J.G. Brennan

Changes in structure, density and porosity of potato during dehydration

Abstract Structural changes in potato during drying were studied by light microscopy. It was observed that the degree of shrinkage of potato during low-temperature drying is greater than at high-temperature drying. Shrinkage also affects the physical properties of materials, such as the density and porosity. In the early stage of drying the density increased as the moisture content decreased, reaching a peak and then decreased with further decrease in moisture content. The density at a given moisture content decreased with increasing drying temperature. A model was developed to represent the relationship between the density and moisture content. The total porosity increased steadily as moisture content decreased in the early stages of drying and then increased sharply towards the end of drying. The percentage changes in thickness, length and width of the potato samples during drying increased linearly with decreasing moisture content.

Moisture sorption isotherm characteristics of potatoes at four temperatures

Abstract Moisture equilibrium data of potatoes by desorption and adsorption were determined at 40, 50, 60 and 70°C. The experimental procedure was a gravimetric method. Isosteric heats of water desorption and adsorption were calculated by applying the Clausius-Clapeyron equation to the sorption isotherms at different temperatures. Seven mathematical models were used to fit the experimental data. A nonlinear least-squares regression program was used to evaluate the constants of the seven desorption and adsorption isotherm models.

A mathematical model of simultaneous heat and moisture transfer during drying of potato

Abstract A mathematical model of simultaneous heat and moisture transfer is proposed for the prediction of moisture and temperature distributions during drying in a slab-shaped solid. The model took into account the effect of moisture-solid interaction at the drying surface by means of sorption isotherms of food. Non-constant physical and thermal properties were also incorporated in the model. The model was applied to the air drying of potatoes. A finite difference method (Crank-Nicolson) was used in the solution of simultaneous heat and moisture transfer equations at different times during drying. When the experimental results were compared with those obtained from the finite difference method, good agreement was found.

Instrumental and sensory evaluation of crispness: I—In friable foods

Using the Constant Loading Rate Texture Testing instrument it was possible to extract from the deformation/time and force/time curves together with the sound emitted from the samples during the test, those variables included in both the definition of crispness and the assumption that in the case of friable foods crispness may be regarded as ‘compressive brittleness’. Significant correlations (P<0·01) were obtained between sensory crispness and the instrumental variables: equivalent sound level Leq (r = 0·701), ratio of work done during fracture to total work WF/WT (r = 0·878) and fracture or collapse rate (r = 0·570). A poor correlation between sensory crispness and both sensory hardness (r = 0·398) and instrumental fracture force (r = 0·018) indicates that a hard friable food material is not necessarily a crisp one, whereas the highly significant correlation between sensory crispness and both sensory sound intensity (r = 0·908) and instrumental Leq (r = 0·701) indicates the importance of sound in the sensory and instrumental evaluations. The results of regression analysis showed a number of almost equally well fitting equations with the ratio WF/WT and Leq as the dominant variables in predicting sensory crispness in friable foods.

Optimisation of high temperature puffing of potato cubes using response surface methodology

One cubic centimetre potato cubes were blanched, sulfited, dried initially for between 40 and 80 min in air at 90 °C in a cabinet drier, puffed in a high temperature fluidised bed and then dried for up to 180 min in a cabinet drier. The final moisture content was 0.05 dwb. The resulting product was optimised using response surface methodology, in terms of volume and colour (L*, a* and b* values) of the dry product, as well as rehydration ratio and texture of the rehydrated product. The operating conditions resulting in the optimised product were found to be blanching for 6 min in water at 100 °C, dipping in 400 ppm sodium metabisulfite solution for 10 min, initially drying for 40 min and puffing in air at 200 °C for 40 s, followed by final drying to a moisture content of 0.05 dwb.

The influence of moisture content and temperature on the specific heat of potato measured by differential scanning calorimetry

Abstract Differential Scanning Calorimetry was used to determine the specific heat of potato. The effects of temperature and moisture content on specific heat were investigated. Specific heat increased quadratically with moisture content over the range 0–4·13 (g water/g solid) and linearly with temperature varying from 40 to 70°C. A model was proposed to describe the effects of moisture content and temperature on the specific heat of potato. Good agreement was found between specific heat values determined experimentally and predicted by the model.

Thermal conductivity of potato as a function of moisture content

Abstract To measure the thermal conductivity of potato, a line-source probe system was developed. The thermal conductivity of potato was determined at various moisture contents in the temperature range of 40–70°C, using the heated probe method. The thermal conductivity of potato decreased with the decrease in moisture content. Temperature had little effect on the thermal conductivity. Thermal conductivity data were correlated with moisture content by a semi-logarithmic equation.

A proposed mechanism of high-temperature puffing of potato. Part I. The influence of blanching and drying conditions on the volume of puffed cubes

Potato cubes were blanched and partially dried in a cabinet drier prior to puffing in a Torbed 500, high-temperature fluidised bed drier. It was established that formation of a partially dried layer (PDL) on the surface of the cubes was necessary to achieve puffing. The influence of blanching and drying conditions on this layer was investigated. It was concluded that blanching for 2 min in water at 100°C followed by drying in air at 90°C for 40 min resulted in the maximum increase in volume of the puffed cubes.

Moisture sorption isotherm characteristics of plantain (Musa, AAB)

Abstract Moisture sorption isotherms of fresh, pre-treated and freeze-dried plantain, at 40°C, 50°C and 60°C, were constructed using the static gravimetric method. The pre-treatment involved blanching and infusion in 40° Brix sucrose at 4°C for 16 h. The isotherms of the fresh plantain were of BET type II, while those of the freeze-dried samples were BET type I. Hysteresis was observed between the adsorption isotherm of the freeze-dried plantain and the desorption isotherm of the fresh samples. At low values of a w , below 0.40, the moisture contents of the pre-treated samples were lower than that of the fresh plantain, at corresponding values of a w . However, above a w 0.50 the reverse applied. In both desorption and adsorption an increase in temperature resulted in lower equilibrium moisture contents at corresponding values of a w . The monolayer moisture contents of the moist-infused and freeze-dried samples were lower than those of the fresh material. Of the five models tested the Henderson, S. M. (1952, Agricultural Engineering 33 , 29–32) and GAB (1981) versions fitted best to the isotherms of the fresh material. The Iglesias & Chirife model (1978) was the best fit to the isotherm of the infused material. The isosteric heat of sorption for the fresh plantain, ranged from 47.4 kJ/g mol at 18% moisture, dry solids basis, to 73.2 kJ/g mol at 2% moisture, dry solids basis. The comparable range for the freeze-dried material was 42.2–635 kJ /g. mol.

Air-drying characteristics of plantain (Musa AAB)

Abstract The drying behaviour, shrinkage and moisture distribution within cylindrical pieces of plantain, of varying thickness, and with different air temperatures, were studied in an experimental hot-air drier. By means of response surface methodology the influence of air temperature and thickness of the pieces on the drying rates was established. Air temperature had the greatest influence on drying behaviour. The activation energy for air-drying of plantain was estimated as 38.81 kJ (g.mol) −1 . Shrinkage during drying was followed by changes in volume and dimensions of the pieces. Change in volume could be well described by a core drying model, while change in dimensions was related linearly with moisture content. A simulation, based on Ficks diffusion equation, was used to predict the distribution of moisture within the plantain pieces during drying. The predicted values related well to those obtained experimentally at moisture contents less than 1.20kg (kg d.s.) −1 . This type of model could be used to determine local moisture contents rather than average moisture contents in food pieces, thus predicting more accurately the susceptibility of the food to spoilage.