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Abstract [Objectives] This paper was to clarify the effects of different drying methods on the physiological activities of kiwifruit， so as to provide theoretical bases for the drying processing of kiwifruit.
[Methods] With kiwifruit as the research object， the effects of hot air drying， microwave drying， and vacuum freeze drying on the active ingredients in dried kiwifruit were studied， and antioxidant activity of the ingredients were determined through the scavenging capacity of 1，1phenyl2picrylhydrazyl free radicals （DPPH）， hydroxyl radical scavenging capacity and the antioxidant capacity of ferrous ions.
[Results] Vacuum freeze drying greatly reduced the physiological activities of kiwifruit， followed by microwave drying. Hot air drying was the best choice for dehydration of kiwifruit. Under the same conditions， the total flavonoid content of kiwifruit from hot air drying was 70.12 mg/ml， and the DPPH· free radicals scavenging ability of the treatment solution was the strongest， with the scavenging rate reaching 9.42%. The OH radicals scavenging rate was the highest at the concentration of 0.015 mg/ml， reaching 69.92%. The absorbance reached the maximum at the concentration of 0.025 mg/ml， and the ferrous ion reduction capacity was also the greatest with the absorbance of 2.542.
[Conclusions] Hot air drying could well maintain the shape and nutrients of kiwifruit， making it a proper drying method for kiwifruit processing.
Key words Different drying methods; Flavonoid; Antioxidant activity; Kiwifruit
Kiwifruit is grown all over the world and is widely used as medicine and food raw materials[1]. The regional and seasonal properties of kiwifruit affect its sales season and time. Therefore， the postharvest preservation processing has become an urgent problem for the development of kiwifruit industry[2-3]， and drying is one of the effective methods to prolong the storage period of kiwifruit[4]. There are many drying methods for kiwifruit. The traditional methods of dehydration drying include microwave drying and hot air drying. Microwave drying is not easy to control the time and temperature. If the temperature is too high， the kiwifruit slices are over dried， leading to great losses of nutrient. On the other hand， it is hard to maintain the quality of dry products in hot air drying. Compared with these 2 drying methods， vacuum freeze drying can maintain the shape and nutrient content of the original kiwifruit as much as possible[11-12]. The study on the content of antioxidant substances and their antioxidant capabilities in the products from different drying methods can provide bases for the election of drying methods and quality evaluation. In this study， vacuum freezing， hot air and microwave drying methods were applied to process fresh kiwifruit， and the effects of different drying methods on the antioxidant capacity were evaluated by hydroxyl radical determination， flavonoid content determination， DPPH and ferrous ion reduction method， in order to provide theoretical basis for the drying processing of kiwifruit. Materials and Instruments
Test materials were fresh kiwifruit， purchased from a fruit shop in Linfen City. The kiwifruit for the test was fresh with uniform size and certain hardness， which was stored at normal temperature for later use. The test reagents were as follows： Folin phenol， methanol， ethanol， rutin， sodium nitrite， sodium hydroxide， 1，1phenyl2picrylhydrazyl free radical （DPPH）， ferrous sulfate， absolute ethanol， sodium carbonate， aluminum nitrate， hydrogen peroxide， potassium ferricyanide， trichloroacetic acid， ferric chloride， ascorbic acid and 80% methanol.
Instruments for the test included 101 electrothermal blowing dry box of Beijing Ever Bright Medical Treatment Instrument Co.， Ltd.; FA2204B electronic balance of Techcomp Shanghai Instrument Co.， Ltd.; WFJ7200 visible spectrophotometer of Unico （Shanghai） Instrument Co.， Ltd.; HH6 digital display constant temperature water bath of Jintan Changzhou Instrument Manufacture Co.， Ltd.
Test Methods
Sample preparation
Fresh kiwifruit with uniform size and hardness was selected， peeled and cut into thin and even slices.
