Optimization of Vacuum Impregnation with Aqueous Extract from Hibiscus sabdariffa Calyces in Apple Slices by Response Surface Methodology: Effect on Soluble Phenols, Flavonoids, Antioxidant Activity, and Physicochemical Parameters

Abstract

Vacuum impregnation (VI) of natural extracts is often used as a pretreatment for fruit dehydration. Apple slices were subjected to VI [X_VP: vacuum pressure (−0.4 to −0.2 mbar), X_IT: impregnation time (2–10 min), and X_RT: restoration time (1–3 min)] of Hibiscus sabdariffa (HS) calyces aqueous extract and optimized using response surface methodology (RSM). Total soluble phenols (TSP) and flavonoids, antioxidant capacity, and physicochemical parameters were evaluated before and after vacuum impregnation. Also, optimized VI apple slices were heat air-dried and characterized for all the mentioned parameters. Under the experimental conditions, all vacuum-impregnated apple slices increased in TSP content, with impregnation time, restoration time, and the interaction between impregnation time and vacuum pressure being the key factors. According to RSM, the optimal VI conditions for TSP (R² = 0.99) were X_VP −0.4 bar, X_IT: 6.73 min, and X_RT 3 min. VI also improved flavonoid and antioxidant activities (DPPH and ABTS) of apple slices and promoted changes in total soluble solids, pH, titratable acidity, water activity, moisture, and color (luminosity, a*, and b*) parameters. Additionally, vacuum-impregnated apple slices (under optimized conditions) were further dehydrated, resulting in an increase in soluble phenols, flavonoids, and antioxidant activity. VI with HS extract is an effective alternative for developing dehydrated apple slices with an increase in antioxidant compounds.

1. Introduction

Owing to the accelerated modern lifestyle and alterations in traditional dietary patterns, several studies have indicated that approximately 40% of daily energy intake is derived from foods with high caloric content and poor nutritional value, such as extruded or fried products. The high consumption of these processed foods is associated with factors contributing with overweight, obesity, and metabolic disorders globally [1,2]. In this context, there is a trend toward consuming healthy and nutritional foods rich in bioactive compounds with beneficial contributions to consumers [3]. Among the foods in high demand by consumers are fresh-cut and dehydrated fruits and vegetables in the form of snacks or preparing infusions, which are considered convenient and nutritious products, mainly because of their fiber and antioxidant compound contents [4]. In general, this type of product has been widely accepted by consumers, and it is estimated that by 2025, the market value will be 98 billion dollars, with a projected annual increase of 5.8% [5]. Nonetheless, dehydrated products are an alternative to reduce post-harvest fruit and vegetable losses, which are estimated to exceed 40% of the total production worldwide [3].

Apples are one of the most significant and extensively cultivated raw materials for numerous food products globally, both in terms of economic value and production scale, and are consumed in fresh, processed, or minimally processed, including dehydrated products. However, a major drawback of the conventional dehydration process (oven drying) for fruit and vegetables is the oxidation/degradation of vitamin C and bioactive compounds (polyphenols, anthocyanins, and carotenoids), which promote notable color changes and reduce their health benefits and sensory quality [5]. In this context, diverse technologies have been developed based on matrix engineering that enable efficient incorporation of physiologically active components of natural origin into different food matrices. This mitigates the limitations of conventional dehydration processes and improves the functional properties of the dehydrated fruit products, among which vacuum impregnation stands out [6].

Vacuum impregnation (VI) is a processing technology that uses the porous structure of food matrices and is mainly applied as a pretreatment before the dehydration process. During VI, the food is submerged in a solution and subjected to negative pressure to remove gas occluded in the food matrix (vacuum application). Upon restoration of atmospheric pressure, the resulting positive differential pressure drives the transport of the immersion solution into the free spaces of the food matrix via hydrodynamic mechanisms and relaxation phenomena due to pressure changes [6]. This technology is easy to implement, cost-effective, and avoids thermal treatments, thereby preserving valuable compounds in the treated food and impregnation solutions [7]. VI represents a technologically viable alternative for the functionalization of dehydrated vegetables, tubers, and fruits through the incorporation of essential oils, minerals, vitamins, water activity depressants, pH regulators, antimicrobials, texturizers, probiotics, prebiotics, and natural extracts with antioxidant activities [7,8,9,10]. Dried Hibiscus sabdariffa calyx is a potential source of antioxidants among natural extracts with antioxidant properties.

H. sabdariffa (HS), also called roselle, karkade, or jamaica, is a member of the Malvaceae family and is cultivated globally (often in tropical areas). HS calyxes are used for their therapeutic effects in folk medicine and have been studied in clinical trials in both healthy individuals and those with chronic non-communicable diseases such as hypertension, dyslipidemia, and type 2 Diabetes Mellitus [11]. These benefits are mainly attributed to the bioactive molecules (phenolic acids, flavonoids, and anthocyanins). In contrast, in food applications, HS calyxes are valued for their sensory attributes, pigments, and antioxidant properties [11,12]. HS aqueous extracts have been used as impregnation solutions to study the transport kinetics of hibiscus anthocyanins in apple cubes [12] and papaya cubes [4]. Additionally, VI has been applied to various food matrices, including potatoes [10], partially dehydrated strawberries [13], and chockberry fruits [14], prior to dehydration. Furthermore, a variety of natural juices and natural extracts have been utilized as impregnating solutions to enhance the content of phenolic compounds, anthocyanins, and antioxidant capacity in food matrices, including Vaccinium myrtillus juice [13], pear-apple based juice [14], and C. sinensis aqueous extracts [15].

In general, VI enhances the sensory properties and antioxidant content of dehydrated or minimally processed fruits. Research has focused on optimizing the VI variables, including vacuum pressure, treatment time, restoration time after treatment, type and concentration of the impregnation solution, and the size, thickness, and geometry of the fruit. Studies agree that each VI process involving a new food matrix or modifications to the impregnating solution should be optimized to maximize the advantages offered by this technology [6,16,17,18]. In this context, the response surface methodology (RSM) has been investigated to develop models for optimizing vacuum impregnation processes. RSM has been used to optimize grape juice incorporation into apple tissues using a Box-Behnken design, considering vacuum pressure, juice concentration, and processing time [19]. Barat et al. [20] also used RSM to optimize the vacuum impregnation of apple slices with sucrose. Moreover, the vacuum impregnation of apple tissues with black carrot concentrate was optimized by RSM coupled with a rotatable central composite design [21]. Thus, this study aimed to optimize the vacuum impregnation conditions (vacuum pressure, impregnation time, and restoration time) of apple slices with H. sabdariffa aqueous extracts, using response surface methodology. The effect on soluble phenol and flavonoid content, antioxidant capacity, and physicochemical parameters before and after vacuum impregnation and dehydration procedures in apple slices were also evaluated.

