332 Produce Degradation: Reaction Pathways and their Prevention
that could result from using too much of a single organic acid. Citric acid is often
used in dipping solutions for its antioxidative and pH-lowering effect in the treatment
of sliced or peeled fruits and vegetables^ (Soliva and Martin-Belloso, 2003). Inhibition
of microorganisms by organic acids depends upon several factors, including reduc-
tion in pH, the ratio of undissociated and dissociated species of the acid, chain
length, cell physiology, and metabolism. Two or more of these elements may interact
to create either a microstatic or a microbicidal effect.^ Microorganisms differ in their
sensitivity to various acids. For example, lactic acid bacteria are not only tolerant
to weak lipophilic acids, but also produce them as metabolites. Acetic acid also has
a microbicidal effect against lactobacilli but only a bacteristatic effect against bacilli
(Doores, 1993). Acidification also increases the sensitivity of microorganism to heat
treatment and to other preservation treatments. Various synergistic effects are known
to occur among organic acids. Examples are the effects of mixtures of ascorbic and
sorbic acids, and with lactic acid^ (Doores, 1993; Bibek, 1996).
Some plants naturally contain compounds with antimicrobial properties. Essen-
tial oils of spices (vanillin, cinnamol, eugenol, carvacol, thymol, and essential oils
of oregano, thyme, and rosemary); humulone and lupulone in hops; allicin and alliin
in garlic, onion, and horseradish; allyl isothiocyanate in mustard and other plants;
and tomatidine in unripe tomatoes and other substances are known to inhibit the
growth of microorganisms. The use of natural preservatives is usually limited by the
flavor of the compounds or extracts and also by their potential toxicity^ (Bibek, 1996;
Cherry, 1999; Velísek,ˇ 1999). An exception can be the application of hexanal, which
is naturally produced by enzymatic oxidation of fatty acids after peeling or cutting
in damaged cells. When used in the package atmosphere of apple slices stored at
4°C, the treatment caused strong inhibition of mesophilic bacteria, molds, and yeasts
and prolongation of the lag phase of psychrotrophic bacteria (Lanciotti et al., 1999).
The addition of antibiotics (natamycin, tylosin, and bacteriocins — antibiotics
of Gram-positive bacteria, which include nisin, pediocin, sakacin, and subtalin) is
not very common in preservation of fruit products. They may be used as one of the
additives for the preservation of canned low-acid vegetables to control anaerobic
spore-forming microorganisms (tylosin). Bacteriocins act against Gram-positive bac-
teria (streptococci, bacilli, clostridia, and other anaerobic spore-forming bacteria)
(Hurst and Hoover, 1993).
The new trend in application of preservatives is antimicrobial packaging, which
can take several forms, including:
- Addition of sachets or pads containing volatile antimicrobial agents into
packages. - Incorporation of volatile and nonvolatile antimicrobial agents directly into
polymers. - Coating or adsorbing antimicrobials onto polymer surfaces.
- Immobilization of antimicrobials to polymers by ion or covalent linkages.
- Use of polymers that are inherently antimicrobial.
Active packaging with various types of plastic films is used for modified atmosphere
packaging of fruits and vegetables, as previously discussed, but antimicrobial packaging