Antioxidants from a heated histidine‐glucose model system. I: Investigation of the antioxidant role of histidine and isolation of antioxidants by high‐performance liquid chromatography

Abstract

Abstract The radical‐combining activity of Maillard reaction products [MRP (aq) ], produced by heating d ‐glucose and l ‐histidine (3:1) in a 0.1 M phosphate buffer for 10 h at 105°C (final pH 6.53), was estimated directly by means of a diphenylpicrylhydrazyl radical (DPPH · ) method. Additionally, the indirect methods of peroxide values changes (oven test), hexanal formation, and protection factors (Rancimat method) were determined on a lipid model system that consisted of sunflower seed oil/water (1:2), emulsified with 3% (w/w) Tween 40. Results from the DPPH · method showed a potential antioxidant activity of MRP (aq) , which was confirmed by the indirect methods. Surprisingly, histidine in solution alone (heated or not) exhibited an antioxidant activity greater than or similar to the MRP (aq) activity in the indirect methods with the lipid model system, in contrast to the results from the DPPH · method. The suitability of various solvents for extraction of potential antioxidant compounds from freeze‐dried MRP (aq) was examined, and ethanol extracts showed the greatest activity by the DPPH · method. Consequently, the ethanol extract of freeze‐dried MRP (aq) was separated by means of preparative reverse‐phase high‐performance liquid chromatography (HPLC) with a 0.05 M phosphate buffer (pH 4.4)/water/acetonitrile gradient system. The antioxidant activity of the eluate was measured through the DPPH · method, and a fraction (Fraction A) with antiradical activity was further purified by preparative HPLC. Fraction B was collected, and its freeze‐dried residue exhibited potent antiradical activity, significantly greater than that of the same level of n ‐propyl gallate.