Sample drying
First， 10.000 g of pretreated fresh kiwi fruit slices were taken respectively for hot air drying， vacuum freeze drying and microwave drying. The slices were evenly laid out in the air dry oven， and dried at the temperature of 60 ℃. The weight was measured every 10-20 min until the water content dropped to 10%-12%. For vacuum freeze drying， the slices were placed on the clean object tray in the vacuum freeze dryer， followed by setting the temperature and time of the vacuum freeze dryer. In the first stage， the slices were in the freezing treatment at -30 ℃ for 1 h， the second stage， vacuum pumping treatment at -10 ℃ for 1 h， and finally the third stage， sublimation drying processing for 1 h at 30.0 ℃. As for the microwave drying， the fresh kiwifruit slices were placed on the clean turntable in the microwave oven， followed by processing for 1 min at the power of 1 000 W.
Extraction of antioxidants from kiwifruit
The kiwifruit samples treated in 3 different drying methods were labeled， and then extracted using 70% ethanol in a water bath at 70 ℃. The ratio of material to ethanol in the extraction was 1：10， and the extraction time was 2 h.
Determination of kiwifruit extract content
Standard curve plotting
First， 0.062 5 g of rutin standard was taken， dissolved in 80% methanol solution， and then set to the constant volume of 250 ml， obtaining rutin standard solution with the mass concentration of 0.25 mg/ml， which was stored at room temperature in dark. Then， 0.0， 2.0， 4.0， 6.0， 8.0， 10.0 ml of the standard solution was taken， and added with 1.5 ml of 5% NaNO2 solution， which was then placed still for 6 min after fully mixing. Next， after adding 1.5 ml of 10% Al（NO3）3 solution， the mixture was placed still for 6 min after fully mixing， and then 20 ml of 5% NaOH solution was added. Finally， the mixture was set to constant volume of 509 ml using 80% methanol solution， fully mixed. After the solution placed still for 15 min， the absorbance was measured at the wavelength of 510 nm. After 3 repetitions， the average vale was taken. Afterwards， the standard curve was plotted with the absorbance value as the ordinate and the rutin standard solution mass concentration as the abscissa[6]， obtaining the regression equation： Y=0.168X+0.001 （R 2=0.996）.
Determination of flavonoid content in the sample
Accurately 1 ml of each sample liquid was taken and marked. According to the curve plotting steps of rutin standard， the absorbance of the testing sample liquid was measured at the wavelength of 502 nm using a spectrophotometer， and the yield rate of flavonoid in the sample liquid was obtained based on the above regression equation.
Determination of DPPH· free radicals scavenging ability by different sample liquid
Accurately 2.5 mg of DPPH· was taken， dissolved in anhydrous methanol and diluted into a 100 ml volumetric flask， obtaining the DPPH· solution with the mass concentration 25 of μg/ml， which was then stored in the dark （0-4 ℃）[7]. Then， 0.1 ml of different sample solution was taken and added with 3.9 ml of DPPH· methanol solution with the mass concentration of 25 μg/ml. After quickly mixing， the absorbance （516 nm） was measured at 0， 5， 10， 15， 25， 35， 45， 55， 65， and 75 min until the absorbance was relatively stable， and the kinetic curve of the pigment scavenging DPPH· was plotted[8].
