Diacetyl-a Review: Part I-Analytical and Biochemical Considerations: Part Ii-Brewing Experience

Abstract

Diacetyl and 2,3-pentanedione are normal products of yeast metabolism and are formed in every brewery fermentation.The desired level in the final beer depends on the particular flavour aimed for but, in all types of beer, flavour defects are caused by excessive concentrations of diacetyl and many brewers might be happy to have no diacetyl in the beer.Recent improvements in analytical techniques show that many of the problems associated with diacetyl are due to the occurrence of compounds which can give rise to diacetyl in the finished beer. These compounds include the so-called "precursor," acetolactic acid, but possibly other compounds such as the bisulphite addition com pound of diacetyl are also involved.Study of the factors affecting diacetyl formation and removal by yeast shows how the concentration of diacetyl in beer can be controlled, and the processes at present used to regulate the diacetyl concentration in beer are described. The yeast strain used, the condition of the pitching yeast, the wort composition, the detailed management of the fermentation and the treatment of the beer during packaging and storage can all affect the diacetyl content of the beer.

A Review of Flavour Formation in Continuous Beer Fermentations*

“…Diacetyl has a strong "butterscotch" aroma in concentrations above the flavour threshold around 0.1-0.15 ppm in lager beers 114 . The now accepted pathway is that diacetyl results from the chemical oxidative decarboxylation of excess α-acetolactate leaked from the valine biosynthetic pathway to the extracellular environment.…”

The Production of Secondary Metabolites with Flavour Potential during Brewing and Distilling Wort Fermentations

“…Both of these VDKs (especially diacetyl) will have a significant effect on the flavour, aroma and drinkability of beer [85]. Diacetyl imparts a butterscotch-like stale milk flavour to beer with a threshold usually reported to be around 0.1-0.2 mg/L in lagers and 0.1-0.4 mg/L in ale for most tasters [3,4,15,86].…”

“…Diacetyl metabolism during brewing has been the subject of significant polemic over the years but a comprehensive review, earlier this decade, by Krogerus and Gibson [86] has discussed the history and current situation regarding diacetyl biosynthesis and subsequent metabolism during wort fermentation. Although microbial contamination can be responsible for diacetyl formation [7,85], yeast metabolism and its management are the major routes and this is why it is discussed in detail here. It has already been discussed that diacetyl and 2,3-pentandione are formed indirectly because of valine and isoleucine biosynthesis.…”

“…The non-oxidative decarboxylation of acetoin can be enhanced by heating under anaerobic conditions and by maintaining a low redox wort potential [91]. Diacetyl Although microbial contamination can be responsible for diacetyl formation [7,85], yeast metabolism and its management are the major routes and this is why it is discussed in detail here. It has already been discussed that diacetyl and 2,3-pentandione are formed indirectly because of valine and isoleucine biosynthesis.…”

The Role of Small Wort Peptides in Brewing Fermentations

“…Low molecular weight peptides are considered to act as foam negative effectors 3 . Beers containing simple peptides resulted in a significant decrease in foam stability 22 . A possible mechanism by which small peptides destabilize beer foam proposes that a limiting number of sites at the gas/liquid interface exist within foams, which are available for occupation by surface active molecules.…”