Managing Wine Quality

Oenology and Wine Quality

Andrew G. Reynolds, Andrew G. Reynolds

• Éditeur: Woodhead Publishing
• Année: 2010
• ISBN:
  9781845699987
• Sujet: Technik & Maschinenbau
• Sous-sujet: Lebensmittelwissenschaft

Many aspects of both grape production and winemaking influence wine sensory properties and stability. Progress in research helps to elucidate the scientific basis of quality variation in wine and to suggest changes in viticulture and oenology practices. The two volumes of Managing wine quality review developments of importance to wine producers and researchers. The focus is on recent studies, advanced methods and likely future technologies.Part one of the second volume Oenology and wine quality opens with chapters reviewing the impact of different winemaking technologies on quality. Topics covered include yeast and fermentation management, enzymes, ageing on lees, new directions in stabilisation, clarification and fining of white wines and alternatives to cork in wine bottle closures. Managing wine sensory quality is the major focus of part two. Authors consider issues such as cork taint, non-enzymatic oxidation and the impact of ageing on wine flavour deterioration. The volume concludes with chapters on the management of the quality of ice wines and sparkling wines.With authoritative contributions from experts across the world's winemaking regions, Managing wine quality is an essential reference work for all those involved in viticulture and oenology wanting to explore new methods, understand different approaches and refine existing practices.

• Reviews the impact of different technologies on wine quality
• Discusses yeast and fermentation management, enzymes and ageing on lees
• Considers issues surrounding wine sensory quality including cork taint and the impact of ageing on flavour deterioration

Part I

Winemaking technologies and wine quality

1

Yeast fermentation management for improved wine quality

G. Specht, Lallemand, USA

Abstract:

The yeast and alcohol fermentation management practices reviewed in this chapter are applicable for most alcohol fermentations that winemakers will encounter. The way the yeast is prepared at the yeast production facility and managed during rehydration and throughout the early phases of alcohol fermentation by the winemaker will have a substantial influence on the yeast’s ability to conduct a problem-free fermentation and the resulting wine quality. In spite of improved winemaking technology, there is still much to learn about properly managing yeasts and conducting healthy stress-free fermentations as sluggish and stuck fermentations are still a universal problem.

Key words

yeast

stuck fermentations

yeast nutrition

alcohol fermentation

yeast inoculation

1.1 Introduction

Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making Champagne.

(Kurt Vonnegut, Breakfast of Champions)

Alcoholic fermentation is the process by which a microorganism (yeast) converts sugar into alcohol and carbon dioxide gas. Most winemakers can appreciate that yeast is not just some unnecessary ingredient but that it is a living organism whose role is so critical that without yeast there is no wine. These diverse single-celled organisms can be divided into 60 broad genera such as Saccharomyces and over 700 more tightly defined groups called species such as cerevisiae. Meaningful selection of yeast was begun about a century and a half ago by Louis Pasteur who demonstrated the role of live yeast in the fermentation process.

The first Saccharomyces yeasts were selected for winemaking roughly 80 years ago; the main criteria for selection were their ability to perform reliably and complete the fermentation. It was through the persistence of the wine institutes that the starter culture technique was refined and liquid yeast starter cultures were accepted by winemakers. In the mid-1960s, active dried oenological yeast starter cultures were developed (Degré, 1993), and these have gained mainstream acceptance for use by winemakers in the majority of wine made today. Since the 1980s, universities and oenological research institutes have based their selection strategy of naturally-occurring wine yeast on their research focus and available resources. Some notable examples of these yeast selection strategies would include the competitive factor (Barre and Vézinhet, 1984), ecological origin (Delteil and Lozano, 1995), varietal volatile compounds (Tominaga et al., 1998), breeding techniques and genetic engineering of the yeast, as covered in Chapter 2.

1.2 Yeast and fermentation management and wine quality

Among the selected Saccharomyces available as active dried yeast, there are substantial differences in their kinetics and aptitude to achieve a complete fermentation. These differences come from the yeast’s nitrogen and oxygen needs, some of which have been characterized (Sablayrolles et al., 2000), and their ability to tolerate juice conditions. Another important influence on the yeast fermentation kinetics is the individual yeast’s ability to compete against the microbial soup of indigenous yeast present in the juice or must. Under winery conditions, competition between yeast for the uptake of nutrients and the influence of the competitive factor is a practical key point in the choice of yeast. The method of yeast biomass propagation prior to dehydration has also been demonstrated in winery scale fermentations to have a direct impact on the performance of the yeast during fermentation and on the resulting wine quality (Bohlscheid et al., 2007b). More details on this will be shown in a later section of this chapter.

