Advances in flotation technology have led to worldwide interest in and growing acceptance of this technique. South Africa is no exception, seeing that many South African winemakers are convinced that flotation is acceptable at various cellar sizes and not just at the cooperative level. Advantages of flotation include time and cost savings, less pollution and faster production flow. This article will explore the differences between settling and flotation of grape must, with special focus on the latter. Interviews with winemakers from four local wine cellars also provided valuable insight.
Must clarification is an integral part of white wine production and involves the removal of turbidity causing particles. The simplest form of must clarification is settling as it requires no specialised equipment or training and thus appeals to a large audience. Flotation, however, offers numerous other advantages when compared to settling.
Regardless of the downstream clarification method, must has to be treated with a pectinolytic enzyme first. These enzymes hydrolyse pectins, which leads to increased settling/flotation rate, juice yield, filtration rate and ultimately wine clarity. Pectins are naturally occurring compounds which occur in the grape cell wall. These structural components carry a negative charge and coat positively charged proteins in grape must, thus inhibiting agglomeration and sedimentation. Haze causing particles can vary in size and very small particles will stay in suspension due to their net negative charge. Bigger particles, despite their net negative charge, will settle out. Pectinase action exposes positively charged proteins, after which flocculation and settling can take place (see Figure 1).
FIGURE 1. Mechanism of pectinase action.
The use of a pectinase with high activity is important, regardless of the chosen wine clarification method. The pectin content of grapes depends on factors such as grape ripeness, vintage conditions, grape variety and grape health. Varieties such as Muscat have a higher pectin content and require higher enzyme dosages. Flotation of press juice and juice from Botrytis infected grapes are often problematic and also require higher enzyme doses.
The driving force during settling is gravity, which makes this clarification method simple and widely used. Turbidity causing particles settle to the bottom of the liquid, after which the clear juice can be racked off the sediment.
These additives have a large scope in winemaking, but in this article specific focus will be put on them as flotation additives. Clarifying agents can be added with an inline dosing pump and serve the purpose of increasing electrostatic interactions, which improves the size and rate of flock formation. Flotation efficiency is influenced by the clarification agent type and dosage and will be evident in terms of flocculation speed, final turbidity, foam volume and flock size. Table 1 lists several clarification agents and some of their characteristics.
Flotation is also called reverse settling, since sediment moves upward and thus against gravity. Rising gas bubbles adhere to solid particles, which make them lighter than the must and cause them to rise to the surface. A fining agent such as gelatine coats the foam particle and aids flock formation. With regard to apparatus requirements, a flotation tank is linked to a pressurisation pump, gas supply and dosage pump (for example gelatine addition). With a modified wine tank, clarified juice can be racked off from the racking valve and foam can be pumped away.
Various flotation gases such as nitrogen (N2), carbon dioxide (CO2), oxygen (O2), argon (Ar) and mixtures of oxygen and nitrogen are utilised. Winemakers concerned about must oxidisation typically opt for inert gases (argon or nitrogen). An important aspect of flotation remains pressurisation, which is between five and seven bars. Above seven bars, bubble migration speed is too high and they will not adhere to solid particles.
Questions on flotation were posed to the winemakers and cellar masters from Diemersdal Estate (Juandré Bruwer), Nitida Cellar (Daniel Keulder), Perdeberg Winery (Albertus Louw) and Vergelegen Wines (André van Rensburg).
Question: Why do you do flotation?
Albertus Louw: Chenin blanc is a big focus for us and the success rate in delivering a clear juice by means of flotation is crucial. Flotation is also faster than settling and we get a higher juice yield, even with grapes that have thicker skins and more tannins. Losses when settling free run juice is 12% and 6% with flotation.
Daniel Keulder*: Apart from improved logistics, time savings and decreased juice losses (compared to settling), I felt that my juice was less oxidised before fermentation and my wine definitely had a greener tint. I also didn’t have to wait a day for the juice to get to 15°C before I could inoculate.
* 2016 was the first year that flotation was done at Nitida Cellar.
Question: Do you do traditional settling at your cellar?
Juandré Bruwer: Yes, reserve wines and smaller volumes are settled out, because we have smaller tanks for these smaller volumes.