Determination of OH· radical scavenging ability by different sample liquid
First， a stoppered 10 ml graduated test tube was added with 2.0 ml of 6 mmol/L ferrous sulfate solution， 2.0 ml of sample solution with different concentrations， and 2.0 ml of 6 mmol/L hydrogen peroxide， respectively. After fully mixing and placing still for 10 min， the tube was added with 2.0 ml of 6 mmol/L salicylic acid solution. Then， after fully mixing and placing still for 30 min， the absorbance value A2 at 510 nm was measured， and the absorbance value A1 of 2.0 ml of the sample solution with 2.0 ml of absolute ethanol was measured， as well as the absorbance value A0 of 2.0 ml of salicylic acid with 2.0 ml of absolute ethanol. A total of 3 parallel tests were conducted to calculate the scavenging rate[13]. The scavenging rate for OH· was P（%）=[1-（A2- A1）/A0]×100%. Where： A2 is the absorbance of 2.0 ml of the sample solution with 2.0 ml of ferrous sulfate， 2.0 ml of hydrogen peroxide and 2.0 ml of salicylic acid; A1 is the absorbance of 2.0 ml of the sample solution with 2.0 ml of absolute ethanol; A0 is the absorbance of 2.0 ml of salicylic acid with 2.0 ml of absolute ethanol.
Determination of ferric ion reducing capacity
Successively 2.5 ml of 0.2 mol/L phosphate buffer solution with pH of 6.6， 2.5 ml of 1% potassium ferricyanide and 1 ml of sample solution with different concentration was mixed， and then bathed for 20 min at 50 ℃. After quick cooling， 1.0 ml of 10% trichloroacetic acid solution was added， followed by adding 2.5 ml of distilled water and 1.0 ml of 0.1% ferric chloride solution， and after fully mixing and placing still for 10 min at room temperature， the absorbance at 700 nm was determined. Ant 3 parallel tests were conducted. The reducing capacity of ferric ion was determined according to the value of the absorbance， and a larger absorbance value indicated stronger reducing capacity[5]. Ascorbic acid was used as a standard antioxidant[9]. Results And analysis
Total flavonoid content in 3 different drying methods
The content of total flavonoid of the samples after treatment by different drying methods was shown in Fig. 1. As shown in Fig. 1， the total flavonoid content was different for different drying methods at the same condition. The content of total flavonoid from high to low was in the order of vacuum freeze drying > hot air drying > microwave drying. Vacuum freeze drying had the smallest effect on the content of flavonoid in the sample liquid of kiwifruit， and the content of flavonoid in the sample liquid reached 84.56 mg/ml. On the other hand， the fresh kiwifruit sample liquid was undried， which was subjected to the greatest effect of wet weight， so the flavonoid content was the lowest.
Fig. 2 Kinetic curves of DPPH· free radical scavenging by different solutions
Scavenging rate of OH· free radicals in extracts from different drying methods
As shown in Fig. 3， as for scavenging abilities of the 4 sample solutions， the sample solution from hot air drying had the most significant effect on scavenging hydroxyl radicals， and its scavenging rate was greater than the scavenging rate of the sample solution from any other drying method in the range of 0.005-0.025 mg/ml.
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In the range of 0.005-0.025 mg/ml， there was no significant change in the scavenging rate with the concentration between the undried and hot air dried extraction， and when the concentration was 0.015 mg/ml， the scavenging rate of hot air drying reached the highest of 69.92%. Under the same concentration， the scavenging rates of the extracts from vacuum freeze drying and microwave drying were lower than that from hot air drying. For the extract from vacuum freeze drying treatment， the scavenging rate increased with the concentration in the range of 0.005-0.020 mg/ml， and reached the highest at the concentration of 0.02 mg/ml. For the kiwifruit extract from microwave drying， the scavenging rate increased significantly with the increased of concentration in the range of 0.005-0.015 mg/ml， and the scavenging rate reached the maximum of 68.92% at the concentration of 0.015 mg/ml. On the whole， the scavenging rate of the extract from hot air drying was the highest at the concentration of 0.015 mg/ml， reaching 69.92%.
Determination of ferric ion reducing capacity of sample liquid treated by different drying methods
The ferric ion reducing capacities of the sample liquids and ascorbic acid were shown in Fig. 4. As shown in Fig. 4， the absorbance values of the sample liquids treated by the 3 drying methods were higher than those of the undried extracts and the standard ascorbic acid. In terms of the increase， the sample liquid treated with microwave drying was greater than that with hot air drying， and in terms of the absorbance value， the sample liquid with hot air drying treatment was higher than that with microwave drying treatment from the beginning to the end. Within the measured concentration range， the absorbance value of the sample liquid treated with hot air drying increased with the increase of the concentration， and the ferric ion reducing capacity also enhanced with the increase of the concentration. The absorbance value could reach as high as 2.542.
