| Plant-based yogurt has attracted much attention of consumers in recent years because it does not contain cholesterol and is suitable for patients with lactose intolerance and vegetarian consumers.The use of plant-based slurries to replace milk to develop lactic acid bacteria fermented beverages has tended to become a research hotspot.China has abundant walnut resources,and walnuts own high nutritional value.Here lactic acid bacteria were used to ferment walnut milk to achieve the dual advantages of walnut nutrition and lactic acid bacteria fermentation.In view of the problems of low acidity,long fermentation time,strong graininess and poor flavor in walnut milk fermentation,firstly the paper studied the effect of walnut milk formula on the quality of fermented milk.Then the influence of walnut protein hydrolysis on the fermentation properties of walnut milk and textures of walnut fermented milk were investigated.Finally,the improvement for the flavor of walnut fermented milk was explored.The research results can provide theoretical basis and practical guidance for the development of walnut fermented milk products.The main research contents are as follows:Firstly,the effect of walnut milk formula on the quality of fermented milk was investigated.The sugar composition and fermentation characteristics of reconstituted milk,soy milk and pure walnut milk were comparably determined.The results showed that the total amount of fermentable sugars in walnut milk was very small,and the acidity of walnut fermented milk was very low.Therefore,it was necessary to add carbon source to promote fermentation.By adding glucose and sucrose,it was found that sucrose in the walnut milk system was more conducive to acid production than glucose,and the optimum addition amount of sucrose was determined to be 7.0%through sensory evaluation.The fermentation research of walnut milk under different protein concentrations(1.0%～3.0%)was investigated,and it was found that the protein concentration of walnut milk was proportional to the acidity and buffering capacity(R~2>0.995).By adding buffer salt to increase the buffering capacity of walnut milk,it was found that the time for walnut milk with protein concentration of 1.5%to reach the fermentation end point(10 h)was shortened by 58.3%.The walnut milk was centrifugally defatted to reduce the fat content,and it was found that centrifugal defatting had little effect on the fermentation characteristics of walnut milk,but the texture characteristics became weaker and the sensory score was low.Then,five proteases(papain,alkaline protease,bromelain,flavor protease and protein glutaminase(PG enzyme))were used to enzymatically hydrolyze walnut milk,and their effects on the fermentation characteristics of walnut milk and the quality of fermented milk were investigated.After protease hydrolysis,the TCA soluble nitrogen content of walnut milk increased significantly.The TCA soluble nitrogen content of alkaline protease group was the highest(18.09%),and the free amino acid content of flavor protease group was the highest(18.94 mg/100m L).The fermentation time of walnut fermented milk was significantly shortened by protease enzymolysis treatment,and the order from short to long was papain<alkaline protease<bromelain<PG enzyme<flavor protease<no enzymatic hydrolysis.Compared with the non-enzymatic hydrolyzed group,the time for the walnut milk after papain hydrolysis to reach the end of fermentation was shortened by 62.5%,and it only took 6 hours.Except for PG enzyme,the texture of walnut fermented milk after enzymatic hydrolysis by other proteases was better,and the hardness and consistency decreased,reducing the sense of granularity.Then the fermentation characteristics of walnut milk with different protein concentrations after papain enzymatic hydrolysis were studied.It was found that too high or too low protein concentration would not shorten the fermentation time of walnut milk.The effect of different additions of papain on the quality of walnut fermented milk was further investigated.It was found that with the increase of papain addition,the TCA soluble nitrogen content of walnut milk increased accordingly,while the acidity of fermented milk did not increase accordingly,and the hardness,consistency and cohesion of fermented milk decreased accordingly.The walnut milk with papain addition of 0.1%(E/S)fermented the fastest.Finally,the effects of enzymatic hydrolysis and yogurt starter on the volatile flavor components and quality of walnut milk and its fermented milk were studied.The results showed that the key flavor compounds of walnut milk were hexanal,2-octenal,1-octen-3-ol,(E,E)-2,4-nonadienal,nonanal,n-octanal,pentanal,(E)-2-heptenal and 2-pentylfuran,of which the contribution rate of hexanal was more than 50%.The content of aldehydes and alcohols in walnut milk increased after enzymatic hydrolysis by papain,and produced(Z)-6-nonenal,(E,E)-2,4-decadienal,n-butanol and more ketones.After papain hydrolysis,the total amount of volatile flavor substances and aldehydes in walnut fermented milk was significantly reduced,but the total amount of ketones was less,and 2,3-butanedione and esters were not produced.The fermentation characteristics of five kinds of yoghurt starter cultures(A,B,C,D,E)in walnut milk and the texture characteristics and volatile flavor of fermented milk were further compared.The fermentation rate of starter cultures D in walnut milk was the fastest and of starter cultures E was the slowest.Starter cultures D helped to improve the hardness,consistency and water holding capacity of walnut fermented milk,and starter cultures A helped to improve the cohesion and viscosity index of walnut fermented milk.In terms of flavor components,there were significant differences of the total volatile flavor substances and ketones in walnut fermented milk prepared with different yoghurt starter cultures.Among them,2,3-butanedione was not detected in the walnut fermented milk prepared from the starter cultures C and D,while it was more abundant in the A,B and E groups. |
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