A THREE PARAMETER EQUATION FOR FOOD MOISTURE SORPTION ISOTHERMS

A three parameter equation for moisture isotherms was developed and fitted to 31 food sorption isotherms. The GAB and the Peleg's models were fitted to the isotherms for comparison. Residuals and root mean square were used for assessment of the goodness of fit. The proposed equation gives fit to 28 isotherms but not as good as that obtained with the Peleg's model but much better than that found with the GAB model. Nevertheless the new equation gives higher probability of good fit to experimental data than the other two equations tested. Moreover, the new equation gives water content approaching infinity when a_w= 1, a property which is not offered neither by the Peleg's nor the GAB model.     

WATER ACTIVITY AND MOISTURE SORPTION ISOTHERMS OF GAZİANTEP CHEESE

Water activity of Gaziantep cheese prepared in the laboratory was determined as 0.95 by using the Proximity Equilibration Cell method. Adsorption and desorption isotherms were obtained at 12, 20 and 30C and 5, 20 and 27C, respectively. Halsey, Chung-Pfost, Guggenheim-Anderson-deBoer, and cubic and quadratic polynomial equations were used to evaluate the adsorption and desorption data. Parameter estimation was carried out. GAB and quadratic polynomial equations were the best equations for defining the adsorption and desorption data, respectively.     

ASSESSMENT of A SEMI-EMPIRICAL FOUR PARAMETER GENERAL MODEL FOR SIGMOID MOISTURE SORPTION ISOTHERMS1

Published sigmoid moisture sorption isotherms (0 < a_w < ～ 0.9) were fitted by the four parameter models m = k₁a_wⁿ¹+ k₂a_wⁿ² where m is the moisture contents (dry basis), a_w the water activity and the k and n values are constants (n₁ < 1 and n₂ > 1). Not surprisingly, the model had the same or better fit than the GAB model. In contrast with both the BET and GAB models, the proposed model is not based on the assumption that there exists a well-defined monolayer of absorbed water. At a_w < ～ 0.4 and n₁ > ～ 0.55, however, the model produces a practically linear a_w/[m(1 – a_w)] vs a_w plot, of the kind used to calculate the monolayer moisture with the BET model. The proposed model can be a convenient means to catalog both sigmoid and nonsigmoid isotherms, and used to calculate the equilibrium water activity of dry mixtures with equations solving software.     

RHEOLOGY AND THERMAL PROPERTIES OF SYRUPY COMPOSITES OF SACCHARIDES AND MILKFAT

Viscous syrups with between 20 and 34% moisture were made from homogenized slurries of sugars, nonfat dry milk, anhydrous butteroil and emulsifiers. Homogenized mixtures were condensed under vacuum at 45C for 35 min, to form ordered structures with a syrup-like consistency. Complex viscosity determined through dynamic spectrometry increased with increasing fat content. At low frequency, high fructose corn syrup and maltose-based syrups showed more solid-like behavior (G" > G") while the sucrose-based syrup was more liquid (G" > G'). Thermal analyses showed all composites to have a uniform melting range (95–140C), whereas the original components melted in the range 20–200C. The rheological and thermal properties suggest that these syrups could be useful confectionery ingredients.     

MOISTURE SORPTION ISOTHERMS FOR KARINGDA (CITRULLUS LANATUS (THUMB) MANSF) SEED, KERNEL AND HULL

Adsorption-desorption behaviors of karingda (Citrullus lanatus (Thumb) Mansf) seed, kernel and hull for nine equilibrium relative humidities (ERH) ranging between 11 and 96% at temperatures of 10, 20, 30, 40 and 50C were studied following a static equilibriation technique using saturated solutions of various salts. Under both the adsorption and desorption processes, the equilibrium moisture contents (EMC) of the hull were found to be highest followed by those of the seed and the kernel at all the corresponding temperatures and ERH values. Analysis of these data using four sorption models (modified Henderson, modified Halsey, modified Chung-Pfost and Guggenheim-Anderson-de Boer) and taking the temperature dependence of the respective coefficient into consideration, it revealed that both the Chung-Pfost and the GAB models were acceptable in describing EMC-ERH relationships for karingda seed, kernel and hull over the entire range of temperatures. The excess heat of sorption of all the samples, estimated from the Clausius-Clapeyron equation, decreased exponentially with the increase in moisture content of the same.     

ESTIMATION of HEAT of MOISTURE SORPTION and IMPROVED CRITERIA FOR EVALUATING MOISTURE SORPTION ISOTHERM EQUATIONS FOR FOODS

Heat of sorption of moisture on foods can be estimated from sorption isotherm equations having both moisture and temperature as independent variables. Sorption thermodynamics dictate that the heat of moisture sorption should decrease with increased moisture concentration, or increased temperature of food, provided no biological, chemical or other changes occur during sorption. This fact, and the Clausius-Clapeyron equation were used to derive two analytical formulae to evaluate the reliability of moisture sorption equations in the calculation of heat of sorption. As examples, Chung and Pfost, modified Harkins and Jura, modified Henderson, and Guggenheim-Anderson-De Boer equations at specified conditions were analyzed.     

