WINETECH Technical Yearbook 2019

amino acid content compared to 3.5% for the branched-chain amino acids. Proportionally, Cabernet Sauvignon and Merlot had the lowest aromatic amino acid content, with only 4.1% and 4.2%, respectively. These cultivars were also observed to have amongst the lowest proportions of branched-chain amino acids (Cabernet Sauvignon 2.8% and Merlot 2.6%), along with Chardonnay 2.6%. Moreover, Roussanne was found to have, on average, the highest aromatic amino acid content, in both absolute terms (264 mg/L) and proportionally (13.4%). In terms of the branched-chain amino acids, Cinsaut and Roussanne were found to have the highest proportions, with 4.6% and 4.5% being found, respectively. Therefore, as Roussanne contains high concentrations of these precursor molecules (both aromatic and branched-chain amino acids), it can be identified as a cultivar with a lot of aromatic potential in terms of the production of fusel

alcohols and esters. However, these positive aroma compounds are only produced when the total YAN concentration is capable of fulfilling the full biosynthetic requirement of the yeast. As Roussanne has been identified as a cultivar which has a very low total YAN content (average 132 ± 34 mg N/L, Petrovic et al. , 2019), this cultivar will most likely require nutrient supplementation in the form of DAP or complex nutrients to realise its full aromatic potential. LEAST ABUNDANT AMINO ACIDS Ornithine (average 2 mg/L, range 0.4-4.8 mg/L), glycine (3.3 mg/L, 1.1-5.3 mg/L), methionine (3.6 mg/L, 0.7-9.9 mg/L), and lysine (3.9 mg/L, 2.3-6.1 mg/L) were found to have the lowest concentrations, both in terms of the overall average, as well as per cultivar. The low concentration of ornithine is most likely due to its central role in nitrogen metabolism, acting as a precursor molecule for the formation of the most abundant

amino acids, arginine and proline. As Saccharomyces cerevisiae , the most widely used yeast for fermentation, is not able to efficiently metabolise glycine and lysine, these amino acids are considered as a poor source of nitrogen for this yeast. However, these amino acids may be assimilated by some non- Saccharomyces yeasts. TAKE-HOME MESSAGE Combining the knowledge of YAN levels with the amino acid profile can be useful when deciding on nutrient additions. For example, Merlot and Cabernet Sauvignon were found to have the lowest proportions of aromatic and branched-chain amino acids, with Roussanne having the highest proportion of these precursors of fruity and floral aromas. As Cabernet Sauvignon and Merlot have been found to have very low total YAN concentrations, these cultivars would in most cases require nitrogen supplementation to ensure the completion of fermentation.

However, the addition of complex nutrients (which may contain varying concentrations of these branched-chain and aromatic amino acids) may be a more beneficial supplementation strategy for these cultivars compared to ammonia addition in the form of diammonium phosphate (DAP). On the other hand, as Roussanne already has high concentrations of these precursor molecules, the addition of (more cost-effective) DAP may be sufficient to ensure not only the completion of fermentation, but to ensure the formation of favourable organoleptic qualities in the final wine. SUMMARY This type of composition profiling is especially relevant in terms of identifying and meeting nitrogen demands of various yeast strains, Saccharomyces and non- Saccharomyces. This is the first time such extensive work focused on amino acid composition was undertaken.

– For more information, contact Astrid Buica at abuica@sun.ac.za.