2.1. Ultrasound-Assisted Extraction of Soluble Polyphenols from Soursop Samples
The experimental data for soluble polyphenols (SP) obtained by UAE from soursop peel, columella, seeds, and pulp are shown in
Table 1. Statistical differences (
p < 0.01) were observed between treatments (experimental conditions: X
ET = extraction time (5, 10, and 15 min), X
PC = pulse cycle [0.4, 0.7, and 1 s], and X
SA = sonication amplitude [40%, 70%, and 100%]) and raw materials (seeds, peel, columella, and pulp). The extraction of SP from soursop pulp and by-products was dependent on the experimental conditions. The highest SP contents from peel were 157.40 and 156.29 mg/g DM in samples treated for 5 and 15 min (X
PC, 0.7 s; X
SA, 100%), respectively. Under the same experimental conditions (X
ET = 5 min, X
PC = 0.7 s, X
SA = 100%), the highest SP content in seeds was obtained (28.38 mg/g DM), and comparable values were observed in powder seeds after 10 min of exposure at 100% sonication amplitude (X
SA) and 0.7 s pulse cycle (X
PC). In contrast to peel and seeds, the highest SP values for columella (143.35 mg/g DM) and pulp (23.62 mg/g DM) were observed when the lowest values for the experimental conditions were applied (X
ET = 10 min, X
PC = 0.4 s, X
SA = 40% and X
ET = 5 min, X
PC = 0.7 s, X
SA = 40%, respectively). León-Fernández et al. [
22] reported an SP content of 35.6 mg/g (DM) in soursop pulp extract after applying extraction with an ultrasonic bath (3 h, 42 kHz at 25 °C) and using methanol as solvent. In this work, a 11-fold reduction in extraction time was achieved compared with the report of León-Fernández et al. [
22]. Several studies have shown that UAE could be used as an efficient method for the extraction of polyphenolic compounds from plant material [
13,
16].
Other authors have argued that the mechanical effects generated by UAE modify the morphology of the cell walls, and a more porous surface is obtained, thus facilitating the extraction [
23]. Thus, the components in the free state migrate from the material to the solution. In addition, the ultrasonic treatment accelerates the molecular motion of the solution and hence helps to quickly and effectively combine the components with the solution [
24].
On the other hand, peel and columella showed higher (
p < 0.01) SP content than pulp and seeds after UAE. These results may be attributable to the composition and complexity of each matrix and because the peel protects the fruit from external factors, and the columella is a conduit of nutrients and protects the seeds [
25]. Similar trends were previously reported by Albuquerque et al. [
26] who reported higher phenolic content in peel (1.7–1.9 g/kg) than in pulp (0.30–1.20 g/kg) and seeds (0.30–0.40 g/kg) from
Annona cherimoya Mill.
The significant interaction effects (
p < 0.01) of the UAE parameters on the SP content in peel (
Figure 1A), seeds (
Figure 1C), columella (
Figure 1E), and pulp (
Figure 1G) are shown in three-dimensional response surface plots, where the shapes contour plots (elliptical form) indicate the interactions between the corresponding variables [
27]. Results indicated that SP extraction by UAE was observed in the whole experimental domain and in all vegetal material studied, independent of the extraction time, pulse cycle, and sonication amplitude used. However, the highest SP content from peel (
Figure 1A) and seeds (
Figure 1C) can be obtained at 15 and 5 min, respectively, with medium pulse cycles (0.7 s) and sonication amplitude (100%). Conversely, the highest SP values in columella (
Figure 1E) and pulp (
Figure 1G) can obtained at lower experimental conditions (X
ET = 7.5 min, X
PC = 0.4 s and X
ET = 5 min, X
PC = 0.7 s, respectively), in particular for the sonication amplitude (X
SA, 40%). In addition, the Pareto charts (
Figure 1B,D,F,H) show the effect of independent variables on the extraction of SP at a confidence level of 95%. The effect (negative or positive) of the variables on SP extraction is dependent on each matrix, and the effect of the independent variables (and their interactions) on each vegetal material could be ranked as follows: for pulp, X
ET > X
SA > X
PC; peel, X
PC > X
SA > X
ET; columella, X
PC > X
SA > X
ET; and seeds, X
ET > X
PC > X
SA. Xu; and Pan [
28] reported that extraction efficiency for all-trans-lycopene from red grapefruit is a function of the independent variables (extraction time, temperature, solvent/material ratio, ultrasonic intensity, and duty cycle). As in this study, the UAE conditions for SP compounds from soursop samples were dependent on the plant matrix [
25].
