How volatile composition facilitates olfactory discrimination of fat content in beef and pork

Foods differing in fat content can be distinguished through olfaction alone. The mechanisms underlying the ability of humans to discriminate between foods differing in fat content through olfaction are underexplored. In this study, beef and pork samples were prepared (raw and roasted) with low (muscle tissue; raw: 2-5%; roasted: 5%), medium (muscle tissue with lard; raw: 25-30%; roasted: 36-44%), and high (lard; raw: 40-42%; roasted: 69-70%) fat content. Olfactory triangle discrimination tests and ranking tests were performed to explore whether humans can discriminate and rank fat content of the samples through orthonasal olfaction. Headspace-Solid Phase Micro Extraction-Gas Chromatography-Mass Spectrometry (SPME-GC-MS) was used to characterize the volatile compound composition of the headspace of samples differing in fat content. Partial least-squares regression and partial least squares-discriminant analysis were performed to determine the volatile compounds that were responsible for olfactory fat content discrimination. We found that fat content in both raw and roasted samples can be distinguished through orthonasal olfaction. Perceived odor differences did not always contribute to olfactory identification of fat content. Roasted beef and pork meats with higher fat content had more abundant fatty acids, aldehydes, and ketones. Phthalic acid, isobutyl 2-ropylpentyl ester, and carbon disulfide facilitated the olfactory discrimination of fat content in raw pork and beef samples. 2-Methyl-propanal, benzaldehyde, 1-hydroxy-2-propanone, 2,3-pentanedione, 2,5-octanedione, and 2-butanone contributed to odor differences of roasted beef samples differing in fat content. We conclude that beef and pork samples differing in fat content differ in volatile compound composition of the headspace, and that these differences facilitate discrimination between samples differing in fat content based on olfaction alone.

A point of view on human fat olfaction - do fatty derivatives serve as cues for awareness of dietary fats?

Fat (triglycerides) consumption is critical for the survival of animals, including humans. Being able to smell fat can be advantageous in judging food value. However, fat has poor volatility; thus, olfaction of fat seems impossible. What about fatty acids that comprise fat? Humans smell and discriminate medium-chain fatty acids. However, no conclusive evidence has been provided for the olfactory sense of long-chain fatty acids, including essential acids such as linoleic acid (LA). Instead, humans likely perceive the presence of essential fatty acids through the olfaction of volatile compounds generated by their oxidative breakdown (e.g., hexanal and γ-decalactone). For some people, such scents are pleasing, especially when they come from fruit. Nonetheless, it remains unclear whether the olfaction of these volatiles leads to the recognition of fat per se. Nowadays, people often smell LA-borne aldehydes such as E,E-2,4-decadienal that occur appreciably, for example, from edible oils during deep frying, and are pronely captivated by their characteristic "fatty" note, which can be considered a "pseudo-perception" of fat. However, our preference for such LA-borne aldehyde odors may be a potential cause behind the modern overdose of n-6 fatty acids. This review aims to provide a view of whether and, if any, how we olfactorily perceive dietary fats and raises future purposes related to human fat olfaction, such as investigating sub-olfactory systems for detecting long-chain fatty acids.

Olfactory discrimination of fat content in milks is facilitated by differences in volatile compound composition rather than odor intensity

The mechanisms underlying the ability of humans to olfactorily discriminate fat content in milks remain unknown. In this study, we found that fat contents (0.5, 1.5 and 3.5% fat) can be discriminated by olfaction in commercially available pasteurized milks (p < 0.05) but not in ultra-high temperature processed (UHT) milks. The composition of volatile compounds of pasteurized milks differed with fat content, whereas that of UHT milks differing in fat content was similar. Principal component analysis revealed that differences in volatile compound composition of pasteurized milks differing in fat content contribute to olfactory discrimination. In UHT milks, acetoin and 2-heptanone may mask odor differences leading to indistinguishable odors. No differences were observed regarding perceived odor intensity of pasteurized milks or UHT milks differing in fat content. We conclude that the olfactory discrimination of fat content in pasteurized milks is facilitated by differences in volatile compound composition rather than odor intensity.

Investigation of human flavor perception and discrimination of the fat content in food using DR A-Not A and 3-AFC methods

This study examined human flavor perception and discrimination of powdered milk samples with various fat contents using two different sensory discrimination methods, DR A-Not A and 3-AFC. DR A-Not A was expected to be more effective. Using skim milk and whole milk powder, five different samples with various fat contents were prepared. An independent samples design was used to compare DR A-Not A and 3-AFC. Each subject performed 24 repeated tests consisting of comparisons of four different test samples from the reference sample. Signal detection d' results showed that sample discrimination was possible using DR A-Not A, but not 3-AFC. Moreover, the just noticeable difference (JND) was calculated using the results of DR A-Not A. The calculated JND was 0.47% (w/v), and the Weber fraction was 0.82. These results confirm that the DR A-Not A method is more effective for studying the human sensitivity to the fat content in food.

