Odor Descriptors and Aromatic Series During the Oxidative Aging of Oloroso Sherry Wines

Abstract

The aroma of Oloroso sherry wines obtained by industrial oxidative aging was studied by gas chromatography-olfactometry analysis. The aromagram exhibited five different odorant zones (OZ) separated by no-odor intervals of at least 6 min, in which no odor was detected. Based on the perceived aroma frequency the most distinctive series for OZ₁ was fruity. The OZ₂ and OZ₃ were mainly represented by chemical and fatty series, respectively, and the spicy and empyreumatic series were the most representative for the OZ₄ and OZ₅, although exhibiting few notes. Except for the vegetable, floral, and empyreumatic series, the remainder increased the number of odorant notes perceived during aging.

Keywords:

• Sherry wines
• Oloroso wine
• Oxidative aging
• Aroma series
• Odor descriptors
• Sniffing

INTRODUCTION

Sherry white wines can be classified into two different categories, depending on the way they are aged in oak casks. Thus, so-called “Fino” wine is obtained by biological aging over a long period (5–6 years) under the action of flor yeasts, which develop aerobically on the surface of wines containing 15–15.5% ethanol.[1,2] Fino wine exhibits special sensory features, including a pale yellow color, a slightly bitter flavor, and complex aroma. This last feature is developed during its biological aging, largely as a result of the action of flor yeasts and the contribution of volatile compounds extracted from the wooden casks where the wine is aged. In addition, flor yeasts protect the wine from chemical browning, which helps to preserve its pale color throughout the aging period,[3] and contact of the wine with the casks’ wood causes some of the wood components to be extracted from which this wine acquires its characteristic bouquet. On the other hand, “Oloroso” sherry wine is obtained by fortifying the must with ethanol prior to aging, in order to prevent growth of flor yeasts and ensure exclusively oxidative aging. Under these chemicals conditions, Oloroso wine develops a dark color and its aroma evolves differently from that of Fino wine.[4–6 6

Essentially, the industrial aging method used in the area involves storing the wine in 500 L American oak casks, which are stacked in rows called escalas to form a criaderas and solera system. The casks in each escala contain the identical same wine at the same aging stage. The first escala, called solera, is the closest to the ground and contains the oldest wine. A fraction from 1/4 to 1/5 of its contents is periodically withdrawn for bottling. After each withdrawal, the barrels in the solera are replenished with wine from the second escala, also called first criadera, which in turn is replenished with wine from the third escala (second criadera) and so forth. The topmost escala, contains young wine from the year's vintage.

The aroma profile of wine is extremely complex. Some analytical techniques, such as gas chromatography–mass spectrometry, have revealed the presence of more than 800 volatile compounds—many, however, are at very low concentrations.[7–9 9 In any case, instrumental techniques can detect chemicals, but are unable to identify those with a perceivable odor; in fact, only a small number of volatile compounds are odor-active and contributors to wine aroma.[10–15 15 Gas chromatography–olfactometry (GC–O), also known as “sniffing,” is a highly useful technique for establishing aroma profiles, as it allows the discrimination of odor-active compounds in complex matrices.[16–21 21 In some cases, the discriminating ability of GC-O is more than adequate; in most, however, establishing the specific contribution of each individual odor compound to the overall aroma requires using an alternative method. To this end, the GC–O is being increasingly used in combination with sophisticated olfactometric methods to estimate the sensory contribution of odor active compounds.[22–24 24 In this work, changes in the total number of odor descriptors detected by olfactometry and aroma series during oxidative aging in Oloroso sherry wines were studied with a view to estimating their contributions to the aroma profile of the wines, as well as to establishing the sequence of odorant series constituting their aroma fingerprint.

MATERIALS AND METHODS

Wines

Oloroso sherry wines (grape cv. Pedro Ximenez) selected by expert tasters of the Quality Regulation Board as the most representative among the wines produced in 15 wine cellars in the Montilla-Moriles region were used. The wines were subjected to oxidative aging for 0 (young wine), 3, 4, 6, 9, or 14 years. The oldest wine is marketed as a typical high quality Oloroso sherry wine. Because the concept of vintage is not applicable to sherry wines, the aging times were calculated following commercial criteria (by considering the age and volume of the mixed wines in the oak casks). One sample per aging time (or escala) was obtained from each cellar. All wines of identical age were mixed and immediately analyzed by triplicate. The Oloroso wine is produced by the typical industrial aging process, known as criaderas and solera, in American oak casks. The cask wood is about 25 years old, with a medium toasted level, a high density, and low porosity and permeability.

