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p r a c t i c a l w i n e r y & v i n e ya r d a U G U S t 2 0 1 4 63 while those that form very little volatile phenol concentrations are referred to as POF-negative yeasts. Above their sensory threshold, volatile phenols are responsible for the loss of freshness and fruity charac- ter —and, in a worst-case scenario, medic- inal smells in a white wine. Unless the cinnamoyl esterase side activity has been removed by enzyme purification, it is always present in com- mercial enzymes, with some being less than others. The higher its content in the particular enzyme used, the greater its potential negative impact on wine aroma since the sensory threshold of volatile phenols listed above is ~770 µg/L. Juice fining before fermentation Phenolic acids (especially caftaric acid) and the flavonoids catechin and epicate- chin play a particular role in final wine aroma. These compounds are capable of becoming oxidized and forming qui- nones, which can capture certain aromas, especially volatile thiols. Most commercial wine yeasts are POF-positive. Careful consideration must be given to the selection of enzymes when using these wine yeasts. The use of purified enzymes, however, minimizes the risk that the total vinyl phenol content will increase above the aroma threshold, even if POF-positive yeasts are used. Only some granular and liquid enzymes are CE-free by process puri- fication, while other preparations may be produced with low CE activity. When these phenolic compounds are oxidized pre-fermentation, the quinones that are formed can be eliminated by the glutathione thiol naturally present in grape juice. This phenomenon occurs before the alcoholic fermentation yeasts have revealed the aromas, but the reaction undoubtedly results in a considerable loss of glutathione. This can be seen by the accumulation of the glutathione-caftaric acid adduct identified as grape reaction product (GRP) in an HPLC phenolic anal- ysis of must or juice. Ultimately, the result- ing wine is more susceptible to oxidation when the glutathione levels are reduced. When must is handled under strict inert conditions, the phenolic compounds are not oxidized and the glutathione is preserved. During primary fermentation, reductive conditions allow these com- pounds to remain stable and allow the varietal aromas to be revealed. After fermentation, however, exposure to oxygen through transfer to barrels, blending, stabilization or filtration may allow the phenolic compounds to become oxidized and thus form reactive qui- W I N E M A K I N G W inemakers are challenged with allowing a specific lot of grape juice to achieve its full potential as wine and main- tain that potential for as long as possible. Not all grapes are of equal quality, however, so while it is true that the best grapes in the world need very little intervention, others may require more "input" to achieve magic in a bottle. Throughout the winemaking process, winemakers can combine their exper- tise with a range of intervention and enological products in order to achieve the best result possible. This article highlights five key winemaking deci- sions that can impact the flavor-aroma profile of aromatic white wines. Enzyme selection for skin contact, settling and flotation The use of enzymes forms an integral part of winemaking. There are a variety of enzymes available to winemakers in both liquid and granulated forms. Enzymes from various suppliers differ from each other in their pectinase content, especially polygalacturonase levels, as well as overall concentration and side activities. Pectinase is not a single enzyme but a combination of many enzymes involved in the degradation of plant pectin and cell walls. The proportion of specific enzyme activities in a commercial enzyme product will determine the best application, whether the use is for extrac- tion of grape skin flavor-aroma compo- n en ts, de-j uicin g an d press yield improvement or post-press rapid depec- tinization for better solids management through settling or flotation solids- removal processes. In addition to the main enzyme activi- ties, commercial enzyme preparations can contain additional side activities. One of the side activities, cinnamoyl esterase (CE), can be detrimental to Karien O'Kennedy, Laffort South Africa, Maryam Ehsani, Laffort France, and Peter Salamone, Laffort USA BY White wine aroma optimization white wine aroma in the presence of grape phenolics and certain yeast strains. 3 Phenolic acids occur naturally in grape juice in two forms: as an ester with tar- taric acid (coutaric, fertaric and caftaric acids) and in a free form (coumaric, feru- lic and caffeic acids). Cinnamoyl esterase in commercial pectinase preparations can convert the esterified phenol acids to their free forms, which are toxic to many microorganisms. Wine yeasts overcome this toxicity by converting these free hydroxycinnamic acids to less toxic com- ponents via decarboxylation, forming 4-vinylphenol from p-coumaric acid and 4-vinylgaiacol from ferulic acid. Wine yeasts with the ability to form these volatile phenols are referred to as POF-positive (phenolic off-flavor) yeasts, 2