| Although a lot of work has been carried out on the effect of ultrasound treatment on whey proteins and its ability to enhance the structural,functional and chemical properties,very little is known about the digestion behaviour of the ultrasound treated proteins in solid food systems.Again,few studies have studied the effect of soy-whey matrix and filler on the gel properties.This study sought to observe the structural properties and behaviour of ultrasound pre-treated whey protein emulsion gels treated at different frequencies and power levels under simulated digestion.More so,soy-whey protein prepared at different ratios and protein concentrations were used to study the role of plant-animal protein filler and matrix on gel properties.Structural properties such as texture profile,water holding,swelling capacity,rheology,gel solubility and microstructure were evaluated.Analysis of simulated digesta was done using SDS-PAGE,free amino acid and released peptide concentration.The first study focused on the effect of ultrasound frequency mode(mono,dual,and trifrequency)and power(0-300 W)on whey protein’s structural properties(intrinsic fluorescence and sulfhydryl content)was studied.The heat-set emulsion gels were made from ultrasoundmodified whey protein after which the whey protein emulsion gel(WPEG)properties and digestion pattern were investigated.Ultrasound pretreatment by the mono and dual frequency within power range of 180–240 W enhanced the textural and rheological properties of WPEG,while those treated with triple-frequency saw a decline beyond 180 W.The greatest increase in hardness and storage modulus was observed in the WPEG treated with dual frequency ultrasound at 240 W,which was 3.07 and 1.41 times greater than the control.From the results of the confocal laser scanning microscope(CLSM),the microstructure of WPEG prepared using DFU exhibited a homogeneous and denser network compared to the control.Due to the modifications induced by the ultrasound treatment in the structure and microstructure of the gels,protein hydrolysis was delayed during the first 30 mins of gastric digestion in the dual-frequency ultrasound pretreated gel,in contrast to control.As opposed to the control,the WPEG produced by ultrasonic pretreatment had a higher rate of release of free amino groups throughout the intestinal phase.The results showed that modifying the structure of whey protein with dual frequency ultrasound was more efficient at improving the gel properties of WPEG,and ultrasound power should be considered in the application of ultrasound pretreatment for producing protein gels.The fine network of WPEG prepared with whey protein pretreated by ultrasound resulted in better hardness and storage modulus.Partial unfolding of the protein induced by ultrasound pretreatment might make the whey protein more susceptible to the digestive enzyme.Our results could provide new insights for using ultrasound as the potential processing tool on designing specific protein emulsion gels as the delivery system for nutrients.The next chapter studied the effect of sonicated mixed protein ratios at the interface on the mechanical,microstructure and digestion kinetics of emulsion-filled whey protein gels.The filler or interfacial composition were mixed at different soy(SPI)to whey(WPI)ratios(SW)of 10:0,8:2,6:4,4:6,2:8 and 0:10.The fillers were dissolved in whey protein solution,and heated to form emulsion-filled gels.The ratio of plant and animal protein at the interface during emulsion formulation affected the mechanical and microstructural properties of emulsion-filled whey protein gels.Increasing whey protein ratio increased zeta potential,gel hardness and storage modulus(highest at SW 0/10)while decreasing swelling capacity,demonstrating the active role of whey protein in strengthening the network of the gels.On the microstructure,with increasing SPI ratios,smaller flocculation and coalescence were observed on the surface of the gels compared to the uniform particle sizes and distribution in high WPI filled gels(SW 0:10;2:8).Subjecting the acid and base consumption during simulated digestion into a biphasic model,the results showed that increasing SPI facilitated pepsin diffusion and reaction rate.At the end of the intestinal phase,SW 0:10 had the lowest reaction rate,suggesting the high resistance of this gel to digestion.The results indicate that mixing soy and whey protein fillers at different ratios can provide emulsion-filled gels with improved and diverse properties that can be applied in various food applications.In the third experiment,the effect of mixed soy and whey protein matrices on gel properties and digestion kinetics was investigated.Different matrix protein concentrations(8-14%)with a composite SW ratio of 5:5 was screened using gel hardness and storage modulus.The better performing gel(13%)was selected for matrix composition studies.Soy-whey(SW)composite protein(13%)mixed at different ratios(SW 0/10,3/7,5/5,7/3,10/0)were dispersed into another soy-whey(SW 6:4)composite emulsion and gelled thermally.Hardness decreased with increasing SPI ratio(p < 0.05),the highest been found at SW 0:10 and the weakest at 10:0.The ratio of SPI and WPI had no effect on the water holding capacity(p > 0.05).For swelling ability,apart from SW 10:0,which had a capacity of 60%,all the other gels had less than 10% ability,which levelled after 4 hours.Increasing SPI ratio increased disulfide bond and reduced hydrogen bonds within the gel network.The microstructure displayed a loose gel network with increasing SPI as the number of aggregates increased.Pepsin diffusion was faster in SW10:0 and the least in SW 0:10 though it had a higher HCl consumption.The hard and compact nature of this gel limited the accessibility of the pepsin to cleavage sites.Mixing soy and whey at different matrix ratios could be used to enhance the textural,rheological and consequently,the digestion kinetics of mixed plant-animal gel systems.The study concludes that multi-frequency ultrasound and power enhanced the structural and gel properties of whey emulsion gels.The use of plant and animal protein mixtures as emulsified droplets(filler)and as matrix for emulsion filled soy-whey gels produced different effects on the gel properties.Thus,using plant-animal protein filler and matrix at different ratios,and varying the protein concentrations can be considered for its application in the food industry as the world moves towards more sustainable production and raw material acquisition for food security and environmental friendly production. |
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