3.5. Changes of drip loss, vitamin C content and TSS of litchi pulp
The changes in nutritional quality of frozen litchis could be
determined by comparing key quality indices (Kaushik et al., 2013).
This study analyzed vitamin C and TSS to determine the effect of
freezing methods on nutrient preservation in frozen litchis. It was
found that little drip loss occurred in the IF samples (p < 0.05)
during the whole storage time, while drip loss increased to 4% in AF
samples after 180 days of storage (Fig. 6a). This can be easily
explained in the light of the relationship between drip loss and
microstructure integrity. The vitamin C content and TSS decreased
gradually in both IF and AF samples (Fig. 6b and c). After 180 days a
drastic loss (40e60%) of vitamin C was detected in AF samples,
likely due to rapid leaching out via drip loss, whereas less than 20%
loss occurred in the IF samples. It was noticed that most of the
vitamin C loss in IF samples occurred before and during the freezing
process rather than storage time, indicating that optimization of
pretreatment and freezing processes are the key steps to preserving
vitamin C levels, as well as the other nutrients. Additionally, the
application of microwave thawing played a critical role in mini-
mizing nutrient loss due to rapid and uniform thawing (Li & Sun,
2002b) and reduced thawing time (Mead et al., 1986). This could
effectively avoid the damaging effects of increased temperature,
enzymatic activity and microbe growth on loss of vitamin C, sugar
and other nutrients during thawing. Also, microwave thawing itself
also has a sterilizing effect and can reduce enzyme activity. As a
result, application of IF process plus water shift microwave thawing
radically resolved the problem of drip loss in frozen litchi fruit.
With the same thawing method, the quality changes obtained from
AF and IF were different. It therefore can be concluded that the
main differences of quality changes obtained from IF and AF litchis
with the same storage and thawing processes were due to the
different freezing procedures.
3.6. Sensory quality evaluation of litchi fruit after IF and frozen
storage
Sensory evaluation is important for food marketing. Litchi fruit,
as a characteristic subtropical fruit, is famous in both domestic and
overseas fruit markets for its delicate flavor and marketable value. It
is therefore of interest to perform sensory evaluation testing for the
quick frozen litchis. Particularly, because most consumers are
familiar with the flavor and taste of litchis stored at normal tem-
perature, it is important for producers to make more precise and
effective market positioning and promotion plans by employing
sensory evaluation. The IF samples showed similar hardness,
chewiness and taste than the fresh fruit, but significantly (p < 0.05)
lower scores on aroma (Table 1). Yet, the IF samples outperformed
the fresh ones in color quality (Table 1, Fig. 7), consistent with the
previous experimental results of a
*
value and L
*
value. Meanwhile,
there was a significant difference of appearance between IF and AF
litchi pericarp/pulp (Fig. 7). Overall, the total sensory score of the IF
samples was 19.20, very similar to that of the fresh fruits (21.70).
Sensory quality provides an important reference for producers to
determine shelf-life and to establish consumption guidance for
marketing. Frozen litchis, as a new kind of quick-frozen fruit
product, also will benefit from a good market strategy. According to
the results, at least 6 months of storage time for IF litchis was
readily achievable under the given storage conditions, and they
have the best flavor within 3 h after thawing