Modelling moisture sorption isotherms for maize flour

The sorption isotherm of food material is pertinent in the processing and storage of food products. Adsorption and desorption isotherms for maize flour were investigated using the static gravimetric method over the range of temperature (27–40 °C) and water activity (a_w) (0.10–0.80) commonly experienced in the tropical environment. The experimental data were compared with five widely recommended models in the literature for food sorption isotherms (GAB, modified GAB (MGAB), modified Oswin (MOE), modified Henderson (MHDE), and modified Chung–Pfost (MCE)). The GAB, MGAB, and MOE models were found to be acceptable in predicting the moisture sorption isotherms for maize flour. Overall, the MGAB appears to be most suitable for fitting the adsorption and desorption moisture isotherms data for the maize flour.     

Changes in maize starch water sorption isotherms caused by high pressure

The effect of high pressure (300 MPa, for up to 60‐min duration) on water sorption (adsorption and desorption) properties of maize starch suspensions was studied. Increase in the pressurisation time (5–60 min) augmented linearly the water‐holding capacity of starch by 2.7‐fold. Adsorption and desorption isotherms moved to higher moisture contents with the increment of pressurisation time. These movements caused the hysteresis effect extent to decrease/increase with the pressurisation time for lower/higher water activity values. While no particular tendency was found for the effect of pressurisation time on energy‐related constants, Cb (BET) and Cg (GAB), the K value decreases linearly, being always higher for sorption. The monolayer value obtained using the BET model was lower compared with the GAB model, increasing linearly in both cases with the pressurisation time. The results obtained in this study are of possible technological importance for food products containing starch.     

Isohalic sorption isotherms

The sorption behaviour of dried salted cod ( Gadus morrhua ) can be predicted from the physical properties of sodium chloride solutions and the sorption characteristics of fish muscle.     

Isohalic sorption isotherms.

Isohalic sorption isotherms of cod ( Gadus morrhua ) are used, in conjunction with growth data for dun mould ( Wallemia sebi ), to predict the shelf life of dried salted fish in tropical climates. Measured shelf-lives of several fish species processed and stored by traditional methods are in agreement with the predictions.     

Isohalic sorption isotherms.

The sorption behaviour of a dried salted tropical fish, lisa (Xenomugil thoburni), produced under semi-commercial conditions was investigated. It was found to be similar to that of freeze-dried cod (Gadus rnorrhua) published earlier. The results confirm that the procedure for predicting the water activity of cod muscle from the physical properties of sodium chloride solution and the sorption characteristics of fish muscle can also be applied to tropical fish processed commercially.     

Modified classification of sorption isotherms

A new equation is developed in this paper which is further used to divide sorption isotherms into classes which are connected to surface water effects and/or solution water effects and their combinations. It is shown that sorption isotherms can be successfully classified using the new equation into three basic classes I, II (most frequent) and III which roughly correspond to the original Brunauer’s classes. The equation is also used for further division of class II into three subclasses. Criteria for these classifications are presented.     

Model analysis of sorption isotherms

The equation developed by Blahovec and Yanniotis, which is based on surface adsorption and solution water, was applied for fitting experimental sorption data for starchy and high protein foods, fruits and vegetables, nuts, legumes, and seeds. Analysis of sorption isotherms shows that surface adsorption is more important than solution water in the isotherms for cereals, potatoes, legumes and seeds, while in vegetables, fruits, meat, milk products and some nuts solution water is more important. The ratio of solution water to surface adsorption increases as water activity increases, it is higher for the adsorption isotherm than the desorption isotherm at any water activity value and decreases as temperature increases.     

Moisture sorption isotherms of potato slices

Water vapour sorption isotherms of Norchip, Russet Burbank and Norland potatoes were determined by the salt solution technique at 10, 25 and 40°C. The adsorption isotherms of potatoes were of sigmoid shape and were affected by drying method, temperature and sugars addition. The freeze dried products adsorbed more water vapour than the vacuum dried materials. Addition of glucose, sucrose or lactose to the potatoes caused the equilibrium moisture content to decrease in the low and intermediate water activity ranges. The sorption was temperature independent in the a _w range 0.11–0.35 and showed a hysteresis loop at intermediate moisture levels.     

Moisture sorption isotherms of green chilli

The adsorption and desorption equilibrium moisture contents of green chilli were determined experimentally in relative humidity range of 11‐97% and at the temperatures of 20, 30, 40 and 50°C. The experimental procedure used was a dynamic method with periodically recording of sample mass. The effect of temperature on adsorption and desorption isotherms was found significant. Hystereses were observed for entire range of relative humidity and hysteresis loops decreased with the increase of temperature. Nine sorption isotherm models were fitted the experimental data at the temperatures of 20, 30, 40 and 50 °C. The modified Smith equation was the best fitted equation to the experimental data for relative humidity range of 11‐97% for the adsorption and desorption isotherms of green chilli.     

