Effect of Transglutaminase Post-Treatment on the Stability and Swelling Behavior of Casein Micro-Particles

Abstract

Casein microparticles are produced by flocculation of casein micelles due to volume exclusion of pectin and subsequent stabilization by film drying. Transglutaminase post-treatment alters their stability, swelling behavior, and internal structure. Untreated particles sediment due to their size and disintegrate completely after the addition of sodium dodecyl sulfate. The fact that transglutaminase-treated microparticles only sediment at comparable rates under these conditions shows that their structural integrity is not lost due to the detergent. Transglutaminase-treated particles reach an equilibrium final size after swelling instead of decomposing completely. By choosing long treatment times, swelling can also be completely suppressed as experiments at pH 11 show. In addition, deswelling effects also occur within the swelling curves, which enhance with increasing transglutaminase treatment time and are ascribed to the elastic network of cross-linked caseins. We propose a structural model for transglutaminase-treated microparticles consisting of a core of uncross-linked and a shell of cross-linked caseins. A dynamic model describes all swelling curves by considering both casein fractions in parallel. The characteristic correlation length of the internal structure of swollen casein microparticles is pH-independent and decreases with increasing transglutaminase treatment time, as observed also for the equilibrium swelling value of uncross-linked caseins.

Influence of Post‐Treatment Temperature on the Stability and Swelling Behavior of Casein Microparticles

“…A parallel model approach, which assumes that a cross‐linked casein fraction restricts the swelling of noncross‐linked caseins in the core by forming an elastic network, can be used to describe all swelling curves of CMPs subsequently cross‐linked with TGase. [ 37 ] Complex swelling kinetics with overshoot has been described previously for other polymer systems, with the effect attributed to the ionic contribution. [ 38,39,40 ] Subsequent heating could also affect the stability and swelling of supramolecular casein structures, such as CMPs, which are held together largely by temperature‐dependent hydrophobic interactions.…”

“…It was accounted for this by introducing the maximum swelling degree V ∞, and second‐order swelling kinetics originally used to describe the swelling of gelatin and cellulose in an aqueous medium [ 48 ] but also successfully used recently to simulate swelling kinetics of casein fibers, and CMPs treated with TGase. [40, 37 ] As a result, the rate coefficients of the swelling steps are no longer comparable and are now denoted as RC Ia* and RC Ib*. The rate of volume change for volume I within the parallel swelling model is $$$$\begin{equation}\frac{{d\;{\rm{volume\;}}{I_t}}}{{dt}} = \mathop \sum \limits_{i = a,b} {\rm{RCI}}\_{i^*} \cdot \sigma _t^i \cdot {\left( {{V_\infty } - {\rm{volume}}\;{I_t}} \right)^2}\end{equation}$$$$…”

“…[ 20 ] Our studies showed that CMPs without cross‐links completely decomposed after adding SDS, while CMPs treated with TGase remained stable due to additionally introduced isopeptide bonds. [ 37 ] The stabilization also results in CMPs expanding to an equilibrium state rather than entirely decomposing during swelling. As a new feature in the swelling curves, overshooting occurs during rapid swelling at pH 14, which is more pronounced the longer the treatment time with the crosslinking enzyme.…”

Citrate effect on the swelling behaviour and stability of casein microparticles

“…Casein films cross-linked with high methacrylated casein also showed stronger swelling in alkaline medium compared to those in acidic media 31 . In addition, the swelling kinetics also showed overshooting, which can also be observed with CMPs, but only if covalent cross-links with transglutaminase were subsequently inserted 32 . As the pH in the human stomach is highly acidic and near neutral in the duodenum 33 , the pH-dependent stability and swelling properties of CMPs can be favorable for the controlled release of the orally supplemented bioactive compounds.…”

Nearly Reversible Expansion and Shrinkage of Casein Microparticles Triggered by Extreme pH Changes

“…The expansion process can be suppressed or increased by increasing the free calcium concentration in the medium or by removing colloidal calcium phosphate by pre-treatment with the calcium chelator citrate [ 24 , 25 ]. A temperature post-treatment or an enzymatic cross-linking with the enzyme transglutaminase stabilizes the microparticles, so that they swell to an equilibrium final value, instead of disintegrating [ 26 , 27 ].…”