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 °C target die temperatures, giving extrudates with pH values of 5.6, 6.8, and 7.4. Freeze-dried equimolar solutions of glucose and glycine were heated either dry or after equilibration to ∼13% moisture at 180 °C in a reaction-tube system designed to mimic the heating profile in an extruder. Volatile compounds were isolated onto Tenax and analyzed by gas chromatography−mass spectrometry. For the extrudates, total yields of volatiles increased with decreasing pH at 180 °C, reached a maximum at pH 6.8 at 150 °C, and increased with increasing pH at 120 °C. Amounts increased with temperature at all pH values. Pyrazines were the most abundant class for all sets of conditions (54−79% of total volatiles). Pyrroles, ketones, furans, oxazoles, and pyridines were also identified. Yields of volatiles from the reaction-tube samples increased by >60% in the moist system. Levels of individual classes also increased in the presence of moisture, except pyrazines, which decreased ∼3.5-fold. Twenty-one of the compounds were common to the reaction-tube samples and the extrudates. Keywords: Maillard reaction; extrusion cooking; glycine; glucose; pH; temperature; volatiles; modeling