Using bentonite as a treatment to reduce soluble proteins has shown promising results both on Vitis vinifera variety Lemberger and on an interspecific hybrid variety Maréchal Foch (Springer et al., 2016). It has been hypothesized that by reducing the content of soluble proteins from the must before alcoholic fermentation would help increase the level of tannins in the finished wine. In interspecific hybrid wines, the tannin content is relatively low compared to Vitis vinifera wines and some studies have been showing the potential role of pathogen-related proteins in this low content (Springer et al., 2016 Springer & Sacks, 2014 Watrelot, 2021 Watrelot & Norton, 2020).
It is well known that proteins are compounds that can interact with phenolic compounds, leading to the red wine astringency perception and to precipitation of tannins.
Schematic representation of the structure of bentonite and the concept behind cation exchange and action of bentonite on positively charged proteins. This will lead to interaction through electrostatic bonds with wine proteins positively charged, or any other wine compounds positively charged (Figure 1). Bentonite, which is negatively charged but depending on the type of bentonite, would bear cations such as sodium (Na +), calcium (Ca 2+) which will be exchanged with the positively charged proteins present in the wine. To treat this haze issue and remove heat-unstable proteins, a fining agent, bentonite, is commonly used. Some other compounds and matrix lead to haze formation in wines, including pH, phenolic compounds content and type, polysaccharides and sulfate content (Moroney, 2018). These proteins are resistant to some chemical reactions happening during the production of wine and therefore remain in wines as they do not precipitate. As a response to a pathogen, pathogen-related proteins (PR), thaumatin-like proteins, and chitinases are produced by grapes. However, in cold-hardy red wines, the content of tannins is generally very low, which does not reduce the proteins’ content. Heat-unstable proteins are a common concern mainly in white wines as red wines tend to contain tannins that interact with proteins, which naturally reduce haze formation. The cause of a visible haze can be over-fining, leading to some excess of material that does not precipitate or heat-unstable proteins remaining in wine (Waterhouse et al., 2016). One issue observed in cold-hardy wines is the formation of haze in bottled wines with about 43 % of the wines, with a fail heat test (survey of Iowa Wine Quality carried out by the Midwest Grape and Wine Industry Institute in 2020). I normally sweeten with either white grape concentrate or apple concentrate (yep right from the supermarket) and add sorbate at the same time to prevent further fermentation, then let it settle out before filtering etc, and then I filter as fine as I can go.In winemaking, the fining step, which corresponds to adding material(s) to the wine, is important to remove any undesired compounds from the wine. Keep in mind that if you plan to sweeten and use sorbate, make sure to add sulfite at the same time or you will really have a cloud on your hands. 5 microns and although it takes a while you get amazing results, and if you are filtering from the carboy you wont stir up the sediment until the end and then you will be pulling it through a filter anyway so it wont get into the rest of the wine anyway. I normally just use a fine filter meaning medium, but I have also gone superfine IE. Alternatively you could filter once it is all settled, that is what I usually do with apple and I find it really polishes the wine so that it sparkles nicely. Try to get it as cool as you can and make sure it is easily racked from where it is because much of the sediment in apple is very fine and takes forever to fall out but is easily disturbed and will swirl up when the carboy is lifted to rack it.
You can not filter it out, but you will end up with a much better product in the end anyway if you wait it out. Apple takes a while a few months on the inside, up to a year on the outside. The haze is going to be there for a while, so learn to live with it.
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