2. Materials and Methods

The experiments were conducted in two sections (A and B). Section A aimed to achieve the highest vacuum impregnation of soluble phenols from aqueous extracts of H. sabdariffa calyces into apple slices using response surface methodology. Furthermore, the total flavonoid content, antioxidant activity (ABTS and DPPH), and physicochemical parameters (water activity, total soluble solids, color, titratable acidity, moisture content, and pH) were evaluated. Section B involved the experimental validation of vacuum impregnation conditions, followed by the dehydration of vacuum-impregnated apple slices with aqueous extracts of H. sabdariffa calyces, comparing before and after vacuum impregnation and dehydration procedures, and evaluating the effect on soluble phenols, flavonoids, antioxidant activity, and physicochemical properties.

2.1. Plant Material and Chemical Reagents

Gala red apples and Hibiscus sabdariffa calyx were procured in a single purchase from a local market. The apples were washed and disinfected using sodium hypochlorite solution (200 ppm). Subsequently, they were manually peeled and sliced (5 mm thickness). H. sabdariffa aqueous extracts were prepared by boiling dehydrated calyx (20 g) in potable water (500 mL) for five minutes, and then allowed to cool to ambient temperature. The aqueous extract and solids were separated by decantation. The H. sabdariffa calyx extract exhibited a pH value of 2.67 ± 0.13, titratable acidity of 0.53 ± 0.01% citric acid, total soluble solid content of 1.88 ± 0.20 °Brix, luminosity of 27.44 ± 0.23, a* of 34.07 ± 0.25, b* of 3.62 ± 0.07, soluble phenols of 2968 ± 89.07 mg GAE/100 mL of extract, total flavonoids of 278.04 ± 15.69 mg CE/100 mL of extract, antioxidant properties by DPPH of 8228.18 ± 47.71, and ABTS of 7115.03 ± 210.65 mmol Trolox equivalent/100 mL of extract.

Chemicals reagents of analytical grade were acquired from Sigma-Aldrich (St. Louis, MO, USA) or Karal Co. (Guanajuato, Mexico). Ferric chloride hexahydrated, Folin-Ciocalteu phenol reagent 2 N, acetic acid, sodium nitrite, 2,2-Azinobis-(3-ethylbenzothiazoline-6-sulfonic acid), sodium acetate, 2,4,6-tripyridyl-s-triazine, aluminum chloride, catechin, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid, distilled water (dH₂O), gallic acid, methanol, 1,1-Diphenyl-2-picrylhydrazyl.

2.2. Part A

2.2.1. Experimental Design

A 3³ Box-Behnken design (comprising fifteen combinations of factors and levels in randomized order) was employed to determine the optimal conditions for VI of aqueous extracts of HS into apple slices, using the soluble phenol content as the response variable. For this, the vacuum pressure (−0.2, to −0.6 bar), impregnation time (2–10 min), and restoration time (1–3 min) were considered as factors.

2.2.2. Vacuum Impregnation Process

The VI procedure was performed in a sealed chamber (Vevor, Shanghai, China) coupled with a manometer and connected to a vacuum pump (3 CFM and ¼ HP), as recommended [12] with modifications. Apple slices (100 g) were weighed and placed in a vacuum chamber, and HS aqueous extract (500 mL) was placed in the chamber to maintain the mass-to-solution ratio (1:5 w/v). The VI experimental runs were performed according to statistical design conditions. After treatment, the apple slices were carefully removed from the vacuum chamber, gently dried on a filter paper, and weighed. Three replicates were used for each treatment. The impregnation degree (ID%) was calculated as follows [17]:

$$\mathrm{I}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{g}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e} (\mathrm{\%})={\displaystyle \frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}}\times 100$$

(1)

where m₁ and m₀ are the masses of the apple slices before and after the vacuum impregnation, respectively.

2.2.3. Extraction and Quantification of Total Soluble Phenols

Total soluble phenols (TSP) were extracted from apple slices post-VI by combining the sample (2 g) with an 80:20 methanol: water solution acidified with 2% HCl 2 N (20 mL). This mixture was shaken at 80 rpm for 60 min in a 360° rotating mixer (RML-80 PRO, Huzhou City, China) and centrifuged at 6000× g for 10 min at 4 °C (Hermle Z32HK, Wehingen, Germany) [22]. TSP quantification followed the Montreau method [23] with modifications, using 2 N Folin-Ciocalteau’s reagent (FCR), and spectrophotometrically measured at 750 nm in an Accuris Microplate reader (SmartReader MR-9600, Denver, CO, USA). The extract (12 µL) was mixed with FCR (12 µL), sodium bicarbonate solution (7.5 mg/mL, 116 µL), and dH₂O (164 µL) in a 96-well microplate. TSP content was informed in mg of gallic acid equivalent per 100 g of fresh apple weight, derived from a gallic acid reference curve (R² = 0.998).

2.2.4. Response Surface Methodology

The TSP experimental results were fitted to a second-order polynomial equation to obtain regression coefficients. The general/abbreviated model applied to RSM analysis is shown (Equation (2)):

$$\mathrm{Y}=b_0+\sum a_i{X}_i+\sum a_{ii}{X}_1^2+\sum a_{ij}{X}_i{X}_j$$

(2)

The estimated response (Y) was the total soluble phenols, b₀ = intercept coefficient, a_i = linear coefficients, a_ii = quadratic coefficients, a_ij = interaction coefficients, X_i and X_j: Independent variables were vacuum pressure, impregnation time, and restoration time (in coded values). The fitness of the model was assessed using R² (coefficient of determination), R-adjusted, F-ratio and lack of fit tests and p-value from the analysis of variance.

