Changes in Milk Protein Functionality at Low Temperatures and Rennet Concentrations

Abstract

This study aimed to evaluate the influence of low-concentration rennet on the chemical, rheological characteristics, and protein fractions of skim milk (SM) at 4 ± 1 °C. Skimmed milk (SM) was divided into four lots of 500 mL, and diluted rennet (1:10,000) was added at different levels at 4 ± 1 °C. The treatments included control (no rennet), T1 (0.001 mL/rennet), T2 (0.01 mL rennet), and T3 (0.1 mL rennet) treatments, which were incubated for 24 h. The sampling was performed at 0, 1, 2, 6, 12, and 24 h, and the SM after incubation time was heated to 73 °C/16 s to denature the rennet enzyme. Skim milk samples (SMS) (control and rennet-added samples) were evaluated for proximate composition, capillary gel electrophoresis (CGE), hydrodynamic diameter, zeta potential, and rheology at 0, 1, 2, 6, 12, and 24 h. Foaming ability, foaming stability, water-holding capacity (WHC), oil emulsifying activity (OEA), and emulsion stability (ES) were performed at 0, 12, and 24 h of incubation time. There was a significant (p < 0.05) increase in non-proteins by 0.50% and in non-casein nitrogen by 0.81% as incubation progressed. The results showed that aggregation or curd was not formed during storage time. The CGE data indicated that increasing the rennet concentration had a significant (p < 0.05) effect on decreasing κ-CN, and breakdown increased at higher levels of rennet usage. There was a significant (p < 0.05) increase in the hydrodynamic diameter and a decrease in the zeta potential values in rennet-added samples at the end of the incubation time (24 h). The rheological results showed no changes in the storage modulus (G'), loss modulus (G″), or viscosity values. Increasing the rennet amount and storage time led to a significant (p < 0.05) decrease in the foaming ability and foaming stability and a significant (p < 0.05) increase in the oil emulsifying activity and emulsion stability of rennet-added SMS. This study concluded that milk protein functionality can be changed without aggregating or curd formation, and rennet milk can be processed.

Keywords: low temperature; milk protein functionality; rennet; skim milk.

Figure 1

Capillary gel electrophoretogram of skim milk sample include the following: (a) control during 24 h of incubation; T1, T2, and T3 during at 0, 1, 2, and 6 h of incubation; (b) T1 at 12 h of incubation; (c) T1 at 24 h of incubation; (d) T2 at 12 h of incubation, (e) T2 at 24 h of incubation; (f) T3 at 12 h of incubation; and (g) T3 at 24 h of incubation. AU = absorbance units.

Figure 2

κ-CN breakdown rate % of skim milk sample (T1, T2, and T3) at 12 and 24 h of incubation. T1 = milk with rennet (0.001 mL/100 mL of milk); T2 = milk with rennet (0.01 mL/100 mL of milk); and T3 = milk with rennet (0.1 mL/100 mL of milk). Bars not sharing common letters are significantly different (p < 0.05).

Figure 3

Loss modulus (Pa) of skim milk samples (control and rennet-added samples) at different incubation times ((A): 0 h, (B): 1 h, (C): 2 h, (D): 6 h, (E): 12 h, and (F): 24 h).

Figure 4

Storage modulus (Pa) of skim milk samples (control and rennet-added samples) at different incubation times ((A): 0 h, (B): 1 h, (C): 2 h, (D): 6 h, (E): 12 h, and (F): 24 h).

Figure 5

Viscosity (mPa·s) of skim milk samples (control and rennet-added samples) at different incubation times ((A): 0 h, (B): 1 h, (C): 2 h, (D): 6 h, (E): 12 h, and (F): 24 h).

Figure 6

Foam ability of skim milk samples (control and rennet-added samples) at different incubation times. (A): H0, representing the initial height of the skim milk samples before conducting the foaming tests. (B–D): H1, indicating the height of skim milk samples after performing foaming tests at 0 h, 12, and 24 h of incubation time, respectively.