Preventing protein haze in bottled white wine

Transcription

1 Waters,Alexander, Muhlack, Pocock, Colby, O Neill, Høj & Jones Preventing protein haze in white wine 215 Preventing protein haze in bottled white wine E.J. WATERS 1,5, G. ALEXANDER 1,2, R. MUHLACK 1,3, K.F. POCOCK 1, C. COLBY 3, B.K. O NEILL 3,P.B.HØJ 1,2,4 and P. JONES 1 1 The Australian Wine Research Institute, PO Box 197, Glen Osmond, SA 5064, Australia 2 School of Agriculture and Wine, The University of Adelaide, PMB 1, Glen Osmond, SA 5064, Australia 3 School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia 4 Current address: Australian Research Council, GPO Box 2702, Canberra, ACT 2601, Australia 5 Corresponding author: Dr Elizabeth Waters, facsimile: +61 (8) , Elizabeth.Waters@awri.com.au Abstract Slow denaturation of wine proteins is thought to lead to protein aggregation, flocculation into a hazy suspension and formation of precipitates. The majority of wine proteins responsible for haze are grapederived, have low isoelectric points and molecular weight. They are grape pathogenesis-related (PR) proteins that are expressed throughout the ripening period post véraison, and are highly resistant to low ph and enzymatic or non-enzymatic proteolysis. Protein levels in un-fined white wine differ by variety and range up to 300 mg/l. Infection with some common grapevine pathogens or skin contact, such as occurs during transport of mechanically harvested fruit, results in enhanced concentrations of some PR proteins in juice and wine. Oenological control of protein instability is achieved through adsorption of wine proteins onto bentonite. The adsorption of proteins onto bentonite occurs within several minutes, suggesting that a continuous contacting process could be developed. The addition of proteolytic enzyme during short term heat exposure, to induce PR protein denaturation, showed promise as an alternative to bentonite fining. The addition of haze-protective factors, yeast mannoproteins, to wines results in decreased particle size of haze, probably by competition with wine proteins for other non-proteinaceous wine components required for the formation of large insoluble aggregations of protein. Other wine components likely to influence haze formation are ethanol concentration, ph, metal ions and phenolic compounds. Abbreviations BSA bovine serum albumin, HPF haze protective factor, MW molecular weight, MWCO molecular weight cut off, pi isoelectric point, PR pathogenesis-related, VvTL1 Vitis vinifera thaumatin-like protein Keywords: haze, heat, protein instability, turbidity, sediment, protein, pathogenesis-related, thaumatin-like, chitinase, Botrytis, powdery mildew, Uncinula, bentonite, proteolytic enzymes, mechanical harvesting, ripening, maturity, heat test, grape, Vitis vinifera, white wine, haze protective factor, mannoprotein Introduction In the new era of proteomics and functional genomics, research on wine proteins is still battling with the old problem of protein haze and attempting to invent new methods to solve this issue. Proteins constitute one of the three main macromolecular components of grape juice and white wine, polysaccharides and polymeric phenolic compounds being the others. In this review, proteins are defined as polypeptides with MW greater than 10 kda and with more than 50% of their mass as protein. Yeastderived mannoproteins in wines are predominantly carbohydrate in nature (Llaubères et al. 1987, Saulnier et al. 1991, Dambrouck et al. 2003) and are not considered in this review as wine proteins but as polysaccharides. A specific group of the total grape-derived proteins resists degradation or adsorption during the winemaking process and remains in finished wine if not removed by the commonplace commercial practice of bentonite fining. While bentonite is effective in removing the problem proteins (Blade and Boulton 1988, Høj et al. 2000, Achaerandio et al. 2001, Ferreira et al. 2002), it is claimed to adversely affect the quality of the treated wine under certain conditions, through the removal of colour, flavour and texture compounds (Høj et al. 2000). Furthermore, because of bentonite s considerable swelling and poor settling characteristics, anecdotal evidence demonstrates that from 3% to 10% of the wine volume is taken up by the bentonite lees (Tattersall et al. 2001). While this wine might be recovered through filtration, oxidation typically reduces the quality of this wine to the point where it cannot be blended back with the original product. In addition, handling and disposal of spent bentonite continues to be of concern, because of high labour input and associated costs, occupational health and safety issues, and the wine

2 216 Preventing protein haze in white wine Australian Journal of Grape and Wine Research 11, , 2005 industry s environmental responsibilities and legislative requirements. It is estimated that the cost of bentonite fining to the wine industry worldwide is in the order of US$ m per annum (Høj et al. 2000). The development of improved bentonite fining technologies, or alternatives, that are economically viable and which maintain wine quality would therefore be highly desirable, and this is an area that warrants further research. The sensory properties of wine proteins themselves remain largely unknown, although one report suggests that they have no effect on wine sensory properties (Peng et al. 1997). However, the main concern with residual wine proteins is the possible risk of formation of an unsightly haze that is absolutely dependent on the presence of protein the focus of this review. The origin of wine protein The origin of wine proteins has been the subject of some debate since characterisation of these compounds began in the late 1950s. Bayly and Berg (1967) fermented a model juice solution and concluded that the contribution to wine protein levels by the yeast was negligible. Lee (1986) also suggested that the major source of wine protein is the grape and that the level of total protein is influenced by variety, stage of maturity and climate. Hsu and Heatherbell (1987a) arrived at the same conclusion using a sensitive technique known as lithium dodecyl sulfate