Quantification of salt content in commercial fish sauces through reagent-free flow-injection method

Salt content monitoring in fish sauce is important for quality control of the fish sauce manufacturing. This work describes a green analytical method (AGREEprep score: 0.68, AGREE score: 0.74) for determination of salt in commercial fish sauce by using a simple spectrophotometric flow-injection (FI) system. Dilution of the fish sauce samples with deionized-distilled water was imperative to minimize the interference from color and other matrices. The method was developed based on schlieren effect which uses light refraction at the boundaries between the salted sample solution and water carrier. The schlieren signals were monitored at 890 nm and the peak height corresponded to salt concentration (express as %w/v NaCl) in the diluted sample solution. Under optimal conditions, the proposed method successfully determined salt contents without any chemical employment in ambient temperatures (24-28 °C) environment. Standard addition curves provided a linear working range between 5 and 25 %w/v with good determination of coefficient (r² > 0.99). The relative standard deviation (RSD) for analysis in authentic fish sauce samples were less than 5 % and the recoveries were between 92 % and 119 %. The limit of detection and limit of quantitation were 0.6 %w/v and 2 %w/v, respectively. According to the paired t-test, there was no significant difference between the results from the proposed method and the conventional titration method (t_observed 2.10 < t_{critical of two-tail} 2.16 at P = 0.05). The proposed method has great advantages of environmentally friendly, cost-effective for analysis, and safety by avoids the risks associated with direct contact with chemical reagents.