| In recent years,because the unique natto kinase of natto and its related products has important research value,which has a variety of medical care effects such as preventing hypertension and treating thrombosis,people pay more and more attention to them.In this paper,a new natto compound beverage was prepared by inoculating Bacillus natto with soybeans and skimmed milk powder as raw materials.By exploring the preparation technology and quality of natto compound beverage,it not only improved the taste and enhanced the activity of natto kinase,but also further developed the diversified market of natto compound beverage and filled the blank of natto compound beverage market.The main research contents were as follows:（1）In this paper,Bacillus natto was extracted and screened from four kinds of popular natto（Black Sprite natto,Alexandrite natto,Yanjing natto and Binli natto）.The target strain was screened by casein culture medium,and it was found that strain BN-1-2 had the highest activity.The experimental results of the physiological,biochemical and molecular biological identification showed that the characteristics of strain BN-1-2 were consistent with those of Bacillus subtilis.Therefore,strain BN-1-2 was selected as the strain required for subsequent research.（2）The preparation of natto compound beverage took soybeans and skimmed milk powder as raw materials,used the BN-1-2 screened in this experiment for fermentation,and strictly controled the process conditions.The fibrinolytic activity and sensory score of natto kinase were used as indicators to judge.It was concluded from the single factor experiment that the comparatively significant factors affecting natto beverage were the ratio of fermentation substrate,the inoculation amount of Bacillus natto,fermentation temperature and fermentation time,while the effect of initial p H on the quality of the final product could be ignored.Response surface optimization（RSM）was used to optimize the fermentation process of natto beverage.The optimal fermentation conditions were as follows: the volume of soy milk was 1:1,the inoculation amount of Bacillus natto was 2%,and fermentation at 37℃ for 16 h.The optimized drink had good flavor and the nattokinase enzyme activity was up to 205.46 U/m L,with sensory score of 44.31.（3）In order to improve the stability of the natto compound beverage,the single stabilizer was selected by taking the centrifugal sedimentation rate as the index,and the static stratification rate was measured for evaluation.It was found that xanthan gum,konjac gum and sodium carboxymethyl cellulose had an obviously stabilizing effect on the natto compound beverage.Then the interaction between different stabilizers was studied.Through the compounding experiment,the best stabilizer addition combination was obtained: adding 0.20% sodium carboxymethyl cellulose,0.15% konjac gum and 0.05%xanthan gum.The compound beverage prepared according to this proportion had strong stability and would not appear stratification within the validity period.Finally,the storage experiment of natto compound beverage was carried out,which was found that the quality of the compound beverage was better when storing at 4℃ and drinking within one month.（4）The quality of natto compound beverage before and after fermentation was determined.The appearance of natto compound beverage was milky white,the texture was uniform without precipitation,and the taste was smooth and delicate,with soybean milk aroma.After fermentation,the content of polypeptides in the beverage increased,which was more conducive to human absorption.The content of active substances increased,especially the content of flavonoids and polyphenols respectively reached 3.29 mg/ml and5.45 mg/ml.In addition,it was rich in a variety of essential amino acids,of which the content of sweet and fresh amino acids increased after fermentation.There were 25 kinds of volatile flavor compounds in the compound beverage,particularly,alcohols,esters and ketones were the main contributors to the flavor characteristics of the products.Finally,the microbial index of natto compound beverage was determined according to the national food safety standard,which was found that the compound beverage met the requirements of microbial index of fermented milk in GB 19302-2010. |
