More than 800 volatile compounds have been identified in wine.¹ Some of these compounds can be associated with varietal characteristics or are generated during fermentation, while others are considered undesirable when they occur.² Volatile compounds become part of the wine mix by different sources. In grape sugars, for example, fermentation releases primary metabolites ethanol and CO₂, secondary metabolites esters, acids and higher alcohols, non-volatile grape-derived precursors such as monoterpenes, norisoprenoids and some thiols that are released by enzymatic action by bacteria and yeasts, and esters and diacetyl³ from the action of malolactic bacteria.

The effects of crop level reduction on berry composition are normally an increase in sugar level (Brix) and a corresponding increase of ethanol. Crop reduction may increase free and bound terpenes,⁴ individual monoterpenes and norisoprenoids,⁵ anthocyanins and phenols,⁶ as well as increasing volatile acids.⁷ Delay of harvest also is linked to an increase in Brix by a reduction in berry weight due to dehydration processes.⁸ In addition to Brix, phenolics⁹ and aroma compounds¹⁰ are either concentrated or new ones are produced. The drying of fruit also generates shrinkage, which modifies the shape and dimension of the transported products.¹¹

This change or reduction of volatiles and polyphenols is not only due to concentration but to changes in metabolism.¹² Dehydration by controlled processes can reduce ethyl acetate and acetic acid,¹³ and increase ethanol and acetaldehyde, among other compounds.¹⁴ Wines made from dehydrated grapes normally contain more terpenes and norisoprenoids.^8,13 Grapes that undergo dehydration are susceptible to microbial spoilage, leading to Botrytis cinerea-derived increases in higher alcohols, and production of high amounts of other alcohols such as glycerol, arabitol and mannitol.¹⁵ Sour rot can reduce terpenes; for example, free geraniol, nerol and linalool concentrations declined and trans-furan linalool oxide, benzyl alcohol, 2-phenylethanol increased in one study of Riesling.¹⁶

We chose to experiment with different harvest dates to determine whether keeping a full crop with an extended harvest date might have a greater positive impact on wine volatile composition than to reduce the crop level. In many situations, our growers already have a balanced vine (i.e., <10 Ravaz Index) and don’t need to drop crop for the sake of balancing the vine. The overall objective for this project was to determine the impact of harvest date and crop control on the wine volatile composition of four grape cultivars—two whites (Riesling and Pinot Gris), and two reds (Cabernet Sauvignon and Cabernet Franc)—commonly produced in the Niagara Peninsula region of Ontario, Canada.

Description of methods
Experimental design: Two crop levels imposed at véraison, full crop (FC) and half crop (HC), as well as three harvest dates (including two harvest dates after regular harvest) were combined in a factorial treatment arrangement containing six treatment combinations. Harvest date treatments were T0 (normal commercial harvest), T1 (three weeks after T0), and T2 (three weeks after T1). Wines were produced from each treatment replicate using standard methods.17 During 2011 and 2012, analysis of volatile compounds in wine samples by gas chromatography–mass spectrometry (GC-MS) was performed to determine whether differences existed between the two crop levels and between harvest dates.

Sample preparation: Aroma analysis by GC-MS was carried out in 2011 and 2012 wines for the four grape cultivars based on Bowen & Reynolds18 with adjustments. A 30ml sample was taken from each wine treatment replicate and kept at 4º C under N2 inert gas until analysis. In dup­licate, 100 µL of an internal standard, prepared with 10 µL of 98% 1-dodecanol (Aldrich; Oakville, Ontario) in 10ml of 100% ethyl alcohol was poured into 10ml volumetric flasks to
volume. The prepared sample was transferred into a 10ml Gerstel extraction vial.

A 10 mm stir bar ( the Gerstel Twister) coated with polydimethylsiloxane (PDMS; 0.5 mm film thickness) was added to the sample and stirred for one hour at 1,000g for extraction at room temperature. After extraction the stir bar was removed, rinsed, dried and then placed in a 4ml amber vial at 4º C until analysis the same day. The stir bar was then inserted into an extraction glass tube inside the thermal desorption unit attached to GC-MS. GC-MS conditions and conditioning of material were identical to Bowen & Reynolds.18 Stir bars used for extraction were previously conditioned before use every time to avoid any cross contamination. After analysis, each stir bar was kept overnight in a solution of 80:20 acetonitrile/ methanol, allowed to dry and then placed at 250º C for two hours with a constant flow of N2 inert gas.