RSM for the extraction of SP from soursop pulp, peel, columella, and pulp by UAE as a function of extraction time, pulse cycle, and sonication amplitude was developed using a multiple regression technique. A second-order polynomial equation (
Table 2) for each material was obtained, which described the effect of the test variables as well as the combined effect of all test variables on the response [
21]. The analysis of variance of the SP content showed that the experimental data from pulp, columella, peel, and seeds have good correlation (R
2 = 0.77, R
2 = 0.94, R
2 = 0.91, and R
2 = 0.97, respectively), and thus, adequate adjustment of the experimental data to the model was observed (lack of fit,
p > 0.05), except for pulp, which showed weak adjustment (R
2 = 0.77) (
Table 2). The lack of fit demonstrated the fitness of the model, giving an indication of approximation to the real system. Comparable results were reported by Bashi et al. [
29] during optimization (extraction temperature, pH, liquid/solid ratio, and extraction time) of UAE of phenolic compounds from
Achillea biebersteinii using RSM (R
2 = 0.85).
Some coefficients (
Table 3) were not significant (
p > 0.05) to the model: for peel, X
ET and X
SA2*X
ET, in pulp, X
SA, X
ET, X
SA*X
PC, X
SA2*X
PC, and X
SA2*X
ET. On the other hand, all coefficients for seed and columella were significant (
p < 0.05) to the model.
Pan et al. [
16] mentioned that when UAE is applied in plant materials (e.g., pomegranate peel) the intensity level and pulse duration has a prominent effect on the polyphenolic extraction yield; and a long extraction time had a negative effect. However, the authors mentioned that efficacy of treatment depended on the matrix composition and the cellulose-hemicellulose-lignin ratio. In addition, a slight increase in temperature from 1 to 2.88 °C (
Table 1) for all UAE runs was observed in a normal reaction when UAE was applied as a consequence of interaction of cavitation and extraction time. This temperature increase can be also attributable to heat transference from cavitation bubbles, which may cause a gradual temperature increase in the medium and facilitate extraction of the compounds. Similar trends were reported previously during UAE (extraction time, 2 min; pulse cycle, 0.7 s; sonication amplitude, 55%) of polyphenolic compounds from muicle leaf extracts [
16]. However, there was no drastic increase in temperature because the temperature was controlled during the experiments.
2.2. Evaluation of Model Reliability and Comparison of Ultrasound-Assisted Extraction with Conventional Extraction
The optimum UAE conditions for SP extraction from
A. muricata peel, seeds, columella, and pulp are shown in
Table 4. To verify the reliability of the model, treatments were performed using the optimal conditions for each sample as recommended by Aydar et al. [
19]. The experimental values of the SP content (
Table 5) for all samples were in agreement with the predicted values under the optimal conditions (see
Table 4). Therefore, an efficient extraction of polyphenols from soursop samples using UAE was possible, as has been reported by other studies where RSM was used to optimize the extraction of polyphenols by UAE from pomegranate [
17], orange [
18] peel, muicle [
16], and starfruit leaves [
13].
The SP content from peel and columella, hydrolyzable polyphenol (HP) content from peel and seeds, and condensed tannin (CT) content from pulp were significantly higher (
p < 0.01) when UAE was used compared with conventional extraction (
Table 5). However, there was no significant difference (
p > 0.01) in the SP content from seeds and pulp, HP content from columella and pulp, or CT content from peel, seeds, or columella for both extraction methods; however, the extraction time was significantly reduced to 45–55 min. Also, the total polyphenol content was higher using UAE in all samples except pulp, and the yield of phenolic content from peel and columella was higher under UAE (32.34–37.51%) than for organic aqueous extraction (14.87–16.68%). Cvjetko Bubalo et al. [
30] reported an increase (more than 30%) in the extraction of SP in grape peel when the samples were treated by UAE (15 min, 60 W, 20 °C) compared with conventional extraction. Velásquez-García et al. [
31] added a pretreatment with ultrasound in the extraction of bioactive compounds from mesocarp of chontaduro (
Bactris gasipaes) using supercritical CO
2, which did not increase the extraction yield, but the operation time was shorter than for conventional extraction. Sousa et al. [
32] found that UAE was inefficient for extracting ellagitannins. These authors mentioned that this can be justified because the UAE process produces a large amount of energy, which may lead to cleavage or oxidation of molecules, mainly those with a higher molecular mass.
The effect of ultrasound is attributed to interaction with the plant material, altering its physical and chemical properties, and to a cavitation effect, which facilitates the release of extractable compounds and enhances mass transport by disrupting the plant cell walls [
15]. Our results are in agreement with Jovanović et al. [
33], who compared UAE and maceration extraction methods for extraction of polyphenols from
Thymus serpyllum L. herb. They reported that the polyphenolic yield obtained by UAE was higher (22.81 mg/L) than the maceration extraction method (18.12 mg/L), and indicated that UAE could be used for extraction of polyphenols in the future due to the high yield of polyphenolic and flavonoid content, short extraction time, and low degradation of the extracted compounds compared with other extraction methods.