This winemaking treatment simulated an example of how the presence of leaves and MOG in machine-harvested fruit can modulate the aroma and flavour of the resultant wine. The treatment used in the 2018 Cabernet trial simulated a low level of MOG and was added at ~1%. The MOG added included leaves, petioles and some canes, and was added to the ferment post-crushing.
Whether grapes are hand-picked or machine harvested, there will always be a certain amount of non-grape material that finds its way into the picking crates or bins. This can include other vine material such as leaves, petioles and canes, as well as stones, sticks, strainer wire, caterpillars, millipedes and stink bugs.
What is the impact of MOG?
The impact of MOG in ferments will vary depending on the type of material present. Recent research by Capone et al. (2018) looked at the impact of grape stems and leaves on ‘green’ characters in Shiraz wine. The methodology included fermenting Shiraz fruit with the addition of grape leaves and stalks at a level which is considered typical for machine-harvested fruit. The study was able to determine that the presence of leaves alone did not contribute to green flavour and found that the stems were a bigger contributor to ‘green’ aroma character and resulted in increased concentrations of methoxypyrazines . The wines made with added stems were also rated higher in astringency, colour intensity and acidity.
Other impacts which can arise from MOG include those from insects and bugs. The AWRI helpdesk has been involved in a number of investigations over the years where the presence of millipedes in bunches during harvest has caused an unpleasant aroma character. Further research is needed to identify the aroma compound(s) involved and any options for remediation.
How much MOG is too much?
Most wineries will inspect picking bins upon arrival and have specifications for the amount of MOG that they will accept. An example of a classification of different levels of MOG was published as part of the Australian wine industry code of conduct (see table below).
Table adapted from Winegrape assessment in the vineyard and at the winery (compiled by Wendy Allan)
Allan, W. 2003. Appendix 1: Australian winegrape load assessment: a visual guide. In (S. Collings ed.) Growing quality grapes to winery specifications: quality measurement and management options for grapegrowers. Adelaide, SA: Winetitles, pp. 79-101.
Cowey, G. 2012. Ask the AWRI: How and why identify matter other than grapes. Aust. N.Z. Grapegrower Winemaker (580): 77-78.