Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY LOW TEMPERATURE 471
and pathogenic bacteria start to grow and their spores germinate at this range. Just
from the spoilage aspect, a 6- to 8-h temperature abuse (12�C) of a vacuumpackaged,
refrigerated, low-heat-processed meat product can reduce its expected
shelf life of 8 weeks by ca. 7 to 10 d. A fluctuation in temperature of a frozen food
increases microbial death and injury due to repeated damaging solution effect and
mechanical damage from larger ice crystals that form during repeated freezing and
thawing. Dead microbial cells can also lyse, releasing intracellular enzymes, many
of which (e.g., proteinases and lipases) can act on food components and reduce the
acceptance quality of food (see Chapter 21).
The rate of cooling of a food is also very important for effective control of the
growth of pathogenic and spoilage microorganisms. A slow rate of cooling of foods
has been implicated as a major cause of foodborne diseases (Chapter 23). This can
occur by trying to cool a large volume of hot or warm food in a big (deep) container
in a refrigerator or overstuffing refrigerators with hot or warm foods. During thawing
of a frozen food (such as an uncooked chicken), rapid thawing is desirable in order
to control microbial growth, especially growth of pathogens. If the food is thawed
slowly, the temperature on the food surface will soon increase, thereby allowing
microbial growth, even when the inside is still frozen.
Refrigerated foods have limited shelf life, and, with time, microorganisms grow
and spoil the products. In frozen foods, microorganisms (only cells, not spores)
slowly die. However, even after long storage, some survive in frozen foods.
B. Nature of Food
Composition, pH, A w, and presence of microbial inhibitors (preservatives) in a food
can greatly influence growth, sublethal injury, and viability of microorganisms during
storage at low temperature. A food with higher solid content (especially high proteins,
carbohydrates, and lipids, but low ions), pH closer to 7.0, higher A w, and the
absence of microbial inhibitors facilitate growth and survival of microorganisms at
refrigeration temperature and inflict less injury and cause less death at frozen temperature.
Thus, the shelf life of refrigerated foods can be increased by using one or
more of these factors, such as low pH, low A w, incorporation of suitable microbial
inhibitors, and, when possible, vacuum or modified air packaging (see Chapter 40).
In packaged frozen foods, ice may form in the packages (package ice), especially
if the storage temperature fluctuates. During thawing, the ice melts and is absorbed
by the food, resulting in an increase in the A w in a localized area (e.g., in a bread)
and making it susceptible to microbial growth after thawing.
C. Nature of Microorganisms
Although some microorganisms can grow at as low as –10�C, many mesophilic and
thermophilic bacterial cells can be sublethally injured and may die with time at low
temperatures above freezing. At temperatures below –10�C, vegetative cells of
microorganisms can sustain sublethal injury and die. In general, Gram-negative or
rod-shaped bacteria are more susceptible to the damaging effect of freezing than
Gram-positive or spherical-shaped bacteria. Also, cells from the early exponential
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