382 Produce Degradation: Reaction Pathways and their Prevention
12.2.2 PECTINOLYTIC MICROORGANISMS
By isolating representative samples of colonies from count plates and testing the
isolates for pectinolysis, 10 to 20% of isolates among the mesophilic bacteria on
shredded lettuce were found to be pectinolytic [14]. A high proportion of Pseudomo-
nas (20 to 60%) was found to be pectinolytic in many samples of shredded carrots
and shredded chicory salads [33,34]. In contrast, Brocklehurst et al. [30] counted
only 10^6 to 10^7 pectinolytic bacteria/g^1 by direct plating on Hankin media, for a total
mesophilic flora count of 10^8 to 10^9 CFU/g. Pectinolytic isolates were usually
identified as Pseudomonas fluorescens of different biovers, P. paucimobilis, P. vir-
idiflava, P. luteola, Xanthomonas maltophila, Flavobacterium spp., Cytophaga spp.,
or Vibrio fluvialis [33–36]. Some pectinolytic fungi (Mucor spp. and Sclerotinia
sclerotiorum) and yeasts (Trichosporon spp.) have also been isolated from shredded
carrots [33,37].
12.2.3 FOODBORNE PATHOGENS
Raw fruits and vegetables may harbor many potential foodborne pathogens [38–41].
Listeria monocytogenes has been isolated in lettuce heads [42] in England (9% of
samples contaminated [43]) and Spain (7.8% of sample contaminated [44]) and in
potatoes and radishes in the U.S. (25.8 and 30.3% of samples contaminated, respec-
tively [45]). Vegetables were found to carry 10^2 to 10^4 CFU/g of cytotoxic and
hemolytic Aeromonas spp. in the U.S. [46]. Fresh fruits and vegetables can also play
a role in the transmission of Salmonella spp. [47]. An investigation in the U.S. after
an outbreak of Salmonella poona found that 1% of melon rinds from fruits imported
from Mexico were contaminated with the bacteria [48]. Similarly, in an outbreak of
Salmonella caused by the consumption of bean sprouts in England, bacteria were
isolated on the bean seeds used by the producer [49]. Vegetables sampled in the
field or in retail outlets were contaminated with Salmonella spp. at frequencies of
7.5% in Spain [50] and 8 to 63% in the Netherlands [51]. In Egypt, vegetables and
salads were found to be contaminated with Shigella spp., Salmonella spp., and
S. aureus (10^3 CFU/g, [52]. C. botulinum spores were isolated from 13.6% of
cabbage samples [53] and from onion skin [54], garlic [55], and many other vege-
tables, as reviewed by Notermans [56]. B. cereus was isolated from vegetable seeds
and from seed sprouts [57].
12.2.4 YEAST AND MOLD
The low pH (< 4.5) of most fruits means that spoilage is caused mainly by fungi.
On the other hand, the pH range of most vegetables varies between 5.0 and 7.0 and
thus spoilage may be caused by either fungi or bacteria, although the former are the
most important group [1]. In terms of their spoilage characteristics fungi are often
somewhat arbitrarily divided into two groups: the plant pathogens, which infect
plants before harvesting, and the saprophytic fungi, which attack commodities after
harvesting. An important property of most spoilage organisms, both fungal and
bacterial, is their ability to secrete pectolytic enzymes, which soften and disintegrate
plant tissues. Thus, the growth of fungi on fruits and vegetables usually results in