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A Technical Guide for Wine Producers
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Fructose Utilisation By Anchor Red Wine Yeast Strains
Neil Jolly1, Karien O'Kennedy2 and Graham Reid2
1ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Fructose, wine yeast, glucose-fructose ratio
Introduction
Stuck and lagging fermentations are found periodically in wine industries. There are numerous causes, including incorrect choice of yeast strain, incorrect yeast rehydration procedures, incorrect temperature control during fermentation, inadequate yeast nutrition and high ethanol levels during the latter part of fermentation. Another reason may be an imbalance between glucose and fructose, especially near the completion of high sugar fermentations. During fermentation the glucophilic Saccharomyces cerevisiae preferentially uses glucose above fructose (Reed & Peppler, 1973; Margalith, 1981; Cason & Reid, 1987; Cason et al., 1987) and if the glucose-fructose ratio (GFR) drops to below 0.1, fermentation can stop (Gafner et al., 2000).
At veraison, glucose concentrations in grapes are normally higher than fructose concentrations (Boulton et al., 1996). As grapes ripen, the fructose levels increase, with a concomitant decrease in GFR and overripe grapes have higher fructose than glucose concentrations (Amerine & Thoukis, 1958; Kliewer, 1965; Kliewer, 1966; Snyman, 2006). According to Kliewer (1965) ripe grapes are defined as being between 20° and 24°B. Above 24°B is considered overripe. However, today’s modern winemakers in pursuit of optimal ripeness and fruity styled wines are redefining ripeness and often harvest grapes at higher degrees of Balling. The locality of the vineyard (terroir), viticultural practices and grape cultivar obviously play an important role in determining at what sugar level the grapes are ripe. Harvesting at high sugar concentrations presents additional challenges for yeasts with regard to osmotolerance at the start and ethanol tolerance at the end of fermentation. Higher ripeness in most cases translates to a lower GFR (Amerine & Thoukis, 1958; Ough & Amerine, 1963; Kliewer, 1965; Kliewer, 1966; Jolly, unpublished data, 2007). The GFR of grapes at harvest can range from 1.07 to 0.78 in the range of 16.4 to 27.0°B (Amerine & Thoukis, 1958; Snyman, 2006).
It was shown that fermentations starting with an initial lower GRF (ca. < 0.8) are at a higher risk of leading to a stuck fermentation (Jolly, 2007). Fortunately, strains of the wine yeast S. cerevisiae differ in their ability to utilise glucose and fructose (Cason et al., 1987; Berthels et al., 2004), so the correct choice of yeast will limit this risk.
The aim of this study was to evaluate a range of Anchor Yeast red wine yeasts for their ability to utilise high and low GFR musts at normal and high fermentation temperatures.
Materials & Methods
Laboratory-scale fermentations were carried out in 500 ml aliquots of previously frozen, non-sterile white grape must (21.5°B, 0.39 g/L volatile acidity, 3.17 g/L total acidity, pH 4.07) at an ambient temperature of 22° and 28°C, respectively. A yeast nutrient (Nutrivin Super, Anchor Bio-Technologies, Cape Town) was added at 0.5 g/L to all the must samples. Fructose (30 g/L) was added to a portion of the must to decrease the GFR from approximately one to below 0.8. The Anchor Yeast red wine yeasts (Table 1), one French red wine yeast (reference 1) and an Anchor Yeast white wine yeast, N 96 (reference 2) were rehydrated as per manufacturer’s recommendations and inoculated at 20 g/hL. All the fermentations were done in duplicate and the fermentation vessels with tightly fitting fermentation caps were placed on an orbital shaker. Glucose and fructose analyses for determination of the GFR were done by Koelenhof Wynlaboratorium, Stellenbosch using an enzymatic technique. Residual sugar analyses were done according to the Rebelein method (SAWLA, 2002).
Results & Discussion
In this study the laboratory-scale fermentations mimicked commercial-scale fermentations. The placement of the fermentation vessels on an orbital shaker emulated the natural turbulence found in large fermentations as a result of the generation of CO2 (Henschke, 1990). The tightly sealed fermentation caps ensured that no oxygen entered the fermentation vessels. The data presented in Table 2 shows that at 22°C the yeasts were able to ferment the normal GFR must to dryness (< 3 g/L sugar), but a few yeasts appeared to struggle in the low GFR must.
The yeasts were also able to ferment the normal must at 28°C. However, the yeasts were generally less efficient in fermenting the higher Balling/low GFR (fructose-added) must to dryness at 28°C. The higher fermentation temperature puts greater stress on the yeasts. In addition, it is most likely that ethanol also plays a role, as ethanol is more toxic at higher than lower temperatures (Casey & Ingledew, 1986; Van Uden, 1989). At 28°C the Anchor Yeast hybrid red wine strains (prefixed NT) generally showed better utilisation of sugar than the two natural isolates (WE 372 and the French reference yeast). The most suitable yeasts (with the least g/L residual sugar) for low GFR must at 28°C were NT 112 and NT 202. In these experiments, the low GFR was on average 0.77, which is a fairly extreme situation. Similar ratios have none the less previously been reported, particularly when grapes are harvested at higher Ballings (Snyman, 2006; Jolly, unpublished data, 2007).
It is also known that NT 112 can produce SO2 under stress conditions (data not shown), which can possibly delay the onset of malolactic fermentation. This is an added bonus for situations where temporary inhibition of MLF is desired (e.g. micro/macro-oxygenation). On the contrary, caution should be taken when NT 112 is used for fermentations where a quick onset of MLF is desired. The NT 202 yeast, on the other hand, has been shown to stimulate MLF (H. du Plessis, personal communication, 2007) and this yeast is therefore recommended for grape varieties where MLF can be problematic, e.g. Merlot.
Recommendations
Based on the results of this study, it is recommended that for high sugar musts (above 24°B) and where a low GFR may be expected, fermentation temperatures should be lowered to limit the toxic effect of the alcohol on the yeast. For additional security the use of an Anchor Yeast NT hybrid strain is recommended.
For further details, please contact Neil Jolly at e-mail: jollyn@arc.agric.za
References
Amerine, M.A. & Thoukis, G., 1958. The glucose-fructose ratio of California grapes. Vitis 1, 224-229.
Summary
There are numerous causes of stuck wine fermentation. One of these is the imbalance between glucose and fructose (more fructose than glucose). A selection of Anchor Yeast red wine yeasts was therefore evaluated at two different temperatures to determine their ability to ferment musts with normal and a low glucose-fructose ratio (more fructose than glucose). It was shown that the normal must could be fermented at both temperatures, but the low glucose-fructose ratio must at the higher temperature led to incidents of stuck fermentation. Two of the yeasts i.e. NT 112 and NT 202 could ferment well under all conditions tested and are therefore recommended for use in situations where a low glucose-fructose ratio may be expected.
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