Green and sustainable techniques to increase the extraction of phenolic compounds from turnip greens by‐products

The work was aimed to optimize extraction and recovery of phenolics from turnip greens by-products using eco-sustainable techniques based on the use of micro- waves, ultrasound, or enzymes and of water as solvent. The optimization of ultrasound- and microwave-assisted extractions was performed using a 4-factors/3-levels and a 3-factors/3-levels Box–Behnken Design (BBD), respectively and the results of the corresponding trials were fitted to a second order polynomial model. The enzymatic extractions were performed using three different commercial enzymatic preparations, optionally combined with an ultrasound treatment carried out according to the central values of the BBD design. The best results obtained from each green technique were compared with those of the conventional solid–liquid extraction performed with a 70% ethanol aqueous solution. Regarding ultrasounds, the conditions able to maximize the total phenolic concentrations of the extracts (solid–water ratio, 50 g/L; T, 45C; t, 40 min; power, 24 W) increased the extraction efficiency by about 75% and reduced the extraction times from 2 h to 10 min with respect to the conventional extraction. Concerning microwaves, the optimized condi- tions (solid-water ratio, 75 g/L; t, 3 min; power, 160 W) allowed the increase of the extraction efficiency by about 48% and the decrease of the extraction times to just a few minutes. For both techniques, the factor that mainly affected extraction yield and antioxidant activity were “solids-to-solvent ratio” and “power,” respectively. According to the results of the enzymatic treatments, the best extraction was that performed with the commercial mix of pectinlyase, polygalacturonase, pectinesterase, and arabinase, which had the same extraction efficiency of microwaves but required longer times (90 min). Furthermore, the combination with ultrasounds did not improve its performances. Extraction technique also affected the phenolic profiles in terms of number and concentration of compounds identified. In conclusion, ultra- sounds and microwaves are the best treatments to maximize extraction yield (the first) and minimize extraction time (the second).