In this article the relationship between glycerol and wine quality is critically reassessed in the context of modern South African (SA) wine styles and cultivars. For this purpose two sets of data were used: a) a quantitative database containing the analytical data of the glycerol levels of some 450 commercial SA wines of which ~90% received Veritas awards (1999 and 2000 Competitions) and were therefore of adjudged quality (Nieuwoudt et al., 2002) and b) a qualitative database containing the opinions of an expert panel of 15 individuals on various aspects regarding glycerol and wine quality (this article). These aspects were communicated by means of a short questionnaire. The panel consisted of individuals involved with the training of winemakers, experts in the field of wine chemistry and leading SA winemakers. All members have previously been involved in the official judging of wine quality. The winemakers on the panel are specialist producers of SA wines and are actively involved with the international marketing of SA export wines. The panel members, several of whom have expressed particular interest in glycerol in wine, were encouraged to express their own personal opinions gained through practical experience. The producing cellars represented diverse geographical winemaking regions of SA, including Paarl, Robertson, Worcester, Cederberg, the Cape Peninsula, Stellenbosch and Hermanus.
Some new perspectives
The results of the expert opinions on the range of questions submitted to the panel are presented in the following sections.
On the perceived importance of glycerol for wine quality and the nature of its contribution the questions were:
1.Is glycerol in your opinion important for wine quality?
2.How does glycerol add to wine quality in your opinion?
The majority (80%) of the panel members were of the opinion that glycerol is important for wine quality. The perceived contribution was generally defined in terms of mouth-feel and texture properties. Glycerol was perceived to ensure consistency in style and to confer “suppleness” to wine and a “roundness” and “smoothness” on the palate. In addition, glycerol was thought to confer “fullness” (also referred to as “viscosity” or “weight”) to wine and to lessen the perception of acidity, particularly in dry white wines. Several panel members were of the opinion that glycerol contributed to the complexity of wine and the length of the finish. Its contribution to sweetness was considered significant, but only when present at high concentrations.
The input of the panel members provided valuable insights with respect to the attributes that are sought in wine. The key issue here is clearly whether these attributes can be positively linked to glycerol. The concept of mouth-feel is one of the most complex, but also least understood sensory attributes of wine (Pickering et al., 1998). The absence of reliable sensory data not only complicates the formulation of a workable definition for mouth-feel, but also the identification of the component(s) in wine responsible for the perceived attributes. No positive relationship between glycerol per se and the mouth-feel attributes of wine has yet been established and several factors other than glycerol have been implicated in mouth-feel. These include the ethanol concentration, the yeast cell wall mannoproteins, barrel maturation, yeast autolysis, the yeast strain used, as well as phenolic compounds in red wines (Ribereau-Gayon et al., 1998; Deltail & Jarry, 1992). Furthermore, at the concentrations at which glycerol is normally found in wine, the impact that it could have on the viscosity of wine would probably not be perceived by even the most experienced tasters (Noble & Bursick, 1984). Against this background it is quite possible that the perceived contribution of glycerol to mouth-feel can easily be over-emphasised.
On particular wine styles and optimal glycerol concentration the questions were:
1.Which wines do you think would benefit from glycerol?
2.What would you say is the optimal level of glycerol in any wine of your choice?
The panel members suggested that several white wine styles could benefit from glycerol. These were Chardonnay (especially barrel fermented wines), the fuller-styled Chenin blanc and Sauvignon blanc wines, as well as Semillon and Viognier. The optimal glycerol concentrations suggested ranged from 5 – 7 g/L for the white wines, although a few members felt that much higher concentrations of 10 – 15 g/L would be favourable. Glycerol was considered to be important for the quality of the noble late harvest wines. Several panel members were of the opinion that the red cultivars, particularly Cabernet Sauvignon and Pinot Noir, would also benefit from glycerol and optimal concentrations of 5 – 8 g/L were suggested. In general, the opinion was that the quality of the delicate-styled wines would be negatively affected and the fuller-styled wines (red and white) would be positively affected by glycerol. “Putting a figure to the optimal concentration of glycerol in a particular wine style however, does not make much sense” according to Manuel Bolliger, previously from Cape Point Vineyards, and in his opinion, “optimal is the quantity derived from physiologically ripe fruit”.
In our assessment of glycerol levels in SA wines, no major differences between mean glycerol levels of wines of different quality ratings were observed for the dry white and dry red styles respectively (Table 1). The glycerol concentrations in the noble late harvest wines were, however, significantly associated with quality. Unfortunately, no analytical data were available for Semillon, Viognier and Pinot Noir, and these cultivars should be included in future studies. The perception that there is a strong positive correlation between glycerol concentration and the quality of the dry white and dry red wines is therefore not supported by the analytical data. Within the concentration ranges reported, no major differences between mean glycerol levels of the delicate-styled and the fuller-styled Chardonnay wines were found (Table 2). The perception that glycerol has a negative impact on wine quality in the delicate-styled wines, was therefore also not supported by the analytical data obtained in this study.
Taking into account that glycerol is strongly associated with mouth-feel, the overall opinion of the panel members clearly suggests that the mouth-feel properties of the white wine styles in particular, need to be improved. It follows naturally that the impact of higher glycerol levels on the quality of these wines should be investigated. In two studies where glycerol-overproducing yeast strains were used to produce Chardonnay wines, the quality of the experimental wines was, however, rated less favourably than the control wines, although glycerol levels in excess of 15 g/L were formed in some cases (Prior et al., 2000; de Barros Lopes et al., 2000). It should be noted that the attainment of the high glycerol levels (10 – 15 g/L) suggested by some panel members would have major implications on the carbon flux during yeast glycolysis. Reports in the literature show that the higher levels of glycerol formed are also coupled with elevated levels of other metabolites, notably acetic acid. Current genetic studies are focussed on decreasing the acetic acid accumulation of these strains (Eglington et al., 2002).
