Piotr P. Lewicki

Some remarks on rehydration of dried foods

Inconsistencies in measuring procedures and calculations of rehydration indices for dried foods are shown in the paper. A new method to calculate rehydration ability is proposed, which is based on the capacity of dried material to absorb water and to hold solubles inside the matrix. Rehydration indices of dried potato, apple and root of parsley are expressed by methods described in the literature and the new method. Simplistic and straightforward interpretation of the new indices is shown. Analysis of variance shows which variables must be precisely measured in order to keep the variance of the new indices as small as possible.

Effect of pre‐drying treatment, drying and rehydration on plant tissue properties: A review

Abstract The paper presents the effect of pre‐drying treatment, drying and rehydration on plant tissue properties. Pre‐drying procedures are divided into chemical and non‐chemical treatments. The effect of sulphur dioxide, calcium and sodium chlorides on plant tissue and its behavior during drying and rehydration is discussed. Sugars, organic acids, biopolymers and surface active agents are also used to pretreat plant material before drying. Sugars at low and high concentrations as compounds modifying plant tissue properties are thoroughly discussed. Blanching and freezing are pre‐drying treatments which strongly affect tissue properties. Blanching at low and high temperature affects tissue properties in a different way. The effect of drying on plant tissue properties is discussed under the following headings: shrinkage, porosity, biopolymer structure, and biochemical activity. Rehydration following drying to plant material. Water imbibition causes swelling and build up of swelling stresses. The damage to...

Water as the determinant of food engineering properties. A review

Water affects safety, stability, quality and physical properties of food. The influence of water on physical properties of food is dependent on the state of water in food. The state, expressed as water activity, is briefly discussed in the paper. Further, the influence of water on such physical properties as rheological, thermal, mass transfer, electrical, optical and acoustic is presented in details. 2003 Elsevier Ltd. All rights reserved.

Effect of Drying on Microstructure of Plant Tissue

Abstract Drying causes many chemical and physical changes in plant tissue. Physical changes are mostly due to stresses developed in the tissue and are pronounced by macro- and microalterations of size, shape and internal structure. The extent and direction of these changes depend on the mode of drying. Convective drying of apple tissue results in numerous breaks of cell walls and formation of many microcavities. In consequence, the dominating cross-sectional area of cells moves toward values smaller than those observed in raw apple. Cavities formed during convective drying can be pictured in 2D plane as pentagons joined together or ellipses with large ratio between axes. Puff-drying forms porous structure with cavities twice as large as in raw apple. The process homogenizes the structure, and spread of cell sizes is even smaller than that observed in raw apple. Broken cell walls are clearly visible in microphotographs of sliced samples. Many large cavities with cross-sectional area exceeding 40,000 µm2 are characteristic of puff-dried apple while such areas are hardly found in raw apple. The structure of freeze-dried apple is similar to that formed during puff-drying. Large irregular cavities and broken cell walls are clearly seen in microphotographs. However, larger cavities are present in freeze-dried apple than in puff-dried one. Cross-sectional area of cavities in freeze-dried apple can be simulated by pentagons joined together or elongated ellipses.

Raoult’s law based food water sorption isotherm

Abstract A new model of water sorption isotherm is developed on the basis of Raoult’s law. It is assumed that water present in food occurs in two states, as free water with properties of the bulk water and as water of hydration. Hydrated molecules are considered as new entities with molecular weights larger than those of non-hydrated molecules. Hydration reduces the free concentration of water and thus affects water activity in solution. Application of the developed equation to food sorption data showed that it gives approximation of sorption isotherms much better than that offered by the GAB model. Moreover, it predicts infinite adsorption at a w =1 , the property which is not offered by the GAB equation. The new equation makes it possible to interpolate isotherms at high water activities close to one. The probability that the new equation will fit the food isotherm with small RMS is higher than 90% and substantially exceeds that found for the GAB model. As a two-parameter model it makes substantial improvement over the three-parameter GAB equation.

Effect of pre-treatment on convective drying of tomatoes

Tomatoes of two varieties were dried by convection at 60 °C. Skin was removed and quarters of tomatoes were pre-treated either by soaking in CaCl2 solution or by soaking in this solution followed by osmotic dewatering. Another batch of tomatoes was treated by osmosis in hypertonic solution containing sucrose and calcium chloride. Pre-treatment of tomatoes with calcium chloride increased the rates of convective drying and osmotic dewatering as well. Convective drying of pre-treated tomatoes was 20% shorter than that of raw material. Pre-treatment with CaCl2 increased by 20% the amount of water removed during osmotic dehydration and facilitated infiltration of sucrose. Treatment with CaCl2 followed by osmotic dewatering was more effective than osmotic treatment done in the presence of calcium chloride. Rehydration properties of pre-treated and dried tomatoes were poor. It was suggested that interactions of calcium with polymers stiffened the structure, which on one hand improved drying processes, but on the other hand restricted polymer hydration and swelling during rehydration.

