380 Produce Degradation: Reaction Pathways and their Prevention
12.1 INTRODUCTION
As soon as vegetables and fruits are harvested physiological changes occur, and
some of these lead to a loss in quality. Respiratory activity involving the breakdown
of carbohydrates by the plant enzymes continues and the changes induced, whether
advantageous or deleterious, are markedly influenced by the maturity of the plant
when harvested; thus, plants can usually be stored for lengthy periods with little change
in quality if they are harvested at the right time. Many fleshy fruits such as bananas are
harvested before maturation and ripening continues thereafter, but citrus fruits only
ripen satisfactorily on the tree [1]. However, although spoilage can be induced by
autolytic enzymes, it is caused more usually by the activities of microorganisms.
The number of microorganisms on vegetables received from the field is highly
variable. All green plants posses microflora, which normally subsist on the surface
of vegetables. Soil, water, air, insects, and animals all contribute to the microflora
of vegetables. The relative importance of these sources differs with the structural
entity of the plant (e.g., leaves have greater exposure to air, whereas root crops have
greater exposure to soil). High aerobic plate counts occur in vegetables that are in
contact with soil, such as garlic, and on vegetables grown above the ground, such
as spinach. The activities of humans have important effects. For example, the use
of pesticides to control insects often limits the spread of microorganisms. Similarly,
cultivation, either by hand or mechanically, introduces or distributes microorganisms
into ecological niches from which previously they were absent. Finally, the intro-
duction of human or other animal waste material into the water or soil has an obvious
impact on the flora of vegetables [2].
The microbial contamination of raw vegetables usually occurs on exposed sur-
faces, while the internal tissues remain essentially free of microorganisms. The
means by which microorganisms penetrate the tissues has not been established
clearly, but their presence usually is not deleterious to the growing plant. An equi-
librium of coexistence exists, although it can be broken, and spoilage can develop
under certain circumstances. However, harvesting often injures produce, and plant
tissues can rupture. As a result, nutrients that enhance microbial growth are released
and create an entrance to internal tissues. Consequently, the harvested vegetable is
more prone to support microbial growth than is the growing plant. [3]
Most of the microorganisms present on fresh vegetables are saprophytes such
as coryneforms, lactic acid bacteria, spore-formers, coliforms, micrococci, and
pseudomonads derived from soil, water, and air. Fungi including Aureobasidium,
Fusarium, and Alternaria often are present but in relatively lower numbers than
bacteria. To date, the strictly anaerobic organisms that also may be present have not
been well characterized except for certain heat-resistant spore-formers important in
spoilage of canned vegetables.
The predominant microorganisms on healthy, raw vegetables are usually bacteria,
although significant numbers of molds and yeasts may be present. Spoilage organisms
may be on the plant in the field or may be introduced during harvesting and transport.
Most spoilage is caused by fungi, chiefly members of the genera Penicillium, Sclerotinia,
Botrytis, and Rhizupus. It has also been reported that Sclerotinia sclerotium, the cause