The Role of Riboflavin in Beer Flavour Instability: EPR studies and the application of flavin binding proteins

Spectrophotometric method for exploring 3-methyl-2-butene-1-thiol (MBT) formation in lager

The disappearance of riboflavin absorbance at 445 nm from beers or model beers on light exposure is directly linked to light-struck character formation. The addition of (+)-catechin, (-)-epicatechin, tryptophol, or ascorbic acid was able to reduce, but not stop, absorbance loss or light-struck character formation in either model beer or mainstream lager that was exposed to light. When isohumulone was present in model beer, the inhibitory effect of (+)-catechin, (-)-epicatechin, or tryptophol decreased with increasing isohumulone. The spectrophotometric method used in this study is a simple and effective method for determining light-struck susceptibility.

Photooxidative degradation of beer bittering principles: a key step on the route to lightstruck flavor formation in beer

Isohumulones, dihydroisohumulones, tetrahydroisohumulones, and humulinones, important hop-derived bittering compounds in beer, were shown to give rise to reactive triacylmethyl radicals on interaction with triplet-excited riboflavin after spin trapping by 5,5-dimethyl-1-pyrroline N-oxide or 2-methyl-2-nitrosopropane, followed by electron paramagentic resonance spectroscopy combined with spectral simulation. Electron abstraction from the ionized beta-tricarbonyl chromophore, which is common to all five-membered ring hop derivatives, is the initial event on photoinduced degradation. Radicaloid decomposition of isohumulones leads to precursors for 3-methylbut-2-ene-1-thiol, the lightstruck constituent in beer. Interaction of reduced derivatives of isohumulones with triplet-excited riboflavin furnished radical precursors of volatile aldehydes, which may lead to the development of unpleasant stale or cardboard flavors.

Deflavination and reconstitution of flavoproteins

Flavoproteins are ubiquitous redox proteins that are involved in many biological processes. In the majority of flavoproteins, the flavin cofactor is tightly but noncovalently bound. Reversible dissociation of flavoproteins into apoprotein and flavin prosthetic group yields valuable insights in flavoprotein folding, function and mechanism. Replacement of the natural cofactor with artificial flavins has proved to be especially useful for the determination of the solvent accessibility, polarity, reaction stereochemistry and dynamic behaviour of flavoprotein active sites. In this review we summarize the advances made in the field of flavoprotein deflavination and reconstitution. Several sophisticated chromatographic procedures to either deflavinate or reconstitute the flavoprotein on a large scale are discussed. In a subset of flavoproteins, the flavin cofactor is covalently attached to the polypeptide chain. Studies from riboflavin-deficient expression systems and site-directed mutagenesis suggest that the flavinylation reaction is a post-translational, rather than a cotranslational, process. These genetic approaches have also provided insight into the mechanism of covalent flavinylation and the rationale for this atypical protein modification.

Role of riboflavin in beer flavor instability: determination of levels of riboflavin and its origin in beer by fluorometric apoprotein titration

A method for the quantitative determination of riboflavin levels in beer was developed. The method is based on the quenching of riboflavin fluorescence, which occurs when riboflavin binds to the aporiboflavin-binding protein from egg white. The method does not require any pretreatment of the beer before analysis, other than dilution, and proved to be simple, reliable, and sensitive. The lowest concentration that could be detected was approximately 10 nM riboflavin. The possible interference of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) with the determination of the riboflavin content of beer was excluded, because beer contains only a very small amount of FAD (0.03 microM) and no FMN. The riboflavin levels of the types and brands of beer investigated were in the range of 0.5-1.0 microM. The origin of the riboflavin in beer proved to be the malt. Hop and yeast hardly contributed to the riboflavin content of beer. Besides its use in the determination of riboflavin levels, the aporiboflavin-binding protein also provides a way to remove riboflavin from beer, which reduces the light sensitivity and the related lightstruck off-flavor formation in beer.