WINETECH Technical Yearbook 2019

ASTRID BUICA, GABRIELLA PETROVIC & JOSE-LUIS ALEIXANDRE-TUDO: Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: YAN, FAN, ammonia, IR spectroscopy, statistical modelling, vintage effect, cultivar effect. MAY 2019 RAPID AND COST-EFFECTIVE QUANTIFICATION OF YAN CONCENTRATION AND COMPOSITION – VINTAGE AND CULTIVAR EFFECT

Due to the complexity of the grape juice matrix and the increasing consumer demand for quality wines, monitoring the grape juice composition becomes essential. Yeast assimilable nitrogen (YAN), a critical component of the grape juice matrix, is, however, often neglected. This is for the most part due to the lack of rapid, cost- effective and easy-to-use methods available to accurately measure the YAN concentration and composition. Although the formol titration offers a relatively easy method to measure YAN, it is only able to measure the total YAN, and therefore, no knowledge

can be gained on the composition of the nitrogen status of the grape juice matrix. As YAN provides the primary nutrients required for the proliferation of yeast during fermentation, YAN deficiencies have been identified as the main cause of stuck/sluggish fermentations. However, due to the complex metabolic activities of yeast, the types of nitrogenous compounds contributing to the total YAN becomes important. It is known that too little YAN can result in stuck/sluggish fermentations, an increase in hydrogen sulphide which leads to the production of reductive flavours and

odours, an increase in higher alcohols, which can result in the production of undesirable aromas, as well as a decrease in the production of fruity esters that can contribute to the production of quality wine. On the other hand, when YAN concentrations are too high, this can lead to microbial instability, as there are enough nutrients to support the growth of unwanted yeasts and bacteria, which can result in the production

of certain allergens and carcinogens, such as biogenic amines and ethyl carbamate, as well as the production of protein haze. All of this can compromise the quality of the wine and ultimately lead to the production of a low quality wine. HOW DO CALIBRATIONS WORK? Three aspects need to be in place: a reference/standard method, an additional