2.2.5. Quantification of Total Flavonoids

Total flavonoids (from TSP extracts) were quantified spectrophotometrically using a microplate reader. First, the sample extract (100 μL) and 430 μL of 5% NaNO₂ solution were combined and left for five minutes. Then, 30 μL of AlCl₃ solution at 10% were added and left for one minute. Next, 440 μL of NaOH (1 M) were added, and the blend was homogenized. From this mixture, 200 μL were placed in the microplate and read at 490 nm using a microplate reader. A catechin reference curve (R²: 0.9997) was used, and the results were informed as mg catechin equivalents per 100 g of fresh apple weight [24].

2.2.6. Antioxidant Activity

The antioxidant properties (from the SP extracts) were evaluated spectrophotometrically (using a microplate reader and 96-well microplates) through radical scavenging assays, including DPPH and ABTS methods (Section 2.2.3). The DPPH radical assay was conducted by blending 190 µL DPPH solution (190 mM) and 40 µL sample extract. The blend was then homogenized for 30 min at 200 rpm (in the absence of light), after which the absorbance was recorded (517 nm) [25]. The ABTS radical scavenging assay was performed by combining the sample extract (35 µL) and ABTS solution (265 µL, 7 mM). The mixture was homogenized for 10 min at 200 rpm (in the absence of light), after which absorbance (734 nm) was recorded [26].

2.2.7. Physicochemical Properties

Total soluble solids (TSS, °Brix) were measured using a portable digital refractometer (ATAGO, PAL87S, Atago Co., Ltd., Tokyo, Japan). pH was measured using a pH meter (HANNA Instruments, HI 207, Bedford, UK). Titratable acidity was evaluated using a pH meter (HANNA Instruments, HI 207, Bedford, UK) as follows: 10 g of apple slices were blended with distilled water (100 mL) in a food processor (Nutribullet of 900 W), and titration was conducted with NaOH (1 N) until pH values reached 8.2–8.3 (phenolphthalein was used as indicator), and results were informed as % citric acid [4,27]. Activity water (Aw) was measured using a portable water activity meter (VTSYIQI, VTS160A, HeFei Vetus Electronic Technology Co., Ltd., Hefei, China) according to the manufacturer’s instructions. The moisture content (%) was determined using the gravimetric method [28]. Color parameters (Luminosity, a*, and b*) were determined using a portable colorimeter (FRU, WR10QC, Shenzhen, China) on the CIELab* scale. The calculation of the total color difference utilizes the color parameter (Equation (3)) [29].

$$\mathrm{T}\mathrm{C}\mathrm{D}=\sqrt{(L-L_0{)}^2+(a-a_0{)}^2+(b-b_0{)}^2}$$

(3)

where L, a, and b are the color attributes of the apple slices before treatment, and L₀, a₀, b₀ are the color attributes of the apple slices after vacuum impregnation.

2.3. Part B

2.3.1. Experimental Validation of Vacuum Impregnation Conditions Obtained by RSM

The best VI conditions derived from RSM were experimentally validated to assess the model accuracy and compare the predicted and experimental VI of soluble phenols from HS extract in apple slices, as recommended [30].

2.3.2. Impregnation-Dehydration Treatment

Apple slices vacuum impregnated with HS extract under optimal conditions (derived from RSM) were air-dried in a convective drying oven (LUZEREN^®, DGH9070A, ISSE LABS S. A de C. V, Mexico city, Mexico) at 60 °C for 24 h [9]. The apple slices were stored (at room temperature) in hermetic plastic bags until analysis. Untreated apple slices were used as controls.

2.3.3. Characterization of Dehydrated Apple Slices

Dehydrated apple slices (vacuum impregnated and untreated samples) were characterized by physicochemical parameters (SST, pH, TA, aw, moisture content, L*, a*, b*), soluble phenol and flavonoid contents, and antioxidant activity (DPPH and ABTS), following previously described procedures.

2.4. Statistical Analysis

All statistical analyses, including RSM were conducted using STATISTICA v.12.5 software (Statsoft^®, Tulsa, OK, USA). Results are presented as the mean ± standard deviation (n = 9). Statistical significance was determined using ANOVA (p < 0.05) and Tukey’s test (α = 0.05) to compare the means. To assess the differences between untreated and vacuum-impregnated apple slices (under optimal conditions) before and after dehydration, Student’s t-test was employed (p < 0.05). Each experiment and measurement were repeated thrice.

3. Results and Discussions

Fresh apple slices before (C+) and after VI with the HS extract (T1–T15) are shown in Figure 1. The control apple slices exhibited a characteristic yellow color [6]. In contrast, the vacuum-impregnated apple slices displayed different shades of red (characteristic color of HS extract) depending on the experimental conditions. These findings are consistent with previous reports on papaya cubes impregnated with HS extracts, which exhibited different shades of red [4]. Similar trends have been reported in honeydew melon slices and chicken meat cubes impregnated with hibiscus extracts, which exhibited a red-darker color than control samples [12,31].

3.1. Vacuum Impregnation of Soluble Phenols from Hibiscus sabdariffa Aqueous Extract into Apple Slices

In total, fifteen experiments were conducted using a 3³ Box–Behnken design to examine the effect of independent variables on the VI of soluble phenols from the HS extract in fresh apple slices. The experiments were randomized to reduce systematic errors. The results are listed in

Table 1. Vacuum impregnation conditions (vacuum pressure, impregnation time, and restoration time) used for response surface methodology with experimental and predicted values, error rate, and impregnation degree after impregnation of aqueous extracts of Hibiscus sabdariffa calyces in fresh apple slices.

