A SIMPLIFIED ANALYTICAL MODEL FOR FREEZING TIME CALCULATION IN FOODS

A simplified analytical model for the freezing time prediction of simple-shaped foodstuffs was developed. It was assumed in the model that the solution to the unsteady state, unidirectional heat conduction equation with constant thermophysical properties was valid during cooling and freezing. the latent heat effects during freezing were incorporated into an effective diffusivity term. the predictions of the model were compared to the available experimental data on freezing of infinite slabs, infinite cyliners and spheres, and to the experimental data obtained in this research for the freezing of apples. the level of agreement between the predictions and the experimental data was quite satisfactory.     

青豆速冻过程冻结时间的数值计算

青豆近似为球状，采用完全隐式差分法，建立描述近似青豆的球状食品冻结过程传热特性的偏微分方程：通过数值计算获得青豆冻结时间的数值解，并与实验值比较。结果表明：数值法计算结果与实测值吻合较好，具有较高的精度。     

FREEZING TIME CALCULATION FOR PRODUCTS WITH SIMPLE GEOMETRICAL SHAPES

A simple model is proposed to estimate freezing times of foodstuffs of simple geometrical shapes (infinite flat slabs, infinite cylinders, spheres, rectangular parallelepipeds and finite cylinders). This model combines Plank's equation for change of phase period with the unsteady heat transfer solutions for cooling periods before and after phase change. The total freezing time is obtained by the determination and the summation of the precooling, phase change and tempering times. the results produced are at least as accurate as or better than any of the previous methods, including regression formula and finite difference computations. Tables required for fast and accurate predictions of freezing times of foodstuffs with this method are provided.     

A GENERALIZED RHEOLOGICAL MODEL FOR INELASTIC FLUID FOODS1

A versatile four-parameter model is presented for the rheological characterization of inelastic fluid foods. The model has been shown to accurately represent shear stress versus shear rate, and apparent viscosity versus shear rate relationships for several food materials. By appropriate specification of the four parameters of the model, conventional rheological models used in fluid food analyses (power law, Bingham plastic, Herschel-Bulkley, Casson, Heinz-Casson, and Mizrahi-Berk) can be obtained from the generalized equation.     

A REVIEW ON PREDICTING FREEZING TIMES OF FOODS

Heat transfer during freezing of a food material involves a complex situation of simultaneous phase transition and changing thermal properties. Models for predicting freezing times range from relatively simple analytical equations based on a number of assumptions and approximations, to the more versatile numerical methods which require the use of a sophisticated computer. The necessity for having a consistent definition for freezing time, the nature of the freezing process, different prediction models and thermo-physical properties of importance are discussed in view of the mathematics of freezing time computations. In this review, attention has been focused on established analytical models which can be solved without resorting to computer techniques. The review points out the need for further refinement of the existing Plank-type models to facilitate accurate freezing time estimations under a wide range of practical conditions.     

PREDICTION OF FREEZING TIMES FOR FOODS AS INFLUENCED BY PRODUCT PROPERTIES

This investigation utilizes a computer simulation technique to predict the freezing times and temperature history curves for food products. The input information consists of the product properties for temperatures above the initial freezing point, freezing medium conditions and the initial product temperature. It has been established that food products with lower initial freezing points, higher initial water contents and higher initial product densities will have longer freezing times. The prediction of freezing time is most sensitive to the accuracy of the measurement of the product density and the initial freezing point if the freezing point is above ?0.5°C. The influence of the accuracy of unfrozen product thermal conductivity data on the freezing time is not important in the range of 0.45 to 0.55 W/m°K investigated. The combined influence of inaccuracy in measuring these product properties on the freezing time prediction will be significant even if the influence of an individual product property is small.     

14—A SIMPLIFIED KINETIC MODEL FOR THE MECHANISM OF PILLING

A simplified kinetic model describing the mechanism of pilling is proposed. This model involves only three parameters, which relate to the main stages of the process: the total number of pills, the rate of pill formation, and the rate of pill wear-off. These parameters can easily be calculated from the pill curve of the fabric.

The published literature and the authors' own data are used to check the range of application of the proposed model.     

HEAT TRANSFER IN THIN-FILM WIPED-SURFACE EVAPORATION of MODEL LIQUID FOODS

Experimental product-side heat transfer coefficients were studied in a laboratory scale thin-film wiped-surface (TFWS) evaporator under different rotational speeds, liquid flow rates and temperatures of the heating medium. Experiments were conducted with water, and 10, 30, and 45% sugar solutions in order to simulate the conditions during evaporation of fruit juices. Dimensionless correlations for heat transfer coefficient were established. It was shown that the “equivalent diameter” of the liquid bow wave formed in the front of wiper blade could be used as a characteristic dimension. Magnitudes of unstable and stable modes of evaporator performance were determined and different regimes related to the liquid flow were revealed.