Study On Microwave Drying Characteristics Of Banana Slices Changed By Cold Plasma Pretreatment

As one of the most popular fruit in the world,bananas are rich in a variety of nutrients and widely planted in tropical and subtropical regions around the world.In addition to fresh food,different drying processes are usually used to reduce the moisture content in fruit and vegetables,in order to achieve the purpose of extending the shelf life.Owing to the high content of carbohydrate,phenols and a variety of enzymes in bananas,browning is prone to occur during drying,which resulting in deterioration of product flavor,texture,further reducing product quality.However,traditional heat treatment and chemical additives are not suitable for inhibiting browning of bananas.Be a novel non-thermal technology,cold plasma(CP)treatment can inhibit the enzyme activity,which has great potential in the field of food processing.In this study,CP was used for banana slices treating,and the effects of voltage,current,time,and discharge gap on the enzymatic inactivation effect was explored.Banana slices were treated then detected the quality characteristics and changes in the content of anthraquinone products from enzymatic browning.In addition,CP treatment was used as a pretreatment technology to study its effects on drying kinetics,heating uniformity and browning index during microwave drying of banana slices were studied as well.The main research conclusions include:(1)The activities of polyphenol oxidase(PPO)and peroxidase(POD)in banana slices decreased with the increasing of CP treatment voltage,current and time,but increased with the increase of discharge gap.The optimal conditions were as follows:treatment time 2 min,treatment voltage 52.9 V,discharge gap 5.22 mm,and the PPO enzyme activity was measured to be 122.01 U/g and POD was 158.98 U/g after adjustment according to the actual situation.(2)CP treatment reduced the content of anthraquinone.The longer treatment time,the heavier reduction.The liquid chromatography analysis also showed that the total peak area of the treated samples was lower than that in untreated samples,CP treatment could effectively inhibit the enzyme activity in banana slices.Anthraquinone have a certain ability to scavenge OH-,DPPH,and O2-·,the higher the concentration of anthraquinone compounds,the higher the free radical scavenging rate.Stability analysis showed that anthraquinone products were relatively stable in temperature range of 30-60?,but were more sensitive to light and additives.The anthraquinone compounds had the worst stability under ultraviolet light,and the addition of additives would also reduce the stability to different degrees.(3)The activities of PPO and POD enzymes in banana slices decreased significantly with the prolonging of treatment time,but the content of intracellular reactive oxygen increased with the increasing of treatment time.Isoamyl butyrate and isoamyl acetate were the most abundant volatile components in banana slices,and CP treatment could change the content of some volatile components.CP treatment was beneficial to the maintenance of the surface color of banana slices,The Luminance value(L*value)of banana slices treated with CP for 2 min was 76.71 after 5 h's storage,which was higher than untreated sample(74.20).The surface micro structure of banana slices changed from smooth to rough after CP treatment,crevices and cracks occurred as well.(4)The dielectric constant(?')and loss factor(?")of the banana slices after CP treatment were higher than those of the untreated sample,which indicating that the CP treated banana slices can absorb electromagnetic energy and convert it into heat energy more effectively.Using CP as a pretreatment technology can shorten the microwave drying time of banana slices,inhibit enzymatic browning and improve the surface heating uniformity.After 4 min microwave drying,the final moisture content of the samples with 2 min CP treating was 0.65 g/g(d.b.),which was lower than that of the untreated samples(0.84 g/g(d.b.)).In addition,the surface temperature of the samples treated with CP increased slightly,and no significant changes were found in internal temperature.As a pretreatment technology,CP treatment improved the surface uniformity of banana slices during microwave drying.The analysis of browning index showed that CP treatment could inhibit enzymatic browning during drying process.