Effects of ionic strength and sulfate upon thermal aggregation of grape chitinases and thaumatin-like proteins in a model system

Influence of flash heating and aspergillopepsin I supplementation on must and wine attributes of aromatic varieties

The protein instability with haze formation represents one of the main faults occurring in white and rosé wines. Among the various solutions industrially proposed, aspergillopepsin I (AP-I) supplementation coupled with must heating (60-75 °C) has been recently approved by OIV and the European Commission for ensuring protein stability of wines. This study investigates the impact of AP-I either applied independently or in combination with flash pasteurization on the chemical composition of grape must and wines derived from Sauvignon Blanc and Gewürztraminer. The efficacy on protein stability of a complete treatment combining heat (70 °C) and AP-I (HP) was confirmed through heat test and bentonite requirement, although no differences were observed between must heating and HP treatments. However, high-performance liquid chromatography analysis of unstable pathogenesis-related proteins revealed that AP-I supplementation reduced chitinases and thaumatin-like proteins compared to the non-enzymed samples, with and without must heating. Amino acid increase was reported only in HP musts, particularly in Sauvignon Blanc. The concentration of yeast-derived aroma compounds in Gewürztraminer wines was increased by must heating; compared to controls, flash pasteurization rose the overall acetate esters content of 85 % and HP of 43 %, mostly due to isoamyl acetate. However, heat treatments -with or without AP-I- reduced terpenes up to 68 %. Despite the different aroma profiles, no differences were observed for any descriptor for both varieties in wine tasting, and only a slight decrease trend was observed for the floral intensity and the typicality descriptors in heated wines.

Towards the rapid detection of haze-forming proteins

Protein haze in white wine can be a serious quality defect because consumers perceive hazy wines as "spoiled". Unfortunately, a specific method for the detection, or selective treatment, of such proteins in affected wines does not exist. Herein we investigate on the development of an easy-to-use sensor device that allows detection of haze-forming proteins (HFPs). Such a device is expected to overcome the limitations of the "heat test" currently used to assess the protein content in wine and the amount of bentonite needed to remove such proteins. To this aim, three different approaches were explored. Firstly, an impedimetric immunosensor against chitinases was developed and its performance assessed. Secondly, the exploitation of the dual role of HFPs as biorecognition element and analyte to develop an impedimetric biosensor was evaluated, in what can be considered a very unique strategy, representing a new paradigm in biosensing. Lastly, Fourier transform infrared (FT-IR) spectra were collected for various wine samples and chemometric tools such as discrete wavelet transform (DWT) and artificial neural networks (ANNs) were used to achieve the quantification of HFPs. Detection of HFPs at the μg/L level was achieved with both impedimetric biosensors, whereas the FT-IR-based approach allowed their quantification at the mg/L level in wine samples directly. The sensitivity of the developed methods may enable the rapid assessment of wine protein content.

Peptidomics as a Tool to Assess the Cleavage of Wine Haze Proteins by Peptidases from Drosophila suzukii Larvae

Thermolabile grape berry proteins such as thaumatin-like proteins (TLPs) and chitinases (CHIs) promote haze formation in bottled wines if not properly fined. As a natural grapevine pest, the spotted-wing fly Drosophila suzukii is a promising source of peptidases that break down grape berry proteins because the larvae develop and feed inside mature berries. Therefore, we produced recombinant TLP and CHI as model thermolabile wine haze proteins and applied a peptidomics strategy to investigate whether D. suzukii larval peptidases were able to digest them under acidic conditions (pH 3.5), which are typically found in winemaking practices. The activity of the novel peptidases was confirmed by mass spectrometry, and cleavage sites within the wine haze proteins were visualized in 3D protein models. The combination of recombinant haze proteins and peptidomics provides a valuable screening tool to identify optimal peptidases suitable for clarification processes in the winemaking industry.

Recombinant Thaumatin-Like Protein (rTLP) and Chitinase (rCHI) from Vitis vinifera as Models for Wine Haze Formation

Cross-linking net aggregates of thermolabile thaumatin-like proteins (TLPs) and chitinases (CHIs) are the primary source of haze in white wines. Although bentonite fining is still routinely used in winemaking, alternative methods to selectively remove haze proteins without affecting wine organoleptic properties are needed. The availability of pure TLPs and CHIs would facilitate the research for the identification of such technological advances. Therefore, we proposed the usage of recombinant TLP (rTLP) and CHI (rCHI), expressed by Komagataella phaffii, as haze-protein models, since they showed similar characteristics (aggregation potential, melting point, functionality, glycosylation levels and bentonite adsorption) to the native-haze proteins from Vitis vinifera. Hence, rTLP and rCHI can be applied to study haze formation mechanisms on a molecular level and to explore alternative fining methods by screening proteolytic enzymes and ideal adsorptive resins.

