Separation and Identification of Stereoisomers of Isomerized alpha-Acid Derivatives by HPLC/DAD and Electrospray HPLC/MS (1). Patrick L. Ting (2), Susan Kay, and David Ryder, Technical Center, Miller Brewing Company, Milwaukee, WI. (1) This work was presented as a poster at the 2005 ASBC Annual Meeting, Savannah, GA. (2) Corresponding author. E-mail: <Ting.Patrick@mbco.com> J. Am. Soc. Brew. Chem. 65(1):9-14, 2007.

Four classes of advanced isomerized hop products (advanced hop products) are commercially available. They are derivatives of alpha-acids that have undergone stereochemical transformations, resulting in iso-alpha-acids with cis and trans configurations. Their compositions play important roles in providing bitterness, foam, flavor, and flavor stability; each of these derivatives exhibits differences in its bitterness strength, foam enhancement, flavor, and flavor stability. The traditional IBU (international bitterness unit) method for measuring bitterness using UV absorbance neither distinguishes their compositions nor provides clues as to their corresponding sensory perception. A simple high-performance liquid chromatography (HPLC) method using a 100 × 4.6 mm C-18 column and an aqueous meth­anol buffer solution has been developed to rapidly resolve analogs and stereoisomers of iso-alpha-acids (class I), rho-iso-alpha-acids (class II), tetrahydro­iso-alpha-acids (class III), and hexahydroiso-alpha-acids (class IV). This method can also differentiate between these four classes of iso-alpha-acid derivatives. Additionally, we demonstrate that electrospray HPLC/MS (mass spectrom­etry) can be a versatile method for identifying these isomerized hop derivatives. Keywords: Hexahydroiso-alpha-acids, HPLC, HPLC/ESI-MS, Iso-alpha-acids, rho-Iso-alpha-acids, Tetrahydroiso-alpha-acids