Influence Of Ultrasound Assisted Pretreatment And Freezing On Freeze Drying Of Carrots

Freeze dried vegetable/fruit as a kind of high-quality food is increasingly popular,since freeze drying could well preserve the original nutritional value and sensory attributes of the food.However,the long drying time and its high energy cost limit its development in food industry.In this work,power ultrasound was applied into both pretreatment and freezing processes during the production of freeze-dried carrots in order to study the effect of ultrasound on improving the current freeze-drying process of vegetable/fruit.The main contents are as follows:1.The method of ultrasound assist heat combined with ascorbic acid?UHA?was applied in the pretreatment of carrots.Single factor experiment and response surface methodology?RSM?were employed to find that the optimal processing conditions of UHA pretreatment was ultrasonic power density 0.29W/ml,reaction time 3min,pretreatment temperature 60?and ascorbic acid concentration of 1%,while residual activity of peroxidase and polyphenol oxidase were used as indicators.Furthermore,the influence of UHA pretreatment,blanching and ascorbic acid immersion pretreatment on the quality attributes of carrot was also compared.Based on the prerequisite that enough enzyme inactivation was achieved,the results showed that UHA pretreatment not only could significantly increase the Vc contents of carrot,but also could well maintain the original cell structure,which shows its great potential being a mild and efficient way of pretreatment.2.In the study of ultrasound assist freezing process of carrot,a microscopic observation was firstly carried out to exam the effectiveness of contact ultrasound assist nucleation of thin carrot slice.Secondly,an ultrasonic vibrating plate was used to integrate the contact ultrasound into freeze drying process and its influence on ice crystals size was evaluated by CT scanning and voids determination of freeze-dried carrot.Thirdly,the influence of ultrasound assist nucleation process on freezing rate was investigated by analyzing the freezing curves of carrot.The results showed that direct contact ultrasound is applicable to stimulate nucleation in carrot slice at lower supercooling degree and successfully increased the voids size of freeze dried carrot from 67.34±5.06?m to 80.81±3.03?m,while the sublimation time was significantly reduced.In addition to that,ultrasound could also help samples dropped through the characteristic freezing time(t_cf)faster and reduce the total freezing time(t_tf).3.The influence of three different drying methods on carrot quality attributes was compared,namely ultrasound-assisted freeze dying with UHA pretreatment,freeze drying with blanching pretreatment and hot air drying with blanching pretreatment.It was found that samples under ultrasound application have better quality on attributes of structure,Vc content,rehydration rate,etc.,which shows the potential of ultrasound to become a promising method to be applied in the production of freeze-dried fruit/vegetable.In summary,the application of ultrasound in the process of pretreatment and freezing could not only reduce the carrot quality damage caused by conventional pretreatment,but also could greatly enhance the freeze drying rate,reducing energy consumption.Therefore ultrasound application during the production of freeze-dried fruit/vegetable has great prospects in improving both the production efficacy and quality of the final products.