Characterisation and improvement of the rehydration behaviour of high-protein milk powder

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Wu, Shaozong
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University College Cork
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Milk protein isolate (MPI) has poor rehydration behaviours, particular in wetting and dissolution. The rehydration difficulty inhibited the achievement of MPI functionality. Methods were explored to solve rehydration difficulty. Agglomeration was a valid method for enhancing the wetting of dairy powder. Moreover, broadband acoustic resonance dissolution spectroscopy (BARDS) monitored gas release from powder through the alteration of speed of sound in solution. Thus, BARDS was utilised to study the gas release behaviour of agglomerated MPI, and MPI coated with emulsifiers. Meanwhile, as MPI composed mainly by casein, pH of solution affected the status of casein which possibility modify the rehydration process of MPI as well. Real-time measurement of particle size was applied to analyse the effect of pH on rehydrating MPI. Sodium carbonate, as an alkali food additive, was firstly studied at rehydrating dairy powder. However, alkalization of MPI followed by neutralization has been thoroughly investigated. Consequently, considering the effect of calcium chelation on MPI, citric acid was utilised to neutralise MPI from alkalization. Finally, the gelation behaviour of MPI was studied in a different sequence of neutralisation. Our results showed that as the porous structure modified the gas distribution in agglomerated MPI, interstitial air within agglomerates and vacuole air within agglomerates determined the gas release behaviour of agglomerated powder, while the vacuole air in primary powder particle became a crucial barrier for MPI dissolution. The BARDS profile showed that there was faster initial gas release from the agglomerates, indicating better wetting and dispersion ability of the agglomerates. The gas release of MPI during the wetting step was facilitated by lecithin and Tween 80 while the dissolution step was still poor. The wetting behaviour of MPI covered by the emulsifiers was greatly improved except for samples coated with 4% lecithin. The most instant wetting behaviour was observed when coating MPI with Tween at the larger size fractions. The swelling of the primary particle was observed in alkalization, which was ascribed to the swelling of casein micelle aggregates. This swelling of casein micelle aggregates indicates greater water penetration and loosening of the aggregate structure at higher pH, which facilitated the faster dissolution of MPI powder, especially at pH 8.4. As alkalization proved an excellent ability at the dissolution of MPI, sodium carbonate was utilised, which also showed improvement of MPI rehydration. Sodium carbonate was supposed as a calcium chelation as well to modifying the structure of casein micelle. The results showed that wettability worsened with higher sodium carbonate concentrations, due to strong films developing at the powder/water interfaces which inhibited water penetration into the MPI. However, appropriate agitation could effectively wet and disperse MPI. On the other hand, particle size and centrifugal sedimentation studies showed that increasing the sodium carbonate concentration greatly improved the dissolution ability of the MPI powder. The subsequent neutralisation process by citric acid enhanced the effects of calcium chelation due to reaction-product citrate. Besides, alkalization and neutralisation created larger colloidal particle size, which is hypothesised as being a dynamic balance between micellar casein and non-micellar casein. The sequence of neutralisation determined the process of rehydration, which in turn determine the gelation process. As for the gelation time and gel stiffness, performing alkalization firstly before neutralisation had a better result corresponding with increasing mobile casein molecule by improved rehydration, exhibiting homogeneous gel structure.
Dairy powder , Rehydration , pH , Gas , Casein micelle , Post drying
Wu, S. 2020. Characterisation and improvement of the rehydration behaviour of high-protein milk powder. PhD Thesis, University College Cork.