Heat and mass transfer in vacuum drying process of fructooligosaccharides syrup

Abstract Fructooligosaccharides (FOS) is a functional food additive. FOS syrup produced by biological methods can be easily transported, stored and used after drying. In this study, the effects of operating pressure, heating temperature and initial moisture content of FOS syrup on the drying characteristics during vacuum drying were studied. With a visual system, the state changes of the material during the drying process were recorded. The specific heat, viscosity and thermal conductivity of the FOS at different temperatures and moisture contents were measured. From the perspective of drying characteristics, the whole drying process can be divided into four periods: the increasing drying rate period AB, the first falling drying rate period BC, the second falling drying rate period CD and the third falling drying rate period DE. The heat transfer mode in AB and BC was boiling heat transfer, with the material viscosity less than 267.9 Pa·s. In CD period, the heat transfer mode was convection heat transfer with the material viscosity of 267.9 Pa·s to 501.6 Pa·s. In DE period, the material viscosity was greater than 501.6 Pa·s and did not have fluidity, and the heat transfer mode was heat conduction. A multivariate model for the convection heat transfer coefficient was obtained based on the heat balance. The maximum error between the simulation value by the model and the experimental value of the material moisture content during the vacuum drying process was 4.18 %.