Dr. Carien Coetzee
28 June 2020
FOCUS ON H2S: PART 5
In Part 1 of this Focus on H2S series, the implications of fining a reductive wine with copper(II)sulphate were discussed. Cu2+ will bind with H2S to form a copper-H2S complex, however, contrary to popular belief, this complex is not readily removed from the wine by racking or general filtration. The copper-H2S complex, therefore, remains in the wine and during ageing H2S can be released from these complexes, resulting in the latent formation of reductive aroma.
By reducing the need for the use of copper (by managing H2S formation, to begin with), the occurrence of these problematic complexes can be avoided. However, often the unwanted development of reductive H2S odours necessitates the use of copper as a (temporary) remedial action.
Ideally, the complex should be removed from the wine medium after the reaction of the added copper to H2S. Different products show some potential for the removal of these complexes and researchers have been investigating a co-polymer, PVI/PVP, as a potential fining agent.
WHAT IS PVI/PVP
PVI/PVP is a polyvinylimidazole and polyvinylpyrrolidone adsorbent cross-linked copolymer. The polymer is insoluble and slightly hygroscopic. It has a number of functional imidazole groups which connect selectively and scavenge various metals removing it from wine.
PUTTING PVI/PVP TO THE TEST
A recent trial at The Australian Wine Research Institute (AWRI) assessed the impact of two commercially available PVI/PVP co-polymer products on the metal concentration of 38 commercially available white and red wines1.
MATERIALS AND METHODS
Two commercial cross-linked polymers provided by Enartis were tested.
- Stabyl MET (0.35 g/L)
- Co-polymer of PVI/PVP and amorphous silica.
- Claril HM (0.53 g/L)
- Co-polymer of PVI/PVP and pre-activated chitosan (chitosan can aid the removal of heavy metals from wine and is also used as a stabilisation and clarification agent).
For each of the 38 commercial wines, one control sample was left untreated, while two more samples were treated with either Stabyl MET or Claril HM. Samples were mixed for three hours, after which they were filtered using 0.2 μm regenerated cellulose filter.
For the samples treated with Stabyl MET, the copper concentration decreased by an average of 50%1. When treated with Claril HM, the average decrease in copper concentration was slightly higher (59%)1. Therefore, both products showed potential as fining agents for the removal of copper and copper-H2S complexes.
Results from another study also showed that more copper will be removed when a higher addition rate of PVI/PVP is applied2. In general, the addition of 20-50 g/hL3 will effectively remove most, if not all, of the copper (and therefore copper-H2S complexes).
The AWRI study did, however, report large variation in effectivity between the different wines tested and the products were, in general, less efficient in decreasing the copper concentrations in red wines when compared to white wines. Of the two products tested, Claril HM seems to be more successful and consistent for the treatment of different wine styles3 probably due to the dual function of the PVI/PVP and the chitosan.
It is important to note that, for some of the wines, the PVI/PVP products showed no measurable effect on copper concentration. The reason for the difference in effectivity and the inherent wine factors responsible for this variation is unknown and needs further investigation.
THE EFFECT ON THE SENSORY COMPOSITION OF THE WINE
Informal sensory evaluation revealed that the panellists were either not able to distinguish between the control and the treated sample or that the sensory differences reported were not considered to have a negative impact on the perception of the wine1,2.
Another study reported a slight, but significant decrease in the concentration of phenolic compounds after treatment with PVI/PVP2. This adjustment of the phenolic content might affect the mouthfeel and colour of the wine, however sensory evaluation of tested samples showed no notable effect. It seems that the lower molecular weight phenolic molecules are more likely to be removed by PVI/PVP and this could, in fact, improve oxidative and colour stability2.
PVI/PVP can successfully remove copper as well as other metals such as iron and aluminium from wine. By removing copper and copper-H2S complexes, the risk of the latent development of reductive aroma (due to H2S release from metal complexes) can be minimised. Additional advantages such as improved shelf life (due to the minimising the risk of metal related oxidative processes) should also be considered.
The effect of the wine composition on the effectivity of PVI/PVP needs to be further investigated as great variability has been reported. Therefore, conducting fining trials before applying the treatment to larger volumes of wine, comes highly recommended. These trials will not only ascertain the effectivity of metal removal but it will also ensure that desired sensory outcomes are achieved.
RECOMMENDATIONS4 FOR CONDUCTING TRIALS AND USING CLARIL HM AND STABYL MET IN THE CELLAR
- For trials, use a base solution of 5-10% of the polymer in distilled water
- Periodically mix the solution for one hour to ensure proper rehydration
- Use a pipette tip with a wide opening (or cut the first mm of the tip) to prevent clogging and make sure the base solution is homogenous before pipetting
- The fining agent settles rapidly. When conducting trials, ensure sufficient contact time by keeping the fining agent in suspension. When adding to the tank, make sure the wine is being mixed properly to avoid rapid and direct settling.
- The efficiency of the product is dependent on contact time. Mix the sample continuously for 0.5 to 1 hour or periodically for 2 hours. Ensure that the trial contact time is the same as the tank contact time.
- Allow to settle for 24 hours
- Filter through a rough prefilter
- Adhere to provider recommendations
- Hirlam, K.; Scrimgeour, N.; Wilkes, E. Removing Metals from Wine Using PVI/PVP. Aust. Wine Ind. Tech. Rev. 2019, 243.
- Mira, H.; Leite, P.; Catarino, S.; Ricardo-Da-Silva, J. M.; Curvelo-Garcia, A. S. Metal Reduction in Wine Using PVI-PVP Copolymer and Its Effects on Chemical and Sensory Characters. Vitis – J. Grapevine Res. 2007, 46 (3), 138–147.
- Karasek, J. How to Avoid and Treat Reductive Wines. In Enartis webinar; 2020.
- Karasek, J. Analysis and Treatments for Canned Wine. In Enartis webinar – Canned wine: New Information for preparing wines for canning; 2020.
Photo by Michael Ungacta on Unsplash