Low-temperature low-humidity pretreatment followed by RF heating: a novel method of fig preservation

Abstract

Fig (Ficus carica) is an economic crop with significant health importance. Its post-harvest life is very short. Once dried, it is readily infested by insects. The aim of this study is to analyze whether low-temperature low-humidity (LTLH) drying followed by radiofrequency (RF) heating (for drying and disinfestation, respectively) is a treatment suitable for the food industry. During the preliminary study it was found that electrode heights of 180, 200, and 220 mm gave 100% mortality of Oryzaephilus surinamensis for all the time ranges studied. Hence, during the quality evaluation, these electrode heights and slow conveyor speeds (2.5, 5, and 7.5 m h⁻¹) were used. LTLH-dried figs served as a control, and further disinfestation of fig samples using RF heating (10 kW, 40.68 MHz) was performed. At 180 mm and 2.5 m h⁻¹ (sample A), the moisture was most significantly reduced to 12.4%, which was much lower than that of the control (20.2%). However, the protein content fell significantly (by 31.17%), while the phenolic content (282 mg GAE per g) was significantly high after the treatment combination of 180 mm and 2.5 m h⁻¹ (sample A). It was observed that the total phenolic content increased with increasing RF exposure intensity. The carbohydrate, fat, total dietary fiber, titratable acidity, total soluble solids (TSS), water activity, pH, and color values (L*, a*, b*) of the treated figs showed no significant difference (p > 0.05) among the samples. Multi-criteria decision analysis (MCDA) found that sample B (180 mm and 5 m h⁻¹) ranked the highest (MCDA score = 0.62), with a higher normalized score of phenolic content, scavenging activity and moisture compared to the control (second-ranked MCDA score = 0.61). Thus, LTLH pretreatment followed by RF heating at an electrode height of 180 mm and conveyor speed of 5 m h⁻¹ is suitable for quality retention.