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

Kerstin Martha Mensien Burseg 1 2 Hoang Ly Lieu 0 2 Johannes Hendrikus Franciscus Bult 1 2 0 Hogeschool INHolland , PO Box 3190, 2601 DD Delft, The Netherlands 1 NIZO food research B.V. , PO Box 20, 6710 BA Ede, The Netherlands 2 TI Food & Nutrition , PO Box 557, 6700 AN Wageningen, The Netherlands 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. Introduction Oral stimulation with high concentration tastant pulses that are intermitted by low concentration tastant or water intervals (pulsatile stimulation) results in taste intensity ratings that are higher than those observed for continuous stimulation of the same average tastant concentration (Meiselman and Halpern 1973; Busch et al. 2009; Burseg, Brattinga, et al. 2010). Different explanations were suggested for this enhancement (Meiselman and Halpern 1973; Busch et al. 2009; Burseg, Brattinga, et al. 2010). One explanation attributes taste enhancement to taste quality contrast effects with pulseinterval combinations representing qualitative contrasting taste stimuli (Meiselman and Halpern 1973). In that view, the perceived (quality) contrast between pulse and interval invokes a perceptual overestimation of the evaluated taste property leading to an overall taste enhancement. This theory is supported by studies showing the effect of cumulated successive contrasts on taste intensity: After repeated stimulation with one taste quality, the intensity of a successively presented qualitatively contrasting stimulus is perceived as more intense compared with the same stimulus evaluated without preceding contrasting stimuli (Kroeze 1983; Schifferstein and Oudejans 1996). Stimulus contrasts can be achieved by either alternating stimulus qualities or stimulus concentrations (Schifferstein and Oudejans 1996). Supporting the contrast explanation, pulsation studies showed taste enhancement upon alternation of high concentration tastant pulses with low concentration tastant intervals of the same quality (concentration contrasts) (Busch et al. 2009; Burseg, Brattinga, et al. 2010) or by alteration with water intervals (quality contrast) (Meiselman and Halpern, 1973; Burseg, Brattinga, et al. 2010). The water interval is hereby regarded as a separate taste quality as it differs qualitatively from the target stimulus (Meiselman and Halpern 1973). In the present work, we investigated the effect of the magnitude and the quality of the pulseinterval taste contrast on sucrose sweet taste enhancement by pulsation. For this aim, the pulseinterval concentration contrast was varied by increasing the pulse/interval sucrose concentration difference in a stepwise manner (Study 1). In a second study, the pulse sucrose concentration was kept constant but pulses were alternated with citric acid at varying concentrations to create taste quality contrasts. In an earlier study, it was shown that contrast-induced taste intensity enhancement is independent of the concentration if the preceding and target stimulus are of different qualities (Schifferstein and Oudejans 1996). In the current study, this was tested by altering the citric acid concentration in the interval. The interval citric acid concentration was varied according to the subjects individual citric acid detection threshold to achieve equal intensities (below, at and above detection threshold). To that end, individual citric acid detection thresholds were determined for continuous and pulsatile citric acid stimulation. Materials and methods Study 1: sucrose concentration contrast Stimuli A computer controlled gustometer (Bult et al. 2007) was used to deliver taste stimuli intra-orally at a flow rate of 15 mL/min. Stimuli were produced at desired concentrations by running 4 pumps in parallel, mixing a sucrose solution (15%; 0.438 mol/L; w/v) and water (Evian, Danone) at predefined ratios. In total, 6 stimuli were delivered. To generate continuous stimuli, the sucrose concentration (S; in % [w/v]) was kept constant (c) over 40 s at 6% (Sc6%; 0.175 mol/L) or 7% (Sc7%; 0.204 mol/L). In pulsed stimuli, high concentration sucrose pulses (p) and low concentration sucrose intervals (i) were alternated. Keeping the net sucrose concentration fixed at 6% for all pulsed stimuli, pulseinterval sucrose concentration difference (Dp,i = [S]p [S]i in %) were varied between stimuli. The following pulseinterval concentration differences were given: 7.54.5 (Dp,i = 3%), 93 (Dp,i = 6%), 10.51.5 (Dp,i = 9%), and 120 (Dp,i = 12%). The pulse and interval lengths were 2.5 s each to yield 5-s pulsation periods (pulse + interval in seconds, at these period lengths, we previously observed maximum sweetness enhancement [3]). Periods were repeated 8 times to yield 40-s stimuli. The sucrose concentration of each of the 6 stimuli was verified by refractometry (Jasco polarimeter P 1030; Hg lamp: 365 nm). The polarimeter was calibrated with sucrose solutions of known concentrations. Subjects Fourteen subjects (age: 2252, 4 male) were recruited. They were trained on the gustometer taste delivery method. Subjects were instructed to consume only water 1 h prior to the test. Materials and methods used did not require medical ethical approval under Dutch regul (...truncated)