5-3.
MICROBIAL SPOILAGE OF FRESH VEGETABLES
a. General. Since vegetables are harvested from or near the soil, they are
subjected to a mixed flora of soil, as well as airborne microorganisms. In general, the
pH of vegetables is near neutrality, so that bacteria, as well as fungi, cause
b. Celery. Celery plants are infected by strains of Sclerotinia sclerotiorum,
which produce a pink rot at the base of the plant. The affected tissue is water soaked
and soft, and the light-brown lesions have a pinkish-brown border.
c. Cole Crops. Brussels sprouts, cabbage, and cauliflower (cole crops of the
crucifer family) are affected by Rhizopus soft rot. This fungus dissolves the infected
tissue, rendering it mushy. The dead, light brown tissue is often covered by coarse,
stringy mycelium that may bear masses of white to black clusters of spores.
d. Lettuce. There are several conditions that affect lettuce.
(1) Bacterial soft rot. Slime aptly describes this condition. It is caused by
various species of bacteria. When the rot starts at or near the midrib, the first signs are
small, yellowish-tan to rusty colored flecks or spots that enlarge and coalesce. As the
disease spreads, the entire head may become a slippery brown mass.
(2) Downy mildew. Initially, the lesions tend to be small and confined to the
upper surface of wrapper leaves. As the areas enlarge, they turn from light green or
yellowish to brown and become soft.
(3) Watery soft rot. This rot occurs on the lower part of heads. The tissue is
water soaked and light or pinkish brown. A white, cottony mold spreads over the
decayed tissue, and the head eventually becomes a watery mass.
e. Onions.
(1) Bacterial soft rot. Onions may be affected by bacterial soft rot. The
entire onion or individual fleshy scales are water soaked and pale yellow to light brown.
Affected scales are soft and filled with putrid-smelling liquid. This liquid may ooze out at
the neck when the upper part of the affected part is squeezed.
(2) Black mold rot. Black mold rot in onions is caused by Aspergillus niger.
Black powdery spore masses adhere to the outer scale or are lodged between the two
outer scales. The lesions will be sunken, discolored, and shriveled.
MD0723
5-4
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