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Modelling of Open Vat Red Wine Fermenters

 

Beringer Blass Wine Estates Nuriootpa South Australia

Industry contacts:

Mr. Nigel Dolan (nigel.dolan@beringerblass.com.au) 

Alan Edward (alan.edward@beringerblass.com.au)

Moderators:

Dr. Rodney Weber, ADFA  (r.weber@adfa.edu.au)

Dr. Stephen Gay, UQ (s.gay@mailbox.uq.oz.au)

Company Background:

Beringer Blass is a large global wine company with production operations in Australia, California, New Zealand and Italy. Major Australian Brands include Wolf Blass, Jameson’s Run, Yellowglen, Annies Lane, Saltram, Ingoldby, Maglieri, Rothbury and Yarra Ridge. Along with the Wine Industry in general Beringer Blass’s business is seeing sustained double-digit growth, particularly into international markets where it’s products are seen to be not only of high standard and good value, but is perceived as clean and green. An example of this success is in Canada where the Wolf Blass Yellow Label Cabernet, is now the largest selling red wine. Within Beringer Blass the largest Australian winery is The Wolf Blass Winery at Nuriootpa in the Barossa Valley. Current annual production at this winery is over 30 million litres. As part of this production an estimated 25000 tonnes of Red Wine grapes will be crushed and fermented in the 2002 Vintage.

 

 

 

Red Wine Production

The quality and character of red wines relies on polyphenolic compounds that are largely contained within the grape skin which when extracted contribute to the colour, flavour and texture of red wine. Extraction of these elements is achieved by leaving the grape skins in contact with the grape juice while it undergoes the yeast fermentation of natural grape sugars into alcohol. The contact between the skins and the juice is vital to the success of this process.

Various types of vessels ( fermenters ) can be used to contain the young wine during this process, one of which is an Open Vat Fermenter. These are remarkably simple – comprising just a large squat cylindrical container, open at the top. Beringer Blass have established 56 such fermenters, each with a capacity of up to 10 tonnes in a newly constructed Fermenter building. These new fermenters are based on a size and design previously used at older facilities within the company.

Red wine grapes are crushed and pumped into the fermenters where yeast is then added to initiate the fermentation process. By- products of the fermentation of sugar to alcohol are heat and carbon dioxide.

 

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Some of the CO2 attaches to she skins causing them to float, forming a layer or “cap” of skins some 50 cm thick sitting above the fermenting juice. This cap only has its lower surface in contact with the juice much limiting the extraction of the desired polyphenols. If left floating the cap will completely dry out over the seven-day fermentation process and provide little contribution of the potential amount of polyphenols, with pale flavourless wine being the result.

There are two main techniques used to keep the skins fully wetted and in contact with the juice so that maximum extraction can be achieved.

The first uses a “heading down” system which is essentially a slotted lid placed over the cap to hold it below the juice surface and thus constantly saturated. One problem with this system is that being closed there is limited introduction of oxygen into the fermentation causing the oxygen dependant yeast to stress and produce unwanted “off characters” in the wine. The second problem is that being static, this system does not provide any significant flow or movement of juice through or across the cap, which although being saturated will over time compact and offer reduced surface area to the juice.

The second method involves periodically withdrawing juice from the base of the fermenter and delivering it as a spray over the surface of the cap. This keeps the cap wet and provides a flow of juice across and through the cap, thus being effective in the extraction of the polyphenols. The mechanism used for this is a pump, which sucks the juice from the fermenter and returns it through a delivery pipe onto propeller blades (an “irrigator”), which are rotated by the force of the flow and spread the juice across the cap surface in an even distribution. The other advantage of this system is the oxygen ingress, important for yeast activity and stability of colour pickup. From experience to date the optimum frequency and duration of this irrigation of the cap (known as a “pump over”) is once every 6 hours for half an hour for each fermenter. Overly frequent, and in particular, continuous pump over causes physical breakdown of the skins to a degree that makes later separation of the fermented juice (wine) from the solid matter difficult.

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Questions 

Beringer Blass and the industry in general has vast experience in the operation of such fermenters and believes it’s methodologies are close to ideal in extracting the optimum results for red wine character and quality. However no analysis of the system has been conducted to provide knowledge of exactly what is happening in the process. There are a large number of parameters involved in this pump over process, including:

  • Height of the irrigator above the cap
  • Shape and size of the propeller blades
  • Volume or rate of flow
  • Pressure of flow
  • Duration of the pump over, and
  • Frequency of the pump over

There is also the issue of how much crushed grapes are filled into the fermenter, thus varying the fill height and ratio of surface area to juice volume, as well as the thickness of the cap.

Beringer Blass is keen to gain a better understanding of the effect of these variables in the system, and to explore if the current process could be improved. For example, are they getting complete and uniform distribution of irrigated juice over the cap? What is the ideal particle size of the irrigated juice? What is the best flow rate? What is the ideal volume to surface area ratio?

 Additional comparison 

An alternative fermenter commonly in operation at Beringer Blass are roto-fermenters, which are essentially large sealed vessels, similar in configuration to a cement mixer, but larger and sealed to contain the fermenting juice. Slow rotation of the vessel (combined with internal spiraling protrusions) keep the juice in contact with the skins. These fermenters are very efficient at extraction of polyphenols, particularly in large batches, they have the disadvantages of being very expensive to build, and being a closed system suffer from a lack of oxygen ingress and the associated problems that causes.

A comparison of the open vat fermenters and roto-fermenters would also be of significant interest.