Run	Predictors 1			Response Variables		Relative Error (%)	Impregnation Degree (%)
	XVP (bar)	XIT (min)	XRT (min)	Experimental TSP 2	Predicted TSP 3
Untreated control				34.99 ± 0.01 j
T1	−0.2	10	2	100.34 ± 3.13 def	103.86	−3.38	11.72
T2	−0.3	10	1	113.44 ± 1.96 c	107.59	−5.43	12.19
T3	−0.3	10	3	135.21 ± 5.13 b	134.04	−8.27	13.38
T4	−0.3	6	2	105.73 ± 0.99 d	104.59	−1.08	10.53
T5	−0.2	2	3	63.63 ± 1.34 i	60.11	−5.80	9.31
T6	−0.4	2	2	68.66 ± 1.00 hi	65.14	−5.40	10.29
T7	−0.3	6	2	104.82 ± 0.59 d	104.59	−0.21	10.61
T8	−0.2	6	3	97.05 ± 4.25 ef	95.63	−1.48	11.33
T9	−0.3	2	3	95.06 ± 2.33 f	99.16	4.13	10.74
T10	−0.4	6	3	142.73 ± 1.81 a	141.21	−1.07	16.17
T11	−0.4	6	1	113.67 ± 1.79 c	115.19	1.31	11.40
T12	−0.4	10	2	86.51 ± 1.22 g	90.03	3.90	10.37
T13	−0.3	2	1	70.64 ± 1.59 h	73.58	3.99	9.53
T14	−0.3	6	2	103.21 ± 0.79 de	104.59	1.31	10.75
T15	−0.2	6	1	68.19 ± 1.01 hi	69.61	2.03	10.07

Data are presented as the mean ± standard deviation (n = 9). Statistically significant differences (α = 0.05) among the treatments are denoted by lowercase letters in each file. ¹ Vacuum pressure (X_VP); Impregnation time (X_IT), and restoration time (X_RT); TSP: Total soluble phenols; ² Gallic acid equivalents (mg GAE/100 g wet basis); ³ The values were estimated using a 2nd-order polynomial equation (R² = 0.99).

, which includes the experimental conditions for VI, experimental and predicted values, relative error, and impregnation degree.

Statistical differences (p < 0.05) were observed between the untreated (C+) and vacuum-impregnated apple slices (T1–T15). The results showed that the impregnation degree (ID) of TSP ranged from 9.31–16.17%, which is comparable to that of vacuum-impregnated cranberries with ascorbic acid (ID = 11%) [17]. Blanda et al. mentioned that the amount of impregnated solution depends on several factors, including the viscosity of the solution and porosity of the food matrix [32,33]. The highest TSP impregnation content (142.73 mg/GAE/100 g) was achieved at −0.4 bar of vacuum pressure, 6 min of impregnation time, and 3 min of restoration time, while the lowest TSP impregnation was observed under −0.2 bar of vacuum pressure, 2 min impregnation time, and 3 min of restoration time (63.63 mg GAE/100 g), similar to those observed at −0.2 bar of vacuum pressure, 6 min of impregnation time, and 1 min of restoration time (68.19 mg GAE/100 g) or at −0.4 bar of vacuum pressure, 2 min of impregnation time, and 2 min of restoration time. Aguirre-García et al. [4] informed an increase in TSP from 402 to 670 mg/GAE 100 g after 360 min of VI; however, these effects were temperature dependent (45–65 °C). It has been reported that hibiscus extract can improve the phenolic content up to 4.8 times of honeydew melon fruit after VI effects influenced by the geometry and thickness of fruit and the concentration of aqueous extracts [30]. Similarly, Nawirska-Olszanska et al. [14] informed and increase in phenolic compounds in chokeberry fruits after VI (1542.89–2528.58 mg GAE/100 g); however, these results were influenced by the vacuum pressure used (4–8 kPa). In most cases, VI is used to improve the technological functions of fruits through a mass transfer process that facilitates or accelerates the incorporation of desirable bioactive molecules from natural extracts into food products via hydrodynamic forces [32].

3.2. Influence of Vacuum Impregnation Parameters on Soluble Phenols into Apple Slices

The influence of vacuum pressure, impregnation time, and restoration time were evaluated by applying RSM, and the ANOVA and regression coefficient outputs are listed in

Table 2. Analysis of variance using a quadratic model and regression coefficients, with vacuum impregnation conditions on the soluble polyphenols content from aqueous extracts of Hibiscus sabdariffa calyces in apple slices.

Source 1	Analysis of Variance				Regression Coefficients Soluble Phenolic Content β-Coefficient
	SS 2	DF 3	MS 4	F-Value
Mean/intercept	-	-	-	-	48.12 *
XIT	7291.45	1	7291.44	1409.37 *	−42.82 *
XIT2	1963.00	1	1963.00	379.42 *	4.52 *
XRT	4063.72	1	4063.72	785.42 *	−21.61 *
XRT2	1679.09	1	1679.09	324.55 *	6.43 *
XVP	811.60	1	811.59	156.87 *	−425.84 *
XVP2	1462.70	1	1460.69	282.72 *	−1149.13 *
XIT * XRT	5.25	1	5.20	1.015 **	−0.56 **
XIT * XRT2	302.15	1	302.15	58.40 *	0.98 *
XIT2 * XRT	51.77	1	51.765	10.00 *	−0.33 *
XIT * XVP	266.94	1	266.94	51.59 *	−175.64 *
XIT2 * XVP	3747.13	1	3747.13	724.28 *	15.62 *
XRT * XVP	0.03	1	0.031	0.06 **	−0.51 **
Lack of fit	57.05	3	19.01	3.676 **
Pure error	165.55	32	5.174
R-square	0.9905
R-Adjust	0.9881
Total SS	23,643.64

¹ Vacuum pressure (X_VP), Impregnation time (X_IT), and restoration time (X_RT). ² SS, sum of square. ³ DF, degree of freedom. ⁴ MS, means square. * Significant (p < 0.05), ** non-significant (p > 0.05).

. Almost all independent variables and their interactions (X_VP, vacuum pressure, X_IT, impregnation time, and X_RT, restoration time) were significant (p < 0.05), except for X_IT * X_RT and X_RT * X_VP. Yilmaz et al. [21] reported that the restoration period and the interaction vacuum period * restoration period were not significant during the vacuum impregnation of apple tissues with black carrot concentrate and explained that hydrodynamic and relaxation phenomena are related to the vacuum impregnation of solvents into vegetal tissues, but this process takes place during the relaxation period and restoration time has a significant effect on VI effectiveness. Furthermore, during the VI of grape juice into apple cubes, some factors were not significant in the model [19].

