Fundamental Food Microbiology, Third Edition - Fuad Fathir

84 FUNDAMENTAL FOOD MICROBIOLOGY
quantities to support microbial growth. In general, microorganisms preferentially
metabolize carbohydrates as an energy source over proteins and lipids. Thus, microorganisms
growing in a food rich in metabolizable carbohydrates utilize carbohydrates,
but in a food low in metabolizable carbohydrates and rich in metabolizable
proteins they metabolize proteins (after metabolizing the carbohydrates). In a food
rich in both carbohydrates and proteins, microorganisms usually utilize the carbohydrates
first, produce acids, and reduce the pH. Subsequent microbial degradation
of proteins can be prevented at low pH, causing nondegradation of proteins or a
protein sparing effect. In the formulation of processed meat products, added carbohydrates
can provide this benefit.
III. METABOLISM OF FOOD CARBOHYDRATES
Food carbohydrates comprise a large group of chemical compounds that include
monosaccharides (tetroses, pentoses, and hexoses), disaccharides, oligosaccharides,
and polysaccharides. Although carbohydrates are the most preferred source of energy
production, microorganisms differ greatly in their ability to degrade individual carbohydrates.
1–5 Carbohydrates that are degraded at the cellular level as monosaccharides;
disaccharides, and trisaccharides can be transported inside the cell and hydrolyzed
to monosaccharide units before further degradation. Polysaccharides are broken
down to mono- and disaccharides by extracellular microbial enzymes (e.g., a-amylase)
secreted in the environment before they can be transported and metabolized.
A. Degradation of Polysaccharides
Molds, some Bacillus spp. and Clostridium spp., and several other bacterial species
can degrade starch, glycogen, cellulose, pectin, and other polysaccharides by extracellular
enzymes. The mono- and disaccharides are then transported in the cell and
metabolized. Breakdown of these polysaccharides, especially pectins and cellulose,
in fruits and vegetables by microorganisms can affect the texture and reduce the
acceptance quality of the products.
B. Degradation of Disaccharides
Disaccharides of foods, either present in food (lactose, sucrose) or produced through
microbial degradation (maltose from starch), are hydrolyzed to monosaccharides
inside the cell by specific enzymes: lactose by lactase to galactose and glucose,
sucrose by sucrase to glucose and fructose, and maltose by maltase to glucose. Many
microbial species cannot metabolize one or more disaccharides. 1–5
C. Degradation of Monosaccharides
Monosaccharides are degraded (catabolized) by aerobic, anaerobic, and facultative
anaerobic microorganisms via several pathways that generate many types of intermediate
and end products. The metabolic pathways are dependent on the type and