Biochemistry of Some Beneficial Traits ◾ 137
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,
mainly 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 into 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).
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 (monoglycerides
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.
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 13.
QUESTIONS
1. Name two species of yeasts and briefly discuss the use of each in the production of fermented
foods and food additives.
2. List three mold species and indicate their specific uses in food bioprocessing.
3. List the main carbohydrates, proteinaceous and NPN compounds, and lipids in food systems that starter-culture bacteria have at their disposal to metabolize.
4. Discuss how lactic acid bacteria, while growing in milk, are able to transport lactose.
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,
mainly 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 into 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).
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 (monoglycerides
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.
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 13.
QUESTIONS
1. Name two species of yeasts and briefly discuss the use of each in the production of fermented
foods and food additives.
2. List three mold species and indicate their specific uses in food bioprocessing.
3. List the main carbohydrates, proteinaceous and NPN compounds, and lipids in food systems that starter-culture bacteria have at their disposal to metabolize.
4. Discuss how lactic acid bacteria, while growing in milk, are able to transport lactose.