polyacrylamide gel electrophoresis. They conceded, however, that it was possible for yeast cells to secrete proteins during fermentation. In one of the more recent studies, Ferreira et al. (2000) and Dambrouck et al. (2003) used modern immunological techniques to confirm that wine proteins originate predominantly from the grape. A contrasting possibility was raised by Yokotsuka et al. (1991), who analysed the protein profile of Vitis vinifera var. orientalis cv. Koshu grapes as well as the resulting wine made from the same grapes. In conflict with other authors, they found eight wine protein fractions not present in the juice and suggested they had come from yeast. Support for multiple biological sources of wine proteins was also recently presented (Kwon 2004) using nanohigh-performance liquid chromatography/tandem mass spectrometry, although the relative levels of different proteins was not established. Waters and colleagues (Waters et al. 1991, 1992) described three major wine protein fractions: 24, 32 and 63 kda in V. vinifera cv. Muscat Gordo Blanco wine. At equivalent concentration, all fractions produced haze in heat-treated wine, with the 24 kda fraction producing up to 50% more haze than the others. Interestingly, the 63 kda fraction was found to be the most thermostable and contained polysaccharides this breakthrough led to research into haze-protective factors (Waters et al. 1993). By analysing the amino acid structure of a 24 kda and 32 kda protein, Waters et al. (1996) determined that these proteins shared homology with thaumatin and chitinases respectively and were highly similar to other plant pathogenesis-related (PR) proteins. Tattersall et al. (1997) further characterised the 24 kda protein, Vitis vinifera thaumatin-like protein 1 (VvTL1), and found that it is encoded by a member of a multigene family in the grapevine and that it was highly expressed in conjunction with the onset of sugar accumulation and softening in the grape berry. It has also been observed that there is relatively high expression after veraison of chitinase encoding genes in V. vinifera cv. Shiraz grape berries (Robinson et al. 1997). Wine protein characteristics Determination of the complexity of wine protein composition has been an area of ongoing research and one that has improved considerably with modern analytical techniques. Electrophoresis initially enabled separation of different sized wine proteins, with Moretti and Berg (1965) and Bayly and Berg (1967) finding four different protein bands, varying in concentration within and among wines from different cultivars of V. vinifera. These researchers were the first to raise the possibility that certain protein fractions, rather than total protein, might be responsible for protein instability. Somers and Ziemelis (1973) used gel filtration chromatography to separate wine proteins from other constituents and, by using the exclusion limit of their gel, concluded that the wine protein size was between 10 and 50 kda. They assumed (but could not confirm) that smaller polypeptides did not remain part of the recovered protein fraction after separation. Hsu et al. (1987) used polyvinylpolypyrrolidone and XAD-4 to remove phenolic material from white wine before protein analysis and discovered many different protein fractions with a range of kda. In a subsequent study (Hsu and Heatherbell 1987b) it was suggested that low molecular weight proteins (20 30 kda) were important to heat instability of wines, rather than those with high molecular weights. This was later confirmed (Waters et al. 1992). Research into the isoelectric point of wine proteins has often been concurrent with research into wine protein size. The isoelectric point of a protein is the ph at which it has zero net charge and is important because, at wine ph, wine proteins have a net positive charge. This allows their removal by bentonite, a cation exchanger, and might also have importance in interactions between the protein and any non-proteinaceous factors in haze production. Proteins with low isoelectric points (pi) were found to be significant contributors to total wine protein (Moretti and Berg 1965) and to wine haze (Bayly and Berg 1967). Hsu and Heatherbell (1987a) confirmed this work and suggested that the majority of wine proteins have a low pi of , whilst Lee (1986), using Australian wines, suggested that the major protein fractions of wine had a pi of Dawes et al. (1994) fractionated wine proteins on the basis of their pi and found that the five different fractions all produced haze after heat treatment. However, haze particle formation was found to differ between the fractions, leading to a statement that other wine components, such as phenolic compounds, need to be considered to fully understand protein haze. The unusual aspect of wine protein instability is that the proteins responsible for protein haze in the long term are, paradoxically, very stable themselves in the short term and survive the vinification process. It is, therefore,

3 Waters,Alexander, Muhlack, Pocock, Colby, O Neill, Høj & Jones Preventing protein haze in white wine 217 not surprising that wine proteins are highly resistant to low ph and enzymatic or non-enzymatic proteolysis (Waters et al. 1992). The mechanism for the resistance has not yet been fully elucidated, but resistance to low ph and proteolysis are characteristics of plant PR proteins in general. Experimental evidence suggests that it is also characteristic of the grape PR proteins involved, rather than phenolic association or glycosylation (Waters et al. 1995b). Limited proteolytic processing of the wine proteins can, however, occur during white table wine vinification (Waters et al. 1998) and during the Champagne winemaking process (Manteau et al. 2003). These data indicate that there could be scope to exploit this susceptibility to proteolysis under certain conditions and thus prevent wine protein haze. Protein levels in white wine have been reported by several authors and have been shown to differ by variety. Lee (1986) measured the protein content of 14 wines, from different Australian regions and made from different varieties, with the protein