Zhejiang Gongshang Universcity, Hangzhou, China
Human saliva is a fluid naturally secreted in the oral cavity that interacts with food. It can be either unstimulated or stimulated. In this work, we explore the fact that saliva could be an effective emulsifier due to the presence of salivary proteins.
Whole human unstimulated and stimulated saliva from three healthy subjects were collected and analyzed for protein content, surface tension, saliva flow, and pH value. Saliva-oil emulsions were produced both mechanically and orally.
Droplet size distribution showed that with increased mixing speed droplets of mechanically made samples tended to be smaller, d32= 30 ?m for the 4000 rpm, was the smallest, while orally made samples had smallest sized droplets for lowest volume 0.25 ml d32=44 ?m. Zeta potential values for all three orally made samples were similar with values of –20 mV. The microstructures of such emulsions were examined immediately after the emulsification. Results show that stable saliva emulsions can be produced during oral processing of either rapeseed oil or pork fat. Fat had small particles d32=24 ?m, high shear during eating one of the possible explanations for the small size. Proteins from saliva aqueous phase are depleted during the emulsification process, they distribute on the oil-water interface. SDS-Page analysis of US, mechanically and orally made sample showed that protein fractions of 27 and 55 kDa molecular weights were favoured for emulsion formation. Although, other proteins probably contribute to emulsion formation these areas had the biggest loss when compared to normal saliva sample.
This work concludes that human saliva could function as an effective emulsifier and oral emulsification is highly feasible during oral processing of oil/fat. Findings from this work could provide new scientific insight into our understanding of the oral behaviour of free oil/fat and their sensory fat perception.