The Australian Wine Research Institute

Flotation is a clarification process where grape solids are separated in juice using gas (air/nitrogen) bubbled from below to disperse solids from the liquid. Traditional grape must or juice settling takes time, involves chilling which requires high levels of energy and uses valuable tank space or longer periods of time. Flotation or ‘reverse settling’ provides another option for winemakers to speed up the settling and clarification process with many added benefits. It is a proven technique that is employed in other liquid processing industries such as wastewater, mining and fruit juice.

Background

Extraction of juice and modifying the degree of solids in the juice prior to fermentation is a key step in white winemaking. If the juice contains high levels of suspended solids, this can lead to “detrimental effects on wine quality” (Riberau-Gayon et al. 2005), although many winemakers today experiment with varying levels of juice solids to achieve style diversity in their white wines. Reduction of suspended solids prior to fermentation should typically occur quickly, while trying to minimise the amount of juice lost during the process. Flotation is one process that can be used to achieve both of these aims.

A major survey of winemaking practices in Australia (Nordestgaard 2019) showed that flotation is used across all sizes of wineries in Australia, with larger wineries (>1,000 tonnes) more likely to employ this process. It was further reported that this was the “second most common practice nominated by wineries as having a positive impact on their operations in the last five years”.

Practical and logistical considerations

The process of separating suspended solids from grape juice using flotation is described as follows:

• Grapes enter press and or crusher (~12°C)
• Juice/must pumped into holding tank. Chilling is not required.
• Pectic enzymes are added, and juice left for 4 – 6 hours.
• Transfer to flotation tank where it is floated using air/nitrogen with the assistance of flocculation aids (juice 14 – 16°C)
• Rack into tank for fermentation (~16°C)

Major benefits include the speed at which the juice can be clarified and the fact that the juice does not need to be chilled for the process to occur, which results in energy savings. Another benefit is that once the juice is clarified, it can be directly inoculated at the post-floating temperature, further speeding up processing times. Flocculation aids are recommended by manufacturers as these assist in aggregating the solids and enhance gas bubble-particle interaction. These include additives such as bentonite and gelatine.

The AWRI’s winemaking practice survey (Nordestgaard 2019) indicated that nitrogen is the gas most commonly used by wineries for flotation; although, some wineries performed batch flotation of juices with air. The presence of air enhances oxidation and precipitation of phenolics and reduces the browning capacity of the juice (see hyperoxidation). Work done by Sindou et al. (2008) showed that wines made from juice floated with nitrogen had similar total phenolic content, browning capacity and sensory qualities to control wines made from juice that was clarified by standard sedimentation and racking. Wines made from juice floated with air had lower total phenolics and browning capacity than the control wines and showed slight oxidation characters on the palate.

Equipment

The type and size of the equipment required for flotation is dictated by winery size and processing capacity. The AWRI has not performed any trials of flotation equipment and therefore recommends that wine producers discuss their requirements with the different suppliers. A range of different size flotation devices are available, making this a technique that any size winery can use. Choice of correct tank size is also important to ensure enough ullage space is allowed for the lees (also known as ‘flees’) to rise into. A good rule of thumb is to allow for 10% ullage space.

Potential risks

There are very few risks associated with use of flotation for juice clarification. The biggest risk is of foaming occurring during the process if the flow rate of nitrogen gas is not managed correctly.