Calibration compounds and odor activity values: Scan analysis reflected more than 100 volatile compounds in wines from all cultivars. For calibration purposes, 30 compounds were chosen as highest in priority. Three-point calibration curves were created for each compound for quantification. Aromatic standards were obtained from several well-known chemical suppliers. Model wine was used for calibration curves and prepared using 12% (v/v) of pure anhydrous ethanol diluted in Milli-Q water and 5 g/L of tartaric acid with a pH adjusted to 3.6 with 1N NaOH. Each aroma standard was diluted first in pure anhydrous ethanol at 1,000 mg/L and kept at 4º C until analysis, then diluted at different concentrations in model wine. Calibration samples were analyzed in SIM/SCAN mode using the same conditions as described previously with use of the same internal standard. Odor activity values were calculated as a ratio between each concentration obtained by calibration vs. their respective thresholds obtained from literature.

Descriptive sensory analysis
Panel training: Both panel training and descriptive analysis were consistent with practices previously described for white¹⁹ and red wines.¹⁷ Panel training was conducted to create a set of standard descriptors across all tasters. The panel consisted of 12 panelists (six males and six females) ages 23 to 56, and all the participants were students or faculty members from the Brock Un iversity Cool Climate Oenology and Viticulture Institute (CCOVI) with previous tasting experience. Six 60- to 90-minute training sessions were conducted weekly for each cultivar. A final list of aroma, flavor and basic taste descriptors was generated from the most frequently identified descriptors from the initial two sessions.

Aroma standards were made as reference descriptors to define wine characteristics and modified during the training period. All standards were made in neutral Riesling and Cabernet Franc base wines and stored at 4° C. All standards were presented as 30ml samples in ISO wine glasses. Standards represented the “high intensity” anchor term at the far right end of the respective line scales (15 cm). In each training session, panelists were required to independently evaluate the two wine samples and use an unstructured linear scale of 0-15 points to define the intensity of wine attributes. The panel training was carried out in November 2013 for Cabernet Franc and Cabernet Sauvignon, and March-April 2014 for Riesling and Pinot Gris.

Descriptive analysis: The tasting sessions also were scheduled twice per week, and no time limit was required for the panelists. The tastings were arranged in the sensory lab in CCOVI in individual booths, and Compusense software was used for data collection. The wines were kept at 18° C until tasting, and ISO glasses were used for the analysis. The same aroma standards as the training session were available to the panelists. A one-minute break between each sample and a 30-minute break between each flight were programmed in the software. A 15-point linear scale marked with “absent” and “high intensity” at two ends was used for the panelists to indicate the intensity of aromas or flavors.

Each panelist tasted one flight of six wines in duplicate for each of the three fermentation replicates (six flights total) consisting of each crop level x harvest date treatment. Wines were marked with three-digit numbers and presented in a random order for all cultivars. Data collection took place in November/December 2013 for Cabernet Franc and Cabernet Sauvignon, and May/June 2014 for Riesling and Pinot Gris.

Chemical and sensory results for aroma compounds
Pinot Gris: Pinot Gris contained 23 volatile noteworthy compounds. Some were highly odor active, and one would expect them to impact the general aroma in the wine. Differences between treatments occurred in 2011 and 2012 for several volatile compounds. With respect to crop levels, only three—isobutyl alcohol, terpinolene and diethyl succinate (the latter was only detected in HC treatment)—differed between HC and FC in 2011. In 2012, four compounds—1-hexanol, hexyl acetate, ethyl phenyl acetate, and terpinolene—decreased with crop reduction, while ethyl acetate and β-damascenone increased.

More compounds were affected by delayed harvest than by crop reduction. Terpinolene decreased with extended harvest date in 2011 but was only detected in T2 in 2012. (See “Terpenes in Pinot Gris and Riesling.”) Citronellol was only detected in T2 in 2011. Damascenone decreased with harvest date both seasons. As to esters, ethyl acetate was unaffected (see “Ethyl Acetate and Isoamy Acetate”), while isoamyl acetate and ethyl caproate decreased in 2011 relative to delayed harvest date. Higher concentrations were detected at all harvest dates in 2012 compared to 2011. Ethyl caprylate decreased with harvest date in 2012. (See “Ethyl Caprylate and Hexyl Acetate.”) Hexyl acetate was reduced with delayed harvest in both years, finishing with undetected levels at T2. Ethyl phenyl acetate increased both years relative to harvest date, while diethyl succinate decreased in 2011 and increased in 2012.

With respect to alcohols, isoamyl and phenethyl alcohol were quite elevated in T2 wines. (See “Isoamyl Alcohol and Phenethyl Alcohol.”) Isobutyl alcohol was reduced by delayed harvest in 2011. 1-Hexanol increased with delayed harvest, but only in 2012, and concentrations were lower than in 2011. As to acids, octanoic acid was reduced with delayed harvest in 2011 but increased slightly in 2012. Decanoic acid decreased with delayed harvest date both years.

