Effect of ultrasound and high pressure processes on synthetic casein micelles and their use as model acid-milk gels

Abstract

The caseins, as a major protein in milk, have a great influence on milk character. To understand the structure of the casein micelle and improve the quality of the milk, the caseins were used to synthesize the casein micelle, which called synthetic casein micelles (SCMs). This study will show three aspects related to the SCMs, which is the preparation of SCMs; the stability of SCMs under the hydrostatic pressure processing (HHP) and ultrasound (US) treatment; as well as the gelation profile of the SCMs Previous studies were mainly concentrated on the ratio of αs-, β-, and κ-casein in the micelle formation at 37°C. In this study, the effect of formation temperature and additives (whey protein and lactose) were examined. The particle size which was measured by dynamic light scattering (DLS) showed the relation between the size and temperature, which can be described as the higher temperature, the larger size from 5°C to 45°C. The addition of whey protein doesn't affect the size formation, while the micelle size has a little increase when lactose presented at 37°C. The result of calcium and phosphate content in micelle showed no difference from each other. HHP and US treatments are commonly used in sterilization and hormalization respectively.In this study, the SCMs were examined at HHP up to 500 MPa for 10 minutes. The results showed the SCMs were disrupted at the pressure in a range of 200 MPa to 300 MPa, which has a similar result to the micelle in milk. The electronic micrographs showed the size and structure of the micelle became smaller and more irregular, while the tubular structure in the casein surface seems to disappear after HHP treatment, but CCP was still present inside of those fragments. The US treatment had little effect on the SCMs. After 30 minutes sonication, the size only decreased about 13 nm. The electronic micrographs showed that more small fragments were presented in the solution, while the tubular structure in the surface of the micelle became less after the US. As the whey protein didn't affect the SCMs formation, the profile of acid SCMs gel was investigated by heating 90°C for 30 minutes. The particle size had increased when the percentage of whey protein was higher than 10%. The rheometer data showed that the gel's strength increased with the whey protein percentage going up, which suggested that the extra whey protein addition under the heat condition could enhance the acid SCMs gel formation.