Monitoring the Changes of Aroma Components in Jasmine Flowers During Roasting Process by On-line Photoionization Mass Spectrometry

Due to the high specificity and high speed of mass spectrometer, on-line monitoring of the roasting process by mass spectrometry has attracted increasing attention at present. In this work, fresh jasmine flowers were roasted in a home-made tube furnace, and the vaporized aroma components were monitored by an on-line low pressure vacuum ultraviolet photoionization time-of-flight mass spectrometry (VUV-PI-TOF MS) in real time. The jasmine flowers were roasted at four different temperatures (90, 120, 150 and 200 ℃), and the mass spectra of the roasting products were quickly obtained. Since PI produces little or no fragments, nearly all the mass peaks can be attributed to molecular ions. Products identification was fulfilled according to the previous GC/MS results. The contents of these substances had been reported to be positively correlated with the quality of jasmine flowers. The relationship between the roasting temperature and various releasing aroma components was studied by the temperature-programmed mode with a heating rate of 5 ℃/min. Water is abundant in jasmine flowers, whose signal strength first increased slowly, then kept stable and finally decreased slowly with the rise of roasting temperature from 40 to 170 ℃. During this process, the signal strength of most vaporized aroma compositions kept increasing and increased quickly at a temperature higher than 140 ℃. The increasing rates of compounds such as acetone, butanone, ethyl acetate, phenylethyl alcohol, benzoate cis 3-hexenyl and benzyl alcohol were higher than that of indole and benzyl alcohol. The time-evolved profiles of water and four main vaporized aroma compounds in jasmine flowers over 1 700 s were also investigated at 90, 120 and 150 ℃, respectively. With the elapse of time, all substances except butanone had the same trend at 90, 120 and 150 ℃. They rose quickly within a short period of time, then maintained at a lower concentration for a long time. However, when the roasting temperature was 150 ℃, the signal of butanone appeared again at 1 500 s. The results showed that more aroma ingredients were lost at a higher temperature, which was bad for the preservation of jasmine flowers’ quality. However, too low temperature was not favorable for the vaporization of water and the taste of toasted jasmine flowers would be bad. Therefore, 90 ℃ is a highly-recommended roasting temperature. VUV-PI-TOF MS can be used for the evaluation of the roasting process of jasmine flowers, and offer guide for the industrial production.