Fundamental Food Microbiology, Third Edition - Fuad Fathir

306 FUNDAMENTAL FOOD MICROBIOLOGY
excreted in the food environment. Many of the latter group can hydrolyze large
nutrient molecules of food (e.g., polysaccharides, proteins, and lipids) to small
molecules before they are transported into the cells (see Chapter 7 and Chapter 11).
Most foods have some amounts of low-molecular-weight metabolizable carbohydrates
(mono- and disaccharides and their derivatives, such as glucose-6-phosphate),
nitrogenous compounds (small peptides, amino acids, nucleosides, nucleotides,
urea, creatinine, trimethylamine oxide), free fatty acids, and some organic
acids (lactic, citric, and malic acids). Many spoilage microorganisms, particularly
spoilage bacteria, are able to utilize the low-molecular-weight food components to
reach a population of 10 7–9 /g, /ml, or /cm 2 food and cause detectable food spoilage.
Thus, the supply of extra nutrients from the hydrolysis of the macromolecules of
foods by bacterial extracellular enzymes is not necessary for the onset of spoilage
of many foods. In fact, studies with Pseudomonas spp. and Bacillus spp. show that
in the presence of low-molecular-weight nitrogenous compounds, the synthesis of
extracellular proteinases is repressed. When the supply of the small molecules is
used up, the mechanism is derepressed, leading to the synthesis and excretion of
extracellular proteinases. The proteinases hydrolyze the large protein molecules of
food to produce small peptides and amino acids for transport and endogenous
metabolism in the cells, which in turn intensify the spoilage. In general, microbial
food spoilage from the metabolism of low-molecular-weight nutrients occurs at the
early stage of microbial growth; spoilage from the breakdown of macromolecules
by extracellular enzymes appears late in the sequence of events. 1,2
After microbial cells die normally or are killed by nonthermal treatments so that
the intracellular and extracellular enzymes are not inactivated or destroyed, the
enzymes can cause food spoilage even in the absence of viable cells or growth of
microorganisms. Many microbial cells in a food that are subjected to freezing (then
thawing), drying (then rehydrated), modified atmosphere or vacuum packaging,
refrigeration, high hydrostatic pressure, electric pulse field, or high-intensity light,
or are exposed to some preservatives, may die and undergo autolysis to release
intracellular enzymes. If these foods are stored for a long time under conditions that
favor catalysis of one or more intra- and extracellular enzymes, they can undergo
spoilage. Ripening (not spoilage) of cheddar cheese (at low temperature and low
A w) is a good example wherein following death, bacterial intracellular enzymes are
involved in the breakdown of milk nutrients, not to cause spoilage but to impart
desirable product characteristics. In most foods, the initial microbial population is
generally low and spoilage by microbial enzymes in the absence of growth may not
be of practical significance. However, if a product is heavily contaminated with a
high initial microbial load (in the absence of growth) and then subjected to a
nonthermal treatment that kills microorganisms but does not inactivate enzymes,
spoilage of the food by microbial enzymes can occur (Chapter 20). In thermally
processed foods, several heat-stable enzymes of the microorganisms retain their
activity even after the producer cells are killed. During subsequent storage of the
food under favorable conditions, these enzymes can break down the food nutrients
to cause spoilage. Among the heat-stable enzymes, some extracellular proteinases,
lipases, and phospholipases of several psychrotrophic bacteria found in food cause
spoilage of thermally processed dairy products. 1,2