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A Technical Guide for Wine Producers
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Industry-wide investigation into the occurrence and causes of problem fermentations
Sulette Malherbe, Florian Bauer & Maret du Toit
This research is sponsored by Winetech
Introduction
Stuck and sluggish fermentations remain a major oenological concern for winemakers because of the economic impact of fermentation problems. Incomplete or "stuck" fermentations are defined as those leaving a higher than desired residual sugar content at the end of alcoholic fermentation, while slow or sluggish fermentations are characterised by a low rate of sugar utilisation (Bisson, 1999). Although these terms are often associated with alcoholic fermentation, the same problems might also occur in the case of malolactic fermentation. The focus of this article will be on alcoholic fermentation problems.
The current literature related to stuck or sluggish fermentation is substantial. A significant amount of information about the metabolism, physiology, cell biology, and stress adaptation in Saccharomyces cerevisiae has been generated under laboratory conditions. However, extrapolation from such studies has to be done with caution since these conditions do not exactly mimic industrial winemaking conditions.
Nevertheless, those studies have been useful in identifying numerous factors that may affect fermentation rate and cause sluggish or stuck fermentations, including high initial sugar content, nutrient, and in particular nitrogen limitation, ethanol toxicity, organic and fatty acid toxicity, presence of killer factors or other microbially-produced toxins, cation imbalance, temperature extremes, pesticide, and fungicide residues, microbial competition and poor oenological practices (Henschke, 1997; Ingledew and Kunkee, 1985). These factors may induce decreases in the metabolism of the yeast cell and consequently decreases in biomass production, cell viability and fermentation rate. Fermentation problems are rarely due to one single factor in isolation but usually the result of a combination of several factors resulting in a complex problem. The possibility of a synergistic effect amongst these factors renders problem fermentations a rather challenging subject to investigate (Alexandre and Charpentier, 1998).
Metabolic basis of stuck and sluggish fermentations
The metabolic basis of stuck and sluggish fermentation has been fairly well established. The decrease in rate of sugar consumption is correlated with a decrease in sugar uptake capacity. Glucose and fructose consumption are reduced in response to various environmental or cellular stress conditions. Nutrient limitation (macronutrient and micronutrient), low pH, lack of oxygen, lack of adequate agitation, temperature extremes, presence of toxic substances, presence of other micro-organisms, imbalance of cations, and poor strain tolerances (particularly to ethanol or acetaldehyde). All of these have been associated with stuck and sluggish fermentations and have an impact on glucose and fructose transporter expression and activity (Alexandre and Charpentier, 1998).
Causes of stuck alcoholic fermentation
Glucose/fructose ratio
A new project on stuck and sluggish fermentation: What makes this project different?
As part of a Winetech research programme on improving the fermentation performance of yeast (project number IWBT 5/02), this project involves an industry-wide investigation into the occurrence and causes of problem fermentations.
Due to the multiple factors which can lead to slow or incomplete fermentation, previous studies on problem fermentations were mostly limited to investigating one or two factors.
This project is designed to overcome this weakness and to use industry data to assess the causative factors responsible for stuck fermentations in the South African context. The project will aim at establishing a database containing information on the occurrence, all parameters assessed (physical/chemical etc.) and the combination of causative factors. Chemometrics will be used for the statistical multifactorial analysis of this data. With the use of chemometric techniques it is possible to extract maximum information from analytical, spectral and sensory data. Careful application of these chemometrics could lead to the identification of valuable relationships amongst stuck fermentation samples that have gone unnoticed before. This novel approach to problem fermentations will therefore allow the holistic investigation of numerous factors simultaneously.
As conventional chemical analysis is both time consuming and expensive, especially in winery environments that operate hundreds of fermentation tanks simultaneously, infrared spectroscopy could offer an alternative to conventional chemical analysis. Many applications of IR to wine analysis have been found, however references to the monitoring of large scale wine fermentations are scarce (Urtubia et al., 2004), and the monitoring of problem fermentations nearly non-existent. Infrared spectroscopy (IR) can consequently be used for monitoring fermentations since many compounds can be measured quickly from a single sample without prior treatment.
During the 2005 harvest season problem fermentation wine samples were collected or received from various South African wine regions and the cultivar distribution was as follows (see graph):
In order to establish a representative dataset of problem fermentations, as much samples as possible are needed for this study, originating from all wine regions. This is where we need your co-operation in 2006.
The project is therefore industry related (in other words, without the help and involvement of the commercial sector there is no project!). Previous studies on problem fermentations were done in experimental cellars or in synthetic media. Although these studies generated valuable information on the subject, the information is mostly of scientific value and not forcibly related to real industrial conditions. For the project described here the samples are collected directly from commercial cellars and analysed. This is therefore research IN the industry FOR the industry.
The accuracy of the laboratory analysis of a wine sample is only a true reflection of what is happening in a wine if the sample was collected in the correct manner.
The same goes for this project, the results obtained from the data generated is only as representative as the quality and number of samples that were taken. Therefore it is of utmost importance for the significance of this project to have as much samples as possible from all South African cellars.
In the unlikely event that one of your fermentations proceeds slower than normal (sluggish) or gets totally stuck, please contact me to collect a sample before you re-inoculate. This is purely for research purposes and all samples will be handled confidentially.
Any questions or further suggestions for this project are welcome.
Thank you to everyone who already participated during the 2005 season, it was greatly appreciated.
Please contact Sulette at (021) 808-4867 or 084 591 2112 or e-mail sulette@sun.ac.za.
References
Alexandre, H. and Charpentier, C., 1998. Biochemical aspects of stuck and sluggish fermentation in grape must. J. Ind. Microbiol. Biotech. 20, 20-27.
Bisson, L.F., 1999. Stuck and sluggish fermentations. Am. J. Enol. Vitic. 50, 107-119.
Gafner, J. and Schütz, M., 1996. Impact of glucose-fructose-ratio on stuck fermentations: Practical experiences to restart stuck fermentations. Vitic. Enol. Sci. 51, 214-218.
Henschke, P., 1997. Stuck Fermentation: Causes, Prevention and Cure. ASVO Seminar - Advances in Juice Clarification and Yeast Inoculation, 30-41.
Ingledew, W.M. and Kunkee, R.E., 1985. Factors Influencing Sluggish Fermentation of Grape Juice. Am. J. Enol. Vitic. 36, 65-76.
Specht, G. (Lallemand), 2003. Overcoming stuck and sluggish fermentations. Practical Winery and Vineyard, September 2003, 1-5.
Urtubia, A., Pérez-Correa, J.R., Meurens, M. and Agosin, E., 2004. Monitoring large scale wine fermentations with infrared spectroscopy. Talanta 64, 778-784.
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