Online monitoring
of the volatile compounds during the tea roasting
process is crucial to find the optimum roasting conditions and improve
the quality of green tea. In this work, synchrotron radiation photoionization
mass spectrometry (SR-PIMS) was utilized to online monitor the evolved
gaseous compounds during the tea roasting process. By virtue of “soft”
ionization and fast data acquisition characteristics of SR-PIMS, dozens
of aroma compounds including alcohols, aldehydes, furans, and nitrogen-
and sulfur-containing species were detected and identified in real
time. Moreover, 5-hydroxymethylfurfural (5-HMF), the key intermediate
of Maillard reactions, was found with high sensitivity. Evolution
processes of all the products could be observed via the time- and
temperature-resolved profiles in N2 and the air. Dehydration
was found to be the first step during roasting. Oxygen in the air
was found to accelerate the formation rate of various stable species
and intermediates in the course of the thermal treatment of fresh
green tea. The formation mechanisms of evolved compounds such as three
sulfur-containing compounds, i.e., dimethyl sulfide, hydrogen sulfide,
and methanethiol, could be proposed according to the step-by-step
formation process. The time-resolved results were demonstrated to
be applicable in the evaluation of different roasting processes by
statistical analysis. The optimum tea roasting temperature and duration
are proposed to be around 200 °C and 1000 s.