Analytical Methods
In order to assess the performance of the OMD system, the
following parameters were taken into account: the flux
through the membrane and the juice concentration at the be-
ginning and at the end of the process. The flux was measured
by collecting and weighing the permeate in a given period of
time. The initial and final concentrations of the apple and beet
juices and stripping solutions (CaCl
2
and NaCl) were mea-
sured by using an Abbe refractometer (PZO-RL1, Warsaw,
Poland). The refractive index is linearly dependent on the
concentration of salts in solution. Therefore, the water activi-
ties of CaCl
2
and NaCl solutions could be determined from
their respective refractive indexes and calibration curve relat-
ing water activity and salt concentration (Chirife and Resnik
1984
;Serenoetal.
2001
).
The quality of the juices was evaluated following presented
methods. The TPC of both apple and beet juices was deter-
mined using the Folin
–
Ciocâlteu method described elsewhere
(Rózek et al.
2007
). The antioxidant capacity was determined
as the Trolox equivalent antioxidant capacity (TEAC) assay.
The 2,20-azinobis(