Mechanical Injury of Fresh Produce 95
may be used for synthesis of other phenolic compounds (phytoalexins) that aid the
healing process. Immediately after injury, oxidation of preexisting phenolics occurs,
leading to an initial decrease in phenolic content. O-quinones produced usually have
antimicrobial properties and they readily undergo oxidative polymerization. Follow-
ing this, there is a significant increase in phenolic content.^146
When browning occurs, constituent phenols are oxidized to produce a quinone
or quinonelike compound that polymerizes, forming brown pigments. These unsat-
urated brown polymers are generally referred to as melanins or melaninodins. Among
the compounds believed to be important as substrates are chlorogenic acid, neochlo-
rogenic acid, catechol, tyrosine, caffeic acid, phenylalanine, protocatechin, and
dopamine.^147 Some induced proteins in wounded plant tissues are enzymes of phe-
nolic metabolism, such as phenylalanine ammonia-lyase (PAL), whose increased
activity leads to the accumulation of phenolic compounds (e.g., chlorogenic acid,
dicaffeoyl tartaric acid, and isochlorogenic acid) and tissue browning. Wounding of
iceberg lettuce leaves increases PAL activity 6- to 12-fold over 24 h at 10°C and
leads to a threefold increase in the total phenolic content within 3 days.^23 Soaking
cut lettuce in hypertonic solutions appeared to render the tissue insensitive to wound
induction of PAL and caused a reduction in the wound-induced accumulation of
phenolic compounds. The hypertonic solution did not cause the loss of a portion of
the wound signal through efflux of water from the tissue, but rather induced a general
stress-related resistance to further abiotic stresses.^148 The synthesis and accumulation
of phenolic compounds in lettuce as a consequence of wounding can be suppressed
by a heat-shock treatment,^149 although heat shock-treated cut lettuce produced more
phenolic compounds than unwounded lettuce.^150
Bruising pear fruit after 120 days of storage caused a 30% increase in chlorogenic
acid and a 50% increase in catechin, but no increase in p-coumaric acid derivatives.^151
After 3 days of storage of whole heads and excised midrib sections of iceberg, butter
leaf, and romaine lettuce (Lactuca sativa L.) at 5 and 10°C, only 5-caffeoylquinic
acid (chlorogenic acid), 3,5-dicaffeoylquinic acid (isochlorogenic acid), caffeoyltar-
taric acid, and dicaffeoyltartaric acid were detected in wounded lettuce midribs. Of
these four compounds, chlorogenic acid accumulated to the highest level in all three
lettuce types.^152 Storage of root slices of carrot cv. Flakoro at 5 or 20°C resulted in
considerable accumulation of soluble phenols, particularly chlorogenic acid, and of
ethylene. Isocoumarin also accumulated in the peel, particularly at 20°C.^153 The
wound responses observed in cassava include increased activity of PAL, peroxidase
(POD), and polyphenoloxidase (PPO); formation of phenols/polyphenols including
leucoanthocyanidins, catechins, scopoletin, and condensed tannins; and often the
formation of a wound periderm.
4.9 ENZYMATIC EFFECTS
A number of enzymes are known to contribute to flavor biogenesis that could affect
sensory quality and shelf life of fresh fruits and vegetables. Flavor production by
the lipoxygenase (LOX) pathway, for example, is generally quiescent unless trig-
gered by maceration or cell damage.^154 The resulting production of volatile aldehydes