54 Part I: Principles
cultures that can also serve as protective cultures
and inhibit the growth of such harmful microorgan-
isms. The enzyme lysozyme (EC 3.2.1.17) can be an
effective agent for the inhibition of Listeriain food.
Van de Guchte et al. (1992) integrated the gene
responsible for lysozyme formation in a strain of the
bacterium Lactococcus lactis. After genetic trans-
formation, this bacterial strain was able to express
and secrete lysozyme at high levels. The researchers
cloned lysozyme-encoding genes from E. colibacte-
riophages T4 and lambda in wide-host-range vectors
and expressed in L. lactis. Biologically active
lysozyme was produced and secreted by the trans-
genic L. lactisstrains, suggesting that these bacteria
can be used as both a starter and a protective culture
(Van de Guchte et al. 1992).
NATURALSWEETENERPRODUCED
BYMICROORGANISMS
Techniques to enhance flavor in food have been
known for a long time, but only recently has it been
recognized that microorganisms can also be used in
flavor production and enhancement. Today, many of
the techniques for flavoring food and beverages
make use of synthetic chemicals (Vanderhaegen et
al. 2003). With increased public concern about the
danger of using synthetic chemicals, flavors pro-
duced by biological methods, also called bioflavors,
are becoming more popular with consumers (Arm-
strong and Yamazaki 1986, Cheetham 1993). The fla-
vor and fragrance industry is estimated worldwide at
$10 billion per year; and although thousands of natu-
ral volatile and synthetic fragrances are known, only
a few hundred are regularly used and manufactured
on an industrial scale (Somogyi 1996). There are sev-
eral methods for the production of bioflavors includ-
ing (1) product extraction from plant materials and
(2) the use of specific bioengineered microorganisms
for their biosynthesis. Biotechnological production
of bioflavors using microorganisms has certain
advantages such as large-scale production with low
cost, nondependence on plant material, and preserva-
tion of natural resources (Krings and Berger 1998).Figure 3.12.Gene disruption by homologous recombination.