Lactic acid bacteria have been reported to stimulate the immune
system and various studies have described their ability to activate macro-
phages and lymphocytes, improve levels of immunoglobulin A (IgA) and
the production of gamma interferon. These effects may contribute to a
host’s resistance to pathogens and to the antitumour activity noted for
LAB, mainly Lactobacillus acidophilus, in some animal models. An
additional or alternative possible mechanism proposed for the antitu-
mour effect is the observed reduction in activity of enzymes such as
b-glucuronidase, azoreductase and nitroreductase in faecal material
when LAB are ingested. These enzymes, produced by components of
the intestinal flora, can convert procarcinogens to carcinogens in the gut
and their decreased activity is probably due to inhibition of the produc-
ing organisms by LAB.
A number of studies have indicated that probiotics may have a role in
preventing and treating atopic diseases such as atopic eczema and
asthma in children.
High levels of serum cholesterol are established as a predisposing
factor for coronary heart disease. It has been suggested that consump-
tion of fermented milks has a hypocholesterolaemic action and some
have suggested a variety of mechanisms by which this can occur. The
evidence is however weak and it has not proved possible to demonstrate
this effect in a number of trials.
An alternative approach to the consumption of large numbers of
probiotic bacteria is to encourage the growth of indigenous bifidobacte-
ria and lactobacilli in the gut through consumption of prebiotics. These
are defined as non-digestible food components that exert a beneficial
effect on the consumer by selectively stimulating the growth and activity
of certain bacteria in the colon. The most common prebiotics are
polymeric forms of fructose such as inulin, a natural component of
foods such as Jerusalem artichokes, leeks, onions and garlic.
9.4.3 The Malo-lactic Fermentation
LAB can decarboxylateL-malic acid to produceL-lactate in a reaction
known as the malo-lactic fermentation (Figure 9.3). This process is
particularly associated with wines, where malic acid can form up to half
the total acid, and its effect is to reduce substantially a wine’s acidity. It is
particularly encouraged in wines from cool regions which tend to have a
naturally high acidity and, although less desirable in wines from warmer
regions, it is often promoted to provide bacteriological stability to the
bottled product. It may also modify and improve the body and flavour of
a wine.
A natural malo-lactic fermentation can be encouraged by refraining
from sulfiting the new wine and leaving it on the yeast lees (sediment) for
322 Fermented and Microbial Foods