Study On The Relationship Between Color,Phenols And Browning Enzymes In Different Parts Of Persimmon By Microwave Drying

The persimmon has high nutritional value,but concentrated in the collection period and easily perishable and spoiled.Compared with traditional drying methods,microwave drying is characterized by high thermal efficiency,low energy consumption,and safety and hygiene.Our laboratory had conducted a series of studies on microwave drying of persimmon slices,and found that during microwave drying,the color deepened and turned black in the center of the persimmon slices and gradually extended from the inside to the outside with the drying process.Hence,in order to explore the reason for the color change of persimmon slices from the inside to the outside during microwave drying from the perspective of enzymatic browning,this project divided persimmon slices in two directions,the vertical group was divided into three parts from the top to the bottom:A,B and C,and the horizontal group was divided into two parts from the outside to the inside:1 and 2.Studied the relationship between the content of phenolic compounds and browning enzyme activity of different parts of persimmon slices before and after drying under different microwave drying conditions,as well as the relationship between the moisture state and temperature distribution of different parts of persimmon slices during drying and color change.The effect of inhibitors on browning enzyme activity was investigated to find ways to improve the color change of persimmon slices during drying.The research results were as follows:（1）The contents of phenolic substances and browning enzyme activities in different parts of fresh persimmon slices were significantly different.Compared with the parts A,B and C of fresh persimmon slices,the contents of total phenols and total flavonoids were higher at both ends than in the middle.The contents of soluble tannins increased sequentially in part 1,and the trend was the same as the total phenols in part 2.There was an inverse relationship between PPO activity and total phenols,while POD activity increased sequentially.At the same time,compared with parts 1 and 2 of fresh persimmon slices,the contents of total phenols,soluble tannins and total flavonoids were higher on the outside and lower on the inside at the middle and lower ends,but opposite at the top.The activities of PPO and POD were higher on the inside and lower on the outside.The contents of phenolic substances were the lowest in part B2 and the highest in part A2 of persimmon slice.It was speculated that the first color change in part B2 might not be related to enzymatic browning,while the color change in part A2 might be related to enzymatic browning,and the main components involved in browning reaction in this part were total phenols,soluble tannins and total flavonoids.（2）Under various drying conditions,PPO and POD were both inactivated.Compared with the parts A,B and C of persimmon slices,the contents of total phenols,soluble tannins and total flavonoids in different parts were basically reduced in part 1,and higher than in the middle at both ends of part 2 under different drying conditions.Compared with the parts 1 and 2 of persimmon slices,the contents of total phenols,soluble tannins and total flavonoids in different parts of persimmon slices with microwave power of 420 W,drying time of 11 min and loading amount of 2.2 kg/m² were lower on the inside and higher on the outside,while those with microwave power of 490 and 560 W,drying time of 15 min and loading amount of 1.8 kg/m² were lower on the inside and higher on the outside,but the opposite was true in the middle.Correlation analysis showed that the content of total flavonoids in different parts of persimmon slices was significantly positively correlated withΔE value and negatively correlated with L*value.Compared with fresh samples,the contents of total phenols,soluble tannins and total flavonoids in parts A and C2 were decreased,the contents of total phenols and soluble tannins in parts B and C1 were increased,and the contents of total flavonoids were decreased,while the contents of total flavonoids in part B2 was not significantly decreased（p>0.05）,which indicated that enzymatic browning might be more difficult in part B than in parts A and C of persimmon slices.With the increase of microwave power and drying time,the contents of total phenols,soluble tannins and total flavonoids in parts A and B of persimmon slices increased,while the contents of total phenols in part C increased first and then decreased,while the contents of soluble tannins and total flavonoids increased.With the decrease of the loading amount,the contents of total phenols in six parts of persimmon slices increased,the contents of soluble tannin decreased first and then increased,while the contents of total flavonoids in part 1 decreased first and then increased,and the contents of total flavonoids increased in part 2.The color darkening in different parts of persimmon slices was negatively correlated with the content of flavonoids,and the color change occurred first in part B2 of persimmon slices and spread from inside to outside,which might have little relationship with enzymatic browning,but might be caused by non-enzymatic browning during the drying process.（3）The content of free water was the most abundant in different parts of persimmon slices,with parts A and B exhibited similar peak times while part C demonstrated the latest peak time.With the increase of microwave power,drying time and the decrease of loading amount,the T₂ in different parts of persimmon slices moved to the left,and the peaks merge with a reduced peak amplitude.At the same time,the surface temperature of persimmon slices in part B increased first and then spread to parts A and C,with part B exhibited the highest temperature.Under the microwave power of 560 W,drying time of15 min,and loading amount of 1.8 kg/m²,the decrease degree of water in parts A,B and C of persimmon slices was B>C>A,with the lowest water content observed in part B2.Therefore,microwave power,drying time and loading amount were important factors affecting the drying effect of persimmon slices.During the drying process,the water state and temperature distribution in different parts of persimmon slices were uneven.After drying,the water content was the lowest and the temperature was the highest in part B2,resulted in the most severe color change in this part.（4）The three inhibitors inhibited PPO and POD activities in persimmon in the order of ascorbic acid>sodium bisulfite>citric acid.The higher the concentration of ascorbic acid and sodium bisulfite,the stronger the inhibitory effect on PPO and POD activities.The higher the concentration of citric acid,the stronger the inhibitory effect on PPO activity,while the inhibitory effect on POD activity showed a trend of increased first and then decreased.Ascorbic acid was selected to improve the color change of persimmon slices during microwave drying.The improvement effects of different concentrations of ascorbic acid on persimmon slices were obviously different under different drying conditions.The improvement effect of ascorbic acid on persimmon slices was 5 mmol/L>1 mmol/L>10 mmol/L.