Quantification of protein by acid hydrolysis reveals higher than expected concentrations in red wines: Implications for wine tannin concentration and colloidal stability

Protein is reportedly negligible in most red wines, due to its loss following co-precipitation with phenolic substances. A method for protein quantification in red wine was developed which overcame analytical interference from phenolic substances, based on ethanol precipitation, followed by acid-hydrolysis and amino acid quantification. Protein concentration was surveyed in a range of red wines produced from V. vinifera and interspecific (Vitis spp) hybrids, revealing higher than expected concentrations, ranging from 23 mg/L ± 2.57 to 380 mg/L ± 16. The results showed that tannin extracted from grapes remains soluble in wine in the presence of protein even at high protein (>100 mg/L) and tannin (>500 mg/L) concentrations. As a further consequence of this, the particle size and concentration of colloids within high- and low-protein wines were similar, independent of protein or tannin concentration. Higher wine tannin concentration was also correlated with increased heat stability of wine protein.

Starmerella bacillaris Strains Used in Sequential Alcoholic Fermentation with Saccharomyces cerevisiae Improves Protein Stability in White Wines

Haze can appear in white wines as a result of the denaturation and subsequent aggregation of grape pathogenesis-related (PR) proteins. Yeast cell-wall polysaccharides, particularly mannoproteins, represent a promising strategy to reduce the incidence of this phenomenon. The aim of this study was to evaluate the effects of 13 Starmerella bacillaris strains, in sequential fermentation with Saccharomyces cerevisiae, on wine protein stability of three white wines (Sauvignon blanc, Pinot grigio, and Manzoni bianco). The resulting wines were characterized in terms of their chemical composition, content of PR proteins and polysaccharides, and heat stability. In addition, the mannoprotein fraction was purified from six wines, five produced with S. bacillaris and one with S. cerevisiae EC1118 used as control. Generally, wines produced with S. bacillaris strains were more heat-stable, despite generally containing higher amounts of PR proteins. The increased heat stability of Starmerella wines was attributed to the stabilizing effect resulting from their higher concentrations of both total polysaccharides and mannoprotein fractions. In particular, for the most heat unstable wine (Manzoni bianco), the low MW mannoprotein fraction resulted to be the most involved in wine stability. The ability to produce wines with different heat stability was demonstrated to be strain-dependent and was more evident in the most unstable wines. By reducing fining waste, the use of S. bacillaris as an enological starter can be proposed as a new tool to manage wine protein stability for a more sustainable winemaking.

Haze Formation and the Challenges for Peptidases in Wine Protein Fining

To meet consumer expectations, white wines must be clear and stable against haze formation. Temperature variations during transport and storage may induce protein aggregation, mainly caused by thaumatin like-proteins (TLPs) and chitinases (CHIs), which thus need to be fined before bottling of the wine. Currently, bentonite clay is employed to inhibit or minimize haze formation in wines. Alternatively, peptidases have emerged as an option for the removal of these thermolabile proteins, although their efficacy under winemaking conditions has not yet been fully demonstrated. The simultaneous understanding of the chemistry behind the cleavage of haze proteins and the haze formation may orchestrate alternative methods of technological and economic importance in winemaking. Therefore, we provide an overview of wine fining by peptidases, and new perspectives are developed to reopen discussions on the aforementioned challenges.

Potential Contribution of Climate Change to the Protein Haze of White Wines from the French Southwest Region

The aim of this study was to evaluate the role played by climatic conditions during grape ripening in the protein instability of white wines produced in the French southwest region. For this purpose, basic wine analyses were carried out on 268 musts and the corresponding wines, all produced during the 2016, 2017, 2018, and 2019 vintages, with distinctive climatic conditions. Qualitative and quantitative variables were correlated with levels of protein haze determined by heat test (80 °C/2 h) in the wines using analysis of covariance (ANCOVA), principal component analysis (PCA), and classification and regression trees (CART). Our results show that the climatic change, with the increase in temperatures, and the decrease in precipitation during the grape ripening phase, tends to enhance the risk of protein instability in wines. Indeed, the values of pH, titratable acidity, and malic acid concentrations of the musts, which are good indicators of the conditions in which the grapes ripened and of the level of ripeness of the grapes, were also the variables that correlated best with the protein haze. By measuring these parameters at harvest before alcoholic fermentation, it may be possible to predict the risk of protein haze, and thus early and precisely adapt the stabilization treatment to be applied.

