Johannes H.F. Bult

Investigations on multimodal sensory integration: Texture, taste, and ortho- and retronasal olfactory stimuli in concert

Perceptual interactions between odour and oral texture were explored in a study in which a cream odour was presented ortho- or retronasally at well-defined moments whilst milk-like foods with different viscosities, produced by adding a thickener, were present in the mouth. Gaseous (odour) and liquid (texture) pulses were presented using a specially-developed computer-controlled system of air-dilution olfactometry and pumps. Odour pulses, lasting 2 s, were presented either during a 3-s period in which a liquid filled the oral cavity, during a 3-s period in which the liquid was manipulated orally or during the swallowing of the liquid. Subjects rated the intensity of overall flavour, thickness and creaminess. Perceived flavour intensity was reduced with increasing viscosity of the liquid, irrespective of whether or not the odour was presented ortho- or retronasally. The odour stimulus increased the intensities of thickness and creaminess, but only when the odour was presented retronasally that is as if the odour would have originated from the liquid. Furthermore, this enhancement was most pronounced when odours coincided with swallowing, less pronounced when odours coincided with oral manipulation and absent when presented during mouth filling. The results suggest that cross-modal interactions are the rule rather than the exception, provided that multi-modal sensory integration has occurred.

Compensation for age-associated chemosensory losses and its effect on the pleasantness of a custard dessert and a tomato drink

Differences between elderly subjects (n=52, 60-85 years) and young subjects (n=55, 18-35) in their food liking and their olfactory capability were investigated. Two food systems were used: custard desserts and tomato drinks. Flavor enhancement/enrichment, textural change, and/or irritant addition were incorporated as compensatory strategies into these foods. The addition of low concentrations of both cherry flavor and cream flavor to the custard desserts influenced their pleasantness for the majority of the elderly. The addition of cream topping increased the pleasantness of the custard desserts for both the elderly and the young. The elderly equally liked the tomato drinks with no or with low irritant addition, whereas the young generally disliked an irritant addition. However, the food liking of the elderly was not generally increased by these different compensatory strategies. Instead, subgroups were observed for each compensatory strategy, in which applied compensatory strategies led to an increase in product pleasantness. Age-associated losses in olfactory capabilities did not sufficiently explain differences in food liking, as only elderly with similar olfactory capabilities to the young demonstrated a liking of enhanced flavor. The present study does not support the assumption that age-associated impairment in olfactory capability will inevitably lead to changes in food liking.

Sweet taste enhancement through pulsatile stimulation depends on pulsation period not on conscious pulse perception

When aqueous NaCl solutions are tasted at continuously alternating concentrations, overall saltiness ratings exceed those observed for solutions with the same averaged, but non-alternating concentrations. In the present study, this effect is replicated for alternating aqueous sucrose solutions. We tested the hypothesis that enhancement depends on the conscious perception of intensity contrasts. High sucrose pulses were continuously alternated with low sucrose intervals at pulsation periods between 1.5s and 20s. Tastant pulsation enhanced sweetness intensity and this enhancement varied between 8 and 14%, peaking for periods from 4.5s to 6s (Study 1). This range coincided with the average pulsation period at which perceived taste pulses blended into a continuous stimulus, i.e. the taste fusion period (TFP). When comparing intensity ratings of sucrose solutions at individualized pulse periods of 0.5, 1.0 and 2.0 times TFP to ratings for continuous sucrose solutions of the same net concentration, pulsatile stimuli were perceived as significantly sweeter (p<0.01; Study 2). However, sweetness intensity enhancement was the same for all pulsation periods. It was shown that sweet taste enhancement peaks at pulsation periods ranging from 0.5 to 2.0 TFP and that the level of conscious pulsation perception does not affect taste enhancement. The results suggest the introduction of enhancement effects at pre-conscious stages of gustatory processing. Further mechanisms that may account for such pre-conscious effects are discussed.

Sweet taste intensity is enhanced by temporal fluctuation of aroma and taste, and depends on phase shift.

