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Chapter 12

Sensory Analysis and Quantitative Determination of Grape Glycosides The Contribution of These Data to Winemaking and Viticulture

Downloaded by UNIV OF PITTSBURGH on December 23, 2014 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0637.ch012

Patrick J. Williams and I. Leigh Francis Australian Wine Research Institute, P.O. Box 197, and Cooperative Research Centre for Viticulture, P.O. Box 145, Glen Osmond, South Australia 5064, Australia The contribution that glycosidically-bound volatiles make to varietal wine aroma was determined by sensory descriptive analyses. Sensory studies were made on grape glycosides a) hydrolyzed under accelerated conditions i) in vitro and back added to wine, and ii) in situ in wine; b) hydrolyzed under conditions of natural aging. These studies confirmed the role of glycoside hydrolysis in the expression of varietal wine aroma. The sensory data have lent support to the development of a method for quantifying total glycosides in wine grapes, thus giving an indication of juice 'richness'. This assay, made through a determination of the glycosyl-glucose, offers the possibility of an objective pre-harvest measure of grape, and hence potential wine quality.

Recognition of the presence of glycosidically-conjugated flavor precursors infruitsof all major horticultural classes has, in the last few years, stimulated much interest in these compounds. Advances in research on glycosidic flavor precursors of plantderived foods has been the subject of several recent reviews (1-4). Grapes and wines were among the earliest products to be investigated in this field with research centering initially on monoterpene glycosides; this research helped in elucidating the role of monoterpenes as flavor compounds of floral grape varieties (5). The subsequent recognition of glycosides of C13 norisoprenoid compounds and of shikimic acid-derived metabolites as precursors of non-floral grape flavor, was a later development (6). Further aspects of the involvement of glycosides in the flavor of grapes and wines have been recently discussed (7-9). New developments in glycoside research of grapes and wines are: a) the use of formal sensory descriptive analysis to investigate the precise role of glycosides in flavor expression, and b) the possibility of grape and wine quality evaluation through the quantification of glycosides. These developments are discussed here. 0097-6156/96/0637-0124$15.00/0 © 1996 American Chemical Society

In Biotechnology for Improved Foods and Flavors; Takeoka, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

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Sensory Studies on Grape Glycosides Hydrolyzed in vitro and Back-Added to Wine. The first studies in this series determined the sensory effects of accelerated hydrolysis of grape glycosides, in vitro, for three white wine varieties, ie Semillon, Sauvignon Blanc and Chardormay (10) and the black grape, Shiraz (11). The primary aim of these works was to determine, by sensory analysis, whether the isolated grape glycosides on hydrolysis and back addition to a wine medium, produced an aroma that was related to the aroma of wines made from the varieties. It was found that glycosidesfromeach of the varieties, on acid hydrolysis gave aromas that had sensory properties common to wines made from the same juices. While the hydrolysates from the three white varieties showed several shared aroma attributes each was, nevertheless, distinctive. By direct comparison of the sensory properties of the hydrolysates with wines for three of the varieties ie Chardonnay, Semillon (10), and Shirazf/iJ it was evident that glycosidic flavor precursors contribute to important aroma attributes in these wines. In the case of Shiraz, it was further shown that precursor hydrolysates contain aroma compounds that are important to high quality wines of the variety (11). Sensory Studies on Glycosides Hydrolyzed in situ. Having established that glycosides can contribute to wine flavor, the next series of experiments examined processing steps that could be used to accelerate glycoside hydrolysis in white winemaking. A high temperature-short time treatment of either juices or wines produced no discernible effect on wine aroma, as measured by duotrio difference tests. However, storage of wine protected from air at 45°C for several weeks gave wines with aromas clearly different from the controls. Sensory descriptive analysis showed that this thermal treatment produced wines with complex aromas, similar in character to the aroma properties of wines that had been cellar stored for many months (12). Headspace gas chromatographic studies showed that there were significant increases in the amounts of norisoprenoid compounds as a result of the treatment. Headspace analysis also confirmed that the thermal conditions used were sufficient to hydrolyze glycosides and release the same norisoprenoids (13). Furthermore, statistical treatment of the instrumental and sensory data indicated that the concentration of several of the norisoprenoids in the wine headspace was significantly correlated with aroma descriptors from the descriptive analysis study (14). These experiments thus showed that rapid hydrolysis of glycosides in situ can be effected by heat treatment, and that this induces desirable flavor changes. The Effect on White Wine Aroma of Altering the Concentration of Glycosides in Juice. It could be argued that the conditions of accelerated hydrolysis employed in the in vitro and in situ hydrolysis experiments described above were far removedfromthose experienced by a wine during vinification and natural aging. For example, an increased temperature may alter the balance of volatiles released by acid catalyzed hydrolysis compared to the pattern of products that would be obtained at typical cellar



