Effect of pH and temperature on the formation of volatile compounds in cysteine/reducing sugar/starch mixtures during extrusion cooking

Mixtures of cysteine, reducing sugar (xylose or glucose), and starch were e xtrusion cooked using feed pH values of 5.5, 6.5, and 7.5 and target die te mperatures of 120, 150, and 180 degreesC. Volatile compounds were isolated by headspace trapping onto Tenax and analyzed by gas chromatography-mass sp ectrometry. Eighty and 38 compounds, respectively, were identified from ext rudates prepared using glucose and xylose. Amounts of most compounds increa sed with temperature and pH. Aliphatic sulfur compounds, thiophenes, pyrazi nes, and thiazoles were the most abundant chemical classes for the glucose samples, whereas for xylose extrudates highest levels were obtained for non -sulfur-containing furans, thiophenes, sulfur-containing furans, and pyrazi nes. 2-Furanmethanethiol and 2-methyl-3-furanthiol were present in extrudat es prepared using both sugars, but levels were higher in xylose samples. Th e profiles of reaction products were different from those obtained from aqu eous or reduced-moisture systems based on cysteine and either glucose or ri bose.