3. Results and discussion
3.1. Testing principles
The general approach of the suggested test is selective adsorption
of synthetic dyes in a test system resulting in color changes in a
drink sample.
In particular, the main component of the test system was suggested
to be a specifically developed adsorbent that provides an
adsorption of synthetic food dyes and substantially does not
interact with natural dyes of drinks.
Different polymeric adsorbents meeting this criterion were
tested. The test samples were divided into two groups: N - natural
red wines and juices; S - model drinks made by addition to white
wine or apple juice the predetermined amount of synthetic dyes to
give the same color characteristics, i.e. intensity (I) and tint (T) as
measured for the drinks of the first group.
The exemplified compositions of natural drinks (N1eN3) and
drinks containing synthetic dyes (S1eS3), the total amounts of dyes
as well as the color intensity and tint of such drinks are presented in
Table 1.
The said drinks samples were treated with different adsorbents
for 15 min in order to evaluate intensity and tint changes after
treatment. The adsorbents used were different modifications of
activated carbon, microcrystalline cellulose, DEAE-cellulose, polyamide,
chitosan, chitin and Riosorb.
The chitosan containing adsorbent Riosorb was specially
developed in our previous study (Komissarchik & Nyanikova, 2010)
and provides high adsorption activity towards synthetic food dyes
and almost does not adsorb dyes from natural drinks.
The structure of chitosan is shown schematically in Fig. 1.
Fig. 2 illustrates the IR-spectrum of chitosan containing adsorbent
Riosorb.
Riosorb has free amino groups providing interactions with
negatively charged groups of the synthetic food dyes, e.g., sulfo
groups contained in all of the synthetic dyes studied. Nevertheless,
the adsorbent structure does not provide adsorption of natural dyes
contained in the natural drinks.
In order to evaluate the described selectivity, we used color
intensity and tint changes. To combine the color intensity and tint
changes we developed the Synthetic Dyes Index (SDI) expressed as
a product of the relative intensity change with the relative tint
change multiplied by 100.
According to the general approach, the more synthetic dyes are
contained in a drink, the more relative intensity and tint changes
are, and the higher is the SDI value. Hence, the values of SDI are
higher for the drinks containing synthetic dyes when compared to
SDI values obtained for natural drinks.
Based on this logic, we suggest the measure of selectivity to be
the difference in SDI obtained for two drinks having the same initial
color intensity and tint but one containing synthetic dyes and
another made of natural ingredients.
The test results have shown that the most adsorbents either
adsorbed both synthetic and natural dyes (like activated carbon), or
showed low adsorption activity towards synthetic dyes. The best
selectivity was shown for chitosan containing adsorbents. In
particularly, chitosan containing adsorbent Riosorb showed the
outstanding adsorption of synthetic dyes represented by the SDI
values, whereas SDI for natural drinks was the lowest of all the SDIs
obtained for other adsorbents.
The SDI values for the samples of drinks illustrated in Table 1
treated with Riosorb are shown in Table 2.
The results shown in Table 2 indicated that the natural and
synthetic drinks initially having similar color intensity and tint
demonstrated the significantly different SDI values. The color
changes during the treatment were also visually recognizable.