High false positive rates in common sensory threshold tests

Large variability in thresholds to sensory stimuli is observed frequently even in healthy populations. Much of this variability is attributed to genetics and day-to-day fluctuation in sensitivity. However, false positives are also contributing to the variability seen in these tests. In this study, random number generation was used to simulate responses in threshold methods using different “stopping rules”: ascending 2-alternative forced choice (AFC) with 5 correct responses; ascending 3-AFC with 3 or 4 correct responses; staircase 2-AFC with 1 incorrect up and 2 incorrect down, as well as 1 up 4 down and 5 or 7 reversals; staircase 3-AFC with 1 up 2 down and 5 or 7 reversals. Formulas are presented for rates of false positives in the ascending methods, and curves were generated for the staircase methods. Overall, the staircase methods generally had lower false positive rates, but these methods were influenced even more by number of presentations than ascending methods. Generally, the high rates of error in all these methods should encourage researchers to conduct multiple tests per individual and/or select a method that can correct for false positives, such as fitting a logistic curve to a range of responses

Desensitization but not Sensitization from Commercial Chemesthetic Beverages

Sensations such as spiciness or stinging are particularly challenging to assess in sensory evaluation tests, as sensitization (increase in intensity with repeated tasting) and desensitization (decrease in intensity with repeated tasting) phenomena can confound intensity ratings. However, much of the published work on these phenomena are with model solutions or complex meals rather than commercial beverages. Thus, we tested whether we could observe sensitization or desensitization using canned spicy ginger beer (contained chili extract) and seltzer water. Samples were presented in pairs, with a 20 s wait and no rinse within a pair, but a 4 min wait with rinsing between pairs. Pairs of samples were: ginger beer followed by ginger beer, ginger beer followed by seltzer, seltzer followed by ginger beer, and seltzer followed by seltzer. These pairs were intended to allow us to also test for cross-sensitization/desensitization between the two beverages. Tests were conducted both in open cups and capped vials to observe how loss of carbonation influenced sample ratings. Participants tasted all pairs of samples in counterbalanced order and rated samples for intensity of “Spiciness, burning, or stinging sensation,” bitterness, sweetness, sourness, overall flavor, and liking/disliking. Results indicate no sensitization effects. Desensitization, however, likely occurred for both beverages. Further, tasting seltzer and ginger beer together in a pair amplified the “bitterness” of the seltzer water, a likely contrast effect. Overall, while sensitization may not interfere with the sensory ratings for these beverages, contrast effects and desensitization should be considered carefully when planning sensory evaluation tests

Oral Sensations and Secretions

Sensations experienced in the mouth influence food choices, both immediately and in the long term. Such sensations are themselves influenced by experience with flavors, the chemical environment of the mouth, genetics of receptors for flavors, and individual behavior in the chewing of food. Gustation, the sense of taste, yields information about nutrients, influences palatability, and feeds into the human body\u27s preparation to receive those nutrients. Olfaction, the sense of smell, contributes enormously to defining and identifying food flavors (and is experienced even after placing food inside the mouth). Another vital component of food flavor is texture, which contributes to palatability, especially if a food\u27s texture violates a person\u27s expectations. Next, chemesthesis is the sense of chemically induced irritancy and temperature, for example spiciness and stinging. All of these sensations are potentially modified by saliva, the chemical and physical media of the mouth. As a person experiences the culmination of these oral sensations, modified through an individual\u27s own unique saliva, the flavors in turn influence both what and how a person eats

Conditioning of Human Salivary Flow Using a Visual Cue for Sour Candy

Objective Although the “mouthwatering” to sight, smell, or thought of food is commonly accepted in food and nutrition research, the concept of mouthwatering and human salivary flow conditioning is not well accepted in salivary research. The objective of this study was to revisit whether human salivary flow could be classically conditioned to a previously neutral stimulus. Design Sour candy or a non-food control in opaque containers were presented to healthy participants (n = 8). Simple images were consistently paired with container contents. Participants viewed the images for 15 s, then opened the containers and ate (candy) or did not eat (non-food control) the contents. This was repeated 14 times (7 of each stimulus). Order was semi-randomized to ensure one candy and one non-food were presented as the first two and last two stimuli. Saliva was collected with cotton dental rolls during these presentations (first two and last two) after viewing the image for 15 s, but before opening the container. Results Participants were successfully conditioned to increase salivary flow in response to the image that predicted candy, as demonstrated by greater weight of saliva in response to 1) the candy-paired image than the non-food-paired image, and 2) the candy-paired image at the end of the first visit compared with the beginning (when the image had no meaning). However, the effect was attenuated during the second visit. Conclusions We demonstrate classical conditioning of human salivary flow is achievable, but the effect may not persist to a second visit

