Sensitivity to Viscosity Changes and Subsequent Estimates of Satiety across Different Senses Authors: Pellegrino, Robert1,2; Jones, Jourdan1; Shupe, Grace E.1; Luckett, Curtis R.1 1 Department of Food Science, Institute of Agriculture, University of Tennessee, Knoxville, Tennessee 2 Smell & Taste Clinic, Department of Otorhinolaryngology, TU Dresden, Dresden, Germany Corresponding author: Curtis R. Luckett; Department of Food Science, University of Tennessee, 2510 River Drive, Knoxville, TN 37996, U.S., [email protected] Keywords: Viscosity, Satiety, Tactile, Auditory, Vision 1 Abstract While it is widely accepted that texture is a multisensory property, little research has been published regarding how we use senses other than touch to assess texture. In beverages, humans use texture (i.e. viscosity) information to estimate calories and expected satiety. This study was designed to compare and contrast the sensitivity of humans to changes in viscosity and estimated satiety through different sensory modalities. Milk samples of varying viscosities were constructed, and 49 participants were asked to perform a series of 2-alternative forced choice tests and identify which sample was thicker. Sensitivity to viscosity changes across different sensory modalities was determined by having each participant consume the samples, listen to the sample pouring, and observe clear vials of the samples. Using vision, participants were notably less sensitive to changes in viscosity when compared to hearing or oral tactile sensations. Interestingly, oral tactile sensations and hearing were almost identical in their viscosity difference threshold (0.346 cP and 0.361 cP, respectively). Similar patterns were observed when the participants were asked to estimate how full they would be after consuming beverage stimuli varying in viscosity. Expected caloric values and satiation were found to change with thickness level when participants were assessing the stimuli through the auditory or tactile modalities. However, these measures of expected caloric value and satiation did not change as a function of viscosity for visual assessment, suggesting the assessment of caloric density and satiation are linked to specific sensory modalities’ ability to detect viscosity. This study highlights the relative importance of vision, audition, and touch to forming our sensory judgements regarding viscosity and subsequent satiety estimations. Introduction Viscosity represents a primary mechanical characteristic in food and is determined by the molecular components of the food such as water, protein, carbohydrates and fat (Vliet & Walstra, 1980). Consequently, these components and others help cue individuals to changes in viscosity to determine perceptual outcomes. For instance, wine judges use viscosity to determine alcohol content by visually inspecting the speed with which the “legs” descend the inside wall of a glass (Nurgel & Pickering, 2005) and shoppers use the thickness of yogurt to determine fat content (Bruzzone, Ares, & Giménez, 2013). Indeed, humans use viscosity as a strong indicator of nutritional properties of a beverage (McCrickerd et al. 2012). These properties include, caloric density, satiety (fullness), and satiation (fullness over time) . For instance, thick beverages elicit a higher expected satiety than thin beverages (McCrickerd, Chambers, Brunstrom, & Yeomans, 2012), and this perception relates to physiological changes in the body such as increased insulin and pancreatic polypeptide (PP) (Yeomans, Re, Wickham, Lundholm, & Chambers, 2016). The aforementioned findings bolster the concept of a satiety cascade, which states early cognitive and sensory information integrate with post-ingestive signals and behaviors such as a suppressed appetite after eating (Blundell, Rogers, & Hill, 1987). Typically, surface-contacting physical instruments such as rheometers measuring resistance to stress can be used to objectively characterize the viscosity of liquids (i.e. beverages). However, measuring human sensitivity to viscosity is different than many other food related perceptions, in that an absolute threshold cannot be calculated. More clearly, a liquid cannot have a complete absence of viscosity, so to measure sensitivity, difference thresholds can be used. Difference threshold, or just noticeable difference (JND), is the minimum change in stimulus intensity before a change in perception is noted (Lawless & Heymann, 2010). For the purposes of viscosity, this can be thought of as the change in viscosity needed for a population to notice a sample is thicker than another. In determining difference thresholds, a series of 2-alternative force choice tests (2-AFC) are commonly used (Ulrich & Miller, 2004; Ulrich & Vorberg, 2009). Most studies measuring