Microbial Metabolites in Fruits and Vegetables 517
can grow and produce significant amounts skatole, indole, and p-cresol. The con-
tamination of potatoes typically occurs in the field, augmented by tissue damage
and plant stress.
Modified atmosphere packaging (MAP) has been extensively employed to
enhance the shelf life of fresh-cut produce. By modifying the atmosphere to contain
high oxygen and carbon dioxide the activity of aerobes can be retarded. However,
this in turn leads to a different form of spoilage, typically involving yeasts and lactic
acid bacteria. Yeast spoilage in MAP occurs through pectinase activity, gas produc-
tion, off-flavor development, and visible colony formation (Flett, 1992). In vegeta-
bles with a high sugar content (e.g., root crops) lactic acid bacteria can become
established and cause spoilage. A study performed by Jacxsens et al. (2003) provides
a good example of the role of different microbes on the spoilage of MAP vegetables.
With celeriac and grated carrots under modified atmosphere (3% O 2 and 5% CO 2 )
initial spoilage events are associated with degradation of plant cell walls by yeast
and other pectinase-expressing microbes. The autolytic action of endogenous plant
enzymes augments the further breakdown of cell walls, releasing glucose, fructose,
and sucrose. The sugars support the growth of lactic acid bacteria that typically
produce polysaccarides (slime production), CO 2 , and lactic and acetic acid, in addi-
tion to diacetyl.
17.3.3 MICROBIAL METABOLITES AND FRUIT SPOILAGE
The most significant spoilage agent of fruit is fungal activity. Typically, under
conditions that do not favor bacterial growth (e.g., high acidity) fungi can proliferate
with visible deleterious effects. Among the significant fungal diseases, blue mold
caused by Penicillium expansum, gray mold caused by Botrytis cinerea, and Mucor
rot caused by Mucor piriformis are common on fruits.
Molds such as P. italicum and P. digitatum cause spoilage of fruits via the
formation of pigmented rots, in addition to volatiles such as 4-vinylguaiacol. Methyl
esters of acids such as butanoic acid also lead to sulfurous and rancid odors due to
a combination of mold and fruit autolytic pathways. Apples are frequently spoiled
by P. expansum, which causes discoloration and a strong aroma due to the formation
of geosmin.
Aspergillus flavus, Eurotium oxysporum, and Penicillium species have all been
implicated in dried fruit taints caused by formation of 2,4,6-trichloroanisole from
2,4,6-trichlorophenol. The odor of 2,4,6-trichloroanisole can be described as phe-
nolic, iodine, or musty and can be perceived at levels on the order of 20 ppm.
However, there is debate as to whether 2,4,6-trichloroanisole recovered in fruit is
derived from microbial activity, since it has been found in vine fruit devoid of both
microorganisms and the chlorophenol precursor. This doubt was confirmed by the
fact that dried fruit sterilized using either propylene oxide or hydrogen peroxide
produced the highest concentrations of 2,4,6-trichloroanisole. This further underlines
the difficulty of establishing the impact of microbial metabolites on produce quality.
A similar mold metabolite that has the potential to cause a musty taint in packaged
food is 2,4,6-tribromoanisole, derived from the metabolic breakdown of the fungi-
cide 2,4,6-tribromophenol.