Fundamental Food Microbiology, Third Edition - Fuad Fathir

90 FUNDAMENTAL FOOD MICROBIOLOGY
donor (oxidized) and the other as the hydrogen acceptor (reduced). Alanine, leucine,
and valine can be oxidized, whereas glycine, proline, and arginine can be reduced
by this type of reaction (Stickland reaction). The products in this reaction are fatty
acids, NH 3, and CO 2.
The products of microbial degradation of amino acids vary greatly with the types
of microorganisms and amino acids and the redox potential of the food. Some of
the products are keto acids, fatty acids, H 2, CO 2, NH 3, H 2S, and amines. Metabolic
products of several amino acids are of special significance in food because many of
them are associated with spoilage (foul smell) and health hazards. They include
indole and skatole from tryptophan; putrescine and cadaverine from lysine and
arginine; histamine from histidine; tyramine from tyrosine; and sulfur-containing
compounds (H 2S, mercaptans, sulfides) from cysteine and methionine. Some of these
sulfur compounds, as well as proteolytic products of proteinases and peptidases
(both extra- and endocellular) of starter-culture microorganisms are important for
desirable and undesirable (bitter) flavor and texture in several cheeses. The breakdown
of threonine to acetaldehyde by Lactobacillus acidophilus is used to produce
the desirable flavor in acidophilus yogurt. Indole production from tryptophan is used
to differentiate Esc. coli from other coliforms. Also, the amino acid metabolism
profile is used in species identification of unknown bacterial isolates. In addition to
degradation (catabolism) of proteinaceous compounds of foods, the synthesis (anabolism)
of several proteins by some foodborne pathogens while growing in foods is
important because of the ability to produce proteins that are toxins. They include
thermostable toxins of Staphylococcus aureus, thermolabile toxins of Clostridium
botulinum, and toxins produced by some bacteria associated with foodborne infections
(such as Shiga toxin). The ability of some microbial species to synthesize
essential amino acids (such as L-lysine), antibacterial peptides (such as nisin and
pediocin), and enzymes (such as amylases and proteinases) in relatively large
amounts has been used for beneficial purposes in foods.
V. METABOLISM OF FOOD LIPIDS
The main lipids in food are mono-, di-, and triglycerides; free saturated and unsaturated
fatty acids; phospholipids; sterols; and waxes, with the glycerides being the
major lipids. Microorganisms have low preference for metabolizing lipids. Being
hydrophobic, lipids are difficult to degrade when present in a large mass. In emulsion,
they can be metabolized by the microorganisms at the oil–water interphase. Glycerides
are hydrolyzed by extracellular lipases to release glycerol and fatty acids. The
fatty acids then can be transported inside the cells and metabolized by b-oxidation
to initially generate acetyl ~ CoA units before being utilized further. Fatty acids, if
produced at a rapid rate, accumulate in the food. Unsaturated fatty acids can be
oxidized by microbial oxidases to initially produce hydroperoxides and then carbonyl
compounds (aldehydes and ketones).
Some of the microorganisms that are important in food and can release lipases
(hydrolytic enzymes) are found in the following genera: Alcaligenes, Enterobacter,
Flavobacterium, Micrococcus, Pseudomonas, Serratia, Staphylococcus, Aspergillus,