Effect of Pre-Fermentative Maceration and Fining Agents on Protein Stability, Macromolecular, and Phenolic Composition of Albariño White Wines: Comparative Efficiency of Chitosan, k-Carrageenan and Bentonite as Heat Stabilisers

In this work, the effect of pre-fermentative skin maceration (PFSM) on the chemical composition of the macromolecular fraction, polysaccharides and proteins, phenolic compounds, chromatic characteristics, and protein stability of Albariño monovarietal white wines was studied. PFSM increased the extraction of phenolic compounds and polysaccharides and reduced the extraction of pathogenesis-related proteins (PRPs). PFSM wine showed significantly higher protein instability. Sodium and calcium bentonites were used for protein stabilisation of wines obtained with PFSM (+PFSM) and without PFSM (-PFSM), and their efficiencies compared to fungal chitosan (FCH) and k-carrageenan. k-Carrageenan reduced the content of PRPs and the protein instability in both wines, and it was more efficient than sodium and calcium bentonites. FCH was unable to heat stabilise both wines, and PRPs levels remained unaltered. On the other hand, FCH decreased the levels of wine polysaccharides by 60%. Sodium and calcium bentonite also decreased the levels of wine polysaccharides although to a lower extent (16% to 59%). k-Carrageenan did not affect the wine polysaccharide levels. Overall, k-carrageenan is suitable for white wine protein stabilisation, having a more desirable impact on the wine macromolecular fraction than the other fining agents, reducing the levels of the wine PRPs without impacting polysaccharide composition.

Wine Thermosensitive Proteins Adsorb First and Better on Bentonite during Fining: Practical Implications and Proposition of Alternative Heat Tests

Bentonite fining is the most popular treatment used to remove proteins in white and rosé wines. The usual heat test used to adjust the bentonite dose consists of heating the wine during 30 min at 80 °C. At this temperature, all of the proteins are unfolded, and this can lead to an overestimation of the dose. We have shown that proteins adsorb on bentonite in a specific order and, more importantly, that the proteins responsible for haze formation adsorb first. Fluorescence spectroscopy showed that this is due to the structural properties of proteins, which can be classified as hard and soft proteins. Alternative heat tests were performed at a lower temperature (40 °C) and showed a better correlation with accelerated aging. These tests were also less dependent upon the wine pH.

The Secondary Structure of a Major Wine Protein is Modified upon Interaction with Polyphenols

Polyphenols are an important constituent of wines and they are largely studied due to their antioxidant properties and for their effects on wine quality and stability, which is also related to their capacity to bind to proteins. The effects of some selected polyphenols, including procyanidins B1 and B2, tannic acid, quercetin, and rutin, as well as those of a total white wine procyanidin extract on the conformational properties of the major wine protein VVTL1 (Vitis vinifera Thaumatin-Like-1) were investigated by Synchrotron Radiation Circular Dichroism (SRCD). Results showed that VVTL1 interacts with polyphenols as demonstrated by the changes in the secondary (far-UV) and tertiary (near-UV) structures, which were differently affected by different polyphenols. Additionally, polyphenols modified the two melting temperatures (TM) that were found for VVTL1 (32.2 °C and 53.9 °C for the protein alone). The circular dichroism (CD) spectra in the near-UV region revealed an involvement of the aromatic side-chains of the protein in the interaction with phenolics. The data demonstrate the existence of an interaction between polyphenols and VVTL1, which results in modification of its thermal and UV denaturation pattern. This information can be useful in understanding the behavior of wine proteins in presence of polyphenols, thus giving new insights on the phenomena that are involved in wine stability.

White Wine Protein Instability: Mechanism, Quality Control and Technological Alternatives for Wine Stabilisation—An Overview

Wine protein instability depends on several factors, but wine grape proteins are the main haze factors, being mainly caused by pathogenesis-related proteins (thaumatin-like proteins and chitinases) with a molecular weight between 10~40 kDa and an isoelectric point below six. Wine protein stability tests are needed for the routine control of this wine instability, and to select the best technological approach to remove the unstable proteins. The heat test is the most used, with good correlation with the natural proteins’ precipitations and because high temperatures are the main protein instability factor after wine bottling. Many products and technological solutions have been studied in recent years; however, sodium bentonite is still the most efficient and used treatment to remove unstable proteins from white wines. This overview resumes and discusses the different aspects involved in wine protein instability, from the wine protein instability mechanisms, the protein stability tests used, and technological alternatives available to stabilise wines with protein instability problems.

