A survey of the YAN status of South African grape juices and exploration of multivariate data analysis techniques for spectrometric calibration and cultivar discrimination purposes.

ENGLISH ABSTRACT: Yeast assimilable nitrogen (YAN) has been identified as one of the main drivers of wine quality, influencing the production of various aromas and ensuring a successful fermentation to dryness. Due to the number of factors affecting YAN concentration and composition, paired with the complexities of yeast metabolism, more data is required to enable a comprehensive understanding of this important component of the grape juice matrix. Thus, there is a need for simple, rapid, and cost-effective methods to measure YAN status. The main aims of this research were to gain insight into the nitrogen status of grape juices used for commercial winemaking in the South African wine industry, and subsequently, to assist in a more comprehensive understanding of grape juice nitrogen status. Therefore, in Chapter 3, an unsupervised survey of the YAN, FAN, and ammonia concentrations of 805 grape juice samples of various (industrially relevant) cultivars and geographical origins are reported. Subsequently, an overall average of 191 ± 64 mg N/L, 138 ± 46 mg N/L and 53 ± 24 mg N/L was observed for YAN, FAN, and ammonia, respectively. Trends of nitrogen deficiency and excess could be found for various cultivars and geographical origins. Analysis of variance tests and exploratory data analysis techniques such as hierarchical agglomerative clustering and CART analysis established ‘cultivar’ as the most important factor in determining the YAN concentration and composition of the resulting grape juice. In Chapter 4, using the data collected in Chapter 3, plus an additional vintage (2018), the viability of infrared (IR) spectroscopy for the accurate quantification of YAN, FAN, and ammonia was tested. IR spectroscopies compared included: Fourier-transform infrared (FT-IR), Fourier-transform near infrared (FT-NIR) and attenuated total reflection mid-infrared (ATR-MIR) spectroscopy. FT-IR and FT-NIR were found to outperform ATR-MIR in a variety of tasks assigned to each instrument and were deemed robust and capable of accurate quantification as RPDVAL > 2.5 were repeatedly obtained for both spectroscopies. The achievement of accurate calibration models is owed to the large amount of variability included in both the calibration and validation sets and the application of proper external validation strategies. Thus, both industry and research are presented with a simple, rapid and cost-effective method to measure this important component of the grape juice matrix. In Chapter 5, a deeper look into the FAN component of YAN was conducted by quantifying individual amino acids. Overall, proline, arginine, glutamine, alanine, tryptophan and GABA were found to be the most abundant while glycine, lysine, methionine and, ornithine were found to be the least abundant. Subsequently, the discriminatory power of the amino acid profile of the various cultivars were tested. This was done to identify key differences in amino acid profiles which could possibly serve as the basis for further research investigating yeast metabolism and aroma…