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.
Date of this Version
Running, Cordelia and Mattes, Richard D., "Humans are more sensitive to the taste of linoleic and α-linolenic than oleic acid" (2015). Department of Nutrition Science Faculty Publications. Paper 18.