| Caprine milk has poor rheological properties,which usually result in low cheese yield,as well as economic loss to cheesemakers.Several methods had been used to improve its coagulation properties.However,high hydrostatic pressure(HHP)had been rarely used.The effects of high-pressure(HP)treatment(200-500 MPa for 25 min at 25°C)combined with storage temperature(25 and 4°C)on the physicochemical properties of raw caprine milk were studied.Storage of HP-treated and untreated milk samples at 25°C considerably affected the changes in the conformation of milk proteins,which were reflected by changes in the protein sedimentation rate,gradual decreases in the soluble calcium and phosphorus contents,a slight decrease in pH,an insignificant decrease(P > 0.05)in viscosity,and a decrease in the casein hydration level of milk at the end of the storage time.In contrast,the HP-treated and untreated milk samples stored at 4°C demonstrated different characteristics than the samples stored at 25°C.These results could be due to calcium and phosphate association with caseins,which screen charges and reduce the repulsion of micelles during the storage time.With the aim of improving the milk coagulation properties,this study assessed the effects of HP treatment on caprine milk before rennet-induced coagulation.Additionally,the rheological,textural,microstructural characteristics and water-holding capacity of the curds were evaluated as a function of pressure(300,400 and 500 MPa),holding time(20,40 and 60 min),and temperature(20,40 and 60°C)using a response surface methodology.The Box-Behnken design was used to investigate the effect of process variables.The results showed that with an increase in pressure level or holding time,or a decrease in temperature,the rennet coagulation time showed a decreasing trend while curd firming rate and coagulum strength showed the opposite.Also,an increased pressure level,temperature and time resulted in an increase in water-holding capacity and a subsequent decrease in the gel-firmness of the curds.The trend of the results was confirmed by Scanning electron micrographs of the curds.HP could therefore be an alternative technology for improving the coagulation properties of caprine milk.The effect of HP treatment on the physicochemical,structural,and rheological characteristics of microfiltration retentate(MFR)caprine milk prior to spray drying was investigated.In addition,the functional and microstructure properties of micellar casein concentrate(MCC)were determined.Microfiltered/diafiltered milk protein retentates was treated at(25?2)°C for 15 min with a different level of pressure(100-500 MPa),and then spray dried.The results showed that the turbidity and casein size decreased significantly while pH,soluble calcium and phosphorus significantly increased with an increase in pressure level.Further,the structural characteristics of MFR for all samples were studied.There was an increase in hydrophobicity of HP-treated samples compared with the untreated.In contrast,the G`max and yield stress values of the HP-treated MFR gels decreased.The MFR sample treated with HP at 100 MPa had the highest average of the G`max(99.75 Pa),and the lowest value of rennet coagulation time(25.9 min).Moreover,the solubility,emulsifying and foamability of MCC samples increased significantly with HP-treated samples compared to control.Maximum percentages of solubility(80.3),foam capacity(47)and foam stability(79.8)were observed at MCC-400 MPa.However,MCC-400 MPa sample showed the lowest average turbiscan stability index at 1 and 24 h(4.9 and 7.3,respectively).Thus,HP pre-treatment could be used as a good alternative technology to promote MCC products' application in the food industry. |
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