## 3 Results and Discussions
### Effect of Vacuum Impregnation on Total Flavonoids and Antioxidant Activity  
Table 4 shows the total flavonoid (FLA), ABTS, and DPPH values of apple slices vacuum impregnated with the HS extracts. In general, all VI treatments enhanced (\(p<0.05\)) the FLA content and antioxidant capacity (ABTS and DPPH) of apple slices in comparison  
Figure 3: Optimization (**A**) and desirability (**B**) profiles of vacuum-impregnated soluble phenols from _H. subdarifña_ aqueous extracts in apple slices. The red lines in the figure indicate the optimal vacuum impregnation conditions for each predictor.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**Optimal Experimental Conditions**} & \multicolumn{2}{c}{**Prediction of Soluble Polyphenols**} \\  & & **(mg GAE/100 g Fresh Weight)** \\ \hline Vacuum pressure (bar) & \(-0.4\) & \\ Impregnation time (min) & 6.73 & \\ Restoration time (min) & 3 & \\ Optimal response & & 142.39 \\ \(-95\%\) Confidence limit & & 139.2 \\ +95\% Confidence limit & & 149.52 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Optimal vacuum impregnation conditions of aqueous extracts of _Hibiscus subdariffña_ calves in apple slices, obtained by the predicted model.  
to untreated apple slices. The FLA content ranged from 51.93 to 78.05 mg CE/100 g of fresh sample in a dependent VI conditions. [PERSON] et al. [40] informed that VI was effective in introducing FLA compounds (quercetin glycosides) from apple peel extracts into apple slices. [PERSON] et al. [41] reported that during the VI of flavonoids from mandarin juice into apple snacks, the increase in flavonoids depends on the type of compound (hesperidin > narirutin > didymin) as well as the vacuum pressure applied. However, it has been reported that VI of black carrot concentrate does not increase the flavonoid content of apple tissues [21]. According to [PERSON] et al. [14], the FLA content in chokeberry fruit increases after VI of apple-pear juice; however, this effect is dependent on several factors related to the food matrix, impregnation solution, and vacuum conditions, and in some cases, there may be an insignificant increase or decrease in the FLA content.  
Regarding antioxidant activity, apple slices exhibited a significant increase (\(p<0.05\)) in ABTS and DPPH values in an experimental-dependent manner (Table 4). ABTS values ranged from 57.74 to 147.30 mmol TE/100 g of fresh product, where T10 exhibited the highest value and T6 the lowest value. Conversely, the DPPH values ranged from 134.20 to 175.21 mmol TE/100 g of fresh sample, where T3 exhibiting the highest value and T15 the lowest value. In both cases, VI improved the antioxidant properties of apple slices in comparison with the untreated sample (ABTS of 67.98 mmol TE/100 g and DPPH of 82.68 mmol TE/100 g). Comparable trends were observed when apple cubes or slices were vacuum impregnated with blackcurrant infusion or grape juice concentrate [19; 39] and apple cubes with citric acid [27]. Additionally, _H. sabdariffa_ calyx extracts have been reported to improve the antioxidant properties of papaya cubes after VI [4]. Antioxidant activity is closely related to the content of bioactive compounds in the extract, including soluble phenols and flavonoids [36]. However, in some cases, VI may promote a reduction in the antioxidant capacity of the VI products, possibly associated with a leaching process [21].