Microstructure Analysis and Quality Evaluation of Jujube Slices Dried by Hot Air Combined with Radio Frequency Heat Treatment at Different Drying Stages

Abstract

Jujubes have been favored by consumers because of their rich nutrition and wide use. Hot air drying has been commercially and typically used to prolong shelf life and acquire the dried produce. Jujube slices were dried with hot air combined with radio frequency (RF) at different drying stages, namely, early (0–2 h, E-HA + RF), middle (2–4 h, M-HA + RF), later (4–6 h, L-HA + RF), and whole (0–6 h, W-HA + RF) stages. This study aimed to investigate the effects of different RF application stages on the microstructure, moisture absorption rate, color, and ascorbic acid of jujube slices. Compared with the hot air drying (HA) group, the E-HA + RF group obtained the best results among the experimental groups because it reduced the cells with a roundness of less than 0.4 by 5%. Moreover, the M-HA + RF group showed better results than those of other groups, with an 18.6% and 48.8% reduction in cells for a cross-sectional area less than 200 µm2 and a perimeter less than 25 µm, respectively. The minimum total color difference (ΔE = 9.21 ± 0.31) and maximum retention of ascorbic acid (285.06 mg/100 g) were also observed in this group. Therefore, the method of hot air drying assisted by phased RF is viable in the drying industry to improve the quality of dried agricultural products and reduce energy consumption.

Effects of variable-temperature drying on the qualities and sweet-substance profile of Zizyphus jujuba Mill. cv. Junzao

“…4 B,E). The lower temperature in the early drying period of VTD slows down the degradation of cell walls and prolongates the period of shrinkage and collapse of tissue structure caused by moisture gradient and enzyme action ( Yao et al, 2022 ).

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Effect of Infrared-Combined Hot Air Intermittent Drying of Jujube (Zizyphus jujuba Miller) Slices: Drying Characteristics, Quality, and Energy Consumption Dimensions

“…Jujube with an undamaged appearance and consistent color, shape, and size were purchased from the local farmers' market in Shihezi City, Xinjiang, China, and stored in a refrigerator at (4 ± 1) • C. The material was taken out at the start of the drying test, cleaned, dried with absorbent paper, and pitted before the jujubes were cut into thin slices with a thickness of 0.3 cm and a diameter of 2.2 cm with a slicer. The average dry basis moisture content of the samples was determined to be 0.267 g/g [20].…”

Enhancing peach slices radio frequency vacuum drying by combining ultrasound and ultra-high pressure as pretreatments: Effect on drying characteristics, physicochemical quality, texture and sensory evaluation

“…Conventional drying methods, such as hot air drying [11] and vacuum far-infrared drying [12] , mainly relied on internal conduction and surface convection for energy exchange and transfer, not only heats up slowly, but also produces the uneven heating phenomenon of “external heat and internal cooling”, resulting in poor temperature uniformity, drying effect and product quality is difficult to guarantee. Compared with the above dehydration methods, RFV drying technology has the advantages of non-ionization, high efficiency, volumetric heating effects, large penetration depth, and self-equilibrium of water content [13] , [14] . In addition, the penetration characteristics of the radio frequency electric field make the dry products have a certain expansion effect, which greatly improve the springiness, toughness, and flavor of fruits and vegetables.…”