A study of saliva lubrication using a compliant oral mimic

Due to ethical issues and the difficulty in obtaining biological tissues, it is important to find synthetic elastomers that can be used as replacement test media for research purposes. An important example of this is friction testing to understand the mechanisms behind mouthfeel attributes during food consumption (e.g. syrupy, body and clean finish), which requires an oral mimic. In order to assess the suitability of possible materials to mimic oral surfaces, a sliding contact is produced by loading and sliding a hemispherical silica pin against either a polydimethyl siloxane (PDMS), agarose, or porcine tongue sample. Friction is measured and elastohydrodynamic film thickness is calculated based on the elastic modulus of the samples, which is measured using an indentation method. Tests were performed with both saliva and pure water as the lubricating fluid and results compared to unlubricated conditions. PDMS mimics the tongue well in terms of protein adhesion, with both samples showing significant reductions in friction when lubricated with saliva versus water, whereas agarose showed no difference between saliva and water lubricated conditions. This is attributed to PDMS's OSi(CH3)2- group which provides excellent adhesion for the saliva protein molecules, in contrast with the hydrated agarose surface. The measured modulus of the PDMS (2.2 MPa) is however significantly greater than that of tongue (3.5 kPa) and agarose (66–174 kPa). This affects both the surface (boundary) friction, at low sliding speeds, and the entrained elastohydrodynamic film thickness, at high speeds. Utilising the transparent PDMS sample, we also use fluorescence microscopy to monitor the build-up and flow of dyed-tagged saliva proteins within the contact during sliding. Results confirm the lubricous boundary film forming nature of saliva proteins by showing a strong correlation between friction and average protein intensity signals (cross correlation coefficient = 0.87). This demonstrates a powerful method to study mouthfeel mechanisms

eCapillary: a disposable microfluidic extensional viscometer for weakly elastic polymeric fluids

We report a disposable microfluidic extensional viscometer based on an optimized hyperbolic contraction–expansion geometry. This “eCapillary” device works by measuring pressure drop as a function of flow rate while accounting for viscous contribution to the pressure drop. The viscometer operates by applying a constant pressure and using an image-based approach to measure the flow rate. The device is fabricated entirely out of polydimethylsiloxane with no embedded sensors, making it disposable. We tested our approach using weakly elastic polymer solutions whose relaxation times were characterized by dripping-on-substrate rheology. Flow visualization was used to determine the onset of inertioelastic instabilities in the eCapillary device, thereby establishing the operating limits for extensional rheological measurements. Holography-based velocimetry analysis showed that extensional strain rate is uniform in a narrow section of the contraction throat necessitating correction for the shear contribution to the measured pressure drop. We observed the onset of extensional thickening to occur at Deborah number ≈ 1 and found that the apparent extensional viscosities are 2–4\ua0orders of magnitude higher than the shear viscosities. Finally, we compared our data with those from other microfluidic extensional viscometers reported in the literature and found good agreement

Impact of flavour solvent on biscuit micro-structure as measured by X-ray micro-computed tomography and the distribution of vanillin and HMF (HPLC)

The influence of flavour solvent, propylene glycol (PG) and triacetin (TA), was investigated on the micro-structure (as measured by X-ray micro-Computed Tomography, X-ray μCT) and aroma compound distribution (as measured by HPLC) within shortcake biscuits. X-ray μCT scanning showed biscuits made with PG had smaller pores and higher porosity than biscuits made with TA. Vanillin distribution across the biscuits was not homogeneous and was found at higher concentrations in the centre of the biscuits than the edge or bottom. The baked aroma compound 5-hydroxymethyl-furfural (HMF) was present at higher concentrations at the surface of the biscuits where Maillard chemistry is presumed to occur at its highest rate. The type of solvent had a significant effect on the total concentration and distribution of aroma compounds (p < 0.05). TA biscuits retained greater vanillin and more HMF was formed during baking when compared to PG biscuits. The core of TA biscuits had (on a relative scale) a much greater vanillin and lower HMF concentration than PG biscuits when compared to their periphery. Although this may be due to different physicochemical properties of the two solvents and varying levels of interactions with other ingredients, the micro-structure differences indicated by X-ray μCT image analysis illustrate one potential route by which the flavour solvent may be influencing the generation and stability of biscuit aroma compounds