MOISTURE SORPTION and VOLUMETRIC CHANGES of CANOLA DURING HYDROTHERMAL PROCESSING

The moisture content and volume change of canola seeds immersed in water or exposed to steam were measured. Three water temperatures of 22, 50 and 75C and one wet steam treatment (atmospheric pressure ∼ 100C) were used in the experiments. the maximum moisture attained by the canola seed was 0.67 decimal (db) in soaking and 0.54 decimal (db) in steaming. the moisture sorption rates were higher at higher temperatures of soaking. the coefficient of expansion did not vary significantly among treatments and an average coefficient of 1.032 per unit change in moisture content was obtained.     

MOISTURE SORPTION STUDY ON NIGERIAN FOODS: MAIZE and SORGHUM

Adsorption-desorption behavior of maize and sorghum between a_w= 0.10–0.98 at temperatures of 20, 25 and 40C was studied. the grains exhibited type II sorption isotherm. the sorption data were analyzed using six sorption models and the coefficients of determination were between 0.6965–0.9994. the Caurie model gave the poorest fit and the Henderson model the best. Some models were better for adsorption than for desorption. the GAB and BET monolayer moisture contents were not significantly (p > 0.05) different. Monolayer values were temperature-dependent with activation energy from 1.36–10.08 MJ/mol and the maize monolayer values exhibited the highest sensitivity to temperature. Heat of sorption was obtained by applying Claussius-Clapeyron equation to the sorption isotherms at the three temperatures. the heat of sorption decreased with an increase in moisture content and an exponential equation was used to describe the relationship. the processing and storage consequences of this information were discussed.     

MOISTURE TRANSFER PROPERTIES of WILD RICE

Working isotherms for processed wild rice and desorption isotherms for unprocessed wild rice were determined at temperatures ranging from 10 to 43.5°C. the constants for the Guggenheim-Anderson-DeBoer (GAB), Day and Nelson, Chen and Clayton and modified Halsey equations were determined by using a nonlinear optimization technique. the GAB equation showed extremely good fit to the experimental data (less than 5% error). the diffusion coefficients for moisture transport in both broken and whole processed wild rice and for unprocessed wild rice samples were determined at room temperature. the processed broken wild rice kernels had an effective diffusion coefficient of 2.66 × 10^?9 m²/h, whereas the effective diffusion coefficient was 7.08 × 10^?10m²/h for the processed whole wild rice kernels. Unprocessed wild rice had moderate effective diffusivity (1.4 × 10^?9 m²/h). These results predicted quite well the equilibrium water activities and time to equilibrium of wild rice-white rice mixtures using both analytical methods and the finite element method. the transfer of moisture to or from packaged wild rice was also illustrated for different distribution systems.     

PHYSICAL and THERMAL PROPERTIES of GORGON NUT

Bulk density, true density, angle of repose, coefficient of friction on metal surfaces, specific heat, thermal diffusivity and conductivity of gorgon nut were determined using standard techniques for different sizes of nuts at moisture contents and temperatures ranging from 15 to 60% (dry basis) and 25 to 55C, respectively. the physical properties varied quadratically with moisture content. Specific heat increased with moisture and temperature but decreased with the size of the nut; whereas, the thermal diffusivity showed a reverse trend. Thermal conductivity increased with moisture content but did not follow any trend with temperature within the range of the study. the physical and thermal properties data at various moisture contents and temperatures were used to develop equations for different sizes of gorgon nuts.     

EFFECT OF SPRAY-DRIED OR REDUCED-CHOLESTEROL YOLK AND TEMPERATURE ON THE LINEAR VISCOELASTIC PROPERTIES OF MAYONNAISE

The dynamic viscoelastic properties of five mayonnaise products were evaluated at 5–30C and 0,031-31,4 s⁻¹. The composition of the products was held constant, while the type of egg product was varied. Two egg yolks (native and spraydried) and three whole egg products (native, dried-yolk containing and low-in-cholesterol-yolk containing) were used as the emulsifiers. Temperature generally produces a decrease in the viscoelastic functions. A replacement of yolk (native or not) by egg white reduces the viscoelastic properties of mayonnaise. Processed yolk gives rise to greater viscoelastic parameters than native yolk. This effect loses significance when yolk is diluted by native egg white. The low-cholesterol mayonnaise has similar viscoelastic properties when compared to commercial mayonnaise.