Characterization of the relationship between olfactory perception and the release of aroma compounds before and after simulated oral processing

Aroma is an important property of fermented milk, and it directly affects consumer acceptance. However, previous studies have mainly focused on analyzing the composition of aroma compounds in fermented milk in vitro, and the composition may be different from the real aroma composition that stimulates the sense of smell. Furthermore, the relationship between olfactory attributes and the release of aroma compounds was not fully understood. In this study, we selected 6 samples of fermented milk differing in aroma perception intensity based on our pretest. A descriptive sensory analysis focusing on orthonasal and retronasal olfaction of fermented milk was first conducted by semitrained panelists. Artificial saliva was mixed with the fermented milk samples and continuously stirred at 37°C for 15 s to simulate oral processing conditions. Headspace solid-phase microextraction-gas chromatography coupled with quadrupole time-of-flight mass spectrometry was applied to identify the head space composition of 6 kinds of fermented milk before and after the simulated oral processing. Twenty-five volatile compounds were identified in the fermented milks, 15 of which were predicted to have an influence on the olfactory perception of fermented milks during oral processing. Partial least squares regression analysis based on chemical and sensory data was then applied to explore the correlation between sensory perception and volatile aroma release. The results showed that oral processing greatly increased the perception of creamy aroma compounds, such as diacetyl and acetone, but did not increase the perception of dairy sour aroma compounds, such as butanoic acid and hexanoic acid. This study can help improve our understanding of the relationship between olfactory perceptions and the release of volatile aroma compounds under oral processing. It might also contribute to the design of palatable fermented milks catering to specific consumer preferences.

Retronasal Olfaction Test Methods: A Systematic Review

Background

This report produces a bibliographic study of psychophysical tests proposed clinical assessments of retronasal olfaction.

Aims

We review how these tests can be utilized and discuss their methodological properties.

Study Design

Systematic review.

Methods

We undertook a systematic literature review investigating the retronasal olfaction test methods. PubMed, the free online MEDLINE database on biomedical sciences, was searched for the period from 1984 to 2015 using the following relevant key phrases: “retronasal olfaction”, “orthonasal olfaction”, “olfaction disorders”, and “olfaction test”. For each of the selected titles cited in this study, the full manuscript was read and analyzed by each of the three authors of this paper independently before collaborative discussion for summation and analytical reporting. Two reviewers independently read the abstracts and full texts and categorised them into one of three subgroups as follow, suitable, not-suitable, and unsure. Then they cross-checked the results, and a third reviewer decided assigned the group “unsure” to either the suitable group or the not-suitable group. Fifty eight studies revealed as suitable for review by two authors whereas 13 found not suitable for review. The total amount of 60 uncertain (unsure) or differently categorized articles were further examined by the third author which resulted in 41 approvals and 19 rejections. Hence 99 approved articles passed the next step. Exclusion criteria were reviews, case reports, animal studies, and the articles of which methodology was a lack of olfaction tests. By this way excluded 69 papers, and finally, 30 original human research articles were taken as the data.

Results

The study found that the three most widely used and accepted retronasal olfaction test methods are the retronasal olfaction test, the candy smell test and odorant presentation containers. All of the three psychophysical retronasal olfaction tests were combined with orthonasal tests in clinical use to examine and understand the smell function of the patient completely. There were two limitations concerning testing: “the lack concentrations and doses of test materials” and “performing measurements within the supra-threshold zone”.

Conclusion

The appropriate test agents and optimal concentrations for the retronasal olfaction tests remain unclear and emerge as limitations of the retronasal olfaction test technique. The first step to overcoming these limitations will probably require identification of retronasal olfaction thresholds. Once these are determined, the concept of retronasal olfaction and its testing methods may be thoroughly reviewed.

