The Development and Use of a Rapid Analytical Technique for Estimation of Free and Potentially Volatile Monoterpene Flavorants of Grapes

Abstract

A technique has been developed for the rapid assay of monoterpene flavorants of grape juices. Steam distillation of juice at neutrality yielded free aroma compounds of the grape while, at low pH, monoterpenes derived from the polyols and glycosidically bound forms were collected. Reaction of these distillates with a vanillinsulfuric acid reagent gave a color, the intensity of which was proportional to the concentration of monoterpenes in the two fractions. Thus analytical figures for free volatile terpenes (FVT) and potential volatile terpenes (PVT) of the grape were obtained. The presence of sulfur dioxide in juice interfered with the determination, necessitating treatment with H₂O₂ before color development. Other non-monoterpenoid juice constituents did not interfere, except trans-hex-2-en-1-ol, which would have contributed to FVT figures. Experiments with different categories of reference monoterpenes, including polyols and glycoside derivatives, demonstrated that the technique gave quantitative recovery of FVT while yields of PVT were 55 to 80 percent. Solutions containing mixtures of different monoterpene categories on assay gave satisfactory recoveries of FVT and PVT. In a preliminary survey, the assay was applied to samples of mature grapes of several varieties and figures for FVT and PVT correlated with the recognized aroma characteristics of the cultivars examined. The value of the assay was demonstrated in observing the increase in concentration of monoterpene flavor compounds in developing Muscat of Alexandria grapes.