250 ◾ Fundamental Food Microbiology
pickles, salsa, salad dressings, mayonnaise, and fermented sausages. Heterofermentative lactic acid
bacteria (such as Lab. fructivorans, Lab. fermentum, and Leu. mesenteroides) and homofermentative
lactic acid bacteria (such as Lab. plantarum and Ped. acidilactici) have been associated with such
spoilage. (Yeasts and molds are aciduric and thus are also associated with spoilage of such foods.)
Significance of Foods
Food Types
Foods differ greatly in their susceptibility to spoilage by microorganisms. This is mainly because
of their differences in intrinsic factors (A W , pH, O-R potential, nutrient content, antimicrobial
substances, and protective structures). A food with a lower A W (~0.90) or a lower pH (~5.3) is less
susceptible to bacterial spoilage than one with A W of approximately 0.98 or a pH of approximately
6.4. However, molds and yeasts will probably grow equally well under both conditions. The influence of each of the intrinsic parameters on microbial growth has been described in Chapter 6. On
the basis of susceptibility of spoilage, foods can be grouped as perishable (spoil quickly in days),
semiperishable (have a relatively long shelf life of a few weeks or months), and nonperishable
(have a very long shelf life of many months or years). In addition to intrinsic parameters, extrinsic
parameters (storage conditions) play important roles in determining the ease of microbial spoilage
of many foods. 3–5
Food Nutrients
Microbial growth in a food is associated with the metabolism of some food carbohydrates, proteinaceous and nonprotein nitrogenous (NPN) compounds, and lipids. The influences of major
types of carbohydrates (polysaccharides, trisaccharides, disaccharides, monosaccharides, and
sugar alcohols), proteinaceous compounds (proteins, peptides), NPN compounds (amino acids,
urea, creatine, and trimethylamine oxide), and lipids (triglycerides, phospholipids, fatty acids,
and sterols) present in foods on microbial spoilage are briefly discussed here. The metabolic pathways of some of these compounds by microorganisms have also been discussed in Chapter 8 and
Chapter 12. It is evident from previous discussions that microorganisms differ greatly in their
abilities to metabolize different food nutrients (such as the ability or inability to utilize cellulose
and lactose as carbon sources, casein as a nitrogen source, and oxidation of oleic acid). Similarly,
the same nutrient (substrate) can be utilized by different microorganisms by different metabolic
pathways to produce different end products (e.g., glucose metabolized by homolactic and heterolactic acid bacteria). The same nutrient (substrate) can be degraded to produce different end products under aerobic and anaerobic metabolism (respiration and fermentation, respectively). Thus,
glucose is metabolized (catabolized) by Micrococcus spp. aerobically to produce CO 2 and H 2 O and
by Lab. acidophilus anaerobically to produce mainly lactic acid. Saccharomyces cerevisiae metabolizes glucose aerobically to CO 2 and H 2 O but anaerobically to ethanol and CO 2 . Under specific
conditions, some microorganisms can also synthesize (anabolism) polymeric compounds as end
products, such as dextran (polymer of glucose) production by Leu. mesenteroides while metabolizing sucrose. Some microorganisms can also secrete extracellular enzymes to break down large
molecular nutrients (polymers) in a food (such as the breakdown of starch by amylase produced
by some molds). Finally, some microorganisms can synthesize pigments while growing in a food
(such as Micrococcus luteus producing a yellow pigment).
pickles, salsa, salad dressings, mayonnaise, and fermented sausages. Heterofermentative lactic acid
bacteria (such as Lab. fructivorans, Lab. fermentum, and Leu. mesenteroides) and homofermentative
lactic acid bacteria (such as Lab. plantarum and Ped. acidilactici) have been associated with such
spoilage. (Yeasts and molds are aciduric and thus are also associated with spoilage of such foods.)
Significance of Foods
Food Types
Foods differ greatly in their susceptibility to spoilage by microorganisms. This is mainly because
of their differences in intrinsic factors (A W , pH, O-R potential, nutrient content, antimicrobial
substances, and protective structures). A food with a lower A W (~0.90) or a lower pH (~5.3) is less
susceptible to bacterial spoilage than one with A W of approximately 0.98 or a pH of approximately
6.4. However, molds and yeasts will probably grow equally well under both conditions. The influence of each of the intrinsic parameters on microbial growth has been described in Chapter 6. On
the basis of susceptibility of spoilage, foods can be grouped as perishable (spoil quickly in days),
semiperishable (have a relatively long shelf life of a few weeks or months), and nonperishable
(have a very long shelf life of many months or years). In addition to intrinsic parameters, extrinsic
parameters (storage conditions) play important roles in determining the ease of microbial spoilage
of many foods. 3–5
Food Nutrients
Microbial growth in a food is associated with the metabolism of some food carbohydrates, proteinaceous and nonprotein nitrogenous (NPN) compounds, and lipids. The influences of major
types of carbohydrates (polysaccharides, trisaccharides, disaccharides, monosaccharides, and
sugar alcohols), proteinaceous compounds (proteins, peptides), NPN compounds (amino acids,
urea, creatine, and trimethylamine oxide), and lipids (triglycerides, phospholipids, fatty acids,
and sterols) present in foods on microbial spoilage are briefly discussed here. The metabolic pathways of some of these compounds by microorganisms have also been discussed in Chapter 8 and
Chapter 12. It is evident from previous discussions that microorganisms differ greatly in their
abilities to metabolize different food nutrients (such as the ability or inability to utilize cellulose
and lactose as carbon sources, casein as a nitrogen source, and oxidation of oleic acid). Similarly,
the same nutrient (substrate) can be utilized by different microorganisms by different metabolic
pathways to produce different end products (e.g., glucose metabolized by homolactic and heterolactic acid bacteria). The same nutrient (substrate) can be degraded to produce different end products under aerobic and anaerobic metabolism (respiration and fermentation, respectively). Thus,
glucose is metabolized (catabolized) by Micrococcus spp. aerobically to produce CO 2 and H 2 O and
by Lab. acidophilus anaerobically to produce mainly lactic acid. Saccharomyces cerevisiae metabolizes glucose aerobically to CO 2 and H 2 O but anaerobically to ethanol and CO 2 . Under specific
conditions, some microorganisms can also synthesize (anabolism) polymeric compounds as end
products, such as dextran (polymer of glucose) production by Leu. mesenteroides while metabolizing sucrose. Some microorganisms can also secrete extracellular enzymes to break down large
molecular nutrients (polymers) in a food (such as the breakdown of starch by amylase produced
by some molds). Finally, some microorganisms can synthesize pigments while growing in a food
(such as Micrococcus luteus producing a yellow pigment).