Fundamental Food Microbiology, Third Edition - Fuad Fathir

138 FUNDAMENTAL FOOD MICROBIOLOGY
bioprocessed or fermented products and are considered desirable by consumers.
Some of the by-products of fermentation can also be purified and used as food
additives. The production of several of these by-products by some desirable microorganisms
is discussed in this chapter. (See also Chapter 7.)
Before describing the metabolic pathways used by these microbes, it will be
helpful to review the components of foods (substrates) used in fermentation. Before
the substrates are metabolized inside microbial cells, they have to be transported
from the outside environment. It will be beneficial to recognize, in brief, the cellular
components involved in the transport of these substrates.
The important substrates available in the starting materials of fermentation
include several carbohydrates, proteinaceous and nonprotein nitrogenous (NPN)
compounds, and lipids. The important fermentable carbohydrates in foods are starch,
glycogen (in meat), lactose (in dairy products), sucrose, maltose (from breakdown
of starch), glucose, fructose, and pentoses (from plant sources). The proteinaceous
and NPN components include mainly large proteins (both structural and functional),
peptides of different sizes, and amino acids. Lipids can include triglycerides, phospholipids,
fatty acids, and sterols. Microorganisms differ greatly in their ability to
transport these components from outside and metabolize them inside the cells.
II. MECHANISMS OF TRANSPORT OF NUTRIENTS
Nutrient molecules have to pass through the cell barriers — the cell wall and the
cell membrane. However, in most Gram-positive lactic acid bacteria, the main barrier
is the cytoplasmic membrane. The cytoplasmic membrane is made up of two layers
of lipids in which protein molecules are embedded, some of which span the lipid
bilayer from the cytoplasmic side to the cell wall side (Figure 11.1 and Figure 2.2).
Many of them are transport proteins involved in carrying nutrient molecules from
the outside into the cell (also removing many by-products from the cell into the
environment). 1,2
In general, small molecules, such as mono- and disaccharides, amino acids, and
small peptides (up to 8 to 10 amino acids), are transported almost unchanged inside
the cell by specific transport systems, either singly or in groups. Fatty acids (either
free or hydrolyzed from glycerides) can dissolve and diffuse through the lipid
bilayers. In contrast, large carbohydrates (polysaccharide such as starch), large
peptides, and proteins (such as casein, albumen) cannot be transported directly inside
the cell. If a cell is capable of producing specific extracellular hydrolyzing enzymes
that are either present on the surface of the cell wall or released into the environment,
then large nutrient molecules can be broken down to small molecules and then
transported by the appropriate transport systems.
Mono- and disaccharides, amino acids, and small peptides are transported
through the membrane by different active transport systems, such as primary transport
systems (e.g., ATP-binding cassette or ABC transporters), secondary transport
systems (e.g., uniport, symport and antiport systems that use proton motive force),
and phosphoenolpyruvate-phosphotransferase (PEP-PTS) systems. 3 A system can be
specific for a type of molecule or for a group of similar molecules (group transfer),