Riesling: Riesling contained 27 volatile compounds of interest. Crop reduction affected several compounds. Ethyl butyrate, phenyl ethyl alcohol, citronellol, geraniol and β-damascenone were reduced in concentration by cluster thinning in 2012 but were not impacted in 2011. Some compounds were only detected in one year. In 2011, diethyl acetal was only present in FC/T2, while geraniol increased with crop reduction. Hexyl acetate increased with crop reduction in 2011 but was reduced in 2012, and the same occurred for isoamyl acetate and terpinolene. Ethyl phenyl acetate was reduced in cluster-thinned wines in 2011 but was only detected when crop was reduced in 2012.

Delayed harvest date increased ethyl butyrate, isobutyl alcohol, linalool, terpinolene and citronellol, and decreases were noted for hexyl acetate, octanoic acid, hexanoic acid, ethyl caprylate and β-damascenone in both years. Geraniol decreased with delayed harvest date in 2011. As to esters, ethyl acetate increased with delayed harvest, but isoamyl acetate decreased with delayed harvest in 2011 and increased in 2012. Ethyl caprylate and hexyl acetate decreased both years with delayed harvest. As to alcohols, isoamyl and phenethyl alcohols and 1-hexanol increased slightly with harvest date, while 1-heptanol was present only in T2/FC in 2012. Decanoic acid decreased with delayed harvest.

Cabernet Franc: Cabernet Franc contained 22 volatile compounds of interest. Reductions in crop increased citronellol and ethyl phenyl acetate and reduced ethyl heptanoate in both years. In 2012 there were crop reduction-related increases in 1-hexanol, ethyl caprylate, ethyl caproate, 2-phenethyl acetate, isoamyl acetate, ethyl acetate and benzaldehyde, while reductions also occurred in diethyl acetal and diethyl succinate. 1-Heptanol and decanal were detected in 2012 only, with a reduction and increase, respectively, with crop reduction. Ethyl butyrate was the only compound present in both years that increased with crop reduction in 2011.

Diethyl acetal and diethyl succinate increased with increased harvest date in both years, while 1-hexanol, citronel­­lol, β-damas­cenone and decanoic acid decreased. Among esters, ethyl acetate and ethyl caprylate increased with harvest date in 2011, and isoamyl acetate increased in 2012. Oth er esters that increased relative to extended harvest date were ethyl butyrate (2011) and ethyl phenyl acetate (2012; not detected in T1). Ethyl heptanoate was found only in T2 wines.

Among alcohols, isoamyl and phenethyl alcohols were unaffected, isobutyl alcohol decreased, and 1-heptanol was present only in 2012. Among other compounds, octanoic acid decreased with delayed harvest, decanal was only present in FC/T1 (2012), and benzaldehyde was only present in T2 wines both years.

Cabernet Sauvignon: Cabernet Sauvignon contained 21 volatile compounds of interest. Crop reduction reduced 1-hexanol and increased benzaldehyde in both years. Compounds reduced by crop reduction were phenyl ethyl alcohol, octanoic acid, isoamyl alcohol, ethyl caproate, 2-phenylethyl acetate and decanoic acid (2011 only). Diethyl acetal in 2012 was reduced with cluster thinning, while diethyl succinate and ethyl heptanoate increased. Hexanoic acid was only present in HC/T0 in 2011, while it was reduced with crop reduction in 2012. 1-Nonanol, on the other hand, was only present in HC/T2 in 2012 but was lowered with crop reduction in 2011.

Delayed harvest date reduced 1-hexanol, phenyl ethyl alcohol, hexanoic acid and decanoic acid in both years. Among esters, ethyl acetate increased substantially with harvest date in 2011. Isoamyl acetate and ethyl caprylate diminished with harvest date in 2011. Ethyl butyrate was likewise reduced in 2012 but only present at T2 in 2011. Ethyl caproate followed the same trend but was present in all harvest dates in 2011. Ethyl heptanoate was present in 2012 only with an increase between T1 and T2. Diethyl acetate was only present in T0 (2011), and in FC/T2 in 2012. Among alcohols, isoamyl alcohol increased slightly with harvest date in 2012. Phenethyl alcohol decreased relative to delayed harvest. Isobutyl alcohol was only detected in 2012 wines with reductions relative to harvest date. Among other compounds, octanoic acid was reduced with delayed harvest in 2011 only. Citronellol was only detected in 2012 wines with reductions relative to harvest date, and was not present in T2. β-Damascenone was only detected in 2011 wines with a reduction with delayed harvest date.

Sensory analysis
Pinot Gris: Crop level had very few effects on Pinot Gris in both seasons (data not shown). Reducing crop level increased body in 2011; crop reduction in 2012 led to lower levels of lemon and bread (yeasty) flavors and acidity. Harvest date was responsible for several differences including reductions in lemon (2011) and bread aroma (2012), and increases in honey (2011 and 2012) and floral (2012) aromas. Delayed harvest also increased bread (2011), floral (2012), honey (2012) flavors in addition to body and length (both seasons).