Additionally, Table 2 shows the β-coefficients of the model, with most significant parameters (p < 0.05), except for X_IT * X_RT and X_RT * X_VP interactions. The polynomial model (Equation (4)) was refined by removing non-significant terms (p > 0.05) to enhance its predictive capability [30]. The predicted and experimental data demonstrated a strong correlation (R²: 0.9905 and R-adjusted: 0.9881) with a relative error below 10% (Table 1). Furthermore, the model’s efficacy was supported by a non-significant lack of fit (p > 0.05), indicating robust predictive power [34]. Additionally, the R² value is a crucial indicator of model fitness [35]. Yilmaz et al. [21] reported R² values of 0.98 after VI of carrot juice in apple tissue (lack of fit = 0.4189). Similar trends were reported in apple cubes vacuum-impregnated with grape juice for total soluble solids (R² = 0.943, lack of fit = 0.06) and luminosity (R² = 0.955, lack of fit = 0.10). However, for the total soluble phenols, the R² value was 0.694 (lack of fit = 0.06), which did not show an acceptable adjustment [19]. González-Pérez et al. [30] after vacuum impregnation of β-carotene from carrot juice in Pachyrhius erosus, using RSM analysis, reported that polynomial models with R² > 0.8 exhibited adequate adjustments; in contrast, models with R² > 0.6 were unsuitable for predicting experimental values.

The second-order polynomial equation demonstrated that the parameters associated with vacuum impregnation of apple slices comprised linear and quadratic coefficients of impregnation time, restoration time, and vacuum impregnation, as well as the interactions between impregnation time × restoration time and impregnation time × vacuum pressure (p < 0.05).

$$\begin{gathered} \mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{b}\mathrm{l}\mathrm{e} \mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{o}\mathrm{l}\mathrm{s}\left(\mathrm{m}\mathrm{g} \mathrm{G}\mathrm{A}\mathrm{E}/100 \mathrm{g} \mathrm{f}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{h} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\right)=48.12-42.82X_{IT}+4.52{X_{IT}}^2- \\ 21.61X_{RT}+6.43{X_{RT}}^2-425.28X_{VP}-1149.13{X_{VP}}^2+0.98X_{IT} * {X_{RT}}^2-0.33{X_{IT}}^2 * X_{RT}- \\ 175.64X_{IT} * X_{VP}+15.62{X_{IT}}^2 * X_{VP} \end{gathered}$$

(4)

Figure 2 illustrates the effect of impregnation time (Y-axis) and restoration time (X-axis) at different vacuum pressures (−0.4 to −0.2 bar) on the VI of TSP (Z-axis) from HS extract into apple slices. These plots were based on the polynomial model presented in Equation (4). The three-dimensional representations facilitated the visual identification of the area exhibiting the highest VI of TSP in apple slices. It was observed that the impregnated TSP content increased with increasing impregnation and restoration times, independent of vacuum pressure (Figure 2A–C). However, at a higher (−0.4 bar) vacuum pressure (Figure 2C), greater impregnation of soluble phenols was achieved with less impregnation time, considering seven minutes of impregnation time and three minutes of restoration time.

Additionally, the most significant parameters during VI of soluble phenols from the HS extract in apple slices are presented in the Pareto Chart with a 95% confidence level (Figure 2D). The impregnation time (X_IT), restoration time (X_RT), and interaction between impregnation time × vacuum pressure (X_IT² * X_VP) were identified as the primary factors influencing the VI process. Previous research has indicated that increasing the processing time improves grape juice impregnation in apple cubes, whereas higher vacuum pressure facilitates the impregnation process [19]. The restoration time is critical during the VI process because it enhances the relaxation/deformation phase, allowing liquid influx due to capillary pressure [21]. Some authors have reported that long restoration times (>30 min) are needed to improve VI results [36]; however, it has been reported that five minutes of restoration time after VI is sufficient to increase the phenolic compounds in yam bean slices and mentioned that food matrix porosity and solution characteristics are critical factors involved in the effectiveness of VI [37]. Nevertheless, some authors have noted a reduction in the soluble phenol content within the food matrix during VI, associated with an increase in vacuum pressure during impregnation due to the breakage of plant cell walls and the lixiviation of phenolic compounds [38]. Therefore, optimization of the VI process in each food matrix is required.

3.3. Optimization of VI of H. sabdariffa Calyx Extract in Apple Slices

The VI process was optimized by maximizing the TSP content from the HS extract in apple slices, as shown in Figure 3. The best optimized VI conditions were 6.73 min of vacuum impregnation at −0.4 bar of vacuum pressure and 3 min of restoration time (Figure 3A). Additionally, these conditions exhibited a desirability (D) value of 1, indicating very good prediction performance (Figure 3B). It has been reported that D values ≥ 0.7 during optimization processes are acceptable for predicting optimal conditions. Diamante et al. [39] informed a D value of 0.75–0.78 when the VI of blackcurrant-infused apple cubes was optimized.

Table 3. Optimal vacuum impregnation conditions of aqueous extracts of Hibiscus sabdariffa calyces in apple slices, obtained by the predicted model.

Parameter	Optimal Experimental Conditions	Prediction of Soluble Polyphenols (mg GAE/100 g Fresh Weight)
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

lists the optimized VI conditions for obtaining apple slices impregnated with HS extract, with a theoretical optimal response of 142.39 mg GAE/100 g of fresh apple slices for further validation.

3.4. Effect of Vacuum Impregnation on Total Flavonoids and Antioxidant Activity

Table 4. Total flavonoids and antioxidant activity (DPPH and ABTS) of apple slices after vacuum impregnation of aqueous extracts of Hibiscus sabdariffa calyces.

Treatment	Flavonoids (mg CE/100 g Fresh Weight)	ABTS (mmol TE/100 g Fresh Weight)	DPPH (mmol TE/100 g Fresh Weight)
Untreated control	25.36 ± 0.43	67.98 ± 0.64 f	82.68 ± 1.96 f
T1	64.81 ± 2.11 cde	99.74 ± 0.67 cd	166.74 ± 0.94 cd
T2	53.38 ± 4.29 h	135.36 ± 0.52 ab	170.11 ± 2.74 abcd
T3	61.34 ± 2.55 def	142.19 ± 0.38 ab	175.21 ± 4.13 a
T4	53.68 ± 2.26 h	106.32 ± 0.34 c	134.20 ± 1.04 e
T5	54.30 ± 1.26 h	58.17 ± 0.35 e	132.87 ± 1.77 e
T6	60.44 ± 0.57 efg	57.74 ± 0.50 e	171.65 ± 1.11 abc
T7	55.23 ± 0.98 gh	92.96 ± 0.17 d	169.93 ± 2.25 abcd
T8	67.01 ± 0.93 cd	101.20 ± 0.88 cd	168.07 ± 0.41 bcd
T9	68.31 ± 0.96 bc	99.67 ± 0.59 cd	164.65 ± 1.48 d
T10	78.05 ± 1.16 a	147.30 ± 1.29 a	172.84 ± 2.07 ab
T11	69.69 ± 0.22 bc	131.04 ± 1.85 b	167.34 ± 1.16 bcd
T12	73.88 ± 2.51 ab	111.57 ± 1.05 c	165.86 ± 2.08 cd
T13	55.96 ± 2.66 fgh	102.33 ± 0.92 cd	169.95 ± 2.27 abcd
T14	51.93 ± 0.57 h	131.30 ± 0.19 c	167.82 ± 0.34 bcd
T15	52.54 ± 0.78 h	101.24 ± 0.41 cd	134.99 ± 1.20 e

