© The Authors, published by EDP Sciences, 2019

1. Introduction

If consumers want to enjoy a wine, it should be free of tartaric crystals (MARBE-SANS, 2016). First and foremost, a clear product is expected for the wine product. This could simply make it difficult to market the product of wine in the case of visible precipitations in the bottle. In some countries, such as Russia, wine is not marketable with tartaric acid precipitate. In the US, Weinstein precipitate is considered a sign of bad cellar practice (EDER, 2003). Tartaric stabilization is therefore an essential part of winemaking. Basically tartaric stabilisation can be distinguished in subtractive and additive processes. In the subtractive processes, the tartaric acid precipitation is induced in the cellar. This is done either by cold and addition of contact potassiumhydrogentartrate or the precipitate forming components (tartaric acid and/or potassium) are removed by physical processes such as electrodialysis.

In additive processes, stabilization takes place by the addition of crystallization inhibitors, the so-called inhibitors. This can be done by the addition of metatartaric acid, carboxymethylcellulose (CMC), mannoproteins or gum arabic. With the approval of potassium polyaspartate, there is another option for tartaric stabilization. Now the question arises as to how this product compares with the already established agents metatartaric acid and CMC.

Potassium polyaspartate, with the molecular formula [C₄H₅NO₃K]_n is an odorless, light brown powder with usually 90% dry matter. It is completely soluble in water, with the potassium polyaspartate content of the anhydrous substance being at least 98%. For the preparation of potassium polyaspartate, the monomeric L-aspartic acid is used, which is formed by fermentation. Upon heating, monomeric L-aspartic acid is converted to insoluble polysuccinimide. Subsequently, the polysuccinimide is treated under controlled conditions with potassium hydroxide to obtain KPA.

Potassium polyaspartate acts as a protective colloid and thus inhibits tartaric acid precipitation (O.I.V., 2017). The pure potassium polyaspartate is powdery and is dissolved with deionized water and conserved using SO₂. The SO₂ concentration in the finished product is 0.3% (TRIULZI, 2017, BOIANCHINI, 2017).

Regulation (EU) 2017/1961 authorized the treatment of wine with potassium polyaspartate. The maximum quantity may not exceed 10 g/hl. At higher doses, the stabilising effect would not be improved, and in some cases even clouding might occur (O.I.V., 2016). There is no information on the stabilization of calcium tartrate. So far no negative effects on the filtration performance are known. Potassium polyaspartate is offered as a liquid product and does not need to be dissolved like other preparations. However, the wines must be protein-stable (SCHEDLER, 2018).

2. Material and methods

3. Results and discussion

Optical control of tartaric precipitate in the bottles

The results of the optical controls of a tartaric precipitate were comparable for the two varietes. In both varieties after 70 and 120 days at 20 ^∘C storage temperature only in the blank samples with potassium and tartaric acid a Weinsteinausfall was detectable. Also recognizable tartrate precipitate was found at 2 ^∘C storage temperature in the blank sample (with and without the addition of potassium and tartar) and in the wine with addition and CMC as stabilizer. However in the enriched samples with added tartaric acid and potassium especially in the variant stored at 2 ^∘C visible, crystalline precipitations were found in the non treated wines but also in the wines stabilized with CMC and metatartaric acid in both varieties.

Gravimetrical analysis of precipitated potassium hydrogen tartrate (“Weinstein”)

In the case of Chardonnay's unaltered wines, at both storage temperatures (2^∘ and 20 ^∘C) at none of the three measuring points, a tartaric precipitate could be weighed. More meaningful were the results in the wines with the addition of tartaric acid and potassium. At the storage temperature of 20 ^∘C, a high amount of precipitate was measured in the unstabilized sample at all times, whereas all added stabilizers provided stable wines at all times. For the enriched wines with the addition of tartaric acid and potassium at a storage temperature of 2 ^∘C, the untreated sample already exhibited a tartaric precipitate at the first measurement point, which increased markedly at the second and third measurement times (2.2 and 2.5 g/l, respectively). Even the wine which was stabilized with CMC had a slight precipitate after 70 days, which also increased with storage time (0.6 or 1.2 g/l).

The metatartaric acid-stabilized variant was stable for the first two dates, but after 175 days a small cristallisation (0.050 g/l) was observed. Only the variants stabilized with KPA showed no tartaric precipitation over the entire observation period, whereby it should be emphasized that the variant with half the recommended amount also provided sufficient stabilization.

With the unchanged wines of the variety Rheinriesling at a storage temperature of 20 ^∘C in no variant a precipitation of potassiumhydrogentartrate was measurable. In the variants stored at 2 ^∘C, however, in the comparative variants without stabilizer already after 70 days precipitated tartrate was weighed. In the variants stabilized with metatartaric acid, CMC and KPA, no tartrate precipitation was observed. For the enriched wines with the addition of tartaric acid and potassium at a storage temperature of 20 ^∘C, a significant tartaric precipitation of 0.5–1.5 g/l was observed for the non-stabilized control samples at all three times. Very meaningful results were obtained at 2 ^∘C storage. In the comparison sample, a clear tartaric precipitation could be measured at all three measurement times (1.5–2.5 g/l). Even with CMC-stabilized wines, a tartaric precipitate was already evident after 70 days, which increased to about 1.2 g/l after 175 days of storage. Even with the variants stabilized with metatartaric acid a tartic precipitation was observed after 120 and 175 days (0.050 g/l and 1.500 g/l, respectively). While after 120 days storage, the amount of crystals in the metatartaric stabilized variant was significantly lower than in that one treated with CMC, after 175 days of storage hardly a difference between the two wine treatment agents was recognizable. It was noteworthy that in both KPA-stabilized variants no tartaric precipitation could be detected. It can therefore be stated that the new stabilising agent KPA had the best effect of all inhibitors and had a sufficient effect even at half the recommended application rate.

