I've been trying the following method: I'm working with lipase from porcine pancreas type II (10 mg/ml in ultrapure water), p-nitrophenyl laurate 0.01M in ethanol and phosphate buffered saline pH 7.5 at 410 nm. Separated, all the enzyme, substrate and buffer solutions are transparent but the final reaction mixture is not (it looks like a "milky" mixture), is it normal? if not, what should I do to solve it?