Data are presented as mean ± standard deviation (n = 9). Statistically significant differences (α = 0.05) among the treatments are denoted by lowercase letters in each file. CE: Catechin equivalent. TE: Trolox equivalent. T1–T15: The key sample numbers are presented in Table 1.

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 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. Schulze et al. [40] informed that VI was effective in introducing FLA compounds (quercetin glycosides) from apple peel extracts into apple slices. Betoret 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 Nawirska-Olszánska 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].

3.5. Effect of Vacuum Impregnation on Physicochemical Parameters

Table 5. Physicochemical properties of apple slices after vacuum impregnation of aqueous extracts of Hibiscus sabdariffa calyces.

Treatment	pH	TA (% Citric Acid)	TSS (°Brix)	Water Activity (aw)	Moisture (%)
Untreated control	4.01 ± 0.01 a	0.18 ± 0.01 c	15.03 ± 0.35 a	0.94 ± 0.01 bcde	86.08 ± 0.39 ab
T1	3.29 ± 0.21 d	0.39 ± 0.04 ab	9.97 ± 0.61 bcd	0.94 ± 0.01 bcde	87.81 ± 0.34 a
T2	3.31 ± 0.02 d	0.37 ± 0.03 ab	8.83 ± 1.65 e	0.93 ± 0.01 de	85.58 ± 0.60 ab
T3	3.38 ± 0.15 cd	0.43 ± 0.01 ab	8.20 ± 0.10 de	0.93 ± 0.01 de	84.36 ± 0.88 ab
T4	3.32 ± 0.01 d	0.39 ± 0.01 ab	8.23 ± 0.23 de	0.94 ± 0.01 cde	85.53 ± 0.04 ab
T5	3.54 ± 0.02 bcd	0.45 ± 0.01 a	9.93 ± 0.35 bbcde	0.92 ± 0.01 e	87.20 ± 0.34 a
T6	3.46 ± 0.09 bcd	0.45 ± 0.01 a	9.20 ± 0.62 cde	0.98 ± 0.01 a	88.01 ± 0.15 a
T7	3.55 ± 0.04 bcd	0.40 ± 0.01 ab	9.97 ± 1.11 bcd	0.97 ± 0.01 abc	87.80 ± 0.01 a
T8	3.53 ± 0.07 bcd	0.42 ± 0.01 ab	9.90 ± 0.10 bcde	0.97 ± 0.01 abc	87.51 ± 0.03 a
T9	3.52 ± 0.04 bcd	0.37 ± 0.02 ab	9.33 ± 0.30 bcde	0.96 ± 0.02 abcd	86.13 ± 1.33 a
T10	3.64 ± 0.06 bc	0.43 ± 0.02 ab	9.53 ± 0.25 bcde	0.96 ± 0.01 abc	87.09 ± 0.35 a
T11	3.62 ± 0.12 bc	0.41 ± 0.05 ab	9.83 ± 0.70 bcde	0.97 ± 0.02 abc	85.88 ± 0.40 ab
T12	3.49 ± 0.09 bcd	0.38 ± 0.02 ab	9.17 ± 0.49 cde	0.97 ± 0.01 abc	86.52 ± 8.44 b
T13	3.65 ± 0.09 bc	0.40 ± 0.01 ab	9.70 ± 0.45 bcde	0.96 ± 0.01 abcd	86.41 ± 0.20 a
T14	3.55 ± 0.03 bcd	0.40 ± 0.01 ab	9.50 ± 0.17 bc	0.97 ± 0.01 ab	86.86 ± 0.55 a
T15	3.67 ± 0.07 b	0.36 ± 0.02 ab	9.07 ± 0.05 b	0.86 ± 0.01 abc	87.30 ± 0.13 a

Data are presented as mean ± standard deviation (n = 9). Statistically significant differences (α = 0.05) among the treatments are denoted by lowercase letters in each file. TA: titratable acidity. TSS: Total soluble solids. T1–T15: The key sample numbers are presented in Table 1.

lists the physicochemical properties (pH, TA, TSS, aw, and moisture) of apple slices after vacuum impregnation with H. sabdariffa aqueous extract. All evaluated parameters exhibited statistically significant differences (p < 0.05) between treated and untreated apple slices. The VI process resulted in a decrease in the pH values (3.29 to 3.67) of the treated samples compared to the untreated sample (pH of 4.01). Additionally, the TA values of the treated samples significantly increased (p < 0.05) after VI (0.37–0.45% citric acid) in comparison to the untreated sample (0.18% citric acid). Blanda et al. [33] reported a decrease in pH (4.24 to 3.90) and an increase in TA (0.24 to 0.36%) in apple slices vacuum impregnated with ascorbic acid, and similar trends were observed in apple tissues vacuum impregnated with black carrot juice [21]. Previous research has indicated that the physicochemical properties (pH, TA, and TSS) of food matrices under VI are influenced by the intrinsic characteristics of impregnation solutions; for instance, an increase in pH has been observed in vacuum impregnated fresh-cut apples with fructose and calcium lactate [42,43]. A decrease in TSS content was observed in the VI samples (8.20 to 997 °Brix) compared to the untreated sample (15.03 °Brix). Similar trends have been reported after VI of black carrot juice in apple tissues [21] or calcium lactate in apple slices [9]. This phenomenon can be attributed to the low TSS content of the HS extract [9]. During the VI process, certain soluble solids in apple slices may be removed while the extract diluted with water entered the apple slice, reducing the TSS content, as explained by González-Pérez et al. [30] during the VI of Daucus carota extract in Pachyrhizus erosus. In contrast, several authors have reported an increase in the TSS content of vacuum-impregnated fruits; however, in most cases, the impregnation solutions contain added sugars such as fructose or glucose, and this effect could be attributed to the presence of solutes in the solution [4,19]. Nevertheless, it has also been reported that there are no changes in TSS content after VI fresh-cut apples, which is attributed to the TSS content of the solution used [44] and the sample geometry [39].

