Evaluation of Antioxidant-Rich Fruit Extracts to Improve the Bioactive Compounds of Apple Slices

Abstract

This study aimed to evaluate the impregnation process of apple slices with bioactive compounds (BCs) that were obtained from the richest antioxidant fruit powder extracts. Acaí, beetroot, blueberry, cranberry, dragon fruit, and pomegranate powders were assessed on their main BCs, antioxidant capacity (AC), and reducing power (RP) before and after heat treatment to select the extract with the highest and most stable BCs and AC. The effect of pH (3–7) and the total soluble solids (TSSs) (5–15%) on the BCs and AC of selected fruit powder extracts were also evaluated to decide which one should be used to impregnate apple slices. Results indicate that the pomegranate extract presented the highest BCs, AC, and RP among the fruit powders tested, even after heat treatment. Moreover, the TSSs did not affect the BCs from the pomegranate extract; however, pH affected the stability, showing higher values of BCs as the pH was reduced. Apple slices impregnated with pomegranate extract (pH 2) showed an increase in their total phenolic compounds, total anthocyanins, AC, and RP at 25.94, 3085.00, 19.80, and 56.94%, respectively, compared to control apple slices. This study indicates that reducing the pH of pomegranate extracts may increase their antioxidant compounds, showing a higher impregnation of them on apple slices.

1. Introduction

Color is probably the most important sensory characteristic of food products since it may affect consumer preferences and buying intention [1]. In addition, the color might also indicate many other information related to its nature, processing, and healthy characteristics [2]. For example, green, red, and red–blue colors have been associated with healthy, spicy, and antioxidant foods, respectively [3,4], and that is why a colorful salad is preferred over its pale-colored counterpart [5]. Natural colorants, without the toxic effects of synthetic colorants, have gained more attention from food consumers, researchers, and industry due to their therapeutic and medical properties [6]. However, the most important problem with natural pigments is their instability, their less brilliant color compared to their synthetic equivalent [7], and their limited number of available natural food sources [8]. In this respect, fruit and vegetables are not only excellent sources of health-promoting compounds (fiber, antioxidants, minerals, vitamins, etc.) that are widely demanded by consumers because of the various health benefits that they offer—especially in reducing the effects of non-communicable diseases like cancer, Alzheimer’s, diabetes, atherosclerosis, stroke, etc. [9]—but they are also the primary sources of natural pigments, including anthocyanins (red, blue, and purple colors), carotenoids (red, orange, and yellow colors), and betalains (red and purple colors). These pigments add both color and antioxidant properties to food products [10]. However, they pose several challenges throughout the food processing chain, including color changes due to the physicochemical properties of the food products, as well as instability during processing, storage, and distribution [8].

Apples are one of the most consumed fruits worldwide because of their pleasant flavor, firmness, and nutritional characteristics [11]. Their high consumption makes them a significant source of phenolic compounds, which play a crucial role in protecting humans from various diseases linked to oxidative stress [12]. The most abundant phenolic compounds in apples include chlorogenic acid, phloridzin, epicatechin, and quercetin [13]. However, anthocyanins, which have the highest antioxidant capacity among phenolic compounds, are found in limited quantities, particularly in the edible portions of the fruit [14,15]; thus, their inclusion is constantly investigated by different researchers. In this sense, Hernández-Carranza et al. [15] added anthocyanins from Hibiscus sabdariffa extracts to apple slices, increasing the total anthocyanins by 132.9 mg/100 g, which represents a 77.0% increase in total phenolic compounds and a 103.7% increase in antioxidant capacity. Similarly, anthocyanins (38.85 mg/kg) from Hibiscus sabdariffa extract were impregnated in apple slices after 30 min using ultrasound-assisted vacuum equipment [16]. Moreover, anthocyanins from blueberry juice (delphinidin, cyanidin, and malvidin) were vacuum-impregnated into apple discs, which enhanced their antioxidant capacity [17]. In addition, various other antioxidant-rich extracts obtained from beetroot, green tea, black carrot, and chokeberry have been used to improve the bioactive compounds in apples [14,18,19,20].

