3.2.1. Reverse osmosis
The permeate flux in the reverse osmosis process ranged from
40.30 L h−1 m−2 to 0.50 L h−1 m−2, decreasing along of time and
tending to zero, as shown in Fig. 2. This is a classical behavior of membrane
processes carried out in batch mode. This decrease is attributed
not only to the fouling phenomena occurrence, but also to the increase
of juice's osmotic pressure during concentration that makes
the water removal slower. Simultaneously, soluble solid content increased
along the process until reaching the juice's osmotic pressure.
This increase led to higher juice viscosity, increasing the resistance to
mass transfer and decreasing the permeate flux.
With respect to the physicochemical properties, reverse osmosis
promoted an increase in the soluble solids content, reaching a volumetric
concentration factor of 3.72. Samples showed a slight variation
on the pH and an increase in acidity, total phenolics and antioxidant
activity as a consequence of juice concentration (Table 2).
Although phenolic compounds have increased more than three
times, the antioxidant activity was not concentrated in the same proportion.
Considering the results expressed in a dry basis, in order to
eliminate the concentration effects, phenolic loss during the reverse
osmosis process was around 11%, while the antioxidant activity was
reduced in 21%. This indicates that some compounds present in the
apple juice, other than phenolics, which also have antioxidant properties,
may have been lost during processing. The reduction on total
phenolics and antioxidant activity may be related to some oxidation
occurring during concentration or to the passage of some compounds
through the reverse osmosis membrane.
Bánvölgyi et al. (2009) evaluated the effect of concentration by reverse
osmosis on the characteristics of blackcurrant juice. Similarly to
the present work, the authors verified that the concentration factor
achieved for the antioxidant activity was lower than the obtained
for the total phenolic content. Gomes, Costa, Campos, Couri, and
Cabral (2011) also observed that the antioxidant activity did not increase
proportionally to the lycopene content, during the concentration
of watermelon juice by the same process