Table 2 indicates that the effect of the anionic acid species on the sour taste was as determined by structure (Fig 4). The effects will be similar if the structures of anionic acid species are similar. For example, lactate and pyruvate, fumarate and succinate, malate and tartrate, have similar structures, so their threshold were close.Additionally, it seems that hydroxyl or conjugated groups in anionic acid species will intensify the sour taste caused by hydrogen ion. For example, tartrate has two hydroxyl group, so the threshold of tartrate was lower than that of malate which has only one hydroxyl group. In contrast to succinate, fumarate has an unsaturated bond which belongs to a conjugated group, so its threshold is lower than that of succinate.