ABSTRACT
The metabolism of food nutrients, principally carbohydrates, nitrogenous compounds, and lipids, by spoilage bacteria enables the cells to increase in number as well as produce metabolites that can adversely reduce the acceptance quality of a food. A food is considered spoiled when the changes are detectable and the microbial population has reached approximately 10 7–9 /mL/g or/cm2. The changes are brought about by the catalytic actions of a large number of microbial enzymes. Most microbial enzymes are intracellular and act on the nutrients that can be transported inside the cells through several transport mechanisms. A bacterial cell contains many enzymes, many of which are intracellular, and some are extracellular. Intracellular enzymes are involved in the metabolism of small nutrient molecules of food that are transported inside the cells. Many intracellular enzymes can also act on intracellular large molecules, such as endonucleases, mucopeptidase, lipases, and proteinases. Extracellular enzymes, after synthesis, either remain bound to the cell surface or are excreted into the food environment. Many of the latter group can hydrolyze large nutrient molecules of food (e.g., polysaccharides, proteins, and lipids) to small molecules before they are transported into the cells (see Chapters 8 and 12).
