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New technology for volatile acidity
reduction provides remedy for many
stuck fermentations

David Wollan, Gary Baldwin and Clark Smith

Wine Network Australia

Introduction

Recent anecdotal evidence in Australia suggests there has been a resurgence in the incidence of stuck fermentations. This is by no means restricted to inexperienced or sloppy winemakers so there has been much discussion about the origin of the phenomenon. No matter what the cause, all winemakers dread the prospect of watching helplessly as their ferments slow down and eventually stop. In particular we fear the spoilage that may occur simultaneously with this. Recent research work in the USA suggests that, at least in some cases, there may be a technological solution.

Stuck fermentations undoubtedly arise from a wide variety of causes. Many of these are well understood: poor propagation procedures, high alcohol, over clarification, failure to oxygenate musts during the sterol-producing yeast growth phase and must nutrient deficiencies. Toxic factors such as long-chain fatty acid toxicity and bacterial peptide production by virulent Lactobacillus sp. (Huang et al., (1994) may also play a role. Since factors may work in concert and yeast strain susceptibility may vary, effective levels are difficult to discern.

Volatile acidity in arrested musts is often assumed to be an artefact of opportunistic infection by bacteria subsequent to sticking. Some researchers have suggested a more active role (Fugelsang (1993), Pfaff et al., (1978), Schanderl (1959). Controlled studies in replicate model fermentations coupled with commercial experience in the US provide compelling new evidence that acetic acid may be a cause rather than an effect. Moreover, acetic acid removal from stuck lots with elevated VAs appears to enhance refermentation prospects substantially.

There is now available in Australia a gentle, effective process for reducing volatile acidity in wine without harming flavour or other constituents. This treatment which was developed by California firm Vinovation, Inc. involves reverse osmosis filtration coupled with anion exchange. Based on US experience on over one hundred stuck lots, volatile acidity reduction to the 0.6 g/L level was promptly followed by refermentation in approximately 75% of cases.

In order to ascertain that acetic acid reduction itself, rather than some other factor associated with Vinovation's process, is responsible for the observed effect, studies of the phenomenon were undertaken at Napa Valley College in 1995 by Rasmussen et al., (1995).

Principles of the process

The Vinovation process is depicted in Figure 1. It depends on the ability of a semi-permeable membrane to separate from wine a permeate stream containing acetic acid and ethyl acetate, but substantially no flavour or colour. Wine from the tank (1) is recirculated via tangential flow (2) against a reverse osmosis membrane (4) , and a small portion passes through. The "retentate," or that portion which does not pass through the membrane, contains all the flavour and colour, and is returned to the tank. The "permeate," or that portion which passes through the membrane (6), is a colourless, flavourless liquid containing only water, alcohol, acetic acid and ethyl acetate and is absent of vinous character.

The permeate is passed through a weak-base anion exchange resin (7). Ethyl acetate is hydrolysed by the basic conditions within the column. Acetic acid is retained by the resin, while permitting alcohol and water to pass through. The purified permeate is then recombined with the retentate and returned to the tank (5).

The process continues until the desired V A reduction is achieved. The resulting wine is essentially unchanged in volume and flavour composition.

Napa Valley College test protocol

160-litre lots of 1993 Columbia River Chardonnay (23Brix, pH 3.2) and Yakima Valley Merlot (22Brix, pH 3.4) were fermented under normal commercial conditions. DAP was added to correct electrode nitrogen to 125 ppm. 110 ppm S02 was added to discourage bacterial activity. Fermentation was initiated by addition of a heavy yeast inoculum (1 gm/L) of Prise de Mousse. Merlot was divided into eight 20-litre lots and fermented in a submerged cap apparatus at 27C. Chardonnay was fermented to 12oBrix at 15oC and divided into eight 20-litre lots. After 16 days, temperature was held at 20C to encourage completion.

Duplicate acetic acid additions of 1.Og/l, 2.0g/l and 4.0g/l were made to these lots, and untreated control lots were retained. Fermentation was monitored by weight, and residual sugar was assessed at 30 days.

 

Table 1. Chardonnay ferments - Residual sugar (g/l) at 30 days

Level of HOAc addition

Control

1.0 g/l

2.0 g/l

4.0 g/l

Rep 1

Rep 2

Avg.

0.1

0.1

0.1 a

2.0

2.0

2.0 b

3.6

3.4

3.5 c

17.0

16.5

16.8 d

Means shown discriminated at the 1% level of significance.

 

Table 2. Merlot ferments - Residual sugar (g/l) at 30 days.

Level of HOAc

addition

Control

1.0 g/l

2.0 g/l

4.0 g/l

Rep 1

Rep 2

Avg.

0.1

0.1

0.1 a

0.1

0.1

0.1 a

0.1

0.1

0.1 a

15.5

35.0

25.3 b

Means shown discriminated at the 1% level of significance.

 

Experimental findings

Suppression of fermentation occurred in increasing magnitude as a function of acetic acid addition in both experiments (Table 1 and Table 2). Differences in Chardonnay fermentation rate for both replications clearly demonstrate inhibition (Figure 2).

