Magnetic polymers for the reduction of pyrazines in wines

A new remediation method for wines with elevated IBMP levels has been trialled using magnetic polymers, prepared in the same way as ordinary polymers but with the incorporation of iron oxide nanoparticles as magnetic substrates. Magnetic polymers were found to have removed over 40% of the IBMP present in spiked model wine and white wine within ten minutes.

Alkylmethoxypyrazines (MPs) are aroma volatiles noted for their potency and ability to impart sensory characters of green bell pepper, grass, and vegetables to wine. Three grape-derived MPs have been uncovered in recent decades—3-isobutyl-2-methoxypyrazine (IBMP), 3-isopropyl-2-methoxypyrazine (IPMP) and 3-sec-butyl-2-methoxypyrazine (SBMP) —that are mainly located in grape stems, followed by skins and seeds. Another source of MPs in wines originates from the contamination of grapes by Coccinellidae beetles, leading to a wine fault known as ‘ladybug taint’ (LBT). More recently, 2.5-dimethyl-3-methoxypyrazine (DMMP) was identified as another MP compound released by Coccinellidae that contributes to LBT odour. MPs can contribute to the so-called varietal flavours of certain grape varieties including Cabernet Sauvignon, Cabernet Franc, Sauvignon Blanc, and Carmenere; however, high levels (≥20 ng/L) can be overpowering and cause undesirable ‘green’ and ‘unripe’ characters due to the extremely low sensory thresholds of MPs. The detection and recognition thresholds of IBMP determined in red wine are 10 ng/L and 15 ng/L, respectively. The quantity of IBMP is constantly found to be higher than IPMP and SBMP in grapes and wines, and ranges in wines from below 2 ng/L to around 50 ng/L. Consequently, the level of IBMP may be utilised as an indicator of the overall green character potential of grapes and wines.

Grape IBMP concentrations can be affected by grape maturity, sunlight exposure, water status, temperature, vine vigour, and yield. It has been confirmed that greater light exposure for the berries before veraison can decrease IBMP accumulation, but changes during ripening have not been explained entirely. The decrease in IBMP concentration during berry maturation may be mainly driven by dilution due to an increase in berry weight, with no clear degradation pathway of IBMP thus far being elucidated.

Since IBMP is relatively stable during fermentation and ageing, remedial methods are necessary when there are highly elevated IBMP levels, for example, in grapes from cool climate regions or when grapes are picked early to make lower alcohol wines. Several pre- or post-fermentation treatments to remove excessive MPs from juice or wine have been investigated. Must clarification is reported to remove 50% of IBMP in grape juice, but this is not suitable for skin-fermented wines. Fining agents such as bentonite and activated charcoal have little effect on the concentration of MPs in wine and lack selectivity. In recent years, several kinds of polymers have been used to remediate juice or wine with excessive MPs, and silicone and polylactic acid polymers have been reported to be able to remove grape-derived MPs in wine while causing little change in most non-targeted volatile aroma compounds and colour parameters. Addressing the issue of selectivity, molecularly imprinted polymers (MIPs) have been utilised for the extraction of MPs from wine.

MIPs offer a promising alternative to traditional solid-phase sorbents by possessing complementary cavities for target molecules such as IBMP. By association and then disassociation of a template (target molecule or target analogue) during synthesis, this group of polymers gains “memories” of the target molecule and can therefore bind specifically with that compound. Furthermore, MIPs can be made into magnetic forms, termed magnetic molecularly imprinted polymers (MMIPs), through the attachment of magnetic substrates. The major advantage of using MMIPs is they can be directly separated by an external magnetic field instead of by filtration. 

The practical usage of MMIPs, examination of the effects of polymers on other wine volatiles, colour parameters, and sensory properties needs to be further studied.

This article is adapted from its original version:
Chen Liang, David W. Jeffery and Dennis K. Taylor. Preparation of Magnetic Polymers for the Elimination of 3-Isobutyl-2-methoxypyrazine from Wine. Molecules 201823(5), 1140; doi:10.3390/molecules23051140 © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (