Nitrogen is requisite for yeast growth and metabolism during wine fermentation, and contributes to wine flavor development. Understanding how yeast assimilable nitrogen (YAN) impacts wine chemistry and sensory properties helps inform nitrogen management decisions in both the vineyard and winery. This study used Virginia-grown Chardonel, obtained from a multi-year viticulture study evaluating the impact of nitrogen fertilizer on grapevines. The objective of this work is to determine if vineyard treatments resulted in sufficient juice YAN profile differences to impart chemical and/or sensory differences in resulting wines. Grapes were combined from each field replication of six vineyard nitrogen treatments and pressed to generate six juice lots. Each juice was divided in two for triplicate fermentation using one of two Saccharomyces cerevisiae x bayanus yeast strains: EC1118 or QA23 to investigate the interactive effects of YAN and yeast strain. Juice and wine samples were frozen until chemical and UPLC-PDA amino acid analysis. Fermentation replications of each wine were combined, bottled, and stored at 4ºC for four months until evaluation via sensory sorting task with untrained panelists (n=63). Juice YAN concentrations ranged from 157-201mg N/L, a small variation in terms of commercial winemaking. Chemical differences between wines produced with the same juice but different yeast strains were larger in wines that had lower juice YAN concentrations. DISTATIS analysis of sorting results showed differences but no pattern in sensory perceptions of wines based on nitrogen profile or yeast strain. This range of nitrogen profiles shows limited impact of YAN profile on final wine quality. / Master of Science in Life Sciences / Nitrogen is an essential nutrient in grape growing and winemaking. Grapevines use nitrogen as a nutrient to survive and produce grapes for making wine. In winemaking, yeast turn the sugar and nitrogen in grape juice into alcohol via fermentation, using nitrogen to grow and produce other compounds that contribute to the final flavor of the wine. Understanding how nitrogen qualities and quantities in grape juice impact final wine quality is important so vineyard managers can apply nitrogen fertilizer to meet their needs. This thesis evaluates how different nitrogen quantities and qualities in the initial juice and during white wine fermentation impact the chemical and sensory properties of a final wine product. A Chardonel vineyard is being maintained for a multi-year study where the vineyard is divided for grapes in different areas of the plot to receive one of six different nitrogen fertilizer treatments. Grapes were harvested from each treatment and pressed into juice. Each lot of juice was then split in two for fermentation by one of two yeast strains for a total of 12 wines. Wines were bottled and stored for four months until sensory evaluation. Samples of juice and wine were collected and frozen before chemical analysis for nitrogen and other standard wine industry parameters. For sensory analysis, consumers were asked to taste each wine then sort them into groups based on how similar the wines were perceived to be. Chemical differences were found between wines, but they did not translate into sensory differences. Vineyard managers and winemakers can be aware that in Virginia-grown Chardonel, with modest differences in nitrogen from the vineyard treatments, differences in juice and wine chemical parameters did not impact the final wine product.

Thesis (MSc)--Stellenbosch University, 2011 / ENGLISH ABSTRACT: South African Chenin blanc wine is characterised by its style diversity and has been receiving more industry limelight over the past few years. Three dry Chenin blanc wine styles are currently described by the wine industry as Fresh and Fruity (FF), Rich and Ripe unwooded (RR-unwooded) and, Rich and Ripe wooded (RR-wooded). According to the South African wine industry, however, consumers seem to be somewhat confused by these various Chenin blanc wine styles, which have not yet been clearly defined and classified. During this study we investigated the following perspectives of this wine style classification confusion: Currently, no specified sensory method is being used to differentiate between wine styles of a single wine cultivar, such as Chenin blanc. The sorting method can be used as a more cost and time effective classification and profiling method than general quantitative descriptive analysis (QDA). It was therefore investigated whether both uninstructed and instructed sorting, with consumers and wine experts, can be used to classify and describe the Chenin blanc wine styles similarly to QDA. The increase in young wine consumers, Generation Y, introduces new wine marketing opportunities for the wine industry which need to be investigated. Young consumers are not aware of the various Chenin blanc wine styles, which may influence their wine preferences. The influence of wine style knowledge on these consumers’ wine style preferences were investigated during blind and wine style informed preference test. Any segmentation within this Generation Y group was also investigated with regards to their Chenin blanc wine style preferences and/or exploratory data. The following scientific and industrial findings were made: QDA was successfully substituted by instructed and uninstructed sorting tasks. Although the sorting tasks easily differentiated between wooded and unwooded wines, QDA differentiated more successfully between the unwooded wines. Wine tasting experience did not significantly influence the sorting results of the consumers versus the wine experts, but more experienced panellists consistently used fewer descriptors than untrained consumers. All panellists sorted products slightly better when they were provided with wine style sorting instructions. However, there were no significant differences between the instructed and uninstructed sorting task results, i.e. a wine style descriptor list did not improve panellists’ abilities to describe their sorted wine groups. South African Chenin blanc wines can definitely be divided into wooded and unwooded wine styles, i.e. two wine style groups, but the unwooded Chenin blanc wine spectrum formed a wine-style-continuum. This finding is apparent from both the sorting and QDA methods. The unwooded continuum wines can further be differentiated from one another with the use of a fresh-to-mature scale and/or a light-to-full body scale during QDA. Generation Y wine consumers preferred all the Chenin blanc wine styles equally and moderately during the blind tasting, but their knowledge of the wine style concepts significantly increased their preferences for the wines during the informed wine style tasting. The cognitive influence of wine style knowledge is therefore powerful in terms of influencing a consumer’s wine preferences. Other interesting correlations were also made between Generation Y consumers’ wine style preferences and their exploratory data. / AFRIKAANSE OPSOMMING: Suid-Afrikaanse Chenin blanc wyn word gekarakteriseer deur styl diversiteit en is die afgelope paar jaar meer in die kollig. Drie droë Chenin blanc wynstyle word tans deur die wynbedryf beskryf as: Vars en Vrugtig, Ryk en Ryp ongehout en, Ryk en Ryp gehout. Volgens die Suid- Afrikaanse wynbedryf is daar heelwat verwarring is tussen hierdie Chenin blanc wynstyle weens die feit dat hierdie wynstyle nog nie duidelik gedefinieer en geklassifiseer is nie. Tydens die huidige studie het ons die volgende perspektiewe van hierdie wynstyl-verwarring ondersoek: Huidiglik word daar geen gespesifiseerde sensoriese metode gebruik om te onderskei tussen verskillende wynstyle van ’n enkele wyn kultivar, soos Chenin blanc, nie. Die sorteringsmetode kan gebruik word, instede van kwantitatiewe beskrywende analise, as ’n meer koste- en tyd-effektiewe produk klassifikasie en profileringsmetode. DIt was dus ondersoek of beide geïnstrukteerde en ongeïnstrukteerde sorteringstake, met verbruikers en wynkundiges, gebruik kan word om die verskeie Chenin blanc wynstyle kan profileer en klassifiseer soortgelyk soos kwantitatiewe beskrywende analise. Die toename in jong verbruikers, Generasie Y, skep nuwe wynbemarkingsgeleenthede wat ondersoek moet word. Jong verbruikers se onbewustheid van die verskeidenheid Chenin blanc wynstyle mag hulle voorkeure vir hierdie wyne beïnvloed. Die invloed van wynstyl kennis op hierdie jong verbruikers se wynstyl voorkeure was ook ondersoek gedurende ’n blinde en ’n wynstyl-ingeligte voorkeur toets. Enige segmentasie binne hierdie Generasie Y verbruikersgroep was ook ondersoek in verband met hul Chenin blanc wynstyl voorkeure en/of verkennende data. Die volgende wetenskaplike en industriële bevindings is gemaak: Kwantitatiewe beskrywende analise kan suksesvol vervang word deur beide geïnstrukteerde en ongeïnstrukteerde sorteringsmetodes. Alhoewel die sorteringsmetodes maklik gebruik kon word om te onderskei tussen gehoute en ongehoute wyne, kan kwantitatiewe beskrywende analise fyner onderskeid tref tussen die individuele ongehoute wyne. Die wynproe-ervaringsvlak het geen invloed op die sorteringsresultate gehad nie, maar meer ervare paneellede het konsekwent gebruik gemaak van minder beskrywende terme as onopgeleide verbruikers wat baie meer en verskillende soorte terme gebruik het. Beide panele het effens beter gesorteer wanneer hulle spesifieke wynstyl sorteringsinstruksies ontvang het. Daar was egter geen beduidende verskille tussen die geïnstrukteerde en ongeïnstrukteerde sorteringsresultate nie, wat daarop dui dat sorteringsinstruksies en ’n lys van wynstyl beskrywings nie die paneellede se wyse van sortering verander het nie. Suid-Afrikaanse Chenin blanc wyne kan definitief verdeel word in gehoute en ongehoute wynstyle, d.w.s twee wynstyl groepe, maar die ongehoute Chenin blanc wyn spektrum vorm ’n wynstyl-kontinuum. Laasgenoemde is bevind tydens beide die sorteringstake asook kwantitatiewe beskrywende analise. Die ongehoute kontinuum wyne kan verder van mekaar onderskei word met behulp van ‘n vars-tot-volryp en/of ‘n ligte-tot-volmond skaal tydens kwantitatiewe beskrywende analise. Alle jong wyn verbruikers het ewe veel van al die wynstyle gehou tydens die blinde wynstyl voorkeur toets, maar tydens die wynstyl-ingeligte voorkeur toets het hulle voorkeure vir al die wynstyle drasties verbeter. Die kognitiewe invloed van Chenin blanc wynstyl kennis kan dus lei tot hoër voorkeure vir die wynstyle. Ander interessante korrelasies is ook bevind tussen hierdie Generasie Y wyn verbruikersgroep se wynstyl voorkeure en hul verkennende data.