Effects of different pretreatments combined with instant controlled pressure drop drying on the quality of jujube slices

Jujube (Ziziphus jujuba Mill.) is one of the most popular fruits native to China in the subtropical and tropical regions. However, fresh jujubes are susceptible to microbiological spoilage, especially for the rich contents of vitamin C, various amino acids, polysaccharides, and phytochemicals. The dried products are characterized by green natural, better quality with rich nutrition, edible convenience, and broad application prospects. The drying is used to reduce the moisture content to a safe level for the preservation of food products at present. The weight and volume of products are reduced to minimize transportation and storage costs. Among them, Jujube slices are a kind of dried snack with a high vitamin C content. The drying of jujube slices usually includes vacuum freeze, instant controlled pressure drops, vacuum frying, and microwave drying. Instant controlled pressure drop drying has been rapidly developed with creativity, environment-friendly, energy saving and high quality. This study aims to explore the effects of different pretreatments combined with instant controlled pressure drop drying on the quality of jujube chips. Huizu jujubes from the Xinjiang area were dried with three types of pretreatments, namely blanching, ultrasonic and dense phase carbon dioxide (DPCD). A systematic investigation was made on the effects of the three pretreatments on the color parameters, hardness, crispness, dilatability, soluble solids, retention of total sugar, total phenol, total flavonoids and vitamin C content, rehydration ratio, and microstructures. Results showed that: 1) Pretreatment drying shared a significant impact on the color of dried samples. There was a very similar color of jujube crisps under DPCD drying, compared with the fresh ones (ΔE=3.73). 2) DPCD pretreatment was used to keep more vitamin C content, compared with the other two pretreatments (P<0.05). The retention rates of vitamin C by DPCD pretreatment were 14.3%, and 25.0% higher than that of the samples dried by blanching, and ultrasonic, respectively. 3) The lowest hardness and highest crispness were achieved in the jujube crisps dried by DPCD pretreatment. 4) Microstructure detection indicated that a porous structure was observed after all three pretreatments, thus changing the properties of jujube slices. The DPCD pretreatment led to a more uniform porous structure. The reason was that the structure of jujube cells was changed under the action of carbon dioxide, which probably contributed to the higher crispness of the samples dried by instant controlled pressure drop drying. 5) The better sensory was evaluated after DPCD pretreatment. There was a high score of color, shape, taste and texture, compared with the rest. In conclusion, DPCD pretreatment improved the color, brittleness, expansion degree, and retention of nutrition content in the jujube crisps. A more suitable pretreatment can be expected for the differential pressure flash drying of jujube. The findings of the current work can provide the theoretical basis and technical support for the practical application of instant controlled pressure drop drying to produce jujube crisps. © 2024 Chinese Society of Agricultural Engineering. All rights reserved.