460 ◾ Fundamental Food Microbiology
looks to steadily increase in the future. To suit the taste of consumers, many new products are
being developed that are low in fat (caloric); high in fiber, phosphates, and other additives; and
have low amounts of or no preservatives. To achieve the long shelf life and to make these products
safe, extra precautions are being introduced for microbiological control. This has helped many
new or emerging pathogenic and spoilage bacteria, in the absence of competition from associated
microorganisms, to become predominant (Chapter 29). New designs of processing equipment for
high-production efficiency, centralized production of large volumes of products, transportation
of products for long distances in regional storage facilities, retailing conditions, and consumers’
handling of the products have facilitated these so-called new pathogenic and spoilage bacteria in
gaining prominence (Chapter 21). Unless some effective intervention strategies are developed, new
pathogenic and spoilage microorganisms will continue to surface in refrigerated foods stored for
a long time. 1
objectives
The main microbiological objective in low-temperature preservation of food is to prevent or reduce
growth of microorganisms. Low temperatures also reduce or prevent catalytic activity of microbial
enzymes, especially heat-stable proteinases and lipases. Germination of spores is also reduced.
Low-temperature storage, especially freezing (and thawing), is also lethal to microbial cells, and
under specific conditions, 90% or more of the population can die during low-temperature preservation. However, the death rate of microorganisms at low temperature, as compared with that
of heat treatment, cannot be predicted (as D and Z values in heating). Also, spores are not killed
at low temperatures. Thus, foods are not preserved at low temperature in order to kill microbial
cells. Freezing is also used to preserve starter cultures for use in food bioprocessing. This has been
discussed in Chapter 14.
Mechanisms of Cold-induced inactivation
The metabolic activities, enzymatic reactions, and growth rates of microorganisms are maximized
at the optimum growth temperature. As the temperature is lowered, microbial activities associated
with growth slow down. Normally, the generation time, within a certain range, is doubled for
every 10°C (50°F) reduction in temperature. Thus, a species dividing every 60 minutes in a food
at 22°C (71.6°F) will take 120 minutes to divide if the temperature is reduced to 12°C (53.6°F). At
a lower range, generation time can be even higher than double. For example, if the temperature is
reduced from 12°C (53.6°F) to 2°C (35.6°F) and the species can grow at 2°C, the generation time
for the species could be more than 240 minutes. The lag and exponential phases and the germination time (of spores) for some psychrotrophs (mesophilic types) become increasingly longer as
the temperature is reduced to approximately 0°C or even to approximately –1°C (33.8°F). At this
temperature, nongrowing cells of some mesophiles (nonpsychrotrophic) and thermophiles may
be injured and killed, especially if they are stored for a long time (weeks) at 2°C or below and the
foods have low A W , low pH, or preservatives. The rate of catalytic activity of some enzymes also
decreases as the temperature of an environment is reduced.
Water is present in a food as free water and bound (with the hydrated molecules) water. As
the temperature in a food system drops to approximately –2°C (28.4°F), free water in the food
starts freezing and forming ice crystals (pure water freezes at 0°C, but in a food with solutes, it
looks to steadily increase in the future. To suit the taste of consumers, many new products are
being developed that are low in fat (caloric); high in fiber, phosphates, and other additives; and
have low amounts of or no preservatives. To achieve the long shelf life and to make these products
safe, extra precautions are being introduced for microbiological control. This has helped many
new or emerging pathogenic and spoilage bacteria, in the absence of competition from associated
microorganisms, to become predominant (Chapter 29). New designs of processing equipment for
high-production efficiency, centralized production of large volumes of products, transportation
of products for long distances in regional storage facilities, retailing conditions, and consumers’
handling of the products have facilitated these so-called new pathogenic and spoilage bacteria in
gaining prominence (Chapter 21). Unless some effective intervention strategies are developed, new
pathogenic and spoilage microorganisms will continue to surface in refrigerated foods stored for
a long time. 1
objectives
The main microbiological objective in low-temperature preservation of food is to prevent or reduce
growth of microorganisms. Low temperatures also reduce or prevent catalytic activity of microbial
enzymes, especially heat-stable proteinases and lipases. Germination of spores is also reduced.
Low-temperature storage, especially freezing (and thawing), is also lethal to microbial cells, and
under specific conditions, 90% or more of the population can die during low-temperature preservation. However, the death rate of microorganisms at low temperature, as compared with that
of heat treatment, cannot be predicted (as D and Z values in heating). Also, spores are not killed
at low temperatures. Thus, foods are not preserved at low temperature in order to kill microbial
cells. Freezing is also used to preserve starter cultures for use in food bioprocessing. This has been
discussed in Chapter 14.
Mechanisms of Cold-induced inactivation
The metabolic activities, enzymatic reactions, and growth rates of microorganisms are maximized
at the optimum growth temperature. As the temperature is lowered, microbial activities associated
with growth slow down. Normally, the generation time, within a certain range, is doubled for
every 10°C (50°F) reduction in temperature. Thus, a species dividing every 60 minutes in a food
at 22°C (71.6°F) will take 120 minutes to divide if the temperature is reduced to 12°C (53.6°F). At
a lower range, generation time can be even higher than double. For example, if the temperature is
reduced from 12°C (53.6°F) to 2°C (35.6°F) and the species can grow at 2°C, the generation time
for the species could be more than 240 minutes. The lag and exponential phases and the germination time (of spores) for some psychrotrophs (mesophilic types) become increasingly longer as
the temperature is reduced to approximately 0°C or even to approximately –1°C (33.8°F). At this
temperature, nongrowing cells of some mesophiles (nonpsychrotrophic) and thermophiles may
be injured and killed, especially if they are stored for a long time (weeks) at 2°C or below and the
foods have low A W , low pH, or preservatives. The rate of catalytic activity of some enzymes also
decreases as the temperature of an environment is reduced.
Water is present in a food as free water and bound (with the hydrated molecules) water. As
the temperature in a food system drops to approximately –2°C (28.4°F), free water in the food
starts freezing and forming ice crystals (pure water freezes at 0°C, but in a food with solutes, it