Heat and mass transfer during cooking of chickpea : measurements and computational simulation

Abstract

Chickpea is a food legume crop grown in tropical, sub-tropical and temperate regions. World chickpea production is roughly three times that of lentils. Among pulse crops marketed as human food, world chickpea consumption is second only to dry beans. Turkey, Australia, Syria, Mexico, Argentina and Canada are major chickpea exporters.

There are two types of chickpea, namely, the kabuli and the desi. The kabuli type is grown in temperate regions while the desi type chickpea is grown in the semi-arid tropics. Chickpea is valued for its nutritive seeds with high protein and starch content. They are eaten fresh as green vegetables, parched, fried, roasted, and boiled, as snack food, dessert and condiments. The seeds are ground and the flour can be used in soup, dhal and bread. Cooked chickpea is mostly preferred by consumers, especially the kabuli type.

In this thesis, the heat and moisture transfer behavior of kabuli chickpea when subjected to cooking at different temperatures was investigated. The thermo-physical properties of chickpea were studied to develop a model to simulate the temperature distribution and moisture absorption in a chickpea seed when cooked in water.

The thermo-physical properties determined experimentally were thermal conductivity, specific heat, moisture diffusivity, particle density and moisture content. Thermal diffusivity was calculated using the experimental values of thermal conductivity, specific heat and density. The water absorption in chickpea was determined when the seeds were soaked at different temperatures. It was observed that as the temperature of the soaking medium was increased, the rate of moisture absorption also increased. Soaking was done to enhance the gelatinization process during cooking. Cooking experiments were conducted for boiling temperatures ranging from 70 to 98°C for both soaked and unsoaked seeds. It resulted in the soaked seeds being cooked within 40-50 min, whereas the unsoaked seeds took around 250-300 min to cook. The amount of soluble solids lost during the cooking process is also reported which enables to predict the optimum soaking and cooking temperature.

Using linear regression simple models for dependency of thermal conductivity, specific heat, thermal diffusivity and density on temperature and moisture content were developed. The rate of moisture transfer and the center temperature in the seed during cooking was determined experimentally and also simulated with the constant thermal properties found experimentally. The closeness of the simulated and experimental results was proved by appropriate statistical analysis.

Based on the results obtained, it can be understood that soaking the chickpea seeds at temperatures ranging from 25 to 40°C for 8 h and cooking it at higher temperatures ranging from 90 to 100°C will improve the quality of the cooked seed with minimum mass loss. This optimum condition saves both energy and time.