Microbial Ecology of Spoilage 389
It is well known that groups of microorganisms have optimum, minimum, and
maximum pH for growth in produce. As with other factors, pH usually interacts
with other parameters in the produce to inhibit growth. The pH can interact with
factors such as aw, salt, temperature, redox potential, and preservatives to inhibit
growth of pathogens and other organisms. The pH of the produce also significantly
affects the lethality of heat treatment of the plant food. Less heat is needed to
inactivate microbes as the pH is reduced [72]. Some produce, such as fully ripe
tomatoes, is in a pH range (3.9 to 4.5) that prevents or retards growth of enteric
pathogens such as Shigella and E. coli O157:H7. The pH of many vegetables, melons,
and soft fruits is 4.6 or higher, which is suitable for the growth of pathogenic bacteria.
The growth and survival of human pathogens could be affected by the presence of
plant pathogens such as Botrytis cinerea or Penicillium spp. [73]. Growth of post-
harvest fungi in subsurface tissues can alter the pH of plant tissues, allowing the
growth of pathogenic bacteria. Populations of L. monocytogenes inoculated into
decayed apple tissue increased on fruit infected by Glomerella cingulata but not by
Penicillium expansum [73–78]. This difference was attributed, in part, to the increase
in pH of the infected tissues from 4.7 to 7.0 as a result of infection by G. cingulata
compared to a decrease in pH from 4.7 to 3.7 as a result of infection by P. expansum
[73]. Similar results were obtained with E. coli O157:H7 when it was coinoculated
with G. cingulata or P. expansum on apples [75,77].
12.3.2.2.3 Nutrient Content
Microorganisms require certain basic nutrients for growth and maintenance of met-
abolic functions. The amount and type of nutrients required range widely depending
on the microorganism. These nutrients include water, a source of energy, nitrogen,
vitamins, and minerals [84–86]. Varying amounts of these nutrients are present in
foods. Plant foods have high concentrations of different types of carbohydrates and
varying levels of proteins, minerals, and vitamins.
Foodborne microorganisms can derive energy from carbohydrates, alcohols, and
amino acids. Most microorganisms will metabolize simple sugars such as glucose.
Others can metabolize more complex carbohydrates, such as starch or cellulose
found in plant foods. Amino acids serve as a source of nitrogen and energy and are
utilized by most microorganisms. Some microorganisms are able to metabolize
peptides and more complex proteins. Other sources of nitrogen include, for example,
urea, ammonia, creatinine, and methylamines. Examples of minerals required for
microbial growth include phosphorus, iron, magnesium, sulfur, manganese, calcium,
and potassium. In general, small amounts of these minerals are required; thus, a
wide range of foods can serve as good sources of minerals [84–86].
In general, the Gram-positive bacteria are more fastidious in their nutritional
requirements and thus are not able to synthesize certain nutrients required for growth
[86]. For example, the Gram-positive foodborne pathogen S. aureus requires amino
acids, thiamine, and nicotinic acid for growth [86]. Fruits and vegetables that are
deficient in B vitamins do not effectively support the growth of these microorgan-
isms. The Gram-negative bacteria are generally able to derive their basic nutritional
requirements from the existing carbohydrates, proteins, minerals, and vitamins that
are found in a wide range of food [86].