Additionally, a statistically significant (p < 0.05) increase in water activity and moisture content was observed in vacuum-treated apple slices compared with the untreated sample (0.94 and 86%). The water activity ranged from 0.92 to 0.98, whereas the moisture content ranged from 84.36 to 88.01%. These values are consistent with those reported in the literature for fresh apples [19,27,40,43]. In jicama slices vacuum impregnated with Daucus carota extract, the water activity varied according to the experimental conditions, and in some treatments, the aw values of treated samples were higher than those of untreated samples [30]. Assis et al. [9] reported that vacuum impregnation of calcium lactate did not alter the water activity values (0.991–0.996) of apple slices. Conversely, Xie et al. [43] reported that apple slices exhibited a slight decrease in water activity (from 0.980 to 0.978) after vacuum impregnation of fructose syrup at different concentrations. Similar trends were observed by González-Pérez et al. [19] in apple slices vacuum-impregnated with grape juice (decrease from 0.984 to 0.975); furthermore, these effects are dependent on experimental conditions. Additionally, Dincer et al. [12] reported an increase in water content in apple tissues (85.91 to 91%) vacuum impregnated with hibiscus extract and noted that the gained water is associated with the vacuum process that transferred water from the outside to the inside of the apple tissues; however, this effect is influenced by the vacuum pressure and processing time. Similar trends were observed in apple slices vacuum impregnated with calcium lactate and quercetin derivatives, which exhibited an increase in moisture content of up to 30% [9,40].

Table 6. Color attributes of apple slices after vacuum impregnation of aqueous extracts of Hibiscus sabdariffa calyces.

Treatment	L*	a*	b*	TCD	Color
Untreated control	72.22 ± 0.56 a	−3.41 ± 0.18 e	21.30 ± 1.08 bc	-
T1	48.25 ± 0.23 b	15.48 ± 0.38 cd	20.08 ± 0.78 cd	30.59
T2	43.00 ± 0.67 e	15.55 ± 0.57 cd	23.24 ± 0.24 b	34.84
T3	39.54 ± 0.47 fg	14.17 ± 0.59 cd	28.38 ± 0.49 a	38.44
T4	46.29 ± 0.28 jk	16.27 ± 0.58 d	17.82 ± 0.66 ef	41.35
T5	40.34 ± 0.64 c	15.46 ± 0.89 bc	17.57 ± 0.28 efg	32.76
T6	44.53 ± 0.78 f	15.58 ± 0.42 cd	11.80 ± 0.72 i	38.23
T7	40.60 ± 0.52 de	17.28 ± 0.22 cd	11.56 ± 0.36 i	34.96
T8	44.63 ± 0.11 f	16.88 ± 0.19 ab	19.16 ± 0.80 de	37.84
T9	34.44 ± 0.46 d	15.24 ± 0.23 a	15.60 ± 0.54 gh	35.51
T10	36.86 ± 0.8 k	16.55 ± 0.45 cd	17.12 ± 0.61 fg	42.34
T11	36.16 ± 0.11 hi	15.36 ± 0.51 bc	16.98 ± 0.61 fg	40.83
T12	37.99 ± 0.52 ij	15.35 ± 0.28 cd	16.72 ± 0.68 fg	40.91
T13	40.38 ± 0.52 gh	15.58 ± 0.58 cd	12.22 ± 0.54 i	38.76
T14	49.36 ± 0.26 f	15.41 ± 0.46 cd	15.98 ± 0.78 fgh	37.46
T15	48.24 ± 0.26 b	15.53 ± 0.34 cd	14.26 ± 1.08 h	30.44

Data are presented as mean ± standard deviation (n = 9). Statistically significant differences (α = 0.05) among the treatments are denoted by lowercase letters in each file. L*: luminosity; a*: redness/greenness; b*: yellowness/blueness; TCD: Total color difference; color square were obtained processing L*, a*, b* data in a color converter software (http://colormine.org/color-converter, accessed on 18 June 2024). T1–T15: The key sample numbers are presented in Table 1.

shows the effect of vacuum impregnation of the HS extract on the color of apple slices. In general, all vacuum impregnated samples exhibited significant differences (p < 0.05) between the treated and untreated samples for all color parameters (luminosity, a*, and b*). After VI apple slices showed luminosity values ranged from 34.44 to 48.25, a significant reduction compared to the untreated apple slices (72.22). Regarding the a* (14.17–17.28) and b* (15.60–28.38) parameters, the VI samples exhibited changes compared to the control (a* = −3.41, b* = 21.30). In this context, vacuum impregnated apple slices showed appreciable color changes (TCD of 30.59 to 42.34) compared to the untreated sample. These results are in agree with those reported in vacuum-impregnated papaya cubes with HS extracts, which describe a color change from orange to red [4]. Similar trends have been reported in cranberries vacuum-impregnated with sucrose, ascorbic acid, and citric acid; moreover, the highest TCD values were observed in treatments with a higher degree of impregnation [17]. Additionally, it has been reported that color parameters (TCD of 44.20 to 58.80) of apple tissues impregnated with grape juice are significantly affected by vacuum impregnation conditions, particularly vacuum pressure [19].

3.6. Vacuum Impregnation Validation and Dehydration of Apple Slices Vacuum Impregnated

After theoretical optimization (Table 3), experimental validation was performed under optimized vacuum impregnation conditions, including vacuum impregnation of −0.4 bar, impregnation time of 6.73 min, and restoration time of 3 min (

Table 7. Physicochemical parameters, phenolic content, and antioxidant activity of fresh and dried apple slices before and after vacuum impregnation of aqueous extracts from Hibiscus sabdariffa calyces.