level ranging from 18 to 81 mg/l. Some of these wines appeared to have been fined with bentonite prior to analysis. Pocock et al. (1998) reported concentrations in un-fined Australian wines up to several hundred mg/l. Hsu and Heatherbell (1987b) found a range of mg/l in four different un-fined white wines, while a very large variation ( mg/l) was noted by Bayly and Berg (1967). Minute variations in the polypeptide sequence of these residual proteins form the basis of a recently developed method of varietal identification based on mass spectrometry (Hayasaka et al. 2001, 2005). Effect of growing and harvesting conditions on wine protein levels In grapevines, the synthesis of the PR proteins occurs predominantly in the skins of grapes (Igartubura et al. 1991, Pocock et al. 1998) and is regulated in a developmental and tissue-specific manner. In V. vinifera cv. Muscat Gordo Blanco, both the berry-specific expression of the VvTL1 gene and the levels of the corresponding major thaumatinlike protein increased dramatically after the onset of berry softening (veraison) and continued throughout berry ripening (Tattersall et al. 1997). Similar developmental patterns were also found in the expression of genes encoding chitinases, some identical to those involved in wine protein haze (Derckel et al. 1996, 1998, Robinson et al. 1997). An immunological study of V. labruscana cv. Concord also showed that thaumatin-like proteins and chitinases accumulate during berry ripening (Salzman et al. 1998). PR proteins are also found in a number of other fruit such as banana (Clendennen and May 1997), cherry (Fils-Lycaon et al. 1996) and kiwi fruit (Tattersall et al. 1997). In all cultivars of V. vinifera studied so far, thaumatinlike proteins and chitinases are the major soluble protein components of grapes (Peng et al. 1997, Tattersall et al. 1997, Pocock et al. 1998, 2000). The predominance of these PR proteins was evident at all stages of berry development following veraison (Pocock et al. 2000). Importantly, as the concentration of extractable proteins in the berries continually increases during ripening, it can be presumed that the haze-forming potential increases as ripening proceeds (Murphey et al. 1989, Tattersall et al. 1997, Pocock et al. 2000). While PR protein synthesis in healthy grapes appears to be triggered by veraison, this does not mean that the classical PR protein inducers, stress, wounding and pathogenic attack, cannot further modulate the levels of PR proteins in grapes. Grape PR proteins exhibit antifungal activity in vitro against common fungal pathogens of grapevines, including Uncinula necator, Botrytis cinerea, Phomopsis viticola, Elsinoe ampelina and Trichoderma harzianum (Giananakis et al. 1998, Salzman et al. 1998, Tattersall et al. 2001, Jayasankar et al. 2003, Monteiro et al. 2003). It would be tempting to speculate that the antifungal activity observed in vitro reflects the main function of the PR proteins in vivo, and that their expression in fruit after veraison represents a pre-emptive defence mechanism for fruit. (Jayasankar et al. 2003) added further credence to this hypothesis by demonstrating that grapevines regenerated after in vitro selection with E. ampelina culture filtrates had greater disease resistance and high constitutive expression of PR proteins, including VvTL1. Studies in which the synthesis of PR proteins is modified by gene technology would allow us to investigate this hypothesis further. While this will be of intellectual interest, there is currently little chance that the haze problems of winemakers could be solved by reducing expression of PR proteins in grapes as this might well lead to increased disease problems. Increased expression of some PR genes and enhanced concentrations of some PR proteins have been observed in leaves and berries from grapevines infected with pathogens (Renault et al. 1996, Derckel et al. 1998, Jacobs et al. 1999, Bezier et al. 2002, Robert et al. 2002). In glasshouse experiments, Monteiro et al. (2003) observed increased levels of thaumatin-like proteins in berries infected with U. necator compared to uninfected berries. Jacobs et al. (1999) demonstrated that chitinase and β-1,3- glucanase activity increased in grape berries and leaves in response to powdery mildew infection, and that expression of genes, VvChi3, VvGlub, and VvTL2, coding for PR proteins, was also strongly induced. Of these three putative gene products, only VvTL2 has been detected as a soluble protein in grape juices and wines (Waters et al. 1996). A recent study (Girbau et al. 2004) demonstrated that powdery mildew infection of V. vinifera cv. Chardonnay grape bunches resulted in increased levels of a grape minor thaumatin-like protein, VvTL2, in wine. At high levels of infection (>30% of bunches infected), this had a significant impact on the level of haziness in the wine following a heat test. Contrary to expectations that fungal diseases would lead to elevated levels of PR proteins in berries, Marchal et al. (1998) observed that juice from berries infected by B. cinerea showed reduced protein levels, and suggested that proteolytic enzymes from B. cinerea were responsible for this. Secretion of proteases by B. cinerea has been observed in culture media and on fruits such as apple (Zalewska-Sobczak et al. 1981) and tomato (Brown and

