Cold plasma enhances drying and color, rehydration ratio and polyphenols of wolfberry via microstructure and ultrastructure alteration

Wolfberry is very difficult to dry as its surface is covered by thin wax layer. Chemical dipping is frequently used to dissolve it and enhance drying. In current work, cold plasma (CP) was employed and its pretreatment effect (15, 30, 45, 60 s) on drying time, color, rehydration ratio, total phenolic content, total flavonoids content as well as micro-, and ultrastructure of wolfberry dried by hot air drying was explored and compared with that of unpretreated and the pretreated samples using sodium carbonate (SC) solution. Suitable CP processing time could shorten the drying time by 50%. CP pretreatment increased rehydration ratio of dried wolfberry by 7–16%. The L*, a*, and b* of dried wolfberry treated by CP were significantly (p < 0.05) higher than samples without pretreatment, and CP pretreatment decreased ΔE by 18–27%. Phytochemical contents of dried samples increased in the beginning and then decreased with the increase in CP treatment time. Microstructure and ultrastructure observations showed that the cell wall, cell membranes were disintegrated with increase of CP treatment time. Ultrastructure alteration probably facilitated the release and diffusion of moisture and phytochemical compounds entrapped in cells and by thus enhanced the drying and phytochemicals extraction.