A close examination of the d50 versus pressure curves obtained for both formulations reveals the existence of a threshold pressure. Below such pressure value, the reduction of d50 follows the Kolmogorov's theory and it can be represented by Equation (2). Contrary, pressures above the threshold value increases the d50. Similar observation was previously reported by Marco-Moles et al. (2012) who identified two distinctive effects of pressure on the stability of oil-in-water emulsions. These authors found stable emulsions within the pressure range of 70-150 MPa, while destabilization of the emulsions was observed when increasing the pressure up to 250 MPa. The occurrence of coalescence has been thought to be dependent on collision frequency and probability of collision, which are function of physical properties of the emulsion and operating conditions, energy density and flow conditions (Tesch and Schubert, 2002; Jafari et al., 2008). High input energy during emulsification will increase the collision frequency and therefore the probability of coalescence. Floury et al. (2004) performed a numerical simulation of the velocity profiles for water within a HPH unit operated at constant flow rate of 10 L h1 and maximum pressure of 350 MPa. These authors predicted that the fluid velocity dramatically increases with pressure, reaching values up to 200 m s1 at the operating pressures of >300 MPa.
After developing a parabolic velocity profile, the velocity of the fluid rapidly decreases over a short distance with velocities of 0.6 m s1. It seems reasonable to assume that such circumstances (increasing particle velocity and decreasing over short distance) favor the occurrence of coalescence over adsorption.