Pulsatile stimulation enhances taste intensity compared to continuous stimulation with stimuli of the same net tastant concentration. In the present work, we studied the effects of pulsatile delivery of aroma and taste on their combined contribution to taste intensity. Effects on taste perception were evaluated for aroma and taste pulsation and the aroma pulse-taste pulse phase shift. High-concentration sucrose pulses were alternated with water rinses every 2.5s. Four different aroma (isoamyl acetate) versions were presented: (1) no aroma, (2) continuous aroma (3) aroma pulses in-phase and (4) aroma pulses out-of-phase with taste pulses. Aroma-taste combinations were evaluated for sweetness intensity by a 15-member trained panel using time-intensity analysis. Sweetness intensity was enhanced by pulsatile stimulation of sucrose or isoamyl acetate. In addition, taste enhancement by aroma and tastant pulses was additive if both were presented out-of-phase which resulted a sweetness intensity enhancement by more than 35% compared to a continuous sucrose reference of the same net sucrose concentration. Aroma-induced sweetness enhancement can be explained by cross-modal aroma-taste integration. Amplification of aroma-taste integration by pulsatile stimulation may be attributed to a potentiated afferent input of aroma and taste information prior to aroma-taste integration. Alternative mechanisms include the importance of swallowing on aroma-taste integration.

Aroma exposure time and aroma concentration in relation to satiation

The present study investigated the effect of aroma exposure time and aroma concentration on ad libitum intake and subjective satiation. In a within-subject study, thirty-eight unrestrained, healthy female participants (age: 18-39 years; BMI: 18·5-26·0 kg/m²) were asked to consume tomato soup during lunchtime, until they felt comfortably full. Every 30 s, the participants consumed 10 g of a bland soup base while tomato soup aroma was delivered separately through the nose via a retronasal tube that was attached to an olfactometer. This gave the impression of consuming real tomato soup. For each sip, the aroma varied in exposure time (3 and 18 s) and concentration (5 × ), resulting in four different test conditions. Ad libitum food intake and appetite profile parameters were measured. A 9% lower food intake was observed when the participants were exposed to the condition with 18 s exposure time and a high concentration than when exposed to the other three conditions. These results indicate that changing the retronasal aroma release by aroma concentration and aroma exposure time affects food intake.

Investigation of interactions between texture and ortho- and retronasal olfactory stimuli using psychophysical and electrophysiological approaches

UNLABELLED Flavor is a result of the complex combination of olfactory, gustatory and trigeminal sensations perceived during oral processing of foods, including thermal, painful, tactile and/or kinesthetic effects. Aim of this study was to better understand interactions between synchronous tactile (texture) and olfactory (odor) sensations, using a psychophysical and an electrophysiological approach. Texture stimuli were aliquots of lean milk and thickened lean milk. A butter aroma was presented either orthonasally or retronasally after oral processing and before swallowing the oral stimulus or in the absence of an oral stimulus. Eighteen subjects (11 women, 7 men, mean age 24 years), naïve to the expected effects, rated both odor and texture intensity of each stimulus. Event-related potentials (ERP) were obtained from five recording positions. For the psychophysical data, the presence of an oral stimulus increased odor intensity, irrespective of odor presentation route. For the electrophysiological data, both early and late chemosensory ERPs were affected by odor conditions, texture conditions, and their respective interaction. IN CONCLUSION (1) perceptual interactions occurred between food texture and odor, with cross-modal interactions being found for both orthonasal and retronasal odor administration, and (2) these interactions between texture and odor occur at both primary-sensory and cognitive evaluative levels of stimulus processing. The temporal dimension plays then a critical role in the investigation of odor-texture interactions.

Vibromyography of oral processing varies with type of semi-solid food and with sensory judgements

Vibromyography was used to quantify oral activity during the processing of well-characterized semi-solid model foods whilst subjects assessed the intensity of the sensory attributes to thick, creamy, melting, fatty, rough and liking. A series of eleven starch-based vanilla custard desserts was prepared with five different viscosities, three different fat levels, and three different particle sizes. Over 5 sessions, subjects (N=10) took 5 ml spoonfuls of each of the stimuli for 5 s before they rated the strength of the various attributes. The results demonstrated that oral movements varied significantly with the type of semi-solid food and with the type of attribute. Individual subjects displayed a highly idiosyncratic behaviour. Oral movements were significantly related to sensory ratings of liking, creaminess and roughness.

Effects of pulsation rate and viscosity on pulsation-induced taste enhancement: new insights into texture-taste interactions.

