The effect of transglutaminase on reconstituted skim milks at alkaline pH
2018
Lam, Elisa | McKinnon, Ian | Marchesseau, Sylvie | Otter, Don | Zhou, Peng | Hemar, Yacine
The effect of transglutaminase (TG) treatment on the changes in the physico-chemical properties of reconstituted skim milk (10% w/w) under alkaline conditions (pH ∼6.7 to ∼10) was investigated. The casein micelle hydrodynamic radius, the distribution of proteins between the serum and micelles, and the milk viscosity differed between milk treated with and without TG under alkalinisation. When milks were alkalinised from pH ∼6.7 to ∼8, both TG-untreated and TG-treated casein micelles swelled due to the increase in electrostatic repulsions between the negatively charged casein molecules. In milk without TG, the swelling of the micelles resulted in their progressive dissociation, while in the presence of TG-induced crosslinks, most caseins were maintained in their micellar form. When the pH increased from ∼8 to ∼10, TG-treated casein micelles resisted complete micellar disruption and continued to swell, whereas TG-untreated casein micelles disintegrated into smaller particles, with a marked increase in the soluble casein concentration. Under alkalinisation, only the α-lactalbumin present in the serum of TG treated milk was resistant to denaturation and aggregation. In response to all these micellar and serum changes, the viscosity increased for both TG untreated and treated milks. A simple qualitative model taking into account the extent of casein micelle dissociation and the size of the casein micelles was proposed to describe the behaviour of the viscosity as a function of pH. TG crosslinking of skim milk did not affect the partition of calcium, inorganic phosphorus, magnesium and potassium; and milk alkalinisation did not result in new calcium-phosphate phases.
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