Evaluation of the relative bitterness and light stability of reduced iso-alpha acids.

Todd, P.H., Johnson, P.A. and Worden, L.R.

Abstract
The reaction of iso alpha acids with iso pentenyl mercaptain in beer is catalysed by light to give objectionable sun struck or skunky odour. These defects can be prevented by reducing the carbonyl group of the iso alpha acid with borohydride to the p iso alpha acid (dihydro iso alpha acid) or by catalytic reduction of alpha acids to tetrahydro alpha acids which are isomerized on boiling in wort. Hexahydro iso alpha acids have also been prepared by borohydride reduction. In the present report the relative bitterness of these reduced compounds has been compared with iso alpha acids by a critical tasting panel. The absorptions of iso alpha acids and tetrahydro iso alpha acids are well defined but the composition of the other reduced compounds depends on the method of preparation. The extinction coefficients at 1 cm of the preparations used were estimated at 255 nm in 1% solution in methanol with 0.002 N NaOH as: iso alpha acids 520, p iso alpha acids 470, tetrahydro iso alpha acids 480 and hexahydro iso alpha acids 460. Using these values, quantities were added to unhopped beer in amounts equal to 15 ppm iso alpha acids; then after comparison of bitterness fresh samples were prepared of an estimated equal bitterness. With iso alpha acids as 100 the bitterness of p iso alpha acids was about 67, tetrahydro iso alpha acids about 203 and hexahydro iso alpha acids about 115. When the beers were aged and exposed to light the iso alpha acids gave strong mercaptan odour, the p iso alpha acids gave a slight odour but the others were free from this defect. None of the beers developed haze.
Keywords: beer bitterness flavour illumination iso alpha acid light reduction stability