Effect of electrodialysis desalination technology parameters on ratio of desalinization and quality of soy sauce

Abstract: Soy sauce is a kind of nutritional condiment which contains various bioactive components. In the production of soy sauce, a lot of salt has been added in order to prevent the growth of pathogenic microorganisms. However, excessive intake of high salt food is harmful to our health, so the study of the desalination of soy sauce is very valuable. The technology of electrodialysis has applications in the areas of pollution controlling, resource recoverying and chemical processing. At present, no information is available about the effect of electrodialysis desalination (ED) on the changes of flavor compounds and amino acids of ED-treated soy sauce. In this paper, a traditional Chinese soy sauce containing 16 g/100 mL sodium chloride was desalted by laboratory-scale electrodialyser equipped with a stack composed of eight cation and seven anion-exchange membranes, and the impact of operating voltage, flow rate and pH value on desalination was evaluated. The optimal condition of electrodialysis was obtained through the single factor experiment. The result indicated that the salt content could be reduced by 81.6% at the voltage of 9 V, the flow rate of 2.4 cm/s, and the pH value of 4.2 with 19.4% amino acid nitrogen losing. The analysis of variance showed that pH value had a significant effect on the loss of amino acid nitrogen (P<0.05), while the desalination rate was not changed remarkably (P>0.05). After electrodialysis, the changes of free amino acids and volatile flavor compounds were analyzed with amino acid automatic analyzer and GC-MS (gas chromatography-mass spectrometer). The results showed that part of free amino acid and aroma of soy sauce lost. The influence of electrodialysis on the changes of free amino acids in soy sauce was determined under various pH value conditions (3.2, 3.7, 4.2, 4.7 and 5.2). Several kinds of bitter amino acids lost, and histidine, arginine and lysine lost by 23.79%, 26.39% and 28.5% respectively. As their isoelectric point was deviated from the solution pH value, they were positively charged in solution and susceptible to leave the material liquid chamber with the effect of electric field. Besides, some flavor compounds of soy sauce lost, including alcohols and phenols, especially the 4-ethyl-2-methoxy-phenol and 2-methoxy-phenol significantly decreased. However, due to its low odor threshold, it had little effect on the flavor of soy sauce. Since the ED unit was generally operated in the batch mode, it would be feasible to produce soy sauce with arbitrary salt concentration. The results prove that electrodialysis is a feasible method with a remarkable decrease of salt content.