Whole bunch fermentation: adding complexity, or just making ‘green’ wine?

Abstract

AIM: Certain grape varieties contain negligible levels of isobutyl methoxypyrazine (IBMP) in grapes. However, it has long been known that grape stems may have a high concentration of IBMP. The study aimed to discover the extent to which whole bunch addition during fermentation contributes to ‘capsicum’ (green) flavour and aroma in wine by imparting IBMP. It was also of interest to determine whether additional tannin could be extracted from stem contact, which was expected to impact wine astringency.  

METHODS: Pinot noir and Shiraz grapes were either fermented either completely crushed, or as 100% whole bunches. Other treatments had proportional whole bunch addition at 25%, 50% or 75%. Wines were put through descriptive sensory analysis after 10 months and multiple volatile and non-volatile compounds were analysed at the same time.  

RESULTS:  For both varieties, the sensory scores for ‘green capsicum’ and the concentration of IBMP were highly correlated with the proportion of whole bunches in the ferment. Volatile compounds other than IBMP were increased by whole bunch fermentations, and were α-terpineol, β-citronellol and E-2-hexenol. For Shiraz, the wood-derived volatiles guaiacol and vanillin increased with whole-bunch inclusion, while for Pinot noir ethyl cinnamate increased, possibly reflecting that some carbonic maceration occurred. In Shiraz wines, the concentration of tannin and the perception of astringency increased with the inclusion of whole bunches, but this was not observed for Pinot noir. Whole-bunch ferments from both grape varieties had lower levels of polysaccharide in the finished wine, primarily due to a lower contribution of pectic material.     

CONCLUSIONS: This study showed that winemakers can substantially alter wine volatile and non-volatile composition through whole bunch fermentation. The results will be discussed in terms of potential wine style outcomes for the respective grape varieties, whether whole bunch inclusion might improve the complexity of a wine, or simply introduce unwanted ‘green capsicum’ attributes.