Prediction of Prefermentation Nutritional Status of Grape Juice: The Formol Method

The Formol titration is a simple and rapid method for determination of the quantity of assimilable nitrogen in juice (). It provides an approximate, but useful, index of must nutritional status. The procedure consists of neutralizing a juice sample with base to a given pH, adding an excess of neutralized formaldehyde, and retitrating the resulting solution to an end point. The formaldehyde reacts with free amino groups of α-amino acids, causing the amino acid to lose a proton, which can then be titrated. Free ammonia is also titrated. Proline, one of the major amino acids in grapes that generally cannot be used by yeast under wine fermentation conditions, is partially titrated. Arginine, which contains four nitrogen atoms but only one carboxylic acid group, is titrated to the extent of the single acid functionality. Traditionally, barium chloride has been included to precipitate sulfur dioxide so that it does not interfere with the determination. If the juice is unsulfited or if the sulfur dioxide level is less than 150 mg/L, this part of the procedure may be ignored (see Note 1).