Wine yeasts of New Zealand: an investigation into their distribution, contribution to sauvignon blanc aroma and interaction in co-ferments

Abstract

Sauvignon Blanc is the most important grape variety for New Zealand’s economy with exports of ~$740M in 2008. The two varietal thiols 3‐mercaptohexanol (3MH) and 3‐mercaptohexyl acetate (3MHA), reminiscent of tropical aromas like passion fruit and grapefruit, have been identified as key aroma compounds in Marlborough Sauvignon Blanc. These compounds are released during fermentation due to the metabolic action of yeast. This thesis investigates the contribution of natural New Zealand yeast isolates to the characteristic aroma of Sauvignon Blanc. Natural Saccharomyces and non‐Saccharomyces isolates were screened for their 3MH and 3MHA production. Four non‐ Saccharomyces yeasts from two species, Candida zemplinina and Pichia kluyveri, were identified as predominantly producing either 3MH or 3MHA. In subsequent cofermentation experiments with commercial S. cerevisiae strains, the combination of VL3 and P. kluyveri I at a ratio of 1:9 showed remarkable increase in 3MHA production. Analysis of nitrogen usage in this co‐ferment revealed that P. kluyveri I was able to grow on proline as sole nitrogen source in a synthetic medium. Proline is one of the most abundant amino acids in grape juice but cannot be utilized by S. cerevisiae as its degradation pathway requires oxygen. Nothing is known about proline utilization in P. kluyveri but the data presented here suggests a different utilization pathway than that found in S. cerevisiae.