Conclusion
In the determination of total flavonoid content， the flavonoid content in the liquid with vacuum freeze drying treatment was the highest of 84.56 mg/ml. The flavonoid content in the liquid with microwave drying treatment was 60.74 mg/ml， and the flavonoid content in the liquid with hot air drying treatment was 70.12 mg/ml. On the other hand， without drying， the fresh kiwifruit had the lowest flavonoid content due to the effect of water.
The liquid with hot air drying treatment had the strongest DPPH· free radicals scavenging ability， with the highest scavenging rate of 9.42%. On the other hand， the scavenging rate of OH· free radicals in the sample liquid from hot air drying treatment was the highest of 69.92% at the concentration of 0.015 mg/ml.
The absorbance value of the sample liquid treated with hot air drying increased with the increase of the concentration， and the ferric ion reducing capacity also enhanced with the increase of the concentration. The absorbance value could reach as high as 2.542.
In summary， the effects of the 3 drying methods from high to low are in the order of vacuum freeze drying > microwave drying > hot air drying， so the optimal drying method is hot air drying. Hot air drying has the least effect on the physiological activity of kiwifruit， and as the best drying method， it can better maintain the shape and nutrient compositions of kiwifruit， which can well preserve its various functions.
References
[1] ZHAI JL. Problems and development strategies of kiwifruit industry in China[J]. Science and Technology for Development， 2015（4）：521-529.
[2] DAI MJ， DONG M， FEI LJ， et al. Effect of storing property of kiwifruit treated by different microwave[J]. Science and Technology for Food Industry，2014，35（3）：326-330. [3] GU HN， LI Q， CHEN C， et al. Quality change and storage period forecast mode of domestic kiwi[J]. The Food Industry，2014，35（6）：7-10.
[4] ZENG MC， BI JF， CHEN QQ， et al. Weibull distribution for modeling med and short wave infrared radiation drying of kiwifruit slices[J]. Modern Food Science and Technology， 2014， 30（6）： 146-151， 201.
[5] DONG Z， YUAN CL， YAN XY， et al. Progress in research on polyphenol oxidase in grapes and wine[J]. Food Science， 2016， 37（15）：271-277.
[6] GUO WL， LI M， XIE Q， et al. Study on antioxidant activities of grape skin pigment[J]. China Food Additives， 2007（2）： 116-119.
[7] ZHU YH， TAN J. Antioxidation effect of corn peptides[J]. Journal of the Chinese Cereals and Oils Association， 2008， 23（1）： 36-38.
[8] LV YH， SU P， NA Y， et al. Study on antioxidant activities of mulberry pigment in vitro[J]. Journal of Zhejiang University （Agriculture & Life Science）， 2007， 33（1）： 102-107.
[9] DU L， LI XH， CHEN F. Study on antioxidant activities of black peanut skin pigment[J]. Science and Technology of Food Industry， 2012， 33（13）： 100-104..
[10] ZHU CH， GONG Q， LI JX， et al. Research progress of the comprehensive processing and utilization of kiwifruit[J]. Storage and Process， 2013， 13（1）： 57-62.
[11] PENG BZ， YUE TL， YUAN YH. Optimization of technical parameters on vacuum freezedry kiwifruit slice[J]. Transactions of the Chinese Society of Agricultural Machinery， 2007， 38（4）： 98-102.
[12] MENG J， HANG H. Study on extraction and antioxidative activity of the activated component of Juglans regial[J]. Food Science， 2001（12）： 44-46.
