Volatile organic compound release from β-lactoglobulin stabilized emulsion systems under oral conditions

The release of volatile organic compounds (VOCs) from liquid food strongly contributes to the perception of flavour and aroma during food consumption. The VOCs that are odour active can be referred as aroma compounds. Oil-in-water (O/W) emulsions, in particular are known for their capability to carry non-polar compounds through the oil phase. Previously, many studies have been performed on emulsion systems, emphasizing the effects of emulsion structure and composition (e.g. droplet size, oil content, emulsifier type and content) on VOC release. However, there is little knowledge on the relationship between the emulsion systems structure and VOC release under environmental and dynamic oral processing conditions. Earlier studies showed how oral conditions such as saliva, pH and ionic strength can cause the emulsions to flocculate while changing their properties and attributes. In addition, the functionality of β-lactoglobulin (β-lg) as an emulsifier and potential VOC binding protein is not fully known and needs further investigation. The objectives of this research were to investigate the release patterns of different types of VOCs (e.g. family class, polarity, volatility) from primary and multilayer emulsion systems (P-O/W and M-O/W) stabilized with β-lg using static and dynamic headspace analysis, and at the same time to characterize the structural changes occurring in emulsions under oral and processing conditions. The role of the tongue in pre-swallowing mastication with emulsions and other liquid systems was also investigated. Under equilibrium conditions, the release of volatile organic compounds (VOCs) from oil-in-water emulsions stabilized by β-lg was studied. Gas-matrix partition coefficients (K) for different volatile compounds were determined by static headspace gas chromatography (GC). Two indirect methods to measure K were used: phase ratio variation (PRV) and phase ratio calibration (PRC). These two methods were found to be simple and accurate alternatives to measuring K without using external calibration. The VOC release depended mainly on the physicochemical properties and affinity of the compounds to the matrix. The role of β-lg on VOC release was compared with Tween 20, both as emulsifiers. In emulsion systems, the presence of the emulsifier conformation at the interface had an effect on the affinity with intermediate hydrophobic VOCs. This may be attributed to the hydrophobic cavity site of the protein and covalent bonds with the aldehydes. The binding affinity of β-lg to VOCs increased at high pH as the protein had a more flexible and open conformation (with an increase in the surface hydrophobicity (S0) and retinol binding). To investigate the dynamic release of VOCs under oral conditions, a model mouth with an artificial tongue was developed. The role of the tongue in transporting the bolus through the mouth by pressing it against the palate has been widely studied, however, the relationship between tongue pressures generated and VOC release is not clear. Pressure patterns during swallowing were found to vary across subjects which may explain some of the differences in individual flavour perception. The model mouth described here is capable of reproducing actual human tongue pressure patterns by a computer controlled artificial tongue driven by an actuator. The tongue functionality is monitored by pressure and force sensors. The model was designed to incorporate oral features and conditions (e.g. temperature, saliva flow, air flow and appropriate oral cavity volumes) combined with on-line VOC measurement using proton transfer reaction-mass spectrometry (PTR-MS). Three liquid model systems (e.g. aqueous solution, oil, and O/W emulsions) were used to evaluate the model mouth operation. Different tongue pressure patterns were applied to the liquid systems, and the release of VOCs was monitored in real time. The release was significantly more intense for longer tongue pressure duration and was affected by the initial tongue position above the sample surface. The role of saliva (artificial vs. human) and the sample temperature on VOC release was also investigated. The release was enhanced by the addition of saliva containing mucin, and at a higher temperature. A multilayer oil-in-water (M-O/W) emulsion system was compared to a primary oil-in-water (P-O/W) emulsion as a carrier for VOCs under various environmental conditions (pH and salt). The M-O/W emulsion consisted of soy oil coated with β-lg and pectin layers that have opposite charges. The release of VOCs was measured using static and dynamic headspace methods (including the use of the model mouth). The partition coefficients (K) calculated by the PRV method, showed different volatile release profiles between the emulsion types. An increase in VOC release was found for the unstable P-O/W emulsion at pH 5, whereas M-O/W emulsions were stable at the same pH and retained the hydrophobic VOCs. Hydrophobic interactions and hydrogen bonds, with the secondary dense layer of pectin, may be responsible for the improved retention. Increasing pH and ionic strength acts as a VOC release trigger to detach the pectin from the interface. Pectin attachment to β-lg at pH 4 had a significant impact on S0 due to the protein unfolding. The VOC release from the emulsion systems using the model mouth support the results under equilibrium conditions. Addition of artificial saliva (containing 1% wt mucin) caused the emulsion sample to become unstable through bridging and depletion flocculation, mostly observed for P-O/W emulsion. The VOC release was found to increase with the changes in the oil volume phase distribution due to emulsion flocculation. The results of this study contribute to the knowledge on VOC release from emulsions by providing a novel platform to better understand the impact of emulsion structure on aroma release in the mouth. Applying new systems, such as the M-O/W emulsion, could provide useful tools to design a desired delivery system of aroma and other bioactive compounds into the mouth