98 Produce Degradation: Reaction Pathways and their Prevention
role in the H 2 O 2 metabolism. Baicalein, however, predominantly contributed to H 2 O 2
metabolism. Because beta-glucuronidase, cell wall peroxidases, and baicalein pre-
exist in S. baicalensis cells, their constitutive presence enables the cells to rapidly
induce the H 2 O 2 -metabolizing system.
Changes that occur in POD activity in wounded and fresh-cut fruits significantly
contribute to their product quality.182,188–190 In addition to the role played by POD in
the flavor of fresh fruits and vegetables, POD isozymes are involved in many cell
alterations.^191 They appear to influence flesh firmness through catalysis of the cross-
linking between tyrosine residues of the cell wall extensins and ferulic acid substit-
uents of pectins^192 and the synthesis of lignin and suberin polymers.192,193 The anionic
peroxidase associated with the suberization response in potato (Solanum tuberosum
L.) tubers during wound healing is a 45-kDa, class III (plant secretory) peroxidase
that is localized in suberizing tissues and shows a preference for feruloyls such as
those that accumulate in tubers during wound healing. In contrast, the cationic
peroxidase(s) induced in response to wound healing in potato tubers is present in
both suberizing and nonsuberizing tissues and does not discriminate between hydroxy-
cinnamates and hydroxycinnamyl alcohols.^194
Lipoxygenase (linoleate: oxygen oxireductase; EC.1.13.11.12) (LOX) is present
in most plant tissues and, in the presence of oxygen, catalyzes oxidation of polyun-
saturated fatty acids (PUFA) containing a cis,cis-1,4-pentadiene structure. Allene
oxide synthase (AOS; hydroperoxide dehydratase; EC 4.2.1.92) catalyzes the first
step in the biosynthesis of jasmonic acid from lipoxygenase-derived hydroperoxides
of free fatty acids. Rangel et al.^195 demonstrated that wounding causes local and
systemic induction of LOX activity in passion fruit (Passiflora edulis f. flavicarpa)
leaves, while exposing intact plants to methyl jasmonate (MJ) vapor provoked a
much stronger response. Based on immunocytochemical localization studies using
leaf tissue from MJ-treated plants that showed that the inducible LOX was compart-
mented in large quantities in the chloroplasts of mesophyll cells associated with the
stroma, it was suggested that the wound response in passion fruit may be mediated by
a chloroplast 13-LOX, a key enzyme of the octadecanoid defense-signaling pathway.
Conjugated hydroperoxy acids (HPO) produced by LOX catalysis undergo
metabolism by hydroperoxide lyase (HPO lyase). HPO lyase catalyzes the cleavage
of HPO to aldehydes, such as cis-3-noneal and hexanal from linoleic acid HPO and
cis-3,cis-6-nonadienal and cis-3-hexenal from linolenic acid HPO.196,197 Besides
hydroperoxidation, formation of oxoacids and ketodienes is catalyzed by LOX.198,199
Hydroperoxide isomerase could also catalyze the isomerization of hydroperoxide as
an intermediate reaction pathway.^198 Internal bruising may alter aroma volatile pro-
files in fruit tissues. Individual volatile profiles of the three tissues in bruised tomato
fruit were significantly different from those of corresponding tissues in undropped,
control fruit, notably, trans-2-hexenal from pericarp tissue; 1-penten-3-one, cis-3-
hexenal, 6-methyl-5-hepten-2-one, cis-3-hexenol and 2-isobutylthiazole from locule
tissue; and 1-penten-3-one and beta-ionone from placental tissue. Alteration of
volatile profiles was most pronounced in the locule tissue, which was more sensitive
to internal bruising than the other tissues. Changes observed in the volatile profiles
appear to be related to disruption of cellular structures.^200