The importance of reducing sodium intake as a practical preventative measure against cardiovascular disease and stroke is becoming widely known [1,2,3,4]. According to the guidelines for hypertension management, a goal should be to set the salt intake to <6 g/day [5]. However, efforts to promote salt reduction in Japan is insufficient. Many studies have reported that the use of glutamate is an effective way to reduce salt intake without decreasing the palatability of food [6,7,8]. However, the results of previous studies may have been influenced by the flavor and taste of the soup itself [9,10,11], and few studies have considered the effects of free glutamate concentration [12, 13]. Yamaguchi et al. used water as a base medium and reported the interaction between monosodium glutamate (MSG) and the four basic tastes [14]. In their study, solutions with sodium chloride (NaCl) in the range of 0–3.4 × 10–1 mol/L and MSG in the range of 0–8.0 × 10−2 mol/L (0–2.0% NaCl, 0–1.3% MSG) were evaluated. However, few basic research studies have examined the effects of MSG added to water as the base medium with concentrations ≤0.9% NaCl, which is similar to that in typical soups.

In this study, we used distilled water with no taste and no odor as the medium to examine the possibility of reducing salt in ordinary soup preparations (0.9% NaCl). We validated the changes in the taste of the NaCl solution with the addition of umami substances (L-glutamates) using a sensory evaluation test and quantitative analysis. We also compared the effects of four glutamates that have different counter-ions.

The participants were healthy females in a registered dietitian training course at a university, and there were 11 participants (aged 22–64; mean ± SD 30.5 ± 3.7) in Experiment 1 and 11 participants (aged 20–65; mean ± SD 28.7 ± 12.6) in Experiment 2. In Experiment 1, 48 types of NaCl/MSG mixtures (0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9% NaCl and 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6% MSG) were tasted. The participants tasted one series of test solutions with a set MSG concentration on each testing day. On the test day, the participants tasted eight different NaCl/MSG mixtures in order from the lowest to highest NaCl concentrations. In Experiment 2, the sensory evaluation was a single-blind test. Ten types of NaCl/glutamate solutions consisting of 0.3 or 0.6% NaCl alone and 0.3 or 0.6% NaCl plus one of four glutamates, MSG, monopotassium glutamate (MPG), calcium diglutamate (CDG), and magnesium diglutamate (MDG), were evaluated. The free glutamate concentration of the test samples was set at 1.6 × 10–2 mol/L. The participants tasted the 0.3% NaCl mixture on day one and the 0.6% NaCl mixture after 2 weeks.

Sensory evaluation was performed between 10:30 am and 11:30 am. Each solution was tested twice by the participants. Saltiness and palatability were evaluated using a scale with 21 possible ratings. The range was between −5 and +5 in 0.5-point increments. The evaluation baseline (0) of saltiness and palatability was set as the 0.5% NaCl solution. The saltiness of the distilled water was rated as −5, and that of 1.0% NaCl was rated as +5. The average rating of the two evaluations was calculated for each participant. The participants were asked to rinse their mouths with water before and between the evaluations of each sample.

To determine whether differences existed in the average ratings of the eight different NaCl mixtures for each MSG concentration in Experiment 1 and to determine whether differences existed in the average ratings of the five different NaCl/glutamate mixtures in Experiment 2, repeated-measures analysis of variance (ANOVA) was used. Then, the Tukey’s honest significant difference post hoc test was used for multiple comparisons. For all statistical analyses, JMP statistics software (Ver. 14.2, SAS Institute Inc., Cary, NC, USA) was used, and p < 0.05 was considered significant.

As shown in Table 1, repeated-measures ANOVA revealed that there were significant differences among the average ratings of mixtures with different NaCl concentrations (eight types) and all other MSG concentrations (all p < 0.001). For saltiness, the post hoc test results showed that at the same MSG concentration, all of the mixtures with a 0.2% concentration difference showed significant differences (all p < 0.05). For palatability, among all MSG concentrations, the mixture with 0.6% NaCl obtained the highest rating. The post hoc test showed significantly higher ratings for the 0.6% NaCl mixtures, showing the highest rating regardless of MSG concentration, than those of the 0.3 and 0.9% NaCl mixtures (all p < 0.05). The 0.6% NaCl mixtures with 0.1–0.6% MSG received higher ratings than those of the 0.9% NaCl mixtures, which contained approximately the same amount of NaCl as that of ordinary soups. Among all 48 samples, the 0.6% NaCl/0.3% MSG mixture achieved the highest rating.

Table 1 Saltiness and palatability ratings for the 48 NaCl/MSG mixtures
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Figure 1 shows the evaluation results of the 10 different NaCl/glutamate mixtures. For the 0.3% NaCl mixtures, repeated-measures ANOVA showed significant differences in the saltiness and palatability ratings of the NaCl-only solution and the four mixtures (p < 0.001). The post hoc test showed that for all evaluated items, the mixtures with any glutamate received significantly higher ratings than that of the NaCl-only solution (all p < 0.01). Repeated-measures ANOVA showed that there were significant differences among the five different types of 0.6% NaCl/glutamate mixtures in palatability (p < 0.001). The post hoc test showed that palatability demonstrated significantly higher ratings for the mixtures with any glutamate than for the NaCl-only solution (all p < 0.01). Regarding the differences in the palatability of the 0.3% NaCl/four glutamate mixtures, the mixture with MSG received significantly higher ratings than those of the mixtures with CDG and MDG. Among all ten samples, the 0.6% NaCl/0.3% MSG mixture achieved the highest rating.

Fig. 1
figure 1

Saltiness and palatability ratings for NaCl alone and NaCl with four kinds of glutamate (MSG, MPG, CDG, and MDG). Participants scored the mixtures using a 21-point rating scale and compared the mixtures to the 0.5% NaCl solution on saltiness (−5: distilled water, +5: 1.0% NaCl) and palatability (−5: extremely bad, +5: extremely good). The data are shown as the mean ± SE. The significant differences between each rating score are indicated by alphabetic superscripts. A column is significantly different from others that do not have the same alphabetic superscript according to Tukey’s test (p < 0.01). For saltiness ratings of 0.6% NaCl, no superscripts are used due to the lack of a significant difference among the four kinds of glutamate. ¶: p for repeated-measures ANOVA. MSG, MPG, CDG, and MDG indicate monosodium glutamate, monopotassium glutamate, calcium diglutamate, and magnesium diglutamate, respectively

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A limitation of this study concerns the confounding factors. The samples in this study were evaluated in the order of low to high concentrations, i.e., they were not randomized. In addition, because the participants were recruited from a small group of females in a registered dietitian training course at the university, the evaluation results may have been affected. To make the results of this study more generalizable, it would be ideal to increase the number of participants and expand the age and gender of the participants. The results of this study, however, should be important basic data when a larger epidemiological study is conducted.

In this study, NaCl/glutamate mixtures prepared with distilled water were quantitatively evaluated for saltiness and palatability. The results demonstrated that the 0.6% NaCl/0.3% MSG mixture achieved the highest palatability rating and that adding an appropriate amount of an umami substance (L-glutamate) to an NaCl solution improved palatability and contributed to a reduction in salt intake, indicating that the addition of MSG maintains a palatable taste while reducing the NaCl concentration to 0.3% less than that in ordinary soups (0.9% NaCl). These results suggest that glutamate can compensate for palatability loss caused by salt reduction and provide a feasible solution for reducing daily salt intake.