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Diffusion of Water in Starch Materials
Abstract
SUMMARY The diffusion of water in starch materials is a fundamental mass transport process, which is important in the processing and storage of various starch-based foods. The effective diffusivity of water in heterogeneous starch systems is a complex function of the moisture content, the temperature and the pressure. The water diffusivity, estimated from the drying data or moisture-distribution curves, depends strongly on the physical structure (bulk porosity) of the material. Mechanical compression, air pressure, gelatinization or addition of sugars reduced substantially the water diffusivity. Water may be transported by liquid or vapor diffusion, depending on the physical structure of the starch system. The diffusion of water is involved in various physical and chemical processes applied to foods, such as cooking, baking, dehydration and extrusion cooking.
... Bananas exhibit significant inter-varietal variability in their composition, such as sugars, proteins, or lipids contents [1] , which can impact moisture diffusivity. This variation in moisture diffusivity has been observed in starch-based systems when simple sugars are added to the matrix [50][51][52] . Figure 4(A-C) display the validation data for the strain equation (15). ...
The transport and equilibrium properties of water in food mixtures are important in various processing operations and in storage stability of food products. The dough/raisin mixture is a typical mixture of different food materials, used in bakery products. The water diffusivity at 15–70 °C in dough of 20–60% moisture and in raisins of 20–40% moisture was determined from moisture distribution measurements in two contacted cylinders of dough/raisin pulp, applying the diffusion equation. The sorption isotherms of the two materials were determined using a water activity apparatus and the standard gravimetric method. A sharp discontinuity in moisture distribution was observed at the dough/raisin interface with higher concentration in the raisin side, evidently due to differences in water activity of the two materials. The water diffusivity in raisins, estimated from moisture distribution was similar to the diffusivity obtained from drying data (1–5 × 10−10 m2/s). The water diffusivity in the dough was in the same range. The diffusion and equilibrium properties of moisture may control the physicochemical and microbial changes occurring in dough/raisin mixtures.
The method of distance-concentration curves was applied to the estimation of water diffusivities in starches at high temperatures (25-140 °C) and pressures (up to 40 atm). Temperature had a positive (Arrhenius) effect on the water diffusivity. Pressure decreased the water diffusivity and caused an increase of the energy of activation for diffusion of water in starch, especially at pressures of 1–10 bar. Water diffusivities varied from 0.3×10−10 to 30×10−10 m2/s, and they were higher in hydrated starches than in gels, evidently due to the higher porosity. The water diffusivity in hydrated granular starch decreased with increasing moisture content, due to their lower porosity.
The physical structure, developed during drying, affects the transport of water and the quality of processed solid foods. the effective diffusivity of water in starch-based systems was estimated from drying data at moisture 3–50% and temperatures 40-100°C. Hydrated granular or gelatinized starches and their mixtures with sugars or inert particles were used to produce different structures during air drying. the physical structure was characterized by the bulk porosity, estimated from the bulk and solid densities of the samples. the water diffusivity was influenced strongly by the porosity, developed during drying. Mechanical compression, starch gelatinization or addition of water-soluble sugars reduced the water diffusivity in granular starches. Incorporation of inert particles or extrusion cooking resulted in higher water diffusivities, due to higher porosities of the starch/inert mixtures or the expanded extrudates.
The particle density of granular and gelatinized corn starches was determined in the moisture range 0 to 1 kg water/kg dry solids, using a gas stereopycnometer. The bulk porosity (void fraction) of spherical starch samples at various moisture contents was estimated from the bulk and particle densities during air drying at 60°C. The particle density data fitted a polynomial function of moisture content (X), passing through a maximum value of 1500 kg/m3 at X = 0.15. The bulk porosity of the starch samples increased linearly during drying, reaching a value of 0.45 near dryness. Differences in structure between granular and gelatinized starches during drying were observed by stereomicroscopy. The changes in porosity could be related to variations of the effective moisture and thermal diffusivities of starch materials.
The effective moisture diffusivity (D) in starch materials was estimated by the method of slopes of the drying curve and a computer simulation technique. Drying data (moisture vs time) were obtained on slabs and spherical samples of hydrated and gelatinized starches in an air-dryer operated at 60–100°C and air velocity 2 m/sec. The two methods gave similar results in high-amylopectin starch gels of low porosity, where liquid diffusion might predominate during drying. Considerable differences between the two methods were found in hydrated granular starches and in porous high-amylose gels, where vapor diffusion might be the main water transport mechanism.