The effect of pre-fermentation must treatments of Pinot gris on wine quality and stability parameters

Abstract

Pinot gris is an emerging grape variety which is creating considerable interest in the New Zealand wine industry. It is deemed to have potential within the market.

Small-scale winemaking trials were used to investigate the effects of reductive (control), oxidative and skin contact pre-fermentation winemaking conditions on the composition, stability and quality of Pinot gris wine. Results indicate that the skin contact treatment (14°C for 7 hours) produced increases in must and wine pH, a greater yield of juice at pressing and a wine with a greater extract. The wine derived from the oxidatively treated must showed a reduced total sulphur dioxide requirement, compared to the control.

At fermentation, the oxidative treatment started and completed fermentation the fastest (4.9 days). The skin contact treatment was the next fastest to complete fermentation, reaching its maximum rate of fermentation in 6.1 days. The reductive treatment took the longest to reach the maximum rate of fermentation (7.1 days).

The most protein stable of the wines was the treatment derived from oxidatively processing the must. Precipitation of the quinones by enzymatic oxidation also led to the removal of proteins, resulting in a wine with a reduced potential to form a haze.

Increased colour parameters were apparent for the wine processed with must skin contact. A visual difference existed at the bottling stage for this treatment compared to the other two treatments. In addition, the skin contact treatment had increases in browning (A420nm) and pinking (A520nm) with age. There was no significant difference found in the spectrophotometric analyses of browning (A420nm) and pinking (A520nm) between the reductive and oxidative treatments at bottling or with 11 months bottle age. However, the reductive treatment visually appeared slightly darker in colouration with age compared to the oxidative treatment.

There was a large variation in the phenolic composition of the three treatments. The skin contact treatment showed the greatest concentration of total phenols (A280nm - 4), with increases in the flavonoid fraction ((A280nm - 4) - 2/3 (A320nm) - 1.4)). The oxidative treatment had a reduced phenolic content, due to the reduction of non-flavonoid content (A320nm - 1.4).

HPLC analysis showed that the hydroxycinnamates, caffeic acid and caftaric acid concentrations were greatly reduced in the oxidative treatment. The concentration of the grape reaction product (GRP) was higher in this treatment at the bottling and aged stages. The increase in this compound from must to bottling indicated that glutathione depletion had not occurred as quickly in comparison with the other two treatments. The drop in the concentration of GRP from bottling to aged in this treatment indicated that GRP was involved in reactions with caftaric acid quinone resulting in the conversion of the quinone to caftaric acid. Caftaric acid increased in concentration over time in the oxidative treatment. This caftaric acid would then be available to undergo chemical oxidation with the regeneration of caftaric acid quinone. With the reduced phenolic content, it is suggested that the level of caftaric acid quinone present is lower in the oxidative treatment. This would result in fewer quinone polymers and thus a greater colour stability for this treatment.

Pinot gris wine derived from oxidatively processed must had a reduced fruit aroma but more aroma complexity than that made by reductive techniques. Pre-fermentation processing of Pinot gris with skin contact leads to a wine of increased colour, aroma, bitterness and astringency characteristics.