We used the TasteSynth system (Kaji and Miyashita, 2021). The configuration of the system is shown in Figure 2. The system is composed of PC software and a current output device that can output and stop the electrical stimulation waveform using the PC software. The current output unit is a constant current circuit with noise-reduction circuit.

A chopstick-shaped device was used to present the electrical stimulation waveforms. The device and its structure are shown in Figure 3. The chopstick device had a copper electrode on the hand side and a titanium electrode on the mouth side, and each electrode was connected to the current output device independently. When food is ingested, a circuit is formed in the human body, and electrical stimulation can be applied to the tongue.

Participants placed a saltwater gel sample on their tongue using a chopstick device and evaluated the saltiness intensity and discomfort caused by the electrical stimulation at the time cued by the experimenter. A 100 mm VAS was used to evaluate the saltiness intensity (Figure 4). The VAS was set as follows: water gel sample at the left end, 0.56% saltwater gel sample at the center, and 1.12% saltwater gel sample at the right end. Participants were asked to check the saltiness of the samples in advance. The participants were instructed to draw a line on the VAS sheet by hand, where the salty taste was closest to the one they perceived at the time the experimenter signaled. Additionally, the participants were asked to rate the discomfort on a 5-point scale as follows: 1: none, 2: low (not bothersome), 3: mild (bothersome in daily life, but not unbearable), 4: moderate (bearable in tests, but unbearable in daily life), 5: severe (unbearable). During the evaluation, it was possible to check the taste as many times as was desired. However, the gel sample that was used once for taste check was not used again, and a new gel sample was used each time.

The following eight conditions were used in the preliminary Experiment 1.

1) 0.56% saltwater gel sample [proposed waveform (0.5 mA)]

Three sets (18 trials) of eight conditions were conducted. In each set, the above conditions were performed in order from top to bottom, because the participants had an understanding of electric taste and could perceive the conditions.

Additionally, the following two conditions were used in the preliminary Experiment 2.

1) 0.56% saltwater gel sample [proposed waveform (0.8 mA)]

Three sets (six trials) of two conditions were conducted. As in preliminary Experiment 1, the above conditions were performed from top to bottom.

The saltwater gel samples were cubes of approximately 1.5 cm per side at room temperature and were provided on white paper plates. The concentrations of the saltwater gel samples were determined by referring to the literature (Makino, 1998; Matsuda, 2017), 0.56% saltwater gel sample was set as the salt concentration of low-sodium food, and 0.80% saltwater gel sample was set as the salt concentration of ordinary food. The saltwater gel samples were prepared by boiling and dissolving a solution of 0.80 wt% or 0.56 wt% sodium chloride and 0.80 wt% agar (“powdered agar S-7” manufactured by Ina Food Industries), and then cutting into 1.5 cm squares.

The waveform, as shown in Figure 1, Figure 5, was used for the electrical stimulation. In preliminary Experiment 1, we presented the proposed waveform, the waveform that reverses from the cathode to the anode (without the ease-in process and gradual stimulation process of the proposed waveform), and the conventional waveform that was used in related studies. In preliminary Experiment 2, the proposed waveform with the maximum current set to 0.8 mA and the waveform that reverses from the cathode to the anode (without the ease-in process and gradual stimulation process of the proposed waveform) are presented.

In the electrical stimulation conditions, the stimulation was presented in the following order. First, the experimenter confirmed that the participant placed the saltwater gel sample on the tongue correctly and then began the output of the electrical stimulation. Second, the experimenter signaled the participants at approximately 1.2 s after the start of the stimulation waveform presentation to evaluate the taste. In the no stimulation condition, the experimenter cued the participants when approximately 1.2 s had elapsed after confirming that they had placed the gel sample correctly on their tongue and asked them to evaluate the taste of the timing.

First, the participants checked the saltiness of the three VAS reference points. Subsequently, the task was initiated. When evaluating saltiness, participants were instructed to stick chopsticks into the gel, keep the food on their tongue, and lick it without chewing. The 0.56% saltwater gel sample, which was the center of the VAS, could be tasted at any time during the experiment. Participants were instructed to drink water between each task to prevent confusion.

