Experimental study on in-situ viscosity measurement of milk fermenting to yogurt by laser-induced capillary wave method

We have developed a new technique for measuring liquid viscosity using a pulsed carbon dioxide laser as a heating source. In this method, interfering laser beams heat the liquid surface and generate a laser-induced capillary wave (LiCW) caused by spatially sinusoidal temperature distribution. The temporal behavior of LiCW is detected by a diffracted probe beam at the heating area. The LiCWdynamics provide information on the thermophysical properties of the liquid, such as viscosity and surface tension. In this paper, we have (1) measured nine Newtonian liquid samples with viscosity ranging from 0.304 ≃ 7080 mPa•s and obtained viscosity within approximately ±10% deviation from the reference values, and (2) measured the apparent viscosity of milk fermenting to yogurt and obtained signals that change significantly during fermentation. The relative increase of apparent viscosity was more than 100-fold, and this order of magnitude agreed with previous measurements acquired by conventional methods.