a

Semillon juice (21 L) passed through a glass column packed with XAD-2 resin (5.0 kg)

After Treatment A the XAD-2 resin was washed with water (3 x 1 L), eluted with ethanol (1.5 L); the glycosidic eluate was evaporated to near dryness in vacuo, extracted with Freon 11, and taken up in 25% v/v aqueous ethanol (240 ml), which was added to Thompson Seedless juice (21 L).

As for treatment B, with the glycosidic eluate in 25% aqueous ethanol (240 mL) added to Semillon juice (21 L).

Thompson Seedless with added Semillon glycosidic isolate

Semillon with added Semillon glycosidic isolate

Conditions

Semillon XAD-2 treated

Description

Treatment

Semillon untreated

Thompson Seedless untreated

Semillon untreated

Description

A l l juices were obtainedfroma commercial winery and filtered before use.

B

Code

Semillon juice (21 L) to which was added 25% v/v aqueous ethanol (240 ml).

Thompson Seedless juice (21 L) to which was added 25% v/v aqueous ethanol (240 ml)

Semillon juice (21 L) passed through a glass column packed with glass wool.

Conditions

Control

Table I. Summary of juice treatments for the preparation of wines made with and without glycosides

a



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temperatures. Also, the base wine medium may have influenced the aroma of the samples presented for sensory analysis in the in vitro experiment. Importantly, neither experiment examined any possible sensory effects of fermentation on the glycosides. Whilst the effects of fermentation on glycosylated constituents in juice is unknown, studies monitoring the concentration of monoterpene glycosides before and after fermentation, indicate that yeast do not assimilate the glycosides (8, 15). To explore these issues, small-scale fermentations were carried out with Semillon and Thompson Seedless juices that had been treated so that the glycoside concentration in the juices was altered. Sensory analysis of the samples was performed to determine what effect these treatments had on the aroma of the finished wines. Experiments to Determine the Aroma of Wines Made With and Without Glycosides. Small-scale fermentations were carried out with Semillon juice that had been treated using XAD-2 resin so that the glycosidic fraction was either absent, intact, or augmented (14). In addition, a Thompson Seedless juice was fermented with and without added Semillon glycosides. Table I gives a summary of the juice treatments. Experiment A consisted of intact Semillon juice as a control, with a volume of Semillon juice that had been passed through a bed of XAD-2 resin as the treatment. The effectiveness of the XAD-2 resin in removing glycosides from the juice in this experiment is described below. The treatments in experiments B and C were the appropriate juice with a quantity of Semillon glycosidic isolate added, so that the treated Thompson Seedless juice in B contained Semillon glycosides at single strength, and the treated Semillon juice in C contained glycosides at double strength. For each experiment, the controls and treated juices were divided into three replicate lots for fermentation. Malolactic fermentation did not proceed in any of the treatment or control samples except for one replicate for treatment B which showed a malic acid decrease of 20%. After fermentation, the samples were bottled and cellar stored at 18±4 °C. Difference Testing at Different Stages of Cellar Storage. To determine whether any flavor change for the wines had occurred as a result of the juice treatments, duo-trio aroma difference tests were done between control replicates and treatment replicates after 6, 12, 20, and 27 months cellar storage. The first three series of comparisons were done between randomly selected pairs of control and treatment replicates. The results of the difference tests indicated that after 12 months aging in the bottle there was no significant difference in aroma between wines made from the control juices and wines made from the treated juices. Testing of the wines after 20 months storage, and also after 27 months storage - when 45 out of a possible 48 combinations of pairs of control and treatment samples were compared - showed that for each of the three experiments there were significant differences in aroma between the control and treated wines. To assess the effect of the juice treatments on wine flavor, sensory descriptive analysis was undertaken by a trained panel of 11 judges, scoring a subset of the samplesfromthese experiments in duplicate. There was insufficient volume of sample