4 218 Preventing protein haze in white wine Australian Journal of Grape and Wine Research 11, , 2005 Adikaram 1983). Girbau et al. (2004) also examined the impact of infection of grapes with B. cinerea in the vineyard and showed that infection resulted in marked decreases in the levels of PR proteins in the berries. Similar although less dramatic trends of reductions in protein levels were seen in laboratory experiments in which otherwise healthy berries were inoculated with B. cinerea (Girbau et al. 2004). Although these grapes were not vinified in this study, it was expected that the difference in protein levels between clean and infected fruit would also be observed in wines made from clean and infected fruit. The reduction in protein levels in the juice from Botrytis-infected grapes did not appear to be an artefact of poor extraction into juice due to shrivelling or desiccation of the berries, but could, as suggested by Marchal et al. (1998), be due to proteolytic degradation of grape PR proteins by enzymes of B. cinerea. Protein levels were also reduced in juice when B. cinerea was grown in this medium (Girbau et al. 2004). If these effects are due to the activity of proteolytic enzymes from B. cinerea, these enzymes have the potential to replace bentonite fining for protein stabilisation in oenology, a goal of many research efforts worldwide. It has been shown that wounding, a classical PR protein inducer, increased chitinase activity in V. vinifera leaves (Derckel et al. 1996) and berries (Derckel et al. 1998). The effect of mechanical harvesting, a commercial activity inducing wounding, on the levels of PR proteins in grapes is, therefore, of interest. Paetzold et al. (1990) observed that mechanically harvested grapes produced must with increased protein content compared to that of hand harvested fruit. They suggested that less protein was lost in complexes with phenolics from must from mechanically harvested fruit because the lack of stems during crushing led to lower polyphenolic content in the must compared to that from hand harvested fruit. Dubourdieu and Canal-Llaubères (1990) showed that wine made after maceration of destalked grapes for 18 hours contained more protein than wine made after immediate pressing of whole bunches. Whether this increase in protein was due to the removal of stalks or the wounding of grapes that occurs during destalking or maceration was not elucidated. Pocock and colleagues (Pocock et al. 1998, Pocock and Waters 1998) examined the impact of mechanical harvesting on the PR proteins in grapes and wine. Mechanical harvesting together with prolonged transport of the fruit resulted in higher PR protein levels in the juice and wine. Indeed mechanical harvesting of white grapes and subsequent transport was found to double the amount of bentonite required for prevention of protein haze when compared to fruit harvested by hand and transported from the same vineyard (Pocock and Waters 1998). This does not appear to be a result of increased protein synthesis, as comparisons among hand harvested berries, mechanically harvested intact berries and the predominant form of mechanically harvested fruit a mixture of broken fruit and juice indicated that little if any protein was produced as a result of stress caused by mechanical harvesting. Increases in protein content of juice from mechanically harvested fruit thus appear to be due to extraction of protein from skins rather than a physiological wounding response by the berry. The effect of water stress imposed during some commercial viticultural management practices on the expression of PR proteins in grapes has been examined by analysing the PR protein content of V. vinifera cv. Shiraz berries from a replicated irrigation trial (Pocock et al. 2000). The lack of irrigation gave clear physiological signs of vine water stress but did not lead to elevated levels of PR proteins in the berries. At a fixed amount of protein per berry, however, the protein concentration in the juice from water stressed berries was higher than that from irrigated berries because berries from irrigated vines were larger and thus berry solutes were less concentrated. This effect of water stress on berry size is a general phenomenon (Smart and Coombe 1983) and it is likely that anecdotal reports that haze problems are greater in drought years are due to changes in berry sizes in these years rather than a direct physiological response of the berries to water stress in the form of enhanced PR protein production. Oenological control of protein instability with bentonite Batchwise addition of bentonite, a montmorillonite clay, is universally employed throughout the wine industry for the prevention of wine protein haze, in a process known as bentonite fining (Blade and Boulton 1988, Høj et al. 2000, Ferreira et al. 2002). The adsorption of wine proteins onto bentonite is due to the cationic exchange capacity of the bentonite clay. Wine proteins are positively charged at wine ph, and thus can be exchanged onto bentonite, which carries a net negative charge (Blade and Boulton 1988, Høj et al. 2000, Ferreira et al. 2002). A range of bentonites commercially available in Australia were evaluated by Leske et al. (1995). The sodium products were shown to be generally more effective than the calcium-based materials on a per unit weight basis. The sensory evaluation by Leske et al. (1995) of wines treated with bentonite showed no significant differences between the control and the fined samples. Similarly, using difference testing, Pocock et al. (2003) reported that bentonite fining of a Chardonnay and Semillon wine had no effect on wine aroma and palate. This contrasts with previous findings of Miller et al. (1985) that demonstrated reduced concentration of aroma compounds after bentonite addition to juice, must or wine. More recently Pollnitz et al. (2003) elegantly confirmed that aroma compounds can be absorbed by bentonite, as did Cabaroglu et al. (2003), although the later study found no sensory effect of bentonite fining of V. vinifera cv. Muscat Ottonel or Gewürztraminer wine. Rankine (1989) stated that bentonite fining results in the loss of aroma and flavour, and Martinez-Rodriguez and Polo (2003) expand this conclusion to sparkling wines when bentonite is added to the tirage solution. This has led to the widespread conclusion, throughout both literature and industry, that bentonite fining at typical addition rates has a detrimental effect on wine aroma and flavour, despite the fact that the conclusions of Miller et al. (1985) were influ-

