3.2. Microstructural analysis
The integrity of microstructure serves as an important criterion
for quality of frozen foods (Neri et al., 2014; Yoon, 2002) and mainly
depends on the size and distribution of ice crystals formed during
freezing process and frozen storage. By examining fruit micro-
structure, the extent of tissue damage during the freezing process
can be determined (Yoon, 2002). This allowed us to optimize the
freezing procedure to meet the requirements for higher product
quality. Compared with the fresh samples (Fig. 2a1), the structural
integrity of cell wall of pericarp was better maintained in IF samples
(Fig. 2c1 and c2) than in the AF samples (Fig. 2b1 and b2). Similarly,
in IF samples the microstructure of the litchi pulp was undamaged,
and the cellular integrity did not seriously deteriorate during frozen
storage of up to 180 days (Fig. 3c3). In contrast, AF samples showed
significant damage shortly after the freezing process and visible
damage increased with prolonged storage time (Fig. 3 b2 and b3).
Storage duration influenced AF samples more significantly than IF
samples. This may be due to temperature fluctuations during
storage, causing recrystallization and the growth of ice crystals
(Aleong et al., 2008; Knight et al., 1988; Lv et al., 2014). Altered
microstructural integrity provides a reasonable explanation for
changes observed in many quality indices in frozen foods. A com-
plete microstructure reflects good preservation of natural
morphology and structure of frozen foods (Herrero, Carmona,
Garcia, Solas, & Careche, 2005; Yoon, 2002), and at the same time
also indicates a reduction in drip loss in thawing because of
reduced structural damage, and subsequently less nutrient loss. For
vegetal products with are sensitive to color changes, good
microstructural integrity helps reduce the possibility of enzymic
damage, and the resulting discoloration (Duan, Wu, & Jiang, 2007).
Therefore, microstructural integrity is an important factor for
frozen foods. According to the data, we conclude that IF led to
reduced damage to the microstructure of litchi pericarp and pulp,
which could account for the maintenance of attractive red color,
good texture and high nutritional quality after storage times of six
more months. On the other hand, quality of the AF litchi samples
was unacceptable, and microstructure was not maintained. Similar
results have been reported in frozen pork (Ngapo, Babare, Reynolds,
& Mawson, 1999b) and potato (Sun & Li, 2003).
Microstructural integrity of frozen litchis was closely related to
the size and distribution of ice crystals which mostly depended on
the freezing rate. According to the information about the micro-
structural integrity of pericarp and pulp tissues in Figs. 2 and 3, the
range and average size of ice crystals in IF and AF litchi samples
could be determined as: AF > 200
m
m > IF