Microbial Spoilage Of Fresh Fruits

a. General. A large and varied population of microorganisms, including the spores of many types of fungi, contaminates the surface of fruits during the growing season. Relatively few of the fungi are capable of attacking fruit before harvest. The ripening process increases the susceptibility of fruit to invasion. Fruits have a low pH that inhibits most bacteria. The acid-tolerant bacteria are mainly gram-positive Lactobacillus and Leuconostoc. Fruits are usually spoiled by acid-tolerant yeasts and molds.

b. Soft Rot and Blue Mold Rot. Penicillium species are important spoilage organisms of most fruits.

(1) Soft rot. P. digitatum and P. italicum cause soft rot in citrus fruit.

(2) Blue mold rot. P. expansum causes blue mold rot of deciduous fruit. Blue mold rot consists of soft, watery, tan to light brown areas that are readily gouged out of the flesh of the fruit. The tissue has a moldy or musty odor and flavor. There are typical bluish-green spores on the fruit. It is the most important storage decay of apples.

c. Black Rot. Black rot in apples, citrus fruits, bananas, and pineapples is due to species of Alternaria as well as other molds. The tissue becomes soft and watery.

d. Brown Rot. The brown rot of fruits has unsunken, decayed areas turning dark brown to black in the center. The mold spore masses are yellowish gray. The skin clings tightly to the center of the old lesion.

e. Anthracnose. Anthracnose is a defect with scattered black or dark brown sunken spots covering firm decayed tissue. With moist conditions, there are pink spore masses on the spots. Tomatoes are especially susceptible to this defect.

f. Fungal Rot and Gray Mold Rot. The loss of berries occurs with dehydration, discoloration, and overripe, mushy, damaged, moldy, or spoiled fruit. Common deteriorative conditions are fungal rot and gray mold rot. Berries are spoiled primarily by Botrytis and Mucor.

g. Fermentation Defect. The species of several genera of yeasts (Saccharomyces, Pichia, Torulopsis, and/or Candida) can cause a fermentation defect in fruits.

h. Summary. A summary of microbial conditions of fruits is found in figure 5-1.

FRUIT

CONDITION

Apples

Fermentation

Apricots

Gray mold rot

Bananas

Storage rot, Black rot, Crown rot

Berries

Fungal rot, Gray mold rot

Citrus fruit

Soft rot, Black rot

Fresh fruit, in general

Blue mold rot, Black mold rot, Souring, bitter flavor, Soft rot

Peaches

Brown rot

Muskmelons

Alternaria rot, Bacterial spot, Bacterial soft rot, Blue mold rot

Tomatoes

Blossom end rot, Bacterial spot, Late blight rot, Anthracnose, Soil rot, Sour rot

Watermelons

Anthracnose, Black rot, Stem-end rot

Figure 5-1. Microbial conditions of fruits.

Figure 5-1. Microbial conditions of fruits.

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