Mechanical and Temperature Effects on Shelf Life Stability of Fruits and Vegetables - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 52

TABLE 2.4
Effects of Mechanical Injury on Tomatoes

Stage of
Ripeness Mechanical Injury Effect

Bruising All stage of

ripeness or
maturity

Random and uneven ripening; uneven appearance and quality
Water soaking
Metabolic and morphologic breakdown of bruised tissue 62,63

Impact-Bruising Mature-green High respiratory activity

High ethylene production
Ripening of damage areas
Decrease in titrable acidity 64

Light-pink Increase in off-flavors

Less tomato-like flavor 65

discoloration than fruits harvested at maturity. For cucumbers, mechanical damage
has been known to render pickled products unacceptable for marketing. It causes
physiological breakdown during storage and processing which is typified by slough-
ing off of the placental tissue. 57 Tissue softening in cucumber involves the induction
of cell wall-degrading enzymes, 58 anodic peroxidase enzymes, 59 and changes in sugar
composition of cell-wall polysaccharides. 60 Mechanical injury due to rough handling
of citrus fruits is seen as discoloration in the rind due to oil spotting and oleocellosis.
Following the breaking of oil cells in the peel, the oil is exuded, damaging the
surrounding cells and serving as the entry point for microorganisms. 61 Although not
a serious reason for quality loss among tomatoes, mechanical injury often causes
subsequent deteriorative changes such as shrinkage, attack of microorganisms, and
decay. Effects of mechanical injury during handling of tomatoes are summarized in
Table 2.4 . Ceponis and Butterfield 66 reported that predominant decay organisms
infect tomatoes at mechanical injury sites. Sargent and co-workers 42 showed that
internal bruising caused by handling impacts disrupts the normal ripening of the
tomato locular gel. Bruising in potatoes is manifested as blackspot that appears days
after the injurious load has been applied. For this to occur, the tuber tissue (cell
walls and cell membranes) must be ruptured and oxygen must react with certain
chemical compounds in the cytoplasm. Upon cell rupture, certain cytoplasmic phe-
nols are freed and oxidized by the enzyme phenol oxidases, to form blue-black
melanin pigments via the red colored dopachrome. 40 Gray and Hughes 67 gave the
following series of reactions after a detrimental load was inflicted on the tuber:

tyrosine → 3,4-dihydroxy-phenylalanine (DOPA) → phenylalanine,

3-4-quinone → 2-carboxy-2,3-dihydro-5,6-dihydroxyindole →

2-carboxy-2,3-dihydroxy-indole-5,6-quinone →

5,6-dihydroxy-indole-5,6-quinone → melanin

Thefirst two reactions are catalyzed by polyphenol oxidases, and the remainder
can be non-enzymatic. This discoloration was initiately thought to arise solely from