Fundamental Food Microbiology, Third Edition - Fuad Fathir

CONTROL BY ANTIMICROBIAL PRESERVATIVES 503
effective. The antimicrobial action is most likely produced by their adverse effect
on the cell membrane and enzymes. Their antimicrobial effectiveness increases in
the presence of sorbate but decreases in foods with high lipids and at low temperature.
K. Chitosan
Chitosan, a polycationic polymer, is obtained by alkaline hydrolysis of chitin from
the shells of Crustaceae. It has many applications in foods, including food preservation,
because of its antimicrobial capability. It causes destabilization of the cell
wall and cell membrane functions and is effective against bacteria, yeasts, and molds.
L. Ethylenediaminetetraacetate (EDTA)
The sodium and calcium salts of EDTA at 100 ppm are approved for use in foods
to chelate trace metals in order to prevent their adverse effect on food quality. At a
low dose (5000 ppm), EDTA appears to have no toxic effect and mostly passes
through the GI tract unabsorbed. By itself, EDTA may not have much antimicrobial
effect, but because of its ability to chelate divalent cations, it can destabilize the
barrier functions of the outer membrane of Gram-negative bacteria and, to some
extent, the cell wall of Gram-positive bacteria. In this way, it enhances antibacterial
action of other chemicals, especially those that are membrane acting, such as surfaceactive
compounds, antioxidants, lysozymes, and bacteriocins. EDTA is also inhibitory
for germination and outgrowth of spores of Clo. botulinum. In the presence of
divalent cations in the food environment (e.g., dairy products), the effectiveness of
EDTA is greatly reduced.
M. Lysozyme
The enzyme lysozyme (a muramidase) is present in large quantities in some foods
such as egg white and shellfish (oysters and clams), as well as in small amounts in
milk and some plant tissues. It hydrolyzes the mucopeptide layer present in the cell
wall of Gram-positive bacteria and in the middle membrane of Gram-negative
bacteria. However, Gram-negative bacteria become sensitive to the lysozyme effect
only after the barrier function of the outer membrane is destabilized by chemical
(e.g., EDTA) and physical (e.g., freezing or heating) stresses. The antimicrobial
effect is manifested by the lysis of cells. Lysozyme is most effective at pH 6.0 to
7.0 and at concentrations of ca. 0.01 to 0.1%. It can be used directly to control
Gram-positive bacteria and with EDTA and other similar compounds to control
Gram-negative bacteria. It has been used in wine (sake) to prevent growth of undesirable
lactic acid bacteria.
N. Monolaurin (Glycerol Monolaurate)
Monolaurin, the ester of lauric acid and glycerol, is one of the more effective
bactericidal agents among the different derivatives of lauric acid tested in foods. Its
effectiveness in deboned meat, chicken sausages, minced fish, and other foods has
been observed against undesirable bacteria, particularly the anaerobes. It also
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