Article by Enology-Grape Chemistry at Virginia Tech
Molly Kelly, Enology Extension Specialist, Virginia Tech.
We have previously reported on the effect of foliar and soil nitrogen applications on Petit Manseng grape wine composition. This review reports on the influence of these same applications on the concentration of free and bound volatiles in Petit Manseng fruit using two hydrolytic techniques.
Some grape-derived aroma and flavor compounds exist as free volatiles that possibly contribute to aroma and flavor, and some as non-volatile bound conjugates. Bound components include terpenes, lactones, C13 norisoprenoids, alcohols, esters and shikimic acid compounds (Ibarz et al. 2006). These precursors (including glycosides) can be released and form aroma and flavor compounds by different mechanisms. Bound glycosides can be cleaved enzymatically (Allen et al. 1991), resulting from the action of enzymes that are endogenous in the fruit and/or added during processing. Additionally, acid-catalyzed reactions during winemaking can result in the formation of various aroma and flavor compounds such as norisoprenoids and terpenes (Ugliano et al. 2006). Sensory analysis of acid and/or enzyme hydrolysates has shown their impact on aroma and flavor profiles (Webster et al. 1993).
The formation of free and bound aromas and flavors may be influenced by nitrogen fertilization (Bell and Henschke 2005, Webster et al. 1993). The relationships between vineyard nitrogen and development of aroma and flavor compounds have been evaluated (Castor and Guymon 1952, Chen 1978), but not for the cultivar Petit Manseng.
Vines were planted in 2008 in Dobson, North Carolina (elevation 2,000 feet), using 101-14 MGT rootstock, cordon-trained, and spur-pruned on a vertically shoot positioned (VSP) trellis. Four treatments were applied each season with six replicates of six vines each and included
(1) control – no nitrogen or sulfur applications
(2) nitrogen (calcium nitrate) at 30 kg/ha applied to soil just after flowering
(3) 15 kg/ha of urea nitrogen in two foliar applications prior to véraison
(4) 15 kg/ha of nitrogen (as urea) and 5 kg of micronized sulfur (microthiol) in two applications prior to véraison.
It should be noted that wine is a complex matrix, with wine and juice aromas and flavors resulting from many interactions among a large number of compounds. Compounds may show synergistic (one compound enhances the perception of another) and/or antagonistic (one compound suppresses the perception of another) interactions (Gustav et al. 2011). These interactions can determine the overall aroma and flavor of wine, without being recognized for their aroma threshold (Etievant 1991). Aroma and flavor compounds in mixtures almost always show reciprocal suppression, in which each compound decreases the perceived intensity of the others (Laing et al. 1984). Some aroma and flavor increases may be due to volatiles binding to proteins, resulting in a decrease in their suppressive effect on other free volatiles (Jones et al. 2008).
The current research showed that both acid and enzymatic hydrolysis demonstrated potential to release bound aroma and flavor compounds. Acid hydrolysis released more terpenes with nitrogen applications than did enzymatic hydrolysis, while ester content increased in both treatments with nitrogen-treated vines. Esters are known to contribute floral and fruity aromas and flavors to wine, while terpenes contribute citrus, floral and sweet aromas and flavors.
This study demonstrated that seven major groups of aroma and flavor compounds are potentially responsible for the varietal aroma and flavor of Petit Manseng. The use of various winemaking practices to release glycosidically-bound aroma and flavor compounds may be needed for expression of full aroma and flavor potential. Data presented in this study may be useful in establishing recommendations for vineyard nitrogen fertilization for Petit Manseng.
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