Identification of Aroma Compounds

The identification of each aroma compound was carried out by its retention time, co-elution with a standard (purity >99%, Sigma Aldrich, Munich, Germany), and confirmed by mass spectrometry (Hewlett-Packard 5972 MSD, Palo Alto, CA, USA). The scan mode was used with a mass range from 39 to 300 amu and a scan rate of 1.6 scan/s. The chromatographic column, injector and oven temperatures, carrier gas and its flow were the same as those used for the sniff described below.

Gas Chromatography-Olfactometry Analysis

Samples of 100 mL of wine were adjusted to pH 3.5, 150 μg of 2-octanol was added as an internal standard and then extracted with 100 mL of freon-11 (Sigma-Aldrich Quimica, S.A., Madrid, Spain) in a continuous extractor for 24 h (liquid-liquid extractor for use with solvents with higher density than sample). After concentrating the freon extracts to 0.2 mL in a micro-Kuderna-Danish concentrator, the GC–O analyses were carried out in a Hewlett-Packard-5890 series II gas chromatograph (Hewlett-Packard, Agilent Technologies, Palo Alto, CA, USA) equipped with a sniffing port (Olfactory Detector, part. No. 093500, SGE-International, Ringwood, Australia) connected by a flow splitter to the column exit. The GC effluent was split 1:2 between the Flame Ionization Detector and the sniffing port. Humidified air was added in the sniffing port at 33 mL/min. Three microliters were injected into the chromatograph equipped with a HP-INNOWax fused silica capillary column of 60 m × 0.32 mm × 0.25 μm thickness (Agilent Technologies, Palo Alto, CA, USA). The oven temperature program was as follows: 5 min at 45°C, 1°C/min up to 185°C and 30 min at 185°C. Injector and detector temperatures were 275 and 300°C, respectively. The carrier gas was helium at 70 kPa and split 1:30. Three trained judges, one woman and two men, selected for their ability to generate accurate terms as well as experience in CG–O sherry wines, performed the extract sniffing. The panelists took turns for 15 min and again up to 140 min (total time of aromagram). Samples were sniffed three times, as previously mentioned, one session per day, and the aroma descriptors and approximate times were recorded.

RESULTS AND DISCUSSION

Table 1 shows the results of the GC–O analysis of the wine during its biological aging. Based on the high complexity of the odor profile, some compounds may be associated to two or three different notes.[25,26] As in the previous work, the most common odor descriptors were classified into aroma series in order to reduce the number of variables to be interpreted in establishing the aroma profile for the wines.[1,27] As can be seen, a total of 45, 49, 56, 56, 56, and 67 aroma notes were detected in the sixth (0 yr), fifth (3 yr), fourth (4 yr), third (6 yr), second (9 yr), and first escala (14 yr), respectively. Odor notes were grouped into five different odorant zones (OZ) with an interval between them of more than 6 min without detecting any odor. OZ₁ spanned the times from 8.7 to 39.4 min in the olfactogram, and from OZ₂ to OZ₅ the time ranged 48.7–57.3, 64.9–104.5, 117.9–125.7, and 133.5–136.5 min, respectively. OZ₁ and OZ₃ in combination contained 81% of all aroma notes detected at the different aging stages; therefore, they may be the most useful for olfactometric analysis. The two zones were followed by OZ₂, with 10% of notes, and by OZ₄ and OZ₅, with the remaining 9%.

OZ₁ contained odors associated to the fruity, chemical, fatty, empyreumatic, and vegetable series, the former two being the most typical for this zone. Fruity notes (strawberry, apple, melon, pear, peach, and hazelnut) were due to the presence of acetaldehyde, ethyl acetate, 1,1-diethoxyethane, ethyl isobutanoate, ethyl butanoate, isoamyl acetate, ethyl hexanoate, and ethyl lactate, in addition to five unidentified compounds. The older wines exhibited a greater number of fruity aromas mainly due to strawberry, apple, and peach notes (unidentified, 12.9 min), which were only detected after 6 years of aging (third escala), and also strawberry and acid apple notes (unidentified, 9.1 and 26.0 min), which were only perceived in the oldest wine (14 yr, first escala). All descriptors in the chemical

Table 1  Retention time (Rt), odor descriptors and odorant series for the compounds detected by sniffing in Oloroso wines during the oxidative aging

