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.
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