Abstract: [Objective] The hydrolysate of whey protein powder contains α- amino small molecule substances of the content in product is relatively high. Find a detection method to detect the content of this part of the substance,and report the proportion of protein as a reference value for the degree of hydrolysis.[Method] Kjeldahl nitrogen determination method was used to detect total protein content,while using ninhydrin and α- amino groups take color reaction,read the absorbance value of the reaction solution,and substitute it into the working curve to obtain the content of α- amino small molecule substances of the ratio amino to total protein is the content （in protein）.[Result] α- amino of small molecule content （in protein） standard deviation results is 1.91,with a relative labeling deviation of 3.5%. The method has good repeatability and the recovery rate also meets the requirements for spiked recovery. [Conclusion] Ninhydrin colorimetric method for detecting the presence of α- amino small molecule substances have obvious advantages and can be used in combination with Kjeldahl method to monitor the protein hydrolysis effect.

Key words: Kjeldahl method, Ninhydrin visual response, α- amino, standard rings, absorption