Bentonite 101: Clarification and Heat/Protein Stabilization
Last updated: 10/2021
Applies to: winemakers using bentonite for clarification (during settling or flotation) and for heat stability. This article covers what bentonite is, how it works, and differentiates between types of bentonite and their relative uses. See end for recommendations about Scott Labs bentonites.
WHAT IS BENTONITE?
Bentonite, considered a clay, is formed from the decomposition of volcanic ash and is mined from specific regions around the world. Bentonite for winemaking is supplied as a dried, granular clay.
Upon microscopic examination bentonite is composed of small plates (or flakes) of silicon and aluminum oxide, along with sodium (Na) and calcium (Ca) cations. These plates contain many stacked sheets organized into a crystal lattice. When bentonite is dry the lattice structure is closed. However, when bentonite is dispersed in water, this causes the plates to separate, ultimately forming a large open lattice structure with an enormous surface area.
When the lattice structure opens it exposes the exchangeable sodium or calcium ions, ultimately giving bentonite an overall negative charge. The method used for rehydrating bentonite is crucial to the lattice structure remaining open and the sodium and calcium cations being able to react with positively charged molecules such as some proteins.
TYPES OF BENTONITE & THEIR USES
Bentonite has two main functions in winemaking which are clarification and removal of haze-forming (heat-unstable) proteins and there are two main types of bentonites: sodium bentonites and calcium bentonites. All bentonites contain both sodium and calcium ions but are each named after the ion that is most exchangeable. The types are further differentiated by how well the lattice structure opens after rehydration. These properties determine their relative suitability for each of the winemaking functions.
Sodium Bentonite for Protein Removal
Natural sodium bentonite has similar levels of exchangeable sodium and calcium ions. Sodium bentonite lattice structures open very well after rehydration, allowing a high surface area of exchangeable ions. This allows sodium bentonites to efficiently bind positively charged heat-unstable proteins in juice and wine, forming a neutral floccule that settles out over time.
Calcium Bentonite for Clarification
Natural calcium bentonite has a high percentage of exchangeable calcium ions, very few exchangeable sodium ions. Calcium bentonite lattice structures open less but flocculate better, making them well-suited for clarification and lees compaction. Due to their less open lattice structure, calcium bentonites have a lower ability to bind proteins.
Bentonite and Fining Agent Blends
Bentonite is also often found blended with other clarification agents like PVPP and Casein. These agents are fluffy and do not settle quickly. When blended with bentonite, these agents can settle faster with an added clarification boost from bentonite.
PROPER BENTONITE PREPARATION
Every bentonite needs to be properly swelled in order for it to work as intended. Proper swelling is affected by a couple factors:
Hard water is particularly problematic when using Calcium-based bentonites. Water hardness refers to the level of mineral ions in water. The more mineral ions dissolved in water the harder the water. Calcium-based bentonites will not swell to max capacity if swelled in hard water. If calcium-based bentonite is preferred then soft (demineralized) water must be used. Sodium-based bentonites are minimally affected by the minerals in hard water.
If bentonite is prepared in an acidic solution or wine, it coagulates immediately and loses up to 50% of its efficacy. The swelling differences of different bentonites under different conditions is demonstrated in the image below (Figure 1):
Figure 1: This figure shows bentonite swelled in solutions of varying acidity and water hardnesses (Degree of Hardness or °dh). The higher the °dh value, the harder the water. In this case, only pure sodium bentonite (GRANUBENT PORE-TEC) swells completely when swelled in hard water. To swell completely, NACALIT® PORE-TEC (a blended Na/Ca bentonite) needs soft water and even in soft water, it only exhibits a limited ability to swell due to the calcium content.
In the acidic medium of a 1% tartaric acid solution (as "model wine"), the swellability is generally reduced across all bentonites. This is to demonstrate that bentonite should always be swelled in water and not wine.
Choosing a Bentonite
Should you choose to use a bentonite from Scott Labs, we represent Erbslöh bentonites. These bentonites are highly specialized and respectful of wine quality and sensory characteristics. The right bentonite for your wine will depend on your goal:
- Sodium-based bentonite for the removal of haze-forming proteins during static settling.
BLANCOBENT UF (if crossflowing)
- Using this specialized bentonite in conjunction with crossflow filtration allows for protein stabilization and filtration to occur in fewer steps (please check product page for more information).
FERMOBENT® PORE-TEC (if fermenting on bentonite)
- Sodium/calcium blended bentonite for use during fermentation. Fermenting on bentonite can be good for tank fermented whites that need early protein stabilization and preservation of aromatics, especially in high protein varietals or in difficult seasons. For more information see Fermenting on Bentonite).
- Calcium/sodium blended bentonite for clarification and gentle protein removal during static settling.