The Role of Polyphenols in Quality, Postharvest Handling, and Processing of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 276

(Lea and Timberlake, 1978; Spanos and Wrolstad, 1992). Therefore, oxidation of juice,
which contains high levels of procyanidins, tends to promote formation of haze. An early
oxidation of juice in the presence of pulp may prevent the formation of haze in the fin-
ished juice products (Pilnik and deVos, 1970; Spanos and Wrolstad, 1992). In this case,
oxidation of procyanidins leads to the tanning of the pomace and significant removal of
these constituents. However, excessive pulp oxidation can reduce the color and flavor of
apple juice (Lea and Timberlake, 1978). In the case of fermented pear juice and cider,
the condensed and polymerized oxidation products of phenolics result in sediments that
can be removed by filtration to reduce the astringency of the beverages (Vamos-Vigyazo,
1981).
12.7.5 Control of enzymatic browning by regulation of phenylalanine
ammonia-lyase activity
Regulation of phenylalanine ammonia-lyase (PAL), the key regulatory enzyme of shikimic
acid pathway that catalyzes the conversion of phenylalanine to trans -cinnamic acid, plays a
pivotal role in phenolic synthesis. The correlation between increases in the corresponding
PAL gene/protein expression/activity and increases in phenolic compounds in response to
different stimuli has been well established (Hahlbrock and Scheel, 1989). Control of PAL
activity, and thereby the biosynthesis of phenolic compounds at the site of injury to the fruit,
is also important in controlling enzymatic browning caused by postharvest and processing
steps (Martinez and Whitaker, 1995). Activity of PAL is stimulated under conditions such
as wounding, light, low temperature, and pathogens, thereby, the PAL activity regulates
the phenyl propanoid pathway. There are some other enzymes that may also participate
in increasing the phenolic production during cold storage such as C4H, CQT, and 4CL.
Hence, the increased content of phenolics provides an opportunity for PPO resulting in
more browning. It has also been reported in some fruits that the increase in PAL activity
takes place during cold storage, but PPO activity remains the same. This suggests that the
accumulated phenolics might undergo nonenzymatic reactions in the presence of metal ions
such as iron stored as ferritin (Lattanzio, 2003).
12.8 Other biochemical regulation of phenyl propanoid pathway
It has been observed that differential subcellular distributions of cinnamic acid arising
from the activities of differentially localized PAL isoforms could partition phenylpropanoid
biosynthesis into different branch pathways including those that bypass the C4H reaction
(e.g., 2-hydroxylation of cinnamic acid) (Achnine et al., 2004). Interestingly, this work sug-
gests that changes in the subcellular location of enzyme isoforms might provide an extra and
unsuspected level of metabolic regulation. More than 16 cytochrome P450 monooxygenases
have been recently shown to be involved in all these branch pathways. Downregulation of
C4H in transgenic plants (Reddy et al., 2005) and C4H mutation (Ruegger and Chapple,
2001) resulted in reduction in chlorogenic acid, flavonoid, and lignin biosynthesis, which
proves that C4H constitutes a rate-limiting step for channeling carbon flux into the phenyl
propanoid pathway (Anterola et al., 2002). Recently, Miziak et al. (2007) found that 2-
amino-4-bromoindane-2-phosphoric acid acts as a potent inhibitor of PAL activity in vitro
and of anthocyanin biosynthesis in vivo. Cinnamaldehyde was also shown to inhibit PAL