Studies on browning after cooking and polyphenols in barley grains

Barley grain is often pearled to remove the outer layers and processed into rolled barley or rice-shaped barley, which are generally cooked with rice as 'mugi-gohan' in Japan. In order to expand barley consumption, it is important to reduce undesirable properties for mugi-gohan and to increase useful constituents that are good for health. Barley products, when cooked and kept warm, easily undergo browning, which is undesirable. One of the objectives of this study was to investigate the causal constituents and factors of browning. A second objective was to elucidate the major anthocyanins from purple waxy hull-less barley and the changes in these constituents during maturation and after harvest to explore the availability of anthocyanins. First, using 42 varieties, the degree of browning, the browning index (BI), was estimated from the difference in Hunter's whiteness value of pearled barley pastes before and after heat treatment. BI was correlated significantly with the polyphenol content of pearled flour (r = 0.766, p < 0.001). Of the constituents of the crude polyphenol fraction, the contents of prodelphinidin T1 (PDT1), prodelphinidin B3 (PDB3), procyanidin B3 (PCB3) and (+)-catechin were correlated with the BI (r = 0.400, 0.595, 0.599, 0.384, respectively; p < 0.05). To examine the effects of inorganic elements on the browning reaction, pearled barley pastes were heated with or without salts, including inorganic elements or chelators of various concentrations. Browning was enhanced by iron chloride and copper chloride and inhibited by zinc chloride, calcium salts (chloride, nitrate, acetate) and three chelators (DTPA, EDTA, phytate) in a dose-dependent manner. To evaluate the effects of phenolic compounds on browning, various amounts of proanthocyanidins, (+)-catechin or related phenolic compounds were added to aqueous barley extracts or barley pastes and heated. In barley extract, PDT1, PDB3, PCB3 and (+)-catechin elevated absorbance at 420 nm after heating in a dose-dependent manner. In barley paste, PDT1 and PDB3 decreased the Lsup(*) value and increased the asup(*) and bsup(*) values of the paste dose-dependently after heating, and PCB3 and (+)-catechin did so to a lesser extent. These compounds promoted the browning more effectively than other related phenolic compounds. To evaluate browning of cooked barley kernels, pearled kernels were cooked and further incubated in a test tube. The asup(*) value of the flour was measured after lyophilization and pulverization, and the increase in the asup(*) value during incubation after cooking (delta asup(*)) was used as an indicator of browning. PDB3, PCB3 and (+)-catechin in cooked kernels decreased during incubation after cooking. In six proanthocyanidin-free lines, polyphenol and flavanol contents of pearled kernels were very low and browning was barely observed. The browning of cooked kernels was significantly correlated with the flavanol content in pearled kernels as well as the flavanol loss during incubation after cooking (r = 0.716, 0.838, p < 0.01, respectively) in 10 varieties. From grains of purple waxy hull-less barley cv. Daishimochi, cyanidin 3-O-(3,6-di-O-malonyl-beta-D-glucopyranoside) and other 3 anthocyanins were isolated and identified. These anthocyanins were most abundant at 35 days after flowering when the dry weight of grains was at a maximum, and their levels decreased during the physiological drying period and drying process after harvest. About two-thirds of these anthocyanins were distributed into bran when grains were pearled to 90% yield. These results indicated that flavanols, such as proanthocyanidins and (+)-catechin, are the major factor causing the browning of cooked barley, and that their autooxidation causes browning during cooking and incubation after cooking. These results also show basic findings on harvest time and processing methods to isolate anthocyanins identified from purple waxy hull-less barley.