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A Technical Guide for Wine Producers
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Stuck and sluggish ferments - An efficient solution to restart
Olivier Fernandez1, Céline Bajard-Sparrow1, Céline Fauveau1, Patrice Pellerin1 and Peter Lankhorst2 and Ockert Augustyn1
1 DSM Food Specialties - Montepellier - France
Stuck and sluggish fermentations are still a major concern, waste of time, and in the worst cases lead to irreversible wine spoilage. Problem fermentations are now better known, which makes it possible to develop prevention tools, including selected yeast strains and winemaking practices such as the addition of nitrogen and oxygen during mid-fermentation (Sablayrolles and Al, 1996), or the use of complete activators based on inactivated yeast (Lebrun and Pellerin, Revue onologues).
Although problems are more likely to occur with high alcohol, ethanol is far from being the sole cause. The chances of not consuming sugars to dryness increase with high sugar content, low yeast available nitrogen, late fungicidal treatments, difficult to ferment varieties and a lack of control over the winemaking process. The combination of several of these factors will increase the probability of the occurrence of a problem.
The multiplicity of causes makes stuck and sluggish fermentations quite unpredictable and restart procedures are often not guaranteed and difficult. With Extraferm© yeast hulls used in combination with Fermichamp© (selection n° 67J INRA Narbonne), DSM proposes an original curative solution.
Must detoxification with Extraferm© yeast hulls
During fermentation, wine yeast produces several auto-toxic compounds such as ethanol, carbon dioxide and medium-length chain saturated fatty acids (C6, C8 and C10) which inhibit sugar transport (Larue and Al, 1982; Salmon and Al, 1993). The objective of must detoxification is to remove these compounds and create more favourable conditions for yeast's growth activity.
Yeast hulls not only detoxify, but also provide nutrients and yeast "survival factors" such as fatty sterols and long-chain unsaturated fatty acids. These compounds increase the restarting yeast strain viability at the end of fermentation.
An innovative process
Traditional yeast hulls can transmit a yeast odour to the wine, even when used at low dose rates (from 20 g/hl). Moreover, their effectiveness in detoxification is sometimes limited.
DSM research developed an innovative production process, with the objective of obtaining highly adsorbent yeast hulls that would not transmit any odour to the treated wine even at high dose rates.
The yeast hulls resulting from this innovative process are marketed under the brand Extraferm©. This new product was established according to the international oenological Codex method in DSM laboratory. A model decanoïc hydro-alcoholic solution, containing 3 mg/l of acid, was treated with increasing amounts (100 to 800 mg/l) of yeast hulls. The fatty acids were detected by NMR, after homogenization and 20 hours at ambient temperature. The analysis showed Extraferm© hulls had better adsorption capacity of toxic compounds (figure 1). In a model solution 40 g/hl of Extraferm© hulls eliminated 30% of the decanoïc acid.
Laboratory tests revealed more properties of interest in winemaking:
Restarting stuck fermentations in production conditions
Production scale trials in France, Germany and Portugal confirmed the technical interest of DSM yeast hulls. Of all the restarting procedures tested, treatment with 30 g/hl of Extraferm© (figures 3 and 4) gave the best results, shorter re-fermentation duration (13 days) and lower residual sugars and volatile acidity.
The absence of yeast odour in treated stuck wines was confirmed. Wine characteristics were preserved for dose rates up to 100g/hl (the legal maximum amount being fixed at 80g/hl). Re-fermented stuck wines were submitted to sensory analysis to evaluate the "yeast odour" (0="no perceived yeast odour", 5="very marked yeast odour"). The results (table 1) confirm the effectiveness of DSM innovating production process.
Fermichamp©, a high alcohol tolerant fructophile yeast to restart fermentations
Even after adsorption of yeast inhibitors, an arrested must remains hostile for yeast development as it contains high quantities of ethanol and very poor nitrogen and nutrient levels. Moreover, the fermentable sugar ratio is considerably modified by the first fermentation. Although grape musts are composed of glucose and fructose in equal quantities, glucose is fermented in a preferential way. The fructose/glucose ratio thus increases throughout fermentation. With most yeast strains, fructose accounts for 70% of fermentable sugars at mid-fermentation and over 95% when residual sugars are below 20g/l.
In light of the specific conditions of a stuck wine, the qualities of an adapted yeast to restart fermentations can be defined:
Comparison of the effectiveness of four yeast strains to restart stuck fermentations
In a study carried out in collaboration with the Research units of INRA Pech-Rouge and Montpellier, four yeast strains recommended to restart stuck fermentations were compared.
An arrested must was obtained by incomplete fermentation of a highly clarified Chardonnay (5 NTU), de-oxygenized, without any nutrient addition (nitrogen 300 mg/l) for an initial potential alcohol of 12,5 %. This must was inoculated at 5 g/hl with a yeast strain known for its weak performances. An arrested fermentation followed with the following characteristics: 11,8 %vol. alcohol and 15,5g/l residual sugar.
The must was sulphated at 5g/hl, racked, divided into 1 litre fermentors and then inoculated with the various restarting yeast strains at 30 g/hl. The tests were duplicated. Of all strains, INRA Narbonne n° 67 J, also known as Fermichamp© proved the most efficient (figure 5).
Fructophile yeast, the right choice
The majority of strains used to restart fermentation belong to the Saccharomyces cerevisiae bayanus subspecies, according to the old yeast classification. Despite their resistance to ethanol, these strains, like the majority of S. cerevisiae, have a greater affinity for glucose resulting in failure to complete fermentations to dryness. This difficulty increases for low residual sugar content musts where fructose is largely predominant.
The strain no 67 J, not only has an alcohol tolerance above 16% but also has a greater capacity to ferment fructose than most selected oenological yeast strains.
A close monitor of the respective glucose and fructose contents during fermentation proved that the use of 67 J allowed an almost simultaneous consumption of both sugars (table 2). The combination of DSM production technology allowing to achieve maximum yeast viability and Fermichamp© enhanced affinity for fructose makes this strain the ideal yeast to restart stuck fermentations.
Interest of the combined use of an adapted strain and efficient yeast hulls
The Chardonnay must described above was used to compare 2 methods of restarting stuck fermentations:
For the 4 strains tested, the complete method allowed faster and more complete sugar consumption, the best results being obtained with Fermichamp© (figure 6). The positive effect of yeast hulls treatment was empathised with the other yeast strains (figure 7), proving the positive impact of yeast hulls in restarting stuck fermentations and this independently from their cause.
Conclusion
When, in spite of all the precautions taken, a stuck fermentation occurs, it is essential to restart it in a timely fashion in order to avoid wine spoilage. With the joint use of Extraferm© yeast hulls and the fructophile yeast Fermichamp©, DSM offers a solution that proved to be the most efficient both in lab and production conditions.
Acknowledgements: the authors wish to thank both Evelyne Aguerra from the Experimental Unit of Oenology INRA Pech-Rouge and Christian Picou of the Research Unit Sciences for the oenology of the INRA Montpellier for their support in the realisation of the tests.
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