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

Beer is one of the most popular drinks in the world. Foam is a unique characteristic of beer, contributing to mouth
feelings and visual perceptions. Therefore, it would be worthwhile to study the solution and interfacial properties of
beer 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 based
on 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 malted
barley and iso-humulone isomerized from hops during boiling. Barley malt was mashed and boiled as brewing
process. Hops were boiled in buffer and iso-humulone was extracted. The solution properties were investigated
through AF4, DLS, cryo-TEM, and microscopic characterisation. The interfacial properties were investigated by using
surface techniques, e.g. ellipsometry, tensiometry.
The results showed iso-humulone itself formed nano-particle and bound with protein to form aggregate, primarily with
protein Z and, to a lesser extent, with LTP. Iso-humulone condensed at interfaces, and increased by wort salt. Wort
protein formed a monolayer with a salt screening effect. Iso-humulone showed reversible adsorption, while protein
showed irreversible adsorption. Sequential addition and pre-mixed solution showed different structures and
properties. 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 was
proposed 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 synergistic
effect was found on protein and iso-humulone mixture. After LTP was fractionated from wort, it was found that the
layer of protein Z and iso-humulone was stiffer than that of LTP with iso-humulone, which might be due to stronger
interaction between protein Z and iso-humulone.