Oenological yeasts offer winemakers an additional tool for differentiating their wines (Eglinton et al., 2003). Determining which yeast to use, and using them in a controlled manner, can help to achieve the subtle aroma and flavour diversity that many winemakers seek (Henschke, 1997b; Heard, 1999; Lambrechts and Pretorius, 2000; Howell et al., 2005).

The secondary metabolism of the specific yeast produces compounds that have great importance in the wine analytical and sensory profiles. There are substantial differences among strains of Saccharomyces in terms of their production of esters, sulphur compounds, varietal compounds such as β-damascenone, and polysaccharides, as well as the impact of their mannoproteins on the volatility of certain compounds and on the stability of pigments and polyphenols (Delteil and Jarry, 1992).

With such large possible analytical differences among yeasts, it is logical that correspondingly large sensory differences have been demonstrated as early as the mid-1980s. Sensory differences due to the yeasts may come from their interaction with grape flavour precursors or be due to the yeast metabolites (Pretorius et al., 2008). Yeast mannoproteins released in the fermentation also have a sensory impact on the aromatic expression of varietal volatile compound (Wolz, 2005). In multivariate sensory studies with principal component analysis (PCA) of sensory analysis results, different yeast strains in red wines gave sensory differences with magnitudes as great as those obtained from grapes at different stages of ripeness, or varying maceration lengths; techniques generally recognized as having a big impact on the wine style (Delteil, 2001). These sensory differences may become even more apparent with ageing (Dumont et al., 1994).

It is critical for the winemaker to consider using suitably selected yeast that can grow and express its metabolic activity under the given juice or must conditions. For example, a yeast showing good tolerances to low temperatures and highly clarified juices for making a lower alcohol aromatic white wine would probably not ferment very well under higher temperature and higher alcohol potential conditions in a concentrated red must and vice versa.

The yeast’s impact on wine quality can also be associated with its production of off-flavours such as volatile acidity, acetaldehyde, ethyl acetate and negative sulphur compounds. The prevention of off-flavour formation will be addressed in later sections of this chapter that will cover proper yeast handling, feeding and other good fermentation practices that contribute to yeast health and vitality.

1.3 Yeast rehydration and handling

Proper rehydration is perhaps the most critical phase in using active dried yeast. When selected yeasts are produced in an active dried form, the goal is to get a very high and viable cell population prior to a series of water removal steps until 5–8% moisture remains in the yeast powder. This low moisture level is necessary to ensure a good shelf life in order to conserve the yeast’s potential activity for more than 36 months. These drying stages remove not only extracellular water, but also most of the water within the cell and bound to the cell’s organelles, causing the yeast cells to shrink and desiccate. With extremely low water activity, there is almost no metabolic activity. To be functional again, the dried yeast cells must reabsorb all of their water. When the dried yeast comes into contact with water (or any other liquid), the cells literally act like dried sponges and suck up the needed water in seconds.

In winemaking, the grape juice is a very hostile medium for Saccharomyces: high osmotic pressure, low pH and often high SO₂. Most selected yeasts resist these conditions when their membrane and their intracellular components are in good physiological conditions. Therefore, before inoculating a grape juice, it is absolutely necessary to bring back vital water at the right temperature to the active dried yeast cells in order for their membrane and intracellular components to reorganize properly. Not only will yeast cells not disperse very well if not properly rehydrated, they can lose a large amount of cellular components, reducing the efficiency of oxygen and nutrient transfer to the cells (Henick-Kling, 1988). This impedes yeast growth and activity leading to sluggish and stuck fermentations. The use of clean potable water at the right temperature for the right length of time would appear to be the most simple and effective yeast rehydration protocol.

Proper yeast rehydration gets them off to a good start and helps to ensure that the yeast will stay healthy throughout the fermentation. Degré (1993) proposed a general recommendation procedure for active dr...