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Effect of Salt Addition on the Micellar Composition of Milk Subjected to pH Reversible CO₂ Acidification

Laboratoire de Génie Biologique et Sciences des Aliments, Université Montpellier II, 34095 Montpellier Cedex 5, France

Corresponding author:
S. Marchesseau; e-mail:
marchesseau{at}arpb.univ-montp2.fr.

Response surface methodology was used to investigate the effect of salt supplementation on the micellar composition of reconstituted skim milk subjected to acidification by CO₂ pressure to pH 5.8, followed by depressurization under vacuum. Using a Doehlert design, calcium and phosphate were added to skim milk in the range of 0 to 25 mmol/kg and 0 to 16 mmol/kg of milk, respectively, and the pH was adjusted to 6.65 ± 0.02. After carbonation, the milk sample was depressurized, and the pH returned to its initial value without modification of the ionic strength. Micellar composition was assessed by the concentration of micellar Ca, P, Mg, and protein, and the buffering properties of milk. The second order polynomial models satisfactorily predicted the effect of salt supplementation on the micellar composition (R²_adj > 0.75). Added calcium was the most determinant factor, and favored the removal of Ca, P, Mg, and proteins from the soluble phase to the micellar phase when this addition was less than 17.5 mmol/kg of milk. Above this concentration, only the concentration of micellar Ca increased. The buffering response surface showed that the amount of micellar calcium phosphate increased to a maximum upon addition of 17.5 mmol of Ca/kg. By comparison with a control sample (supplemented but untreated skim milk), changes were essentially due to salt supplementation and not to the CO₂ treatment. We suggest that Ca formed micellar calcium phosphate when added at a concentration less than 17.5 mmol/kg; whereas above this concentration, Ca bound directly to micellar proteins.

Abbreviation key: BCA = bicinchoninic acid, MSD = mean standard deviation, RMSE = root mean square error, RSM = response surface methodology

Key Words: salt supplementation • CO₂ acidification milk • micellar mineral

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