Managing Wine Quality

Viticulture and Wine Quality

Andrew G. Reynolds, Andrew G. Reynolds

• Publisher: Woodhead Publishing
• Year: 2010
• ISBN:
  9781845699284
• Topic: Technologie et ingénierie
• Subtopic: Sciences de l'alimentation

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 suggest changes in viticulture and oenology practices. The two volumes of Managing wine quality review developments of importance to wine producers, researchers, and students. The focus is on recent studies, advanced methods and likely future technologies.The first volume Viticulture and wine quality opens with chapters reviewing current understanding of wine aroma, colour, taste and mouthfeel. Part two focuses on the measurement of grape and wine properties. Topics covered include instrumental analysis of grape, must and wine, sensory evaluation and wine authenticity and traceability. The effects of viticulture technologies on grape composition and wine quality attributes are the subject of part three. Terroir, viticultural and vineyard management practices, fungal contaminants and grape processing equipment are among the areas discussed.With authoritative contributions from experts across the world's winemaking regions, Managing wine quality: Volume1: Oenology and wine quality is an essential reference for all those involved in viticulture and oenology wanting to explore new methods, understand different approaches and refine existing practices.

• Reviews current understanding of wine aroma, colour, taste and mouthfeel
• Details the measurement of grape and wine properties through instrumental analysis, must and wine, and sensory evaluation
• Examines viticulture and vineyard management practices, fungal contaminants and processing equipment

Part I

Understanding grape and wine sensory attributes

1

Volatile aroma compounds and wine sensory attributes

V. Ferreira, University of Zaragoza, Spain

Abstract:

This chapter presents a revision of our knowledge and understanding of the role played by the different aroma chemicals in the positive aroma attributes of wine. In Section 1.1, some basic concepts concerning the characteristics of aroma chemicals, such as thresholds, odour activity values (OAVs) and the relationship between the intensity of odour and the concentration are presented. After this, a systematic approach to classifying the different aroma chemicals of wine is presented. One basic idea is that all wines share a common basic aromatic structure formed by ethanol and 27 different aroma compounds, most of them by-products of fermentation. The mixture of those products has the typical vinous aroma and exerts an aroma-buffering effect with the ability to suppress the effect of many odorants added to it, particularly those with fruity characteristics. The ability of the different odour chemicals to break such a buffer, and hence transmit a different aroma nuance to the wine, and the relationship between the transmitted aroma nuance and the aroma of the chemical are used to define the different roles played by aroma compounds on wine aroma. These roles can be as impact compounds, major contributors, net contributors, subtle aroma compounds, aroma enhancers and aroma depressors. The subjects can be individual aroma chemicals or well-defined mixtures of molecules sharing chemical and odour properties (aroma families). Different examples of the aroma chemistry behind some of the most relevant wine aroma nuances from simple or complex wines are also presented and discussed.

Key words

wine

aroma

flavour

odour activity value

odour-concentration relationship

aroma buffer

impact chemicals

aroma families

aroma enhancers

aroma suppressors

1.1 Introduction: basic properties of aroma chemicals

All of us have experience with odours and with some odour chemicals and, for those involved with wine and other high-quality foods, the description of the odour and flavour attributes of a product is something familiar or even trivial. The words we use to describe the odour attributes of a product come from our personal experience and take the name of other products with which more or less everybody is familiar and whose aroma is clear, explicit and supposedly unambiguous.

However, most of us have received nearly no education about the way in which the chemical aroma molecules work to create an aroma. On the contrary, it is likely that we will have received some contradictory or even biased information about the role of chemicals in aroma. On the one hand, we know that some chemicals have clear and easily recognizable aromas that match nearly perfectly the aroma attributes of a given product (e.g. vanilla and vanillin, or corked wine and trichloroanisole – TCA), which suggests that single aroma molecules can drive the aroma attributes of a product. However, this happens only in a very limited number of cases, and we should be ready to accept the idea that, most often, the aroma nuances are the result of a complex equilibrium between different aroma chemicals. On the other hand, some complex products such as wine, coffee or cacao have in their composition more than 1000 volatile molecules (Maarse and Vischer, 1989), and therefore, one can be tempted to think that the final aroma perception is the result of the interactions of several hundred volatiles, which means that we have almost no chance to understand, model or predict the aroma of these products. In this case, fortunately, things are not that complicated and, even in those complex products, the numbers of chemicals really contributing to the different aroma nuances are limited (let's say some tens). It is also true that we are lacking well established tools to deal with the aroma of complex products and that we have to rely mainly on empirical evidence, but since the late 1990s there has been some progress that makes it possible to propose a first approximation of the roles played by the different aroma chemicals of wine in the perception of its different sensory attributes. This chapter deals with this question and will try to present more or less systematically a number of ideas, concepts and facts that will help in understanding the complex relationships between wine odour chemicals and wine odour nuances. Before dealing with those ideas, we will present some basic concepts about aroma chemicals.

1.1.1 Volatile compounds and aroma chemicals

An odour chemical is a chemical compound able to interact with our olfactory epithelium and elicit an odorous sensory response. As our olfactory epithelium is located in the nostrils in the upper interior part of our nose, only volatile molecules can reach this point. So far, all aroma compounds known at present are volatile compounds (Leffingwell, 2002). The opposite is also true, and nearly all volatile molecules (excluding permanent gases, water and a limited number of simple molecules) have some aroma. However, there is an extraordinary variability in the range of concentrations at which a volatile compound is really able to impact the pituitary and elicit the sensory response. While some simple molecules, such as ethane, chlorotrifluoromethane or ethanol, will only be smelled when the concentration in the air is as high as 1 g/m³ (van Gemert, 2003), there are some others, such as bis(2-furfuryl) disulphide or 1-methoxy-3-methyl-3-butanethiol (Guth and Grosch, 1991), which can be perceived at concentrations in the air as little as 0.1 ng/m³: i.e. differences in sensitivities are up to 10 orders of magnitude. When a volatile compound is at concentrations well below (let's say 10-fold) its threshold, then its odour contribution can be considered null and the molecule can no longer be, strictly speaking, considered an aroma compound.

1.1.2 Thresholds, odour activity values (OAVs) and I/logC

Thresholds in air are related to the real ability of the odorant to impact the pituitary, but are not related to the potential relevance of the aroma compound in a given product. This is because some odorants are so strongly retained or dissolved in the matrix of the product that they are released to the headspace in contact with the product to a very limited extent. This is the reason why thresholds in water are more useful for predicting the potential importance of a given aroma compound in an aqueous product. In water, some hydrophilic compounds, such as vanillin, are so well dissolved and hence are poorly released to the air that, even if we are very sensitive to them (the threshold in air is very low), a large concentration is required to get a clear odour signal (the threshold in water is very high) (van Gemert, 2003).

Thresholds are simple values and collecting them is relatively easy. This is the reason why, together with the odour description of the chemical, these values are almost the only parameters defining the behaviour of a given odour chemical which are used and tabulated. Many researchers rely then on the quotient between the concentration of a compound in a given product and the threshold of that compound in a matrix similar to that of the product. This parameter is known odour activity value (OAV) or aroma value (Grosch, 1993). However, the threshold, and hence the OAV, is just a first and rough approximation to the potential sensory importance of a given aroma compound in a product. As a first approximation, we can say that being at a concentration above threshold (OAV > 1) is necessary, but it is not enough, particularly in complex products. We can also say that all aroma chemicals present in a product at a concentration one order of magnitude below the threshold (OAV < 0.1) are irrelevant to the aroma of that produ...