viscosity perception in humans have concentrated on only oral tactile sensations; however, texture evaluation is multisensory in nature. As defined by the International Standards Organization (ISO), texture is “all the mechanical, geometrical and surface attributes of a product perceptible by means of mechanical, tactile, and, where appropriate, visual and auditory receptors”. Texture sensations, such as crispness, have been shown to be dependent on multiple sensory modalities. Crispness is heavily dependent on the acoustics during chewing (Vickers & Bourne, 1976) while vision aids with expectations and surface texture (Chen, 2007). Currently, the role of various sensory modalities in constructing viscosity judgements and expected satiety is unknown. Two experiments were conducted to understand how we use sensory information from different modalities to assess beverage thickness, satiety, satiation, and caloric density. The first experiment was designed to determine the sensitivity of auditory, oral tactile, and visual sensory input to changes in viscosity. The second experiment was designed to assess how the three sensory modalities from experiment 1 (audition, oral tactile, and vision) are used to estimate the nutritional properties of a beverage. Experiment 1 Materials and Methods Participants Forty-nine volunteers (32 women) participated in this study with ages ranging from 21 to 64 (32.2 ± 11.2). Based off a prescreener, only individuals that reported a liking for dairy products with moderate, to high consumption were chosen to participate. Individuals who reported visual, auditory, or gustatory/olfactory impairments (chronic or acute) were not selected to participate. Additional exclusion from participation included pregnancy, abnormal oral health, and dietary restrictions (e.g. dairy allergy). All participants were asked to not smoke or eat 1 hour prior to the start of the study. All participants signed an informed consent and were compensated for their time spent participating. This experiment (and the follow-up) protocol was conducted according to the Declaration of Helsinki for studies on human subjects and approved by the University of Tennessee IRB review for research involving human subjects (IRB #16-03417-XP). Preparation of stimuli Iota-carrageenan (IC) was used to thicken milk at several concentrations, ranging from 0.00% to 0.111% (w/v). To account for possible effects of reference viscosity, two samples were used as the reference milk 1.714 cP and 1.768 cP. Both references were the same for each modality condition in the study, visual, auditory, and oral tactile. The milk stimuli were made my mixing IC with 1% milk (Great Value, Wal- Mart, Bentonville, AR) and heated under constant agitation to 63 °C. The milk was then cooled to 25 °C in an ice bath, with constant agitation. Next, the thickened milk was homogenized for 3 minutes using a high-speed homogenizer (T25 Ultra Turrax, IKA Works, Staufen, Germany), and cooled to 4 °C. Lastly, the mixture was homogenized again for 3 minutes. The samples were stored at 4°C prior to serving. The viscosity of each sample was measured with a rheometer (AR 2000, TA Instruments, New Castle, Delaware, USA) using parallel plate geometry (40mm diameter). Measurements were at 25°C, using a shear rate of 143 s-1. This procedure was also used to measure the viscosity of different milk types, reduced-fat milk (1% and 2%), half-half, and heavy cream (Great Value, Wal-Mart, Bentonville, AR). Stimuli presentation For the visual condition, 30 mL scintillation vials were filled with 20 mL of milk at a specific IC concentration. These vials had a white top with a clear glass body through which the milk was visible and easily moved by titling the vial back and forth. Vials were presented at 4 °C. To produce the audio, 150 mL of each milk stimuli at 4 °C was poured from a height of 20 cm into a 400 ml beaker. The sound produced, was recorded using a microphone (Microsoft PnP, Redmond, WA) connected to a PC running Audacity 2.1.0. The samples were poured at a rate in which the pour was completed in 5.0 ± 0.5 sec. Each sample was recorded multiple times and only files that exhibited traits of a continuous pour were used. The raw audio files were treated with a noise reduction (12 dB, 6 sensitivity, 3 bands). For the oral tactile condition, 20 mL of a milk was placed in a 60 mL plastic black container with a fitted top. Milk samples were served at 4 °C. Procedure The tests were spread over a three-day period, one for each of the three modalities – visual, auditory, and oral tactile. A series of two-alternative force choice (2-AFC) tests were used to determine the sensitivity for each modality. In this method, two milk samples were presented at the same time, either in vials,