Alberto M. Sereno

Modelling shrinkage during convective drying of food materials: a review

Shrinkage of foodstuffs is a common physical phenomenon observed during different dehydration processes. These changes affect the quality of the dehydrated product and should be taken into consideration when predicting moisture and temperature profiles in the dried material. The aim of this work is to give a physical description of the shrinkage mechanism and present a classification of the different models proposed to describe this behaviour in food materials undergoing dehydration. The models were classified in two main groups: empirical and fundamental models. Empirical models are obtained by means of regression analysis of shrinkage data. Fundamental models are based on a physical interpretation of the structure of food materials and try to predict dimensional changes due to volume variation of the different phases in the food system along the drying process. Several models referred to in this work were compared with experimental data on air drying of apple, carrot, potato and squid flesh. Average relative deviations between experimental and predicted values of shrinkage found were in most cases less than 10%. For some materials, models that neglect porosity change tend to show larger deviations.

Quantification of microstructural changes during first stage air drying of grape tissue

Abstract Microstructural changes in cells of Ruby grape ( Vitis vinifera ) quarters were monitored during first stage of convective air drying, under a stereo-microscope. A gradual overall shrinkage of grape cells was observed during the process. The cellular parameters: area, perimeter, major and minor axis length, Feret diameter, elongation, roundness and compactness, were quantified by image analysis. It was verified that cell dimensions suffered modifications during drying, but their shape remained unchanged. These microstructural changes showed a smooth exponential decrease with time, and a first-order kinetic model was satisfactorily fitted to the data. Temperature increased the rate of cellular shrinkage and this effect followed an Arrhenius type behaviour. Increasing temperature from 20 to 60 °C resulted in a 350% increase of the area change rate. For the parameters related to cellular dimensions, the magnitude of the values were 10 −3 min −1 for the rate of change at 40 °C and 3 kJ/mol for the activation energy.

SHRINKAGE OF APPLE DISKS DURING DRYING BY WARM AIR CONVECTION AND FREEZE DRYING

ABSTRACT Volumetric and thickness shrinkage evaluated by direct measurement and n-heptane displacement were determined during convective and freeze drying of Golden delicious apples. For convective drying, the influence of blanching and diameter/thickness ratio of the apple disks used were analysed at different levels of moisture content under constant conditions. It was found that shrinkage of dried samples, both by convection and by freeze-drying, is anisotropic to a level which depends on sample geometry (ratio diameter/thickness) used. Blanching did not affect shrinkage results. Based on results obtained a new model to predict bulk density of materials during drying is proposed, showing a better fit to experimental data than previous models reported in the literature. This model was further used to predict changes in apple porosity during drying.

Glass transitions and state diagrams for typical natural fruits and vegetables

Abstract Differential scanning calorimetry (DSC) was used to measure phase transitions and unfreezable water in fresh samples of onion, grape and strawberry after equilibration at different relative humidities. From the DSC trace for each product, the glass transition and melting temperatures, specific heat capacities, latent heats of melting and unfrozen water contents were obtained. It was found that annealing of the sample was necessary to allow devitrification of water and a minimal amount of non-frozen water in the amorphous matrix. The effect of freeze-concentration was observed in an increased glass transition temperature, a gradual disappearance of the ice formation exotherm, and a consequent increase in the ice-melting endotherm. State diagrams for these food samples were defined. The Gordon-Taylor equation was able to predict the glass transition temperature for the water-food systems studied, from the corresponding pure component values.

Prediction of water activity of osmotic solutions

Models to correlate and predict water activity in aqueous solutions of single and multiple solutes, including electrolytes, relevant for osmotic processing of foods are reviewed. During the last decade a significant number of theoretical thermodynamic models that are applicable to these systems have been developed and published. Though their use is still limited, their performance is in general very good, similar to the best traditional empirical equations. Their predictive character together with built-in capabilities to work at different temperatures and in some cases pressure suggests that an increased effort to their wide use should take place. It was found that predictions of water activity in aqueous solutions may easily be made with average relative deviations of less than 2%; this value is of the same order or in some cases less than the typical error of current instrumentation available to measure water activity.