					Escala (years)
	Rt (min)	Odor descriptors	Odorant series	Compound	6^a (0)	5^a (3)	4^a (4)	3^a (6)	2^a (9)	1^a (14)
OZ1	8.7	Pungent, fruity	Fr	Acetaldehyde	+*	+	+	+	+	+
	9.1	Strawberry, apple, toasted	Fr, E	n.i.	−	−	−	−	−	+
	9.9	Strawberry, apple, melon	Fr	Ethyl acetate + 1,1-Diethoxyethane	+	+	+	+	+	+
	10.8	Apple, strawberry, pear	Fr	Ethyl isobutanoate	+	+	+	+	+	+
	11.5	Butter, biscuit	Fa	2,3-Butanedione	+	+	+	+	+	+
	11.6	Vinous, buttery	Ch, Fa	Methyl butanoate	−	−	−	+	+	+
	12.7	Apple, strawberry, pear	Fr	Ethyl butanoate	+	+	+	+	+	+
	12.9	Strawberry, apple, peach	Fr	n.i.	−	−	−	+	+	+
	13.7	Strawberry, peach, glue	Fr, Ch	n.i.	+	+	+	+	+	+
	15.4	Fruity	Fr	Isoamyl acetate	+	+	+	+	+	+
	20.0	Alcohol, solvent	Ch	Isoamyl alcohols	+	+	+	+	+	+
	22.0	Strawberry, fruit-dried	Fr	Ethyl hexanoate	−	+	+	+	+	+
	26.0	Acid apple	Fr	n.i.	−	−	−	−	−	+
	27.7	Herbaceous, fresh	V	n.i.	−	−	−	−	−	+
	33.6	Pear, melon, toasted	Fr, E	n.i.	+	+	+	+	+	+
	35.0	Hazelnut	Fr	Ethyl lactate	+	+	+	+	+	+
	39.4	Vinous	Ch	n.i.	−	−	−	−	−	+
OZ2	48.7	Strawberry, melon	Fr	n.i.	−	−	−	−	−	+
	51.3	Melon, damp	Fr, Ch	n.i.	+	+	+	+	+	+
	54.0	Vinous	Ch	n.i.	−	−	−	−	−	+
	55.0	Glue	Ch	n.i.	+	+	+	+	+	+
	57.3	Fruity, fruit-dried	Fr	Linalool	−	+	+	+	+	+
OZ3	64.9	Cheese, buttery, milky	Fa	Butanoic acid	−	−	+	+	+	+
	70.1	Cheese, milky, rancid	Fa	n.i.	+	+	+	+	+	+
	75.6	Cheese	Fa	3-Methylbutanoic acid	+	+	+	+	+	+
	79.0	Fresh, tea	V	Methionol	+	+	+	+	+	+
	84.6	Sweet, flowers	Fl	2-Phenylethanol acetate	+	+	+	+	+	+
	88.9	Minty	B	n.i.	−	−	−	−	−	+
	95.9	Minty, medicinal	B	n.i.	+	+	+	+	+	+
	100.5	Rose, talc	Fl	2-Phenylethanol	+	+	+	+	+	+
	104.5	Fruity, wood, toasted	Fr, E	Z-Oak lactone	−	−	+	+	+	+
OZ4	117.9	Toasted bread, wood, smoked	E	n.i.	+	+	+	−	−	−
	120.7	Anise, spicy	S	n.i.	−	−	−	+	+	+
	125.7	Toasted, wood	E	n.i.	+	+	+	−	−	−
OZ5	133.5	Curry	S	Sotolon	−	−	+	+	+	+
	136.5	Acid apple, lemon, burnt wood	Fr, E	n.i.	+	+	+	−	−	−
*The “+” indicates that sensation was detected by at least two sniffers and the “–” when the odor was detected only by one or no sniffers.
Fr: fruity; E: empyreumatic; Ch: chemical; Fa: fatty; B: balsamic; V: vegetable: Fl: floral; S: spicy; n.i.: not identified.

series (vinous, glue, alcohol, and solvent) were due to the presence of methyl butanoate, isoamyl alcohols, and two unidentified compounds at 13.7 and 39.4 min, respectively. Chemical notes were present from the beginning—by exception, vinous notes were only detected in the third and lower escalas (6 yr and older wine). Fatty notes (butter, biscuit, and buttery), which were detected at 11.5 and 11.6 min—the last one only in the wines older than 6 yr—were due to 2,3-butanedione and methyl butanoate. The toasted aroma was the sole odor associated to the empyreumatic series; this note was detected at 33.6 min (unidentified compound) throughout the aging process and at 9.1 min (unidentified compound) in the first escala (14 yr). Finally, the first escala exhibited herbaceous and fresh notes (unidentified, 27.7 min) associated to the vegetable series.

