Production of functional food ingredient by enzyme and ultrasound assisted extraction from lemon waste

Abstract

Lemon (Citrus limon) belongs to the family of Citrus fruits which are commonly consumed in daily diet due to their high nutritional value, sensory properties and low price. A large portion of lemon is discarded after processing by the food industry as waste up to 50-70% of fruit weight depending on the cultivar. Citrus wastes are currently discarded to the environment or used for animal feed and biofuel production and these practices cause loss of valuable natural resources. Lemon peel constitutes a large portion of industrial lemon wastes contain several bioactive components such as phenolic acids and flavanones which exhibit several health benefits including antioxidant, antiobesity, antiinflammatory, antihypertensive, antihypercholesterolemic, antimicrobial, antidiabetic and anticarcinogenic activity. Most of the studies in the literature have focused on the extractable phenolics; however, significant amounts of phenolics in fruit peels are also present in non-extractable form as entrapped or bound in the cell matrix. Non-extractable phenolics are protected from harsh environmental conditions in the fruit matrix. They represent a potential uncovered resource for bioactive phenolics that could be used in food, nutraceutical, pharmaceutical and cosmetic industries. Determination of the distribution and amounts of phenolic compounds in extractable and non-extractable fractions can help in upcycling of lemon wastes. Recovery of non-extractable phenolics require use of an additional hydrolysis method to that used for extractable phenolics that needs to be optimized for an efficient production. Lemon peel constitutes a cheap potential resource for bioactive phenolics with antioxidant, antihypertensive and antidiabetic activities. Recovery of these bioactive compounds from lemon waste with green technologies will contribute to the efforts towards sustainable food systems and circular bioeconomy. The objectives of this study were (i) measurement of concentration and antioxidant activity of extractable phenolic compounds present in local lemon varieties, (ii) application of different extraction methods including conventional heat-, acid-, base-, ultrasound-, enzyme-, and ultrasound-enzyme-assisted extractions for maximum recovery of non-extractable phenolics found in lemon peel, (iii) determination of the antioxidant, antihypertensive, and antidiabetic activities of extractable and non-extractable fractions of phenolics in lemon peel (iv) production of an encapsulated functional food ingredient from lemon peel extract. The first part of the thesis is focused on determination of phenolic compounds and their antioxidant activity in different local lemon varieties. In addition, the solvent type and solid:solvent ratio for an efficient extraction were determined. Interdonato, Lamas, Kara lemon and Kütdiken lemon varieties used in the fruit juice industry in Türkiye were chosen for the study. Lemon peels were cut into small pieces and dried with a freeze dryer. Dried peels were ground into a powder and then sieved. Lemon powder was extracted with ethanol/water (50:50, v/v) or distilled water with a solid:solvent ratio of 1:10 or 1:20 (g/mL). The resulting extracts contained extractable phenolics. Total phenolic content (TPC), total flavonoid content (TFC) and antioxidant activity by 2,2ʹ-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and cupric reducing antioxidant capacity (CUPRAC) assays were carried out. As a result of the analyses, TPC in lemon peels was found between 12.06±0.22-23.63±0.11 mg GAE/g dry matter, while the TFC was found between 10.02±0.99-30.10±0.33 mg RE/g dry matter. TPC of samples extracted with 50% aqueous ethanol was higher than that of the samples extracted with distilled water. Antioxidant activities of lemon peels were found in the range of 30.54±0.19-60.64±2.78 mg TE/g dry matter and 38.20±1.09-72.63±0.73 mg TE/g dry matter by ABTS and CUPRAC methods, respectively. It was revealed that Kara lemon had the maximum antioxidant activity, followed by Lamas, Interdonato, and Kütdiken variety. Considering all the results, 50% aqueous ethanol as a solvent, a sample:solvent ratio of 1:20 (g/mL), and the Lamas cultivar were chosen for further studies. The choice of the Lamas species was motivated by the fact that it is a local species unique to Mersin Province, Erdemli District, and is readily available. In the second part, effect of different extraction methods for recovery of non-extractable phenolics (NEP) in Lamas lemon peel was investigated. For this purpose, conventional heat-, acid-, base-, ultrasound-, enzyme-, and ultrasound-enzyme-assisted extractions were applied to the residue obtained after the extraction of extractable phenolics. To determine the maximum amount of non-extractable phenolics in the residues, acid and alkaline hydrolyses were applied. Alkali and then acid hydrolysis allowed the highest recovery. Preliminary experiments were performed to find the optimal parameters in all extraction methods. Preliminary trials were carried out for the conventional heat-assisted extraction method, with temperatures of 23, 40 and 60°C, duration of 40 min, 60 min, and 6 h. The best result was obtained by extraction at 40°C for 1 h. Ultrasound-assisted