Wine Chemistry and Biochemistry

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8D Yeasts and Wine Flavour 373


tions evaluated by chemical and/or sensory techniques. Nevertheless, the process


of screening and evaluation of new yeasts remains a relatively inefficient process,


since various fermentation characteristics are also assessed in addition to the target


flavour property. Basic fermentation properties include vigour, attenuation, nutrient


requirements, flocculation, and foaming, and freedom from off-flavour formation,


such as H 2 S, acetic acids, high ethyl acetate and high SO 2. Use of robotic technolo-


gies will greatly facilitatehigh throughput screening; however key developments


are still required for multiple characterisations of large numbers of strains. High


throughput metabolomic approaches, with which the volatile fraction of fermenta-


tion metabolites produced can be determined by GC techniques (Howell et al. 2006)


or the non-volatiles by NIR techniques (Cozzolino et al. 2006) coupled to powerful


chemometric tools, are still not suitable for routine work or are inaccessible to many


researchers.


Advanced yeast genetic techniques will bring about the greatest developments


in yeast and wine flavour evolution (Pretorius 2000; Pretorius and Bauer 2002).


Screening yeast using gene arrays techniques, for appropriate gene markers which


are associated with flavour metabolism could reduce the need for extensive metabo-


lite screening (see for example Marullo etal. 2007). Several key genes relating to


ester and long-chain polyfunctional thiol formation, degradation and transforma-


tion have recently been discovered and manipulated to improve the flavour mod-


ulation properties of yeasts (Howell et al. 2005; Subileau et al. 2008; Swiegers


et al. 2006, 2007; Thibon et al. 2008). More appropriate regulation of gene expres-


sion, or modulation of a metabolic pathway,will further facilitate creation of strains
with optimised flavour profiles. Controlling production of off-flavours such as H 2 S


and undesirable mercaptans are important targets (Linderholm et al. 2008; Suther-


land et al. 2003), as is the control of acetic acid production (Pigeau and Inglis 2005;


Saint-Prix et al. 2004). Disconnecting environmental factors from flavour metabolic


pathways will produce strains with more stable flavour properties in musts of vari-


able nutrient content, and lower the risk of nutrient linked off-flavour formation.


Such targets are likely to require application of systems biology approaches that


can integrate genome expression profiles and enzyme catalytic reactions (Borneman


et al. 2007). Further ahead, the introduction of new metabolic pathways into yeast


could be envisaged, leading to the production of compounds which are otherwise


grape-derived. Such an example is the cloning of key genes into yeast to produce


resveratrol (Becker et al. 2003), although this particular compound has no flavour


significance. Strains developed by geneticengineering approaches will have limited


impact in those countries in which consumers and governments regulate their use


in grape and wine production; those countries continue to develop strains for wine


production by conventional genetic breeding techniques (Chambers et al. 2008).


Another recent growth area has been the selection of autochthonous yeasts, which


can promote the regional characteristics of wines in order to overcome the apparent


problem of homogenous or industrial wines. A recent focus has been to select yeasts


on the basis of several yeast volatile aroma compounds but with improved under-


standing of the key odorants of wine the focus will move to those grape-derived
compounds that drive varietal character, as summarised in Table 8D.1. These strains

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