Riesling: Crop level had no impact on Riesling aroma in either year, but crop reduction reduced apple/pear, peach/apricot and honey flavors in 2011, in addition to decreasing sweetness and increasing acidity. A slight decrease in acidity relative to crop reduction was measured in 2012. Numerous aroma and flavor attributes were impacted by harvest date (“See Harvest Date and Riesling” on page 159). Delayed harvest increased peach/apricot (2011 and 2012), mango (2011 and 2012), honey (2011) and floral (2012) aromas, and decreased green apple (2011) and grassy (2011) aromas, while apple/pear was highest for T1 wines (2011). Delayed harvest led to increased peach/apricot (2011 and 2012), mango (2011 and 2012), floral (2012), honey (2012), and vanilla flavors (2012), in addition to greater body and length (2011), and enhanced sweetness plus reduced acidity (2012). Sweetness was reduced and acidity increased by delayed harvest in 2011. Delayed harvest also increased apple/pear and green apple flavors in T1 wines (2011).

Cabernet Franc: Reduced crop led to reductions in red fruit aroma and flavor in 2011, but the response was opposite in the much warmer 2012 season. Crop reductions also reduced vegetal aroma and dried fruit flavor in 2012. Numerous harvest date-related responses occurred (“See Harvest Date and Cabernet Franc” on page 159); these included increases in dried fruit (2011), earthy (2011), red and dark fruit (2012) aromas, and decreases in red and dark fruit (2011), herbaceous (2011), vegetal (2012) and earthy (2012) aromas. Flavor attributes that increased with delayed harvest included: dried fruit (2011 and 2012), spicy (2011) and dark fruit (2012), while red fruit (2011) declined. Body (2011) likewise increased relative to delayed harvest, while astringency and acidity were highest in T1 wines in 2011.

Cabernet Sauvignon: Few crop level effects were measured with the exception of increases in dark fruit aroma (2011) and reductions in astringency, bitterness and length (2012). Most of the sensory effects in Cabernet Sauvignon were similar to those measured in Cabernet Franc. The majority of harvest date effects were confined to 2011; delayed harvest increased dried fruit (2011), earthy (2011) and dark fruit (2012) aromas, and reduced red and dark fruit (2011) and herbaceous (2011) aromas (data not shown). Delayed harvest also led to increases in dried fruit (2011) and dark fruit (2012) flavors as well as bitterness, body and length (2011) and astringency (2011 and 2012); the latter mouthfeel-related responses were primarily confined to T1 wines. Red fruit and herbaceous flavors decreased with delayed harvest in 2011.

Lessons learned
Viticultural treatments imposed by this study had impacts on wine aroma composition, and delayed harvest made a greater impact in aroma compounds than crop reduction, possibly due to higher availability of sugars and other chemical metabolites. Varietal aroma compounds such as terpenes in white wine cultivars increased in concentration, making them more varietal-like and intense, while ?-damascenone decreased with delays in harvest date.

The vintage impacted primary aroma compounds, with higher concentrations in 2012 than 2011 in some cases, perhaps linked to the higher Brix values in 2012 in all cultivars. Riesling was less affected by vintage, since higher concentrations for these compounds were detected in 2011 than 2012.

Esters were always linked to delayed harvest date more than crop reduction. In some cases concentrations declined (e.g., ethyl caproate, isoamyl acetate and hexyl acetate), while increases occurred for other compounds (e.g., ethyl caprylate). The effect of vintage was evident, with higher concentrations in 2012 in isoamyl acetate and ethyl caproate in white cultiva rs, but higher concentrations in 2011 for ethyl caprylate. Volatile acids were reduced with delayed harvest date.

Higher alcohols sometimes were impacted by delay of harvest, with reductions in phenyl ethyl alcohol and hexanol. For hexanol, this reduction was likely beneficial in red wines since decreased concentration would reduce the grassy-green odor that it characterizes. Higher isobutyl alcohol in Riesling could be linked to the presence of sour rot in grapes and increases in nonanol in Cabernet Sauvignon with delayed harvest date, particularly T2, might suggest the presence of B. cinerea.

Extended harvest had a greater impact on most wine aroma compounds compared to crop reduction. This is linked not only with higher Brix, but also changes in other metabolites that either increased or decreased with delayed harvest. Climatic conditions and vintage played important roles in the development and presence of volatile components, which are important considerations to take into account when delaying harvest is chosen.

Andrew G. Reynolds is professor of viticulture at the Cool Climate Oenology and Viticulture Institute at Brock University in St. Catharines, Ontario.