5 Waters,Alexander, Muhlack, Pocock, Colby, O Neill, Høj & Jones Preventing protein haze in white wine 219 enced by many factors and should not be considered general (Miller et al. 1985). Similarly, the findings of Leske et al. (1995), who conclude that lack of significant differences observed in this trial suggests that the negative effect of fining may not be evident to a consumer in a commercial situation and the magnitude of any such [negative sensory] effect will presumably depend on the individual wine, the bentonite chosen, and the rate of addition chosen are, therefore, not surprising. A number of studies have indicated that different protein fractions require distinct bentonite concentrations for protein removal and consequent heat stabilisation (Duncan 1992, Ferreira et al. 2002). Bentonite fining has been shown to remove higher pi ( ) and intermediate molecular weight (MW kda) proteins first (Hsu and Heatherbell 1987b). However, these represent only a small portion of the soluble proteins. Proteins with a MW of kda, and with wide pi range ( ) were highly resistant to removal by bentonite fining (i.e. required significant bentonite addition), and typically remained in protein-stabilised wine (Hsu and Heatherbell 1987b). Hsu and Heatherbell (1987b) concluded that it is necessary to remove lower pi ( ), lower MW (12.6 kda and kda) proteins, which contain glycoproteins and represent a major component of proteins present, to protein stabilise wines. Contrary to these findings, a study by Dawes et al. (1994) found that there was no bentonite selectivity based on isoelectric point, and that bentonite fining resulted in the removal of all the different protein fractions. Further, the amount of protein depletion (across all protein fractions) observed in this study corresponded linearly with the level of bentonite addition (percentage reduction in protein concentration per g/l of bentonite added ranged from 70 89%). These different conclusions in the published literature might be attributed to the different methods used to fractionate proteins and assess their levels. Several authors have investigated the extent of adsorption of standard and model proteins by bentonite. Gougeon and colleagues (Gougeon et al. 2002, 2003) studied the adsorption of two homopolypeptide preparations with average MW around 20 kda onto a synthetic bentonite. Their data suggested that these polypeptides tended to unfold and take on a more random coil structure upon adsorption. Using a range of physical measures Gougeon and colleagues (Gougeon et al. 2002, 2003) also hypothesised that the polypeptides were primarily adsorbed near the edges of the bentonite sheets rather than within the interlayer spaces between the sheets. The adsorption of the standard protein, bovine serum albumin (BSA) by bentonite in model wine solutions was studied by Blade and Boulton (1988). Adsorption was shown to be independent of temperature, but varied slightly with protein content, ph and ethanol content. In another study (Achaerandio et al. 2001), bentonite adsorption was evaluated with three proteins (BSA, ovalbumin, lysozyme) in a model wine solution. The effect of ethanol content and protein molecular weight on the adsorption capacity of bentonite was also studied. Adsorption capacity tended to increase with increasing ethanol concentrations with regard to adsorption of BSA and lysozyme, however no change was observed for ovalbumin. Blade and Boulton (1988) showed that attainment of maximal adsorption was rapid, and complete within 30 seconds of the addition (this being the shortest interval which could be attained practically). This is consistent with an earlier study (Lee 1986), in which Gewürztraminer wine fined with bentonite was rendered stable one minute after bentonite addition. In comparison, bentonite fining in a winery setting typically takes one to two weeks, depending on the tank size and rate of bentonite addition used (Audrey Lim, pers. comm.). Rapid adsorption of protein onto bentonite might provide an opportunity to employ a continuous contacting process design. An understanding of the adsorption kinetics and mass transport processes involved is crucial to the design and development of such a continuous system. The swelling properties and small particle size of suspensions of commercial bentonite are obvious limitations to a continuous design approach, as is the regeneration of the bentonite adsorbent. Bentonite regeneration refers to the desorption of adsorbed wine protein from the bentonite surface, and would permit bentonite to be reused. However, a commercial process for bentonite regeneration does not currently exist, and thus bentonite is only used once before being discarded. An early study (Armstrong and Chesters 1964) investigated the effect of ph on the desorption of pepsin from bentonite. In this study, 62.8% of the pepsin (pi 2.8), which had been adsorbed onto the bentonite at ph 3.0, was desorbed by raising the ph to 5.2 using sodium hydroxide. In a more recent study (Churchman 1999), the batch treatment of bentonite-protein complexes with a range of bases at a variety of different concentrations, durations and agitation methods was examined. The greatest degree of desorption was achieved with sodium carbonate at a low solid:solution ratio and with agitation. Similar results were obtained with sodium hydroxide at ph 12 and 13 (protein released per gram bentonite = 52 and 65 mg/g respectively). However, from the study it was concluded that the use of alkaline ph and salts was ineffective for substantial removal of protein from bentonites. Instead, the study suggested protein degradation followed by a treatment to displace the products of protein breakdown from the bentonite was required. For example, a batchwise treatment that employed hydrogen peroxide, sodium carbonate and photo-oxidation with UV radiation were found to be effective in removing residual protein from bentonite. A key observation from both studies is that protein was desorbed from bentonite by addition of a base to increase the ph. However, in both of these studies, the desorption of proteins from bentonite was only investigated for batch systems. As adsorption and desorption are equilibrium processes, it is hardly surprising that residual protein was retained on the bentonite surface. In contrast, in a continuous flow system, fresh reactant (e.g. NaOH) would be continually fed to the adsorption system, while ion exchange products (in this case, desorbed protein and hydroxide) are continually transported away from the system, driving the exchange process to completion.

