A.K. Datta

Development and validation of heat and mass transfer models for freeze-drying of vegetable slices

The objective of this work is to develop and validate mathematical models based on heat and mass transfer principles for freeze-drying of vegetable slices. Carrot samples of 1, 2 and 3 mm thickness were freeze dried to obtain drying curves. Developed mathematical models were tested against experimental data to establish that the freeze-drying process is mass transfer controlled. Average values of mass diffusivity were also obtained for samples of three different thicknesses. Capsicum was freeze-dried using two types of circular disk samples obtained from whole capsicum using a cork borer. In one set of samples the natural protective outer membrane was left as it is and in another set the layer was removed prior to freeze-drying. The resulting difference in drying behaviour was insignificant. The drying process was also found to be very slow for capsicum. Mushroom samples on the other hand showed a drying behaviour very similar to 2 mm thick carrot samples.

Thermophysical properties of concentrated reconstituted milk during processing

Abstract Design of continuous heat exchange equipment for making khoa, a popular base ingredient of sweetmeats in India, requires accurate thermophysical properties of concentrated reconsituted milk over a wide range of concentrations and temperatures. The specific heat, thermal conductivity, and apparent viscosity of milk were determined between concentrations of 40% and 70% and temperatures of 35°C and 65°C. The results were analysed to obtain these properties as functions of temperature and concentration. Viscosity data were analysed to obtain the consistency coefficient and flow behaviour index. The last two parameters were further analysed and expressed as functions of temperature and concentration. The flow behaviour index was found to be independent of temperature.

Determination of specific heat and thermal conductivity of mushrooms (Pleurotus florida)

Abstract The specific heat and thermal conductivity of mushrooms (Pleurotus florida) was determined for a moisture content (MC) range of 10.24–89.68% w.b. (wet basis) and temperature range of 40–70°C. A third variable of bulk density (BD) was added with its three levels (111.06, 383.49, and 655.86 kg/m3) to see its effect in combination with other two input variables on thermal conductivity of mushrooms. Both the thermal properties increased almost linearly with the increasing levels of input variables. Multiple regression models with high R2 values were developed to correlate these properties as a function of input variables. An analysis of variance (ANOVA) revealed that the moisture content had a highly significant effect on specific heat and thermal conductivity of mushrooms. Bulk density was also found to be highly significant affecting thermal conductivity of mushrooms.

A computer based iterative solution for accurate estimation of heat transfer coefficients in a helical tube heat exchanger

An iterative technique is developed and reported for accurate estimation of heat transfer coefficients in a helical triple tube heat exchanger. Based on the experimental temperature rise of whole milk in a helical triple tube heat exchanger, accurate values of film heat transfer coefficients and overall heat transfer coefficients based on the outside area of the innermost tube and the inside area of the middle annulus are obtained from first principles. Three different flow rates of milk were used giving rise to three Reynolds numbers. The procedure described can be applied to any heat exchanger with minor modifications, if necessary.

Computer-aided design and performance evaluation of an indirect type helical tube ultra-high temperature (UHT) milk sterilizer

Abstract A laboratory model indirect type helical tube ultra-high temperature (UHT) milk sterilizer was designed and tested. The helical triple tube heat exchanger modeling and simulation was carried out for heating milk from 90°C to 150°C in the annulus of the triple tube using steam in the innermost and outermost tubes. The holding section was designed based on an Arrhenius model to reduce B. stearothermophilus microorganisms in the milk by 8 log cycles at a sterilization temperature of 150°C with a residence time of 2.64 s. The helical double tube cooling section modeling and simulation was carried out to cool milk from 150°C to 90°C by using tap water. The lengths of heating, holding and cooling sections were found to be 2.28, 1.0 and 2.08 m, respectively. The set-up was tested for UHT processing of milk at the flow rate of 135 l/h. The microbial effectiveness of the process was tested separately with inoculated samples of B. subtilis and B. stearothermophilus microorganisms and found to be effective during processing as well as during storage of the UHT processed milk for two months. The quality attributes (i.e. color and viscosity) of the UHT processed milk were also tested during the processing and storage periods. Overall there was no marked change in quality attributes for which the processed milk can be considered unacceptable.

Heat transfer coefficient in laminar flow of non-Newtonian fluid in tubes

Abstract Heating of non-Newtonian fluids is of particular importance to food processing systems. However, excessive pumping demand generally precludes development of turbulence in tubular arrangements. So, laminar flow is all that can be developed in heat transfer equipment handling non-Newtonian materials. The paper presents a theoretical model of heat transfer, which is dependent only on the flow behaviour index of the fluid. Increase in flow behaviour index produces a lower convective heat transfer coefficient and for real non-Newtonian fluids Nusselt’s number approaches a fixed value corresponding to a flow behaviour index of 4.

Prevention of moisture migration in coated biscuit

Abstract Moisture migration in a newly developed product called fondant coated biscuit makes the biscuit soggy. A suitable continuous drying method was developed to make the biscuit attain the crispness of uncoated biscuits. Moisture sorption isotherms were obtained for fondant, biscuit and fondant coated biscuit separately to establish the stable moisture levels as indicated by BET and GAB equations. Drying was carried out at four different temperatures and drying data were analysed to obtain heat and mass transfer coefficients and diffusivity values from the period of constant drying rate. The diffusivity value was also obtained from the period of decreasing drying rate.

Graphical and Computational Analysis of Creep Curves

ABSTRACT GRAPHICAL and computational methods were developed to analyze creep behavior of biological materials. Both of these methods were based upon the assumptions that linear viscoelasticity and the four element Burgers model apply to the biological materials. Compression creep data for three different materials were used to test the accuracy and effectiveness of the two methods. The results indicated that these methods are potentially useful for analyzing any observed behavior representing exponential change.

Quantitative Analysis of Major Phytochemicals in Orthodox tea (Camellia sinensis), Oxidized under Compressed Air Environment

This study describes major changes in phytochemical composition of orthodox tea (Camellia sinensis var. Assamica) oxidized under compressed air (CA). The experiments for oxidation were conducted under air pressure (101, 202, and 303 kPa) for 150 min. Relative change in the concentrations of caffeine, catechins, theaflavins (TF), and thearubigins (TR) were analyzed. Effect of CA pressure was found to be nonsignificant in regulating caffeine concentration during oxidation. But degradation in different catechins as well as formation of different TF was significantly affected by CA pressure. At high CA pressure, TF showed highest peak value. TR was found to have slower rate of formation during initial phase of oxidation than TF. Even though the rate of TR formation was significantly influenced by CA, a portion of catechins remained unoxidized at end of oxidation. Except caffeine, the percent change in rate of formation or degradation were more prominent at 202 kPa.

Estimation of Thermal Properties and Heat Transfer Study during Continuous Processing of Rice in Milk

Cooking rice and milk simultaneously is the basis of some popular cereal-based dairy products (e.g., rice-pudding, kheer). A conceptual design for continuous processing of rice in milk is being developed and is a helical coil heat exchanger to cook the rice-milk mixture, which is otherwise prepared as a batch process. The present study investigates the thermo-physical properties of the rice-milk mixture as functions of temperature and composition. In addition, a fundamentals-based heat transfer model is presented that includes the effects of milk fouling to estimate the drop in temperature of the product at the outlet, the degree of fouling, and fouling thickness that would build up with time and length of the helical coil. Fouling thickness increases towards the outlet and the rate of increase in fouling thickness decreases with time. At the outlet, a significant drop in temperature is observed (11°C) as an adverse effect of milk fouling.