Effect of pH, temperature, and moisture on the formation of volatile compounds in glycine/glucose model systems

Mixtures of glycine, glucose, and starch were extrusion cooked using sodium hydroxide at 0, 3, and 6 g/L of extruder water feed, 18% moisture, and 120 , 150, and 180 degreesC target die temperatures, giving extrudates with pH values of 5.6, 6.8, and 7.4. Freeze-dried equimolar solutions of glucose an d glycine were heated either dry or after equilibration to similar to 13% m oisture at 180 degreesC in a reaction-tube system designed to mimic the hea ting profile in an extruder. Volatile compounds were isolated onto Tenax an d analyzed by gas chromatography-mass spectrometry. For the extrudates, tot al yields of volatiles increased with decreasing pH at 180 degreesC, reache d a maximum at pH 6.S at 150 degreesC, and increased with increasing pH at 120 degreesC. Amounts increased with temperature at all pH values. Pyrazine s were the most abundant class for all sets of conditions (54-79% of total volatiles). Pyrroles, ketones, furans, oxazoles, and pyridines were also id entified. Yields of volatiles from the reaction-tube samples increased by > 60% in the moist system. Levels of individual classes also increased in th e presence of moisture, except pyrazines, which decreased similar to3.5-fol d. Twenty-one of the compounds were common to the reaction-tube samples and the extrudates.