6 220 Preventing protein haze in white wine Australian Journal of Grape and Wine Research 11, , 2005 Continuous flow contactors, such as continuous stirred tanks, packed bed columns and fluidised beds, are commonplace in many chemical industries, as is continuous catalyst regeneration (Fogler 1992). Therefore, it is postulated that effective regeneration of bentonite could be achieved by treatment with a base such as sodium hydroxide in a simple continuous flow system. Protein removal techniques alternative to bentonite A number of alternative techniques to bentonite fining have been investigated for the removal of proteins from wine, but have shown limited success to date. Techniques investigated include ultrafiltration, addition of proteolytic enzymes, flash pasteurisation and use of alternative adsorbents. A number of studies have investigated the effectiveness of ultrafiltration as a bentonite substitute for protein stabilisation (Hsu et al. 1987, Flores et al. 1990). A progressive increase in membrane retention of soluble protein is observed with a decrease in molecular weight cut-off (MWCO) of the membrane, with up to 99% of wine proteins being retained with a 10 kda MWCO membrane filter. Hsu et al. (1987) showed that 3 to 20 mg/l of protein frequently remained in ultrafiltration permeate, corresponding to periodic occurrence of heat instability. Although protein stability was not always achievable with 10 or 30 kda MWCO membrane filters, an 80 95% reduction in bentonite requirement was observed. The presence of residual proteins in the filtrate, together with high set up and running costs, and possible loss of organoleptic compounds (Miller et al. 1985, Simpson 1986), has rendered ultrafiltration unattractive for commercial practice thus far. Batch addition of both endogenous and exogenous proteolytic enzymes has also been studied as an alternative protein depletion method (Feuillat and Ferrari 1982, Heatherbell et al. 1984, Lagace and Bisson 1990, Duncan 1992, Waters et al. 1992, Dizy and Bisson 1999). However, proteolytic enzymes have been shown to be ineffective in conferring haze protection under normal winemaking temperatures (Heatherbell et al. 1984, Waters et al. 1992, Waters et al. 1995b), which is consistent with the strong proteolytic resistance of PR proteins. The crystal structure of thaumatin-like PR proteins shows that these proteins are compact and globular in nature, with few exposed loops accessible to proteases (Tattersall et al. 2001). Modification of winemaking procedures to induce protein unfolding might, therefore, provide an avenue for proteolytic activity as a method of preventing protein haze. Flash pasteurisation was investigated by Ferenczy (1966) and judged to have too detrimental an effect on wine quality. However, a study by Francis et al. (1994) indicated clearly that a short period of heating to 90ºC did not adversely affect the sensory properties of white wines. A recent study (Pocock et al. 2003) confirms this. This study showed that short term heating reduced the bentonite requirement to between 50% and 70% of the level required for the untreated wines, without affecting the sensory profile. Further, proteolytic enzyme addition during short term heat exposure (to induce protein denaturation, as stated above) reduced the bentonite requirement to between 30% and 60% of the level required for the untreated wines, again without affecting the sensory profile. This process shows significant promise, but further research is required to optimise it on a large scale. Weetall et al. (1984) postulated that the use of immobilised phenolic compounds might successfully stabilise wines against protein haze formation. Phenolic compounds such as proanthocyanidins (condensed tannins) are known to interact with proteins over a wide ph and temperature range. The capacity of immobilised proanthocyanidins from grapes to remove wine proteins was investigated (Powers et al. 1988). The immobilised proanthocyanidins bound proteins from an un-fined Gewürztraminer wine, resulting in a protein stable wine as judged by a trichloroacetic acid turbidity test. Several regenerating solvents for the proanthocyanidin-agarose column were investigated, with alkaline solutions being the most effective. However, the use of immobilised proanthocyanidins appears limited by a reduction in proteinbinding capacity after a small number of regeneration cycles. A range of alternative adsorbents, including other clays, ion exchange resins, silica gel, hydroxyapatite and alumina, have been evaluated (Sarmento et al. 2000) for their ability to stabilise white wines by studying the characteristic absorption isotherms of BSA in model wine. Some of the ion exchange resins showed favourable behaviour in packed bed applications. Metal oxide materials also show promise as alternatives to bentonite fining (Pachova et al. 2002, Pashova et al. 2004a,b). Haze-protective factor An alternative to the removal of haze-forming wine proteins involves the addition of polysaccharides to wines. An investigation of fractionated wine macromolecules from Muscat Gordo Blanco wine showed that certain polysaccharide-rich fractions apparently protected some wine proteins against heat-induced haze formation (Waters et al. 1991). The isolated polysaccharide-rich fractions, known as haze-protective factors (HPFs), showed little hazing when heated, and this was observed regardless of whether the polysaccharide component was present in isolation, or together with the proteins that normally accompanied it in wine (Waters et al. 1991, 1993). A haze-protective mannoprotein was isolated and purified from 600 litres of Carignan Noir wine in milligram amounts (Waters et al. 1994a) and is referred to as Hpf1p. Hpf1p is apparently analogous to the HPF isolated from Muscat Gordo Blanco wine described above (Waters et al. 1993). Carbohydrate and amino acid analysis has indicated that the mannoprotein is derived from the yeast cell wall (Waters et al. 1993, 1994a). Investigation of methods to extract Hpf1p from yeast and immunological studies of the location of Hpf1p demonstrated that Hpf1p was located in the cell wall (Dupin et al. 2000a,b). A second haze-protective mannoprotein (Hpf2p) has also been

