Abstract:
Vitis vinifera L. cv. Pinot noir is New Zealand’s most important red grape variety and second
most planted variety after Sauvignon blanc. Pinot noir is a notoriously difficult grape to work
with – in the vineyard it is susceptible to disease and climatic changes, and in the winery, it is
a challenge to extract the compounds responsible for colour and then ensure colour stability
during aging. The appearance of a wine, of which colour is a crucial component, can shape a
consumer’s enjoyment and perception of flavour while drinking it. Research has shown that
consumers prefer deeper colour in red wines and therefore the development of tools to achieve
greater colour intensity in Pinot noir wines could be extremely useful for winemakers and the
wine industry.
This research explores the impact of interspecies yeast flocculation and sequential inoculation
on Pinot noir colour. Six commercial non-Saccharomyces yeast species, LAKTIA (Lachancea
thermotolerans), CONCERTO (Lachancea thermotolerans), GAÏA (Metschnikowia
fructicola), FROOTZEN (Pichia kluyveri), BIODIVA (Torulaspora delbrueckii) and
PRELUDE (Torulaspora delbrueckii), and two commercial Saccharomyces cerevisiae strains,
VL3 and RC212, were used in microfermentations of synthetic grape must, both individually
and in every possible combination of non-Saccharomyces and S. cerevisiae. Sedimentation rate
assays at the end of fermentation were performed to determine how well each individual yeast,
and yeast combination, flocculated. The most flocculant individual yeast was BIODIVA and
the most flocculant S. cerevisiae and non-Saccharomyces pairings were VL3 + BIODIVA and
RC212 + BIODIVA, suggesting that mixed species flocs may have formed.
These yeast combinations, along with S. cerevisiae controls of VL3 and RC212 alone, were
used in a 20 L-scale Pinot noir winemaking trial. Both UV/visible spectrophotometric
measurement of colour intensity, and sensory evaluation of wine appearance performed by
human participants, found that the mixed species fermentations resulted in wines with greater
colour intensity compared to the controls. The final VL3 + BIODIVA wines were found to be
deeper in colour than the VL3 control wines and while the final RC212 + BIODIVA wines
were not found to be different from the RC212 control wines, the control juice used at the
beginning of the experiment was significantly deeper in colour intensity than the juice used for
the sequential inoculations.
Two methods of measuring anthocyanins in the wines, the Adams-Harbertson assay and high
performance liquid chromatography (HPLC), confirmed that the mixed fermentation wines
with higher colour intensity, had lower anthocyanin concentrations than the less intense control
wines, despite anthocyanins being the principal source of colour in young red wines. This result
is likely due to the phenomenon of copigmentation, which can result in wines displaying a
deeper colour intensity than would be expected based on their anthocyanin content. The
increases in colour intensity may be due to increased flocculation during primary fermentation,
between the highly flocculant BIODIVA and the two strains of S. cerevisiae. A further trial
found that the non-Saccharomyces yeast strains used in the sedimentation rate trial adsorbed
significantly more pigments from Pinot noir skins than the S. cerevisiae strains, with BIODIVA
adsorbing the most. Grape pigments are adsorbed onto yeast cell wall mannoproteins and
previous research has suggested that BIODIVA and other strains of T. delbrueckii have a high
concentration of mannoproteins compared to other yeast species. Given that flocculant yeasts
have differences in cell wall mannoprotein composition compared to non-flocculant yeasts, the
cell wall could be a crucial component behind the mechanism involved in the greater colour
intensity of wines inoculated with BIODIVA. Further research is required to confirm this
hypothesis and confirm that flocculation is the cause of colour intensity changes or if there is
another aspect of BIODIVA metabolism or mixed species interactions resulting in enhanced
colour intensity and copigmentation, such as increased production of acetaldehyde or pyruvic
acid.
This work expands on recent studies exploring the benefits of sequential inoculation and yeast
flocculation and the relationship between yeast behaviour and red wine colour. There are
multiple avenues for future research to refine our understanding of mixed-species interactions,
including co-flocculation, between yeast species and uncover the mechanisms responsible for
the impact of sequential inoculations and flocculation on red wine colour, in particular for Pinot
noir, but with further application for other red wine styles.