Unformatted text preview: t of oxygen. Therefore, the growth of certain
microorganisms can be controlled by controlling the amount of oxygen in
the environment. For example, vacuum packaging inhibits the growth of
microorganisms that require oxygen. Also, the storage life of some fruits
can be extended by reducing the oxygen level in the storage room.
Microorganisms in food products can be controlled by (1) preventing
contamination by following strict sanitation practices, (2) inhibiting growth
by altering the environmental conditions, and (3) destroying the organisms
by heat treatment or chemicals. The best way to minimize contamination cen58933_ch04.qxd 9/10/2002 9:13 AM Page 241 241
CHAPTER 4 in food processing areas is to use fine air filters in ventilation systems to
capture the dust particles that transport the bacteria in the air. Of course,
the filters must remain dry since microorganisms can grow in wet filters.
Also, the ventilation system must maintain a positive pressure in the food
processing areas to prevent any airborne contaminants from entering inside
by infiltration. The elimination of condensation on the walls and the ceiling of the facility and the diversion of plumbing condensation drip pans of
refrigerators to the drain system are two other preventive measures against
contamination. Drip systems must be cleaned regularly to prevent microbiological growth in them. Also, any contact between raw and cooked food
products should be minimized, and cooked products must be stored in
rooms with positive pressures. Frozen foods must be kept at 18°C or below, and utmost care should be exercised when food products are packaged
after they are frozen to avoid contamination during packaging.
The growth of microorganisms is best controlled by keeping the temperature and relative humidity of the environment in the desirable range.
Keeping the relative humidity below 60 percent, for example, prevents the
growth of all microorganisms on the surfaces. Microorganisms can be destroyed by heating the food product to high temperatures (usually above
70°C), by treating them with chemicals, or by exposing them to ultraviolet
light or solar radiation.
Distinction should be made between survival and growth of microorganisms. A particular microorganism that may not grow at some low temperature may be able to survive at that temperature for a very long time
(Fig. 4–34). Therefore, freezing is not an effective way of killing microorganisms. In fact, some microorganism cultures are preserved by freezing
them at very low temperatures. The rate of freezing is also an important
consideration in the refrigeration of foods since some microorganisms
adapt to low temperatures and grow at those temperatures when the cooling rate is very low. Refrigeration and Freezing of Foods
The storage life of fresh perishable foods such as meats, fish, vegetables,
and fruits can be extended by several days by storing them at temperatures
just above freezing, usually between 1 and 4°C. The storage life of foods
can be extended by several months by freezing and storing them at subfreezing temperatures, usually between 18 and 35°C, depending on the
particular food (Fig. 4–35).
Refrigeration slows down the chemical and biological processes in foods,
and the accompanying deterioration and loss of quality and nutrients.
Sweet corn, for example, may lose half of its initial sugar content in one
day at 21°C, but only 5 percent of it at 0°C. Fresh asparagus may lose
50 percent of its vitamin C content in one day at 20°C, but in 12 days at
0°C. Refrigeration also extends the shelf life of products. The first appearance of unsightly yellowing of broccoli, for example, may be delayed by
three or more days by refrigeration.
Early attempts to freeze food items resulted in poor-quality products
because of the large ice crystals that formed. It was determined that the rate
of freezing has a major effect on the size of ice crystals and the quality,
texture, and nutritional and sensory properties of many foods. During slow Z Z Z Microorganisms Frozen
food FIGURE 4–34
Freezing may stop the growth of
microorganisms, but it may not
necessarily kill them. Freezer
–18 to –35°C Refrigerator
1 to 4°C Frozen
foods FIGURE 4–35
Recommended refrigeration and
freezing temperatures for
most perishable foods. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 242 242
of freezing Freezing End of
freezing) Time FIGURE 4–36
Typical freezing curve of a food item. TABLE 4–5
Thermal properties of beef
Quantity Typical value Average density
3.14 kJ/kg · °C
1.70 kJ/kg · °C
Latent heat of fusion
0.41 W/m · °C
(at 6°C) freezing, ice crystals can grow to a large size, whereas during fast freezing
a large number of ice crystals start forming at once and are much smaller in
size. Large ice crystals are not desirable since they can puncture the walls
of the cells, causing a degradation of texture and a loss of natural juices
during thawing. A crust forms rapidly on t...
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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