The results of the evaluation of the saltiness intensity for each condition are shown in Figure 6. The bar graph shows each participant’s average value of the evaluation for each condition. The results of the discomfort caused by electrical stimulation are shown in Figure 7.

The results of the evaluation of the saltiness intensity for each condition are shown in Figure 8. The bar graph shows each participant’s average value of the evaluation for each condition. The results of the discomfort caused by electrical stimulation are shown in Figure 9.

This study was conducted with the approval of the Research Ethics Committee for Research Involving Human Subjects of Meiji University Educational Corporation and the Research Ethical Review Committee of Kirin Holdings Company, Ltd. Before the experiment, the participants were fully informed about the purpose of the study, and written consent was obtained. Participants were allowed to withdraw their consent at any time during the experiment and discontinue the study.

Participants self-reported that they were currently or had been following a low-sodium diet in their daily lives. The experimental lasted from the middle of August to the beginning of October. Participants were recruited from the registered monitors of a research company. The total number of participants in the experiment was 36 (age range 43–65 years, mean age 58.3 years, 23 males and 13 females, duration of salt reduction 2–300 months). One participant had missing data, and four had problems in performing the experiment, such as not being able to eat with chopsticks or licking gel samples; therefore, the experiment was stopped before the start of Experiment 1. Thus, 31 participants were included in Experiments 1 and 2. Participants were paid 5,000 yen as an honorarium.

The following three conditions were used.

1) 0.80% saltwater gel sample (no stimulation)

The participants performed six sets (18 trials), each with three conditions. To account for order effects, each condition was evaluated an equal number of times for all possible orders. These conditions were not disclosed to participants during the experiment. The salt water gel samples were subjected to the same conditions as described in Section 2.2.3.

For electrical stimulation, we used the stimulation waveform described in Section 2.1. Participants experienced the proposed waveform (0.5 mA) before performing the task. If the participants did not experience any severe discomfort, they performed the experiment using the proposed waveform (0.5 mA). However, when the participants stated that the electric stimulation caused them severe discomfort, the experiment was conducted using the proposed waveform (0.3 mA). The presentation method of the electric stimulation was the same as that described in Section 2.2.3.

The participants were trained on the timing of the evaluation and how to describe their taste. First, the participants experienced three salty taste points, which were the criteria for the VAS. Next, the participants experienced a 0.56% saltwater gel sample (with electrical stimulation) under the same conditions as in this test and then performed the task. When evaluating saltiness, participants were instructed to stick chopsticks into the gel, keep the food on their tongue, and lick it without chewing. After a 3-min break, the task of this experiment was started. In this experiment, a 3-min break was provided for every two sets. The 0.56% saltwater gel sample, which was the center of the VAS, could be tasted at any time during the experiment. Participants were instructed to drink water between each task to prevent confusion.

Participants dipped the tip of the chopstick device into a cup of low-sodium miso soup and sipped it, evaluating the discomfort caused by electrical stimulation at the time the experimenter signaled. Additionally, they freely described the overall taste and deliciousness of food at that time. The evaluation method for discomfort was the same as that described in Section 2.2.2.

The following two conditions were used.

1) no stimulation

The electrical stimulation and presentation methods were the same as those described in Section 2.5.2. Participants evaluated each condition once. The experiment was conducted in the following order: no stimulation followed by electrical stimulation. These conditions were not disclosed to participants during the experiment.

For the low-sodium miso soup, we used commercially available miso soup diluted to 1.5 × the prescribed amount (using seasoned miso from “low-salt miso soup supervised by Tanita Shokudo” manufactured by Marukome). The salt concentration of the miso soup was measured using a salt meter (“SO-304 Electronic Salt Meter” manufactured by TANITA), and the salt concentration was 0.30%. Miso soup was served at room temperature in a white-paper cup.

Participants were given instructions, then asked to perform the task before writing a comment about their experience. The instructions were as follows. First, the participants placed the tip of the chopstick device on the miso soup in the cup. The miso soup was then sipped from the cup. The participants were then instructed to taste the miso soup while maintaining the miso soup in the cup connected to their mouths. The participants were asked to comment on the experience by answering two questions: 1) whether there was any change in taste or deliciousness between two conditions and 2) how they felt after tasting the miso soup. The total duration of Experiments 1 and 2 approximately 1 h.