Analytical characterization of monovarietal rhubarb (Rheum rhabarbarum) juices

摘要

The production of rhubarb juices, nectars, spritzers, and rhubarb wines is a growing market. The aim of this work was the chemical characterisation of nine monovarietal rhubarb juices from six different varieties and different vintages. The juice yields after pressing were consistently high between 80 and 90 % (kg juice/100 kg mash). The soluble solids were between 2.73 and 4.05 (o)Brix. Total sugar differed in a wide range (6.5-18.8 g/L); the sugar fraction was consisting of glucose (2.9-8.5 g/L), fructose (2.9-8.8 g/L) and minor amounts of sucrose (0.4-1.7 g/L). Main organic acids were malic acid (10.9-16.9 g/L), oxalic acid (3.0-7.6 g/L), and citric acid (0.7-2.2 g/L). Potassium was the mineral with the highest concentrations (2814-3895 mg/L). Also magnesium concentrations were comparably high and ranged from 53 to 136 mg/L. In contrast to other fruit and vegetable juices rhubarb juices contained no calcium, because exceeding oxalic acid precipitated rapidly with calcium to insoluble calcium oxalate. Particularly the natural sodium values are very high in comparison to other fruit and vegetable juices (5-202, mean 51 mg/L). Other striking parameters were chloride (139535 mg/L) and nitrate (341-1544 mg/L). Total polyphenols ranged from 219-459 mg/L, the antioxidant capacity from 2.6-5.5 mmol/L Trolox. Naturally occuring ascorbic acid was present in small amounts of 1067 mg/L. Flavanols (catechin, procyanidins B1, B2, C1) were present in concentrations from 17-63 mg/L, flavonols from 8-19 mg/L. Quercetin-3-rutinoside was the major flavonol. Special emphasis was given to the determination of anthocyanins in skins and flesh. Main anthocyanins were delphinidin-3-rutinoside, cyanidin-3-glucoside, and cyanidin-3-rutinoside; delphinidin-3-glucoside and carboxypyrano-cyanidin-3-rutinoside were only found in trace amounts. Highest concentrations were found in the skins (320-1286 mg/kg) whereas in the flesh the amounts were substantially lower (0-63 mg/kg). The pasteurised juices contained 11-40 mg/L anthocyanins.