As is well-known, red–blue fruits and vegetables are associated with high bioactive compounds and antioxidant capacity [21], which is why fruits such as berries, beetroot, pomegranate, acaí, and black carrot, among others, have been currently investigated and, in many cases, added to food products for antioxidant fortification [14,19,20,22]. However, according to the authors’ knowledge, a comparative study of the bioactive compounds and antioxidant capacity of red–blue fruits associated with health benefits, as well as the effect of physicochemical factors on their stability and potential applications in common food products, has not been conducted yet. Therefore, this study aimed to impregnate apple slices with the richest antioxidant fruit extract to improve their antioxidant composition. To fully achieve this purpose, the listed topics were covered: (a) evaluation of the stability of bioactive compounds and antioxidant capacity of red–blue fruit extracts before and after heat treatment, (b) assaying the effect of pH and total soluble solids (TSSs) on the bioactive compounds and antioxidant capacity of the richest antioxidant fruit extract, and (c) utilizing the richest antioxidant fruit extract at selected pH and TSSs to impregnate bioactive compounds with antioxidant capacity into apple slices.

2. Materials and Methods

The present study was conducted in three steps (Figure 1): Step (1) Selection of the richest antioxidant powder fruit extract. Step (2) Evaluation of the effect of pH and TSSs on the stability of bioactive compounds and antioxidant capacity of the richest antioxidant fruit powder extract. Step (3) Impregnation of the bioactive compounds from the extracts with the highest antioxidant properties into apple slices.

2.1. Vegetal Materials and Reagents

Blueberry, cranberry, dragon fruit, and pomegranate (whole fruit) powders were acquired from Maia Organicos; acaí and beetroot powders were obtained from TGB Superfoods and Healthy Superfoods, respectively. Powders were stored in their package at 20 ± 3 °C during all the experiments. Apples (Malus domestica L.) cv “Gala” were obtained from a local supermarket in Puebla, Mexico. Fruits selected free from physical and microbiological apparent damage were washed, disinfected using hypochlorite solution (150 ppm), and gently dried using absorbent paper. All chemical reagents and solvents used in this study were obtained from Sigma-Aldrich, Inc. (Toluca, Mexico), and Reyma (Puebla City, Mexico), respectively.

2.2. Evaluation of Bioactive Compounds and Antioxidant Capacity of Antioxidant-Rich Fruit Powder

Antioxidant extracts were obtained by placing 1.0 g of each powder in 100 mL of distilled water. The extraction was conducted at 20 ± 3 °C for 1 h following the methodology reported by Hernández-Carranza et al. [23]. Extracts were cotton-filtered and immediately used for bioactive compounds and antioxidant capacity assays or for the treatments mentioned below.

2.3. Effect of Heat Treatment on the Bioactive Compounds and Antioxidant Capacity of Antioxidant-Rich Fruit Powder

Ten mL of each powder extract were submitted to heat sterilization (120 °C for 15 min) using a vertical and automatic autoclave (BMX-30R, Luzeren, Mexico) to evaluate its effect on the visual color stability, bioactive compounds, and antioxidant capacity of the extracts.

2.4. Effect of the pH and TSSs on the Stability of Bioactive Compounds and Antioxidant Capacity of Selected Fruit Extracts

The effect of pH and TSSs on the bioactive compounds and antioxidant capacity of the selected fruit powders was assessed using citric acid and sucrose, respectively. After conducting this, the extracts were immediately modified in their pH (3, 5, and 7) and TSSs (5, 10, and 15%) following a face-centered central composite design (Design Expert Program 6.0.6, StatEase Inc., Minneapolis, MN, USA). The experimental design resulted in 13 experiments, including 9 different experiments and 4 replicates of the central point. The effect of the variables was analyzed by linear regression with the following response surface model:

$$Y={\beta }_0+{\beta }_1\times pH+{\beta }_2\times TSS+{\beta }_3\times {pH}^2+{\beta }_4\times {TSS}^2+{\beta }_5\times pH\times TSS$$

(1)

where Y is the predicted response; β₀, β₁, β₂… are the coefficients of the model; and the pH and TSSs were the studied factors.

