Australasian Science Magazine

Credit: ugurv/Adobe

By Chen Liang & David Jeffery

Magnetic polymers have been applied to winemaking to demonstrate their potential as a treatment to remove off-flavours. How does it work and what is the effect?

Wines produce an abundance of different aromas, flavours, tastes and mouthfeel sensations due to the innumerable natural chemical compounds present from the grapes and other influences. Through the art and science of winemaking, these compounds will ideally contribute to a balanced and enjoyable wine-consumption experience.

However, that balance can be disturbed by excessive amounts of a certain compound or group of compounds that originate from grapes or from external sources. A classic example is cork taint, which results from undesirable compounds such as 2,4,6-trichloroanisole in wine corks. These are among a range of exceedingly potent aroma compounds that can impart their odour to an Olympic swimming pool with just one drop!

Another class of potent odorants, this time from grapes, is known as the methoxypyrazines. This includes 3-isobutyl-2-methoxypyrazine (IBMP), which contributes a “green” aroma and flavour reminiscent of green capsicum to grapes and wines. When present within certain ranges, IBMP adds desired “varietal” aroma and flavour to some grape varieties, typically cabernet sauvignon and sauvignon blanc. Being perceptible to the senses at only 10 ng/L (10 billionths of a gram per litre) in red wine, high concentrations of IBMP (>20 ng/L) may mask the pleasant fruity and floral bouquets of wines, leading to overpowering vegetative and herbaceous attributes.

An excessive amount of IBMP in wine may be caused by underripe grapes, especially grapes produced in cool climates, as the IBMP concentration in grapes is mainly affected by maturity and sunlight. The concentration of IBMP in grape berries decreases as grapes ripen, and increased sun exposure before the onset of grape ripening may decrease the amount of IBMP.

Intriguingly, methoxypyrazines like IBMP can also arise due to a phenomenon known as “ladybug taint”, where species of ladybird beetles infest the fruit and impart these same types of aroma compounds.

The concentration of IBMP in harvested grapes is a good indicator of the amount that will be present in finished wines because IBMP is readily extracted from grapes and relatively stable during most ordinary winemaking processes. Thus, if the chance has been missed to try to manage IBMP in the vineyard, postharvest methods are needed to remove excessive IBMP from grape juice (white winemaking), must (grape juice and solids used in red winemaking) or wine.

Few approaches to removing IBMP exist, and novel treatment options are still required. Some polymers used in food packaging are known for “scalping” flavour compounds, so their strong sorption ability can remove IBMP and other taint compounds from wine.

Additionally, polymer particles attached to magnetic substrates have been used to extract heavy metals, pesticides and hormones from food samples by applying a magnetic field. We have been examining whether magnetic polymers could work in winemaking to extract undesirable components, such as excessive IBMP, without affecting other desirable characters too much.

To answer this question, we prepared magnetic polymers and tested them as a remedial treatment for IBMP in a research-scale winemaking trial. We took cabernet sauvignon grape must, and due to the relatively low abundance of IBMP in this particular fruit, we added small amounts of concentrated IBMP solution to reach an estimated concentration of 30 ng/L.

Since some components responsible for the aroma of wine are derived from the fermentation process (yeast transforms sugars in grape juice into alcohol and other metabolites, including aroma compounds), we assumed that treatment with magnetic polymers prior to fermentation would have less effects on the wine’s overall aroma attributes. We therefore investigated the timing of magnetic polymer addition, adding powdered magnetic particles into grape must before fermentation and to the untreated finished wine after fermentation.

Two hours after adding the magnetic polymer, we placed a permanent magnet in direct contact with the grape must to extract magnetic polymers added before fermentation. For the post-fermentation treatment, a permanent magnet was applied externally next to the glass bottle to trap magnetic polymers so that the wine could be decanted to another container.

Using an analytical technique called gas chromatography–mass spectrometry, we found that wines treated with magnetic polymers had significantly less IBMP than the untreated control wines, with up to 74% removal of the initial IBMP concentration. Treatment of wines post-fermentation removed 20–30% more IBMP than the treatment added to must pre-fermentation. Accordingly, a panel of trained assessors smelling the wines found that the “capsicum” and “fresh green” aroma attributes associated with IBMP were less evident for magnetic polymer-treated wines than control wines.

We also analysed other wine aroma compounds to evaluate the effect of magnetic polymers on a wine’s volatile profile more generally. We found that magnetic polymers added to must before fermentation decreased these other aroma compounds by 20–30% compared with control wines, whereas treatment applied after fermentation decreased aroma compounds by 40–60%.

Despite the concentration differences of these other aroma compounds, the assessors perceived the overall aroma intensity and fruity attributes of all wines to be the same. However, potentially as a result of the greater loss of fruity aroma compounds, wines treated with magnetic polymers post-fermentation were notable for their “solvent” aroma (i.e. aromas typical of ethanol and other alcohols formed during fermentation). Although the odours of fermentation-derived alcohols can be somewhat pungent, the impact is milder and does not mask other pleasant wine bouquets as much as the presence of IBMP.

We also measured wine colour parameters, and found that magnetic polymers added after fermentation removed more of the red pigments from wines than pre-fermentation treatment. Both approaches were different to the control wines.

These results indicate that it may be more beneficial to add magnetic polymers to remove IBMP before fermentation rather than to wines at the end of fermentation.

Overall we’ve concluded that magnetic polymers could work in winemaking to extract excessive IBMP concentrations without overly affecting other desirable characters if applied to juice/must before fermentation. Furthermore, after remediation with magnetic polymers, unpalatable wine with excessive “green” characters could be blended to make high quality wines or may become an easy-drinking, good quality wine in its own right.

While we found that magnetic polymers can remove excessive IBMP during winemaking, how does this approach compare with the use of other polymers as a post-fermentation treatment, such as polylactic acid (PLA) film used in food packaging? After immersing PLA film in untreated wine for 2 hours, we transferred the wine to another container to separate it from the film. While our results showed that the PLA treatment removed 18% of the initial IBMP concentration, sensory analysis found that the treated wines could still be perceived as vegetative and herbaceous. This difference in performance for a given treatment time may be due to the large surface area and strong sorption ability of magnetic polymers compared with PLA.

Despite the lower amount of IBMP removal, for other aroma compounds and pigments the PLA film had similar effects to magnetic polymers added pre-fermentation; decreases were much less than when magnetic polymers were added post-fermentation. Overall, we determined that magnetic polymers were more effective in removing excessive IBMP than the PLA film, and could improve wine quality more effectively than the PLA film, especially when applied before fermentation.

In this first-of-a-kind study, we demonstrated the use of magnetic polymers in winemaking to ameliorate wines with excessive IBMP concentration. Such an approach using magnetic polymers could conceivably be applied to the removal of other taints or undesirable compounds from wine, and the effectiveness of magnetic polymers could be further improved as well.

This innovative technology provides an alternative solution to amend “faulty” wines. Magnetic separation may therefore be beneficial for industry-scale winemaking compared with existing powder-additive treatments that require sedimentation and filtration for their removal.