On the interaction between wort protein and iso-humulone : A study of surface rheology, interfacial structures and particle formation with the relevance to beer foam

Sammanfattning: Beer is one of the most popular drinks in the world. Foam is a unique characteristic of beer, contributing to mouthfeelings and visual perceptions. Therefore, it would be worthwhile to study the solution and interfacial properties ofbeer and assess them with regards to foam.The main aim of this thesis was to evaluate the main foam-promoting components of beer in terms of solution,interfacial properties and their interactions with regards to the properties of foam. The material was prepared basedon basic brewing steps for a representative model.The amphiphilic components are mainly boiled protein (mainly LTP of 9.7 kDa and protein Z of 43 kDa) from maltedbarley and iso-humulone isomerized from hops during boiling. Barley malt was mashed and boiled as brewingprocess. Hops were boiled in buffer and iso-humulone was extracted. The solution properties were investigatedthrough AF4, DLS, cryo-TEM, and microscopic characterisation. The interfacial properties were investigated by usingsurface techniques, e.g. ellipsometry, tensiometry.The results showed iso-humulone itself formed nano-particle and bound with protein to form aggregate, primarily withprotein Z and, to a lesser extent, with LTP. Iso-humulone condensed at interfaces, and increased by wort salt. Wortprotein formed a monolayer with a salt screening effect. Iso-humulone showed reversible adsorption, while proteinshowed irreversible adsorption. Sequential addition and pre-mixed solution showed different structures andproperties. Their mixture had a synergistic effect on lowering surface tension and increasing dilatational modulus,indicating that the combination of both produced a stiff film. The protein layer consisted of both LTP and protein Z.The mixture of LTP and iso-humulone exhibited a lower dilatational modulus than individual solutions. Thus, it wasproposed that the stiff film originated from the interaction of protein Z and iso-humulone.In conclusion, there was an interaction between protein and iso-humulone at interface and in solution. A synergisticeffect was found on protein and iso-humulone mixture. After LTP was fractionated from wort, it was found that thelayer of protein Z and iso-humulone was stiffer than that of LTP with iso-humulone, which might be due to strongerinteraction between protein Z and iso-humulone.