7 Waters,Alexander, Muhlack, Pocock, Colby, O Neill, Høj & Jones Preventing protein haze in white wine 221 isolated by ethanol precipitation of a chemically defined grape juice medium fermented by the winemaking strain of S. cerevisiae, Maurivin PDM (Stockdale 2000). Putative structural genes have been identified for Hpf1p and Hpf2p, from their amino acid sequence (Waters et al. unpublished, Stockdale 2000). The haze-protective effect of yeast mannoproteins was independently confirmed by Ledoux et al. (1992) when they isolated a mannoprotein fraction from yeast cell walls that was shown to protect against haze. Indirect support was given by the fact that white wine aged on yeast lees had reduced haze potential and lower bentonite requirements than white wine aged without lees contact. Moine- Ledoux and Dubourdieu (1999) showed that the mannoprotein responsible was a fragment of invertase from S. cerevisiae. The susceptibility of wine to haze formation and hence bentonite requirements can be reduced by the addition of this fragment. Other glycoproteins have also been shown to have haze-protective activity including whole yeast invertase (McKinnon 1996, Moine-Ledoux and Dubourdieu 1999, Dupin et al. 2000a), a wine arabinogalactan protein (Waters et al. 1994b), gum arabic and an apple arabinogalactan protein (Pellerin et al. 1994). The exact mechanism by which mannoproteins afford haze protection is unclear. Waters et al. (1993) determined that the presence of mannoproteins in a wine decreased the particle size of the haze rather than preventing aggregation of the wine proteins. It was shown that with the addition of an unpurified yeast mannoprotein fraction, the particle size of the haze formed upon heating was reduced from 30 µm to 5 µm, making the haze barely detectable to the naked eye (Waters et al. 1993). Dupin et al. (2000b) showed that a haze-protective factor, invertase from S. cerevisiae, was present in the wine after heating and removal of the haze. It was suggested that, since the majority of the haze-protective factor was in the supernatant, these factors act by competing with other wine proteins for other non-proteinaceous wine components ( Factor X ) required for the formation of large insoluble aggregations of protein. Protein haze formation and its prediction Protein haze in bottled wines presents itself as an amorphous fluffy white suspension or sediment composed primarily of protein. It can be mistaken for evidence of microbial spoilage and because consumers expect white wine to be sparkling clear, this sort of instability is to be avoided. The full mechanism of protein haze formation is not fully understood despite much research world-wide on this commercially significant problem. Slow denaturation of wine proteins is thought to lead to protein aggregation, flocculation into a hazy suspension and, finally, formation of precipitates (Bayly and Berg 1967, Hsu and Heatherbell 1987a, Waters et al. 1991, 1992). It seems likely that further advances in our understanding of protein haze will come from investigating the mechanism of its formation and the key factors involved. Some of these factors are illustrated later in the review. A key tool both for researchers and industry practitioners is the ability to measure the protein stability or haze potential of a wine. Generally this involves inducing a haze by heat or other methods, then measuring the haze produced. The classic test is described by Pocock and Rankine (1973), where wine is subjected to 80 C for six hours, cooled and the wine inspected visually for haze using a strong beam of parallel light. This test for inducing a haze has been almost universally adopted for measuring protein stability or haze potential despite the fact that some consider it too harsh a test, potentially leading to an excessive use of bentonite. This view is not supported by the work of Dawes et al. (1994) in which the haze levels in wines after storage correlated well with the results from this predictive test. Nevertheless, a more rapid method to predict haze formation would be an advantage. Many variations of this test are described in the literature (Berg and Akiyoshi 1961, Bayly and Berg 1967, Hsu et al. 1987). Methods based on reactions with trichloroacetic acid, ethanol or the proprietary Bentotest reagent are not often used. Measuring the haze once it has been produced has also been carried out in a number of ways. The Pocock and Rankine test relied on a visual inspection this might be suitable for checking the stability of a commercial wine, but not for differentiating between hazes. Spectrophotometric methods seem the most widely used, but this might be due to the availability of such equipment in most laboratories. Absorbance has been measured at a range of wavelengths including 540 nm (Waters et al. 1991) and 650 nm (Dawes et al. 1994). Transmittance is rarely employed although Fenchak et al. (2002) described its use at 633 nm. Turbidimeters or nephelometers are often not commonly available laboratory equipment, but might provide a more accurate assessment of haze. Authors such as Lagace and Bisson (1990), Siebert and colleagues (Siebert et al. 1996a,b) and Yokotsuka et al. (1991) have used these methods successfully. Non-proteinaceous factors in protein haze formation There are several lines of evidence in the literature that suggest that wine components other than protein are absolutely required for the formation of visible protein haze in white wine subjected to the industry standard heat test (80ºC, six hours). For example, several authors (Moretti and Berg 1965, Bayly and Berg 1967, Dawes et al. 1994) have suggested that total protein content failed to correlate with heat stability. More concrete evidence of the involvement of non-proteinaceous components has come from studies that have attempted to reconstitute the process of haze formation in model wine solutions. Yokotsuka et al. (1983) were first to show that proteins isolated from grape must (although denatured in the process) did not produce a visible haze in the absence of particular wine components. Likewise, Waters et al. (1995a) noted that proteins that had been rendered free of procyanidins would precipitate in wine but not in model wine (a solution free of phenolics and other nonproteinaceous wine components). Despite a body of evidence for the absolutely required involvement of a non-proteinaceous wine component in the formation of a visible protein haze, no concrete proof

