Microbial transglutaminase (mTGase) is an acyltransferase that predominantly catalyses the formation of covalent cross-links between protein-bound glutamine and lysine residues, referred to as isopeptide bonds. This typically results in protein polymerisation.
The enzymatic polymerisation of caseins, the major protein fraction in milk, has been studied for decades because of its potential to modify physical properties of fermented dairy products. It was suggested that cross-linked caseins form denser gel networks, resulting in higher gel stiffness and increased water holding capacity. However, other studies indicated that there is an optimal cross-linking degree and that prolonged incubation with mTGase results in converse effects.
The aim of this research was to elucidate the mechanisms that cause these alterations of the gelation properties. Using non-micellar casein preparations at 27 g·kg-1 protein as model systems, structure-function-interrelationships were studied by molecular characterisation in combination with rheological studies of acid-induced gels.
The results suggested that casein molecules self-associate in aqueous solutions and that cross-linking occurs predominantly within distinct casein particles. These cross-links contributed directly to the stiffness of acid-induced gels as indicated by an increased maximum storage modulus. However, in the presence of ions, introduced either prior to or after cross-linking, the highest value was shifted to shorter incubation times. This was attributed to an increased inflexibility of the casein particles with ongoing internal cross-linking, which made them incapable of conformational changes to compensate for the screening of attractive electrostatic interactions through other non-covalent interactions.
The findings provide important information for the direct application of mTGase in milk as well as on the utilisation of cross-linked caseinates as additives in food processing. Further studies should be conducted at casein concentrations below self-association as well as above close packing of casein particles to include other cross-linking mechanisms. Moreover, potential applications in the non-food sector should be ascertained.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:36163 |
| Date | 15 November 2019 |
| Creators | Raak, Norbert |
| Contributors | Rohm, Harald, Corredig, Milena, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | 10.1016/j.idairyj.2016.10.015, 10.1007/s11483-017-9483-6, 10.3390/separations5010014, 10.1007/s11483-019-09601-2, 10.1016/j.fbio.2019.01.016, 10.1016/j.foodhyd.2018.01.037 |
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