Question: Which flotation agents do you use and at what dose?
Juandré Bruwer: Potato protein at 5 g/hℓ.
Daniel Keulder: Gelatine at 8 g/hℓ and 20 g/hℓ (press juice), but I would like to experiment with plant protein in 2017. I don’t use bentonite before fermentation.
Albertus Louw: Gelatine at 10 g/hℓ and bentonite at 20 g/hℓ (free run and press juice).
André van Rensburg: Potato protein at 5 g/hℓ. No bentonite is used before fermentation.
Question: Which gas do you use?
All the winemakers: Nitrogen.*
* Albertus Louw uses air for press juice.
Question: How does flotation differ for various varieties?
Juandré Bruwer: We only flotate Sauvignon blanc and Chardonnay and there is no difference.
Albertus Louw: Colombar is the easiest to flotate. Sauvignon blanc, which is harvested earlier and has a thinner skin flotates well due to lower pH and higher acid. In our experience, Chardonnay and Chenin blanc are the most difficult; the grapes are smaller with thicker skins.
André van Rensburg: Chardonnay and Sauvignon blanc flotate well (Chardonnay being the best) and Sémillon poorly. For this reason Sémillon is whole-bunch pressed and not flotated.
Question: Is it more difficult to flotate press juice? Do you use a higher gelatine dosage? Do you add bentonite and how much?
Daniel Keulder: I flotate press juice with gelatine at 20 g/hℓ. I had two tanks that would not flotate (harvest 2016) and had to settle them out.
Albertus Louw: Yes. Press juice is centrifuged and bentonite is added at 20 g/hℓ. Flotation is then done with gelatine at 10 g/hℓ. Total SO2 and pH adjustments are made (if needed) prior to flotation.
André van Rensburg: It’s slightly more difficult. The dose of plant protein stays the same (5 g/hℓ) and no bentonite is added before fermentation. More enzyme and PVPP is used.
Question: Do you do a pectin test prior to flotation?
Albertus Louw: Yes.*
* The other respondents answered “no”.
Question: Please describe your enzyme usage prior to flotation?
Daniel Keulder: I add an extraction enzyme at 2 g/hℓ during crushing and then allow six to eight hours skin contact.
André van Rensburg: A skin contact enzyme is added during crushing and a settling enzyme is added after pressing to the tank, which is left for eight to 12 hours. Chardonnay gets 1 mℓ/hℓ, Sauvignon blanc 2 mℓ/hℓ and Sémillon 4 mℓ/hℓ of the settling enzyme.
Question: At which temperature do you flotate?
Juandré Bruwer: 12 – 14°C.
Daniel Keulder: 17°C.
Albertus Louw: 15 – 18°C.
André van Rensburg: 10 – 12°C.
Question: Which machine do you use?
Albertus Louw: Velo.*
* The other respondents answered “Della Toffola”.
Flotation offers the winemaker several advantages, compared to settling. Time savings, increased yield, perceived higher quality and environmental considerations contribute to this method’s popularity. The general consensus is that premium wine can also be made with flotation.
DSM communication on flotation, 2009. Article at http://www.oenobrands.com/files/PDF/Ingredients-News/Flotation-News-EN.pdf.
Lourens, K. & Pellerin, P., 2004. Enzymes in winemaking. Article at http://www.wineland.co.za/technical/enzymes-in-winemaking.
Mierczynska-Vasilev, A. & Smith, P.A., 2015. Current state of knowledge and challenges in wine clarification. Australian Journal of Grape and Wine Research 21: 615 – 626.
Morris. J.R. & Main, G.L., 1995. Fining agents for wine. Article at http://www.uark.edu/depts/ifse/grapeprog/articles/nmc14wg.pdf.
O’Kennedy, K., 2015. Op is die nuwe af. WineLand, February, 72.
Sindou, E., Vaimakis, V., Vaimakis, T. & Roussis, I.G., 2008. Effect of juice clarification by flotation on the quality of white wine and orange juice and drink – short communication. Czech Journal of Food Science.
– For more information, contact Bernard Mocke at email@example.com (from the 1st of February 2017 at firstname.lastname@example.org).