Based on statistical analysis, the punicalagin content was significantly (p ⩽ 0.05) affected by the single effect of time and interaction effect of time and intensity level; while the single effect of intensity level and duty cycle depicted insignificant (p > 0.05) effects on the punicalagin content (Table 1). A high R2 value (0.986) represented that the final reduced model could accurately predict the changes of punicalagin content as a function of ultrasound variables (Table 2). Fig. 3b shows the positive interaction effect of extraction time and ultrasonic wave intensity on the content of punicalagin. The punicalagin content was gradually increased with increasing of intensity level at a fixed extraction time; while the punicalagin content was greatly increased by extending the extraction time at a fixed intensity level. For instance, the increment in punicalagin content was 6 mg/g (from ∼140 to 146) when the intensity level was increased from 52.5 W/cm2 to 105 W/cm2 during 10 min of extraction period. However, extending the extraction time from 2 to 10 min at intensity level of 105 W/cm2 led to a 16 mg/g increment in the punicalagin content (from ∼130 to 146 mg/g). Numerical optimization also represented higher positive effect of extraction time than intensity level on the punicalagin content (Fig. 3c).
Optimization of the extraction process, in order to obtain the highest content of ellagic acid from pomegranate peel, is an important contribution to the food and pharmaceutical industries. The result of this study also indicated that ellagic acid content varied from 10.12 to 22.53 mg/g at different PUAE conditions. The lowest ellagic acid content (10.12 mg/g) was quantified after 2 min extraction, whereas the highest content was obtained when the extraction was carried out for 10 min. The ellagic acid content obtained under optimum PUAE condition in this study was considerably higher compared to Çam and Hışıl (2010) who obtained 1.25 mg/g ellagic acid in the extract of pomegranate peel which was prepared by applying pressurized water extraction for 10 min. The quantified value of ellagic acid in this study was also higher than the previous findings of Masci et al. (2016), who reported that 11.85 mg/g of ellagic acid in Israeli pomegranate peel extract was obtained by stirring in ethanol for 24 h. The variation in ellagic acid contents could be due to different extraction methods and conditions as well as different varieties of pomegranates used in these studies.
As displayed in Fig. 3d, the ellagic acid content was increased by prolonging the extraction process. The single effect of extraction time was the only significant (p ⩽ 0.05) factor affecting the content of ellagic acid in pomegranate peel extract (Table 1). In fact, the extraction of ellagic acid was noticeably time dependent (F-ratio = 71.84). This observation was in agreement with the findings reported by Jerman, Trebše, and Mozetič Vodopivec (2010). These researchers reported that the higher content of phenolic compounds from olive fruit was obtained by prolonging the duration of ultrasound extraction.
In this study, a very low content of gallic acid was quantified in pomegranate peel extract. In fact, the highest content of gallic acid in the extract was 0.051 mg/g which was much lower than the punicalagin and ellagic acid contents. Elfalleh et al. (2011) quantified gallic acid content in 6 different types of Tunisian pomegranate peel after extraction by stirring in methanol overnight. They reported that the gallic acid contents were between 1.09 and 1.31 mg/g and this was extremely higher than the gallic acid content (0.051 mg/g) determined in the current study. Table 1 showed that the interaction effect of extraction time and intensity level significantly (p ⩽ 0.05) influenced the gallic acid content; while the other terms did not noticeably affect the gallic acid content. The final reduced model showed high R2 value (0.926) when it was fitted based on gallic acid content (Table 2). Accordingly, about 92% of the variation in the gallic acid content could be explained as a function of significant PUAE variables. In order to visualize the interaction effect of time and intensity level on gallic acid content, a response surface plot was constructed (Fig. 3e).
The results illustrated that the gallic acid content was substantially increased by simultaneously extending the extraction time and increasing the intensity level. The maximum gallic acid content (0.051 mg/g) was obtained when the highest intensity level (105 W/cm2) and longest time (10 min) were applied for extraction (Fig. 3e). Fig. 3f showed that time had a more positive significant effect than intensity level on the recovery of gallic acid. The punicalagin and gallic acid contents were considerably affected by interaction effect of extraction time and intensity level. The most probable reason might be attributed to the application of ultrasound energy. In fact, the appl