Commercial Winemaking Production Series
Fining with Bentonite
Clay is one way to remove proteins that could make wine hazy
By Christian Butzke Enology Professor
Department of Food Science Purdue University
The grape berry contains a large variety of nitrogen compounds, mainly amino acids, peptides (short amino acid chains) and proteins (long amino acid chains). They serve various biological functions within the grape such as enzymes, cell wall components, etc. The nitrogen content of grapes varies greatly by variety, rootstock, vintage, climate, pruning and crop levels, fertilization practices, etc.
Amino acids are soluble, and wine yeast can use them to grow and ferment the grape’s sugars into alcohol. Amino acids, together with ammonium ions, are referred to as yeast- available nitrogen (YAN). Peptides and proteins are not considered YAN because they cannot be metabolized by yeast. Their solubility decreases with the wine’s alcohol content. This may lead to precipitation of agglomerated proteins in the form of a visible amorphous haze. This effect is accelerated or triggered by exposure to elevated temperatures, e.g., when a customer buys a bottle of wine in the tasting room and leaves it in the car over the weekend. Protein hazes cannot be tasted; they are a purely aesthetic, visual problem in wine. However, while it is a natural effect, most consumers prefer a wine free from unappetizing-looking protein instabilities.
Winery lab bench tests
The winemaker’s options to prevent protein
Bentonite clay in different forms can irreversibly adsorb various sizes of proteins and has been the protein- fining agent of choice. It takes about six times the quantity of clay to take out the relevant amounts of protein. Since the protein content of wine ranges from around 10 to 300 mg/L, bentonite additions range from 60 to 1,800 mg/L.
Heat exposure, such as a high temperature–short time (HTST) treatment, can denature proteins in unfermented juices and limit the need for additional fining of the wine. An HTST treatment, similar to a milk
The information in this publication is based on material from Winemaking Problems Solved, edited by Christian Butzke, Woodhead Publishing Ltd.