As a necessary step for the precipitation of proteins, three samples
containing 10 g of yogurt or milk drink was blended with
0.5 mL of K4Fe(CN)63H2O (0.35 mol L1) and 0.5 mL ZnSO47H2O
(1 mol L1) in a 50 mL Falcon tube. The solution was agitated with
a tube shaker (PA162 PACHANE) immediately after each addition
and left to stand for 10 min. After this time interval, 10 mL of solution
methanol:ammonium hydroxide (2 mol L1) (80:20) was
added, the solution was stirred and centrifuged using a centrifuge
(Cientec CT-5000DR) at 6000 rpm and 4 C for 10 min. The centrifugal
step was repeated twice more, each time with an addition
of 10 mL methanol:ammonium hydroxide, in order to extract the
maximum possible colors from the samples.