Drying of shiitake mushroom by combining freeze-drying and mid-infrared radiation

• Total drying time and quality characteristics of combined FD and MIRD was compared with FD.
• The combination of FD followed by MIRD saves 48% drying time compared to FD.
• FD–MIRD products showed the analogous qualities with FD samples.
• The combination of mid-infrared dying with freeze drying causes an increase of sulfur compounds.

Mid-infrared drying (MIRD) was applied before or after freeze-drying (FD) of shiitake mushroom to shorten the drying time, to enhance the rehydration, and to better preserve the aroma compounds and color. The effect of application of MIRD before freeze drying (MIRD–FD) and after freeze drying (FD–MIRD) on drying time, color, rehydration ratio, apparent density, microstructure and aroma compounds was measured, explained and compared with the effect of FD on these parameters. The results showed that the combination of FD (for 4 h) followed by MIRD saves 48% time compared to FD while keeping the product quality at an acceptable level. The MIRD–FD combination was found to be inferior compared to the FD–MIRD as the former tended to produce products with a collapsed surface layer and poor rehydration capability. The combination of MIRD with FD had a significant effect on aroma retention and caused an increase of sulfur compounds such as dimethyl, trisulfide and 1,2,4-trithiolane.