Fundamental Food Microbiology, Third Edition - Fuad Fathir

IMPORTANT FACTORS IN MICROBIAL FOOD SPOILAGE 265
unchanged during rapid growth. However, if the carbohydrates are present in limited
concentrations, then after the carbohydrates are used up the microorganisms usually
start using NPN, small peptides, and other proteinaceous compounds. For example,
yeasts growing in a fruit juice containing relatively high amounts of metabolizable
carbohydrates (fructose, glucose, sucrose) will produce either CO 2 and H 2O (aerobically)
or alcohol and CO 2 (anaerobically). However, Pse. fluorescens growing
aerobically in fresh meat with limited amounts of glucose will first metabolize it
and then start metabolizing free amino acids and other NPN compounds. If it is
allowed to grow for a long time, it will produce extracellular proteinases to break
down meat proteins to produce small peptides and amino acids for further metabolism.
With time, it may even be able to produce lipases to break down meat lipids
and use up some fatty acids. In a food (such as milk) containing large amounts of
both carbohydrates (lactose) and proteins, a lactose-metabolizing microorganism
will preferentially utilize the lactose and produce acid or acid and gas (Lac. lactis
will produce lactic acid and Leuconostoc spp. will produce acid and gas), but a
microorganism unable to utilize lactose will use the NPN and proteinaceous compounds
for growth (Pseudomonas spp.). The spoilage patterns of these bacteria will
be quite different.
In a mixed microbial population, as present normally in a food, availability and
amount of metabolizable carbohydrates greatly affect the spoilage pattern. Fresh
meats, because of a low level of glucose, are susceptible to spoilage through microbial
degradation of NPN and proteinaceous compounds. However, if a metabolizable
carbohydrate (such as glucose, sucrose, or lactose) is added to meat, metabolism of
carbohydrates will predominate. If lactic acid bacteria are present as natural microflora
and the growth environment is favorable, they will produce enough acids to
arrest the growth of many normal microflora that preferentially metabolize NPN and
proteinaceous compounds (e.g., Gram-negative pyschrotroph). This is commonly
known as the protein-sparing effect (proteins are not metabolized). In the formulation
of many processed meat products, metabolizable simple carbohydrates (usually
glucose) are used to produce the protein-sparing effect. 1,2
D. Microbial Growth in Succession
Intrinsic and extrinsic factors or environments of a food dictate which, among the
mixed microbial species normally present, will multiply rapidly and become predominant
to cause spoilage. However, as the predominant types grow, they produce
metabolites and change the food environment. In the changed environment, some
other species, initially present but previously unable to compete, may be in a
favorable position to grow rapidly and again change the food environment further
to enable a third type to grow rapidly. If sufficient time is given, the predominant
microbial types and the nature of spoilage of a food can change. Sequential growth
of Lactococcus spp., aciduric lactose-negative Bacillus sp., and Gram-negative rods
(such as Pseudomonas spp.) in a milk sample can be used as a hypothetical example.
Initially, rapid growth of Lactococcus spp. (able to metabolize lactose) under a
favorable growth condition will reduce the pH from original 6.5 to 5.0 and reduce
the growth rate of many other microbial species present. As the pH drops below 5.0,
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