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to allow all of the treatments to be assessed, and so only two wines were analysed, ie the treatment wine from experiment A (made from Semillon juice that had been treated by XAD-2 resin to remove the glycosides), and the treatment wine from experiment C (made from Semillon juice with added glycoside isolate) (see Table I). From Figure 1, which shows a plot of the mean descriptive analysis data for the two wines, it is clear that differences between the two were considerable. The wine that had been fermented from juice treated so as to have its glycosides removed, was rated as lower in all attributes than the augmented sample, except for the attribute grassy. In particular, the attributes lime, honey, oak, and toasty were significantly enhanced for the latter sample. Importantly, these were characterizing features shown in the descriptive analysis studies of Semillon hydrolyzed glycosides in the in vitro and in situ studies and also of bottled-aged Semillon wines (14). A significant difference between these samples was also observed for the floral attribute, however judge inconsistency for this attribute, as measured by an analysis of variance, meant that the validity of this conclusion was doubtful. This work demonstrated that increasing or decreasing the concentration of glycosides of juice affects wine aroma after an aging period. Because the hydrolysis of glycosides occurred under natural conditions in this study, the sensory effects that were observed are likely to be the same as those that would occur in a Semillon wine during conventional vinification and storage. In the earlier in vitro and in situ studies it was possible that, due to differences in activation energy among the glycosides and their aglycons, hydrolysis at elevated temperatures may have produced volatile compounds in proportions different from that obtained in hydrolysis at cellar temperature, which may have been largely responsible for sensory effects observed. From this study, in which the variable of accelerated hydrolysis is removed, it is evident that the glycosides alone are major contributors to aroma. It is further evident that a period of cellar storage provides time for slow acid-catalysed hydrolysis of the glycosides to occur resulting in the release of volatiles and the expression of varietal aroma. Observation of this delayed expression also suggest that the grape glycosides that release aroma volatiles apparently pass through the fermentation unchanged, without being either degraded or hydrolyzed by the fermentation process. Quantification of the Glycosylated Secondary Metabolites Through a Determination of the Glycosyl-Glucose (G-G). In formally establishing the link between the aroma characteristics of hydrolyzed grape glycosides and wine aroma, this sensory research has given strong support to the results of earlier studies into the chemical composition of the volatiles released from the glycosides (16-20). Together these sensory and compositional studies show the importance of glycosylated secondary metabolites of grapes to potential wine flavor, thus to wine quality and, by implication, to grape quality. It is logical, therefore, that an analysis of the glycosylated secondary metabolites, could give an objective measure of quality. This concept means that, in parallel with research that is necessary to investigate individual flavor molecules and to develop methods to quantify each one of these and its precursor(s), some more rapid method of analysis of the total glycosylated flavor precursors is appropriate.


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floral

toasty Figure 1. Sensory profile plot of mean aroma intensity ratings (n=l 1 judges x 2 replicates) for attributes of a wine made from Semillon juice treated with XAD resin , and a wine made from Semillon juice with glycoside isolate added . Also shown are significance levels where ns, *, **, *** indicate not significant, and significant at p

Sensory Analysis and Quantitative Determination ... - ACS Publications

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