Effects of CD36 Genotype on Oral Perception of Oleic Acid Supplemented Safflower Oil Emulsions in Two Ethnic Groups: A Preliminary Study

Previous studies demonstrate humans can detect fatty acids via specialized sensors on the tongue, such as the CD36 receptor. Genetic variation at the common single nucleotide polymorphism rs1761667 of CD36 has been shown to differentially impact the perception of fatty acids, but comparative data among different ethnic groups are lacking. In a small cohort of Caucasian and East Asian young adults, we investigated if: (1) participants could detect oleic acid (C18:1) added to safflower oil emulsions at a constant ratio of 3% (w/v); (2) supplementation of oleic acid to safflower oil emulsions enhanced perception of fattiness and creaminess; and (3) variation at rs1761667 influenced oleic acid detection and fat taste perception. In a 3-alternate forced choice test, 62% of participants detected 2.9 ± 0.7 mM oleic acid (or 0.08% w/v) in a 2.8% safflower oil emulsion. Supplementation of oleic acid did not enhance fattiness and creaminess perception for the cohort as a whole, though East Asians carrying the GG genotype perceived more overall fattiness and creaminess than their AA genotype counterparts (P < 0.001). No differences were observed for the Caucasians. These preliminary findings indicate that free oleic acid can be detected in an oil-in-water emulsion at concentrations found in commercial oils, but it does not increase fattiness or creaminess perception. Additionally, variation at rs1761667 may have ethnic-specific effects on fat taste perception.

CD36 is involved in oleic acid detection by the murine olfactory system

Olfactory signals influence food intake in a variety of species. To maximize the chances of finding a source of calories, an animal’s preference for fatty foods and triglycerides already becomes apparent during olfactory food search behavior. However, the molecular identity of both receptors and ligands mediating olfactory-dependent fatty acid recognition are, so far, undescribed. We here describe that a subset of olfactory sensory neurons expresses the fatty acid receptor CD36 and demonstrate a receptor-like localization of CD36 in olfactory cilia by STED microscopy. CD36-positive olfactory neurons share olfaction-specific transduction elements and project to numerous glomeruli in the ventral olfactory bulb. In accordance with the described roles of CD36 as fatty acid receptor or co-receptor in other sensory systems, the number of olfactory neurons responding to oleic acid, a major milk component, in Ca²⁺ imaging experiments is drastically reduced in young CD36 knock-out mice. Strikingly, we also observe marked age-dependent changes in CD36 localization, which is prominently present in the ciliary compartment only during the suckling period. Our results support the involvement of CD36 in fatty acid detection by the mammalian olfactory system.

Oleogustus: The Unique Taste of Fat

Considerable mechanistic data indicate there may be a sixth basic taste: fat. However, evidence demonstrating that the sensation of nonesterified fatty acids (NEFA, the proposed stimuli for "fat taste") differs qualitatively from other tastes is lacking. Using perceptual mapping, we demonstrate that medium and long-chain NEFA have a taste sensation that is distinct from other basic tastes (sweet, sour, salty, and bitter). Although some overlap was observed between these NEFA and umami taste, this overlap is likely due to unfamiliarity with umami sensations rather than true similarity. Shorter chain fatty acids stimulate a sensation similar to sour, but as chain length increases this sensation changes. Fat taste oral signaling, and the different signals caused by different alkyl chain lengths, may hold implications for food product development, clinical practice, and public health policy.

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.

Detecting fat content of food from a distance: olfactory-based fat discrimination in humans

The desire to consume high volumes of fat is thought to originate from an evolutionary pressure to hoard calories, and fat is among the few energy sources that we can store over a longer time period. From an ecological perspective, however, it would be beneficial to detect fat from a distance, before ingesting it. Previous results indicate that humans detect high concentrations of fatty acids by their odor. More important though, would be the ability to detect fat content in real food products. In a series of three sequential experiments, using study populations from different cultures, we demonstrated that individuals are able to reliably detect fat content of food via odors alone. Over all three experiments, results clearly demonstrated that humans were able to detect minute differences between milk samples with varying grades of fat, even when embedded within a milk odor. Moreover, we found no relation between this performance and either BMI or dairy consumption, thereby suggesting that this is not a learned ability or dependent on nutritional traits. We argue that our findings that humans can detect the fat content of food via odors may open up new and innovative future paths towards a general reduction in our fat intake, and future studies should focus on determining the components in milk responsible for this effect.

Shared retronasal identifications of vapor-phase 18-carbon fatty acids

The long-chain 18-carbon fatty acids linoleic, oleic, and stearic acids, retronasally in vapor phase, are discriminated from blanks and each other. However, ability to linguistically identify them was unknown. To explore this, a Focus Group and then Check-All-That-Apply measures gave 9 identifiers for the 3 fatty acids plus phenylethyl alcohol (PEA) and geraniol. Next, participants selected 1 of the 9 identifiers from a computer-based display. It was found that the modal identification for linoleic acid was 23% "Rubbery" (next 18% "Oily" and "New Plastic"), oleic acid was 21% Oily (next 19% Rubbery), and stearic acid was 43% Rubbery (next 22% New Plastic), but linoleic acid received ~40% food-related identifiers. Geraniol was 96% "Lemon," and PEA was 67% "Flowers." Identifications for fatty acids differed significantly (P ≤ 0.05) from those for geraniol for most participants (86%) and from those for PEA for 59% of participants. Stearic acid identifications differed significantly from those for linoleic and oleic acids for 32% of participants. However, identification for linoleic acid differed significantly from those for oleic acid for only 14% of participants. Overall, retronasal vapor-phase stearic acid was identified differently from other 18-carbon fatty acids by a substantial minority of participants, but linoleic and oleic acids were not, suggesting that these 2 vapor-phase 18-carbon fatty acids can be identified retronasally as a group but not separately.

