1. Introduction
Calcium is present in wine at levels ranging from 30 to 200 mg/L [
1,
2]. Its concentration depends not only on the amount naturally present in grapes and other parts of the bunch (endogenous origin) but also on technological operations (exogenous origin). These operations include fining with calcium bentonite, which can increase calcium content. Sodium bentonite can also increase calcium content, but to a lesser extent [
3]. Other factors contributing to calcium levels in wines include deacidification with calcium carbonate [
4] and storage in poorly coated cement tanks [
5]. Wines with calcium levels above 70–80 mg/L are considered at risk for calcium tartrate instability [
4]. This instability results in the formation of crystalline calcium L-tartrate, which appears as colorless or white, bipyramidal or rhomboid crystal deposits. In some cases, co-deposits such as phenolic and protein material, quercetin crystals, or yeast cells may also be present [
6].
Calcium-induced instabilities are among the main problems encountered in bottled wines, and the precipitation of calcium tartrate (CaT) is becoming increasingly common due to the increase in calcium levels in grape must caused by climate change. Although CaT is an insoluble salt, CaT-induced instability, while less frequent than that caused by KHT, is more difficult to control and predict, as it is a slow phenomenon [
7]. The time required for the spontaneous nucleation of CaT is much longer than for that of KHT [
7], resulting in slower precipitation [
8]. Usually, CaT precipitation occurs after aging for several years and almost always after bottling [
8,
9,
10,
11]. In addition to higher calcium levels, an increase in pH results in an increase in tartaric acid in the form of tartrate ions (T
2−). According to Abguéguen and Boulton [
7], polyphenols, proteins, and polysaccharides can act as inhibitors of CaT precipitation by interfering with the energy barrier for crystal growth. Tannins, proteins, and high-molecular-weight compounds can delay or even inhibit nucleation by binding to calcium and/or tartrate ions, thereby reducing their concentration in solution and decreasing the degree of supersaturation or blocking the formation of nuclei. The use of sterilizing membrane filtration systems immediately before bottling the wines and the subsequent removal of natural protective colloids can explain the delayed appearance of some calcium tartrate (CaT) in bottled wines, despite being considered stabilized during bottling [
7,
12]. McKinnon et al. [
6] showed that the initial step in the precipitation of CaT involves the formation of a soluble species of CaT, which can aggregate and lead to nucleation. The crystallization of CaT is a phenomenon similar to that observed for KHT. However, preventing the appearance of CaT crystals in bottled wines is more challenging since CaT’s solubility is not significantly affected by low temperatures, rendering cold stabilization technologies ineffective for preventing CaT precipitation [
5]. Therefore, nucleation is the limiting step in CaT crystallization in wine. Unlike KHT, the primary nucleation of CaT is not induced by lowering the temperature and increasing supersaturation [
7,
8]. This can be explained by the insufficient activation energy in the process to initiate crystal formation through cooling [
13]. To address this, the use of CaT seed crystals is of interest since they induce nucleation, making surface integration the limiting phase of the process [
7]. Crystal growth occurs as a second-order reaction and is influenced by factors such as alcohol content and ionic strength [
7,
14]. Several studies [
7,
8] have shown that increasing the ethanol concentration decreases the solubility of CaT. McKinnon et al. [
14] and Cole and Boulton [
15] found that an increase in ethanol content leads to a decrease in the induction time (the time associated with the beginning of the nucleation process) and an increase in the rate of CaT crystallization, which may be due to the decrease in the CaT solubility product caused by the higher ethanol concentration. Abguéguen and Boulton [
7] found that a decrease in alcohol content reduces the growth rate of CaT crystals. Several studies [
7,
8,
11,
14] have demonstrated the strong dependence of CaT precipitation on pH. Solutions with higher pH tend to precipitate more CaT, increasing the probability of CaT instability. This can occur, for example, after malolactic fermentation, which raises the wine’s pH.
Currently, the stability of calcium tartrate is often estimated based on the calcium concentration found in wine. Many publications indicate 80 mg/L for white and rosé wines and 60 mg/L for red wines as threshold values above which wine is considered unstable, but these values should be considered with some reservations [
5]. For example, red wine with a calcium content of 60 mg/L and a pH of less than 3.5 does not produce any precipitate, whereas at a pH of 3.7 or higher, it is very likely to form abundant sediments of crystals [
5]. At pH values greater than 3, the percentage of ions in the T
2− form increases until reaching a maximum value at a pH of about 6.5 [
16]. It is known that the stability of CaT cannot be predicted from a cold stability test and that wines subjected to CaT precipitation are almost impossible to stabilize, even if kept at low temperatures for long periods [
10]. Traditionally, attempts to assess the stability of wine with respect to CaT precipitation were based on the calculation of a wine concentration product and its comparison with the solubility product [
17,
18]. This approach is recognized as having little value as a predictor of CaT instability [
19]. One major limitation to the use of the concentration product approach relates to the method of determining the calcium concentration. Generally, the calcium concentration is measured by atomic absorption spectrophotometry, which overestimates the actual ionized calcium concentration [
20]. An alternative approach for assessing the potential instability of wine with respect to calcium tartrate precipitation is based on the so-called mini-contact process [
19,
21]. In this procedure, the wine is stirred in the presence of CaT seed crystals while the conductivity of the solution is monitored. A decrease in conductivity indicates the loss of CaT from the wine. The procedure is time-consuming and is highly dependent on the quality of the seed crystal surface. Although KHT and CaT share similar crystallization systems, the addition of potassium bitartrate crystals does not induce calcium tartrate crystallization. However, the crystallization of CaT can induce the crystallization of KHT [
22] or even facilitate the simultaneous stabilization of both KHT and CaT [
13]. Therefore, the more specific method developed by Abguéguen and Boulton [
7] was used. This method uses micronized calcium tartrate as a precipitation nucleus to induce the precipitation of calcium tartrate when its concentration in wines is above the saturation point. By determining the variation in wine calcium levels due to precipitation, a more specific measure of calcium tartrate instability can be obtained.
Traditionally, CaT instability in wine is addressed by removing calcium through electrodialysis [
23,
24] or treatment with cation-exchange resins [
24,
25], by adding D,L-tartaric acid to remove calcium by forming calcium D,L-tartrate [
24,
25], or by adding micronized calcium tartrate [
24]. However, there are some limitations to these technologies, such as long processing times and expensive equipment. Enological stabilizing additives, which offer the advantages of simple operation and low cost, have been used. Stabilizing additives in wine that are allowed by the EU and OIV and are efficient for calcium tartrate instability include metatartaric acid [
24] and sodium carboxymethylcellulose [
24]. Nevertheless, metatartaric acid is a short-term solution because, over time, it can slowly hydrolyze into tartaric acid, which can increase the risk of precipitation [
12,
26,
27]. These additives are also used for potassium hydrogen tartrate instability [
24], but potassium polyaspartate, another widely used additive for potassium hydrogen tartrate instability, has been described as increasing calcium tartrate instability in unstable wines [
28]. Therefore, evaluating alternative natural calcium tartrate stabilizers is important for expanding the range of available options for calcium tartrate stabilization in wines. Considering the chemical characteristics of alginic acid, such as its strong anionic character and affinity for calcium ions, this polysaccharide was studied for the first time to evaluate its potential to stabilize calcium tartrate. This polysaccharide is already authorized for use in the clarification of wines with no application limit [
24] and is considered a processing aid by the OIV, making it a clean-label option for calcium tartrate stabilization.