Collaborative study for determination of protein in seasoning soy sauce

Abstract

       Protein content is a key requirement for the quality of seasoning soy sauce according to the notification of the Ministry of Public Health. There are various methods available for protein determination in food but no specific method for seasoning soy sauce exists. Since method for protein determination is an empirical method which gives the result that is method-dependent, different method may give different result. This can lead to the disputes among users of the analytical data. It is obvious that a standardized method is needed especially when the result is to be used in law enforcement. The aims of this study is to validate the selected method for protein in seasoning soy sauces using collaborative trial in order to ensure inter-laboratory reproducibility and to characterize the performance of the method that can be used as acceptance criteria when laboratories apply this method in routine work. In this study, Kjeldahl method using block digestion with a copper catalyst followed by steam distillation and titration was validated by single laboratory prior to the collaborative study. The collaborative study involved a series of experiments in 14 selected competent laboratories. Five blind duplicate samples of seasoning soy sauces and sweetening seasoning soy sauces with protein content varied from 1.8 g/100g to 12 g/100g were used. The analytical data were statistical analyzed using Cochran’s test, Grubbs’ test and ANOVA. The results showed as follows: repeatability standard deviation (S_r), repeatability relative standard deviation (RSD_r,%) and repeatability limit (r) ranged from 0.03-0.16, 0.57-1.72 and 0.09-0.46 respectively. Reproducibility standard deviation (S_R), reproducibility relative standard deviation (RSD_R, %) and reproducibility limit (R) ranged from 0.07-0.21, 1.39-3.78 and 0.19-0.59 respectively. It can be concluded that this method is reproducible and appropriate for the determination of protein in seasoning soy sauces. This method will be proposed for adoption as the standard method attached to the related food standards in the Notification of the Ministryof Public Health.