Humans are more sensitive to the taste of linoleic and α-linolenic than oleic acid

Health concerns have led to recommendations to replace saturated fats with unsaturated fats. However, addition of unsaturated fatty acids may lead to changes in the way foods are perceived in the oral cavity. This study tested the taste sensitivity to and emulsion characteristics of oleic, linoleic, and α-linolenic acids. The hypothesis tested was that oral sensitivity to nonesterified fatty acids would increase with degree of unsaturation but that in vitro viscosities and particle sizes of these emulsions would not differ. Oral taste thresholds were obtained using the three-alternative, forced-choice, ascending method. Each participant was tested on each fat 7 times, for a total of 21 study visits, to account for learning effects. Viscosities were obtained for the blank solutions and all three emulsions. Results indicate lower oral thresholds to linoleic and α-linolenic than oleic acid. At higher shear rates, 5% oleic and linoleic acid were more viscous than other samples. More-dilute emulsions showed no significant differences in viscosity. Particle sizes of the emulsions increased very slightly with increasing unsaturation. Together, the emulsion characteristics and oral sensitivity data support a taste mechanism for nonesterified fatty acid detection. A major contributor to cardiovascular disease is a diet high in saturated fatty acids (33). Replacement of saturated fatty acid with mono- or polyunsaturated fatty acids (MUFA or PUFA, respectively) may improve blood lipid profiles, decrease markers for cardiovascular disease, and improve insulin responses in insulin-resistant or type 2 diabetic patients (34, 35). Thus the type of dietary fatty acids should be a critical consideration in evaluation of the healthfulness of high-fat foods. Oleic, linoleic, and α-linolenic acids are unsaturated fatty acids with one, two, and three double bonds, respectively. Oleic and linoleic acids are common in liquid vegetable oils, such as safflower, canola, and olive oils, while α-linolenic acid is predominantly found in fish oil. The PUFAs, linoleic and α-linolenic acids, are ω-6 and ω-3 fatty acids, respectively, and humans lack endogenous desaturases to create the double bonds at these positions of the alkyl chain. Thus these fatty acids are considered essential fatty acids and must be obtained from the diet. As different molecular structures of fatty acids influence health outcomes, structural differences could also influence affinity for various receptors, including proposed fatty acid taste receptors in the human mouth, as demonstrated for G protein-coupled receptor (GPR) 120 (4, 10, 12). While dietary fat, primarily present as triacylglycerol, has traditionally been valued for textural contributions to food, evidence indicates that nonesterified fatty acids (NEFA) are effective taste stimuli in the oral cavity (11, 20, 32). Large variability has been observed in NEFA oral sensitivity, which can be modified by dietary fat intake or by weight status (20, 23–26). However, most of the human work has tested only oleic acid. New data obtained through improved techniques and multiple tests per NEFA indicate that human oral sensitivity to varying NEFA differs according to properties of the alkyl chain (19). The current study is designed to evaluate differences in human sensitivity to NEFA that vary in degree of unsaturation, but not chain length. Previous studies have observed lower oral thresholds for linoleic than oleic acid (23) or no difference between these two fatty acids (5). Data from another report show a lower oral fatty taste threshold for α-linolenic than linoleic acid, which is, in turn, lower than that for oleic acid, but means and standard deviations to test for significant differences were not reported (10). Notably, none of these previous reports tested individuals multiple times with individual NEFA. Data published on oleic acid indicate that individuals may learn the taste of oleic acid over multiple tests, leading to lower thresholds in later visits than in the first test (29, 31). While learning effects are not always observed or may be blunted by using nonnaive participants (19), multiple visits should be conducted because of the high variability of sensory threshold data and the high occurrence of false positives, which would artificially lower threshold values (18). Thus the present study was designed to observe not only whether oral sensitivity to NEFA increases with greater unsaturation of the alkyl chain, but also whether multiple tests would give more consistent data on this relationship. Our hypotheses were as follows: 1) humans would be most sensitive to the taste of α-linolenic acid, followed by linoleic acid and then oleic acid, and 2) learning effects would be observed over multiple tests, particularly in naive participants. We expected these learning effects to attenuate over the course of the 21 visits (7 visits per NEFA) conducted in the study. Because of ongoing concerns of controlling for emulsion texture in NEFA taste experiments, data on particle size distributions and rheology of the samples were also collected and analyzed. We hypothesized that there would be no difference in particle size among the emulsions of different NEFA and that viscosity would be similar among the emulsions and the blank solutions

Dose–response functions and methodological insights for sensory tests with astringent stimuli