Effect of osmotic dewatering on rheological properties of apple subjected to convective drying

Apple v. Idared cut in cubes was dewatered by osmosis and subsequently dried by convection. Dewatering caused substantial changes in mechanical properties of apple tissue. Shrinkage due to osmotic pre-treatment was small but stiffness was half the value obtained by convective drying of apple cubes to the same final water content. In a viscous-elastic model of apple tissue, osmotic treatment increased the share of viscous element. Concentrated sugar solution in surface layers of pre-treated apple cubes modified the effect of convective drying on rheological properties of the tissue in comparison with those observed during drying of raw apple. Hardening of the surface was observed at the final stages of drying. The more water was removed the difference between osmosed and raw apple, both dried by convection, was the less evident. It is suggested that apple undergoing convective drying is composed by three compartments: surface layers, underneath tissue with lost turgor and structure defects and the core with properties similar to those of raw apple. Osmotic dewatering affects the surface compartment, hence its properties influence response of the material to stress.

Quality of Infrared Dried Apple Slices

Abstract The aim of this work was to compare quality of apple slices dried by near infrared heating and convection in such parameters in which final material temperature in both methods was similar. The infrared drying was done at the distance between the emitters (with total power of 7.875 kW/m2) and heated surface equal to 10, 20, and 30 cm. Flow of ambient air was set at 0.5, 1.0, and 1.5 m/s. Convective drying was done in the same dryer using hot air at 65 and 75°C flowing with velocity 1.5 m/s. Quality attributes measured in this work included: color, kinetics of water adsorption, mechanical properties, and microstructure. It was stated that the changes in chromaticity coefficients are not dependent on the mode of heat supply, but are related to the final temperature of the dried material. Luminance of dried apple slices was affected by temperature as well. Final material temperature, not the way heat is supplied, could be responsible for the differences in the ability of dry apple slices to adsorb water. The similar correlation was stated for mechanical properties: slope of initial part of the deformation curve (crispness), breaking force (hardness or crispness), and work of breaking were all related to the final material temperature. Microstructure of convective and infrared dried apple were different but it seems that the drying rate can be responsible for observed differences.

Water sorption isotherms and their estimation in food model mechanical mixtures

Mechanical mixtures of macromolecules and simple solutes were investigated with respect to their water adsorption capabilities. The additivity principle was applied to predict sorption isotherms of mixtures on the basis of the sorption behaviour of pure components. Mechanical mixtures of macromolecules and mixtures of biopolymers and simple solutes yielded isotherms which were superimposable with those predicted at the low water activity range. In the multilayer water absorption range, predicted isotherms in most cases overestimated the water contents of mixtures in comparison with experimental values. The reasons for the observed differences between experimental and predicted values are discussed. These may include the ordering of the water in contact with macromolecule surfaces, swelling, conformational changes, polymer-polymer interactions, multiple hydrogen bonds, binding of ions, crosslinking, competition for water and plasticization of amorphous regions. The rates of these changes are limited and long time-scale experiments may be needed to observe equilibrium sorption isotherms of mechanical multicomponent mixtures. It is suggested that lack of the additivity of sorption processes should be rather a rule, not an exception in the case of multicomponent mixtures.

Rheological properties of raisins: Part II. Effect of water activity

Abstract Rheological properties of Thompson seedless raisins were measured in relation to the water activity of the berry using the compression-relaxation test. The water sorption isotherm of raisins was determined according to the COST 90 procedure. It has been found that the rheological properties of raisins are strongly dependent on water activity. At water activities lower than 0.25, the raisin behaves as a brittle body. It is inferred that sugars in dry raisins are amorphous and the properties of the glass are pivotal to the characteristics of the material. In this state the probability of breakage during compression is high. At water activities above 0.3, a plasticizing effect of water is manifested. The raisin becomes a viscoelastic body but until aw = 0.4, the elastic component is not strong because of the high viscosity of the interior of the berry. Further uptake of water and increase of aw leads to dilution of the sugar solution and increased elasticity of the skin. The relaxation behaviour of compressed raisins changes around aw = 0.4. The elasticity of the skin is sufficiently high to prevent breakage of raisins during the compression test at stresses lower than 0.3 MPa. Water activity higher than 0.7 weakens the skin and eases of the flow of diluted sugar solution. Deformation of the berry is easy and the probability of breakage increases strongly.