Valorization of plum peels for recovery of phenolic compounds by novel extraction technologies: Characterization, optimization, encapsulation, and functional food application

Abstract

During the last two decades, the use of fruits and vegetables and their wastes as sources of phenolic compounds became widespread due to the various health benefits of these components such as antioxidant, anticancer, anti-diabetic, neuroprotective, anti-inflammatory and antimicrobial effects. The richness of the extracts in phenolic compounds is too much related to the source from where they are extracted as well as the extraction method used and its parameters. In this regards, novel extraction techniques were developed and optimized in order to improve the quality and the yield of extraction comparing to conventional method. However, after extraction, phenolic compounds become prone to degradation through different reactions that may reduce their functional properties and stability. Thus, encapsulation became widely adopted nowadays to protect these bioactive compounds from degradation, to facilitate their storage and to allow their gradual release in the correct time and place (in food products and/or in the human body). Spray drying is one of the best methods for the encapsulation of phenolic compounds but its efficiency strongly depends on the process parameters and the coating materials utilized. In this context, plum (Prunus domestica) peels were selected as the source of phenolic compounds for this research due to the limited studies dealing with the extraction and encapsulation of its phenolic compounds according to the literature. In this research, microwave assisted extraction and ultrasound assisted extraction of phenolic compounds from plum peels were optimized and compared to conventional solvent extraction in terms of total phenolic compounds and total antioxidant capacity. The spray drying encapsulation process was optimized using response surface methodology and the physicochemical and thermal properties as well as the storage stability of the encapsulated powder obtained under optimum conditions were evaluated. Utilization of encapsulated powder in functional food application and the effect on its sensory and physicochemical properties, bioaccessibility of phenolic compounds and storage stability of the food matrix were investigated. Conventional solvent extraction (CSE) was optimized using response surface methodology (RSM). The effect of four independent variables namely extraction temperature (20-60°C), ethanol concentration (40-80%), liquid/solid ratio (30:1-70:1 mL/g) and extraction time (30-120 min) on the extracted amount of total phenolic compounds (TPC) was investigated. Results showed that only temperature and liquid/solid ratio affected significantly (p<0.05) the TPC values. The determination coefficient (R2) was equal to 98.54%. The optimum extraction conditions were as follow: extraction temperature, 60 °C; ethanol concentration, 50%, liquid/solid ratio, 70 mL/g; and extraction time, 30 min. Under these optimal conditions, the experimental value of TPC (37.92 mg GAE/g dw) was in strong agreement with the predicted one (37.89 mg GAE/g dw). For the optimization of microwave assisted extraction (MAE) of phenolic compounds from purple plum (Prunus domestica) peels using response surface methodology, a Box Behnken Design (BBD) was arranged and the effect of three independent variables namely microwave power (140-700 W), irradiation time (30-150 s) and liquid/solid ratio (30:1-70:1) on total phenolic compounds (TPC) and total antioxidant capacity (CUPRAC) of extracts was investigated. The three factors showed significant effects (p<0.05) on TPC and CUPRAC values. The determination coefficients (R2) for TPC and CUPRAC were 99.07% and 99.64% respectively and the lack of fit wasn't significant (p>0.05) for both responses. The optimum extraction conditions were as follows: microwave power, 700 W; irradiation time, 135 s and liquid/solid ratio, 70 mL/g. Under these conditions, the experimental values of TPC (37.77 mg GAE/g dw) and CUPRAC (93.84 mg TEAC/g dw) were in strong agreement with the predicted ones (37.16 mg GAE/g dw for TPC and 93.66 mg TEAC/g dw for CUPRAC). As for the RSM optimization of ultrasound assisted extraction (UAE), the Box Behnken Design (BBD) was arranged with three independent variables which are ultrasonic power (10, 35 and 60%), ultrasonication time (0.5, 3 and 5.5 min) and liquid/solid ratio (30:1, 50:1 and 70:1) and the dependent variables were the total phenolic compounds (TPC) and total antioxidant capacity (CUPRAC) of extracts. The three factors showed significant effects (p<0.05) on the response variables TPC and CUPRAC. The determination coefficients (R2) for TPC and CUPRAC were 97.35% and 98.51% respectively. The optimum extraction conditions were as follows: ultrasonic power, 40%; sonication time, 5.5 min and liquid/solid ratio, 70 mL/g. The experimental values of TPC (38.75 mg GAE/g dw) and CUPRAC (94.81 mg TAEC/g dw) measured under these optimum conditions were in strong agreement with the predicted values (38.04 mg GAE/g dw for TPC and 94.42 mg TEAC/g dw for CUPRAC). Comparing to conventional solvent extraction (CSE), MAE at optimum conditions gave similar TPC, CUPRAC, DPPH and FRAP values, higher ABTS value and reduced the extraction time from 30 min for