The a* values are the chromaticity coordinates that represent the red
(+)/green (−) color attributes. Guava powder is almost colorless
whereas pitaya powder possesses a strong reddish color. The red color
dominates with progress in dissolution and changes in a*—values are
as shown in Table 5. The EFA formulation attained the highest a* value
within 80 min of dissolution, whereas NSD and KCL were not able to
attain the same a* value even after 140 min of dissolution. Since tablets
containing effervescent agents eroded faster (Fig. 3), color diffusion was
also faster for these tablets during dissolution. At the end of complete
dissolution, chroma values for tablets dissolved in HCl solution were almost similar, while chroma values of the tablets dissolved in distilled
water and buffer solution were slightly higher than those dissolved in
HCl solution (data not shown).
The experimental data on vitamin C release suitably fits the
Korsmeyer Peppas equation with a goodness fit of unity (Table 5). The
equation parameters were obtained by plotting vitamin C releases in
log–log coordinates for 60 min of dissolution by repeating the same experiments with 10 tablets using a dissolution tester to increase the ratio
of solutes (vitamin C) to solution.
The value of exponent time (n) for NSD, KCL and EFA tablet formulations were within the 0.15–0.3 range with dissolution in distilled water,
HCl and buffer solution. Hence, the release exponent in the Korsmeyer–
Peppas model suggests that the mechanism of release of vitamin C from
the tablet form was a Fickian type. The Fickian type indicates a Fickian
diffusion where release occurs by the usual molecular diffusion of the
tablets due to a chemical potential gradient. Therefore, the rate of diffusion of mixed fruit tablets is not easily changed by physical agitation but
is strongly affected by the composition of the tablets [16].
3.4. Sensory evaluation
Taste and aroma of foods are major considerations after appearance.
They stimulate the tongue sensory and determine acceptability by consumers. Tablets containing 1% effervescent agent (EFA) dissolved in
fresh water and 10% sugar solution were subjected to sensory evaluation (Fig. 4). This test typically indicated that consumers prefer to take
fruit drinks having balanced sweetness. A significant intensity of sweetness of mixed fruits is eliminated after they undergo a long period of
freeze drying process. Also the bulk content of the tablet needs to be optimized in terms of balanced sweetness and acidity to serve the tablet as
ready to drink simulated fresh juice. However, scores on individual parameters like sweetness, smell and taste were very poor (data not
shown). For an in-depth study, further dissolution and sensory evaluation was carried out with higher amounts of EFA in both individual
and mixed fruit tablets (Table 6). The results revealed that with increasing sugar in the dissolution media, sensory scores increased with respect to all sensory properties, except color (Table 6). Color remained
unchanged with elevated concentrations of sugar solution. Among the
fruit tablets, guava obtained the lowest scores irrespective of sugar concentration, whereas scores for pitaya increased with increasing sugar
concentration. However, sensory scores for pitaya fruit tablets were
best at 20% sugar solution.
3.5. Microbial stability
The results showed that the numbers of colonies in each type of
sample were ‘Too Few to Count’ (TFTC), i.e. the numbers were less
than 30 colonies in 10−1 dilution and there were no colonies in the
10−2 to 10−5 dilutions (Table 7). It revealed that there were hardly
any bacteria and fungi growing on the tablet samples during the
1 month of storage. Adiba et al. [4] had found similar results for storage of 11 months of food tablets made from dates and spirulina.
These observations proved that fruit powder in the form of tablets
were not readily susceptible to microbial spoilage during storage.
This may be attributed to the very low surface area exposed fo