[13]YANG XS. Extraction of crude flavonoids from Abelmoschus manihot （L.） flower[J]. Chemical Engineer， 2011（10）：31-33.
[Methods] With kiwifruit as the research object， the effects of hot air drying， microwave drying， and vacuum freeze drying on the active ingredients in dried kiwifruit were studied， and antioxidant activity of the ingredients were determined through the scavenging capacity of 1，1phenyl2picrylhydrazyl free radicals （DPPH）， hydroxyl radical scavenging capacity and the antioxidant capacity of ferrous ions.
[Results] Vacuum freeze drying greatly reduced the physiological activities of kiwifruit， followed by microwave drying. Hot air drying was the best choice for dehydration of kiwifruit. Under the same conditions， the total flavonoid content of kiwifruit from hot air drying was 70.12 mg/ml， and the DPPH· free radicals scavenging ability of the treatment solution was the strongest， with the scavenging rate reaching 9.42%. The OH radicals scavenging rate was the highest at the concentration of 0.015 mg/ml， reaching 69.92%. The absorbance reached the maximum at the concentration of 0.025 mg/ml， and the ferrous ion reduction capacity was also the greatest with the absorbance of 2.542.
[Conclusions] Hot air drying could well maintain the shape and nutrients of kiwifruit， making it a proper drying method for kiwifruit processing.
Key words Different drying methods; Flavonoid; Antioxidant activity; Kiwifruit
Kiwifruit is grown all over the world and is widely used as medicine and food raw materials[1]. The regional and seasonal properties of kiwifruit affect its sales season and time. Therefore， the postharvest preservation processing has become an urgent problem for the development of kiwifruit industry[2-3]， and drying is one of the effective methods to prolong the storage period of kiwifruit[4]. There are many drying methods for kiwifruit. The traditional methods of dehydration drying include microwave drying and hot air drying. Microwave drying is not easy to control the time and temperature. If the temperature is too high， the kiwifruit slices are over dried， leading to great losses of nutrient. On the other hand， it is hard to maintain the quality of dry products in hot air drying. Compared with these 2 drying methods， vacuum freeze drying can maintain the shape and nutrient content of the original kiwifruit as much as possible[11-12]. The study on the content of antioxidant substances and their antioxidant capabilities in the products from different drying methods can provide bases for the election of drying methods and quality evaluation. In this study， vacuum freezing， hot air and microwave drying methods were applied to process fresh kiwifruit， and the effects of different drying methods on the antioxidant capacity were evaluated by hydroxyl radical determination， flavonoid content determination， DPPH and ferrous ion reduction method， in order to provide theoretical basis for the drying processing of kiwifruit. Materials and Instruments
Test materials were fresh kiwifruit， purchased from a fruit shop in Linfen City. The kiwifruit for the test was fresh with uniform size and certain hardness， which was stored at normal temperature for later use. The test reagents were as follows： Folin phenol， methanol， ethanol， rutin， sodium nitrite， sodium hydroxide， 1，1phenyl2picrylhydrazyl free radical （DPPH）， ferrous sulfate， absolute ethanol， sodium carbonate， aluminum nitrate， hydrogen peroxide， potassium ferricyanide， trichloroacetic acid， ferric chloride， ascorbic acid and 80% methanol.
Instruments for the test included 101 electrothermal blowing dry box of Beijing Ever Bright Medical Treatment Instrument Co.， Ltd.; FA2204B electronic balance of Techcomp Shanghai Instrument Co.， Ltd.; WFJ7200 visible spectrophotometer of Unico （Shanghai） Instrument Co.， Ltd.; HH6 digital display constant temperature water bath of Jintan Changzhou Instrument Manufacture Co.， Ltd.
Test Methods
Sample preparation
Fresh kiwifruit with uniform size and hardness was selected， peeled and cut into thin and even slices.
