Operationally defined solubilization of copper and iron in human saliva and implications for metallic flavor perception

Abstract

Metals such as copper and iron cause unpleasant taste perceptions. Metallic compounds come in contact with saliva before delivery to taste receptors. Therefore, it is assumed that interactions between saliva and metallic compounds affect the perceptions of metals. The aim of this study was to determine the solubilization of metals in saliva and to examine whether or not perceptions of metallic flavor are influenced by metal solubility in saliva. Ten trained panelists evaluated the sourness, bitterness, astringency, electric sensation, and rusty nail-like retronasal aroma of copper sulfate (CuSO4·5H2O, Cu) and ferrous sulfate (FeSO4·7H2O, Fe) dissolved in ultrapure water at different concentrations. Total and soluble metals were measured in the subjects’ saliva collected after tasting the samples using an inductively coupled plasma spectrometer. Approximately, 4.5–6.4% of Fe and 4.0–6.6% of Cu were retained in the saliva after expectoration. The proportion of soluble metal to total metal retained in saliva decreased from 0.68 to 0.29 for Cu and 0.019 to 0.0016 for Fe, as the metal concentration increased. In particular, Fe was solubilized in saliva at a maximum level of 4.5–4.6 μM, regardless of the metal concentration of the solution. The perceived intensities of sensory attributes showed positive linear relationships with log concentrations of total Cu, soluble Cu, and total Fe, but they did not have any relationship with soluble Fe. These results indicate that sensory perceptions of Fe were influenced mainly by the total Fe retained in saliva, whereas the perception of Cu was affected by soluble Cu as well as total Cu.