Study On The Coagulation Properties Of Bovine Milk And The Quality Of Fermented Milk Products

The experiment used rheology, texture science, microstructure, near infrared spectroscopy, sensory evaluation and other evaluation methods to study factors that affect acid and rennet coagulation properties and relationships between coagulation properties and the qualities of Cheddar cheese and yogurt. The main results are as follows:（1） Differences in raw milk, such as somatic cell count, lactation, content of calcium and phosphate salts and additional calcium amount all had effects on coagulation properties. Mid-lactation fresh milk which had low somatic cell count with a ratio of calcium and phosphorus content between 1-1.5 had higher rennet coagulation hardness and good capacity to hold water. Between the addition amount of calcium from 0 to 0.07%, water holding capacity first increased then decreased and rennet coagulation time decreased. The strongest curd occurred at 0.02%.（2）A method which improved from others to detect curd syneresis had good repeatability (1.236E-05min^-1) and reproducibility (2.249E-05min^-1). The amount of whey separation increased as the heating temperature became higher. Higher temperature processing caused low degree of curd syneresis. The value of syneresis constant rate k first increased and then decreased as initial milk pH decreased from 6.4 to 5.6. The lower the final pH of acid coagulation was, the greater change of whey separation amount caused by heat treatment was.（3） Among the measured milk samples, the higher the fat content was, the weaker the ability to holding water and the greater the viscosity was. Relationship between coagulation time and rate showed inverse function as y （coagulation time）= 17.236/x+22.508 （R= 0.72）, yield and protein content showed y （yield）= 2.855x-0.232 （R= 0.83）. In acid curd, if the capacity to water holding was weaker, whey will sooner precipitation from the curd. The correlation coefficient was 0.985;（4）The storage and loss modulus during rennet and acid coagulation can be predicted by near-infrared models, the best correlations were 0.97,0.99 and 0.98,0.99; The near-infrared models established by the initial state of the milk to rennet coagulation properties RTC, CRFmax, viscosity and protein content had correlations between 0.66-0.99. The near-infrared models established by dynamic 1670nm data to predict rennet coagulation time and maximum firming rate had correlation of 0.95;（5）Protein and fat content of raw milk can affect the yield, texture and flavor of Cheddar, the lipid was the main influencing volatile substance. When 4% amount of cream was added, electronic nose results showed the biggest difference. Among Cheddar cheese texture properties, hardness, cohesiveness, elasticity, viscosity and chewiness had various degrees of correlation. The higher the hardness and adhesiveness of the rennet curd were, the higher the hardness, the viscosity of Cheddar cheese were. The higher the adhesiveness was the better elasticity the cheese had. The scores of form and appearance from sensory evaluation of Cheddar were positively correlated with the elasticity, viscosity and hardness of Cheddar with coefficients of 0.4 above. The scores of form and appearance also significant related with a 0.642 correlation coefficient.（6）Acid curds showed different coagulation properties and electric nose results with the use of different ferment and whey protein powder. The capacity of yoghurt to hold water was the highest when additional content of whey protein was 1%, but the viscosity became lower with higher content of whey protein. The sensory lubrication level and the particle sensation of yoghurt and were significantly related with the capacity to hold water and whey separation time. The correlation coefficient of sensory viscosity and physical viscosity is 0.849.（7）High-intensity ultrasound treatment such as 1000W,180s can improve reconstituted milk acid coagulation properties.300s ultrasound treatment of milk increased the capacity to hold water to 50% and significantly improved the viscosity. Ultrasonic pretreatment can be possibly used for improving milk coagulation properties and quality of milk products.