Chardonnay lots with the highest level of acetic acid were stuck at 17 g/L sugar. Control lots were split off, and the remainder was treated by the Vinovation process combining reverse osmosis and anion exchange to remove acetic acid.

 

Table 3. Stuck Chardonnay - Effect on VA and RS of acetic acid removal.

Unprocessed

Control

After acetic acid removal

Re-inoculated

Un-inoculated

Initial

7 days

VA (g/l)

RS (g/l)

RS (g/l)

4.10

16.7

12.0

0.43/0.44

16.7/16.7

0.1/0.1

0.27/0.29

16.7/16.7

0.9/1.4

 

Results are shown in Table 3.  Controls were re-inoculated but failed to go dry. Treated lots also re-inoculated achieved complete dryness in 7 days. Un-inoculated treated lots achieved nearly complete dryness in this period.

Hypothesis for the mechanism of toxicity

Suomalainen (1953) provided evidence from in vitro pH-sensitive dye studies with Saccharomyces cerevisiae which suggest a mechanism for toxicity. The Napa Valley College paper (Rasmussen (1995)), develops this and advances a plausible mechanism for the toxic action of acetic acid: If the lipid bilayer of a yeast cell is indeed permeable to the small, uncharged acetic acid molecule but impermeable to acetate ion, then the pH differential inside and outside the cell in pH 3 to 4 solution would enable massive proton transport into cells as acetic acid diffuses, literally pickling cell contents. The paper predicts that the phenomenon would attenuate at higher pH, and also that Lactobacillus tolerance probably involves a membrane-bound enzyme for facilitated diffusion of acetate ions.

Other applications for reverse osmosis

Initially introduced into the wine industry by membrane suppliers as a method for producing non-alcoholic wine, reverse osmosis was not initially associated with quality wine production. Today, applications of reverse osmosis are exploited by over 300 top wineries in the USA, Switzerland and, just recently, Australia (Figure 3). While volatile acidity reduction has been the main use of this technology, there are other related processes that have found a place in modern winemaking.

Reverse osmosis filtration is widely used in juice concentration. Where late season rain has diluted grape sugar and flavour, using this process it is possible to extract water from juice. This reduces the risk of allowing grapes to hang longer in rain-prone regions.

The technology of water removal by RO can also be effectively utilised for rapid, reliable cold stabilisation with minimal refrigeration; bentonite introduction without water addition; and winery water purification.

The innovation which has attracted the greatest interest amongst Californian winemakers is the use of the process for minor alcohol adjustment. Reverse osmosis permeate alcohol and water can be separated by distillation without adverse effects on flavour. By recombining either the alcohol or the water fraction, the system allows removal of either pure alcohol or pure water from the wine. In either case, flavours become concentrated slightly in the resulting wine.

The idea behind both of these is that during ripening, optimum sugar level does not always correspond with maximum flavour accumulation. In cool climates, premium wine production is hampered by ripening difficulties and rain, which can result in low alcohol wines with diluted flavours. Concentration at the juice stage is one answer to this problem but if this is not possible, water can be removed later by the distillation process described above.

In warmer climates such as California and parts of Australia when grapes are allowed to ripen to full flavour maturity, they often have very high sugars and hence, potential alcohols. In order to obtain, for instance, Chardonnays of mature flavour and Cabernets possessing rich soft tannins, winemakers must often pick fruit at excessive sugar and, eventually, alcohol. If this excessive alcohol could be reduced without dilution, then a better balanced, more palatable wine would result. This process of making small corrections in alcohol content is used in the USA on an estimated 10% of premium wine production. The winemakers thus achieve control of alcoholic content independent of flavour.

 

Note: The Vinovation process is the subject of international patents. The Australian and New Zealand sub-licence for these are held by Wine Network Australia Pty Ltd. For further information please contact WineNet at 565 Burwood Road, Hawthorn, Vic 3122. E-mail: winenet@winenet.com.au

 

References

Fugelsang, K.C., M.M. Osborn, and C.S. Muller. Brettanomyces and Dekkera Implications in Wine Making. Beer and Wine Production. American Chemical Society, Washington, D.C. (1993).

Huang, Y.C., C.G. Edwards, and J.C. Peterson. Relationship between stuck fermentation of grape juice and inhibition of wine yeast by lactic acid bacteria. Presented at the Annual Meeting of the Northwest Chapter of the American Society for Enology and Viticulture, Lake Chalan, WA (1994).

Pfaff, H.S., M.W. Miller, and E.M. Mrak. The Life of Yeasts (2nd. Ed.). Harvard University Press, Cambridge, Massachusetts (1978).

Rasmussen, J.E., Schultz, E., Snyder, R.E., Jones, R.S., and Smith, C.R. Acetic Acid as a Causative Agent in Producing Stuck Fermentations, in American Journal of Enology and Viticulture., pp. 278-280, Vol. 46, No. 2, 1995.

Schanderl, H. Die Mikrobiologie des Mosts und Weines. Eugen Ulmer, Stuttgart, (1959).

Suomalainen, H., and E. Oura. Buffer effect in fermentation solutions. Exp. Cell Res. 9:355-359 (195).

 

{This article was originally published in the 1997 Annual Technical Issue of 

The Australian Grapegrower & Winemaker}

 

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