The Emulsifying Properties Of Human Saliva And Implications To The Oral Behaviour And Sensory Perception Of Free Oil/Fat

Food oral processing is a very important step during food consumption as this is the first step of the food digestion process and more importantly,the only step where we perceive and develop a sensory preference of a food product.As a major food component,oil/fat has important influences on the structural and sensory properties of food products.While the presence of oil/fat in food is generally preferred by consumers,the controlling oral mechanisms of oil/fat sensation and perception is still little known.Therefore,the main aims of this Ph D project were to understand the behaviour of dietary oil/fat during oral processing and to reveal oral mechanisms of the sensation of this important food component.The role of saliva in oral behaviour,human fat perception and oral sensation of dietary free oil/fat was the main focus of this study.The thesis begins with a comprehensive review of literature on the basic facts surrounding oral processing of lipids,human saliva as a complex biological fluid,fat replacers and various theories of oil/fat perception(Chapter 1).Experimental works were conducted in three parts and results are reported accordingly in three major chapters.The first experimental study was on the determination of the dominating sensation cues of oil/fat sensation.Out of all sensory cues,texture and aroma were found to be the dominant ones in the oral sensation of oil/fat(Chapter 2).Results showed that judged by taste,texture and aroma cues rapeseed oil,lard and oleic acid were recognised as natural oil with 21,23 and 21 positive answers,respectively.These cues also correlated very strongly with finger tactile ones and aroma with Spearman's correlation being 0.811 and p-value narrowly missing significance with it being0.58.Silicone oil was judged as natural oil by taste and tactile cues with 23 positive answers indicating a strong texture aspect to the fat perception.As the main task of this study,saliva-oil/fat interactions were conducted both in vitro and in vivo involving a number of healthy individuals where interesting results and facts were obtained(Chapter 3).By gently mixing dietary oil/fat with saliva both in vitro and in vivo,we observed stable emulsion.In vitro emulsions showed the smallest particle size(d₃₂=28.8?m)for high mixing speeds(3000 rpm for 5%emulsion)and particle size(d₃₂=30.4?m)for even higher mixing speeds(4000 rpm for 10%emulsion).Also in situ emulsions showed the smallest particle size for 0.25 ml oil samples(d₃₂=44.2?m)and zeta potential values showing moderately stable emulsion(?=–20.7 m V).This emulsion,when made in the oral cavity,indicating that the oral physiology,tongue movements and saliva proteins and characteristics are responsible for saliva-oil emulsion making.This leads to a conclusion that human saliva can be an efficient emulsifier and oral emulsification could occur once free oil/fat is mixed with saliva under gentle oral processing conditions.Analysed saliva had flow rates ranging from1.23 to 0.34 g/min with p H values reaching values of 7.8 to 6.8.Results showed that in situ emulsions had particle sizes ranging from d₃₂=44.6?m to d₃₂=103?m,while zeta potential values were from?=–41.0 m V to?=–0.3 m V.Of course,the emulsification capability of human saliva varies greatly among the general population.This could lead to different states of the emulsified bulk oil/fat found inside the mouth among different individuals.For this reason,a hypothetical salivary emulsification threshold has been proposed,which could be the fundamental cause of the sensory variation of fat sensation.To further explore the underpinning mechanisms of oral emulsification,properties of salivary monolayer have been studied using quartz crystal microbalance with dissipation(QCM–D)technique and results were reported(Chapter 4).Quartz crystal microbalance with dissipation(QCM–D)is a technique where the properties,characteristics and dynamics of film formations are monitored and researched.In the experiments adsorption of different salivary components to a hydrophobic(non-polar)surface was observed and monitored.It was discovered that low weight molecular components adsorb to the surface before the high weight ones do.This drives the viscoelasticity of the salivary film formed for normal unstimulated saliva(D=15 ppm)compared to the rigid one(D=1.5)formed from the dialyzed saliva.This was confirmed when tensiometry measures showed unstimulated saliva to be more viscoelastic(E'=34.8;E''=7.8)than dialyzed one that was more rigid(E'=42.8;E''=5.2).Effects of p H,ionic strength and salt(Na Cl)on a formed salivary film adsorbed to a non-polar surface were also investigated.The“swelling”and“collapsing”of the adsorbed saliva films were observed in the several experimental setups.The usual salivary film thickness(d=25.0 nm)was swollen at p H5(d=65.0 nm),while ionic strength of 1 M induced similar changed(d=68.0 nm),0.5 M(d=38.0 nm)and 0.02 M inducing the collapsing of the layer(d=10.0 nm).The influence of salt(Na Cl)showed that when introducing salt concentrations from 0.1 to 0.001 M the film showed moderate“swelling”(d=22.5 nm to d=26.0 nm)with an electrostatic“collapse”when water was introduced(d=12.5 nm).A very different behaviour was noticed for salt concentrations from 0.001 to 0.1 M with the film showing“collapsing”(d=20.0 nm to d=11.5nm),with water further decreasing the layer(d=6.0 nm).Data reported here propels our knowledge and the complex relationship between saliva and human fat perception.It shows that human saliva can be a natural emulsifier during oral processing of oil/fat.Even though oral emulsification of oil/fat generally occurs to human subjects,the capability of oral emulsification varies due to the different properties and capacities of saliva among individuals.The oral physiology parameters could also play a role in the emulsification process.Results from this work provide new pieces of evidence and mechanisms for the oral behaviour of oil/fat.While implications of the current findings may require further studies,it is hoped that findings from this work will lead to new thinking about the controlling mechanisms of oral sensation and perception of oil/fat.