WINETECH Technical Yearbook 2019

quality, seeing that certain flavour and colour compounds can be removed. Also notable is the effect of harvest method (e.g. hand harvested vs machine harvested grapes) on specific haze forming proteins, as well as the effect of drought on the concentration and composition of said proteins. These factors will influence the amount of bentonite to be used. WHAT IS IT AND HOW DOES IT WORK? Bentonite is a clay mineral that functions as a cation exchanger in wine. Due to its negatively charged surface, molecules with a positively charged surface (e.g. proteins) can be absorbed to its surface. It is mainly composed of montmorillonite which has a multilayer structure of aluminium hydro silicate forming platelets. Depending on the prevalence of Ca 2+ , Na + or K + cations that are complexed in the interlayer region, different types of bentonite will exhibit variations in swelling and absorbing behaviour. Depending on the dominant cation – sodium or calcium – a bentonite can be classified as sodium bentonite or calcium bentonite. Careful trials should be carried out at winery level to ensure usage of the most suitable variation, seeing that both have advantages and disadvantages. WHEN SHOULD IT BE USED? As with all wine fining actions, an important consideration is timing, seeing that there might be more to bentonite fining than

just protein removal. Some studies propose the treatment of juice, some propose addition during fermentation, while others suggest that bentonite fining should be carried out once the wine stage has been reached. The conflicting results suggest that winemakers must carry out fining trials with the assistance of their chosen supplier in order to best manage fining outcomes. One study showed that not only was bentonite fining most effective at removing proteins from wine when applied to finished wine, but that it also significantly reduced levels of varietal thiols in Sauvignon blanc must. Clearly this suggests that bentonite fining is best carried out post-fermentation, but yet another study on Sauvignon blanc and Riesling showed that bentonite fining during the juice stage or during fermentation required less bentonite and was thus more effective. The early addition of bentonite also favoured fermentation rate and it is suggested that this practice also has a lower impact on the organoleptic properties of wine. This study recommends a two-stage addition of bentonite – first during the juice or must stage and then to wine, if required. In fact, several studies showed that the pre- fermentative use of fining agents, such as bentonite, can have a significant impact on wine aromatic profile and that its use should complement the desired wine style, as well as consider grape origin. That being said, guesswork should be eliminated by doing

proper bench trials to determine the right amount of bentonite to be used. Here, input from the supplier is of cardinal importance. THE PROS AND CONS OF BENTONITE FINING Bentonite is a relatively cheap fining agent, non-toxic, easy to work with and very effective at removing haze forming proteins from wine, but the winemaker should be aware of several possible negatives associated with bentonite fining. These include: ● Stripping of aroma/flavour and colour from wine when excessive amounts of bentonite is used. ● Economical losses due to unnecessary e x c e s s i v e b e n t o n i t e f i n i n g , o r unsuccessful bentonite fining that then leads to protein haze formation. ● The loss of wine that is trapped in bentonite lees (up to 10% of total wine volume). ● Loss in quality of wine recovered from bentonite lees. ● Uncharacteristic aroma profile in certain varietals. ● Environmental impact of bentonite waste. The two main types of bentonite used for wine fining is sodium- and calcium bentonite, each with unique properties, advantages and disadvantages. Sodium bentonite has high swelling capacity, which allows for a high

exchange capacity (protein removal), but the lees is slowly formed, more diffuse and aroma removal from juice or wine is high. Calcium bentonite requires more stirring to disperse and has a lower swelling and exchange capacity, but shows rapid and compact lees formation. Because this type of bentonite produces more compact lees, it is also more suitable as a riddling agent in ‘Méthode Champenoise’. Another positive aspect is minimal aroma removal and unlike sodium, bentonite does not release sodium ions into juice or wine. SUBSTITUTES FOR BENTONITE Two very interesting fining alternatives to bentonite are worth a mention. Proctase, a protease originally developed for human use, is one of these. It effectively removes turbidity forming proteins from wine, but its cost and the requirement for heating and cooling equipment (wine needs to be briefly heated to 70°C and then cooled down in order for the enzyme to do its job) restricts its viability during winemaking to bigger cellars. Chitosan, a fungally derived compound, was shown in a study to significantly remove chitinase from an aromatic white wine at a dosage of 1 g/L. This study also showed that while thaumatin fractions were not significantly reduced, heat stability of said wine was significantly improved. Also noteworthy is that free terpenols, with the