Lychee (Litchi) - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 235

1988b; Chen and Hong, 1992; Jiang and Fu, 1999a). The
antioxidant capacity of the pericarp also decreases during
storage (Duan et al., 2007). These changes decrease the
ability of harvested lychee fruit pericarp to eliminate active
oxygen species, generated in association with enhanced
perioxidation of membrane lipids. Consequently, the fruit
deteriorates and becomes senescent.

thocyanin compounds in pericarp tissues of lychee fruit
decreased during storage. Underhill and Critchley (1994)
suggest that anthocyanins are decolorized at high vacuo-
lar pH before degradation. The oxidative product of PPO-
mediated degradation of anthocyanins, 4-methylcatechol,
further accelerated enzymatic browning by PPO (Jiang,
2000; Ruenroengklin et al., 2009a). A probable mecha-
nism is that anthocyanins are first hydrolysated, forming an
anthocyanidin which may then be oxidized by PPO and/or
POD while the oxidation of phenolics by PPO to products
such as 4-methylcatechol could lead in turn to acceler-
ated degradation of anthocyanidin, resulting in enzymatic
browning of lychee fruit (Fig. 13.2). In addition, the degra-
dation of lychee anthocyanins by reactive oxygen species
which could form the oxidation of endogenous phenolics by
PPO could participate in the enzymatic browning reaction
of harvested lychee fruit during storage (Ruenroengklin
et al., 2009b).

Pericarp browning
The bright red color of lychee fruit is due to the presence
of anthocyanins in pericarp tissues, mainly cyanindin-
3-glucoside, cyanindin-3-galactoside, pelargonidin-3-
glucoside, and pelargonidin-3,5-diglucozide (Zhang et al.,
2000, 2004). After harvest, lychee fruit rapidly lose their
red color and brown, resulting in a reduced market value
(Huang et al., 1990; Underhill and Critchley, 1994). The
major disorder is the rapid browning of pericarp from a
bright red color of harvested lychee fruit (Paull and Chen,
1987; Holcroft and Mitcham, 1996). The browning is
associated with water loss and injury (insect stings) (Jiang
and Fu, 1999a; Somboonkaew and Terry, 2010a). The
browning associated with insect stings may go through the
pinkish-white inner surface of the pericarp. A breakdown
(softening or loss of turgidity) of lychee flesh occurs in
senescent fruit after prolonged storage and overmaturity.
Skin browning has mainly been attributed to the oxida-
tion of polyphenols and the degradation of anthocyanins by
polyphenol oxidase (PPO) (Huang and Wang, 1990; Jiang,
2000; Zhang et al., 2001; Liu et al., 2010). PPO activa-
tion is associated with moisture loss, and treatments that
reduce desiccation and also slow browning (Jiang and Fu,
1999a). The PPO enzyme has been isolated and purified
from lychee pericarp, and its pH and temperature optima
were 6.5 and 70 ◦ C, respectively (Jiang et al., 1997, 1999).
However, PPO activities measured during storage are of-
ten inconsistent. Lin et al. (1988a) demonstrated a rapid
increase in its activity during the first 48 hours of storage,
Zauberman et al. (1991) found no change in PPO activ-
ity during the same period, and Underhill and Critchley
(1993) reported a reduction in PPO activity. Although PPO
involvement in skin browning is widely accepted, a role for
POD has been recognized. Lin et al. (1988b) and Huang
and Wang (1990) reported high POD activities during stor-
age. Similarly, Zhang et al. (2005) recently recorded in-
creased POD activity during browning, further supporting
a role for POD. It is generally accepted that anthocyanins
and phenols in the lychee fruit skin are progressively de-
graded and that browning is due to the formation of poly-
meric brown pigments (Jiang et al., 2006b). Zhang et al.
(2000, 2001) reported that the concentrations of major an-

Browning triggers

Altered membrane fluidity and permeability

Cellular decompartmentation

of enzymes and substrates

Polyphenol oxidase

Phenolics

& peroxidase

Increased

oxidation

Anthocyanin pigments

Brown polymeric pigments

Figure 13.2. A scheme to account for enzymatic
browning of harvested lychee fruit during storage.