8 222 Preventing protein haze in white wine Australian Journal of Grape and Wine Research 11, , 2005 singling out any one wine component or particular wine conditions has yet been presented. Several wine components have been suggested, however, and evidence for the ability of such components and factors to modulate the intensity of heat-induced haze is abundant. ph It is fair to say that the effect of ph on protein haze formation in wine has been incompletely studied to date and that much of the published work has not directly involved wine. Siebert and colleagues (Siebert et al. 1996a, Siebert and Lynn 2003) studied the effect of ph on the formation of protein-polyphenol complexes in a model system that mimicked beer, wine and fruit juice. They concluded that maximum haze occurred at ph when ethanol was 12%, with less haze at lower and higher ph values. The significance of these results for white wine needs to be questioned, given the model solution employed gelatin and catechin as the protein and phenolic compounds respectively. Gelatin is not present in unprocessed white wine and might, therefore, not be a suitable model compound. Another study using a model wine (Fenchak et al. 2002), came to similar conclusions regarding ph. However, Mesquita et al. (2001) employed actual wine samples rather than a model wine to obtain different results to those above. The white wine they used became increasingly heat stable as the ph rose from 2.5 to 7.5, suggesting that ph does play an important role in protein haze formation and that a high ph reduces the potential to form protein haze in response to heat. However, it is not clear what the magnitude of importance of ph variation on haze level is within the range encountered in wine. Ethanol The alcohol content of white wines can vary quite substantially between styles, so it is a little surprising that so few studies have investigated the link between protein haze formation and alcohol content. Studies with beer (Siebert et al. 1996a, Siebert and Lynn 2003) have shown alcohol to have an influence only at ph 4, where increasing alcohol concentration firstly reduced and then increased haze. Alcohol concentration had little influence at ph 3. Mesquita et al. (2001) added extra alcohol to white wine samples (0.5, 1 and 2% v/v) and found alcohol had no influence, confirming the results found in beer. Polysaccharides Wine polysaccharides originate from the grape or from yeast, and their effects on protein haze are numerous. Pellerin et al. (1994) comprehensively tested 15 different polysaccharides and concluded that they either did not affect or increased haze during heat testing. Similarly, Mesquita et al. (2001) showed that polysaccharides increased protein instability, particularly at moderate to high temperatures. However, the level of polysaccharide in this study (over 17 g/l) was much greater than that reported in wines (Doco et al. 2003). A multifactorial study (Fenchak et al. 2002) showed a particular polysaccharide (pectin) to be important in haze formation. However, since pectolytic enzymes are commonly used in white winemaking, one could argue that pectin does not represent a good model polysaccharide. Conversely, Waters and colleagues (Waters et al. 1993, 1994a,b) describe the effects of the yeast-derived mannoprotein haze-protective factor that protects wines from protein haze. Haze-protective factor is discussed above, and this polysaccharide is seen as an exciting prospect for preventing protein haze formation in white wine. Despite this interest, research into the involvement of grapederived polysaccharides in protein haze formation appears to be lacking and presents some opportunities. Metal ions Metal ions, particularly copper and iron, are more often associated with hazes of non-protein origin and their role in protein haze formation is very poorly understood. McKinnon (1996) used inductively coupled plasma analysis to measure metal ion levels in a wine and in a haze and came up with some new findings that have not been investigated further. He suggested that divalent ions might have a role to play, since a reduction in these ions by ion exchange chromatography led to an increase in haze. This result was complicated by the ion exchange process releasing sodium ions into the wine, making conclusions difficult to draw. Besse et al. (2000) measured free and total copper ion concentration in wines before and after heat treatment and haze removal. Copper concentrations were found to decrease after haze removal, suggesting copper was part of the protein precipitate. The role of copper is certainly not clear from this study and it might be that it is bound to the protein and exerts no influence. The role of metal ions in protein stability is very poorly understood and requires further study in order to confirm any quantitative importance. Phenolic compounds Of the non-proteinaceous factors potentially involved with white wine protein haze formation, phenolic compounds have had the most research dedicated to them and substantial evidence exists to suggest that their interactions with proteins are significant. Several researchers (Oh et al. 1980, Siebert et al. 1996b) have suggested a hydrophobic mechanism for the interaction, with a conceptual model proposed in which the protein has a fixed number of phenolic binding sites. More of these sites are exposed when the protein is denatured, such as during heating. Koch and Sajak (1959) were among the first investigators to determine that isolated grape protein was associated with tannins. Several authors (Moretti and Berg 1965, Bayly and Berg 1967) suggested that total protein content failed to correlate with heat stability, and Dawes et al. (1994) suggested that wine phenolics might need to be considered to explain this. Somers and Ziemelis (1973) found that up to 50% of wine protein is bound to flavonoid material. They used this information to explain the variations in protein stability found by Bayly and Berg (1967) and concluded that protein haze is due to the fractions of residual wine proteins that have been rendered