## 3 Results and Discussion
### Impact of Ultrasound Applications  
The wines whose fermentation had been stopped (SL25, SL50 and SL75) exhibited a significantly lighter red color than traditional wines. This effect is a consequence of the \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Final Wine** & **RW** & **SL25** & **SL50** & **SL75** \\ \hline Sugar concentration (g/L) & 2.11 \(\pm\) 0.12 a & 27.13 \(\pm\) 0.69 b & 52.84 \(\pm\) 0.74 c & 77.23 \(\pm\) 0.91 d \\ Alcoholic strength (\% vol) & 14.68 \(\pm\) 0.03 a & 11.63 \(\pm\) 0.01 b & 10.32 \(\pm\) 0.04 b & 8.94 \(\pm\) 0.02 c \\ pH & 3.76 \(\pm\) 0.03 a & 3.71 \(\pm\) 0.01 a & 3.72 \(\pm\) 0.02 a & 3.73 \(\pm\) 0.03 a \\ Tittatable acidity & 6.66 \(\pm\) 0.04 a & 5.64 \(\pm\) 0.07 ab & 5.24 \(\pm\) 0.03 b & 5.11 \(\pm\) 0.06 b \\ Acetic acid (g/L) & 0.40 \(\pm\) 0.01 a & 0.28 \(\pm\) 0.01 b & 0.22 \(\pm\) 0.01 b & 0.14 \(\pm\) 0.01 c \\ Malic acid (g/L) & 2.79 \(\pm\) 0.02 & 2.83 \(\pm\) 0.05 & 2.71 \(\pm\) 0.08 & 2.65 \(\pm\) 0.07 \\ Lactic acid (g/L) & 0.17 \(\pm\) 0.01 & 0.19 \(\pm\) 0.01 & 0.19 \(\pm\) 0.01 & 0.17 \(\pm\) 0.01 \\ \hline \hline \end{tabular} Mean value and standard deviation (x \(\pm\) SD) (\(n\) = 3). Different superscripts (a, b, c, d) in the same row indicate statistical differences (\(p<\) 0.05) according to the Student–Newman–Keuls test.  
\end{table} 
Table 2: Measured values of the main enoological parameters in the final wines. temperature decrease applied to stop the alcoholic fermentation. Wines produced as sweet wines are kept at a temperature of 2 \({}^{\circ}\)C after the fermentation halt, during the 5-day period in which the ultrasound treatment is applied before pressing them. The reference wine, RW, continued alcoholic fermentation at 22 \({}^{\circ}\)C under the same conditions of daily punch-downs, so the extraction of phenolic compounds, particularly anthocyanins, continues for a period that is nearly twice as long as in the case of the SL75 wine, for example. The application of ultrasound in the case of the SL25, SL50, and SL75 wines aimed to compensate for this shorter extraction time. Therefore, ultrasonic treatments were applied to the wines still containing the grape skins in an attempt to enhance the extraction of anthocyanins from the skins into the wine. As can be seen in Figure 2, the initial intensities of red color, before the application of the ultrasounds, ranged between 48% absorbance for the SL75 wine and 61% of the SL25 wine vs. the RW. The progress of the extraction was monitored, Figure 2 shows the 3 types of sweet wines, regarding their relative absorbance at 520 nm during the 10 ultrasound cycles. The RW wine, which is the assay processed as red dry wine, is the experiment that presents the highest absorbance and is given a value of 100. The SL25 assay showed an increase in the intensity of the red color after the application of the ultrasonic treatment, reaching its maximum absorbance at cycle 10, with 71% in respect to the RW. The use of ultrasound resulted in an absorbance increment of 16% for the SL25 wine. On the other hand, SL50 and SL75 experienced greater increments than SL25, with 23% for SL50 and 27% for SL75 after 7 ultrasound cycles. However, after the first 7 cycles, their absorbance at 520 nm began to decrease. In general, the wines that initially presented a low color intensity increased their absorbance by at least 16%, with the greatest increases exhibited by the wines that had started with the lowest levels of absorbance, particularly SL75. It was also observed, regarding the evolution of the absorbance, that its increment was not unlimited and that it decreased, most likely due to the oxidation or degradation of the anthocyanins that actually took place in the SL50 and SL75 wines. This effect has already been described in the production of sweet red wines with extended macroations, where malvidin derivatives experienced decreases of up to 20% in 2-4 day macroations after initial increased levels [46].