The examination of fatty acid taste with edible strips

The objective of this study was to determine whether humans could detect long-chain fatty acids when these lipid molecules are delivered to the oral cavity by edible taste strips. For suprathreshold studies, up to 1.7 μmol of stearic acid or linoleic acid was incorporated into 0.03 mm thick, one-inch square taste strips. Normalized taste intensity values for stearic acid were in the barely detectable range, with values equal to, or slightly above control strips. One-third of test subjects described the taste quality as oily/fatty/waxy. Approximately 75% of test subjects could detect the presence of linoleic acid when this fatty acid was incorporated into dissolvable strips. Normalized taste intensity values for linoleic acid were in the weak to moderate range. The most commonly reported taste quality responses for linoleic acid were fatty/oily/waxy, or bitter. When nasal airflow was obstructed, the perceived taste intensity of linoleic acid decreased by approximately 40%. Taste intensity values and taste quality responses for linoleic acid were then compared among tasters and non-tasters of 6-n-propylthiouracil (PROP). Individuals who could detect the bitter taste of PROP reported higher taste intensity values for linoleic acid compared with PROP non-tasters. However, taste quality responses for linoleic acid were similar among both PROP tasters and PROP non-tasters. These results indicate that humans can detect long-chain fatty acids by both olfactory and non-olfactory pathways when these hydrophobic molecules are delivered to the oral cavity by means of edible taste strips. These studies further show that genetic variation in taste sensitivity to PROP affects chemosensory responses to the cis-unsaturated fatty acid (linoleic acid) in the oral cavity.

Oral cavity discrimination of vapor-phase long-chain 18-carbon fatty acids

Linoleic, oleic, and stearic fatty acids, presented vapor-phase retronasally, were discriminable from blanks and each other, but the same concentrations, oral-cavity-only (OCO), were not discriminable from blanks. It remained possible that higher concentrations might be discriminable OCO. To evaluate this, participants attempted to discriminate undiluted linoleic, oleic, or stearic acids, vapor-phase OCO, from blanks. For each fatty acid, participants received 5 stimulus delivery containers (SDCs) in 2 trials; 4 SDC held blanks, the fifth, a fatty acid. As a "positive control" in 2 trials, participants received vapor-phase OCO peppermint extract and blanks. For all trials, the task was to select the 1 different SDC. It was found that the 1 different SDC was selected in 24% of stearic, 32% of linoleic, 47% of oleic acid, and in 92% of peppermint trials; discriminations (the 1 different SDC selected in both trials) occurred in 0%, 16%, 26%, and 84% of pairs, respectively. Correct selections for oleic acid differed from chance, P = 0.0004, but not for linoleic acid, P = 0.125, or stearic acid, P = 0.345, Bonferroni corrected. Vapor-phase oleic acid can be an oral cavity trigeminal stimulus, linoleic acid might be (uncorrected P = 0.0384), but vapor-phase stearic acid cannot be.

Potential mechanisms of retronasal odor referral to the mouth

The current study took a first step toward elucidating the sensory input that drives retronasal odor referral to the mouth. In 2 experiments, subjects performed odor localization tasks under various oral-nasal stimulation conditions that allowed us to assess the effects of direction of airflow, taste, and tactile stimulation on retronasal odor referral. Subjects reported the locations of perceived odors when food odorants were inhaled through the mouth alone or in the presence of water or various tastants in the mouth. The results indicated that when perceived alone, vanilla and soy sauce odor were localized 54.7%: 26.4%: 18.9% and 60.0%: 21.7%: 18.3% in the nose, oral cavity, and on the tongue, respectively. The localization of odors alone was not significantly different from when water was presented simultaneously in the mouth, indicating that tactile stimulation itself is not sufficient to enhance odor referral. However, the presence of sucrose, but not other tastes, significantly increased localization of vanilla to the tongue. Likewise, only NaCl significantly augmented referral of soy sauce odor to the tongue. These data indicate that referral of retronasal odors to the mouth can occur in the absence of a either taste or touch but that referral to the tongue depends strongly on the presence of a congruent taste.