Fundamental Food Microbiology, Third Edition - Fuad Fathir

148 FUNDAMENTAL FOOD MICROBIOLOGY
peptides and metabolizable proteins in the environment are hydrolyzed by the proteinase
and peptidase enzymes, located mainly on the cell wall of these species, to
small peptides and amino acids and then transported, many by active group transport
systems, inside the cells. Inside the cells, peptides are converted to amino acids, and
the amino acids are then metabolized differently to produce many different end
products, which are excreted in the environment. Some of the metabolic processes
involve decarboxylation (generates amines, some of which are biologically active,
such as histamine from histidine), deamination, oxidative reduction, and anaerobic
reduction. Amino acid metabolism by lactic acid bacteria and different associative
bacteria during fermentation can produce diverse products, many of which have
specific flavor characteristics. Some of these include ammonia, hydrogen sulfide,
amines, mercaptans, and disulfides. Many of them, in low concentrations, contribute
to the desirable flavor of different fermented foods. Proper hydrolysis of food proteins
by the proteolytic enzymes of starter microorganisms is important for the desirable
texture of many fermented foods (such as in cheeses). Rapid hydrolysis of proteins
can result in production and accumulation of some specific hydrophobic peptides that
impart a bitter taste to the product (such as in some sharp Cheddar cheeses).
V. TRANSPORT AND METABOLISM OF LIPID COMPOUNDS
Many starter-culture bacteria metabolize lipids poorly. However, molds have better
lipid metabolism systems. Triglycerides and phospholipids are hydrolyzed outside
of the cells by lipases, produced by the microorganisms, releasing fatty acids and
glycerol and glycerides (mono- or diglycerides). Fatty acids can diffuse through the
membrane into the cells and be metabolized. Some fatty acids are incorporated in
the membrane. Hydrolysis of glycerides, especially those with small fatty acids such
as butyric acid, can cause hydrolytic rancidity of the products. Oxidation of unsaturated
fatty acids by microorganisms, especially molds, can produce many flavor
compounds, either desirable or undesirable.
VI. CONCLUSION
Many safe and food-grade microbial species are used to produce fermented foods
and food additives. They metabolize the carbohydrates, proteins, and lipids in the
food by specific pathways and produce metabolites that in turn bring about the
desirable acceptance characteristics of the fermented foods and food additives. The
metabolic pathways are regulated by many enzymes, the genetic information of
which is coded in the genes located in the chromosome and plasmid in a cell. The
nature of genes and the mechanisms by which the genetic codes are translated into
enzymes in lactic acid bacteria are discussed in Chapter 12.