2.5. Impregnation and Drying Processes of Apple Slices

Extracts with the highest stability (pH and TSSs) in their bioactive compounds and antioxidant capacity were selected and applied to apple slices to enhance their bioactive compounds and antioxidant capacity. Disinfected apples were manually sliced into 0.5 cm thick pieces using a stainless-steel knife and were then treated in three steps: (1) A pre-dried process at 60 °C for 4 h. This step aimed to reduce the moisture content of the apple slices, thereby decreasing the impregnation time for bioactive compounds and enhancing the antioxidant capacity of the selected extract. (2) Immersing the apple slices in the selected extract at a ratio of 1:5 (apple slices to antioxidant extract) for 5 min at 20 ± 3 °C. This duration was chosen based on the observation that there was no significant change (p > 0.05) in the bioactive compounds and antioxidant capacity during the impregnation kinetics tested over various time points (0, 1, 3, 5, 10, 20, and 40 min) at 20 ± 3 °C. As a control, apple slices immersed in distilled water were subjected to the same conditions as those exposed to the extracts. (3) Fully drying apple slices in a food dehydrator (4 h at 60 °C) to reach a moisture content of 15 ± 2% dry basis (oven dried at 105 °C). After drying, the slices were ground using a coffee grinder, were passed through a sieve (180 µm), and stored at 20 ± 3 °C for antioxidant quantification. To obtain apple powder extracts rich in bioactive compounds and antioxidant capacity, the methodology outlined in Section 2.2 was followed. It is important to note that the total betalains or total anthocyanins were measured, depending on which extract exhibited the highest levels of bioactive compounds and antioxidant capacity, in the impregnated apple slices.

2.6. Total Phenolic Compounds of Antioxidant-Rich Fruit and Apple Powders

The total phenolic compounds of extracts were evaluated according to the methodology proposed by Aparicio-Fernández et al. [24]. Briefly, 1 mL of extract was mixed with 1 mL of Folin–Ciocalteu (0.1 N) reagent, and after 3 min, 1 mL of Na₂CO₃ (0.5%) solution was added. The reaction was left for 30 min at 20 ± 3 °C and kept in a dark environment, and the absorbance was read at 765 nm using a Jenway UV-Vis spectrophotometer (model 6405, Staffordshire, UK). Total phenolic compounds were quantified using a standard curve of gallic acid (GAE), and the results were expressed as mg GAE/g db (dry basis).

2.7. The Total Betalains in Fruit Powders That Contain Them in Their Composition

The total betalains (betacyanins plus betaxanthins) in beetroot and dragon fruit were evaluated following the methodology proposed by Aparicio-Fernández et al. [24]. In brief, 5 mL of extract was mixed with McIlvaine buffer until an absorbance between 0.9–1.0 at 535 nm for betacyanins and 483 nm for betaxanthins was reached. For the quantification of betalains, a molecular weight of 550 g/mol and a molar extinction coefficient of 60,000 L/mol cm were used, while for betaxanthins, a molecular weight of 308 g/mol and a molar extinction coefficient of 48,000 L/mol cm were employed. Data were used as per Equation (2):

$$B ={\displaystyle \frac{A\times DF\times MW\times 1000}{\epsilon \times 1}}$$

(2)

where B is the betacyanin or betaxanthin content in db (mg/g), A is the absorbance, DF is the dilution factor, MW is the molecular weight (g/mol), 1 is the cell path (1 cm), and ε is the molar extinction coefficient. The total betalains is the sum of both betacyanins and betaxanthins.

