Combined Pre-treatment of Freeze–Thaw and Ultrasonic-Assisted Aqueous Ethanol for Hot Air Drying of Watery Kimchi Cabbage Waste: Effects on Drying Efficiency, Physicochemical and Microbiological Characteristics, and Microstructure

Purpose Watery kimchi cabbage waste (KCW) worsens soil and groundwater pollution and increases soil salinity. Quick decay limits its storage, transportation, and recyclability. Therefore, developing technologies for controlling moisture and reducing microorganisms are necessary to improve its recyclability. Methods The effects of pre-treatment using freeze–thaw (FT) or aqueous ethanol (AE) immersion with or without ultrasonication (US), combined with hot air (HA) drying at 50 °C, on the drying properties and microbiological and physicochemical characteristics of watery KCW were investigated. Results After 10 min of AE/US and FT + AE/US pre-treatments, the water and solid loss were 13.39% and 15.20% and 5.15 and 6.17%, respectively. FT + AE/US pre-treatment shortened the HA drying time required for the moisture ratio of KCK to reach 0.1 by approximately 66%, compared to that in the control. In KCW samples pre-treated with FT + AE/US, the total aerobic bacteria and yeast and molds reduced to 3.05 and 1.73 log CFU g −1 , respectively, after HA drying. Staphylococcus aureus , total coliforms, and total lactic acid bacteria were undetectable (l log CFU g −1 ). Lewis, Henderson and Pabis, and Page models described changes in the moisture ratio during HA drying, with coefficient of determination, chi-square, and root mean square error values of ≥ 0.865, ≤ 0.006, and ≤ 0.077, respectively. Conclusion Combining FT + AE/US and HA drying removes moisture and salt and improves the microbiological safety and recyclability. Graphical Abstract