Abstract
Proteins in whey were denatured by a heat treatment of 5min at 95°C. Near isoelectric pH, protein solubility dropped markedly but not at neutral pH unless unmasked calcium concentration exceeded a threshold, itself pH dependent. Protein and lactose contents had little influence on insolubilization.Calcium in whey was masked by citrate and phosphate. Phosphoserine, phytate, and ethylenediaminetetraacetate also masked calcium. It was possible to obtain the same pH-insolubilization curve from whey and from demineralized whey mixed with whey ions (calcium 8mM + magnesium 3mM + citrate 8mM with or without phosphate 14mM).Unmasked calcium was calculated in a first approximation asCalcium + magnesium – citrate (in mM) at pH 6.0Calcium + magnesium – citrate – phosphate (in mM) at pH 8.0Although the stoichiometries are not always equimolar, these approximations were satisfactory in practice to predict protein solubility in demineralized whey (with or without recalcification), in unconcentrated and concentrated whey, and in ultrafiltration whey protein concentrates. The unmasked calcium of whey protein concentrate or dialyzed whey protein is highly dependent on conditions used in ultrafiltration or dialysis, and particularly on pH.The use of Rowland's noncasein nitrogen test for denatured whey protein measurement is only valid under restricted conditions. A model is proposed.
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