extraction (UAE) was applied at the amplitude level of 20 and 50%, temperature of 23 and 50°C, and duration of 5, 10, and 15 min. The optimum parameters obtained for maximum TPC, TFC and antioxidant activity were 50% ultrasound amplitude, the sample inlet temperature of 23°C and the extraction time of 15 min. When the effects of process parameters on bioactive compounds in ultrasound-assisted extraction were investigated, it was determined that the effect of ultrasound amplitude was statistically significant on TPC, TFC, and antioxidant activity with both ABTS and CUPRAC methods. Effect of extraction time had a significant effect TFC and antioxidant activity. In enzyme-assisted extraction, a lysing enzyme from Aspergillus sp. containing cellulase and pectinase, β-glucosidase, and a mixture of lysing enzyme and β-glucosidase were used. The residue was incubated at 40°C for 15 min, 60 min and 6 h. Hydrolysis with lysing enzyme for 60 min was found sufficient to release all phenolics. Ultrasound-enzyme-assisted extraction was carried out to determine if ultrasound in combination with enzyme would improve the extraction of phenolics. While in the first approach, enzyme and ultrasound were applied simultaneously, in the second way, ultrasound was applied to the residue first and then incubated with the enzyme. Application of ultrasound first and then enzyme hydrolysis provided higher TFC and antioxidant activity by ABTS and CUPRAC assays. Bioactivities of non-extractable phenolics from lemon peel obtained by conventional heat-, enzyme-, ultrasound-, and ultrasound-enzyme-assisted extractions were evaluated in comparison with those of the extractable phenolics. The antioxidant, angiotensin-I-converting enzyme (ACE), and α-amylase inhibitory activities and phenolic profile of the phenolic fractions were analyzed. While the extractable fraction had higher TPC, ascorbic acid content, and antioxidant activity, phenolic profile analysis indicated that the non-extractable fraction contained higher concentrations of phenolics, especially hesperidin and hesperetin. The concentrations of hesperidin and hesperetin in the non-extractable fraction were 270.9 mg/100 g dry weight and 415.9 mg/100 g dry weight, respectively, which were about two-fold higher than those present in the extractable fraction. Moreover, ACE and α-amylase inhibitory activities of non-extractable fraction were stronger than those of the extractable fraction. Total phenolic content, total flavonoid content, and antioxidant activity were increased by enzyme and ultrasound treatments compared to those by conventional heat treatment. However, ACE inhibitory activities of all the non-extractable fractions were similar while α-amylase inhibitory activity was higher in ultrasound- and ultrasound-enzyme-treated fractions. While ultrasound-assisted extraction slightly improved the yield of non-extractable phenolics, enzyme-assisted extraction yielded two-to four-fold increases in the amounts of individual phenolic compounds compared to heat-assisted extraction. Non-extractable phenolic fraction obtained by enzyme-assisted extraction from lemon peel was found to have a significant potential as an antihypertensive and antidiabetic agent. In the third part, encapsulation of the phenolic extract obtained by enzyme-assisted extraction with maltodextrin and whey protein concentrate was studied. Two maltodextrins with 6 and 19 dextrose equivalent (DE) were used. Two different drying methods including foam mat drying and foam mat freeze drying were applied. Encapsulated extract was analysed for encapsulation efficiency, moisture content, water activity, and glass transition temperature. In addition, encapsulated extract was stored at 40°C and 75% relative humidity for 5 d to determine storage stability. TPC, antioxidant activity, hesperidin content, ACE inhibitory activity, and α-amylase inhibitory activity were measured at the beginning and end of storage. Maltodextrin with 6 DE along with whey protein concentrate was found to be the best wall material. Foam mat-dried sample with maltodextrin 6DE showed the highest encapsulation efficiency, hesperidin content, storage stability and lowest hygroscopicity, while foam mat-freeze-dried sample with maltodextrin 6DE exhibited the highest TPC and antioxidant activity. There was no difference among ACE inhibitory activity of the samples before and after storage. ACE inhibitory activity was reduced after storage in all the samples. On the other hand, -amylase activity of the samples did not show a clear trend as a result of interference from the wall materials. Based on these findings, both foam mat-drying and foam mat freeze-drying can be chosen as the drying method in encapsulation. However, foam mat drying can be preferred as a practical method with low equipment and energy costs and short processing time. In this study, an encapsulated functional ingredient including both extractable and non-extractable phenolic compounds from lemon peel with significant antioxidant, ACE and α-amylase inhibitory activity was developed. The ingredient can be utilized for development of antihypertensive and antidiabetic functional foods or food supplements. Further in vitro and in vivo studies on stability, bioavailability, toxicity and health effects of the ingredient are recommended before its use.