Fundamental Food Microbiology, Third Edition - Fuad Fathir

264 FUNDAMENTAL FOOD MICROBIOLOGY
dextran and different types of exopolysaccharides or too many microbial cells resulting
in confluent growth), and accumulation of liquid (purge accumulation; in fresh
and processed meats due to breakdown of structures holding the water of hydration).
Some of these changes also occur from the effect of microbial metabolites on food
pH. Production of organic acids by microorganisms, causing lowering of food pH,
can reduce the water-holding ability of food (such as growth of some lactic acid
bacteria in low-fat, high-pH-processed meat products). Similarly, production of basic
compounds by microorganisms in a food can shift its pH to the alkaline side and
reduce its acceptance quality (such as decarboxylation of amino acids in some lowheat-processed
meat products with the production of amines, shifting the pH to basic,
and changing product color from light brown to pink in some processed meats;
Chapter 20).
Table 18.1 lists some of the end products of microbial metabolism of food
nutrients that are attributed to food spoilage. The end products vary with the nature
of metabolism (e.g., aerobic respiration, anaerobic respiration, or fermentation;
Chapter 7 and Chapter 11). It is evident that many of these metabolites are able to
produce the changes associated with microbial food spoilage (change in odor, gas
formation, or slime formation). 1–3
C. Utilization of Food Nutrients
Almost all foods contain some amounts of carbohydrates, proteinaceous and NPN
compounds, and lipids that are available for use by microorganisms during growth.
However, the characteristics of food spoilage differ greatly because of differences
in the nature and the amount of a specific nutrient present in a food, the type of
microorganisms growing in the food, and the nature of metabolism (respiration or
fermentation). In general, for energy production, microorganisms prefer to use
metabolizable monosaccharides, disaccharides, and large carbohydrates first; followed
by NPN, small peptides, and large proteinaceous compounds; and finally
lipids. However, again metabolic characteristics depend on whether a particular
species can use a specific carbohydrate (such as the ability or inability to utilize
lactose) and the concentration of it present (limiting or high concentration). Also,
with any nutrient, small molecules are used first before large molecules (polymers).
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
Table 18.1 Some End Products from Microbial Metabolism of Food Nutrients
Food Nutrient End Products
Carbohydrates CO2, H2, H2O2, lactate, acetate, formate, succinate,
butyrate, isobutyrate, isovalerate, ethanol, propanol,
butanol, isobutanol, diacetyl, acetoin, butanediol,
dextran, levans
Proteinaceous and NPN CO2, H2, NH3, H2S, amines, keto-acids, mercaptans,
compounds
organic disulfides, putrescine, cadaverine, skatole
Lipids Fatty acids, glycerol, hydroperoxides, carbonyl
compounds (aldehydes, ketones), nitrogenous bases