Study On Water Absorption Model And Structural Changes During Precooking Of Black Beans

Black beans were widely developed into a variety of products due to their high contents of nutrients and plant active ingredients.However,black beans contain the thick seed coat and hard texture,which causes great inconvenience to the cooking process.Therefore,proper pretreatment of black beans before cooking is essential when cooked with rice together.In this paper,the distribution of water and the relaxation time of hydrogen protons in black beans were analyzed by low-field nuclear magnetic resonance technique?LF-NMR?during soaking;Different treatments were used to study the effect of soaking on the black beans,and Fick diffusion model and Peleg model were used to study the effect of black beans on water absorption;Response surface methodology was applied respectively to optimize the technology of black beans precooking,and to detect the changes of nutrients and internal structure before and after precooking.The main research contents are as follows:Firstly,the LF-NMR was used to study the water distribution and proton mobility of black beans during soaking.The different soaking temperature caused the black beans to absorb the water at different rates,but the final saturated moisture contents were similar.During the study of proton mobility,it was found that there were three kinds of protons T₂₁,T₂₂ and T₂₃ in black beans.Prolonging the soaking time and increasing the soaking temperature could make the relaxation time curve shift to the right,and the proton mobility increased with the increase of soaking temperature.When studying the proportion of the three types of proton clusters,it was found that M₂₁ and M₂₃ both decreased with the increase of the soaking time,while M₂₂increased from about 1%to 90%,indicating that the increased content of water for soaking was mainly free water.Comparing the relaxation time of different temperatures,it was found that high temperature could make the hydrogen proton relaxation time reach a stable state in a short time,which was consistent with the increasing trend of moisture content.The results of magnetic resonance imaging?MRI?showed that water infiltrated into the seed along the cell wall disorderly.Subsequently,the effects of ordinary soaking and ultrasonic treatment on the water absorption characteristics of black beans were compared by Fick diffusion model and Peleg model.Both Fick diffusion model and Peleg model could well fit the moisture content of black beans with the two treatments,and the correlation coefficient R² was greater than 0.9873.The diffusion coefficient of water in Fick diffusion model increased along with the temperature,and the diffusion coefficient of water in the two soaking methods reached the maximum value of17.72 and 14.22 at 70?.At the same time,according to the relationship between the effective diffusion coefficient and the absolute temperature,it was found that the activation energy of black beans decreased from 36.40 kJ/mol to 32.22 kJ/mol by ultrasonic treatment.Taking the water content as the study object,the effects of the two treatments on the K₁ value of Peleg rate parameter were studied.It was found that the K₁ value decreased linearly with the change of temperature.Scanning electron microscopy?SEM?was used to examine the seed coat of black beans after soaking.It was found that the holes and cracks on the surface of the seed coat were deepened after the ultrasonic treatment,which increased the channels for the water penetration and reduced the soaking time.Finally,response surface analysis?RSM?was used to optimize the processing technology of precooked black beans,and the changes of internal structure and nutrients of black beans before and after cooking were studied by differential scanning calorimetry?DSC?,SEM and fourier transform infrared spectroscopy?FT-IR?.The response surface results showed that soaking at 50?for 2 h and cooking for 60 minutes could make the black beans gelatinization degree reach 76.98%and the sensory score reached 86.31,which could achieve to be cooked with rice together.FT-IR analysis found that the secondary structure of black bean proteins changed after pre-treating,?-helix structure and?-turn structure increased,and?-sheet structure decreased,which indicated that more ordered structures changed during pre-treating;SEM observations of black bean powder revealed that the particles after pre-treating aggregated and agglomerated after heated,in the same time the volume became larger,and the pores bacame larger.DSC results showed that the black bean protein was denatured and the endothermic peak disappeared after pre-treating.