2.8. The Total Anthocyanins in Fruit Powders That Contain Them in Their Composition

The content of total anthocyanins in the acai, blueberry, cranberry, and pomegranate was determined according to the pH differential method reported by Vega-Arroy et al. [25]. Five mL of extract was mixed with hydrochloric acid (1 M) or sodium hydroxide (1 M) to attain a pH of 1 or 4.5, respectively. The absorbance of the extracts was read at 520 and 700 nm using a UV-Vis spectrophotometer. Total anthocyanins were expressed as cyanindin-3-glucoside equivalents and calculated using Equations (3) and (4):

$$A={\left({Abs}_{520}-{Abs}_{700}\right)}_{pH=1}-{\left({Abs}_{520}-{Abs}_{700}\right)}_{pH=4.5}$$

(3)

$$TA={\displaystyle \frac{A\times DF\times MW\times 1000}{\epsilon \times 1}}$$

(4)

where A is the difference in absorbance, DF is the dilution factor, MW is the molecular weight of cyanidin-3-glucoside, 1 is the cell path (1 cm), and ε is the molar extinction coefficient (26, 900 L/mol cm).

2.9. Antioxidant Capacity (DPPH Assay) of Antioxidant-Rich Fruit and Apple Powders

The antioxidant capacity of extracts was determined following the methodology proposed by Hernández-Carranza et al. [23]. Thus, 1 mL of extract was mixed with 1 mL of DPPH radical (0.004%), and this was allowed to stand for 30 min in a dark environment at 20 ± 3 °C. The absorbance was measured at 765 nm using a UV-Vis spectrophotometer. The antioxidant capacity was evaluated using a standard curve of Trolox, and the results were expressed as mg Trolox/g db.

2.10. Reducing Power of Antioxidant-Rich Fruit and Apple Powders

Reducing power was assessed following the methodology proposed by Dorman et al. [26]. As such, 1 mL of extract was mixed with 2.5 mL of potassium hexacyanoferrate solution (1% w/v) and 2.5 mL of phosphate buffer (0.2 M), and it was then left to stand for 30 min at 50 °C. Afterward, 2.5 mL of trichloroacetic acid solution (10% w/v) were added, and the mixture was centrifuged for 10 min at 1000 rpm. Lastly, the supernatant was taken and mixed equally with water and 0.5 mL of ferric chloride solution (0.1% w/v). Absorbance was read at 700 nm using a UV–Vis spectrophotometer. Reducing power was expressed as mg of ascorbic acid (AA)/g db.

2.11. Statistical Analysis

Both treatments and assays were conducted in triplicate. All results were analyzed by comparison of means (α = 0.05) through analysis of variance (ANOVA) using Tukey’s test in Minitab 15 software (Minitab Inc., State College, PA, USA).

3. Results and Discussion

3.1. Bioactive Compounds and Antioxidant Capacity-Rich Fruit Powder

In this study, the visual color stability, bioactive compounds, and antioxidant capacity of red–blue fruit powders, namely acai, beetroot, blueberry, cranberry, dragon fruit, and pomegranate, were evaluated before and after heat treatment (120 °C for 15 min) (Figure 2 and Figure 3). Figure 3A shows that the total phenolic compounds in the extracts presented values higher than 10 mg GAE/g in the following ascending order: cranberry, acai, beetroot, dragon fruit, blueberry, and pomegranate. Furthermore, Pearson correlation indicated that the anthocyanins and/or betalains were primarily responsible for the antioxidant capacity (R > 0.904) and reducing power (R > 0.629) of the evaluated powder extracts. The pomegranate extract had the highest total phenolic compounds (>180 mg GAE/g) and total anthocyanins (>1.90 mg cyanidins/g), giving the highest antioxidant capacity evaluated by both DPPH radical inhibition (274.89 ± 18.39 mg Trolox/g) and reducing power (933.96 ± 57.30 mg AA/g). On the other hand, the cranberry extract showed the lowest total phenolic compounds (anthocyanins were not detected) and antioxidant capacity (DPPH and reducing power assays).

