52
Modern Food Microbiology
While the growth of anaerobes is normally believed to occur at reduced values of Eh, the exclusion
of O 2 may be necessary for some anaerobes. When Clostridium perfringens, Bacteroides fragilis, and
Peptococcus magnus were cultured in the presence of O 2 , inhibition of growth occurred even when
the medium was at a negative Eh of −50 mV.
52 These investigators found that growth occurred in
media with an Eh as high as 325 mV when no O 2 was present.
With regard to the effect of Eh on lipid production by Saccharomyces cerevisiae, it has been shown
that anaerobically grown cells produce a lower total level, a highly variable glyceride fraction, and
decreased phospholipid and sterol components as compared to aerobically grown cells.
41 The lipid
produced by anaerobically grown cells was characterized by a high content (up to 50% of total acid) of
8:0 to 14:0 acids and a low level of unsaturated fatty acid in the phospholipid fraction. In aerobically
grown cells, 80–90% of the fatty acid component was associated with glyceride, and the phospholipid
was found to be 16:1 and 18:1 acids. Unlike aerobically grown cells, anaerobically grown S. cerevisiae
cells were found to have a lipid and sterol requirement.
Nutrient Content
In order to grow and function normally, the microorganisms of importance in foods require the
following:
1. water
2. source of energy
3. source of nitrogen
4. vitamins and related growth factors
5. minerals
The importance of water to the growth and welfare of microorganisms is presented earlier in this
chapter. With respect to the other four groups of substances, molds have the lowest requirement,
followed by Gram-negative bacteria, yeasts, and Gram-positive bacteria.
As sources of energy, foodborne microorganisms may utilize sugars, alcohols, and amino acids.
Some microorganisms are able to utilize complex carbohydrates such as starches and cellulose as
sources of energy by first degrading these compounds to simple sugars. Fats are also used by microorganisms as sources of energy, but these compounds are attacked by a relatively small number of
microbes in foods.
The primary nitrogen sources utilized by heterotrophic microorganisms are amino acids. A large
number of other nitrogenous compounds may serve this function for various types of organisms.
Some microbes, for example, are able to utilize nucleotides and free amino acids, whereas others
are able to utilize peptides and proteins. In general, simple compounds such as amino acids will be
utilized by almost all organisms before any attack is made on the more complex compounds such as
high-molecular-weight proteins. The same is true of polysaccharides and fats.
Microorganisms may require B vitamins in low quantities, and almost all natural foods have an
abundant quantity for those organisms that are unable to synthesize their essential requirements. In
general, Gram-positive bacteria are the least synthetic and must therefore be supplied with one or
more of these compounds before they will grow. The Gram-negative bacteria and molds are able to
synthesize most or all of their requirements. Consequently, these two groups of organisms may be
found growing on foods low in B vitamins. Fruits tend to be lower in B vitamins than meats, and this
Modern Food Microbiology
While the growth of anaerobes is normally believed to occur at reduced values of Eh, the exclusion
of O 2 may be necessary for some anaerobes. When Clostridium perfringens, Bacteroides fragilis, and
Peptococcus magnus were cultured in the presence of O 2 , inhibition of growth occurred even when
the medium was at a negative Eh of −50 mV.
52 These investigators found that growth occurred in
media with an Eh as high as 325 mV when no O 2 was present.
With regard to the effect of Eh on lipid production by Saccharomyces cerevisiae, it has been shown
that anaerobically grown cells produce a lower total level, a highly variable glyceride fraction, and
decreased phospholipid and sterol components as compared to aerobically grown cells.
41 The lipid
produced by anaerobically grown cells was characterized by a high content (up to 50% of total acid) of
8:0 to 14:0 acids and a low level of unsaturated fatty acid in the phospholipid fraction. In aerobically
grown cells, 80–90% of the fatty acid component was associated with glyceride, and the phospholipid
was found to be 16:1 and 18:1 acids. Unlike aerobically grown cells, anaerobically grown S. cerevisiae
cells were found to have a lipid and sterol requirement.
Nutrient Content
In order to grow and function normally, the microorganisms of importance in foods require the
following:
1. water
2. source of energy
3. source of nitrogen
4. vitamins and related growth factors
5. minerals
The importance of water to the growth and welfare of microorganisms is presented earlier in this
chapter. With respect to the other four groups of substances, molds have the lowest requirement,
followed by Gram-negative bacteria, yeasts, and Gram-positive bacteria.
As sources of energy, foodborne microorganisms may utilize sugars, alcohols, and amino acids.
Some microorganisms are able to utilize complex carbohydrates such as starches and cellulose as
sources of energy by first degrading these compounds to simple sugars. Fats are also used by microorganisms as sources of energy, but these compounds are attacked by a relatively small number of
microbes in foods.
The primary nitrogen sources utilized by heterotrophic microorganisms are amino acids. A large
number of other nitrogenous compounds may serve this function for various types of organisms.
Some microbes, for example, are able to utilize nucleotides and free amino acids, whereas others
are able to utilize peptides and proteins. In general, simple compounds such as amino acids will be
utilized by almost all organisms before any attack is made on the more complex compounds such as
high-molecular-weight proteins. The same is true of polysaccharides and fats.
Microorganisms may require B vitamins in low quantities, and almost all natural foods have an
abundant quantity for those organisms that are unable to synthesize their essential requirements. In
general, Gram-positive bacteria are the least synthetic and must therefore be supplied with one or
more of these compounds before they will grow. The Gram-negative bacteria and molds are able to
synthesize most or all of their requirements. Consequently, these two groups of organisms may be
found growing on foods low in B vitamins. Fruits tend to be lower in B vitamins than meats, and this