Effect of Natural Air-drying on the Quality and Flavor of Raw Mustard Roots

In order to investigate the effect of natural air-drying on the quality and flavor of raw mustard roots, the moisture content, water activity, hardness, crispness, protein content, amino acid nitrogen content, total plate count and total lactic acid bacteria were measured and compared before and after air-drying of the raw mustard roots. The changes of volatile flavor substances were analyzed by solid-phase microextraction gas chromatography-mass spectrometry (SPME-GC-MS). The results showed that the hardness of mustard roots after natural air-drying was significantly enhanced and the crispness was reduced (P<0.01). In addition, the total plate count and total lactic acid bacteria of the raw mustard roots after air-drying significantly increased compared with the mustard roots before air-drying (P<0.01). Furthermore, the protein content increased remarkably from 2.3 g/100 g before air-drying to 2.5 g/100 g after air-drying (P<0.01). The moisture content and the amino acid nitrogen content of raw mustard roots respectively decreased from 91% and 0.27 g/100 g before air-drying to 88% and 0.24 g/100 g after air-drying (P<0.05). A total of 24 and 18 volatile flavor substances were respectively detected from the mustard roots before and after air-drying through GC-MS. Moreover, the types and contents of volatile substances in raw mustard roots before and after air-drying significantly changed, among which the relative contents of sec-butyl-isothiocyanato and benzenepropanenitrile in the air-dried raw mustard roots respectively increased to 1.362% and 65.870%. Nineteen characteristic volatile markers (VIP>1, P<0.05) were screened via partial least-squares discriminant analysis (OPLS-DA), including 4-methylpentyl isothiocyanate, 3-phenylpropyl isothiocyanate and 3-butenyl isothiocyanate, etc. The present study would provide a certain theoretical basis for optimization of dehydration and quality improvement of mustard roots.