After heat treatment, the Pearson correlation exhibited a higher value (R > 0.872,

Table 1. The Pearson correlation of the bioactive compounds and antioxidant capacity of antioxidant-rich powder extracts.

	TPCs	TAs|TBs	AC	RP
TPCs a	1	0.591 †/0.888 ‡	0.625/0.967	0.808/0.872
TAs b|TBs c		1	0.904/0.931	0.629/0.975
AC d			1	0.403/0.898
RP e				1

^a TPCs: total phenolic compounds. ^b TAs: total anthocyanins. ^c TBs = total betalains. ^d AC: antioxidant capacity. ^e RP: reducing power. ^† before heat treatment. ^‡ after heat treatment.

), indicating that heat treatment did not affect the impact of bioactive compounds on the antioxidant capacity of powder extracts. Nevertheless, the values of the bioactive compounds and antioxidant capacity were affected depending on the powder extracts. In this sense, the dragon fruit powder extract showed the lowest antioxidant stability, displaying a higher decrease in phenolic compounds (80.99%), betalains (90.41%), antioxidant capacity (54.34%), and reducing power (89.65%). On the contrary, although a significant decrease (p < 0.05) of 48.68 and 87.23% in the total anthocyanins and reducing power was observed in the pomegranate extract, it presented the highest total phenolic compounds and antioxidant capacity (p < 0.05), even after heat treatment. In this sense, Zhao and Yuan [27] have pointed out that anthocyanins appear to be responsible (72%) for the antioxidant capacity of pomegranate pulp; however, the peel contains a higher antioxidant capacity than pulp [28], being tannins (heat stable compounds), the most important phenolic compounds, which may explain the maintenance of the color and antioxidant capacity of the pomegranate extract, even with a reduction of the anthocyanins after heat treatment. Moreover, Mena et al. [29] have evaluated the effect of thermal treatment (65–90 °C for 30–60 s) on pomegranate juice, and the results indicate that the total phenolic compounds, anthocyanins, and antioxidant capacity were not affected by the process applied. Therefore, according to the higher bioactive compounds and antioxidant capacity, even after thermal treatment, pomegranate powder was selected for further study.

3.2. Effect of pH and TSSs on the Stability of Bioactive Compounds and Antioxidant Capacity of Pomegranate Extract

Although pH and the TSSs are two of the most important factors in food quality and processing, and even though pomegranate is one of the most studied products in its bioactive compounds and antioxidant capacity (to the best of the authors’ knowledge), the effect of these factors on the antioxidant stability has not been studied yet. Therefore, in this study, the effect of pH (citric acid) and TSSs (sucrose) on the antioxidant stability of pomegranate extract was evaluated in the range of 3–7 and 5–15 (%), respectively. The results are presented in

Table 2. The bioactive compounds and antioxidant capacity of the pomegranate extracts obtained under different conditions of pH and total soluble solids (TSS) ^a.

Treatment	pH	TSSs (%)	TPCs b (mg GAE/g)	TAs c (mg Cyanidin/g)	AC d (mg Trolox/g)	RP e (mg AA/g)
1	3	5	125.70 ± 0.92 a	2.27 ± 0.01 b	203.44 ± 4.18 ab	510.70 ± 4.48 a
2	7	5	69.20 ± 6.32 b	0.26 ± 0.05 d	142.52 ± 11.47 cd	265.74 ± 2.56 c
3	3	15	118.61 ± 3.22 a	2.39 ± 0.10 b	203.62 ± 3.84 ab	614.85 ± 2.31 bc
4	7	15	82.04 ± 3.66 b	0.05 ± 0.04 e	110.23 ± 28.81 d	453.94 ± 4.99 bc
5	3	10	118.49 ± 3.77 a	2.65 ± 0.13 a	220.89 ± 3.97 a	718.57 ± 4.00 bc
6	7	10	67.77 ± 2.77 c	0.02 ± 0.03 e	174.04 ± 17.99 bc	497.48 ± 2.31 bc
7	5	5	123.47 ± 9.40 a	1.50± 0.06 c	207.57 ± 1.34 ab	466.10 ± 1.11 bc
8	5	15	120.81 ± 3.60 a	1.70 ± 0.03 c	163.30 ± 12.43 c	344.63 ± 4.62 bc
9	5	10	118.12 ± 4.45 a	1.70 ± 0.04 c	163.30 ± 12.91 c	390.23 ± 5.47 b

