Luis Mayor

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.

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.

Determination of Particle Density and Porosity in Foods and Porous Materials with High Moisture Content

A simple gas pycnometer was developed to obtain the volume of porous food materials. The volume is obtained by measuring the change in pressure experienced by an amount of compressed gas filling a constant volume reference chamber when it expands into a second chamber containing a sample of the material to be tested. From such pressure change and the knowledge of the volumes of the two chambers the volume of the sample solid matrix is determined. The main difference between the proposed pycnometer and the typical helium pycnometer is the sequence of chambers; in the latter gas compression is made in the sample chamber, producing evaporation of water and volatiles when the gas is expanded into the reference chamber. For these reasons the helium pycnometer is only recommended for volume measurements of bone dry solids. The performance of the gas pycnometer was assessed by measuring the volume of different solids (non-porous metallic cylinder, glass spheres, porous metallic cylinder, sintered sand cylinder and apples) and by comparing the results with values from other methods. The gas pycnometer reproducibility of 0.019%, obtained with dried porous materials, is excellent when compared with a commercial helium pycnometer. The gas pycnometer proposed in this work can be easily built and offers reliable results of particle volume for any type of solids, specifically foods and other materials with high moisture content.

Osmotic Dehydration Kinetics of Pumpkin Fruits Using Ternary Solutions of Sodium Chloride and Sucrose

The objective of this work was to obtain experimental data and modeling of osmotic dehydration kinetics of pumpkin fruits (Cucurbita pepo L.) with aqueous NaCl/sucrose solutions. For this purpose, effective diffusion coefficients for water, sucrose, and NaCl were calculated by means of a simple model based on Ficks second law. Water loss achieved 80%, sucrose 13%, and NaCl 6% of the initial sample weight. Effective diffusion coefficients ranged from 0.58–1.40 × 10−9 m2/s, 0.75–1.23 × 10−9 m2/s, and 2.60–4.11 × 10−9 m2/s for water, sucrose, and NaCl, respectively. The proposed model gave a good correlation of the experimental data. The quality of the operation was evaluated by analysis of the values of WL/SG ratio.

Effect of Temperature and Volume Changes on Effective Diffusivities of Sucrose and NaCl during Osmotic Dehydration of Vegetable Tissue

Osmotic dehydration of pumpkin (Cucurbita pepo, L.) fruits was carried out with binary solutions of sucrose and NaCl at different temperatures and solute concentrations. Water loss and solids gain kinetics were experimentally determined and fitted using a diffusional model. Pumpkins samples were considered as finite cylinders and the analytical solution of the unsteady diffusion equation was used considering the external resistance to the mass transfer negligible. The influence of shrinkage and temperature on the effective diffusion coefficients was also assessed in this work.

Mass Transfer Analysis during Osmotic Dehydration of Pumpkin Fruits Using Binary and Ternary Aqueous Solutions of Sucrose and Sodium Chloride

Osmotic dehydration experiments of pumpkin with binary aqueous solutions of sucrose, sodium chloride and ternary solutions with both solutes at 298 K were carried out. Weight reduction, water loss and solute acquisition kinetics were determined. Experimental data were fitted employing a diffusional model considering samples as spheres and the external resistance to the mass transfer negligible. The model gave as parameter of fitting an effective diffusion coefficient for each component transferred (water, sucrose and sodium chloride) for each experimental condition assayed. Correlations between the effective diffusivity and solute concentration were established for binary and ternary systems.

Effective Diffusion Coefficients during Osmotic Dehydration of Vegetables with Different Initial Porosity

Chesnut and pumpkin fruits were dehydrated with osmotic solutions of sucrose and NaCl at 25°C. These food materials have different structure, composition and porosity. Water loss and solids gain kinetics were experimentally determined and modeled using a diffusional model. In spite of the several mass transfer mechanisms taking place along with diffusion during osmotic dehydration, the modeling was satisfactory and involved effective coefficients of diffusion useful to quantify the different mass transfer fluxes. Water and sucrose transfer rates during osmotic dehydration with sucrose solutions are independent on the initial food material characteristics; however they seem to be related with the permeability of these components to a sucrose layer formed in the surface of the samples. In the case of osmotic dehydration with sodium chloride solutions, the coefficients of diffusion show a dependence on food material characteristic and higher values of these coefficients for pumpkin (more porous material) were found.