Oral sensations from iron and copper sulfate

Abstract

In previous work, retronasal smell was an important cue for perceiving metallic sensations from ferrous sulfate (FeSO₄). The purpose of this study was to examine whether FeSO₄ produced oral sensations as well as retronasal cues and whether astringency contributed to the oral sensations. In the first study, discrimination of FeSO₄ from water was confirmed at concentrations at or above 5 mM without retronasal flow. A second study examined whether FeSO₄ and CuSO₄ could be detected on a non-gustatory region by tactile sensations. Astringency was perceived when the copper sulfate solution was applied to non-gustatory surfaces between the gum and the upper lip with or without retronasal cues. However, rated astringency for FeSO₄ was less than that for CuSO₄, and not significantly different from water. Other gustatory or tactile sensations from iron salts may contribute to their oral sensations and the possibility remains that at least one part of metallic sensation may be a metallic taste.

Introduction

Some divalent salts, such as those of iron, copper and zinc are described as having a metallic taste or flavor [1], [2]. This is particularly true of ferrous sulfate (FeSO₄), which has been proposed as a prototypical metallic compound and as a reference standard in food sensory evaluation [3]. Because of its high bioavailability, ferrous sulfate is also commonly used in foods and dietary supplements as an iron-fortifying agent. However, the sensory profiles of divalent salts are complex, involving tastes, retronasal smell, and oral tactile properties such as astringency [2], [4], [5] rather than a simple taste or flavor. Hettinger et al. [6] proposed that the metallic flavor of ferrous sulfate is not a true gustatory quality but is likely to be due to olfaction. They observed that the frequencies of reports of metallic taste were decreased when the nose was occluded, eliminating retronasal smell. Subjects often have difficulty in identifying the locus of the stimulation when stimuli are introduced into the mouth [7]. Hollingworth and Poffenberger [8] and Murphy et al. [9] recognized that under uncertainty, people will report a retronasal olfactory sensation as a taste. These odor cues can be effectively eliminated by nasal occlusion [9]. Lawless et al. [1] confirmed Hettinger et al.'s results for FeSO₄ but reported that retronasal metallic smell was less important for copper and zinc salts, which were characterized as more bitter and astringent. The solutions of FeSO₄ had no smell when the headspace over the solutions was sniffed (orthonasal smell was absent), suggesting that the olfactory stimulus arises from rapidly generated lipid oxidation products in the mouth.

Very few chemical stimuli are truly monogustatory in quality, and FeSO₄ is no exception. Several results indicate the possibility of sensing FeSO₄ by tactile or gustatory mechanisms. Schiffman [10] measured detection thresholds for FeSO₄ in the range of 0.147 mM when the nose was occluded with the type of clips used in spirometry (personal communication). This implies that detection was not by retronasal smell but by taste or tactile sensations. At the recognition threshold, subjects described the sensations from iron and zinc sulfate as “metallic.” In a duo–trio discrimination procedure, 0.5 mM FeSO₄ was not discriminated from water with the nose occluded [1]. In contrast, discrimination of 0.3 mM CuSO₄ from water was possible with and without nasal occlusion. However, in a same/different task, a greater frequency of subjects judged 1 mM FeSO₄ “different” from water (as opposed to water against itself) even with their nose occluded [1]. If FeSO₄ has oral qualities, it could involve taste or tactile sensations or both. Furthermore, they could vary with concentration.

Oral astringency is a quality sometimes confused with gustatory sensations. In her history of research on taste, Bartoshuk [11] described how Aristotle, and Galen after him, considered astringency as a ‘taste’, along with the qualities of ‘harshness’ and ‘pungency’. In a recent paper, Green [12] discussed the psychophysical, chemical and physiological bases of astringency and concluded that oral astringency is of tactile origin and is caused principally by the precipitation of salivary mucoproteins, which impairs the natural lubrication of oral surfaces. Breslin et al. [5] asked subjects to rate the intensity of astringency after they applied an alum solution to the non-gustatory surface between the upper lip and gums. Oral astringency was evoked on this non-gustatory region, confirming it's tactile nature. Whether FeSO₄ and CuSO₄ would have a similar effect is not known.

The purpose of this study was to improve our understanding of the types of sensations generated by FeSO₄ and CuSO₄ that are often described as having metallic taste or flavor as an extension of the previous study [1]. In particular, the possibilities of oral and tactile sensations from those stimuli were tested. The first study examined whether FeSO₄ produced oral sensations by determining whether it could be discriminated from water at different concentrations when the nose was occluded. CuSO₄ was not included in the first experiment below because oral discrimination with the nose closed was previously demonstrated [1]. In a second study, the potential contribution of astringency to the oral perception of FeSO₄ and CuSO₄ was examined using Breslin et al.'s [5] procedure of stimulating non-gustatory areas between the gum and lip. If astringency is perceived in the non-gustatory area, this would support a tactile cue to explain the discrimination of ferrous salts from water. If not, then the possibility remains of a true gustatory cue in the perception of ferrous sulfate and that such sensations contribute to at least part of its “metallic taste”.