| Puerariae Radix is rich in nutrients,including starch,dietary fiber,carbohydrates,minerals and rich isoflavone.Pueraria powder is a kind of powder product which uses fresh Puerariae Radix as raw material,it includes the process of cleaning,peeling,slicing,color protecting,blanching,cooking,drying,crumbling and so on.Its flavor quality and processing properties are greatly affected by the production technology and drying methods.In this study,Puerariae Radix was used as material.Firstly,the basic characteristics of polyphenol oxidase and its control was studied.Secondly,the effects of hot air drying,vacuum drying and vacuum freeze drying on the physical and chemical properties,processing properties,oxidation resistance,aroma components and sensory evaluation of the whole powder were compared.At last,the best drying method was selected according to the comprehensive indexes,and the whole process was optimized by single factor and response surface experiments,which provided the theoretical basis for the production and processing of pueraria powder.The results are as follows:(1)The optimum pH value of polyphenol oxidase(PPO)was 4.5.The relative activity of PPO was higher than 40% when the pH value in the range of 3.5-5,the activity of PPO decreased rapidly when the pH was lower than 3 or greater than 5.5.The activity of PPO was the highest at the temperature of 45?,and the enzyme activity decreased significantly with the increase of temperature(P<0.05).High temperature treatment could rapidly inactivate the enzyme in a short time,100? treatment for 80 seconds and 90? for 160 seconds could result in loss of enzyme activity.The single inhibitor can inhibit the activity of PPO in different degrees,and the activity of PPO decreased with increasing inhibitor concentration.The residual activity of PPO decreased from 64% to 43.3% when the concentration of sodium isoascorbate was increased from 0.1% to 0.5%.The residual activity of PPO decreased from 75.3% to 61.3% when sodium chloride concentration increased from 0.5% to 2.5%.When the concentration of citric acid and calcium chloride was 0.4%,the residual enzyme activity of PPO was respectively 59% and 67.3%,and the enzyme activity was not significantly reduced when the concentration was further increased(P>0.05).The optimal ratio was obtained through orthogonal experiment: sodium chloride1.5%,sodium isoascorbate 0.5% and citric acid 0.8%,the residual enzyme activity of PPO was 10.19%.According to this proportion,color difference value ?E decreased gradually with with the extension of color protection time.The color difference value was 28.90 for 30 minutes,the color difference value was not significant with further extension of time(P>0.05).Select 30 minutes is more appropriate.(2)The effects of different drying methods on the main nutritional components of pueraria powder were significantly different(P<0.05).Hot air drying powder reducing sugar content was the highest(4.33g/100g).The protein of vacuum freeze-drying powder was the highest(6.89g/100g).The content of starch and fat in vacuum drying powder was the highest,which were 60.68g/100 g and 1.57g/100 g,respectively.The iodine blue value of hot air drying powder was 9.96,which was significantly higher than that of vacuum drying and vacuum freeze drying(P<0.05).There was a significant difference in the color difference valueunder different drying methods(P<0.05),and the hot air drying had the greatest influence on the color of the whole powder,its color difference value was 29.56.According to the particle size and scanning electron microscope analysis,the average particle size of powders with different drying methods had significant difference(P<0.05).Vacuum drying and vacuum freeze drying were easier to maintain the integrity and uniformity of the powder particles.From the view of processing characteristics,there were significant differences in solubility,water absorption and oil absorption capacity,emulsifying property and freezing and tha wing rate of water in different drying methods(P<0.05).The solubility of hot air drying was highest(30.18g/100ml),the lowest was vacuum freeze-drying(18.84g/100ml).Vacuum drying water absorption capacity was 7.82ml/g,vacuum freeze drying oil absorption capacity was 1.15ml/g,which was the highest of the three ways(P<0.05).The emulsification and emulsion stability of hot air drying were 5.49% and 3.36% respectively,which were significantly higher than those of vacuum drying and vacuum freeze drying(P<0.05).There was no significant difference in emulsion stability between vacuum drying and vacuum freeze drying(P>0.05).The vacuum freeze drying had the lowest freezing thawing rate(P<0.05)and the best stability.From the view of gelatinization properties,the viscosity curve of hot air drying and vacuum drying powder did not exhibit obvious peaks and troughs,and the pasting temperature were 72.5? and 75.75? respectively.The peak viscosity(2109)and disintegration(782)of vacuum freeze drying were the highest,and the final viscosity of vacuum drying was the lowest(1296).From the view of digestive characteristics,after 4 hours of digestion,the total digestibility of vacuum drying powder was the highest(40%),followed by hot air drying(38%),the lowest was vacuum freeze-drying(36%).(3)The content ofantioxidants was significantly affected by different drying methods(P<0.05).Contents of total flavonoids and total phenolics in the pueraria powder with vacuum freeze drying were 2.46 g/100 g and 1.37 g/100 g respectively,and showed to be significantly higher than the other drying methods(P<0.05).The processing of each link had different degrees of influence on antioxidants.The total flavonoids were decreased by 14.5%,12% and 12.5%(P<0.05)after color protection,blanching and cooking.The total phenolics decreased by 8.6%,6.3% and 33.3% respectively,but there was no significant difference between color protection and blanching treatments(P>0.05).The DPPH free radical scavenging rate,hydroxyl radical scavenging rate and ABTS radical scavenging rate of puerariae powder were significantly increased with the increase of powder amount(P<0.05).The scavenging effect of vacuum freeze-drying was the strongest,and the scavenging rate of DPPH free radical was 82.80% when the powder content was 10 mg.The scavenging rate of DPPH free radical was significantly higher than that of hydroxyl radical and ABTS free radical(P<0.05).The reduction ability of different drying methods powder had significant difference(P<0.05),vacuum freeze drying powder had the highest force,the absorbance value was 1.354 when the amount of powder was 10 mg.The iron ion chelating ability: Vacuum freeze drying>vacuum drying>hot air drying.With the increase of powder amount,the change tendency of the chelating ability of vacuum drying and hot air drying was similar.Vacuum freeze drying powder could chelate 2.54 mL ferric ion solution when the amount of powder is 10 mg.There was no significant difference between the hot air drying and vacuum drying(P>0.05).(4)The SPME-GC-MS method was used to detect 90 kinds of aroma compounds in fresh Puerariae Radix and pueraria powder.For fresh Puerariae Radix,hot air drying,vacuum drying and vacuum freeze drying were respectively got the 43,68,66,64 kinds of volatile aroma components.The main aroma components of fresh Puerariae Radix were 1-Hexanol,leaf alcohol and hexanal,relative content of esterswere only 0.34%.The main aroma components of pueraria powder were aldehyde ketones and esters,and the content of alcohol in aroma components was less,including 1-Octen-3-ol,linalool and 1-Pentanol and alpha-Terpineol etc.Compared with fresh Puerariae Radix,the aldehydes of hot air drying,vacuum drying and vacuum freeze drying respectively increased 14.35%,13.86% and 20.43%.Compared with hot air drying,vacuum drying and vacuum freeze drying had better retention and increase of aroma components in fresh Puerariae Radix,and reduce some of components which have the counter function to the flavor.Sensory evaluation results suggested the pueraria powder made by vacuum drying method was 64.70,the hot air drying was 59.68,and the vacuum freeze drying was 59.10.The drying time,main nutritional components,iodine blue value,color difference value,antioxidant substances and sensory score were used to comprehensively evaluate the different drying methods.Hot air drying scored 20.15,vacuum drying was 22.25,vacuum freeze drying was 19.53.The total value of vacuum drying powder was the highest,and the technology of vacuum drying was optimized.(5)According to the comprehensive evaluation scores,vacuum drying was selected to optimize the process.Taking the iodine blue value as the evaluation index,the blanching time,cooking temperature,cooking time and drying conditions was optimized by a single factor and response surface experiments.The regression model was calculated by response surface: Y=7.83-0.10A-0.077B+0.51C-0.37D+0.34AB+0.18AC-0.098AD-0.083BC+0.078BD+0.56CD+0.43A~2+0.62B~2+3.00C~2+0.88D~2.Optimum technological conditions of pueraria powder by vacuum drying were: blanching time was 155 seconds,cooking temperature of 90 to maintain 9.5 minutes,drying temperature of 63 to maintain 8.3 hours.Under these conditions,the iodine blue value was 7.62,which was consistent with the predicted value.It proved that the response surface method was a reasonable and feasible method to optimize the drying parameters of pueraria powder. |
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