Abstract: The objective of this work was to explore the changes in the particle size of casein micelles, transmittance and protein solubility in skim milk induced by high pressure combined with mild heat treatment (≤ 50 ℃). Fresh skim milk was treated for 10–30 min at different temperatures (room temperature, 30, 40 and 50 ℃) and pressures (0.1–700 MPa). The particle size was measured by a Marvin laser nanometer, the transmittance was detected by spectrophotometry, and the content of soluble protein was determined by the Coomassie brilliant blue G250 method. Results showed that the transmittance of skim milk treated at pressure levels not exceeding 100 MPa was not affected by temperature, pressure and treatment time. The transmittance of skim milk treated at room temperature and in the pressure range of 200–700 MPa increased with the increase in pressure and pressure-holding time, and the maximum percentage increase of 1 011% was observed when skim milk was treated at 700 MPa for 20 min. Upon treatment at 30–50 ℃ and in the pressure range of 200 to 700 MPa, the transmittance first increased and then decreased with the increase in pressure, and the maximum increase of 537% occurred upon treatment at 500 MPa and 40 ℃ for 10 min. The transmittance was affected by temperature and pressure-holding time, and was higher than that of the untreated control. The median particle size (Dx(50)) of skim milk treated at room temperature decreased with the increase in pressure from 0.1 to 400 MPa, and then remained stable in the range of 400–700 MPa, which was smaller than that of the untreated control. Upon treatment at 30–50 ℃ and in the pressure range of 0.1–700 MPa, the Dx(50) showed a first increasing and then decreasing trend with the increase in pressure, and remained stable in the range of 400–700, 300–500 and 200–400 MPa, which was only slightly affected by treatment time. The content of soluble milk protein (S-Pro) in skim milk treated by high pressure showed an increasing trend, and it was affected by pressure, time and temperature. After treatment at 30 ℃ and 500 MPa for 30 min, the maximum percentage increase in the S-Pro content of 83.55% was obtained. The content of soluble protein at pH 4.6 (S-Pro-pH 4.6) was not affected by temperature, pressure and time when the pressure was low than 100 MPa. For each temperature, S-Pro-pH 4.6 content decreased with the increase in pressure in the range of 200–700 MPa and treatment time. The correlation analysis showed that the transmittance was correlated with Dx(50), and this correlation was weaker with increasing temperature. The transmittance was positively correlated with S-Pro content and negatively correlated with S-Pro-pH4.6 content. There was a negative correlation between S-Pro and S-Pro-pH 4.6 contents. Therefore, ultra-high pressure combined with mild heat treatment can cause changes in the transmittance, particle size and protein solubility of skim milk, and these changes are correlated with each other to some extent.

Key words: ultra-high pressure; mild heat; skim milk; transmittance; particle size; protein solubility; correlation