Studies of iso-alpha-acids: analysis, purification, and stability.

Abstract

The female cones of hop (Humulus lupulus L.) are added to beer, providing taste and flavour and contributing to the stability of foam. The main constituents of hop related to these properties are generically known as alpha-acids. During the brewing process, these acids are isomerized, resulting in the formation of three pairs of trans-/cis-iso-alpha–acids. The aim of this thesis project was to develop a method for the analysis (Chapter 2 and 7) as well as the purification of iso-alpha-acids (Chapter 3-5). Moreover, the stability of iso-α-acids need to be improved (Chapter 6) since these compounds degrade easily in the light and in the presence of oxygen. We have shown that β-CD is effective to separate cis-isomers from trans-isomers and at the same time increases the stability of the complexed compounds. The pure individual iso-alpha-acids can now be produced from pure alpha-acids, obtained from a CPC separation, followed by isomerization and beta-CD separation of the two isomers. These compounds can be used as reference standard for analytical purposes to replace the DCHA-salts of trans-iso-alpha-acids. It also opens the possibility to use pure iso-alpha-acids in brewery research. An HPLC system using these pure individual iso-alpha-acids as calibration standard was developed as well. The application of 2D J resolved NMR has been developed and seems promising to detect a wide range of compounds in beer.​​​​​​​