Optimization of extraction method in potato peel waste and evaluation of antioxidant activity and phenolic content in various PEF treated potato varieties

The industrial processing of agricultural products generates large quantities of food waste. These wastes are either destroyed by causing environmental pollution or used as animal feed or fertilizer. In recent years, food, pharmaceutical and cosmetic industries focused on the evaluation of these by-products as natural antioxidants due to their bioactive compounds. This topic comes into prominence, especially since synthetic antioxidants, which have a large place in the food industry, exhibit carcinogenic and toxic effects. Potato (Solanum tuberosum L.) is one of the most important staple crops which grows all over the world. Given the yearly consumption of 50-60 kg per person and the production of approximately four million tons per year, potato is one of the most produced and consumed basic foods in Turkey. It plays a substantial role in human nutrition. Besides potatoes are consumed directly, processed potato products, such as French fries, chips, mashed potato, crisps, starch have an important place in the industry. In processing, 10–25% of the raw product is discharged as waste. Potato is a source of various bioactive compounds such as starch, dietary fiber, amino acids, minerals, vitamins and phenolic compounds. Phenolic compounds are synthesized by potato plant as protection response from bacteria, fungi, viruses, and insects. This study involves two stages; at the first stage, Innovator cultivar was used for the optimization of extraction conditions of phenolic compounds. Potato peel samples of Innovator cultivar were obtained from Pizzoli factory in Italy. The optimization of the extraction by ultrasound-assisted extraction (UAE) of phenolic compounds from potato peels was carried out by a Box-Behnken Design (BBD). The current design contained 27 experimental runs with three levels (-1, 0, 1) for each factor. The independent factors for the optimization of UAE were: ethanol-water ratio (50%, 75%, 100%), temperature of the bath (20 oC, 35 oC, 50 oC), time (15 min, 30 min, 45 min) and sample/solvent (s-s) ratio (1/20, 1/55, 1/90). Furthermore, Response Surface Method (RSM) was used to find out the combined effect of the factors on the quantification of the sum of phenolic compounds (SPC) via HPLC-DAD-ESI-MS (high-performance liquid chromatography-diode array detector-electrospray ionization interface-mass spectrometry). For Innovator variety potato sample, experimental values varied from 0.21 to 6.09 mg/g d.w. When the data is checked for each run, the lowest values were found to be working with 100% ethanol while the highest values were found to be working with 50% ethanol. Moreover, the most important effect on extraction of phenolic compounds from potato peel was ethanol-water ratio (% (v/v)). Temperature could be range between 30 and 40 oC and time could be varied from 30 to 40 min. Considering these premises, optimal conditions were selected as follows: 55% (v/v) ethanol-water ratio, 35 oC for temperature of bath, 35 min of extraction time and 1/90 sample-solvent ratio (w/v). The second part of the study; PEF treated Bintje, Fountaine, Daisy, and Challenger varieties were also provided from Pizzoli factory like Innovator variety. By using optimum extraction conditions on non-PEF treated Innovator cultivar and on these four other cultivars (Binje, Challenger, Daisy and Fountaine) which treated with PEF, the antioxidant properties and total phenolic contents of potato peels were evaluated. HPLC-DAD-ESI-MS was used to identify and quantify phenolic compound content. The profitability of antioxidants present in the potato peels was also evaluated by using in vitro antioxidant activity assays – 1,1'-diphenylpicrylhydrazyl (DPPH), 2,2'-azinobis(3-ethylbenzothiazolin)-6-sulphonate (ABTS). The results, obtained under the optimum conditions, indicated that the total phenolic content was found to be 7.23, 6.15, 5.04, 3.23 and 2.48 mg / g dry weight in the Daisy, Fountaine, Innovator, Bintje, Challenger, respectively. However, antioxidant capacity was found to be in the order Fountaine> Daisy> Innovator> Bintje> Challenger for both methods (DPPH and ABTS). Non-PEF treated Innovator cultivar showed higher phenolic content and antioxidant capacity than PEF treated Bintje and Challenger cultivars. In the analysis of total antioxidant capacity, similar to the literature DPPH method gave lower results than ABTS method. When the phenolic profiles of five potato peels were examined, the highest values were found to be chlorogenic acid and its derivatives. Chlorogenic acid constitutes half of the total phenolic compounds in all potato varieties.