The effect of monosodium glutamate on parotid salivary flow in comparison to the response to representatives of the other four basic tastes

Abstract

Parotid salivary flow was recorded from eight fit and healthy subjects using modified Lashley cups connected to an instantaneous flow meter in response to gustatory stimuli. The gustatory stimuli were monosodium glutamate (MSG), sodium chloride, sucrose, magnesium sulphate and citric acid. Stimuli were applied for 30 s, and repeated after the flows had returned to baseline following the rinse. Subjects were a significant source of variation for salivary response to each different test stimuli (p < 0.001). The normalised salivary flow showed a strong correlation to concentration for all test stimuli (p < 0.0001). The parotid salivary flow to MSG (umami) showed a dose-dependant response in which both Na⁺ and glutamate ions contributed. The overall order of relative salivary flow responses from highest to lowest flows was citric acid (sour) > MSG (umami) > NaCl (salt) > sucrose (sweet) > = magnesium sulphate (bitter). The relative responses of the peak salivary flows showed the same ordered relation. The peak salivary flow provided a greater contribution to the response to citric acid, NaCl and MSG compared to the response to sucrose and magnesium sulphate.

Introduction

The “tasting” of a food after it has been placed in the mouth and the accompanying reflex salivation induced by that food has a number of different aspects. The term “taste”, used in this context, is an imprecise term which is variously used to describe the detection of molecules by receptors on gustatory receptor cells, refer to the description of the quality perceived by an individual following this detection and describe the overall experience of food that is much more than stimulation of gustatory receptors, but also the texture, flavour release, olfaction and mouth feel.

In this study, we have used simple solutions of molecules to provide stimulation of gustatory receptor cells on the tongue and palate, and have described these as gustatory stimuli. The term taste is used in the broader context to describe the perception of this stimulation following detection. The reflex salivary response to food in the mouth is a response to gustatory, masticatory, and olfactory stimulation and has an important role for mastication, for swallowing and for normal taste [1].

Four “basic qualities of tastes” of bitter, salty, sour and sweet have been well established as perceptual descriptions. A fifth basic quality, umami, has been proposed with strong evidence supporting its existence, possessing as it does distinct taste profiles and electrophysiological responses [2]. Monosodium glutamate (MSG) is often used as an example of umami gustatory stimuli, although it has been argued that MSG should not be used as a term synonymous for umami [3]. Fat has also been suggested as a basic taste [4] but the orosensory perception of fat may involve gustatory, olfactory or textural cues [5].

Basic taste qualities have been shown to be associated with a dose–response reflex parotid salivary secretion. This has been shown for salt, sweet, bitter and sour in dogs [6] and rabbits [7], for salt, sweet, and sour [8], [9], [10] and for salt, sweet, sour and bitter in humans [11]. From the dose response curves it is possible to see a very similar pattern of sour > salt > sweet [8], [9], [10], [11]. Only one study, with the response to quinine (bitter), showed this response curve to be just below that of salt [11]. These response curves were constructed from different groups of individuals [11], and so direct comparison within the same group of subjects to these four basic tastes was not possible. The dose responses in animal studies [6], [7] differ from that observed in humans. Fats have not shown a similar dose-dependent salivary flow suggesting that they may not be gustatory stimuli [12].

The position of the salivary response to monosodium glutamate relative to the other basic tastes has not been established. A dose–response has been reported for increasing concentrations of MSG (umami) in chicken broth and isohumolones (bitter) in beer [13], but the level of response is also affected by the salivary response to the food. The only other study to show a response to MSG had both limited subjects and data and provided contradictory results between two experiments [14]. The salivary response to MSG may, however, be simply due to the Na⁺ ion concentration, and not be affected by the glutamate ion.

Overall, the relative response of the gustatory–salivary reflex to the four common basic tastes has only been established in the same subjects in animals, and has not been demonstrated in the same individuals in human subjects. Furthermore, the relative response of the gustatory–salivary reflex has not considered the salivary response to the basic taste of umami. The aims of this study were to characterise the parotid salivary flow response to MSG and compare this to the response to gustatory stimuli representing the four other basic tastes in the same subjects.