Microbial Ecology of Spoilage 381
of pink rot of celery, produces photoxins, which cause a blistering cutaneous reaction
in field workers who handle the produce [4].
Bacteria are responsible for approximately one-third of the total microbial spoil-
age loss of vegetables. Such spoilage may be due to bacteria that cause soft rots and
other rots, spots, blights, and wilts. Soft rots, occurring during transport and storage,
are usually caused by coliforms, Erwinia carotovora, and certain pseudomonads,
such as Pseudomonas fluorescens (marginalis). Organisms causing rots other than
soft rot include corynebacteria, xanthomonads, and pseudomonads. Often infection
can occur in the field, thereby permitting invasion of the plant tissue by soft-rot
organisms as a result of trauma induced during subsequent transport and storage [5].
Soft rot of potatoes by Clostridium has also been reported.
12.2 NATURE OF MICROORGANISMS
ON FRUITS AND VEGETABLES12.2.1 SAPROPHYTIC MICROORGANISMS
The microbial species that prevail on fruits and vegetables are commonly found on
plants in the field or after harvest and probably originated from the epiphytic
microflora of the raw materials. P. fluorescens, E. herbicola, and E. agglomerans
are major components of the epiphytic microflora of many vegetables [6–10]. An
average Leuconostoc spp. population of 2.5 × 104 CFU/g was found on plants by
Mundt et al. [11], although other studies indicate lower frequencies [12] (1 to 33%
in various crops). Yeast species have been identified from raw fruits and vegetables
[6, 13–15]. Similarly, pectinolytic P. fluorescens, pectinolytic Xanthomonas spp.,
Cytophaga spp., and Flavobacterium spp. have been isolated from various unproc-
essed vegetables sampled in retail outlets [16–18]. More generally, pectinolytic
fluorescent pseudomonds are well-known agents of soft-rot diseases of leafy vege-
tables [19–25] and can account for an important fraction of epiphytic microflora:
104 pectinolytic Pseudomonas cells/cm–2 have been counted on white cabbage leaves
[6]. Counts as high as 10^9 CFU/g^1 have been noted in fresh soil [26], although lower
values have also been reported [27]. It is significant that the coliform population on
vegetables was reduced by disinfection of irrigation water, whereas the number of
pectinolytic bacteria was unaffected [28]. The presence of pseudomonads on vege-
tables presumably does not depend on external contamination, because they are
likely endemic.
In contrast, Erwinia spp., a major cause of soft-rot diseases of vegetables [29],
have only been sporadically isolated from minimally processed fresh vegetables
(MPF) (E. carotovora was found in only few samples by Brocklehurst et al. [30]).
Similarly, P. cichorii, an important disease agent of lettuce and chicory [31], has not
been reported in MPF vegetables. Processing, sorting, and trimming of raw material
to remove all decayed or diseased parts could explain the low frequency of such
plant pathogens in processed products. In addition, soft-rot Erwinia are rarely present
in high numbers in the environment [29], are not isolated from soil samples by direct
plating on selective media [26], and are usually recovered only after enrichment [32].