Cordelia A. Running

Fat taste in humans: Sources of within- and between-subject variability

Non-esterified fatty acids (NEFA) are reportedly detectable through taste mechanisms in the human oral cavity. However, wide variability has been observed in NEFA taste sensitivity between and within subjects as well as across research groups. Some of this variability may be due to the hydrophobic nature of the NEFA and the methods used to make stimuli emulsions. As NEFA are poorly soluble in water, emulsification is necessary for delivery of stimuli to taste receptors. However, properties of emulsions may also be detected by somatosensory cues complicating attribution of sensory findings to taste. Additionally, learning (improved test performance) has been observed when using traditional tests for measuring sensitivity to NEFA, which may contribute greatly to within-subject variability if not standardized. Factors such as sex, diet, and BMI have been proposed to affect NEFA taste sensitivity, but the degree to which these individual factors influence NEFA detection thresholds remains to be fully established. Improved knowledge of stimulus properties and individual sensory capabilities will be needed to further evaluate the posited taste component to human oral fat detection. Progress in this area should facilitate the translation of findings on how NEFA taste may contribute to or reflect food choice and chronic disease risk.

Mechanisms and effects of “fat taste” in humans

Evidence supporting a “taste” cue from fat in the oral cavity continues to accrue. The proposed stimuli for fat taste, non‐esterified fatty acids (NEFA), are released from food through hydrolytic rancidity and lipase activity derived from foods or saliva. NEFA must then be released from the food matrix, negotiate the aqueous environment to reach taste cell surfaces, and interact with receptors such as CD36 and GPR120 or diffuse across cell membranes to initiate a taste signal. Knowledge of these processes in non‐gustatory tissues should inform understanding of taste responses to NEFA. Additionally, downstream effects of oral triglyceride exposure have been observed in numerous studies. Data specific to effects of NEFA versus triglyceride are scarce, but modified sham feeding trials with triglyceride document cephalic phase responses including elevations in serum lipids and insulin as well as potential, but debated, effects on gut peptides, appetite, and thermogenesis. In this review, we highlight the mechanisms by which NEFA migrate to and interact with taste cells, and then we examine physiological responses to oral fat exposure.

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.

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.

Degree of free fatty acid saturation influences chocolate rejection in human assessors

In foods, free fatty acids (FFAs) traditionally have been viewed as contributing an odor, yet evidence has accumulated that FFAs also contribute a unique taste (“oleogustus”). However, minimal work has been conducted using actual foods to test the contribution of FFA to taste preferences. Here, we investigate flavor, taste, and aroma contributions of added FFA in chocolate, as some commercial manufacturers already use lipolysis of triglycerides to generate unique profiles. We hypothesized that small added concentrations of FFAs would increase preferences for chocolate, whereas higher added concentrations would decrease preferences. We also hypothesized a saturated fatty acid (stearic C18) would have a lesser effect than a monounsaturated (oleic C18:1), which would have a lesser effect than a polyunsaturated (linoleic C18:2) fatty acid. For each, paired preference tests were conducted for 10 concentrations (0.04% to 2.25%) of added FFAs compared with the control chocolate without added FFAs. Stearic acid was tested for flavor (tasting and nares open), whereas the unsaturated fatty acids were tested for both aroma (orthonasal only and no tasting) and taste (tasting with nares blocked to eliminate retronasal odor). We found no preference for any added FFA chocolate; however, rejection was observed independently for both taste and aroma of unsaturated fatty acids, with linoleic acid reaching rejection at lower concentrations than oleic acid. These data indicate that degree of unsaturation influences rejection of both FFA aroma and taste in chocolate. Thus, alterations of FFA profiles in foods should be approached cautiously to avoid shifting concentrations of unsaturated fatty acids to hedonically unacceptable levels.

Individual Differences in Multisensory Flavor Perception

Abstract Flavor is perceived differently across individuals. While there are certainly common percepts that all humans can relate to, the degree to which we each experience these sensations from particular foods or objects can vary. Some of this variability comes from differences in our experiences: how familiar we are with, what we have been told about, or the personal value we assign to an item or sensation. However, environment and experience do not explain all of the differences in flavor perception. Biology is a large determinant of the array and intensity of sensations that an individual experiences. Understanding this biological variability can assist in the study of human ingestive behavior and the design of food products. This chapter reviews the current state of knowledge on individual differences in flavor perception, specifically the variation in tactile, chemesthetic, odor, and taste sensations. The genetic or biological roots of these differences are discussed, as well as how these differences may or may not alter food preference and intake.

Sip and spit or sip and swallow: Choice of method 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.

Session 3 Discussion: The microstructure of eating

The Microstructure of Eating.

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 bodys 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 foods texture violates a persons 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 individuals own unique saliva, the flavors in turn influence both what and how a person eats.

Chemical stability and reaction kinetics of two thiamine salts (thiamine mononitrate and thiamine chloride hydrochloride) in solution

Two types of thiamine (vitamin B1) salts, thiamine mononitrate (TMN) and thiamine chloride hydrochloride (TClHCl), are used to enrich and fortify food products. Both of these thiamine salt forms are sensitive to heat, alkali, oxygen, and radiation, but differences in stability between them have been noted. It was hypothesized that stability differences between the two thiamine salts could be explained by differences in solubility, solution pH, and activation energies for degradation. This study directly compared the stabilities of TMN and TClHCl in solution over time by documenting the impact of concentration and storage temperature on thiamine degradation and calculating reaction kinetics. Solutions were prepared containing five concentrations of each thiamine salt (1, 5, 10, 20, and 27 mg/mL), and three additional concentrations of TClHCl: 100, 300, and 500 mg/mL. Samples were stored at 25, 40, 60, 70, and 80 °C for up to 6 months. Degradation was quantified over time by high-performance liquid chromatography, and percent thiamine remaining was used to calculate reaction kinetics. First-order reaction kinetics were found for both TMN and TClHCl. TMN degraded significantly faster than TClHCl at all concentrations and temperatures. For example, in 27 mg/mL solutions after 5 days at 80 °C, only 32% of TMN remained compared to 94% of TClHCl. Activation energies and solution pHs were 21-25 kcal/mol and pH 5.36-6.96 for TMN and 21-32 kcal/mol and pH 1.12-3.59 for TClHCl. TClHCl degradation products had much greater sensory contributions than TMN degradation products, including intense color change and potent aromas, even with considerably less measured vitamin loss. Different peak patterns were present in HPLC chromatograms between TMN and TClHCl, indicating different degradation pathways and products. The stability of essential vitamins in foods is important, even more so when degradation contributes to sensory changes, and this study provides a direct comparison of the stability of the two thiamine salts used to fortify foods in environments relevant to the processing and shelf-life of many foods.