A simplified model for the prediction of drying rates for foods

Abstract A simplified mathematical model based on basic physical and transport properties, such as mass and heat diffusivities, is proposed for the prediction of the behaviour of foods during drying. The model takes into account the effect of moisture-solid interaction at the drying surface by means of any sorption equation available and the change in solid density due to shrinking. Fouriers and Ficks laws describe the transfer of heat and mass in the solid. At the surface, mass and heat balances together with the chosen sorption equation are used to represent the vaporization of water. Temperature and moisture-content profiles are obtained by integration of the resulting set of partial differential equations, an implicit-finite-differences algorithm being used. The model, which predicted experimental results for the drying of apple slices and carrot cubes to within 1·1% of the moisture content and 12% of the drying rate, can therefore be used for the determination of drying profiles.

Phase diagram for freeze-dried persimmon

Phase transitions of freeze-dried persimmon in a large range of moisture content were determined by differential scanning calorimetry (DSC). In order to study this transitions at low and intermediate moisture content domains, samples were conditioned by adsorption at various water activities (aw=0.11–0.90) at 25°C. For the high moisture content region, samples were obtained by water addition. At aw≤0.75 two glass transitions were visible, with Tg decreasing with increasing water activity due to water plasticizing effect. The first Tg is due to the matrix formed by sugars and water. The second one, less visible and less plasticized by water, is probably due to macromolecules of the fruit pulp. At aw between 0.80 and 0.90 a devitrification peak appeared after Tg and before Tm. At this moisture content range, the Gordon–Taylor model represented satisfactorily the matrix glass transition curve. At the higher moisture content range (aw>0.90), the more visible phenomenon was the ice melting. Tg appeared less visible because the enthalpy change involved in glass transition is practically negligible in comparison with the latent heat of melting. In the high moisture content domain Tg remained practically constant around Tg′ (−56.6°C).

Glass transitions and state diagrams for fresh and processed apple

Differential scanning calorimetry (DSC) was used to measure phase transitions in samples of Golden Delicious apples after freeze-drying and osmotic drying in a sucrose solution. From DSC traces, glass transition (Tg) and melting (Tm) temperatures were obtained and used to plot the state diagrams for the two types of samples. The Gordon‐Taylor equation was able to predict the dependence of the glass transition temperature on moisture content. Before calorimetric analysis, dehydrated samples were equilibrated under a wide range of different relative humidities (aw 0.12‐0.93) and sorption isotherms determined. Experimental sorption isotherms agreed with previous results reported in the literature. # 1999 Elsevier Science B.V. All rights reserved.

Evaluation of mass transfer coefficients and volumetric shrinkage during osmotic dehydration of apple using sucrose solutions in static and non-static conditions

Abstract Effects of temperature, solution concentration and solution flow rate on osmotic dehydration/impregnation rate observed during immersion of apple cylinders in sugar solutions at low temperature (⩽25 °C) are presented. Water and sucrose mass transfer coefficients are calculated and correlated using Magee’s model; an extension of the model is proposed to account for solution flowrate; in both cases average relative deviations of less than 3% are obtained. Analysis of results obtained suggests that solute gain by the sample be controlled by diffusion inside the material while water loss is governed by mixed internal–external flow. Additionally, volumetric shrinkage of apple cylinders treated by osmotic solutions is predicted by measuring the change of the moisture content or the net mass loss due to the treatment. Volume changes observed in samples were correlated linearly with moisture content (dry basis) and with the net change in sample weight. These results suggest that shrinkage be essentially due to water removal/solid gain and offer a simple way to predict such changes during industrial processing.

Microstructural Changes during Drying of Apple Slices

Abstract In recent years some effort has been done trying to relate microstructural changes of dehydrated foods with macroscopical properties and quality factors of the processed product. This work assesses the microstructural changes of apple slices submitted to convective drying. Apple slices (25.7 mm diameter, 2.6 mm thickness) were dried in an oven at 70°C on a screen sample holder in order to allow mass transfer in any side of the sample. Microphotographs of the same sample were taken at determined time intervals using a stereomicroscope. The objects in these photographs were identified and classified in three size groups: (a) the cells group, containing objects with an area less than 0.03 mm2; (b) the mixed cells-intercellular spaces group, with an area between 0.03 and 0.06 mm2, and (c) the intercellular spaces group, including objects with an area higher than 0.06 mm2. Geometrical parameters of such objects, namely size (area, perimeter, equivalent diameter, and major and minor axis length) and shape parameters (compactness, elongation, and roundness) were evaluated. Shrinkage of cells and intercellular spaces was very clear during drying, observing a decrease of size with moisture content. Concerning shape factors, compactness remained constant, roundness slightly decreased during drying, and elongation increased in the final stage of the process. Macroscopical changes of the entire disc followed the same behavior as microscopical ones, suggesting that changes at the two scale levels are strongly related.