130 Produce Degradation: Reaction Pathways and their Prevention
5.2.2.4 Effect of Gas on Microorganisms
The literature on the effect of MAs on the microbial spoilage of produce is well
documented. There exists a large variety of research papers, reviews, and book
chapters devoted to the effect of atmospheric compositions, packaging films, and
temperature on the microbial population of produce. The reader can also refer to
Chapters 13 and 16 of this book for detailed information about microbial spoilage
of produce. In the light of scientific research and “commercial conventional wisdom”
[61], it can be concluded that the initial microbial load of produce is a major factor
that affects the quality and shelf life of fresh-cut fruits and vegetables. Bolin et al.
[62] have shown that produce that has a high initial microbial load is more prone
to spoilage than produce with a low microbial load. The relation between microbial
load and spoilage has led scientists to develop predictive shelf-life models that are
based on the growth of microbial populations and their relationship to temperature,
atmospheric composition, etc. [63–66]. Therefore, controlling microbial spoilage of
produce in MAP by MAs can be considered as a supplementary application to the
temperature control and disinfection processes aimed at decreasing initial microbial
loads, such as washing with chlorinated water [67–69], gamma irradiation [70],
electrolyzed water [71], frozen acidic electrolyzed water [72], UV-C [73], and ozone
[74,75]. On the other hand, the studies carried out by Babic et al. [76–78] show that
the correlation between high microbial loads and spoilage is not always valid. In
another words, produce might have good quality ratings at the end of the shelf life
but have high microbial loads. Intactness of the produce, lack of injury, and strict
temperature control can be counted as the main factors that prevent spoilage by
opportunistic pectinolytic microorganisms even at high populations [61]. The fre-
quency of contradictory results in the literature indicates the difficulty of microbial
management in fresh-cut and MAP technologies, which need to be well “tailored”
[79]. The complex interaction and dynamic structure of microorganisms, bacteria,
yeasts, and molds that make up the microbial flora of the produce and their close
relations with produce physiology and morphology force scientists and producers
to think in a multidimensional manner. Microflora are often produce-specific and
have a dominant group due to their competitive nature [80]. Disinfection processes,
selective action of MAs on microorganisms, and temperature play important roles
in the final structure of microflora. Bennik et al. [79] have shown that the psycho-
tropic pathogen Listeria monocytogenes grew better on disinfected than on undis-
infected chicory endive due to the loss of competitive bacteria.
The gases used in MAP, namely O 2 and CO 2 , have different modes of action on
microorganisms. In commercial applications, atmospheres low in O 2 and high in
CO 2 concentrations are preferred, not only due to their beneficial effects on the
produce’s sensorial and physiological quality, but also to their potential in controlling
microbial growth [24]. When it is used at or below 21%, the effect of O 2 on
microorganisms in MAs is limited to the creation of aerobic conditions that inhibit
the growth of strict anaerobic microorganisms. Nevertheless, most bacteria in fruits
are Gram-negative (see Chapter 13), and microorganisms such as pseudomonads
and Leuconostoc spp. will not be affected by the reduction of the O 2 gas concentration
[81]. Bennik et al. [79] showed that Pseudomonas counts on chicory endive at 8°C