Effects of three drying technologies on drying characteristics and quality attributes of jujube crisps

Abstract: With rich contents of vitamin C, various amino-acids, polysaccharide and phytochemicals, jujube is widely planted and consumed in China. Seasonality and perishability of fresh jujube demand to apply preservation technologies, such as vacuum-fried, but the favored product after this processing has the potential risk on our health because of its higher oil content and low nutrient retention. To realize product diversification and satisfy requirements of novel-health products for human beings, it is essential to exploit new processing method of jujube. In order to reduce the oil content of jujube crisps, improve the retention rate of vitamin C and explore new drying techniques for jujube crisps production, fresh jujube slices (thickness of 4 mm) in crisp ripe stage were dried with 3 new types of drying technologies, namely air impingement drying (temperature of 70 ℃, air velocity of 8 m/s), medium and short infrared wave drying (temperature of 70 ℃, air velocity of 8 m/s) and pulsed vacuum drying (temperature of 80 ℃, vacuum and normal pressure retention time of 10 and 2 min, respectively). Effects of the 3 drying technologies on the drying characteristics, color parameters, retention of vitamin C content, crispness, rehydration ratio and microstructures were investigated. Results showed that: 1) All drying process of the three drying methods showed the falling rate period without constant speed stage, and the shortest drying time was obtained under air impingement drying technology, which the drying time was only 105 min. 2) Effective moisture diffusivities were 1.55×10-9, 1.03×10-9, and 0.89×10-9 m2/s for air impingement drying, medium and short infrared wave drying, and pulsed vacuum drying, respectively. 3) Drying methods had a significant impact on the color of dried samples, and the color of jujube crisps under pulsed vacuum drying was very similar to that of the fresh ones. 4) Pulsed vacuum drying could keep more vitamin C content compared to the other two drying technologies. The retention rates of vitamin C were 51.5%, 49.0%, 66.6% for the samples dried by air impingement drying, medium and short infrared wave drying, and pulsed vacuum drying, respectively.5) Compared with air impingement drying and medium and short infrared wave drying, jujube crisps dried by pulsed vacuum drying had the highest crispness. 6) Microstructure detection indicated that all the 3 drying technologies could make a porous structure and thus change the properties of jujube slices. The pulsed vacuum drying led to a more uniform porous structure, which probably contributed to the higher crispness and rehydration ratio of the samples dried by pulsed vacuum drying. In conclusion, although pulsed vacuum drying technology took a relatively long drying time to get the same moisture ratio, the qualities of jujube crisps by pulsed vacuum drying were much better than those by air impingement drying and medium and short infrared wave drying. The findings of current work can provide theoretical basis and technical support for the practical application of pulsed vacuum drying to produce jujube crisps.