This study analyzed the effects of Argon APCP batch and flow systems applied to a red wine and it was focused on its color and phenolic properties. For that purpose, the treatments were applied with two different powers, in batch and flowing, and during several minutes. Variability between starting wines was mainly due to the fact that treatments were performed in six days of two consecutive weeks. The initial wine suffered some type of evolution during those days, which made it that times zero or control samples had different physicochemical parameters like the pH. This made us consider each treatment as totally independent.
3.1. Impact of APCP Treatments on Physical and Color Parameters
Results of the average physical parameters measured in wines before and after the six Argon APCP treatments are shown in Table 2
. The temperature of the control and treated samples hardly varied during the application of most of the treatments. In batch systems, the temperature did not change significantly after 10 min of treatment, the same result was observed in low flow systems after 25 min. The temperature was only significantly increased with time in the treatments linked to high flow systems, although it varied only from 19.7 °C to 20.4 °C when applying 60 W and from 20.7 °C to 21.8 °C when applying 90 W. This result demonstrated the cold character of APCP treatments despite the applied energy [20
]. The pH parameter measures the hydrogen ion concentration of a solution. A decrease of water pH after APCP treatments was demonstrated [21
]. In contrast, inconclusive results about pH variation after APCP have been observed with other products [22
]. In our study, only after the batch treatment of low power (B60) applied to wine from 1 to 5 min did the pH significantly increase to reach around 4 units, which might trigger the microbial spoilage of wine. On another point, the conductivity means that the facility of a liquid media might flow an electric discharge. It has been observed that plasma activated water conductivity is higher than the conductivity found in non-treated water [21
]. In this research, wine conductivity was significantly higher than the control after 3 or 5 min of batch APCP treatments. In contrast, conductivity was reduced after flowing treatments, although this reduction was only significant after 20 min of treatment HF90. This reduction was also observed by Pankaj et al. [15
] after APCP treatment of 80 kV for 4 min in grape juice. In general, a drop in the conductivity of wines is related to a loss in their tartaric stability [23
The color intensity (CI) is an index of the amount of color of a wine. It significantly increased after batch treatments (Table 3
). This increase was of approximately two units for both 60 and 90 W, which is, overall, positive for red wine quality [16
]. However, the CI increase after flow systems was lower (between 0.2 and 0.6 units after 25 min of treatment) and had statistical significance only for LF60, HF90, and LF90 treatments (Table 3
). The lowest hue of a wine means a positive wine evolution, which was observed after 10 min of batch treatments. Flow APCP treatments also resulted in a tonality reduction, although it was only significant in the treatment of the greatest flow and power.
The average total phenolic (TP) compounds, determined by the reaction with the Folin reagent, was significantly higher after batch APCP treatments, varying from 1067 to 1170 with 60 W and from 1067 to 1194 with 90 W (Table 3
). In this way, Herceg et al. [24
] reported an increase in TP in pomegranate juice after Argon plasma treatment. This index is based on the capacity of the phenolics to react with oxidant agents so that it is a total determination of phenolic compounds, but it also expresses the contribution of these compounds to the antioxidant activity of the sample, so its increase is positive for wine quality. However, flowing APCP treatments did not result in a significant modification of TP. Even in the bibliography, contradictory results are found, for instance Lukić et al. [25
] observed a reduction of TP in a Cabernet Sauvignon red wine, which could be due to the possible degradation of these compounds by the plasma mechanism.
3.2. Impact of APCP Treatments on Anthocyanin and Vitisins Contents
The anthocyanins free monomers are the main responsible for the color of young red wines. They were analyzed individually for every sample. The HPLC analyses identified five non-acylated anthocyanins (Table 4
), including 3-o
-glucosides (3-glc) of delphinidin, cyanidin, petunidin, peonidin, and malvidin and 11 acylated anthocyanins (Table 5
), including acetyl glucosides (3-acglc) of delphinidin, petunidin, peonidin, and malvidin; trans-p-coumaroyl glucosides (3-cmglc) of delphinidin, cyanidin, petunidin, peonidin, and cis and trans malvidin and caffeoyl glucoside (cfglc) of malvidin.
Non-acylated anthocyanins represented around 70%, with malvidin-3-o
-glucoside being the majority. Between non-acylated anthocyanins, malvidin derivatives were also found to be the predominant anthocyanin type, while coumaroylated anthocyanins were the major acylated form, which is in accordance with previous studies with the Tempranillo grape variety [25
]. Cyanidin -3- acetylglucoside was not detected in any case. The most important variations for non-acylated anthocyanins were obtained for batch treatments (Table 4
). Thus, APCP static treatment, applied with a power of 60 W, led to a significant increase of delphinidin, petunidin, and malvidin-3-glucosides; the same treatment applied with a power of 90 W enhanced delphinidin and petunidin-3 glucosides. In both cases, the positive increase for the wine quality of these compounds occurred from the first minute of treatment and remained practically constant. This increase could be related to the observed increase in CI exposed previously. As Table 4
shows, there were hardly any variations for the flowing APCP treatments. Only peonidin and malvidin-3-glc decreased significantly after 25 min of the treatment with the lowest flow and power.
Similar to anthocyanins non-acylated, the most important variations for the acylated ones were obtained for batch treatments (Table 5
). Thus, for B60, a significant increase was observed for petunidin and malvidin-3-acglc, for five of the six coumaroylated glucosides, and for malvidin-3-cfglc. The treatment time from which the concentration of these compounds increased varied according to each of them (between 1 min and 10 min). For B90, a lower number of anthocyanins increased their concentration significantly, including petunidin and malvidin-3-acglc, and delphinidin and malvidin-3-trans-cmglc. In this case, as happened with non-acylated anthocyanins, the increase only occurred after one minute of treatment and remained practically constant. As can be observed in Table 5
, dynamic treatments did not produce variations in these compounds, with the exception of petudine-3-acglc, which decreased significantly after 25 min of low flow treatment and 60 watts of power. Elez et al. [26
] applied plasma to sour cherry Marasca and they found a higher concentration of anthocyanins compared to untreated juice for short treatments (3 min). However, results obtained by Lukić [16
] indicated a decrease in the composition of free anthocyanins in a Cabernet Sauvignon red wine. This decrease became more sever with the treatment duration and the frequency of the batch treatment. This could be explained by the degradation of these compounds by the plasma mechanisms.
Due to the difficulty to evaluate every individual compound in the samples, their total contents were studied (Figure 2
). It was demonstrated that treatment of B60 led to an increase of the total acylated, non-acylated, and the total anthocyanins. Applying the highest power of APCP in the batch did not cause this impact. This was because the total anthocyanins content only increased after 1 min of treatment with statistical significance, but this effect disappeared with longer treatments. Being anthocyanins, which are the compounds mainly responsible for red wine color, the batch treatment with the lowest power was the most favorable for the color of the wine. On the other hand, none of the dynamic treatments did not significantly modify the total content of acylated anthocyanins, nor non-acylated, and nor the total anthocyanins content.
In Figure 3
, results of vitisins A and B of every sample are shown. Some anthocyanin derived pigments from malvidin-3-glucoside, such as vitisins A and B are of interest because of their stability in the conditions common in red wines [27
]. Treatment of B60 was the only one that caused a significant increase in vitisin A content after 10 min of treatment, but the lowest content was described 3 min after APCP treatments. However, this effect was not observed in vitisin B content with this same treatment. In this case, flow treatment HF90 caused a significant increase of vitisin B 25 min after treatment. The rest of the treatments did not modify the concentration of these compounds.