Luca Cappellin, Felipe Lopez-Hilfiker and Manuel Hutterli
Assessing consumers’ flavor perception during food consumption using real-time nose-space analysis.
Flavor is central to consumers’ appreciation of food products and consists of both taste and aroma. The latter is related to the concentrations of volatile organic compounds (VOCs) perceived by ortho-nasal and retro-nasal receptors. Aroma is often considered the dominant component of flavor. Depending on the food matrix, it can include different VOC classes such as esters, alcohols, terpenoids, acids, and sulfur-containing VOCs. The VOC mixture can be quite complex, and even extremely small concentrations (in the part-per-trillion range) of some compounds can significantly change the flavor profile.
Nose-space analysis allows direct measurement of volatiles perceived by the consumer. This is achieved by sampling air in real-time from the consumers’ nostrils during food consumption, accompanied by real-time analysis by a direct injection analytical technique. Nose-space analysis is therefore the most direct approach to understand how the concentration of flavor compounds evolves over time and how flavor corresponds to the consumer’s perception.
From an analytical point of view, nose-space analysis is quite challenging. A suitable technique should (1) use direct injection, (2) simultaneously measure all VOCs (potentially thousands), (3), complete the measurement in less than a second, and (4) have a linearity range from sub pptv to ppm level.
Gas chromatographic methods such as GC-MS are far too slow to be suitable, and have difficulty with certain important classes of compounds, such as organic acids. Selected ion flow tube – mass spectrometry (SIFT-MS) has limited sensitivity and, lacking a time-of-flight (TOF) analyzer, can only measure a very limited number of VOCs in a split second. Standard proton transfer reaction time-of-flight mass spectrometers (PTR-TOF) lack the appropriate linear range. Currently, Vocus is the only available instrument that meets all the requirements for nose-space analysis.
Thanks to its cutting-edge time-of-flight technology, Vocus PTR-TOF is capable of simultaneously monitoring nose-space concentrations of virtually all separated flavor compounds in a split second from pptv level to ppm level. Unlike quadrupole-based instruments, Vocus does not compromise between speed and number of measured compounds.
Figure 1 shows a Vocus 2R PTR-TOF with a nose-space analysis experimental setup. PFA tubing is used to conduct the nose-space air, containing VOCs, into the instrument inlet. The instrument measures a full scan of VOC ions at a rate of 5 Hz.
In the example experiment shown in Figure 2, a test subject took four bites of a banana and four bites of a green apple, in sequence. The complete experiment lasted about fifteen minutes. Figure 2 shows the concentrations of several illustrative compounds, including the sum of isobutyl acetate, butyl acetate and other esters; several C6-ketones, acetone, and isoprene. While non-flavor compounds such as acetone and isoprene have a constant behavior, flavor compounds originating from food consumption have distinct temporal profiles. The measurement is reproducible, with distinct flavor profiles visible when comparing the banana and apple nosespace.