Sensations such as bitterness and astringency can limit the acceptance of many purportedly healthy foods. The purpose of this study was to investigate dose–response relationships of various astringent and bitter stimuli in a beverage, and to simultaneously gain additional methodological insight for the effects of wording, repeated tasting, and beverage matrix on these sensations. Untrained participants were presented with samples of a “flavored beverage” or water containing various concentrations of four stimuli (alum, malic acid, tannic acid, and quinine) and were asked to rate intensities of tastes (bitterness, sourness, and sweetness) and astringency subqualities (roughing, drying, and constricting or puckering) using a generalized visual analog scale. Using constricting in place of puckering had no effect on ratings. The effects of repeated tasting and beverage matrix on astringency perception were stimulus‐dependent. This study informs future investigations to understand the psychophysics of tastes and astringency

Different oral sensitivities to and sensations of short-, medium-, and long-chain fatty acids in humans

Fatty acids that vary in chain length and degree of unsaturation have different effects on metabolism and human health. As evidence for a “taste” of nonesterified fatty acids (NEFA) accumulates, it may be hypothesized that fatty acid structures will also influence oral sensations. The present study examined oral sensitivity to caproic (C6), lauric (C12), and oleic (C18:1) acids over repeated visits. Analyses were also conducted on textural properties of NEFA emulsions and blank solutions. Oral thresholds for caproic acid were lower compared with oleic acid. Lauric acid thresholds were intermediate but not significantly different from either, likely due to lingering irritating sensations that prevented accurate discrimination. From particle size analysis, larger droplets were observed in blank solutions when mineral oil was used, leading to instability of the emulsion, which was not observed when emulsions contained NEFA or when mineral oil was removed from the blank. Rheological data showed no differences in viscosity among samples except for a slightly higher viscosity with oleic acid concentrations above 58 mM. Thus, texture was unlikely to be the property used to distinguish between the samples. Differences in oral detection and sensation of caproic, lauric, and oleic acids may be due to different properties of the fatty acid alkyl chains. structural features of fatty acids, predominantly chain length and degree of unsaturation, determine their physiological role in preventing, promoting, or alleviating disease states (3, 16, 29, 42). Generally, polyunsaturated fatty acids and cis-monounsaturated fatty acids are associated with improved health outcomes when substituted for saturated fatty acids (3, 16, 29, 42). Chemically, unsaturation and shorter chain length lead to faster diffusion through cell membranes (30), and long-chain polyunsaturated fatty acids have greater affinity for certain fatty acid receptors, such as G protein-coupled receptor (GPR)120, than saturated or short-chain fatty acids (18, 27). Definitions of “short-chain,” “medium-chain,” and “long-chain” fatty acids vary, but generally short-chain fatty acids are composed of 2 to 4, and sometimes up to 6, carbons, medium-chain fatty acids are composed of 6 or 8 to 10 or 12 carbons, and long-chain fatty acids are composed of 12 or 14 to longer carbon chains. As the alkyl chain length increases, the molecules become less water soluble. Short- and medium-chain fatty acids also diffuse more rapidly across cell membranes than long-chain fatty acids (17). Short-chain fatty acids, such as butyric (C4) and caproic (C6) acids, are present in dairy products, but the bulk of these fatty acids in the human diet are actually byproducts of dietary fiber fermentation by bacteria in the colon (11, 12, 26, 65, 66). Medium-chain fatty acids of 8–12 carbons are found in foods such as palm kernel oil and coconut oil, with some lower concentrations in dairy products (1). Long-chain fatty acids are the most abundant fatty acids in the human diet, as they are prevalent in most triglycerides in food and are vital components of cell membranes. Knowing that structural differences influence the absorption (38) and physiological roles of fatty acids in nongustatory tissues, and given the accumulated evidence that nonesterified fatty acids (NEFA) are effective taste stimuli in humans and rodents (for recent reviews, see Refs. 20, 39, 44, and 59), the concept that structure may alter the taste sensation of NEFA seems probable. While numerous studies have been conducted to investigate the role of different types of NEFA on health outcomes, few have investigated their differential impacts on oral chemosensation in humans. One study (51) showed lower thresholds for linoleic (C18:2) than oleic (C18:2) or lauric (C12) acids, whereas another study (36) showed no differences in thresholds for caproic (C6), lauric, and stearic (C18) acids. Additional studies have reported caproic acid thresholds are lower than linoleic, stearic, and lauric acid thresholds (35) and no difference in sensitivity among oleic, linoleic, and stearic acids (8). However, all of these studies only tested each participant once. New research has shown wide within-subject variability and/or learning effects over time, indicating a need for multiple testing visits to establish reliable taste thresholds for these compounds (57, 58). A study (18) that used a trained panel, who presumably had numerous exposures to the NEFA, tested a variety of NEFA (C10, C12, C18:1, C18:2, C18:3, and C20:4), but that report did not indicate whether the thresholds differed significantly. Thus, clarification is needed for whether oral sensitivity to NEFA differ by fatty acid structure and whether multiple tests per participant are required to document accurate limits of detection for each NEFA (57, 58). Additionally, most NEFA taste studies have used carbohydrate gums and/or mineral oil to mask the textural contribution of NEFA to the blank sample (for a review, see Ref. 44). Textural properties and physical characteristics, such as particle size and emulsion stability, of NEFA emulsions are rarely reported, yet such parameters contribute to the oral sensation of emulsions (13–15, 49, 62, 64). While there is evidence that carbohydrate thickeners mitigate the increase in perceived thickness caused by unstable emulsions (64), the efficacy of mineral oil as a textural masking agent for NEFA has not been studied. Given that mineral oil, unlike NEFA, contains no hydrophilic moieties, this lipid does not form natural micelles. Thus, the physical structure formed in a mineral oil emulsion is different from an emulsion containing NEFA. We thus tested emulsions of NEFA with and without mineral oil as well as “blank” solutions of carbohydrate gums with and without mineral oil to determine what physical effects this lipid has on the samples. The present study was designed to investigate the differences in oral taste thresholds of caproic (hexanoic, C6), lauric (dodecanoic, C12), and oleic (cis-9-octadecenoic, C18:1) acids as well as assess the potential differences in viscosity and particle size for NEFA emulsions with or without mineral oil. The stimuli examined here were 6, 12, and 18 carbon fatty acids and are referred to as short-, medium-, and long-chain fatty acids. While stearic acid would have been a more ideal candidate to maintain the same level of saturation among the tested NEFA, stearic acid is solid until 69°C, a temperature at which sustained exposure could cause thermal burns. The hypotheses tested were 1) emulsion particle sizes would be smaller for mixtures with NEFA than mixtures with mineral oil alone, 2) viscosity would be greater for emulsions containing mineral oil than emulsions not containing mineral oil, 3) viscosity would not be significantly different among NEFA emulsions and the blank, 4) human oral sensitivity to NEFA would increase with decreasing alkyl chain length (sensitivity to caproic acid \u3e lauric acid \u3e oleic acid), and 5) human oral sensitivity to all NEFA would improve over multiple testing sessions