CSE to 135 s. In addition, the optimized UAE produced similar TPC, DPPH and FRAP values, higher CUPRAC and ABTS values and reduced the extraction time from 30 min to 5.5 min. MAE and UAE are easier and less time consuming than CSE. In addition, they can preserve the antioxidant capacity of phenolic compounds in a better way and they are more efficient for large scale extraction. Particularly, MAE extraction is the fastest and the easiest between the three methods. It is less energy consuming than the two other methods and needs less efforts and control during extraction process. For these reasons, MAE was selected for preparing the phenolic extract that was used for encapsulation studies. The HPLC-DAD (Diode Array Detector) analysis of the optimized CSE, MAE and UAE extracts revealed the presence of catechin, rutin, cyanidin-3-galactoside and cyanidin-3-glucoside with slightly different amounts. These molecules are famous for their potential functional properties and health benefits. Optimum conditions of encapsulation of plum peels extract using spray drying was investigated. Maltodextrin percentage in the coating material (30% - 70%), coating to core ratio (2:1 - 4:1) and air inlet temperature (130 °C - 190 °C) were selected as the independent variables according to a Box Behnken design, and their effects on response variables (encapsulation yield, encapsulation efficiency and moisture content) were evaluated. Optimum conditions were a maltodextrin percentage of 60%, a coating to core ratio of 4:1 and an inlet temperature of 190 °C. Under these conditions, the experimental response variables were an encapsulation efficiency of 94.64%, an encapsulation yield of 76.97% and a moisture content of 1.29 % and were in strong agreement with predicted values. Characterization of optimum powder properties was realized by measuring solubility, hygroscopicity, water activity, bulk density, total phenolic content, total antioxidant capacity and color parameters. In addition, advanced analyses like particle size distribution, zeta potential, scan electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) were conducted to further investigate the structure and the stability of the powder. Furthermore, the antioxidant capacity and color stability of the encapsulated phenolic extract in the optimum conditions were evaluated during a storage period of two months. Results revealed that the optimum microcapsules had high solubility and encapsulation efficiency, acceptable electrical and thermal stability and good morphological structure. Microcapsules color and antioxidant capacity were preserved during storage. Cacao cupcakes were enriched with the encapsulated phenolic compounds by replacing 1, 2 and 3% of the flour with the optimum encapsulated powder. The effect of this enrichment on the sensory attributes and storage stability of cakes, and the bioaccessibility of phenolic compounds in this food product were investigated. Results showed that the enriched cakes with 1% phenolic microcapsules were more acceptable than those enriched with 2% and 3%. A light unpleasant bitter taste especially for cakes enriched with 3 % phenolic microcapsules was detected by panelists. The enrichment of cakes did not negatively affect the crust and crumb colors according to the sensory results and color measurements. This enrichment slightly increased the phenolic content of cacao cupcakes and improved their antioxidant capacity. For the storage stability, the peroxide values were low and stable for all cakes and weren't influenced by the addition of microcapsules. The phenolic content, the antioxidant capacity and the color of cakes did not change too much during storage revealing the stability of cakes during 2 weeks at room temperature. The high bioaccessibility of phenolic compounds for control and enriched cakes in both gastric and intestinal phases implied that the produced cakes can be potential sources of phenolic compounds with various health benefits. The conducted studies showed that RSM was effective in optimizing the extraction and encapsulation of phenolic compounds from plum peels. MAE and UAE were shown to be more efficient than CSE. The obtained microcapsules were of high quality with low moisture content, high yield and encapsulation efficiency, and good storage stability. When added to cacao cupcakes, the microcapsules enhanced their phenolic content and antioxidant capacity, without affecting their color or texture. However, they did give a slight bitter taste and aftertaste. Overall, the study suggests that these cakes could potentially provide various health benefits due to their high phenolic content. It is recommended to add the obtained phenolic microcapsules to other products such as cheese, bitter chocolate or beverages due to the absence of using very high temperatures during their process. The phenolic microcapsules can also be used in pharmaceutical products. Besides, the optimum conditions for extraction and spray drying encapsulation can be utilized for phenolic compounds from other similar sources. In addition, further studies trying the encapsulation of phenolic compounds from plum peels using other techniques and other different coating materials are needed in order to effectively mask the unpleasant bitter taste and make their addition to different food products unremarkable and more efficient.