Influence of the reducing environment in the misfolding of wine proteins

While proteins are present in wine at low concentration, and are largely associated with undesirable haze formation in white wines, certain types or fractions make direct and indirect contributions to sensory quality and physical stability. The proteins found in wine represent a small subclass of the total pool of grape proteins that remain soluble in the non-physiological conditions of the wine matrix which is characterised by the presence of alcohol, high acidity, and relatively high levels of phenolic compounds. Although initially stable in these conditions, during storage of white and rosé wines proteins undergo changes leading to haze formation which is considered one of the most relevant non-microbiological defects, and which makes the wine commercially unacceptable. This phenomenon involves the two most abundant proteins present in wines: thaumatin-like proteins and chitinases, both belonging to pathogenesis-related proteins of the grape berry. Haze formation is often triggered by thermal fluctuations occurring during storage of white wines, although the presence of other non-protein-related factors seems to be necessary. Here, we review the characteristics of these two protein families and the factors that influence their solubility with a focus on the disulfide bonds reduction as a possible trigger for the onset of their aggregation.

Characterization of macromolecular complexes in red wine: Composition, molecular mass distribution and particle size

Precipitates were prepared from two compositionally different Pinot noir wines with addition of excess ethanol, and contained primarily polysaccharide, tannin and protein. The ethanol-soluble material was further fractionated into polymeric (tannin) and monomeric phenolics. Tannin associated with precipitates was of a higher molecular mass than that remaining in ethanolic solution. Wine fractions were reconstituted at the ratios of the original wine and analyzed using nanoparticle tracking analysis. The average particle size of the tannin fraction was 75-89 nm, and increased when combined with the precipitate (≅ 200 nm). Addition of the monomeric fraction to the tannin-precipitate complex increased both the incidence and concentration of smaller particles, reducing the average particle size. The formation of aggregates occurred in all fractions and only minor differences in particle size distribution were found between wines. Differences in particle concentration between wines appear to be due to differences in the total concentration of macromolecules rather than compositional differences.

Protein/polysaccharide interactions and their impact on haze formation in white wines

Proteins in white wines may aggregate and form hazes at room temperature. This was previously shown to be related to pH-induced conformational changes and to occur for pH <3.5. The aim of the present work was to study the impact of wine polysaccharides on pH-induced haze formation by proteins but also the consequences of their interactions with these proteins on the colloidal stability of white wines. To this end, model systems and purified global pools of wine proteins and polysaccharides were used first. Kinetics of aggregation, proteins involved, and turbidities related to final hazes were monitored. To further identify the impact of each polysaccharide, fractions purified to homogeneity were used in a second phase. These included two neutral (mannoprotein and arabinogalactan) and two negatively charged (rhamnogalacturonan II dimer (RG-II) and arabinogalactan) polysaccharides. The impact of major wine polysaccharides on wine protein aggregation at room temperature was clearly less marked than those of the pH and the ionic strength. Polysaccharides modulated the aggregation kinetics and final haziness, indicating that they interfere with the aggregation process, but could not prevent it.

Wine protein haze: mechanisms of formation and advances in prevention

Protein haze is an aesthetic problem in white wines that can be prevented by removing the grape proteins that have survived the winemaking process. The haze-forming proteins are grape pathogenesis-related proteins that are highly stable during winemaking, but some of them precipitate over time and with elevated temperatures. Protein removal is currently achieved by bentonite addition, an inefficient process that can lead to higher costs and quality losses in winemaking. The development of more efficient processes for protein removal and haze prevention requires understanding the mechanisms such as the main drivers of protein instability and the impacts of various wine matrix components on haze formation. This review covers recent developments in wine protein instability and removal and proposes a revised mechanism of protein haze formation.

Structure of haze forming proteins in white wines: Vitis vinifera thaumatin-like proteins

Grape thaumatin-like proteins (TLPs) play roles in plant-pathogen interactions and can cause protein haze in white wine unless removed prior to bottling. Different isoforms of TLPs have different hazing potential and aggregation behavior. Here we present the elucidation of the molecular structures of three grape TLPs that display different hazing potential. The three TLPs have very similar structures despite belonging to two different classes (F2/4JRU is a thaumatin-like protein while I/4L5H and H2/4MBT are VVTL1), and having different unfolding temperatures (56 vs. 62°C), with protein F2/4JRU being heat unstable and forming haze, while I/4L5H does not. These differences in properties are attributable to the conformation of a single loop and the amino acid composition of its flanking regions.

