Multisensory modulation of texture perception in apples

Crispness is an important texture parameter influencing sensory evaluation of food, as it works as a predictor of overall food quality. Previous research demonstrated that crispness perception of dry foods could be modulated by the sound the perceiver produced while biting into them (1). Therefore, we investigated whether multisensory effects on crispness perception could be extended to wet food (apples). Additionally, we were interested in verifying whether sound could also modulate perceived hardness, given that hardness (unlike crispness; 2) is considered to mainly rely on mechanical cues. Participants here were asked to bite a series of apple cylinders and to rate their crispness or hardness using a rating scale. Crucially, they heard their biting sound (either realistic or manipulated) through headphones. Measures have been replicated with different apple varieties with the aim to study the sound manipulation effect at different intensity levels for crispness and hardness. In experiment 1, participants evaluated the perceived crispness of cylinders from 3 apple varieties differing in crispness (‘Reinette’-low, ‘Golden’-medium, and ‘Fuji’-high). High frequencies of sound (2-20 kHz) were reduced (-12 dB or -24 dB) or were left unmodified (realistic sound). The results confirmed the role of sound manipulation in crispness perception also in the case of wet food: Apples were perceived as being less crispy when both sound reductions were applied than with the realistic sound. Additionally, apple varieties were correctly perceived as differing in terms of crispness (‘Reinette’<‘Golden’<‘Fuji’). In experiment 2, participants evaluated the perceived crispness and hardness of cylinders obtained from apples differing both in crispness and in hardness (‘Reinette’-low vs. ‘Fuji’-high). Sound manipulation consisted in presenting the realistic sound, or in reducing high frequencies of -24 dB, or else in switching the microphone off (thus dampening the acoustic cues overall). The results for both crispness and hardness confirmed that apples were rated significantly different (‘Fuji’ was perceived as the hardest and most crispy). As for sound, any reduction (-24 dB or microphone off) in the auditory information lowered the perceived crispness of apples as compared to the realistic sound. Interestingly, apples were perceived as being less hard when the microphone was switched off than when the real sound was presented. This supports the existence of multisensory effects of sound also for mechanical texture parameters. In experiment 3, participants underwent the same conditions as in experiment 2 with the sole difference of white noise being continuously presented through the headphones. The volume was such as to try and cover any sounds coming from outside the headphones. The results replicated those obtained in experiment 2 as to the effects of apples both for crispness and for hardness. The effect of sound, though, disappeared for crispness, while it was marginal for hardness (with apples judged as being softer only with the microphone switched off). This research effectively extends to wet foods the study of sound effects on perceived crispness. Additionally, it demonstrates for the first time that such multisensory effects can be highlighted in evaluations considered as being mainly guided by mechanical cues. REFERENCES (1) Zampini, M., & Spence, C. (2004). The role of auditory cues in modulating the perceived crispness and staleness of potato chips. Journal of Sensory Studies, 19, 347-363. (2) Christensen, C. M., Vickers, Z. M. (1981). Relationships between chewing sounds to judgments of food crispness. Journal of Food Science, 46, 574-578.