Could you clarify?

Jaime Gray and Malie McGregor - 02 Oct 2017

The clarification of white grape musts prior to alcoholic fermentation has long since been considered a necessary step in the production process of high quality white wines. With the unavoidable input of time, energy and/or money into the clarification process and the recent interest in trendy “dirty ferment” white wines, one is prompted to ask: why clarify at all?

Freshly pressed grape must contains turbidity made up of solids of varying origin: from soil to grape skins, stems and pulp to precipitating macromolecules such as potassium bitartrate. Fermenting in the presence of these solids can lead to the production of white wines that show decreased varietal aromas, bitter taste and a higher concentration of reductive odours. White wines fermented from clarified must, however, tend to exhibit fresher and fruitier aromas and can contain a lower concentration of higher alcohols.

Clarification of musts is not without its hazards: excessive clarification can result in stuck fermentations due to stripping of nutrient sources in the must and the possible reduction in natural yeast populations that play an important role in spontaneous fermentations.


Must clarification techniques

Several techniques are currently used in industry to clarify must, the simplest of which is sedimentation. The settling of grape must over time and the subsequent racking off of the clarified juice is a practice as ancient as the art of winemaking itself.  Time and gravity are all that is required to separate the denser solid particles within the must from the liquid.  The downfall of this technique is the fact that it is time- consuming. It requires a minimum of one to two days for proper sedimentation to take place, even with the help of modern developments such as cooling and commercial enzymes.

A clarification technique that is quickly gaining popularity is flotation. Considered to be cost effective, fast and efficient, this system involves the addition of fine gas bubbles to the must from the bottom of the tank. These bubbles adhere to insoluble particles within the must, forming complexes of lower density than the surrounding liquid. These aggregates then rise upwards and collect in a foamy blanket at the surface of the must, from under which the clarified juice can be racked. Flotation can be applied with a variety of gases, ranging from relatively unreactive gases such as nitrogen to air. To improve efficacy, flotation should be conducted in conjunction with the use of fining agents such as gelatine and bentonite.

A suitable starting point to assess whether flotation is worthy of the current hype in industry is to investigate how it stands up against the tried, tested and trusted method of cold settling.


A closer look at settling

Being the most natural and simple method of must clarification, settling requires little special equipment and relies predominantly on the force of gravity to separate the denser solid particles within the must from the liquid fraction. Particles smaller than approximately 1µm tend to sediment very slowly, if at all, meaning that there is a practical limit to the efficacy for clarification by settling. It does however mean that there is reduced chance of over-clarification occurring.

As aforementioned, the greatest disadvantage of settling is that fact that it is a time-consuming process. The rate of settling is dependent on the temperature, viscosity and colloidal content (i.e. solid particle size and density) of the must.

Must derived from rotten or Botrytised grapes may be of greater turbidity and require longer settling time, due to the high concentration polysaccharides (especially β- glucanes produced by Botrytis). Modern methods are commonly employed to accelerate the settling process, the two most common of which are cooling and enzyme additions.

Tanks can be cooled to around 5 - 10 °C during settling to reduce the stirring effect of warm convection currents and to suppress the occurrence of spontaneous fermentation. Pectolytic enzymes are often added to musts prior to settling to catalyse the hydrolysis of pectins, thereby reducing the viscosity of the must and the colloidal effect of various macromolecules within the must. This results in a faster rate of settling, improved clarity and filterability of wines, and a more compact deposit of lees.  The activity of these enzymes is known to be influenced by temperature and ethanol, with musts at low temperatures or wines high in ethanol showing decreased pectolytic enzyme activity.


Figure 1

Figure 1: Separation of solid particles from must with time through gravitational sedimentation


Focus on flotation: the science

Flotation is receiving much attention due to the belief that it is quicker, less expensive and more effective than traditional clarification techniques. However, it is sometimes met with apprehension by many commercial wine producers, as its effects on wine quality is not widely known.

The principle behind flotation is based on the theory of particle density and coagulation: air is passed through a tank (containing must or wine) under a pressurized system. The gas that passes through the system binds to the solid particles in the grape must, forming insoluble agglomerations with a lower density than the liquid fraction of the must. These then float to the top of the system and form a foam which can later be removed. The adhesion of air bubbles to solid particles is a result of electrostatic forces and polymeric stabilisation.

