INTERFERING SUBSTANCES IN THE COLORIMETRIC QUANTITATION OF GRAPE MONOTERPENES

The potential for interference by specific C-6 compounds in the colori metric quantitation of grape (Vitis vinifera L.) monoterpenes was inve stigated in model solutions and muscat and neutral-flavored grape cult ivars. The unsaturated C-6 aldehyde 2-hexen-1-al (2HX) showed color ab sorption at 608 mn in distilled water after reaction with an acidified vanillin solution. Absorbance also increased significantly when 2HX w as added to a series of linalool solutions; approximate to 2.5 mg 2HX per liter of a 1-mg.liter(-1) linalool solution increased the absorban ce by >10%. Adding 2.5 mg 2HX per kilogram of 'Gewurztraminer' berry h omogenate significantly increased apparent free volatile terpene (FVT) concentrations to 121% of unadulterated control treatments but did no t affect potentially volatile terpenes (PVT). Adding 2HX also increase d apparent FVT concentration in 'Perlette' and 'Flame Seedless'. Both neutral-flavored table grape cultivars contained some FVT and PVT as a consequence of their muscat ancestries. FVT and PVT quantitation by c olorimetric methods may be subject to significant error if the concent ration of 2HX and other unsaturated C-6 compounds in grape berries or must are >5 mg.liter(-1). However, low concentrations of unsaturated C -6 compounds (<80 mu g.liter(-1)) in British Columbia sines suggest th at there is a low probability of significant interference with this me thod.