96 Produce Degradation: Reaction Pathways and their Prevention
of chain lengths C6 and C9 that is widespread in fruits and vegetables155–157 proceeds
rapidly when plant cells are disrupted in the presence of oxygen. The effects of LOX
on the quality of fresh-cut fruits and vegetables and the effects of the enzyme on
flavor and aroma compounds have been reviewed.158,159
Mechanically stressed fruits and vegetables exhibit visible degeneration of meso-
carp and endocarp during storage.^84 In addition to increases in metabolic rate,
respiration, and gene expression, severalfold increased activity of enzymes such as
polygalacturonase (PG) and polyphenol oxidase (PPO) occurs.^160 Mechanical injury
increases metabolic and pectin esterase activity in cantaloupe melon.^161 Polygalac-
turonase extracts from Phomopsis cucurbitae, a latent infection pathogen, produced
little maceration in netted muskmelon tissue until fruits were 50 days postanthesis
(10 days postharvest). In contrast, PGs from Rhizopus stolonifer, a wound pathogen,
produced high levels of maceration at all stages of fruit development from 20 to 50
days postanthesis.^162 Pectic enzymes have received considerable attention regarding
their involvement in the softening of cell wall components. Firmness retention is an
important quality parameter in fresh-cut produce.163–166 One of the most obvious
changes that occur during the softening of fruits is the progressive solubilization
and depolymerization of pectic substances.167–169 The effect of pectic enzymes on
freshness and shelf life of fresh-cut fruits and vegetables was also recently
reviewed.^159
Most changes in color during the storage of fresh-cut products are enzyme-
mediated. The most common are related to PPO activity located in the chloroplast
thylakoid membranes.170,171 PPO action often results in the formation of highly
reactive quinones that can then react with amino and sulfhydryl groups of proteins
and enzymes as well as with other substrates such as chlorogenic acid derivatives
and flavonoids. These secondary reactions may bring about changes in physical,
chemical, and nutritional characteristics and may also affect the sensory properties
of fruits and vegetables. Quinones contribute to the formation of brown pigments
by participating in polymerization and condensation reactions with proteins.146,172–174
It has been suggested that expression of closely related heterologous genes can be
used to prevent enzymatic browning in a wide variety of food crops without the
application of various food additives. Coetzer et al.^175 demonstrated that PPO activity
of Russet Burbank potatoes could be inhibited by sense and antisense PPO RNAs
expressed from a tomato PPO cDNA under the control of the 35S promoter from
the cauliflower mosaic virus. Their results indicate that expression of tomato PPO
RNA in sense or antisense orientation inhibits PPO activity and enzymatic browning
in the potato cultivar. Tissue printing indicates that the PPO enzyme is distributed
throughout the potato tuber. Following impact injury, both tissue printing and
quantitative electron microscopy revealed that there was no increase in the level
of the enzyme, although there was subcellular redistribution of PPO. This redis-
tribution was first apparent at 12 h after impact, as determined by the use of
confocal immunolocation, and coincided with loss of membrane integrity.^176
Impact-induced blackspots in potato (Solanum tuberosum) tubers are still unex-
plained at the cellular level. Blackspot bruise is a physiological disorder of potato
tubers resulting from mechanical damage to tissues during handling. It is well known