http://paginas.fe.up.pt/~sereno/publ/2004/JFE_ShrinkReview.pdf

Modelling shrinkage during convective drying of food materials: a review

Revue bibliographique sur les transformations physicochimiques provoquees par la cuisson-extrusion sur l'amidon, les lipides, les proteines, les elements mineraux, les fibres, les vitamines, les contaminants et les facteurs antinutritionnels presents dans les aliments

Chemical and nutritional changes in foods during extrusion

Starch gelatinization phenomena is extremely important in many food systems. This review focuses on factors affecting gelatinization characteristics of starch. Important variables which must be considered in design of processes in which starch undergoes gelatinization are heat of gelatinization and temperature of gelatinization. Major interactions are reviewed for the effects of lipids, moisture content, nonionic constituents and electrolytes on these characteristics. Furthermore, treatment of starch-containing systems prior to heating into the gelatinization temperature range can have a significant effect on ultimate gelatinization characteristics.

Influence of time, temperature, moisture, ingredients, and processing conditions on starch gelatinization.

Dependable data on bulk density, volumetric shrinkage due to water loss and porosity are needed to model processes such as drying, packaging and storing. Experimental data are presented for all three properties. It is possible to model the water-loss-based bulk shrinkage coefficient to obtain a predictive equation based on composition of the foodstuff. From this, a generalized correlation is obtained which predicts bulk shrinkage coefficient knowing only the initial moisture content of the food. Porosities for the foodstuffs considered can be predicted through suitable correlations, but there is no generalized equation spanning all foods.

Shrinkage, Porosity and Bulk Density of Foodstuffs at Changing Moisture Contents

Changes in structure, density and porosity of potato during dehydration

The shrinkage in dehydration of root vegetables such as carrots, potatoes, sweet potatoes and radishes was investigated. The samples were dried in flowing hot air. The surface areas were measured by photographing the samples. Three drying models were postulated for the formulation of the relation between the changes of the surface area and the moisture contents. The uniform drying model, which in the case of drying is that the shrinkage in volume equals the volume of water lost by the evaporation, agreed with the measured values during the early stages of the drying. The core drying model by assuming the formation of the dried layer at outer side of material was better in agreement with the experimental results. The semicore drying model, which is the intermediate model between the uniform and the core drying model, and the empirical equations of the shrinkage were also considered.

Shrinkage in dehydration of root vegetables

The mechanism of cooking rice was investigated in this study. The rheological method using the parallel plate plastometer was adopted for measurement of the degree of cooking. The range of temperatures measured ran from 75–150°C. Experimental results showed that cooking rate followed the equation of a first order chemical reaction. We designated the proportional constant as the cooking rate constant, but the slope of Arrhenius plots of the cooking rate constants changed around 110°C. The activation energy of cooking at temperatures below 110°C and above 110°C was about 19,000 and 8,800 cal/mol respectively. The influences of water soaking time before cooking were also studied. We concluded that the cooking process comprises two mechanisms; at temperatures below 110°C the cooking rate is limited by the reaction rate of rice components with water; and at temperatures above 110°C it is limited by the rate of diffusion of water through the cooked layer toward the interface of uncooked core where the reaction occurs. The reaction rate constant and the diffusion coefficient of water were calculated by assuming the core model or shell-type model.

Kinetic studies on cooking of rice

The gelatinization rate of rice and potato starches were investigated. The rheological method, using a capillary tube viscometer, was applied for the measurement of the gelatinization degree. The gelatinization rates were measured for the rice and potato starches at temperature ranges of 70˜85 and 60˜63 °C respectively. The rate parameter in the rate equation was given as the Arrhenius equation. The equivalent values of the activation energy for the rice and potato starches were about 14 and 230 kcal/g-mol respectively. We concluded that the gelatinization rate of starches is limited by the chemical reaction rate and/or the physical transforming rate of starch components with water.

Studies on the gelatinization rate of rice and potato starches

The soaking and the cooking mechanisms of rice were investigated in terms of the mathematical rate equations in this study. The soaking and the cooking rate were measured by using the weighing method, and the soaking rate was examined at temperatures from 8 −50°C, and the cooking rate from 70 – 98.5°C. The rate equation involved two rate parameters: the reaction rate parameter of the rice component with water and the diffusion rate parameter of water, was assumed. The results showed that the cooking rate was mainly limited by the reaction rate of rice components with water at temperatures from 70 – 98.5°C, and the equivalent value of the activation energy of the reaction rate was nearly equal to 20 kcal/mol, though the cooking rate was relatively influenced by the diffusion rate of water in the cooked rice layer at 98.5°C. In the soaking of rice, the values of the diffusion rate parameter at from 8.0 –50°C were smaller than the values at from 110 – 150°C.

Studies on the cooking rate equations of rice

Particles were dried in a vibro-fluidized bed and the drying characteristics in a constant drying rate period are discussed. Ion exchange resin particles were used for sample because they were fluidized smoothly even at a somewhat high moisture content. The effects of vibration on the uniformity of moisture content in the bed were remarkable when air velocities become lower than the minimum fluidization velocity umf. The bed could be dried uniformly and it had no moisture content distributions under the appropriate vibrational conditions, even when air velocities were lower than umf. The uniformity of moisture content in the bed depended upon air velocity, vibrational intensity, and height of bed.

Hideaki Hosaka

Papers

Shrinkage in dehydration of root vegetables

Kinetic studies on cooking of rice

Studies on the gelatinization rate of rice and potato starches

Studies on the cooking rate equations of rice

Drying characteristics of particles in a constant drying rate period in vibro-fluidized bed