where C0
is the initial concentration of anthocyanin (mg/L); Cd
is
the desorption concentration of the solute in the desorption solution
(mg/L); Ce
is equilibrium concentration of anthocyanin (mg/L); Vd
is
volume of the eluent (L); V0
is volume of initial anthocyanin solution
(L).
Analytical procedures
Anthocyanins were directly determined by the pH-differential
method as described by Wrolstad [22]. Samples were diluted with buffer
and the absorbance was read at 520 and 700 nm using a Jasco V-560
UV/VIS spectrophotometer. Anthocyanin content was calculated as
pelargonidin-3-glucoside, with molar absorbance of 22,400 M-1 cm-1
and molecular weight of 433.2 Da.
Total sugar concentration was determined by 3, 5-dinitrosalicylic
acid colorimetry [23]. Protein concentration was determined by
Coomassie Brilliant Blue method using BSA as standard protein [24].
Results and Discussion
Selection of aqueous two-phase system
Partition behavior and stability of anthocyanins were studied
in ATPSs employing ethanol and different phase forming salts
(dipotassium hydrogen phosphate, natrium carbonate and ammonium
sulphate). Ammonium sulphate was selected as the best salt due to
higher stability of anthocyanins in ammonium sulphate (pH 4.3) than
dipotassium hydrogen phosphate (pH 8.2) and natrium carbonate (pH
9.0). The stability of anthocyanins was proved to be affected by the
pH of the system: When pH of anthocyanin solution was above 8, the
pyran ring in structure of anthocyanin was opened [4]. There was less
than 33.5% anthocyanin in those systems comparing with the acidified
ethanol extract.
The effects of ethanol and ammonium sulphate concentration
on the partitioning of anthocyanins were determined on the basis
of phase diagram of ATPS of ethanol/ (NH4
)2
SO4
as shown in figure
1. Comparing with the previous phase diagram [25], the upper limit
line of two phases was added in figure 1. As shown in figures 2 and
3, the partition coefficient and recovery of anthocyanins increased
dramatically with increasing concentrations of (NH4
)2
SO4
and ethanol,
which indicated that anthocyanins tended to concentrate in the top
phase. The partitioning behavior of anthocyanins was similar with
another water soluble pigments-betalain in ATPS consisting of PEG
and (NH4
)2
SO4
[13]. Compared these ATPSs, the partition coefficient
and recovery of anthocyanins in ATPS of 30% (w/w) ethanol and
19% (w/w) (NH4
)2
SO4
were 10.67 and 96.09%, respectively. And the
recovery was the highest. Thus this ATPS was chosen for the ATPE of
anthocyanins in the next study.
Extraction of anthocyanins from V. uliginosum residue
To optimize ATPE, the effects of ATPE strategies on partition
coefficient and recovery of anthocyanins were investigated (Figure 4).
When (NH4
)2
SO4
solution and ethanol were added to V. uliginosum