Effects of mineral salts and calcium chelating agents on the functionalities of milk protein concentrate prepared by ultrafiltration
Functionality of milk protein concentrates can be tailored by modifying state of casein micelles through manipulation of processing conditions including temperature, pH and/or addition of calcium chelators. The objective of this study was to investigate the effect of calcium and calcium chelating agents (EDTA and citrate) on the performance of membrane ultrafiltration (UF) process and the functionalities of resulting milk protein concentrates (MPC). Skim milk adjusted to pH 5.9 was pre-treated with EDTA or citric acid (10, 20 or 30 mmol) and ultrafiltered using a polyethersulfone (PES) membrane at 15°C to five times concentration factor. The membrane performance was measured by the permeate flux during UF process. Used membranes were examined using scanning electron microscopy (SEM). The MPC samples were freeze dried and powders were assessed for physical functionalities including solubility, heat stability and emulsification.
Addition of chilators led to a shift in a protein-mineral equilibrium and calcium dissociation from the casein micelle. The total calcium in the final MPC was reduced (p<0.05) from 191 (control) to 131 mM or 135 mM for skim milk pre-treated with 30 mmol of EDTA or citrate, respectively. The casein micelle particle size was subsequently reduced (p<0.05) from 200 nm (control) to 28 nm or 24 nm for the milk pre-treated with EDTA or citrate at concentrations equal to or greater than 20 mmol. Consequently, solubility of the MPC increased (p<0.05) from 92% (control) to 98% (ETDA, ≥20 mmol) or 98.9% (citrate, 30 mmol); heat stability was also enhanced (p<0.05) from 78% (control) to 83% (EDTA, 20 mmol) or 87% (citrate, ≥20 mmol). The emulsion capacity has increased from 1170 (control) to 1392 or 1459 (g oil/ g protein) (p<0.05) when 30 mmol of EDTA or citrate were added, respectively. Addition of EDTA or citrate hindered the membrane performance as observed by reduced permeate flux from 10.5 kg/h.m2 (control) to 7.9 kg/h.m2 (EDTA, ≥20 mmol) and 8.6 kg/h.m2(citrate, ≥20 mmol) at the start of UF. Consequently UF processing time increased from 5 h (control) to 7 h (EDTA) or 6 h (citrate).
This work has provided new insights into the relationship between calcium, calcium chelators and their influence on the casein micelle size and the physico-chemical properties of MPC produced using UF, and also demonstrated the potential of using EDTA and citrate acid to manipulate MPC product functionality using UF.
Keywords: Milk Protein Concentrate (MPC), Functionality, Ultrafiltration (UF), Membrane, Calcium Chelator, Casein Micelle.