Wine proteins stability

Sarmento, M.R., Oliveira, J.C., Slatner, M. and Boulton, R.B. (2000) Influence of intrinsic factors on conventional wine protein stability tests, Food Control, 11, 423-432. 

Commercial wines are commonly tested for protein stability. Wine proteins, upon denaturation by heat or cold, may cause cloudiness and unsightly deposits after bottling. In addition, proteins may combine with iron and copper salts to form flocculate material in bottled wines. The reaction and absorption of proteins on bentonite is an effective means of removing protein from wines (109, 110, 111). Therefore, fining wines... 

Unfortunately, in spite of the published literature on wine proteins, we do not know the actual protein levels at which table or dessert wines are stable. The changes in protein content during production and processing of wines are still not known with sufficient accuracy to predict their behavior. The winemaker has to depend on empirical tests if he is to produce protein stable wines. Early separation of new wines from their fermentation yeast greatly improves their chances for protein stability by decreasing the release of yeast autolysis products into the wine. 

It is not uncommon among North Coast winemakers to add bentonite, one to three pounds per 1000 gallons, to the fermenting juice. This has been done to reduce fermentation foam and promote protein stability in the finished wine. Ough and Amerine confirmed the action of bentonite in helping stabilize the wine for protein (26). More recently, Vos and Gray (18) observed that bentonite treatment of must, prior to fermentation, reduced protein concentration and H2S development while fermentation conducted in contact with bentonite increased H2S production. 

Protein stabilization of wines is the removal of excess heat-sensitive protein. The process has two stages laboratory determination of degree of heat sensitivity and use of bentonite for removal of excess protein. 

First, laboratory testing is conducted to ascertain the stability of the wine. Like tests for protein stability, tests for determining stability and method for correcting instability vary from winery to winery. Berg (34) suggested that a wine stored at — 4° C for four days, without a bitartrate crystalline deposit, may be considered stable. The wines usually are allowed to warm to room temperature before test results are read. Absence of crystals indicates stability. A quantitative method, the concentration product (36), also can be used to evaluate tartrate stability. 

Next, further need for sensory modification is determined in tasting, and acid adjustment or fining is done if needed to balance or soften the wine. The wine is checked to be sure its bitartrate and protein stability meet the winery s requirements. The amount of SO2 is adjusted, usually to 30-35 mg/L free S02 for bottling. 

Wines are stabilized to prevent cloudiness from a number of causes, such as proteins, metals, colloidal materials, and bitartrates (natural salt of wine). 

Addition of exogenous enzymes is currently the most commonly adopted practice in the wine industry to optimize the rate of enzymatic reactions taking place during winemaking. The large amount of research carried out on this topic has led to the presence of a considerable number of commercial preparations on the market, although in some areas, particularly protein stabilization of white wines, exogenous enzyme-based approaches have been unsuccessful so far. 

A method used to solve the problems of excessive lees and flavor stripping caused by flning wine with bentonite is to ferment in contact with bentonite. Fermentation in the presence of bentonite is an old practice used in Europe for protein stabilization. Such a practice avoids or minimizes the need for subsequent bentonite addition into wine. Fermentation in contact with bentonite has several advantages (1) only juice components are adsorbed onto bentonite and not the fermentation-by products or barrel-aging constituents and (2) fermentation lees have a lower monetary value than do finished wine lees. Thus, protein stabilization or partial stabilization during fermentation may be an important economic consideration. 

Other authors have searched for alternative treatments for the use of bentonites, particularly the yeast mannoproteins owing to the fact that a systematic improvement of protein stability can indeed be observed in white wines during aging on lees in barrels (Ledoux et al. 1992). Aged on lees, new wines become decreasingly... 

Enzyme-extracted mannoproteins from the yeast cell wall added at a dose of 25 g/hL, can reduce by half the bentonite dosage necessary for protein stabilization of a very hazy wine (Table 5.3). During lees autolysis, MP32 is released from the... 

Miller, G.C., Amon, J.M., Gilson, R.L., Simpson, R.F. (1985). Loss of wine aroma attributed to protein stabilization by bentonite or ultrafiltration. Aust. NZ Grapegrower Winemaker, 256, 49-50. 

Moine-Ledoux, V., Dubourdieu, D. (1999). An invertase fragment responsible for improving the protein stability of dry white wines. J. Sci. Food Agric., 79, 537-543. 

Zoecklein B.W. (1988c). Protein stability determination in juice and wine. Virginia Cooperative Extension, Publication No. 463-015. 

Pocock, K.F., Waters, E.J. (1998). The effect of mechanical harvesting and transport of grapes, and juice oxidation, on the protein stability of wines. Aust. J. Grape Wine Res., 4, 136-139... 

Several treatment agents of wine yeast cell walls, sodium caseinate, gelatin, bentonite were evaluated for their potential to bind with aroma compounds. The loss of sensory properties of wine, especially flavor modification, is partly caused by protein stabilization treatments with fining agents or ultrafiltration processing of wine (IS 14). Yeast cell walls are used in sluggish or stuck wine fermentation the effect on fermentation has been explained by the adsorption of toxic fatty acids present in the growth medium (15). Therefore yeast walls are also assumed to bind aroma compounds. 

The inhibiting effect of mannoproteins extracted from yeast on tartrate crystallization is not due to compound MP3 2, the invertase fragment responsible for protein stabilization in wine (Section 5.6.4) (Dnbonrdieu and Moine-Ledoux,... 

Various laboratory tests have been used for many years to assess the risk of protein turbidity before bottling. These tests are based on the instability of proteins under various conditions at high temperatures, or in the presence of tannin, trichloroacetic acid, ethanol or reagents based on phosphomolybdic acid. These tests do not all produce the same results. Some of them overestimate the risk of casse during bottle aging. This may lead to the use of much higher doses of bentonite than would be strictly necessary to ensure the protein stability of the wine. 

Table 5.6. Turbidity levels (NTU) after different protein stability tests carried ont on a Sauvignon Blanc wine during barrel aging on the lees (V. Moine-Ledoux, 1997, unpublished results)...

Thus the heat test (80°C, 30 min) without added tannin is the most effective way of assessing the protein stability of a white wine. 

The effect of prolonged cooling, keeping wine around the freezing point, only causes partial precipitation of the proteins. This treatment is therefore never sufficient to ensure protein stability. 

Ribereau-Gayon demonstrated in 1932 that it was possible to adsorb unstable proteins in wine using kaolin, a negatively charged clay. However, large quantities of kaolin, on the order of 500 g/hl, are necessary to obtain protein stability. This makes kaolin treatment impracticable dne to the volnme of lees it produces and the amonnt of wine lost. 

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