Fundamental Food Microbiology, Third Edition - Fuad Fathir

BIOCHEMISTRY OF SOME BENEFICIAL TRAITS 145
G. Hexose Fermentation by Bifidobacterium
Bifidobacterium species metabolize hexoses to produce lactate and acetate by the
fructose–phosphate shunt or bifidus pathway. 1–3 For every two molecules of hexoses,
two molecules of lactate and three molecules of acetate are produced without generation
of any CO 2 (Figure 11.4). From two molecules of fructose-6-phosphate,
generated from two molecules of glucose, one molecule is converted to produce one
4C erythrose-4-phosphate and one acetyl-phosphate (which is then converted to
acetate). Another molecule of fructose-6-phosphate combines with erythrose-4-phosphate
to generate two molecules of the 5C xylulose-5-phosphate through several
intermediate steps. Xylulose-5-phosphates are then metabolized to produce lactates
and acetates by the method described in the nonoxidizing part of heterolactic fermentation
(also in pentose fermentation; Figure 11.3).
H. Diacetyl Production from Citrate
Diacetyl, a 4C compound, is important in many fermented dairy products for its
pleasing aroma or flavor (butter flavor). It is also used separately in many foods to
impart a butter flavor. Many lactic acid bacteria can produce it in small amounts
from pyruvate, which is generated from carbohydrate metabolism (Chapter 7). 8
However, Lac. lactis ssp. lactis biovar diacetylactis and Leuconostoc species can
produce large amounts of diacetyl from citrate (Figure 11.5). 1,3 Citrate, a 6C compound,
is transported from outside into the cells by the citrate-permease system. It
is then metabolized through pyruvate to acetaldehyde-TPP (thiamin pyrophosphate).
It then combines with pyruvate to form a-acetolactate, which is converted to diacetyl
with the generation of CO 2. Under a reduced condition, diacetyl can be converted
to acetoin, with a loss of desirable flavor. A producer strain can also be genetically
modified to produce excess diacetyl (metabolic engineering). 9
I. Propionic Acid Production by Propionibacterium
The desirable flavor of some cheeses (such as Swiss) in which dairy Propionibacterium
is used as one of the starter cultures is from propionic acid. 1,10 Propionibacterium
generates pyruvate from hexoses through the EMP pathway, and the pyruvate
is used to generate propionic acid (Figure 11.6). Pyruvate and methylmalonyl ~ CoA
produce propionyl ~ CoA and oxalacetate. Propionyl ~ CoA is then converted to
propionate. Oxalacetate is recycled to generate methylmalonyl ~ CoA through succinyl
~ CoA. Propionibacterium also generates acetate and CO 2 (CO 2 contributes
to eye formation in Swiss cheese) from pyruvate.
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IV. TRANSPORT AND METABOLISM OF PROTEINACEOUS
COMPOUNDS AND AMINO ACIDS
Many lactic acid bacteria used in food fermentation have active transport systems for
transporting amino acids and small peptides (ca. 8 to 10 amino acids long). 11,12 Large