Application of Lactic Acid Bacteria and Their Bacteriocins for Food Biopreservation - Food Biopreservation - page 17

3.2 Application of LAB Bacteriocins as Part

of Hurdle Technology

The effi cacy of bacteriocins can improve considerably when applied in combination
with other antimicrobials or barriers. As a matter of fact, food preservation most
often relies on application of several barriers that restrict the survival and prolifera-
tion of microorganisms. These include treatments for inactivation of microorgan-
isms in the raw materials, during processing, or in the fi nished product, acidifi cation,
addition of preservatives, and/or modifi cation of atmosphere composition among
others. The concept of hurdle technology (Leistner 2000 ) is based on the combina-
tion of different barriers acting in different ways on microbial cells, so that the cells
have to activate different repair and adaptation mechanisms in order to survive and/
or proliferate under the imposed selective conditions. Under such varied selective
pressure, the cells will die as a consequence of energy exhaustion and failure to
repair cell damages. Since most bacteriocins act on the bacterial cytoplasmic mem-
brane, they interfere with the generation of energy required to repair bacterial cell
damage. At the same time, cell damage or metabolic constraints imposed by other
hurdles reduce the natural defense mechanisms of bacteria, making them suscepti-
ble at low bacteriocin concentrations that would not be lethal to intact cells. Some
of the hurdles may also destabilize bacterial cell structures such as the outer mem-
brane of Gram-negative bacteria. The outer membrane acts as a selective permeabil-
ity barrier that retains bacteriocins and other antimicrobial substances. When this
barrier is destabilized, bacteriocins (as well as other antimicrobials) can diffuse
much better and reach the bacterial cytoplasmic membrane, where they act
specifi cally.
The scientifi c literature is full of examples where bacteriocins have been tested
in different food systems as part of hurdle technology, with several purposes
(Gálvez et al. 2007 , 2008 ): (1) enhancing the effi cacy of bacteriocins as well as
that of treatments; (2) reducing the required bacteriocin concentration; (3) broad-
ening the spectrum of antimicrobial treatments (for example, to Gram-negative
bacteria); (4) improving the inactivation of bacterial endospores; (5) as an addi-
tional barrier to proliferation of sublethally-injured cells as well as intact cells and
endospores surviving treatments; and (6) as an additional barrier against post-
process contamination. Specifi c examples of these applications will be discussed
in the following chapters dealing with application of bacteriocins in different food
systems. Notwithstanding, a summary of combined treatments and their reported
effects is presented in Table 3.1 .