Microbial aspartic proteases: current and potential applications in industry

Aspartic proteases are a relatively small group of proteolytic enzymes that are active in acidic environments and are found across all forms of life. Certain microorganisms secrete such proteases as virulence agents and/or in order to break down proteins thereby liberating assimilable sources of nitrogen. Some of the earlier applications of these proteolytic enzymes are found in the manufacturing of cheese where they are used as milk-clotting agents. Over the last decade, they have received tremendous research interest because of their involvement in human diseases. Furthermore, there has also been a growing interest on these enzymes for their applications in several other industries. Recent research suggests in particular that they could be used in the wine industry to prevent the formation of protein haze while preserving the wines' organoleptic properties. In this mini-review, the properties and mechanisms of action of aspartic proteases are summarized. Thereafter, a brief overview of the industrial applications of this specific class of proteases is provided. The use of aspartic proteases as alternatives to clarifying agents in various beverage industries is mentioned, and the potential applications in the wine industry are thoroughly discussed.

White wine proteins: how does the pH affect their conformation at room temperature?

Our studies focused on the determination of aggregation mechanisms of proteins occurring in wine at room temperature. Even if the wine pH range is narrow (2.8 to 3.7), some proteins are affected by this parameter. At low pH, the formation of aggregates and the development of a haze due to proteins sometimes occur. The objective of this work was to determine if the pH impacted the conformational stability of wine proteins. Different techniques were used: circular dichroism and fluorescence spectroscopy to investigate the modification of their secondary and tertiary structure and also SAXS to determine their global shape. Four pure proteins were used, two considered to be stable (invertase and thaumatin-like proteins) and two considered to be unstable (two chitinase isoforms). Two pH values were tested to emphasize their behavior (pH 2.5 and 4.0). The present work highlighted the fact that the conformational stability of some wine proteins (chitinases) was impacted by partial modifications, related to the exposure of some hydrophobic sites. These modifications were enough to destabilize the native state of the protein. These modifications were not observed on wine proteins determined to be stable (invertase and thaumatin-like proteins).

Roles of proteins, polysaccharides, and phenolics in haze formation in white wine via reconstitution experiments

Residual proteins in finished wines can aggregate to form haze. To obtain insights into the mechanism of protein haze formation, a reconstitution approach was used to study the heat-induced aggregation behavior of purified wine proteins. A chitinase, four thaumatin-like protein (TLP) isoforms, phenolics, and polysaccharides were isolated from a Chardonnay wine. The same wine was stripped of these compounds and used as a base to reconstitute each of the proteins alone or in combination with the isolated phenolics and/or polysaccharides. After a heating and cooling cycle (70 °C for 1 h and 25 °C for 15 h), the size and concentration of the aggregates formed were measured by scanning ion occlusion sensing (SIOS), a technique to detect and quantify nanoparticles. The chitinase was the protein most prone to aggregate and the one that formed the largest particles; phenolics and polysaccharides did not have a significant impact on its aggregation behavior. TLP isoforms varied in susceptibility to haze formation and in interactions with polysaccharides and phenolics. The work establishes SIOS as a useful method for studying wine haze.

Stability of white wine proteins: combined effect of pH, ionic strength, and temperature on their aggregation

Protein haze development in white wines is an unacceptable visual defect attributed to slow protein unfolding and aggregation. It is favored by wine exposure to excessive temperatures but can also develop in properly stored wines. In this study, the combined impact of pH (2.5-4.0), ionic strength (0.02-0.15 M), and temperature (25, 40, and 70 °C) on wine protein stability was investigated. The results showed three classes of proteins with low conformational stability involved in aggregation at room temperature: β-glucanases, chitinases, and some thaumatin-like protein isoforms (22-24 kDa). Unexpectedly, at 25 °C, maximum instability was observed at the lower pH, far from the protein isoelectric point. Increasing temperatures led to a shift of the maximum haze at higher pH. These different behaviors could be explained by the opposite impact of pH on intramolecular (conformational stability) and intermolecular (colloidal stability) electrostatic interactions. The present results highlight that wine pH and ionic strength play a determinant part in aggregation mechanisms, aggregate characteristics, and final haze.

Roles of grape thaumatin-like protein and chitinase in white wine haze formation

Grape chitinase was found to be the primary cause of heat-induced haze formation in white wines. Chitinase was the dominant protein in a haze induced by treating Sauvignon blanc wine at 30 °C for 22 h. In artificial wines and real wines, chitinase concentration was directly correlated to the turbidity of heat-induced haze formation (50 °C for 3 h). Sulfate was confirmed to have a role in haze formation, likely by converting soluble aggregates into larger visible haze particles. Thaumatin-like protein was detected in the insoluble fraction by SDS-PAGE analysis but had no measurable impact on turbidity. Differential scanning calorimetry demonstrated that the complex mixture of molecules in wine plays a role in thermal instability of wine proteins and contributes additional complexity to the wine haze phenomenon.