The prediction of moisture sorption isotherms for dairy powders

Moisture sorption isotherms were measured for whey protein isolate, high micellar casein and a milk protein concentrate powder. No temperature dependence was observed over the temperature range of 4–37 °C. At 50 °C the powders absorbed less moisture than observed at the lower temperatures. These isotherms were used to predict the isotherms for freeze-dried amorphous lactose/casein/whey protein powders. An isotherm for micellar casein was predicted using a simple additive isotherm model and was used along with isotherms for whey protein and amorphous lactose to predict moisture sorption isotherms for commercial dairy powders. Predicted isotherms compared well with measured isotherms indicating that this simple additive isotherm model is suitable for predicting moisture sorption isotherms of dairy powders. Delayed lactose crystallisation was observed in lactose/whey protein powders when compared to lactose/casein powders over the same water activity range.     

Moisture sorption isotherms of cereals at different temperatures

In this research, moisture sorption isotherms of wheat (Kink and Lancer) barley, rye, oat and corn were determined at 20, 25, 35, 50 and 70 degrees C. The sorption isotherm curves of all cereal samples showed the characteristics of type II isotherm. This indicated that the adsorption occurred in cereal samples was a multilayer adsorption and cereal samples were of a microcapillary structure. In addition, the adsorption in cereal samples decreased as temperature increased.     

The applicability of the GAB model to food water sorption isotherms

Summary  The fitting ability of the GAB equation has been analysed. It has been shown that the GAB model describes well sigmoidal type isotherms when parameters are kept in the following regions: 0.24 7lt; k ≤ 1 and 5.67 ≤ c ≤∞. Outside these regions the isotherm is either no longer sigmoidal or the monolayer capacity is estimated with the error larger than ± 15.5%. Moreover, keeping constants in the above regions fulfils the requirements of the BET model.     

Moisture sorption isotherms of some Nigerian food grains

A fluidized bed apparatus that can be used to determine moisture sorption isotherms of granular food products is described. This apparatus was used to obtain moisture sorption isotherms for maize, cowpeas, groundnuts and soyabeans at temperatures of 30, 40 and 50°C. Hysteresis was observed in the isotherms for the four products and the equilibrium moisture content was lower for the oily grains (soyabeans and groundnuts) at the same value of relative humidity. The Hailwood and Horrobin equation was used to correlate the experimental data and the values of the correlating constants are tabulated. The Brunauer, Emmett and Teller equation was also used to obtain estimates of the monolayer water content for the four grains.     

Solubility-normalized combined adsorption-partitioning sorption isotherms for organic pollutants

Equilibrium sorption isotherms were measured for five different low-polarity organic compounds (benzene, trichloroethene, 1,2- and 1,4-dichlorobenzene, and phenanthrene) over a wide concentration range. The investigated sorbents can be grouped into the following three classes: (1) humic soil organic matter, which shows linear sorption isotherms (solely partitioning, as observed in the peat sample); (2) carbon materials, which were thermally altered (due to their natural history or industrial production) and thus contain a high specific surface area and exhibit nonlinear isotherms, and (3) pure engineered microporous materials (e.g., zeolites and activated carbon), where adsorption is solely due to a pore-filling process. Sorption of all compounds was fitted very well by the Polanyi-Dubinin-Manes (PDM) model, which for sorbents containing humic organic matter (e.g., peat) was combined with linear partitioning. Both the partitioning and the Polanyi-Dubinin-Manes model predict unique sorption isotherms of similar compounds if the solubility-normalized aqueous concentration is used. In addition, an inverse linear relationship between the distribution coefficient (Kd) and water solubility, which was very well confirmed by the data, is obtained. This also leads to unit-equivalent Freundlich sorption isotherms and explains the often observed apparent correlation between sorption capacity at a given concentration (e.g., Freundlich coefficient) and sorption nonlinearity (Freundlich exponent).     

Phosphate sorption isotherms for some Sierra Leone soils

Phosphate sorption isotherms were obtained for three acid highly weathered soils of Sierra Leone, by equilibrating air dry samples for 24 h with 0.01 _M KH₂PO₄ solution at 25 °C. The Langmuir and Temkin equations were used to evaluate the sorption capacities for phosphate. Phosphate sorption maxima of 2439, 2137 and 1429 mg/kg soil and x/log c of 774, 718 and 610 were calculated; where x=phosphate sorbed (mg/kg soil) and c=equilibrium phosphate concentration (μmol dm^?3). Qualitative relationships between these sorption parameters and other soil variables are shown.