Discrepancy of Effective Water Diffusivities Determined from Dynamic Vapor Sorption Measurements with Different Relative Humidity Step Sizes: Observations from Cereal Materials

Abstract

Water diffusivity, a critical parameter for cereal processing design and quality optimization, is usually concentration-dependent. dynamic vapor sorption (DVS) system provides an approach to establishing the relationship between water concentration and diffusivity. However, the usual relative humidity (RH) jump during practical sorption processes is usually greater than that adopted in DVS measurements. Water vapor sorption kinetics of glutinous rice grains, glutinous rice flour and wheat flour dough films were measured using the DVS system to verify if varying RH step sizes can obtain identical diffusivities within the same range. The effective diffusivities were determined according to Fick’s second law. The results revealed that increasing RH step size led to a higher estimated diffusivity, regardless of whether the water concentration gradient or potential chemical gradient was considered a driving force for water diffusion. This finding was further confirmed by a linear RH scanning DVS measurement. The water concentration-dependent diffusivity obtained from a multi-step DVS measurement, according to Fick’s second law, will overestimate the required time for practical cereal drying or adsorption. Thus, this paradoxical discrepancy needs a new mass transfer mechanism to be explained.

Critical Role of Boundary Conditions in Sorption Kinetics Measurements

“…There is a similar issue for the characterization of the sorption kinetics of molecules onto the solid surface of various porous materials such as silica particles or composites sorbents, , TiO2 films, aluminophosphate layers, metal–organic framework, , resins, polymer composites, , zeolites, carbon, or of water penetration in clay or cereals …”

“…Alternatively, a dynamic vapor sorption (DVS) apparatus is often used, ,− in which a nitrogen vapor mixture is flowed around the sample placed in a chamber, while the RH is controlled in the surroundings of the sample but not along the sample surface. Again here, the sample mass variations over time under such conditions are considered to reflect the sorption kinetics of the material.…”

“…When the DVS, the MBV tests, or other tests of the hygrothermal properties of materials have been interpreted theoretically, it appeared necessary to express the boundary condition in a more subtle way than for tests in climatic chambers, now taking into account the air flow along the interface. It is then generally considered that this boundary condition may be formulated by expressing the water mass flux through the interface, i.e., J , in the form J = h ( n ∞ – n s ), in which n s is the RH along the interface. − Note that here h is an unknown coefficient that depends on the flow characteristics and the sample geometry, but it is also often assumed to depend on other material properties, which is illustrated by the fact that it was called the “surface resistance” or “surface emission” coefficient. ,− In addition, n s is generally considered constant during the process, which is somewhat in contradiction with the fact that the water content absorbed in the material evolves over time. More complex boundary conditions were also proposed, such as first-order kinetic equations. , Within the frame of a farsighted work, Thorell finally suggested that the value of this coefficient might be determined from independent tests using a water-saturated sample.…”

“…There is a similar issue for the characterization of the sorption kinetics of molecules onto the solid surface of various porous materials such as silica particles or composites sorbents, 12,13 TiO2 films, 14 aluminophosphate layers, 15 metal organic framework, 13,16 resins, 17 polymer composites, 18,19 zeolites, 20 Carbon, 21 or of water penetration in clay 22 or cereals. 23 Typically, in order to measure the sorption kinetics, a sample of material is placed in a climatic chamber supposed to be able to maintain the air around the sample at a given value of RH, i.e., n ∞ , while the material has been prepared under a different RH, i.e., n 0 . 12, 19,21,[24][25][26] The sorption dynamics of the material is then expected to depend on n ∞ and n 0 , and is described through the relative variation of the sample mass in time.…”

“…[39][40][41][42][43] Note that here h is an unknown coefficient that depends on the flow characteristics and the sample geometry, but it is also often assumed to depend on other material properties, which is illustrated by the fact that it was called the "surface resistance" or "surface emission" coefficient. 23,[44][45][46] In addition, n s is generally considered as constant during the process, which is somewhat in contradiction with the fact that the water content absorbed in the material evolves over time. More complex boundary conditions were also proposed, such as first-order kinetic equations.…”