Milk sample (4.0 mL) was added into 2.5 mL of 3% (v/v) acetic
acid, mixed well and stood for 15 min, diluted to 10.0 mL with
water, mixed, and then centrifuged for 10 min at 1500 rpm. If clarification
was not achieved, extra drops of glacial acetic acid were
added. An aliquot of the supernatant was filtered through a
0.45 lm nylon filter membrane. Samples of orange juice were prepared
by freshly squeezing orange fruit in an all glass assembly
setup. The injection volume was 20 lL.