The Role of Polyphenols in Quality, Postharvest Handling, and Processing of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 277

(Fujita et al., 2006), and interestingly this compound suppresses the browning of cut lettuce
when they are immersed in a solution of the inhibitor. Other chemical compounds such as
1-methylcyclopropene (1-MCP) dramatically inhibits ripening of apple fruit (Rupasinghe
et al., 2000). 1-MCP interacts with ethylene receptors and thereby prevents ethylene-
dependent responses (Rupasinghe et al., 2000). Action of 1-MCP results in decreasing
the PAL and PPO activity, and also lowering the phenolic content. Its application also pre-
vents or delays the softening, the effects of treatment often closely associated with ethylene
production (Rupasinghe et al., 2000). In apple, banana, melon, and pear fruit, inhibition
of ethylene production by 1-MCP was accompanied by lower expression of these genes
(Defilippi et al., 2005). Hexanal can also act as possible inhibitor of PAL (Lanciotti et al.,
2004).
12.9 Selection for apple genotypes with low
postcut enzymatic browning
New genotypes with antibrowning properties have a growing demand for value-added prod-
uct development for niche markets while minimizing the processing cost by eliminating the
use of antibrowning chemical treatments. Several research programs are presently attempt-
ing to develop fruits exhibiting low enzymatic browning (Martinez and Whitaker, 1995;
Marshall et al., 2000). Conventional breeding and genetic engineering methods, such as
antisense RNA and gene silencing techniques, have been investigated as means to develop
new genotypes with antibrowning properties (Martinez and Whitaker, 1995). One possi-
ble approach to reduce the PPO activity and resultant enzymatic browning reactions is to
characterize and inactivate the genes, which code for PPO enzyme. Inactivation can be
accomplished by generating antisense RNAs specific for PPO. Expression of PPO mRNA
might be controlled in this way, a reduction in browning would be accomplished by reducing
the amount of protein formed (Martinez and Whitaker, 1995). Genetically modified versions
of “Golden Delicious” and “Granny Smith” apple fruit have been produced in which 95%
of PPO was silenced by the antisense insertion of polyphenol oxidase genes derived from
apple (Okanagan Biotechnology Inc., 2005). Eden, “SuperMac,” and SJCA16R5A15 are
some of the recently developed apple genotypes using conventional breeding and selection
by Agriculture and Agri-Food Canada—Horticultural Research and Development Center
(AAFC-HRDC), Quebec, Canada. Two hours after slicing and storage at ambient temper-
ature, the whiteness index (WI) values were significantly higher for Eden (Fig. 12.7) as
compared to commercial apple genotypes tested (Joshi et al., 2007). Similar results were
observed for Eden after vacuum dehydration (50 ◦ C for 24 h), while the WI of “Cortland”
and SJCA16R5A15 was greater than “Empire” and “SuperMac.” These results suggested
that Eden offers a potential nonbrowning or minimal-browning characteristic, which may
make it favorable for use in processing apples for either fresh-cut or dried chips. Similar
results were obtained when a range of apple cultivars including “Gala,” “Galarina,” “Spar-
tan” and “Cortland,” and Eden were cut and kept for 24 h at 20 ◦ C (Khanizadeh et al., 2006).
However, the major concern in selecting these cultivars is that these are low in polyphenolics
and hence reduce the nutritional quality and antioxidant capacity of selected cultivars. For
instance, Eden showed the least total antioxidant capacity as estimated by ferric-reducing
ability of plasma and oxygen radical absorbance capacity assays by comparison to other
cultivars.