170 ◾ Fundamental Food Microbiology
General Method of Production
The production of a fermented product has two related yet separate aspects, one involving the
importance of metabolic activities of microorganisms during fermentation and storage of the
product and the other involving the parameters used during processing and storage of the product.
In this chapter, the microbiological aspects are presented. Processing aspects are generally taught
in a food-processing course and are thus covered only in brief here.
Fermentation involves exposing the raw or starting food materials to conditions that favor
growth and metabolism of specific and desirable microorganisms. As the desirable microorganisms grow, they utilize some nutrients and produce some end products. These end products,
along with the unmetabolized components of the starting materials, constitute the fermented
foods having desirable acceptance qualities, many of which are attributed to the metabolic end
products.
Raw (or Starting) Materials
A large number of raw materials from plant and animal sources are used to produce fermented
foods. These include milk (from cows, buffalo, sheep, goats, and mares), meat (beef, pork, lamb,
goat, and fowl), fish (many types), eggs (chicken and duck), vegetables and vegetable juices,
many fruits and fruit juices, cereal grains, tubers, lentils, beans, and seeds. Some are used in
combination.
Microorganisms Used
Many desirable species and strains of bacteria, yeasts, and molds are associated with fermentation of foods. Depending on a product, fermentation may be achieved by a single predominating species and strain. However, in most fermentations, a mixed population of several bacterial
species and strains, or even bacteria and yeasts or bacteria and molds, is involved. When a fermentation process involves a mixed population, the members should not be antagonistic toward
one another; rather, they should preferably be synergistic. Maximum growth of a desirable
microorganism and optimum fermentation rates are dependent on environmental parameters,
such as nutrients, temperature of incubation, oxidation-reduction potential, and pH. In the
fermentation process, if the different species in a mixed population need different environmental conditions (e.g., temperature of growth), a compromise is made to facilitate growth of all
the species at a moderate rate. Depending on the raw or starting material and a specific need,
carbohydrates (dextrose in meat fermentation), salts, citrate, and other nutrients are supplemented. In some natural fermentations, several species may be involved for the final desirable
characteristics of the product. However, instead of growing at the same time, they appear in
sequence with the consequence that a particular species predominates at a certain stage during
fermentation. But analyzing the final product to isolate the species involved in fermentation of
such a food does not give the right picture. Instead, samples should be analyzed at intervals to
determine predominant types at different times and to know the sequences in their appearance.
Finally, some minor flora (secondary flora) can be present in a very low level in the raw material
and the final product and might not be detected during regular analysis. However, they may
have important contributions for the desirable characteristics, particularly some unique aroma,
of the product.
General Method of Production
The production of a fermented product has two related yet separate aspects, one involving the
importance of metabolic activities of microorganisms during fermentation and storage of the
product and the other involving the parameters used during processing and storage of the product.
In this chapter, the microbiological aspects are presented. Processing aspects are generally taught
in a food-processing course and are thus covered only in brief here.
Fermentation involves exposing the raw or starting food materials to conditions that favor
growth and metabolism of specific and desirable microorganisms. As the desirable microorganisms grow, they utilize some nutrients and produce some end products. These end products,
along with the unmetabolized components of the starting materials, constitute the fermented
foods having desirable acceptance qualities, many of which are attributed to the metabolic end
products.
Raw (or Starting) Materials
A large number of raw materials from plant and animal sources are used to produce fermented
foods. These include milk (from cows, buffalo, sheep, goats, and mares), meat (beef, pork, lamb,
goat, and fowl), fish (many types), eggs (chicken and duck), vegetables and vegetable juices,
many fruits and fruit juices, cereal grains, tubers, lentils, beans, and seeds. Some are used in
combination.
Microorganisms Used
Many desirable species and strains of bacteria, yeasts, and molds are associated with fermentation of foods. Depending on a product, fermentation may be achieved by a single predominating species and strain. However, in most fermentations, a mixed population of several bacterial
species and strains, or even bacteria and yeasts or bacteria and molds, is involved. When a fermentation process involves a mixed population, the members should not be antagonistic toward
one another; rather, they should preferably be synergistic. Maximum growth of a desirable
microorganism and optimum fermentation rates are dependent on environmental parameters,
such as nutrients, temperature of incubation, oxidation-reduction potential, and pH. In the
fermentation process, if the different species in a mixed population need different environmental conditions (e.g., temperature of growth), a compromise is made to facilitate growth of all
the species at a moderate rate. Depending on the raw or starting material and a specific need,
carbohydrates (dextrose in meat fermentation), salts, citrate, and other nutrients are supplemented. In some natural fermentations, several species may be involved for the final desirable
characteristics of the product. However, instead of growing at the same time, they appear in
sequence with the consequence that a particular species predominates at a certain stage during
fermentation. But analyzing the final product to isolate the species involved in fermentation of
such a food does not give the right picture. Instead, samples should be analyzed at intervals to
determine predominant types at different times and to know the sequences in their appearance.
Finally, some minor flora (secondary flora) can be present in a very low level in the raw material
and the final product and might not be detected during regular analysis. However, they may
have important contributions for the desirable characteristics, particularly some unique aroma,
of the product.