Fundamental Food Microbiology, Third Edition - Fuad Fathir

238 FUNDAMENTAL FOOD MICROBIOLOGY
E. Production and Purification
Bacteriocin production in lactic acid bacteria is directly related to the cell mass.
Generally, the parameters that help generate more cell mass produce more bacteriocin
molecules. The parameters include nutritional composition, initial and terminal
pH and O–R potential of a broth, and incubation temperature and time. The
species and the strains of a species growing under similar conditions differ greatly
in the amount of bacteriocin production. In general, production of nisin A or
pediocin AcH is much higher than leucocin or sakacin A. A nutritionally rich
medium is always better, and, for many bacteriocins, growing the strains in a
fermentor under a controlled terminal pH produces more bacteriocin. For example,
nisin A production is much higher at pH 6.0, but pediocin AcH production is much
higher at a lower terminal pH of 3.6. It is important to determine the optimum
parameters for growth and bacteriocin production for an unknown strain to obtain
high yield of bacteriocin.
Bacteriocin molecules following secretion by a producer strain remain either
adsorbed on the cell surface or free in the medium, depending on the pH of the
environment. More molecules remain free in the environment at pH 1.5 to 2.0,
whereas at pH 6.0 to 7.0 they remain bound to the cell surface. Based on this pHdependent
adsorbtion–desorbtion bacteriocin on the cell surface of a producer strain,
an effective and easy method has been developed to purify bacteriocin molecules in
large amounts. A second method of purification that involves precipitation of the
molecules by ammonium sulfate precipitation followed by stepwise gel filtration has
also been used. This gives a highly purified preparation, but in small amounts.
F. Applications 8–10
Bacteriocins of food-grade lactic acid bacteria are considered safe food biopreservatives
and have the potential to use them to kill sensitive Gram-positive food
spoilage and foodborne pathogenic bacteria. In foods that can contain injured Gramnegative
bacteria, bacteriocins can also be effectively used to kill them. They are
more effective when used in minimally heat-processed foods in suitable combinations
of two or more (e.g., nisin and pediocin together). Table 16.6 presents the
effectiveness of a preparation containing nisin and pediocin against spoilage pathogenic
and spoilage bacteria in processed meat products during refrigerated storage.
The results show that the bacteriocin preparation effectively reduced the Grampositive
spoilage (Leu. mesenteroides) and pathogenic (Lis. monocytogenes) bacteria
as well as Gram-negative pathogens (Salmonella and Esc. coli O15:H7) during 6
weeks of storage at 4�C.
There are many other food products in which bacteriocins of lactic acid bacteria
can be used effectively as preservatives (Table 16.7). Bacteriocins can also have a
topical therapeutic use. However, before they can be used, research showing their
effectiveness and safety has to be conducted. Finally, approval by regulatory agencies
will be necessary before they can be used as food biopreservatives and topical
therapeutic agents.