M. Shafiur Rahman

TOWARD PREDICTION OF POROSITY IN FOODS DURING DRYING: A BRIEF REVIEW

Four generic trends of pore formation during drying are identified from the literature. The present prediction methods are mainly based on empirical correlations. It is common to correlate porosity with water content by quadratic, polynomial, or exponential forms of equations, which do not provide insight into the physics of the process. The glass transition theory is one of the proposed concepts to explain the process of shrinkage and collapse during drying. However, the glass transition theory does not hold true for all products. Other concepts, such as surface tension, structure, environment pressure, and mechanisms of moisture transport also play important roles in explaining the formation of pores. It is hypothesized that as capillary force is the main force responsible for collapse, so counterbalancing this force causes formation of pores and lower shrinkage.

DESORPTION ISOTHERM AND HEAT PUMP DRYING KINETICS OF PEAS

Abstract Moisture desorption isotherms and thin layer drying kinetices of peas in a laboratory pilot heat pump dryer were measured and modeled. A mesh bottom tray was used and air flow was parallel to the two faces of the thin layer. Air drying temperature, and relative humidity were varied from 25 to 65 °C and 0.20 to 0.60, respectively. The air velocity was 1.5ms−1. A two component exponential model was used to represent the heat pump drying curves. The model parameters were correlated with the temperature and relative humidity. The two component model and correlations for the parameters developed can be used to predict the moisture content of peas during heat pump air drying.

State Diagram of Date Flesh Using Differential Scanning Calorimetry (DSC)

Abstract The state diagram of date flesh was developed by measuring its freezing points, glass transition temperatures, maximal-freeze-concentration condition ( and ), and solute melting points (or decomposition temperature) by Differential Scanning Calorimetry (DSC). The freezing curve and glass transition lines were developed using Clausius-Clapeyron equation by incorporating concept of unfrozen water, and Gordon-Taylor equation, respectively. The developed state diagram of date flesh can be used in determining its stability during storage as a function of temperature and moisture content (such as, frozen and dried conditions) as well as in designing drying and freezing processes.

State diagram of apple slices : glass transition and freezing curves

Abstract The state diagram of apple flesh was developed by measuring and modeling its freezing points and glass transition temperatures. The freezing curve and glass transition lines were developed using Clausias–Clapeyron and Gordon–Taylor models, respectively. The state diagram of apple pieces developed in this work can be used in determining the stability during frozen storage and in dried conditions as well as in designing drying and freezing processes.

Prediction of pores formation (porosity) in foods during drying: generic models by the use of hybrid neural network

Abstract General porosity prediction models of food during air-drying have been developed using regression analysis and hybrid neural network techniques. Porosity data of apple, carrot, pear, potato, starch, onion, lentil, garlic, calamari, squid, and celery were used to develop the model using 286 data points obtained from the literature. The best generic model was developed based on four inputs as temperature of drying, moisture content, initial porosity, and product type. The error for predicting porosity using the best generic model developed is 0.58%, thus identified as an accurate prediction model.

Heat pump dehumidifier drying of food

Although heat pumps have been used extensively in industry for many years, their use for drying, especially foods, has been limited. This article reviews the potential of heat pump dehumidifier (HPD) dryers for use in food drying. HPD dryers offer several advantages over conventional hot-air dryers for the drying of food products, including higher energy efficiency, better product quality, and the ability to operate independently of outside ambient weather conditions. In addition, this technology is environmentally friendly in that gases and fumes are not given off into the atmosphere. The condensate can be recovered and disposed of in an appropriate manner, and there is also the potential to recover valuable volatiles from the condensate.

Pores and physico-chemical characteristics of dried tuna produced by different methods of drying

Abstract Information on pore formation and their characteristics in foods during processing is needed for process design, in estimating other properties (i.e. thermal conductivity, moisture diffusivity), and characterizing the quality of a product. In this study the characteristics of pores in dried tuna (Thunnus tongol) processed by air-, vacuum-, and freeze-drying were measured by mercury porosimetry. Apparent, substance and true densities, porosity and pore size distributions of dried samples were measured to study the pores formed during the drying processes. The apparent density of fresh tuna flesh was 1098 kg/m 3 , while the densities of dried tuna by air-, vacuum-, and freeze-drying were 960, 709 and 317 kg/m 3 , respectively. Porosity of freeze-dried sample was much higher than those the air-dried and vacuum-dried samples. Pores in different samples were characterized as the total intruded volume, total surface area, pore size range and average diameter, and nature of the pore size distribution curves. Peroxide value and color of dried tuna meat were also measured.

Density, shrinkage and porosity of calamari mantle meat during air drying in a cabinet dryer as a function of water content

Abstract The particle density of calamari mantle meat powder decreased with increasing water content and apparent density gave a peak at low water content, then decreased with increasing water content. The shrinkage and porosity were derived from the experimental density data and were correlated empirically. The empirical models were used to predict the density data. An attempt was made to apply the theoretical model proposed by Rahman (1991, Ph.D. Thesis, University of New South Wales, Australia) based on conservation of mass and volume, and modified to include excess volume and air pore formation. The uncertainty in the true density measurement, which is a common difficulty in this area, has made it impossible to predict accurately the excess volume, thus limiting Rahmans model.

State diagram of tuna meat: freezing curve and glass transition

Abstract The state diagram of tuna meat was developed by measuring and modeling its glass transition temperatures and freezing points. Fresh tuna meat was dried in a freeze drier to vary the moisture content from 73.3% to 6.0% (wet basis). Small deformation dynamic oscillation was employed to identify changes in the viscoelastic properties of tuna as a function of solids. Cooling curve method was used to measure the freezing point and end point of freezing. The state diagram yielded maximally freeze-concentrated solutes at 61% solids with the characteristic temperature of glass formation being −54.2 °C. The freezing curve and glass transition lines were developed using the Clausius–Clapeyron equation adjusted with unfreezable water and Gordon–Taylor model, respectively.

Analysis of cooling curve to determine the end point of freezing

Abstract The cooling curve method used for the measurement of freezing point of food is further analyzed to explore whether it can be used to identify the end point of freezing or glass transition. In this method, slope of the cooling curve is determined and plotted as a function of time to identify the end point of freezing ( T ′ m ). Initially, the slope is decreased and then reached a minimum value, which is identified as the nucleation of ice. Then the slope is increased until the end point of freezing. The end point of freezing is identified when the slope starts to decrease from its highest value or plateau. Sucrose solutions and starch gels were used to measure its T ′ m in identifying validity of the proposed method. The measured values of T ′ m by the proposed method is very close to the literature values.