Hydrogen-rich water as a green solvent for the extraction of phytochemicals from agri-food wastes

The valorization of agri-food waste forms a pressing need in the present crop unavailability crisis. Phytochemical extraction may provide an attractive solution to this problem. The use of hydrogen-rich water (HRW) as a solvent for extracting different phytochemicals from various wastes was investigated. The total phenolic content (TPC), total flavonoids content (TFC), total anthocyanins content, and antioxidant activity (DPPH, ABTS+) of HRW extracts were higher than those of pure water, and those of ethanol/HRW (50/50) were higher than those of ethanol/water (50/50) for all waste types. The highest levels of all phytochemicals were found in HRW extracts, followed by ethanol/HRW (50/50). The highest phytochemical contents were shown in HRW samples for all plant wastes as follows: TPC (mg GAE/g extract): 9.04 (tomato peels), 9.32 (carrot), 14.76 (apple peels), 23.88 (lemon peels), and 49.15 (cabbage); TFC, (mg QE/g extract): 9.37 (tomato peels), 10.84 (carrot), 8.96 (apple peels), 9.37 (lemon peels), and 13.31 (cabbage); total anthocyanins (mg QE/g extract): 2.88 (tomato peels), 6.14 (carrot), 4.01 (apple peels), 1.63 (lemon peels), and 40.73 (cabbage); DPPH scavenging activity (mg AE/g extract): 8.47 (tomato peels), 11.06 (carrot), 8.40 (apple peels), 9.85 (lemon peels), and 20.91 (cabbage); and ABTS + scavenging activity (mg TE/g extract): 8.61 (tomato peels), 8.36 (carrot), 11.69 (apple peels), 11.87 (lemon peels), and 24.25 (cabbage). The increase rate was 22–50% for TPC, 16-53% for TFC, 80-390% for total anthocyanins, 9–142% for DPPH scavenging activity, and 14–28% for ABTS+ scavenging activity when HRW was used as a solvent instead of pure water. The rate of increase was 1-23% for TPC, 53-45% for TFC, 36-228% for total anthocyanins, 19-171% for DPPH scavenging activity, and 5-30% for ABTS+ scavenging activity when HRW, mixed with equal amounts of ethanol (50/50), was used as a solvent instead of pure water/ethanol (50/50). The quantities of certain flavonoids and non-flavonoids such as epicatechin, rutin, gallic acid, and p-coumaric acid increased at rates reaching several folds when HRW was used as the solvent. The results of extract color were in line with those of chemical ones, where the highest C* values were observed for HRW, followed by ethanol/HRW (50/50). A positive linear correlation (R2 = 0.61–0.94) between several color parameters (H°, C*, a*) and anthocyanin content was observed. The results showed that using HRW alone or mixed with ethanol significantly improves the extraction of different phytochemicals. The brilliant advantage of this method is that it does not need additional equipment, energy, and toxic solvents, allowing it to be a low-cost and environmentally friendly method.