On the glycerol levels of SA wines the question was:
1.What is your opinion of the glycerol levels of top quality SA wines?
The general consensus amongst the panel members was that the glycerol levels of top quality SA wines were too low. Some members suggested that the SA wines had lower glycerol levels than their Australian counterparts. Data on a recent large-scale assessment of the glycerol levels of Australian wines are unfortunately not available and data reported in the early 1970s were used for the purposes of comparison. Figure 1 shows a comparison between the average glycerol levels of the SA wines used in this study (190 white and 237 red wines) and those of wines from Australia (37 red and 11 white wines; Rankine & Bridson, 1971), California (15 red and 16 white wines; Ough et al., 1972) and New York State (26 red and 37 red wines; Mattick & Rice, 1970). On average the mean glycerol levels of both SA red and white wines were higher than that of Australian wines and were more similar to the average levels reported for the New York State wines. For all countries represented by this data set, red wines had higher average glycerol levels than white wines. It should be noted that the mean glycerol levels reported for white wines were higher in the earlier studies as opposed to later studies, both in California and SA (Table 3). These differences could be explained partly in terms of the worldwide change towards lower fermentation temperatures for white wines (Rib‚reau-Gayon, et al., 1998). The decrease in the mean glycerol levels in Californian red wines (10.6 g/L in 1954 versus 6.5 g/L in 1972) was ascribed to better cap management and picking the grapes at lower sugar contents (Ough et al., 1972). In conclusion, although the glycerol levels of wines of different countries are frequently quoted (together with other data) for the purposes of establishing a benchmark for quality, it is clear that such interpretations should be done in the context of the impact of the fermentation conditions used during a particular period and winemaking region and not merely quality per se.
On factors affecting the glycerol levels and attempts at manipulation of glycerol levels during fermentation the questions were:
1.Which factors do you think contribute to the final level of glycerol in wine?
2.Have you tried practical ways of influencing glycerol levels during fermentation?
Factors listed by the panel members to affect the final glycerol levels of glycerol in wine, included the yeast strain, ripeness of the grapes, fermentation temperature and the degree of Botrytis infection of the grapes. Several panel members have previously tried various practical techniques during fermentation in attempts to increase the final glycerol levels. These included temperature shocks (high and low), the addition of higher levels of SO2, the use of spontaneous fermentations, picking grapes at optimal ripeness and the addition of fresh must during the fermentation process. With respect to the latter technique, the results obtained during a research programme conducted at Jordan Winery in the Stellenbosch district during the 2001 harvest season, are reported in an accompanying article in this issue. In related research projects with which Martin Meinert (Meinert Wines, Stellenbosch) was associated, a variety of strategies aimed at increasing the glycerol levels were evaluated, but only minor increases could be obtained. Martin is of the opinion that glycerol plays a small role in wine quality at the levels normally found in wine. Similar opinions were recently voiced by Pascal Rib?reau-Gayon, who stated that winemakers place too much emphasis on the organoleptic role of glycerol and that the pursuit of winemaking conditions that are more conducive to glycerol production, was in his opinion, of no enological interest (Ribereau-Gayon, et al., 1998).
On the importance of glycerol analyses the questions were:
1.Is glycerol analyses of your wines important to you, and if so, why?
2.Have you had any of your wines analysed for glycerol?
Only a minority (23%) of the winemakers have previously determined the glycerol content of their wines, but more than 75% were of the opinion that glycerol analyses were important to ensure quality. One of the winemakers considered the analyses of the glycerol levels of different blocks of grapes important, particularly in the case of Botrytis infected grapes. In a few instances it was stated that no glycerol analyses have previously been done on the wines, due to the lack of or inaccessibility to analytical facilities. These opinions clearly underline the need for the development of a fast and reliable technique for the analyses of glycerol in SA wines.
The assessment of the contribution of glycerol to wine quality is challenging, particularly in view of the complexity and subjective nature of wine quality. In this study a combined qualitative and quantitative approach was used in an attempt to a) communicate the perceptions of a specialist panel of leading winemakers and other stakeholders in the SA wine industry on this topic and b) to critically reassess the commonly held perceptions regarding glycerol and wine quality. The results obtained in this study certainly provide new insights regarding major aspects of the relationship between glycerol and wine quality, and should stimulate critical thinking, especially with regard to the advantages and possible risks involved in attempts to manipulate the fermentation conditions to achieve higher glycerol levels.
It should also be noted that there is a renewed interest in the glycerol content of wine, not only from a perspective of wine quality, but also in terms of quality control. In several countries glycerol is used, together with other parameters, to verify the authenticity of the fermentation process itself, but also the source of the glycerol present in wine. The assessment of the origin and amount of glycerol in wine should therefore become a general requirement for SA wines.
The authors gratefully acknowledge the permission of the Organising Committee of the SA National Wine Show Association to obtain wine samples during the Veritas Competitions, as well as the willingness of the winemakers to allow their wines to be used for experimental purposes. The enthusiastic support of the members of the evaluation panel is sincerely appreciated. Financial support by Winetech and the THRIP programme of the National Research Foundation is gratefully acknowledged.
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About the Authors:
Helene Nieuwoudt1,2, Bernard Prior1, Sakkie Pretorius2 and Florian Bauer2
1) Department of Microbiology, Stellenbosch University, Private Bag XI, 7602 Matieland, South Africa
2) Institute for Wine Biotechnology, Department of Viticulture and Oenology, Stellenbosch University, Private Bag XI, 7602 Matieland, South Africa