Study On Ultrasound-assisted Thawing For Mango Pulp And Simulating The Processing

Mango pulp is the main processed product of mango and often used as the raw material of mango flavor products.In the process of production and circulation,mango pulp is usually frozen to extend its shelf life.Frozen products need to be thawed before secondary processing,but the rate of thawing by water immersion commonly used in factories is slow,which reduces the efficiency of the production.In this paper,ultrasound was applied to thawing in order to improve the rate.We also optimized the conditions of ultrasonic thawing,and freeze-thaw cycles were repeated under the optimized condition to verify the reliability of the condition.In addition,COMSOL software was used to simulate the process of ultrasonic thawing of mango pulp and predict the thawing time.Ultrasound with frequency conversion was used to keep the temperature field evenly distributed,so as to provide an approach for factories to thaw mango pulp.The main research results are as follows:(1)We explored the effect of ultrasonic intensities on the thawing time,sensory and nutritional quality of mango pulp at the intensities of 0,0.037,0.074 and 0.123W/m L.Compared to water immersion thawing,the time of ultrasonic thawing was reduced by 16-64%.As ultrasonic intensities increased from 0 to 0.074 W/m L,the time of ultrasonic thawing was shorter,but there was no significant difference(p>0.05)between 0.074-0.123 W/m L.The results of sensory quality study showed that there was no significant change in texture and aroma of mango pulp(p>0.05)after thawing at low ultrasonic intensity compared with that after thawing in water immersion.But when the ultrasonic intensity reached a certain level(4 ?-0.074 W/m L;25 ?-0.123 W/m L),the viscosity and some volatile compounds(nonal,dimethyl sulfide and toluene)of mango pulp were increased,and the sensory evaluation score of the texture and aroma became worse.The results of nutritional quality study showed that high intensity(0.074-0.123W/m L)made the content of free gallic acid,caffeic acid and p-hydroxybenzoic acid higher than that of water immersion thawing,but had no significant effect on the content of carotenoids(p>0.05).(2)We explored the effect of ultrasonic temperatures(4 ? and 25 ?)on the thawing time,sensory and nutritional quality of mango pulp.Compared with thawing at 4 ?,the time of thawing at 25 ? was reduced by 51-73%.The smell and texture properties of the thawed mango pulp were better than that at 4 ?.Besides,the content of total phenolic acid was 26.5-58.5% higher,and the content of total carotenoid was8.7-11.0% higher than that at 4 ?.Therefore,the efficiency of ultrasonic thawing at25 ?-0.074 W/m L was high,and the condition did not destroy the sensory and nutritional quality of mango pulp,indicating that it was a good condition for thawing mango pulp.(3)We explored the changes of color,p H,titratable acid,total soluble solid,free phenolic acid and volatile compounds of mango pulp after 1,2,3,5 and 7 times of freeze-thaw cycles by water immersion thawing and 25 ?-0.074 W/m L ultrasonic thawing,respectively.Compared with water immersion thawing,there was no significant change in all properties of mango pulp after 25 ?-0.074 W/m L freeze-thaw cycles(p>0.05)by ultrasonic thawing.With the increase of repeated freeze-thaw times,the content of free gallic acid and mangiferin increased and then decreased,and the difference between the maximum value and the minimum value was only 4.27-10.56%.The other properties had no significant difference(p>0.05).Therefore,it is feasible to apply 25 ?-0.074 W/m L ultrasonic thawing to production,and it will not damage the physical and chemical quality in repeated freeze-thaw cycles.(4)Based on the acoustic module and heat transfer module of COMSOL multiphysics,a three-dimensional simulation model of the sound field and temperature distribution was established during the process of ultrasonic thawing of mango pulp,and experimental verification was performed.The results showed that the RMSE value of simulation value and experiment value was only 0.833-1.233 ?,which verifies the reliability of the model.Therefore,the model can be used to predict thawing time so as to prevent excessive ultrasonic treatment and incomplete thawing.(5)The simulation results of the model showed that ultrasound treatment at a single frequency could lead to local overheating,and as ultrasonic power increased,the highest temperature in mango pulp was higher when thawing was completed.This result could explain the phenomenon of sensory quality deterioration of mango pulp caused by high power.In order to solve the problem of local overheating,ultrasonic thawing with frequency conversion was applied based on the model.The simulation results showed that the time of ultrasonic thawing with frequency conversion was 76.4%shorter than that of ultrasonic thawing with 28 k Hz single frequency.More importantly,the highest temperature in mango pulp thawed by ultrasound with frequency conversion was only 20 ?,while that of single frequency ultrasound is 60 ?,which suggested less local overheating by ultrasound with frequency conversion.Consequently,ultrasonic thawing with frequency conversion was a promising method to solve the problem caused by ultrasonic local overheating,and thus provide a good model to design of ultrasonic equipment.