506 Produce Degradation: Reaction Pathways and their Prevention
17.1 INTRODUCTION
The association of microbes with plants has occurred throughout evolution. Plants
have successfully harnessed the benefits derived from microbial associations through
establishing symbiotic relationships with, for example, mycorrihiza. However, activ-
ity of many other microbes can result in disease (preharvest) and spoilage (posthar-
vest). However, it must be noted that not all plant-associated microbes fall into the
broad classification of beneficial or detrimental since many simply coexist.
The microbial ecology of plants is very diverse and has yet to be fully charac-
terized. Nevertheless, a common feature is that those present on plants at harvest
can significantly affect the subsequent rate and type of spoilage that occurs. There-
fore, when describing the role of microbial metabolites on produce quality the
interaction during plant growth needs to be considered. The complexity of microbial
(bacteria, yeasts, and molds) populations present on plants depends upon several
extrinsic and intrinsic factors (reviewed by Lund, 1992 and Lindow and Brandl,
2003). The surface of leaves represents a harsh environment that exposes cells to
extreme variations in temperature, ultraviolet radiation, and relative humidity. The
high degree of competition that exists between microbes is an additional barrier to
survival along with the activity of preformed and inducible plant defenses. In this
respect the enzymes, metabolic products, and other compounds produced by
microbes are central to their survival and growth on vegetables and fruits. Bacteria
(especially Pseudomonas and Erwinia spp.) tend to dominate the phyllosphere
because they have specially adapted systems to modify the environment in a way
that enhances their persistence. The production of fluorescent or pigmented com-
pounds provides a barrier to the high levels of ultraviolet radiation encountered on
the surface of leaves. The hydrophobic waxy cuticle of plants can inhibit the move-
ment and accessibility of nutrients to bacterial cells. However, biosurfactants pro-
duced by the majority of Pseudomonas spp. decrease the water tension, enabling
relatively free movement across the leaf surface to nutrient sources and natural
openings such as stomata. Pseudomonas are also known to release a toxin called
syringomycin that can produce holes in the plant cell membrane, allowing access
to intracellular nutrients without necessarily resulting in disease symptoms. More
prominent spoilage effects are observed due to the release of enzymes such as
pectinases into the environment that degrade plant walls. The more subtle effects of
microbial metabolites are less well understood. Many of the microbial metabolites
that have a negative impact on product sensory quality are derived from secondary
metabolism. In its most basic definition, secondary metabolism can cover any path-
way that is not essential for cellular function or viability. Common examples of
secondary metabolism include volatiles, antibiotics, toxins, and sideophores.
The microbial ecology of vegetables and fruits also plays a role in product
degradation. Here the activity of one type of microbe can release compounds that
are subsequently catabolized by others. As an added layer of complexity, it should
also be noted that many of the metabolic products encountered in microbes can also
occur in plants. This naturally makes differentiating microbial spoilage from that of
autolytic action problematic. Nevertheless, this chapter aims to provide a broad
overview of the impact of microbial metabolites on vegetable and fruit degradation.