Aroma compounds in Ontario Vidal and Riesling icewines. II. Effects of crop level.

Icewine is a dessert wine of substantial commercial value to the Canadian wine industry. Many grapegrowers crop icewine-designated vines at levels double those for table wines; therefore, the experimental objective was to ascertain whether reducing crop level might impact icewine chemical and aroma compound profiles. Three treatments [control (fully-cropped); cluster thin at fruit set to one (basal) cluster per shoot (TFS); cluster thin at veraison (TV)] were evaluated in randomized block experiments for Riesling and Vidal over two seasons (2003-04; 2004-05). Treatments differed in must pH and titratable acidity (both years) and although wines differed for most standard chemical variables, no clear trends existed. Vidal icewines had the highest aroma compound concentrations in the control and TV (2003) and in TFS (2004). Most Vidal aroma compounds differed with crop level: 17/24 (2003) and 23/24 (2004). Vidal odor activity values (OAVs) were highest for: β-damascenone, ethyl octanoate, cis-rose oxide, 1-octen-3-ol, ethyl hexanoate, isoamyl acetate (2003); β-damascenone, 1-octen-3-ol, ethyl octanoate, cis-rose oxide, and ethyl hexanoate (2004). Principal component analysis (PCA) found β-damascenone, ethyl 2- and 3-methylbutyrate, ethyl isobutyrate, ethyl butyrate and 1-heptanol correlated and associated with the control (2003), but most compounds were positively loaded on PC1 and associated with replicate, not crop level (2004). All Riesling aroma compounds differed with crop level (2003) and 22/23 (2004). Both years, most aroma compounds were highest in TV and lowest in TFS wines. Riesling OAVs were highest for: β-damascenone, ethyl octanoate, and ethyl hexanoate (2003, 2004); cis-rose oxide was highly odor potent (2004). PCA of Riesling showed most compounds loaded on PC1 and associated with TV wines (2003). Freeze/thaw events in November/December appeared more important in aroma compound development than adjustment of crop level, and it is therefore concluded that reduction of crop level in Vidal and Riesling vines would not substantially impact icewine aroma composition.