302 Produce Degradation: Reaction Pathways and their Prevention
diffuse to the intercellular spaces. This causes loss of turgor and softening of the
tissues^ (Poovaiah, 1986; Bolin and Huxsoll, 1989; King and Bolin, 1989). In pro-
cessed fruit and vegetables softening is also caused by hydrolytic changes of mac-
romolecules under acidic conditions, especially during heating.
10.1.4 MICROBIOLOGICAL CHANGES
Fresh fruit and vegetables contain microorganisms coming from soil, water, air, and
other environmental sources. These may also include some pathogens. Vegetables
are fairly rich in carbohydrates (about 5% or more), low in proteins (about 1 to 2%),
and, with the exception of tomatoes, have high pH. Fruits are richer in carbohydrates
(generally 10% and more) and very low in proteins (usually less than 1%); the
majority of fruits have a pH value lower than 4.0. Any damage reduces the natural
protective mechanisms of plant tissues and allows subsequent microbial spoilage.
Different types of molds start the spoilage. High water activity as well as yeasts and
bacteria also favor spoilage. Spoilage of fruits and vegetables is usually caused by
various types of molds from the genera Penicillium, Phytophtora, Alternaria, Bot-
rytis, Rhyzopus, Aspergillus, and others. Yeasts from the genera Saccharomyces,
Candida, Torulopsis, and Hansenula have been associated with fermentation of some
fruits such as apples, strawberries, citrus fruits, and dates. Bacterial spoilage asso-
ciated with the souring of berries and figs has been attributed to the growth of lactic
acid bacteria. Low-acid vegetables and fruits (melon, watermelon, papaya, and
avocado) are often spoiled by the bacterial genera: Pseudomonas, Erwinia, Bacillus,
and Clostridium and others, including other pathogens. The ability of some patho-
genic bacteria such as Salmonella and Shigella to grow on sliced apples, papaya,
watermelon, cantaloupe, and honeydew, especially at ambient temperature, was
proven by several authors (Golden et al., 1993; Leverentz et al., 2001). Microbial
spoilage of fresh fruits and vegetables is generally described by the common term
rot, which is accompanied by changes in appearance, such as with black rot, grey
rot, pink rot, soft rot, and stem-end rot. In addition to changes of color, microbial
rot causes loss of texture and formation of off-flavors. In the case of toxinogenic
molds and bacteria (some species of the genera Bacillus and Clostridium), mycotox-
ins or bacterial toxins could be produced in the product. In fruits with acidic pH,
sporulating bacteria cannot germinate. If such products are to be sterilized, heat
treatment below 100°C is sufficient, because there is no need to inactivate the
thermoresistant bacterial spores. In 1984 a new type of spoilage bacterium, Alicy-
clobacillus acidoterrestris, in aseptically packaged apple juice was reported (Cerny
et al., 1984). Due to its heat resistance (some of its published characteristics are:
D-value at 90°C is about 23 min in apple juice of pH 3.5 and soluble solids 11.4°Brix,
z = 7.7°C) (Splittstoesser et al., 1994) this bacterium should be considered when
products that could be contaminated with this microorganism are to be pasteurized
(Silva and Gibbs, 2001).
Microbial changes of processed fruit and vegetables (pastes, jams, marmalades,
juices, etc.) depend on the properties of the product (i.e., on the conditions of growth
of spoilage microorganisms present). The important factors affecting the safety and
shelf life of such products are: