Study Of Air Jet Impingement Drying Of Okara And Its Quality Change During Storage

Okara is a byproduct from the process of soybean foods,and is rich in proteins,fatty acids and dietary fibers.Wet okara deteriorates rapidly owing to the high moisture and protein content.Drying of wet okara is essential for long-term storage and its value-added applications.In this study,4 drying techniques（vacuum freeze drying,hot air drying,air jet impingement drying and vacuum microwave drying）were performed on okara.Their drying kinetics and the quality of the dried okara was investigated.Air impingement drying was found to be an efficient and appropriated art,and then response surface methodology was used to optimize its drying process.The quality stability of the dried okara during storage was also studied.The major contents and results of the study are as the followings.（1）Okara was dried by vacuum freeze drying（VFD,47 Pa and-20 ^oC）,hot air drying（HAD,80 ^oC）,vacuum microwave drying（VMD,20 kPa and505 W）,and air jet impingement drying（AJID,2 m/s air speed and 80 ^oC）.The drying kinetics,and the properties（the physicochemical properties,the antioxidant properties and the undesirable anti-nutritional factors）of the dried okara were investigated.Page model was suitable to simulate the drying dynamics of okara under VMD,AJID and VFD.To reach the designated moisture content（<10%）,it took the longest time for HAD,and the shortest for VMD,while the drying time of AJID was close to that of VMD.Compared with the okara dried by VFD,VMD,AJID and HAD turned okara darker and more yellow.Among them,product from HAD showed the greatest change.After drying,all the tested anti-nutritional factors reduced significantly.The lowest phytic acid and saponin value were found in the AJID samples,while VMD samples contained the minimum tannin and TIA.The total reducing ability of the okara also decreased considerably after drying,whereas the lowest value was observed in the HAD samples.The decreased DPPH·scavenging activity was found in VMD,AJID and HAD samples.Considering the drying efficiency and overall quality of the dried okara,the air jet impingement drying and vacuum microwave drying are superior to other techniques,and of great potential for commercial application.（2）The process of drying okara in an air jet impingement drier（AJID）was optimized by the response surface methodology and synthetic evaluation method.Air temperature（50,60 and 70 ^oC）,air velocity（1.3,1.8 and 2.3 m/s）,and sample loading density（3,4 and 5 kg/m²）were set as the treatment factors,and drying rate,color,trypsin inhibitor activity,soy isoflavone content and antioxidant activity were evaluated as the responding quality parameters.All treatment variables showed significant effects on the drying rate and soy isoflavone content（P<0.05）.Higher temperature,higher air velocity and lower loading density contributed to higher drying rate.Temperature and air velocity showed quadratic and interactive effect on the antioxidant activity of okara.The optimum conditions for AJID of okara were identified as 70 ^oC,2.3 m/s air velocity,and 3 kg/m² loading density.This process control study provided baseline data for developing effective drying of okara using AJID.（3）The quality stability of okara dried by AJID was evaluated for 60days.Dried okara was stored at 5^oC,25^oC and 40^o C respectively,in the vacuum packed in PE bags.Their moisture content,water activity,chroma indexes（L*,a*and b*value）,protein quality（protein content and solubility）and peroxide value were analyzed at the 0,7^th,30^th,and 60^th day.During the storage,moisture content of okara was lower than 12%,and the Aw values lower than 0.60,which was not suitable for microbial growth.The protein content and protein solubility didn’t change significantly in all the tested samples.Significant change of the chroma indexes was only observed in the b*value of okara stored at 40 ^oC for 30 days.The peroxide value decreased as storage temperature increased.When okara was stored at 25 or 40 ^oC,the peroxide value decreased in the 7^th day,and then kept stable,whereas it didn’t change significantly at 5 oC.The results showed that the okara dried by AJID could be preserved for 2 months and suitable for high-added value application.This study indicated that air jet impingement drying was an efficient technique to drying okara.The optimum conditions for AJID of okara were identified as 70 ^oC,2.3 m/s of air velocity,and 3 kg/m² of loading density.The dried okara could be easily preserved with good quality stability for at least 60 days.