It can also be stated that the results of the optical controls of a tartaric precipitate were in good agreement with the results of the gravimetric determination of the potassiumhydrogentartrate precipitate. This means that all quantitative determined precipitates were not only of analytical interest but also for consumers visible.

Figure 1. Amount of potassiumhydrogentartrate precipitate in wines of Chardonnay enriched with tartaric acid and potassium after 175 days storage at 2 ∘C.

Figure 2. Amount of potassiumhydrogentartrate precipitate in wines of Rheinriesling enriched with tartaric acid and potassium after 175 days storage at 2 ∘C.

Determination of tartaric stability using Checkstab

The analysis of the tartaric stability by means of Checkstab was carried out one week after addition of the stabilising agents in five repetitions per variant. In the case of the Chardonnay, a significant drop in conductivity of around 200 μS was measured in the base wines without addition only in the non-stabilized comparison variant, which indicates instability. This results are in good accordance with the gravimetrically determined tartaric precipitate at a temperature of 2 ^∘C. As expected, the values of the conductivity decrease were higher for the wines with the addition of potassium and tartaric acid, in particular the non-stabilized variant had a very high value of around 320 μS. The variants stabilized with CMC and KPA (50 ml/hl) also showed values of about 90 μS, indicating instability (>60 μS). However, this predicted tartaric precipitation did not happen in practical storage experiment in the variant KPA (50 ml/hl) even at a storage temperature of 2 ^∘C. On the other hand, in the variant stabilized with meta-tartaric acid, which according to the prognosis with Checkstab is relatively stable, it subsequently resulted in a tartaric precipitate at a storage temperature of 2 ^∘C after 175 days of storage.

The results of the Weinstein instability forecast using Checkstab for the variety Rheinriesling were comparable to that of Chardonnay. The analysis showed in the base wine variants without addition of potassium and tartaric acid again only in the non-stabilized variant a critical value of about 200 μS. This was confirmed by the results of storage at 2 ^∘C after 175 days. Similar to the Chardonnay, the conductivity values for the variants CMC and KPA (50 ml/hl) were slightly above the critical value of 60 μS in the variants with the addition of potassium and tartaric acid. However, these forecast values did not agree with the results of the practical storage experiments at 2 ^∘C, since in these cases the wines with metatartaric acid and CMC showed pronounced tartaric precipitation, whereas both variants stabilized with KPA had no crystal turbidity. Thus, it can be stated that the prognosis of the tartaric(in)stability with Checkstab, especially for wines with the addition of a stabilising agent, gives only limited reliable results.

Figure 3. Tartaric instability measured by Minikontaktverfahren with Checkstab for Chardonnay variants.

Figure 4. Tartaric instability measured by Minikontakt-verfahren with Checkstab for Rheinriesling variants.

Residual contents of tartaric acid and potassium after 175 days of storage

In addition to the measurements of potassiumhydrogentartrate precipitatye, the residual levels of tartaric acid and potassium were determined after 175 days of storage after filtering off the tartaric

precipitate in the wines in order to have an additional control of results. For example, in the Chardonnay without the addition of tartaric acid and potassium at a storage temperature of 2 ^∘C in the stabilized variants without tartaric precipitation a tartaric acid content of about 2.2 g/l were detected, while this in the unstabilized reference sample due to the Weinstein precipitation the tartaric content decreased to 1.7 g/l. This is a significant difference (Kruskal Wallis test: p = 0.036). With regard to the potassium content, the values in the non-stabilized variant were also significantly lower (around 500 mg/l) than in the variants with stabilizer addition (around 650 mg/l). A pairwise comparison with Kruskal Wallis test revealed statistically significant differences (p = 0.023) between the potassium levels.

The results were similar for the variety Rheinriesling. Thus, for example, the wines stabilized with KPA still had 3.4 g/l tartaric acid after 175 days at a storage temperature of 2 ^∘C, while the non-stabilized variant only contained about 2.9 g/l due to the tartaric precipitate. Similarly clear results were obtained for potassium, which was in the KPA-stabilized variants without tartaric precipitate around 520 mg/l , while it decreased to around 350 mg/l in the nonstabilised variant with tartaric acid precipitaion. Also the differences of potassium content statistically significant (Kruskal Wallis test: p = 0.098).

References

All Tables

All Figures

	Figure 1. Amount of potassiumhydrogentartrate precipitate in wines of Chardonnay enriched with tartaric acid and potassium after 175 days storage at 2 ∘C.
In the text

	Figure 2. Amount of potassiumhydrogentartrate precipitate in wines of Rheinriesling enriched with tartaric acid and potassium after 175 days storage at 2 ∘C.
In the text

	Figure 3. Tartaric instability measured by Minikontaktverfahren with Checkstab for Chardonnay variants.
In the text

	Figure 4. Tartaric instability measured by Minikontakt-verfahren with Checkstab for Rheinriesling variants.
In the text