246 ◾ Fundamental Food Microbiology
enzymes, in the absence of viable microbial cells, can result from some heat-stable enzymes produced by microorganisms in the foods before heat treatment. In addition, the foods need to be
stored at a temperature for a sufficient length of time for the catalytic activities of the enzymes to
occur to produce the detectable changes.
Significance of Microorganisms
Microbial Types
Raw and most processed foods normally contain many types of molds, yeasts, and bacteria capable
of multiplying and causing spoilage. (Viruses and parasites do not multiply in foods.) As multiplication is an important component in spoilage, bacteria (because of its shorter generation time),
followed by yeasts, are in favorable positions over molds to cause rapid spoilage of foods. However,
in foods where bacteria or yeasts do not grow favorably and the foods are stored for a relatively
longer period of time, such as breads, hard cheeses, fermented dry sausages, and acidic fruits and
vegetables, spoilage as a result of mold growth is more prevalent. Recent advances in anaerobic
packaging of foods have also greatly reduced the spoilage of food by molds and, to some extent,
by yeasts but not by anaerobic and facultative anaerobic bacteria. Thus, among the three microbial
groups, the highest incidence of spoilage, especially rapid spoilage, of processed foods is caused by
bacteria, followed by yeasts and molds. 1–5
Microbial Numbers
To produce detectable changes in color, odor, and texture of a food accompanied with slime formation or gas and liquid accumulation, microorganisms (mainly bacteria and yeasts) must multiply and attain certain levels, often referred to as the “spoilage detection level.” Although they vary
with the type of foods and microorganisms, bacteria and yeasts need to grow and reach approximately 10 7 cells/g/mL, or/cm 2 , of a food from the level present normally in a food. Depending
on the specific nature of spoilage and microbial types, the spoilage detection level can range
from 10 6–8 cells/g/mL, or/cm 2 . Spoilage associated with H 2 S, some amines, and H 2 O 2 formation
can be detected at a lower microbial load, whereas formation of lactic acid may be detected at a
higher microbial load. Slime formation, associated with accumulation of microbial cells, is generally detected at ≥10 8 cells/g/mL, or/cm 2 , of a food. It appears, then, that a food with relatively
higher initial loads of spoilage bacteria (or yeasts) and a storage condition that favors rapid growth
(shorter generation time) will spoil more rapidly than a food with a low initial load of microbes
with a longer generation time. In a hypothetical example (Figure 19.1), the population reached
the spoilage detection level within seven days with a high initial load (ca. 5 × 10 5 /g) as opposed to
20 days with a low initial load (ca. 5 × 10 5 /g) during storage at 12°C. However, when the product
with a low initial load was stored at 4°C (to increase the generation time), it took approximately 55
days for the spoilage bacteria to reach the spoilage detection level. To reduce microbial spoilage of
a food, one needs to aim at achieving both the low initial load and longer generation time of spoilage microorganisms during storage. It has to be recognized that the mere presence of 10 7 cells/g/
mL, or/cm 2 , without growth (e.g., from a massive initial contamination) will not immediately
cause a food to lose its acceptance quality, but such a food will spoil very rapidly following growth
of the contaminants. Bioprocessed foods, in general, contain very high numbers of microorganisms (10 8–9 cells/g or /mL). However, under normal conditions, they are desirable types, and the
enzymes, in the absence of viable microbial cells, can result from some heat-stable enzymes produced by microorganisms in the foods before heat treatment. In addition, the foods need to be
stored at a temperature for a sufficient length of time for the catalytic activities of the enzymes to
occur to produce the detectable changes.
Significance of Microorganisms
Microbial Types
Raw and most processed foods normally contain many types of molds, yeasts, and bacteria capable
of multiplying and causing spoilage. (Viruses and parasites do not multiply in foods.) As multiplication is an important component in spoilage, bacteria (because of its shorter generation time),
followed by yeasts, are in favorable positions over molds to cause rapid spoilage of foods. However,
in foods where bacteria or yeasts do not grow favorably and the foods are stored for a relatively
longer period of time, such as breads, hard cheeses, fermented dry sausages, and acidic fruits and
vegetables, spoilage as a result of mold growth is more prevalent. Recent advances in anaerobic
packaging of foods have also greatly reduced the spoilage of food by molds and, to some extent,
by yeasts but not by anaerobic and facultative anaerobic bacteria. Thus, among the three microbial
groups, the highest incidence of spoilage, especially rapid spoilage, of processed foods is caused by
bacteria, followed by yeasts and molds. 1–5
Microbial Numbers
To produce detectable changes in color, odor, and texture of a food accompanied with slime formation or gas and liquid accumulation, microorganisms (mainly bacteria and yeasts) must multiply and attain certain levels, often referred to as the “spoilage detection level.” Although they vary
with the type of foods and microorganisms, bacteria and yeasts need to grow and reach approximately 10 7 cells/g/mL, or/cm 2 , of a food from the level present normally in a food. Depending
on the specific nature of spoilage and microbial types, the spoilage detection level can range
from 10 6–8 cells/g/mL, or/cm 2 . Spoilage associated with H 2 S, some amines, and H 2 O 2 formation
can be detected at a lower microbial load, whereas formation of lactic acid may be detected at a
higher microbial load. Slime formation, associated with accumulation of microbial cells, is generally detected at ≥10 8 cells/g/mL, or/cm 2 , of a food. It appears, then, that a food with relatively
higher initial loads of spoilage bacteria (or yeasts) and a storage condition that favors rapid growth
(shorter generation time) will spoil more rapidly than a food with a low initial load of microbes
with a longer generation time. In a hypothetical example (Figure 19.1), the population reached
the spoilage detection level within seven days with a high initial load (ca. 5 × 10 5 /g) as opposed to
20 days with a low initial load (ca. 5 × 10 5 /g) during storage at 12°C. However, when the product
with a low initial load was stored at 4°C (to increase the generation time), it took approximately 55
days for the spoilage bacteria to reach the spoilage detection level. To reduce microbial spoilage of
a food, one needs to aim at achieving both the low initial load and longer generation time of spoilage microorganisms during storage. It has to be recognized that the mere presence of 10 7 cells/g/
mL, or/cm 2 , without growth (e.g., from a massive initial contamination) will not immediately
cause a food to lose its acceptance quality, but such a food will spoil very rapidly following growth
of the contaminants. Bioprocessed foods, in general, contain very high numbers of microorganisms (10 8–9 cells/g or /mL). However, under normal conditions, they are desirable types, and the