SUMMARY—An improved method for the rapid determination of free proline in grapes and wine is given. No significant interference from other amino acids in amounts found in grapes and wine would normally be expected. The method is applicable to most biological material as long as lysine, hydroxyproline, tryptophan and glutamine are not present in total amounts greatly exceeding that of proline. Lysine caused greatest interference and this was only 6 error on equal mole amounts. A direct dilution technique instead of the usual extraction of reaction product by benzene increased accuracy and speed of this method. The relative standard deviation between pairs of samples of grape juice averaged about 3 and for wine about 4
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