Gongcheng persimmon is rich in nutritional elements and they are known as the"holy fruit".However,fresh persimmons are prone to soften and rot after harvest,resulting in short shelf-life and huge loss of resources.Drying is a common processing method for fruits and vegetables.Microwave drying is a novel drying technology,which is an efficient processing method compared to traditional hot air drying.When studying the effect of microwave drying on the nutritional composition of persimmon slices,our laboratory found that during the microwave drying process,the color change of persimmon slices started from the center near the core,and spread to the outside of persimmon slices.Finally,the persimmon slices were severely browned inside.In order to explore the reason for the color change from inside to outside during microwave drying of persimmon slices from the perspective of the non-enzymatic browning reaction,this paper divided persimmon slices into 6 parts(The horizontal group was divided into two parts from the outside to the inside:zone 1 and 2,and the vertical group was divided into three parts from top to bottom:A,B,and C;therefore,the six parts were numbered A1,A2,B1,B2,C1,and C2),and the relationship between the main chemical components and color changes in different parts of persimmon slices during different microwave drying conditions,as well as the changes of water state and temperature distribution in different parts of persimmon slices during drying were studied.Meanwhile,by establishing the model system of different parts of fresh persimmon slices,the non-enzymatic browning reaction type was determined,and the control method of color changes in persimmon slices during drying was found.The main results of the paper are as follows:1.The content of chemical components in different parts of fresh persimmon slices was different.Comparing zones 1 and 2,the fructose,water content and total free amino acids showed zone 1<zone 2.Sucrose content showed A1>A2,B1<B2,C1<C2.Ascorbic acid content and sucrose showed opposite trends.Glucose content showed A1<A2,B1>B2,C1<C2.Comparing the A,B and C parts of persimmon slices,the sucrose content showed the trend of A<B<C.Glucose,fructose and water content showed the trend of A>B>C,ascorbic acid showed the trend of A>C>B,and total free amino acids showed the trend of A<C<B.The content of most free amino acids was the least in the A1 part of persimmon slices and the most in the B2 part.Principal component analysis showed that the content of chemical components in different parts of persimmon slices had significant differences.The B2 part of persimmon slices has the lowest content of ascorbic acid,and the highest content of free amino acids,these results indicate that the browning in the B2 part of persimmon slices during microwave drying may be caused by Maillard reaction,but ascorbic acid oxidation did not play a major role.2.With the increase of microwave power and drying time and the decrease of loading amount,the contents of ascorbic acid increased in different parts of persimmon slices,the contents of a*value increased in A2,B1,B2,C1 parts of persimmon slices,while the contents of sucrose and L*and b*values showed a downward trend.The trends of glucose,fructose and total free amino acids were not consistent in different parts of persimmon slices.However,the total amount of free amino acids decreased in the B2 part.The variation in the content of all chemical components was greatest in the B2 part of persimmon slices.Principal component analysis showed that the color change in the persimmon slices when it appeared first may be related to the Maillard reaction,and the deepening of browning and spreading to the outside of persimmon slices may be related to the Maillard and caramelization reactions,with little effect of ascorbic acid oxidation.Meanwhile,with the increase of microwave power and drying time and the decrease of loading amount,the total free amino acids in the B2 part of persimmon slices decreased significantly during drying,but ascorbic acid was the opposite,and free sugars dropped sharply at the end of drying browning.These results suggest that the Maillard reaction would contribute markedly to the browning of persimmon slices,and the oxidative degradation of ascorbic acid might not play an important role,and the caramelization reaction promoted the end point of drying color change.3.In the different parts of fresh persimmon slices,there were mainly three types of water:free water,semi-combined water,and the most closely bound water.During microwave drying,the moisture in different parts of persimmon slices changed dynamically,the T2inversion spectrum as a whole shifted to the left,the moisture content was reduced.In addition,the degree of water loss in different parts of the persimmon slices showed B>C>A,and the most water loss was observed in the B2.After microwave drying,the moisture content was higher in the A2 and C2 parts of persimmon slices and lower in the B2 part.The central temperature of persimmon slices was the highest,and the temperature was lowest at the edges of the A2 and C2 parts of persimmon slices.During microwave drying,the B2 part of the persimmon slices had the highest temperature and the fastest water loss,resulting in non-enzymatic browning occurring first in the B2 part of persimmon slices.4.During heating,the browning degree(A420),intermediate product of the Maillard reaction(A284)and 5-HMF content changed as follows:As heating time increases,there was no significant change in the“ascorbic acid model system”,the other three model systems(“sugars model system”,“sugars+amino acids model system”,and“sugars+amino acids+ascorbic acid model system”)showed a positive correlation with heating time,and the latter two upward trend was more significant.The contents of A284,A420and5-HMF in the“sugars+amino acids model system”in different parts of persimmon slices showed zone 1<zone 2,B>C>A,and the contents of A284,A420and 5-HMF were the highest in the B2 part of persimmon slices.In conclusion,the non-enzymatic browning of different parts of persimmon slices during microwave drying was the result of the combined effects of Maillard reaction,caramelization reaction and ascorbic acid oxidation,among which Maillard reaction was the most important type.The effect of different concentrations(5,25,50,100 mmol/L)of inhibitors(sodium sulfite,citric acid)on the“sugars+amino acids model system”was investigated.The results show that the optimal concentration of sodium sulfite to inhibit browning was 5 mmol/L and that of citric acid was 25 mmol/L.It provides a theoretical basis for preventing browning of persimmon in microwave drying.5.Persimmon slices were soaked in different concentrations(5,25,and 50 mmol/L)of citric acid and then dried by microwave and hot air.The results showed that citric acid could significantly inhibit the browning of persimmon slices.When the concentration of citric acid was 25 mmol/L,the color of persimmon slices was the best. |