Abstract:
Watermelon (Citrullus lanatus) juice is rich in phytochemicals that possess antioxidant
properties which are known to have a positive contribution to human health. Colour, pH, total
soluble solids, lycopene, β-carotene, ascorbic acid, total polyphenolic content and
antioxidants activity are quality properties that characterise watermelon juice. However,
these quality properties undergo some chemical changes throughout the stages of maturity
of the fruit as well as during thermal processing. The aim of this study was to investigate the
effects of maturity and filtration on the physicochemical properties (colour, pH, total soluble
solids) and the phytochemicals (lycopene, β-carotene, ascorbic acid, total polyphenolic
content and antioxidants activity) of fresh watermelon juice. It further investigated
thermosonication effects on these quality properties using response surface methodology
(RSM). RSM considering temperature (25 – 52ºC), time (2 – 10 min) and amplitude level
(24.1 – 60 μm) at a constant frequency of 20 kHz were the independent variables while
colour variables (L*, a* and b*, C* and h*), pH, total soluble solids, lycopene, β-carotene,
ascorbic acid, total polyphenolic content and antioxidants activity were the dependent
variables. Results showed that colour, pH, total soluble solids, lycopene, β-carotene,
ascorbic acid, total polyphenolic content and antioxidants activity of watermelon fruit juice
are maturity dependent. All the physicochemical and phytochemical properties increased
with maturity. The combination of the CIELAB parameters (L*, a*, b*) was clearly explained
by the Chroma (C*) and hue (hº). The (C*) and hº values of the watermelon fruit juice were
significantly (p < 0.05) affected by the stages of maturity. The red colour was observed to be
more saturated in the fully - ripe sample as indicated by the highest Chroma (7.35) values.
The half - ripe sample which was observed to be pink - red had a C* of 5.88 while the lowest
(4.32) value was observed from the unripe sample. The highest hº value was obtained from
unripe sample (83.89), while lower value was observed from the fully - ripe (76.03) sample
because of the saturation of the red colour in the fully - ripe sample. The total soluble solids
also increased with stages of maturity 6.66 to 6.60 at half - ripe while at fully - ripe redness
changed from 9.3 to 8.99 and TSS 8.37 to 8.02. Filtration has shown a significant (p < 0.05)
on the C* and hº values. There was a higher retention of all dependent variables at lower
thermosonication treatment (25ºC; 24.4 μm), especially the lycopene which increased by
127%. Model predictions for the colour properties and phytochemicals were closely
correlated to the experimental results obtained. Prediction models were found to be
significant (p < 0.05) with low standard errors and high coefficients of determination (R2).