	Treatments
Parameter	Fresh Sample (Fresh Weight)		Dehydrated Sample (Dry Weight)
	Untreated Sample	* Vacuum Impregnated Sample	Untreated Sample	* Vacuum Impregnated Sample
pH	3.94 ± 0.03 a	3.26 ± 0.02 b	3.80 ± 0.01 a	3.38 ± 0.01 b
Titratable acidity (% citric acid)	0.18 ± 0.01 b	0.41 ± 0.01 a	0.026 ± 0.02 b	0.032 ± 0.001 a
Total soluble solids (°Brix)	14.13 ± 0.05 a	10.16 ± 0.40 b	18.48 ± 0.17 a	20.05 ± 0.30 b
Water activity	0.94 ± 0.01 b	0.98 ± 0.01 b	0.36 ± 0.01 a	0.37 ± 0.01 a
Water content (%)	87.90 ± 0.22 a	88.23 ± 0.12 a	6.83 ± 1.90 a	6.97 ± 0.35 a
L*	66.98 ± 1.29 a	31.38 ± 0.79 b	74.61 ± 1.33 a	39.69 ± 1.08 b
a*	−2.13 ± 0.12 b	18.66 ± 1.22 a	5.01 ± 0.86 b	21.47 ± 0.62 a
b*	19.71 ± 1.98 a	19.11 ± 0.28 a	30.79 ± 0.57 a	7.73 ± 1.52 b
Total soluble phenols (mg GAE/100 g)	43.27 ± 4.10 b	143.17 ± 2.72 a	593.94 ± 73.94 b	855.85 ± 80.63 a
Total flavonoids (mg CE/100 g)	37.61 ± 4.63 b	56.04 ± 1.52 a	331.84 ± 4.63 b	525.29 ± 28.05 a
DPPH (mmol TE/100 g)	122.51 ± 1.68 b	230.07 ± 3.14 a	433.44 ± 7.70 b	560.49 ± 11.34 a
ABTS (mmol TE/100 g)	104.29 ± 0.77 b	147.99 ± 0.70 a	125.16 ± 1.54 b	150.51 ± 1.93 a

Data are presented as mean ± standard deviation (n = 9). Statistically significant differences (α = 0.05) among the treatments are denoted by lowercase letters in each file. L*: Luminosity; a*: redness/greenness; b*: yellowness/blueness. * Vacuum impregnation was performed under optimal conditions from Box-Behnken design: vacuum impregnation: −0.4 bar; impregnation time: 6.73 min; and restoration time: 3 min.

). The experimental TSP results (143.17 mg GAE/100 g fresh sample) were very close to the predicted values (142.39 mg GAE/100 g fresh sample), demonstrating the adequacy and reliability of the fitted model for vacuum impregnation of TSP from HS extract in apple slices.

Table 7 lists the physicochemical properties, soluble phenol and flavonoid contents and antioxidant activity of fresh and dehydrated apple slices before and after VI with the HS extract. After VI, fresh samples exhibited a decrease (p < 0.05) in pH (3.94 to 3.26), total soluble solids (14.13 to 10.16 °Brix), and luminosity (66.98 to 31.38) and an increase (p < 0.05) in titratable acidity (0.18 to 0.41% citric acid), water activity (0.94 to 0.98), a* (−2.13 to 18.66), total soluble phenols (43.27 to 143.17 mg GAE/100 g fresh sample), flavonoids (37.61 to 56.04 mg CE/100 g fresh sample), DPPH (122.51 to 230.07 mmol TE/100 g), and ABTS (104.29 to 147.99 mmol TE/100 g). However, no differences (p > 0.05) were observed in water content (87.90 to 88.23%) and b* (19.71 to 19.11) parameters. Similar trends have been reported in vacuum impregnated fruits (apple, honeydew melon, cranberries, and Pachyrhius erosus), using diverse solutions containing calcium lactate, apple-pear juice, ascorbic acid, H. sabdariffa aqueous extract, carrot juice, blackcurrant infusion [4,12,14,17,21,30,39].

VI of natural extracts is typically used as a pretreatment for subsequent processing operations, such as dehydration, with the aim of enhancing the health-promoting and quality properties of fruits [45]. As shown in Table 7, VI of HS extract induced significant changes (p < 0.05) in pH (from 3.80 to 3.38), titratable acidity (0.026 to 0.032% citric acid), total soluble solids (18.48 to 20.05 °Brix), luminosity (74.61 to 39.69), a* (5.01 to 21.47), b* (30.79 to 7.73), soluble phenols (593.94 to 855.85 mg GAE/100 g dried sample), flavonoids (331.84 to 525.29 mg CE/100 g dry sample), DPPH (433.33 to 560.49 mmol TE/100 g), and ABTS (125.16 to 150.51 mmol TE/100 g) in dehydrated apple slices compared to the untreated samples [9,27,46]. Moreover, dehydrated samples (untreated and vacuum-impregnated) exhibited low water activity (0.36 to 0.37) and water content (6.83 to 6.97%), indicating that the samples are microbially safe and chemically stable during storage [27]. Figure 4 shows the visual appearance of apple slices before and after vacuum impregnation of the aqueous extract of HS (Figure 4A,B), and before and after dehydration (Figure 4C,D).

4. Conclusions

The optimal conditions for vacuum impregnation of Hibiscus sabdariffa calyx aqueous extract in fresh apple slices were determined by response surface methodology, with −0.4 bar of vacuum pressure for 6.73 min of impregnation time and 3 min of restoration time being selected. These findings indicate that impregnation time, restoration time, and the interaction between impregnation time and vacuum pressure exerted the most significant influence on the impregnation of soluble phenols from H. sabdariffa extract in fresh apple slices. Furthermore, vacuum impregnation enhanced soluble phenol and flavonoid contents and antioxidant activity (DPPH and ABTS), with significant alterations in the physicochemical properties of fresh and dried apple slices. This study highlights that employing statistical tools in conjunction with vacuum impregnation can enhance the physicochemical and antioxidant properties of fresh and dehydrated apple slices. H. sabdariffa aqueous extract serves as a natural colorant with an exotic flavor that can be utilized as an impregnation solution to improve the physicochemical and antioxidant properties of other fruits and vegetables to be dehydrated. This study demonstrated that vacuum impregnation is an effective alternative for developing dehydrated apple slices with an increased content of antioxidant compounds from H. sabdariffa extract. Further studies are needed to standardize the vacuum impregnation of H. sabdariffa extract in apple slices, including other apple varieties, different ripening stages, and concentrations of H. sabdariffa extract.