^a Average ± standard deviation. ^b TPCs: total phenolic compounds. ^c TAs: total anthocyanins. ^d AC: antioxidant capacity. ^e RP: reducing power. Different letters (a, b, c, d and e) in the same column represent statistical differences (p < 0.05).

. As was observed, the total phenolic compounds, total anthocyanins, antioxidant capacity, and reducing power were in the range of 67.77 to 125.70 mg GAE/g, 0.02 to 2.65 mg cyanidin/g, 110.23 to 220.89 mg Trolox/g, and 265.74 to 718.57 mg AA/g, respectively. Similar values (9.8 mg GAE/g fresh weight) of phenolic compounds were obtained by Masci et al. [30] in whole pomegranate extract, and slightly higher values (7.1–17.0 mg GAE/g) were obtained over those reported by Rababah et al. [31] in dry powder of whole pomegranate. Moreover, the total anthocyanins were found to be like the values (2.6 mg/100 g fresh weight) reported by Masci et al. [30] in whole pomegranate extract, confirming that ellagitannins and ellagic acid are the main phenolic compounds in pomegranate peels, while in the pulp, the 3-glucosides and 3,5-diglucosides of cyanidin, delphinidin, and pelargonidin are the most abundant anthocyanins [31]. Similarly, aqueous extracts from the pomegranate husk presented values of antioxidant capacity (4 mM Trolox) and FRAP (3 mM AA) like those obtained in this study, with values of 0.44–0.80 mM Trolox and 1.51–4.04 mM AA for DPPH and reducing power assays, respectively [32].

The response surface models (

Table 3. The regression coefficient of the response surface model used for predicting the effect of pH and the total soluble solids (TSSs) on the compounds and antioxidant capacity of pomegranate extracts.

Compounds	β0	β1	β2	β3	β4	β5	R2
TPCs a	97.03	35.60	−8.78	−5.26	0.32	0.50	0.96
TAs b	1.89	0.30	0.08	−0.08	−0.00	−0.01	0.99
AC c	299.57	−36.32	2.20	2.77	−0.03	−0.81	0.72
RP d	1348.77	−424.15	45.03	35.90	−2.49	2.10	0.74

^a TPCs: total phenolic compounds. ^b TAs: total anthocyanins. ^c AC: antioxidant capacity. ^d RP: reducing power. Bold numbers indicate significant parameter estimates (p < 0.05).

and Figure 4) showed good reproducibility of the experimental data (R² > 0.95) for bioactive compounds. It was found that pH significantly influenced the stability of bioactive compounds and the antioxidant capacity of pomegranate extract. Specifically, the antioxidant capacity increased as the pH decreased, with the lowest values observed at pH 7. This tendency also affected the color of the extracts (Figure 1 shows the reduction in red color at pH 3 due to flavylium cation species to a yellowish color at pH 7 due to pale yellow chalcone) [33]. In this sense, low pH decreases glycoside compounds, forming aglycones, which have more hydroxyl groups that are able to donate hydrogen to reduce oxidative substances [34]. On the contrary, the TSSs in the range evaluated did not affect the stability of the bioactive compounds and antioxidant capacity from pomegranate extracts (p > 0.05), even with the interaction of both factors (pH and TSSs), indicating that sucrose did not interact with antioxidant molecules of pomegranate extract without affecting the antioxidant quantification. Therefore, we can conclude that reducing the pH of pomegranate extracts increases their bioactive compound availability for antioxidant quantification within the range studied. However, it is interesting to note that, despite an increase in the bioactive compounds observed with the reduction in pH, the values obtained with the response surface model did not attain the bioactive compounds and antioxidant capacity values obtained in the pomegranate extract without pH (3.4) modification.

