Greg Howell and Carly Gamble, Vintessential Laboratories
Introduction
The flotation process was originally developed in Australia and has been in use in the mining industry for over a century. It has been used in the wine industry for the clarification of juice for several decades and appears to be gaining renewed popularity due to recent technological improvements. The basics of flotation are discussed in this article, and some conclusions are drawn as to the ongoing growth in the use of this process in the Australian wine industry.
The flotation process
Flotation, as used in the wine industry, is yet another way of clarifying juice. Instead of the solids settling out to the bottom of a tank by gravity (as has been a long standing and common practice), using the flotation process, the solids are agglomerated and rise to the surface of the tank or vessel by the action of gas bubbles. Flotation is primarily used in the production of white and rosé wines.
Advantages of flotation
There are several advantages of the flotation process. These include the increased speed of separation of the solids compared to the traditional sedimentation approach, plus the ability to use it in either a continuous or batch mode. Due to the dense nature of the solids on the surface, juice loss is minimised and the need for expensive juice recovery steps reduced. In regard to speed, one quite recent article states that by using flotation “you can go from pressing to fermentation in a few hours.” (1)
Enzymes required
The use of enzymes in the flotation process is necessary to increase the speed and efficiency of separation and therefore reduce overall flotation time and increase yields.
When grapes are pressed pectins are released into the must. Pectins are complex polygalacturonic acids found in the cell wall of plants. After pressing, juice will contain positively charged proteins which are surrounded by an outer layer of negatively charged pectins. These negatively charged outer layers repel one-another, and result in a stable haze. This increases the overall viscosity of the juice and can impact the efficiency of the flotation process.
Addition of an appropriate liquid pectolytic enzyme preparation will degrade the outer pectin layer by cutting the long pectin chains into molecules of smaller size. The precise location of these cuts within the pectin chain will depend on the type of pectolytic enzyme used. It is therefore preferable to use a blend of enzymes that has been specifically formulated to achieve maximum pectolytic activity for the flotation process (e.g. Rapidase Flotation).
Addition of a pectolytic enzyme preparation should be done as early as possible after pressing with a minimal time for depectinisation of between 2 – 3 hours. This is temperature dependent, the lower the temperature the slower the work done by the enzymes.
This process will expose some of the positively charged inner protein layer and allow particles of opposite charge to attract and agglomerate. This results in a rapid decrease in viscosity of the juice and in conjunction with the subsequent use of clarifying agents, allows particles to be removed more easily by flotation.
Clarifying agents
Following the addition of flotation specific enzymes, clarifying agents are added to complete the process and ensure the overall effectiveness of the operation. As with the addition of the flotation enzyme, these additions must be done prior to fermentation. The typical dosages for the additives are shown below as an indication only, as they are quite variable and all have to be adjusted to achieve the level of clarification required.
1. Bentonite
Bentonite is a widely used additive in the wine industry. Bentonite particles are negatively charged and so attract positively charged particles such as proteins, thus removing them from the must (2). When used in the flotation process it must be used after the addition of the flotation enzyme and where the pectin degradation by the proteinaceous pectolytic enzyme has occurred, otherwise the bentonite will strip out the enzyme and so make it ineffective. The range for dosage is typically 200 – 400 g/kL.
2. Gelatine
This material has a mainly positive charge (2). Its main function is to cause the bentonite to form flocs. It strengthens the flocs, improves cohesion and helps adherence to the gas bubbles. A typical dosage range is 100 – 200 g/kL.
3. Silica (kieselsol)
Silica is used as a colloidal solution and generates larger flocs from the particulates in the juice. It can be used with gelatine but should be added prior to the gelatine. The combination of gelatine and silica provides excellent clarification but floc formation can be a little slower than using bentonite/gelatine. The range of dosage of silica is typically from 100 – 300 g/kL.
Gases that can be used
Gas bubbles are used to make the agglomerated particulates rise to the surface of the vessel where they can be removed leaving the clarified juice below the surface. The size of the floc particles formed is important. If the particle size is too small bubble adherence can be poor and this can result in sub-optimal flotation. If they are too big they can be too heavy and again result in poor flotation.
Several different gases can be used for the flotation, these are mentioned below:
1. Air
Air can be a good choice compared to nitrogen due to its lower cost. The oxygen content of around 20% can lead to some oxidation, but can be of help during the subsequent fermentation. It is very important to purify the air to remove any traces of oil or odours.
2. Argon
As an inert gas it has some great advantages, however cost is usually a prohibitive factor.
3. Carbon dioxide
Although carbon dioxide provides protection from oxidation it does form large bubbles and this can cause some issues with foam removal.
4. Nitrogen
This is the most effective gas for flotation as it forms good size bubbles, is of low solubility and has no side effects. It can however be relatively expensive.
5. Oxygen
Small bubbles are formed but as oxygen is very reactive it can cause unwanted oxidation. It is therefore rarely recommended.
Conclusion
Flotation for use in clarifying juice is not a new process, but is one that has recently gained renewed interest from the wine industry. It is becoming more popular due to improvements in technology and its inherent advantages. The use of flotation systems to speed up and provide a continuous flow system for clarification of juice appears to be undergoing a resurgence of interest. The correct use and dosage of enzymes, plus some or all of the common clarifying agents (bentonite, gelatine and silica) and the appropriate gas, are all critical to the successful operation of wine flotation clarification systems.
References
1. Lansing, R; “Flotation can expedite juice clarification, improve quality”, Wine Business Monthly, 2012, October
2. Mierczynska – Vasilev, A; Smith, P.A; “Current state of knowledge and challenges in wine clarification”, Aust J Grape Wine Research, 21, s1, 2015, pp 615- 626
Greg Howell is the founder and Managing Director of Vintessential Laboratories. He can be contacted by email on [email protected]. Dr. Carly Gamble is the Western Australian State Manager for Vintessential Laboratories and a specialist in enzyme applications. More articles on related topics are available on the Vintessential website: www.vintessential.com.au/resources/articles/
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