Sip and Spit or Sip and Swallow: choice of methods differentially alters taste intensity estimates across stimuli

While the myth of the tongue map has been consistently and repeatedly debunked in controlled studies, evidence for regional differences in suprathreshold intensity has been noted by multiple research groups. Given differences in physiology between the anterior and posterior tongue (fungiform versus foliate and circumvallate papillae) and differences in total area stimulated (anterior only versus whole tongue, pharynx, and epiglottis), small methodological changes (sip and spit versus sip and swallow) have the potential to substantially influence data. We hypothesized instructing participants to swallow solutions would result in greater intensity ratings for taste versus expectorating the solutions, particularly for umami and bitter, as these qualities were previously found to elicit regional differences in perceived intensity. Two experiments were conducted, one with model taste solutions [sucrose (sweet), a monosodium glutamate / inosine monophosphate (MSG/IMP) mixture (savory/umami), isolone (a bitter hop extract), and quinine HCl (bitter)] and a second with actual food products (grapefruit juice, salty vegetable stock, savory vegetable stock, iced coffee, and a green tea sweetened with acesulfame-potassium and sucralose). In a counterbalanced crossover design, participants (n=66 in experiment 1 and 64 in experiment 2) rated the stimuli for taste intensities both when swallowing and when spitting out the stimuli. Results suggest swallowing may lead to greater reported bitterness versus spitting out the stimulus, but that this effect was not consistent across all samples. Thus, explicit instructions to spit out or swallow samples should be given to participants in studies investigating differences in taste intensities, as greater intensity may sometimes, but not always, be observed when swallowing various taste stimuli

Food Chemistry: Experiments for Labs and Kitchens

A manual of kitchen experiments designed to demonstrate the chemical properties of foods and flavors, experienced through the human senses. We share this lab manual with you free of charge in light of the worldwide concerns of the novel coronavirus of 2019, and the COVID-19 disease outbreaks around the world in 2020. Please be warned, this manual is not “complete.” There will be typos. There will be errors. Some labs may not work perfectly. But, we hope you may find it useful—especially if your school was closed or you were quarantined/isolated for the sake of slowing the spread of this global virus. The only thing we ask in return is that you send us a message if you are able to use our experiments. This helps us demonstrate that our work had an effect, which is a key component of an academic career. Dr. Running’s email is: [email protected]. She can also provide instructors with data for analysis, and with the solution keys. Many thanks to Ms. Patsy Mellott, whose sponsorship of the Purdue College of Health and Human Sciences Patsy Mellott Teaching Innovation Award made the development of these labs possible

Session 3 Discussion: The microstructure of eating

The Microstructure of Eating