimes a small increase in refrigeration
temperature may cause a large increase in the growth rate, and thus a
considerable decrease in shelf life of the food (Fig. 4–33). The growth
rate of some microorganisms, for example, doubles for each 3°C rise in
Another factor that affects microbiological growth and transmission is
the relative humidity of the environment, which is a measure of the water
content of the air. High humidity in cold rooms should be avoided since
condensation that forms on the walls and ceiling creates the proper environment for mold growth and buildups. The drip of contaminated condensate onto food products in the room poses a potential health hazard.
Different microorganisms react differently to the presence of oxygen in
the environment. Some microorganisms such as molds require oxygen for
growth, while some others cannot grow in the presence of oxygen. Some
grow best in low-oxygen environments, while others grow in environments
regardless of the amoun...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
- Spring '10