446 Produce Degradation: Reaction Pathways and their Prevention
have been shown to enter leaves of plants via the stomata and into certain fruits via
the calyx or stem scar (Samish and Etinger-Tulczynska, 1963; Samish et al., 1963;
Zhuang et al., 1995). Seo and Frank (1999) demonstrated that E. coli O157:H7
entered lettuce leaves through the stomata during dipping the lettuce in a cell
suspension of this pathogen. Burnett et al. (2000) reported that E. coli O157:H7
were sporadically present in lenticels of intact mature apples immersed in suspen-
sions of this organism. The development of lenticels on mature apples follows
formation of cutin within damaged trichomes or stomata, whose guard cells are
permanently opened as a result of growth-induced stretching. Stomata facilitate gas
exchange in young fruits; however, as lenticels form and loosely packed wax platelets
develop in mature fruits, the lenticels usually close and become impervious to liquids
and gases (Clements, 1935). Interestingly, about 5% of the lenticels of Red Delicious
apples remain open (Clements, 1935).
14.4.1.2 Microbial Damage
Microbial damage to the leaf cuticle will facilitate the entry of bacteria into subsur-
face areas of the leaf tissue. The mechanisms of bacterial entry may involve physical
force or breakdown of the cuticle. These actions are believed to be associated with
fungal penetration of leaves (Kolattukudy et al., 1995). Many filamentous fungi such
as Aspergillus, Fusarium, Penicillium, and Trichoderma can produce a broad spec-
trum of enzymes including amylases, cellulases, and pectinases. Also, some yeasts
can produce pectinases (Blanco et al., 1999). Pectinases are well recognized for their
involvement in softening of fruits and vegetables and the properties of these enzymes
have been comprehensively reviewed (Bateman and Miller, 1996). The involvement
of pectinases in plant–fungi interactions was proposed by De Bary in 1886 (Lang
and Dornenburg, 2000). Breakdown of plant tissue by fungal pectinases and cellu-
lases increases the potential for bacterial penetration into deeper tissues of fruits and
vegetables.
Apart from certain fungal enzymes that degrade fresh produce, some bacterial
enzymes also initiate damage. Bacterial enzymes (cutinases) that degrade the leaf
cuticle have been found in Pseudomonas aeruginosa, Pseudomonas putida, and
Pseudomonas syringae (Sebastian et al., 1987; Fett et al., 1992; Liu and Kolattukudy,
1998). Degradation of seemingly intact fruits and vegetables by certain bacteria such
as Erwinia carotovora causes soft rot. Soft rot lesions in fruits and vegetables can
facilitate entry of other bacteria, including human enteric pathogens such as Salmo-
nella, which may be unable to degrade plant tissue. Vegetables such as tomatoes,
peppers, celery, and potatoes are very susceptible to bacterial soft rot (Lund and
Kelmann, 1977; Bartz and Kelman, 1986; Robbs et al., 1996). Interestingly, Salmo-
nella spp. were isolated from 18 to 20% of vegetables including lettuce, tomatoes,
carrots, sprouts, beans, cantaloupe, and broccoli that had signs of soft rot. This
occurrence of Salmonella spp. was approximately twice that (9 to 10%) which was
found on intact, unblemished samples of those same vegetables (Wells and Butter-
field, 1997). The incidence of bacterial soft rot increased following certain packing-
house procedures including fluming and use of dump tanks (Lund and Kelmann,
1977; Segall et al., 1977).