3.3. Impregnation of Bioactive Compounds from Pomegranate Extracts at Different pH to Apple Slices

Based on the previous results obtained, pomegranate extracts at different pH (2.0, 3.0, 3.4, and 5.0) without adding sucrose were obtained and used to impregnate antioxidant compounds into apple slices as a food matrix. In this sense, as observed in this study, sucrose did not affect the bioactive compounds and antioxidant capacity of the pomegranate extracts.

As mentioned by several authors, the use of solutes in the impregnation process can reduce the inclusion of antioxidant compounds from extracts into fruit products [14,15]. Nevertheless, extracts without pH modification that were reduced to values of 2 were also evaluated for impregnating procedures. As was observed, the highest bioactive compounds and antioxidant capacity were achieved in apple slices impregnated with the pomegranate extract obtained at the lowest pH tested (pH = 2.0), increasing the total phenolic compounds, total anthocyanins, antioxidant capacity, and reducing power to 44.63, 800.00, 37.62, and 70.86%, respectively, compared to control apple slices, as well as to 9.80, 658.55, 7.70, and 27.48%, respectively, compared to apple slices impregnated with pomegranate extract without pH modification (pH = 3.4). It is of paramount importance to note that the total anthocyanins in both extracts and apple slices were significantly higher as the pH reduced, but at medium pH (3–5), this increment did not affect the total phenolic compounds, antioxidant capacity, and reducing power, indicating that there are other compounds in pomegranate extracts affecting these responses (such as, for example, tannins); however, at low pH (1–2), an increase in all antioxidant assays was observed, probably due to the anthocyanin structure being protonated (flavylium cation) at this acid condition, thus showing the ability to donate (H⁺), which can act as a reducing agent for stabilizing the oxidant compounds of antioxidant assays [35,36].

The results obtained in this study are consistent with those reported by Hernández-Carranza et al. [15]. They impregnated apple wedges with Hibiscus sabdariffa extract at a ratio of 1:5 (solid to extract) and observed increases of 77.0% and 103.7% in the total phenolic compounds and antioxidant capacity, respectively. However, they conducted their experiments with a longer impregnation time of 6 h and at a higher temperature of 50 °C, whereas this study used an impregnation time of only 5 min at a temperature of 20 ± 3 °C. In this respect, the impregnation process of the bioactive compounds from solution into the food matrix was influenced by various intrinsic and extrinsic factors. Intrinsic factors include the structure of the sample (whether intact or collapsed), its physicochemical characteristics (such as pH, titratable acidity, total soluble solids, and maturity index), and the initial concentrations of the bioactive compounds present. Extrinsic factors involve the geometry of the system, the physicochemical properties of the surrounding medium, the concentration of bioactive compounds, the temperature, the time, the pressure, and the technology used [37,38]. The findings of this study are highly significant as the apple slices exhibited a marked improvement in their bioactive compounds and antioxidant capacity, along with an appealing red color (Figure 5). This enhancement was achieved by using pomegranate extract with a short impregnation time and by employing green technology.

4. Conclusions

Among the fruit powders rich in antioxidant compounds that were evaluated, pomegranate presented the highest bioactive compounds and antioxidant capacity, regardless of the heat treatment. The total soluble solids in the range evaluated did not affect the antioxidant compounds from pomegranate; however, by reducing the pH of the extract, a significant positive effect was observed in the antioxidant compound quantification. Apple slices impregnated with pomegranate extract under selected conditions significantly enhance their bioactive compounds and antioxidant capacity, giving the apple slices an attractive red color through a short impregnation time, which was achieved by utilizing green technology. Though different studies are necessary for optimizing the impregnation condition of bioactive compounds from pomegranate extract to apple slices, the results obtained in this study clarify the way to promote the inclusion of antioxidant compounds in apple slices.