Measuring and modelling of moisture sorption isotherms and their application to the hygroscopicity evaluation of and thermodynamic computations for food products

The saturated salt solution (SSS) method is the most popular laboratory methodology to measure food MSIs for its remarkable simplicity and acceptable accuracy. However, the SSS method has four disadvantages: (1) lengthy process, (2) limited data points, (3) relatively high cost, and (4) unsafe operation for applying some highly caustic and toxic salts. The last three problems could be bypassed altogether by using the unsaturated solution methods instead. H2SO4 and glycerol solutions are two commonly used unsaturated solutions for humidity control in food MSI measurements. However, the drawback of tedious work has kept the sulfuric acid solution (SSL) method and glycerol solution method (GSL) method from being widely used. This thesis constructed two accurate aw models for H2SO4 and glycerol solutions, respectively. Using these models, experimenters could conveniently and accurately calculate the equilibrium aw levels and no longer need to periodically check and adjust H2SO4 and glycerol concentrations over the entire measurement process. As for MSI modelling, this thesis developed a new two-parameter MSI model based on the nonrandom two-liquid (NRTL) theory. Compared to the most accepted layered adsorption theory, which includes only the effect of water clustering near food surfaces, the NRTL theory considers the effects of both water clustering and food swelling in the description of water adsorption on food. This new model, referred to as the NRTL-solid-liquid-mixture (NRTL-SLM) model, reveals a linear relationship between 1/lnaw and m1. A new index, characterising the magnitude of the slope of the plot of 1/lnaw v. s. m1, was constructed for the hygroscopicity assessment of food products. This new index, referred to as the NHI index, was scaled between 0% and 100%, with the bigger value indicating higher hygroscopicity and 100% representing the hygroscopicity of glycerol. Considering that our proposed NRTL-SLM model is only applicable to a single temperature, we further modify it to incorporate a temperature term. The modified NRTL-SLM (M-NRTL-SLM) model was fitted to the experimental MSI data of seven food products (i.e., grapes, apples, pears, bananas, avocados, potatoes and beef) obtained from three temperatures and compared to 23 other literature MSI models that also include a T term. With the use of the M0NRTL-SLM model, the net isosteric heat of water adsorption (qs) for each of the seven food products was also calculated. In sum, two new MSI measurement methods ( i.e., simplified SSL and GSL methods), two new MSI models (i.e., NRTL-SLM and M-NRTL-SLM model) and one new hygroscopicity index (i.e., NHI index) were developed in the present thesis. In terms of experiments, the MSI curves of seven food products, i.e., grapes, apples, pears, bananas, avocados, potatoes and beef, were measured using the simplified SSL and SSS methods at 10, 25 and 50oC. MSI data of MCC and glycerol at 25oC were taken as reference, and hence, were measured and sourced from literature, respectively. All the experimental and collected MSI data were fitted to our new models, i.e., the NRTL-SLM and M-NRTL-SLM models, and literature MSI models. The monolayer moisture content (m0), NHI index and qs values for each of the seven food products were calculated. The possibilities of utilising m0 and NHI index as hygroscopicity indices for food products were explored.