Fundamental Food Microbiology, Third Edition - Fuad Fathir

GENETICS OF SOME BENEFICIAL TRAITS 163
2. L(+)-Lactic Acid Production
Lactic acid bacteria, such as Lab. acidophilus, Lab. plantarum, Lab. fermentum, Lab
rhamnosus, Ped. acidilactici, and Ped. pentosaceus, that produce proportionately
high quantities of lactic acid in the EMP pathway produce a mixture of both L(+)and
D(–)-lactic acid from pyruvate, because they contain both L- and D-lactate
dehydrogenases. Because L(+)-lactic acid is produced in the body, it is preferred to
D(–)-lactic acid as a food additive, and the strains mentioned are not preferred for
commercial production of lactic acid for use in food. Because many of the species
mentioned are also used to produce fermented foods, studies are being conducted
to produce strains of these species that produce only L(+)-lactic acid. These species
have the two genes, ldh L and ldh D, encoding the two LDHs. Strains of some of
the species mentioned have been developed in which ldh D expressing D-LDH has
been inactivated, enabling them to produce only L(+)-lactic acid.
3. Diacetyl Production by Lac. lactis
Diacetyl is associated with a pleasant butter aroma and is used in many nondairy
products to give desirable characteristics. It is normally produced by some lactic
acid bacteria, including the biovar diacetylactis of Lac. lactis species. Other Lac.
lactis species and strains produce, if at all, very little diacetyl. Metabolic engineering
has, however, enabled development of overproducing diacetyl strains of Lac. lactis.
To develop this strain, first the nox gene (NADH oxidase) from a suitable source is
cloned under the control of the nisin-inducible nis A promoter (NICE) in the Lac.
lactis strain. In the presence of a small amount of nisin, the nox gene in the strain
is overexpressed. In the next step, the gene encoding for a-acetolactate decarboxylase
(ALDB, which converts a-acetolactate to acetoin, see Figure 11.5) is inactivated.
This results in the accumulation of a-acetolactate that is produced from pyruvate.
Under the overexpression of the nox gene and with inactivated ALDB, a-acetolactate
is converted to diacetyl in high levels.
4. Alanine Production from Carbohydrates
Amino acids from proteins are generally used for growth and synthesis in lactic acid
bacteria and do not accumulate in the cells or in the environment. However, by
metabolic engineering, a strain of Lac. lactis has been developed that uses carbohydrate
and ammonium sources to produce higher concentrations of alanine. The
alanine dehydrogenase gene from a suitable source was cloned under the control of
the NICE system. When the cloned vector was introduced in a LDH-negative strain
and the strain was grown in the presence of a small amount of nisin and ammonium
supplements, a large amount of carbohydrates was converted to L-alanine. Because
L-alanine has a sweet taste, this strain can be used as a starter culture to produce
fermented products with different, but agreeable, tastes.
5. Production of Mannitol and Other Polyols
Mannitol, a sugar alcohol, is produced by some strains of lactic acid bacteria, such
as Lac. lactis and Lab. plantarum, in small amounts. It is produced by the reduction
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