Future work

The results presented above suggest that the concept of fermenting different grape berry tissues is certainly informative in determining the potential for a certain tissue to contribute to the pool of volatile compounds in wine (EW experiment) and the actual contribution that tissue is likely to make in a normal fermentation (GP experiment). One drawback of the EW experiment in particular is the difficulty in getting skin and seed tissue from frozen berries, and meant that the amount of grape material used to supplement the model must (10 g in 50 mL) was limited. Ideally, if labour and time were not so restricted, the percentage of grape material in the fermentations would have

only been limited by the concentration of model must that could reasonably be used to equalise sugar and nitrogen and so higher concentrations of the volatile compounds would presumably be observed in the wines. Processes that are less intensive in separation of skin and seeds from frozen grape could allow future similar experiments to have more sensitivity to the contributions of different grape tissues to wine aroma.

The work in this chapter could have been improved by quantitative measurement of the aroma compound concentrations. This would have allowed comparison of the aroma compound concentrations with those of fermentations in the literature. Acquiring

quantitative values would have required construction of a standard curve for each measured aroma compound against an appropriate internal standard. However, this work was not carried out due to restraints on the time and money available.

The usefulness of the work in this chapter could have been increased had a reliable semi-quantitative or quantitative profiling of the lipid composition and amino acid composition of the grape tissues been established. Combined with the work in this chapter, such data would have been able to help explore and confirm some of the

hypotheses and questions outlined. With regard to the future work on lipid composition, Chapter 5 in this thesis provides some of the initial work for analysing the lipid

composition of grape tissues, but does not reliably provide a quantitative or semi-quantitative analysis of the lipidome.

It would also be interesting to assess the variation between parcels of grapes from within the same vineyard, between grapes from different vineyards, between further grape varieties, between grapes at different stages of ripening, and between grapes from different vintages.

Nevertheless, the knowledge gained from berry tissue specific experiments will assist many aspects of viticulture. For example, understanding the impact of grape berry

skin on both grape-derived and fermentation-derived wine volatiles could be

informative for how alterations in berry size or berry skin thickness in the vineyard will ultimately effect wine composition. Furthermore, such experiments may be more sensitive in determining varietal differences in grape composition and the subsequent wines made from these grapes, or how management strategies effect grape composition.

There is much management of light interception in the fruit zone in vineyards and it would be predicted that this would mainly affect the skin of the berries. Tissue specific experiments could also help predict effects of winemaking interventions, such as fermentation time on skins.

It should be acknowledged that ‘real’ Riesling wine making involves fermentation of pressed juice, with the removal of skins and seeds at an early stage. Therefore, the volatile profiles of the skin and seed-derived wines in this study need to be tested against more traditional wine making practices. Despite the artificial conditions in which this study was carried out, the data seems to suggest that these tissues do have potential to influence wine volatile profiles. The results may also help determine

optimum practise for a parcel of grapes in the winery. For example, the type and amount of pressing may influence extraction from skins and seeds which would influence wine volatile composition.

The method of homogenisation is another aspect of the experimental method that will require investigation, as wineries would not get the same level of homogeneity from pre-fermentation treatment of grapes, and therefore may have lower levels of extraction from grapes of the factors that influence wine volatile profiles in this experiment. A future experiment in this vein could involve fermentation with and without the grinding of skins and seeds to see if it influences the wine volatile profile.

The similarity in concentration of most compounds between the crushed and

and Figure 12) would suggest that they will be similar. This similarity could be due to the effective extraction from tissue of factors by carbonic maceration during

fermentation.

The tissue-specific effects on wine volatile composition described in this chapter will stimulate future experiments that will elucidate the cause of these differences. For example, one experiment that would follow on from this study is the measurement of amino acid distribution in the different grape tissues. This would provide a rationale that might explain the differences in volatile profiles of amino-acid derived compounds in wines that utilize different grape tissues. Another experiment that is needed to help explain results in this chapter is measuring the distribution and activity of acyl hydrolases in the different grape tissues. This may help explain the mechanisms by which lipid-derived volatiles compounds varied between wines made from different grape tissues. Furthermore, given that grape metabolites may alter yeast gene expression or biochemistry, there is a unexplored area of work that could be approached in a

number of ways involving natural products chemistry and next generation sequencing or microarray analyses, or even more targeted analysis of specific genes or enzyme

activities in the yeast and how they are altered by grape composition.

Chapter 3 - Fermentation-guided natural products