Oral stimulation with high-tastant concentrations that are alternared with low-tastant concentrations or water rinses (pulsatile stimulation) results in taste intensity ratings that are higher than continuous stimulation with the same average tastant concentration. This study tested the combined effects of taste pulsation rate and viscosity on pulsation-induced taste enhancement in apple juice. According to a tastant-kinetics hypothesis, less pulsation-induced taste enhancement is expected at enhanced pulsation rates in the high-viscous proximal stimulus compared to lower viscous stimuli. High-concentration sucrose apple juice pulses and low-concentration sucrose apple juice intervals were alternated at different pulsation periods (pulse + interval in seconds) every 2.5 s (period length = 5 s) or every 1.25 s (period length = 2.5 s). Pulsed stimuli were presented at two viscosity levels by the addition of pectin (0 and 10 g/L). Sweetness intensities of pulsed stimuli were compared to a continuous reference of the same net but nonalternating sucrose concentration. Sweetness ratings were higher for pulsatile stimuli than for continuous stimuli. In low-viscous stimuli, enhancement depended on the pulsation period and peaked at 5 s periods. In high-viscous stimuli, the same enhancement was observed for both pulsation periods. These results contradict a tastant-kinetics hypothesis of viscosity-induced taste suppression because impaired tastant kinetics by viscosity would predict the opposite: lower pulsation-induced taste enhancement for viscous stimuli, especially at higher pulsation rates. Instead, these observations favor an explanation based on perceptual texture-taste interactions, which predict the observed independence between viscosity and pulsation rate.

Oral texture influences the neural processing of ortho- and retronasal odors in humans

Eating implies mutual interactions between different senses. In the present work we aimed at studying relations between food texture and food odor, using both psychophysical and imaging techniques. Eighteen right-handed healthy human subjects participated to both behavioral and fMRI sessions. Fresh, sweetened milk and a more thickened version were delivered orally; in addition, a buttery-cream aroma was presented ortho- or retronasally. Stimuli were applied using a gustometer and or an air-dilution olfactometer, both computer-controlled. In each session subjects rated separately odor-, taste- and thickness intensities of the stimuli. The behavioral data show that odors, presented through either retro- or orthonasal path, induce a significant flavor enhancement with respect to the no-odor condition. Brain functional data indicated a significant enhancement of the activation of olfactory eloquent areas in favor of ortho-nasal odor presentation while activations of mechanosensory areas were favored by the retro-nasal odor route. As effect of oral stimuli we found a significant correlation between the texture intensity rating vs. the BOLD signal in the supplementary motor area, known to drive subconsciously primed movement, putatively associated in this case with the tongue movement required with the handling of the stimulus. Moreover, we found inhibition of the signal in different sensory specific areas as an effect of the mutual interaction between stimulus qualities. In conclusion, ortho- and retronasal odors differentially affect the neural processing of the texture of oral stimuli.

Sweetness Intensity Enhancement by Pulsatile Stimulation: Effects of Magnitude and Quality of Taste Contrast

Upon stimulation with continuously alternating (pulsatile) taste concentrations, humans report higher average taste intensities than for continuous stimulation with the same average tastant concentration. We investigated the effect of the magnitude of concentration changes (concentration contrast) and the effect of taste quality changes (quality contrast) between alternating tastants on sweet taste enhancement. The perceived sweetness intensity increased with the magnitude of the sucrose concentration contrast: The pulsatile stimulus with the highest concentration difference (average sucrose concentration: 60 g/L) was rated as the sweetest in spite of the fact that the gross sucrose concentrations were identical over stimuli. Moreover, this stimulus was rated equally sweet as a continuous reference of 70 g/L sucrose. On alternation of sucrose with the qualitatively different citric acid, sweet taste enhancement remained at the level observed for alternation with water at citric acid concentration levels up to 3 times its detection threshold. Alternation of a sucrose solution with a citric acid solution at 9 times its threshold concentration, resulted in an attenuation of the pulsation-induced enhancement effect. Upon alternation of citric acid pulses at concentrations around the threshold with water intervals only, no taste enhancement was observed compared with continuous citric acid stimuli of the same net concentration. We propose that the magnitude of pulsation-induced taste enhancement is determined by the absolute rather than relative change of tastant concentration. This explains why 1) pulsation-induced sweet taste enhancement is determined by the magnitude of the sucrose pulse-interval contrast and 2) the alteration of citric acid with water does not enhance taste intensity at detection threshold level.