Effects of microwave-assisted pulse-spouted bed freeze-drying (MPSFD) on volatile compounds and structural aspects of Cordyceps militaris

BACKGROUND: Microwave-assisted pulse-spouted bed freeze-drying (MPSFD) uses microwave heating to substitute the customary conduction heating during freeze drying (FD), resulting in high-quality dried samples with a short drying time. The aim of this study was to determine the effect of MPSFD on volatile compounds and structural aspects of Cordyceps militaris. In order to highlight the effect of MPSFD, FD (recognized for its high-quality products) and hot-air dying (HD; the most commonly used) were also evaluated using headspace solid-phase micro-extraction combined with gas chromatography-mass spectrometry and mid-Fourier transform infrared spectroscopy. RESULTS: Sixty-two components were obtained by FD, 62 compounds by MPSFD and 39 by HD. FD and MPSFD exhibited much better effects on the retention of volatile components of C. militaris than HD. Principal component analysis, hierarchical clustering analysis and E-nose analysis indicated that the volatile compounds of C. militaris dried by FD and MPSFD were similar. The spectral features of FD and MPSFD samples were also similar, whereas the HD sample presented a clearly different spectral feature around a wavenumber of 1640 cm(-1). CONCLUSION: MPSFD exhibited a beneficial effect on aroma and structure retention, with shorter drying time and lower energy consumption. (C) 2018 Society of Chemical Industry