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