516 Produce Degradation: Reaction Pathways and their Prevention
the need for a high protein content to sustain the fermentation the vegetables used
in this process are typically soybeans and other legumes.
The fermentation of fruits and vegetables is considered beneficial due to the
historical acceptance of such products. However, the same fermentations by wild
yeast and contaminating bacteria can lead to spoilage via production of pigmentation,
secondary alcohols, and other volatile products such as diacetyl.
17.3.2 MICROBIAL METABOLITES AND VEGETABLE SPOILAGE
The spoilage of fresh (unfermented) vegetables and fruits is dependent on the type
of produce and the preservation method applied. As noted previously, the main action
of endogenous lytic plant enzymes plays a significant role in limiting shelf life.
Indeed, the autodegradation of plant tissue typically releases nutrients upon which
spoilage microflora can grow.
Vegetables stored under aerobic conditions are spoiled by the action of Gram-
negative obligate aerobes (e.g., Erwinia and Pseudomonas spp). Spoilage by
Pseudomonas spp. is often caused before significant visual damage has occurred
due the accumulation of volatile substances (off-odors) such as ammonia and hydro-
gen cyanide. These are side products resulting from the metabolism of plant-derived
nutrients and both have a very low detection threshold. Through prolonged storage
the action of microbially derived pectinases, along with endogenous plant enzymes,
causes visible rots.
Actinomycetes has been implicated in the spoilage of both beans and mushrooms
due to the formation of 2-methylisoborneol that results in a musty off-flavor. The
growth of Actinomycetes on vegetables is an obvious source of 2-methylisoborneol
but it can also occur if contaminated processing water is used.
Rotting potatoes have a very distinctive odor which is often likened to that of
pig farms. The odor is attributed to the formation of skatole, indole, and p-cresol.
The common spoilage bacteria associated with potato rot are the aerobes Erwinia
carotovora and E. chrysanthemi. Under anaerobic conditions Clostridium scatologenes
TABLE 17.1
Microbes Recovered in Fermented Fruits and Vegetables
Microbe Reaction
Bacteria
Acetobacter aceti Oxidizes a range of organic compounds including ethanol to acetic acid
Streptococcus faecalis Homofermentative producing predominantly lactic acid
Leuconostoc mesenteroides Heterofermentative producing lactate, acetate, ethanol, and carbon
dioxide from sugar fermentation
Pediococcus cerevisiae Homofermentative
Lactobacillus plantarium Homofermentative
Lactobacillus brevis Heterofermentative
Yeast
Saccharomyces cervisiae Produces ethanol under anaerobic conditions