In order to increase the efficacy of clarification, a fining agent can be used. This increases the amount of proteins that will be bound and removed and is known as ‘induced flotation’. This process whereby fining agents are added in conjunction with the gas will form insoluble particles capable of aggregating which can then be removed quickly and easily. The use of flotation without a fining agent will result in the removal of hydrophobic particles while the hydrophilic particles will remain in suspension, as shown in Figure 3A.


Figure 2

Figure 2: Separation of solid particles from the must using a flotation unit to bubble gas from the bottom of the tank


Flotation: gases

The accepted method of flotation currently in the winemaking industry is to perform induced flotation with the aid of nitrogen gas. Winemakers are well aware of the dangers of excessive oxygen exposure during the winemaking process and for this reason, nitrogen is used instead – it still remains highly effective and does not affect the phenolic concentration or sensory quality of the wine. Nitrogen flotation in fact has a better ability to reduce turbidity in wines. Trials have shown that the turbidity o a must following nitrogen flotation is less than half than that of a flotation done with air. Flotation with air decreases the total phenols in the grape must and also lowers the browning capacity of the wine. This is a result of oxidation in the must, the effects of which were then “removed” after flotation as the oxidation products (polymerised phenolic substances) flocculate and are then separated from the clean must. This process is called hyperoxidation.

These wines tend to be lighter in colour and slightly off-balance, while nitrogen- floated wines express similar aroma and characteristics as traditionally clarified wines. Hyperoxidation in wines is useful for sulphur-free wine styles due to its ability to render must more oxidatively stable.

Other gases considered for use are air, argon and carbon dioxide. Air requires purification before use to remove oil traces and off odours which is a time-consuming and costly process. Flotation with air is not ideal for grape must due to the danger of oxidation, especially in the absence of SO2. Argon, an inert gas, is often considered too expensive for use on a commercial scale. CO2 will avoid the danger of oxidation but produces bigger bubbles that are problematic due to larger foam production.


Figure 3A and 3B

Figure 3A: Flotation without the aid of a fining agent removing hydrophobic particles from the must.

Figure 3B: "Induced" flotation using a fining agent forms larger agglomeration with both hydrophylic and hydrophobic particles that separate more effectively from the must


Flotation and fining agents

Flotation works optimally when used in combination with a protein fining agent. There are various agents available for winemakers to use in the cellar. Some of the most commonly used are gelatine, pea and potato proteins, the latter two making the wine suitable for vegetarians.

The clarification efficacy of flotation can be improved when bentonite or silica is used in conjunction with the protein fining agent. It forms a larger insoluble molecule that will separate more successfully as shown in Figure 3B. Bentonite, however, may also remove certain desirable aroma compounds in wines if it is used in too high dosages.


Flotation in the cellar

Flotation is done by linking the flotation unit to the tank at both the racking valve and the bottom valve. Juice moves from the racking valve into the flotation machine where it receives in-line dosing of fining agent and the gas is added before it is pumped back into the tank through the bottom valve. The must is then left for two to three hours to fully separate before the clarified juice is racked from the bottom. The whole flotation process can be completed with four to six hours, allowing the possibility of completing clarification, pressing and inoculation all on the same day.  This saves precious tank space and time in the cellar during harvest. The more condensed lees which form during flotation also decrease the amount of grape juice that is lost during clarification. While sedimentation results in losses of an average of 7 to 10% of juice, flotation only loses approximately 4%.

Flotation is certainly an economically viable practice for industry. The speed of the process, the lower juice losses and the absence of any need for cooling all contribute to financial savings in the cellar. However, flotation does require the presence of a cellar hand to supervise the process unlike traditional sedimentation. The requirement of fining agents for flotation poses a risk of over-clarification in the grape must which should always be kept in mind when determining dosages.


In conclusion

Traditional clarification methods such as sedimentation have shown impressive longevity and efficacy, remaining relevant even in the current modern era of winemaking. However, as technology seeks to optimise winemaking practices, we move towards processes that are faster, cheaper and still uncompromising on quality. Flotation, a method still looked upon with uncertainty by many, shows every sign of fulfilling these criteria and being the next step into the future of must clarification.


Jamie Gray and Malie McGregor were 4th year Viticulture and Oenology students from the Dept. of Viticulture and Oenology, Stellenbosch University at the time of writing this article. The text was edited from its original version by Karien O'Kennedy and proof read by Dr. Debra Rossouw.


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