Sample drying
First， 10.000 g of pretreated fresh kiwi fruit slices were taken respectively for hot air drying， vacuum freeze drying and microwave drying. The slices were evenly laid out in the air dry oven， and dried at the temperature of 60 ℃. The weight was measured every 10-20 min until the water content dropped to 10%-12%. For vacuum freeze drying， the slices were placed on the clean object tray in the vacuum freeze dryer， followed by setting the temperature and time of the vacuum freeze dryer. In the first stage， the slices were in the freezing treatment at -30 ℃ for 1 h， the second stage， vacuum pumping treatment at -10 ℃ for 1 h， and finally the third stage， sublimation drying processing for 1 h at 30.0 ℃. As for the microwave drying， the fresh kiwifruit slices were placed on the clean turntable in the microwave oven， followed by processing for 1 min at the power of 1 000 W.
Extraction of antioxidants from kiwifruit
The kiwifruit samples treated in 3 different drying methods were labeled， and then extracted using 70% ethanol in a water bath at 70 ℃. The ratio of material to ethanol in the extraction was 1：10， and the extraction time was 2 h.
Determination of kiwifruit extract content
Standard curve plotting
First， 0.062 5 g of rutin standard was taken， dissolved in 80% methanol solution， and then set to the constant volume of 250 ml， obtaining rutin standard solution with the mass concentration of 0.25 mg/ml， which was stored at room temperature in dark. Then， 0.0， 2.0， 4.0， 6.0， 8.0， 10.0 ml of the standard solution was taken， and added with 1.5 ml of 5% NaNO2 solution， which was then placed still for 6 min after fully mixing. Next， after adding 1.5 ml of 10% Al（NO3）3 solution， the mixture was placed still for 6 min after fully mixing， and then 20 ml of 5% NaOH solution was added. Finally， the mixture was set to constant volume of 509 ml using 80% methanol solution， fully mixed. After the solution placed still for 15 min， the absorbance was measured at the wavelength of 510 nm. After 3 repetitions， the average vale was taken. Afterwards， the standard curve was plotted with the absorbance value as the ordinate and the rutin standard solution mass concentration as the abscissa[6]， obtaining the regression equation： Y=0.168X+0.001 （R 2=0.996）.
Determination of flavonoid content in the sample
Accurately 1 ml of each sample liquid was taken and marked. According to the curve plotting steps of rutin standard， the absorbance of the testing sample liquid was measured at the wavelength of 502 nm using a spectrophotometer， and the yield rate of flavonoid in the sample liquid was obtained based on the above regression equation.
Determination of DPPH· free radicals scavenging ability by different sample liquid
Accurately 2.5 mg of DPPH· was taken， dissolved in anhydrous methanol and diluted into a 100 ml volumetric flask， obtaining the DPPH· solution with the mass concentration 25 of μg/ml， which was then stored in the dark （0-4 ℃）[7]. Then， 0.1 ml of different sample solution was taken and added with 3.9 ml of DPPH· methanol solution with the mass concentration of 25 μg/ml. After quickly mixing， the absorbance （516 nm） was measured at 0， 5， 10， 15， 25， 35， 45， 55， 65， and 75 min until the absorbance was relatively stable， and the kinetic curve of the pigment scavenging DPPH· was plotted[8].
Determination of OH· radical scavenging ability by different sample liquid
First， a stoppered 10 ml graduated test tube was added with 2.0 ml of 6 mmol/L ferrous sulfate solution， 2.0 ml of sample solution with different concentrations， and 2.0 ml of 6 mmol/L hydrogen peroxide， respectively. After fully mixing and placing still for 10 min， the tube was added with 2.0 ml of 6 mmol/L salicylic acid solution. Then， after fully mixing and placing still for 30 min， the absorbance value A2 at 510 nm was measured， and the absorbance value A1 of 2.0 ml of the sample solution with 2.0 ml of absolute ethanol was measured， as well as the absorbance value A0 of 2.0 ml of salicylic acid with 2.0 ml of absolute ethanol. A total of 3 parallel tests were conducted to calculate the scavenging rate[13]. The scavenging rate for OH· was P（%）=[1-（A2- A1）/A0]×100%. Where： A2 is the absorbance of 2.0 ml of the sample solution with 2.0 ml of ferrous sulfate， 2.0 ml of hydrogen peroxide and 2.0 ml of salicylic acid; A1 is the absorbance of 2.0 ml of the sample solution with 2.0 ml of absolute ethanol; A0 is the absorbance of 2.0 ml of salicylic acid with 2.0 ml of absolute ethanol.
Determination of ferric ion reducing capacity
Successively 2.5 ml of 0.2 mol/L phosphate buffer solution with pH of 6.6， 2.5 ml of 1% potassium ferricyanide and 1 ml of sample solution with different concentration was mixed， and then bathed for 20 min at 50 ℃. After quick cooling， 1.0 ml of 10% trichloroacetic acid solution was added， followed by adding 2.5 ml of distilled water and 1.0 ml of 0.1% ferric chloride solution， and after fully mixing and placing still for 10 min at room temperature， the absorbance at 700 nm was determined. Ant 3 parallel tests were conducted. The reducing capacity of ferric ion was determined according to the value of the absorbance， and a larger absorbance value indicated stronger reducing capacity[5]. Ascorbic acid was used as a standard antioxidant[9]. Results And analysis
Total flavonoid content in 3 different drying methods
The content of total flavonoid of the samples after treatment by different drying methods was shown in Fig. 1. As shown in Fig. 1， the total flavonoid content was different for different drying methods at the same condition. The content of total flavonoid from high to low was in the order of vacuum freeze drying > hot air drying > microwave drying. Vacuum freeze drying had the smallest effect on the content of flavonoid in the sample liquid of kiwifruit， and the content of flavonoid in the sample liquid reached 84.56 mg/ml. On the other hand， the fresh kiwifruit sample liquid was undried， which was subjected to the greatest effect of wet weight， so the flavonoid content was the lowest.
Fig. 2 Kinetic curves of DPPH· free radical scavenging by different solutions
Scavenging rate of OH· free radicals in extracts from different drying methods
As shown in Fig. 3， as for scavenging abilities of the 4 sample solutions， the sample solution from hot air drying had the most significant effect on scavenging hydroxyl radicals， and its scavenging rate was greater than the scavenging rate of the sample solution from any other drying method in the range of 0.005-0.025 mg/ml.
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In the range of 0.005-0.025 mg/ml， there was no significant change in the scavenging rate with the concentration between the undried and hot air dried extraction， and when the concentration was 0.015 mg/ml， the scavenging rate of hot air drying reached the highest of 69.92%. Under the same concentration， the scavenging rates of the extracts from vacuum freeze drying and microwave drying were lower than that from hot air drying. For the extract from vacuum freeze drying treatment， the scavenging rate increased with the concentration in the range of 0.005-0.020 mg/ml， and reached the highest at the concentration of 0.02 mg/ml. For the kiwifruit extract from microwave drying， the scavenging rate increased significantly with the increased of concentration in the range of 0.005-0.015 mg/ml， and the scavenging rate reached the maximum of 68.92% at the concentration of 0.015 mg/ml. On the whole， the scavenging rate of the extract from hot air drying was the highest at the concentration of 0.015 mg/ml， reaching 69.92%.
Determination of ferric ion reducing capacity of sample liquid treated by different drying methods
The ferric ion reducing capacities of the sample liquids and ascorbic acid were shown in Fig. 4. As shown in Fig. 4， the absorbance values of the sample liquids treated by the 3 drying methods were higher than those of the undried extracts and the standard ascorbic acid. In terms of the increase， the sample liquid treated with microwave drying was greater than that with hot air drying， and in terms of the absorbance value， the sample liquid with hot air drying treatment was higher than that with microwave drying treatment from the beginning to the end. Within the measured concentration range， the absorbance value of the sample liquid treated with hot air drying increased with the increase of the concentration， and the ferric ion reducing capacity also enhanced with the increase of the concentration. The absorbance value could reach as high as 2.542.
Conclusion
In the determination of total flavonoid content， the flavonoid content in the liquid with vacuum freeze drying treatment was the highest of 84.56 mg/ml. The flavonoid content in the liquid with microwave drying treatment was 60.74 mg/ml， and the flavonoid content in the liquid with hot air drying treatment was 70.12 mg/ml. On the other hand， without drying， the fresh kiwifruit had the lowest flavonoid content due to the effect of water.
The liquid with hot air drying treatment had the strongest DPPH· free radicals scavenging ability， with the highest scavenging rate of 9.42%. On the other hand， the scavenging rate of OH· free radicals in the sample liquid from hot air drying treatment was the highest of 69.92% at the concentration of 0.015 mg/ml.
The absorbance value of the sample liquid treated with hot air drying increased with the increase of the concentration， and the ferric ion reducing capacity also enhanced with the increase of the concentration. The absorbance value could reach as high as 2.542.
In summary， the effects of the 3 drying methods from high to low are in the order of vacuum freeze drying > microwave drying > hot air drying， so the optimal drying method is hot air drying. Hot air drying has the least effect on the physiological activity of kiwifruit， and as the best drying method， it can better maintain the shape and nutrient compositions of kiwifruit， which can well preserve its various functions.
References
[1] ZHAI JL. Problems and development strategies of kiwifruit industry in China[J]. Science and Technology for Development， 2015（4）：521-529.
[2] DAI MJ， DONG M， FEI LJ， et al. Effect of storing property of kiwifruit treated by different microwave[J]. Science and Technology for Food Industry，2014，35（3）：326-330. [3] GU HN， LI Q， CHEN C， et al. Quality change and storage period forecast mode of domestic kiwi[J]. The Food Industry，2014，35（6）：7-10.
[4] ZENG MC， BI JF， CHEN QQ， et al. Weibull distribution for modeling med and short wave infrared radiation drying of kiwifruit slices[J]. Modern Food Science and Technology， 2014， 30（6）： 146-151， 201.
[5] DONG Z， YUAN CL， YAN XY， et al. Progress in research on polyphenol oxidase in grapes and wine[J]. Food Science， 2016， 37（15）：271-277.
[6] GUO WL， LI M， XIE Q， et al. Study on antioxidant activities of grape skin pigment[J]. China Food Additives， 2007（2）： 116-119.
[7] ZHU YH， TAN J. Antioxidation effect of corn peptides[J]. Journal of the Chinese Cereals and Oils Association， 2008， 23（1）： 36-38.
[8] LV YH， SU P， NA Y， et al. Study on antioxidant activities of mulberry pigment in vitro[J]. Journal of Zhejiang University （Agriculture & Life Science）， 2007， 33（1）： 102-107.
[9] DU L， LI XH， CHEN F. Study on antioxidant activities of black peanut skin pigment[J]. Science and Technology of Food Industry， 2012， 33（13）： 100-104..
[10] ZHU CH， GONG Q， LI JX， et al. Research progress of the comprehensive processing and utilization of kiwifruit[J]. Storage and Process， 2013， 13（1）： 57-62.
[11] PENG BZ， YUE TL， YUAN YH. Optimization of technical parameters on vacuum freezedry kiwifruit slice[J]. Transactions of the Chinese Society of Agricultural Machinery， 2007， 38（4）： 98-102.
[12] MENG J， HANG H. Study on extraction and antioxidative activity of the activated component of Juglans regial[J]. Food Science， 2001（12）： 44-46.
[13]YANG XS. Extraction of crude flavonoids from Abelmoschus manihot （L.） flower[J]. Chemical Engineer， 2011（10）：31-33.