252 ◾ Fundamental Food Microbiology
If a food has carbohydrates that can be fermented by the contaminating microorganisms, then
it will usually be metabolized first. If the metabolizable carbohydrates are present in sufficient
quantities, then the metabolic pathway remains 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, and sucrose) will produce either CO 2 and H 2 O (aerobically) or alcohol
and CO 2 (anaerobically). However, Pse. fluorescence 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, the 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
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, the generation time of Lactococcus spp. becomes longer. However, Bacillus sp., because of its
aciduric nature, can then start multiplication, metabolize proteins, and increase the pH (say 5.8).
In the high pH, the Pseudomonas spp. present initially can then grow by metabolizing NPN and
proteinaceous compounds and increase the pH further by producing basic metabolites (amines,
NH 3 ). This way, the predominant spoilage microorganisms and the metabolites associated with
spoilage (e.g., nature of spoilage) of a food can change if a food is stored for a fairly long time. 3
If a food has carbohydrates that can be fermented by the contaminating microorganisms, then
it will usually be metabolized first. If the metabolizable carbohydrates are present in sufficient
quantities, then the metabolic pathway remains 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, and sucrose) will produce either CO 2 and H 2 O (aerobically) or alcohol
and CO 2 (anaerobically). However, Pse. fluorescence 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, the 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
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, the generation time of Lactococcus spp. becomes longer. However, Bacillus sp., because of its
aciduric nature, can then start multiplication, metabolize proteins, and increase the pH (say 5.8).
In the high pH, the Pseudomonas spp. present initially can then grow by metabolizing NPN and
proteinaceous compounds and increase the pH further by producing basic metabolites (amines,
NH 3 ). This way, the predominant spoilage microorganisms and the metabolites associated with
spoilage (e.g., nature of spoilage) of a food can change if a food is stored for a fairly long time. 3