OZ₃ was mainly characterized by the fatty and floral series, even though it also contained balsamic, vegetable, fruity, and empyreumatic odors. Fatty notes in this zone (cheese, buttery, milky, and rancid) were due to butanoic acid (64.9 min) in the fourth and lower escalas. However, 3-methylbutanoic acid (75.6 min) and an unidentified compound at 70.1 min were detected throughout the aging process. Flower, rose, and talc odors (floral series), which were related to the presence of 2-phenylethanol acetate (84.6 min) and 2-phenylethanol (100.5 min), were present throughout the aging process as well. Minty and medicinal odors (balsamic series) were related to the presence of two unidentified compounds at 88.9 and 95.9 min, respectively. The number of notes in this series remained virtually constant throughout the aging process, with a slight increase in the first escala (14 yr) due to the effect of the supply of the unidentified compound at 88.9 min. In addition, the tea odor (vegetable series) detected in this zone, which has been related to methionol (79.0 min), remained active throughout the aging period. Finally, fruity and empyreumatic odors (wood, toasted) were both due to Z-oak lactone (104.5 min), which was only detected after 4 years of aging (fourth and lower escalas).

OZ₂ was defined by the chemical and fruity series, which contributed a virtually identical number of notes. Damp, vinous, and glue odors, which belonged to the former series, were due to the presence of three unidentified compounds with retention times of 51.3, 54.0, and 55.0 min, respectively; however, the vinous odor was only detected in the oldest wine. Fruity notes in this odorant zone (strawberry, melon, and fruity–dried fruit) were related to the presence of two unidentified compounds (48.7 and 51.3 min) in addition to linalool (57.3 min). The strawberry and melon notes due to the first unidentified compound (48.7 min) were only detected in the oldest wine.

OZ₄ contained only empyreumatic and spicy odors. Toasted bread, wood, and smoked odors were due to two compounds detected at 117.9 and 125.7 min, which became inactive in the third escala (6 yr). On the contrary, anise and spicy odors in this zone (120.7 min) were detected from the third escala. Finally, OZ₅ exhibited spicy, fruity, and empyreumatic odors. The curry odor detected in the fourth (4 yr) and lower escalas was due to sotolon, whereas the acid apple, lemon, and burnt wood notes were related to an unidentified compound at 136.5 min, which, unlike sotolon, only remained active during the first 4 years of aging.

Figure 1 shows the aroma fingerprint for Oloroso wine during its oxidative aging as derived from the number of olfactory detections related to the odorant series. As can be seen, the fruity series was that exhibiting the greatest number of detections, well ahead of all others. The number of fruity odors detected increased during the first 4 years of aging and then there are no changes up to 9 years, after which it increased to the end (14 yr). The fruity series was followed by the empyreumatic, chemical, and fatty series, which remained virtually constant during the first 4 years. The number of empyreumatic notes decreased beyond that point, whereas that of fatty and, especially, chemical notes, increased throughout the remainder of the process. Finally, the floral, balsamic, vegetable, and spicy series were detected once or twice at most. Floral notes remained constant throughout, whereas balsamic, vegetable, and spicy notes increased over the aging period. Balsamic and vegetable notes only increased at a late stage (after 9 years), whereas spicy notes rose after the third year.

Figure 1 Aroma fingerprint of Oloroso wine during the oxidative aging as derived from the number of olfactory detections by sniff. (Color figure available online.)

CONCLUSION

The fruity series is the one that contributed the greatest number of odor notes to the aroma of Oloroso wine, well ahead of all other aroma series, after which the empyreumatic, fatty, and chemical series followed, finishing with the spicy, floral, balsamic, and vegetable series. The number of fruity, chemical, fatty, balsamic, vegetable, and spicy notes increased, empyreumatic notes decreased, and that of floral notes remained constant, during the aging process. These results allow one to detect changes in the aroma profile of Oloroso wines during their oxidative aging, and also to identify the particular active odors which most markedly contributed to their overall aroma.

ACKNOWLEDGMENT

This work was supported by a grant from the Andalusian Government (Spain) (Project AGR-767).