Effects of the types of sugars and buffers, and pH on the formation of Maillard reaction pigments were investigated using the xylose-lysine and glucose-lysine model systems. Sugars (13.3 mM) and lysine (34 mM) were dissolved in acetate, phosphate, and tris(hydroxymethyl)aminomethane (Tris) buffers and heated at 100℃ for 60 min. The profiles of browning, reversed-phase HPLC, sugar-decrease, and gel permeation chromatography of each sample were examined. Browning was stimulated by phosphate and inhibited by Tris. With the rise in pH value, browning occurred more intensively, more sugars decreased, and more melanoidins were formed. Xylose samples turned darker brown than glucose ones. The ratio of absorbance at 400 nm to that at 450 or 500 nm increased in the xylose system with the rise in pH value, while that in the glucose system decreased. Low-molecular-weight pigments, dilysyldipyrrolones, were formed only in the xylose system, while melanoidins were formed in both systems. Lysine stimulated the degradation of sugars, polymerization, and browning in both systems, although more intensively in the xylose system than in the glucose one.

During research on the Maillard reaction between xylose and lysine (Lys), we detected a major peak showing anabsorbance maximum at about 280 nm by diode-array-detection (DAD)-HPLC. In this study, the peak was isolated and identified as 4-hydroxy-5-methyl-3(2H)-furanone (HMFO). This compound accounted for 60 _ 80% of the total area of HPLC peaks (280 nm), and about 20 mg/100 mL of HMFO was produced from a solution containing 200 mg/100 mL of xylose. HMFO was produced not only from xylose, but also from arabinose and ribose, and ribose was the best precursor. When HMFO was heated in a buffer solution with or without Lys, it was decomposed or polymerized, and colored and colorless polymers appeared. Diacetyl and methylglyoxal were the major decomposed dicarbonyl compounds from HMFO; these dicarbonyl compounds are considered to be the major precursors for polymers formed from HMFO.

Pentose is involved in the browning through the Maillard reaction of food derived of plant origin. During research on the Maillard reaction between xylose (Xyl) and lysine (Lys), we detected 4-hydroxy-5-methyl-3(2H)-furanone (HMFO) as a major decomposition product of Xyl. To clarify the chemical pathway of the browning of pentose system, the formation and decomposition of dicarbonyls from HMFO and Xyl were examined. In the HMFO system, HMFO was oxidatively hydrolyzed to form 2-hydroxy-3,4-dioxopentanal, which leads to the formation of methylglyoxal (MGO) and then diacetyl (DA). In the Xyl system, MGO was also the major dicarbonyl degradation product from 1-deoxyxylosone (1-DX). Among Xyl, HMFO, MGO, and DA, MGO turned brown most rapidly in the presence of Lys and formed melanoidin-like brown pigments. In the Xyl system, MGO derived from HMFO and 1-DX most contributed to the browning, although some low-molecular-weight pigments, a colorless polymer, and fluorescent substances were also formed.