It's a beer world after all!
Carbonation is one of the defining features of beer.  The effects of carbonation strongly influence a beer's
mouthfeel, flavor, aroma and appearance.  Beer without carbonation carries the apt description of being "flat"
as the beer is rendered dull and lifeless.  

The two main products of the fermentation of wort sugars are ethanol and CO2 gas.  CO2 is readily soluble in
beer.Carbonation is measured in two ways.  One compares the volume of dissolved gas with the volume of liquid
and the other measures weight of gas in solution compared to the volume of the liquid.  

Traditionally, carbonation to the level required for serving was achieved in the brewery by transferring the beer
into a closed vessel with some residual fermentable sugar, and allowing the fermentation to finish.  In bottles, the
practice of bottle conditioning involves the addition of priming sugar to the finished beer, sometimes with yeast.  

Once the beer is bottled the yeast consumes the priming sugar, gives off CO2 in the bottle, and naturally
carbonates the beer.  Modern production methods allow for CO2 to be added directly to beer using a
porous stone or sintered steel rod.


Haze remains the craze as those cloudy New England IPAs continue to soar in popularity. Haze is actually the
broad term used for turbidity in beer.  The term generally covers all forms of instability in beer in which insoluble
material appears.  As clarity is a desired trait in many types of beer, breweries of these beers work hard to
avoid unwanted haze.  

Haze strictly refers to evenly distributed turbidity throughout the body of the beer.  Precipitates and sediments may
also appear, especially in bottle-conditioned or unfiltered beers. From a technical point of view, there are several
different types of haze.  One form is so-called invisible haze which is caused by very small particles that cannot be
readily detected by the eye but which scatter light in high intensity.   Visible haze is differentiated into chill haze
which develops when beer is chilled to 32F but disappears when beer warms to 68F, and permanent haze, which
is present at all temperatures.

Diacetyl is a flavor compound present in most beers (and many wines).   Imparting aroma characteristics described
as butter, butterscotch, or buttermilk, it is often added to food products to evince a buttery flavor and aroma.
Diacetyl is generated as a by-product of amino-acid metabolism in yeast during fermentation. Yeast excretes a
precursor into the fermenting beer, where it breaks down chemically to produce diacetyl. However, the diacetyl is
subsequently reabsorbed by the yeast cell and converted to a compound with no significant flavor characteristics.
Failure to reabsorb the diacetyl can result in the beer retaining an unacceptably high level of diacetyl.

It is essential that the raw beer is left in contact with the yeast for long enough for the diacetyl to be converted.
Yeasts that separate out too early in the fermentation process, often the result of early application of cooling, can
fail to complete the reabsorption. Diacetyl is particularly unwelcome in lager-style beers and these beers are often
held for a time before cooling is applied, a process known as “warm conditioning” or “diacetyl rest.” For this reason,
lager brewers  allow a short-term rise of fermentation temperature to 60°F or higher at the end of the fermentation.

At low to moderate levels, diacetyl can be perceived as a positive flavor characteristic in some ales and stouts.
The amount of diacetyl produced is yeast strain-dependent but wort composition and fermentation conditions are
also significant contributors to overall diacetyl levels. Diacetyl can also be formed by certain beer spoilage bacteria
(within the group known as lactic acid bacteria), most notably during post-fermentation storage and sometimes in
unsanitary beer lines (that is, plastic tubing not cleaned properly) between a keg of beer and the dispense tap.

Hope you found this month's column of interest.  If you have any questions about beer or brewing
or just want to submit a word for me to discuss here just
e-mail it to me.

Thanks and Cheers!
Carbonation  // Haze // Diacetyl
A new column by
Jack O'Reilly
Jack O'Reilly attended the
famous Siebel Institute/ World
Brewing Academy in Chicago.
I'm very excited to be part of the BeerNexus team.  I think my many years in the
beer business both as a brewer and manger will enable me to explain and
investigate many topics of interest for those who really love craft beer.
Hope you join me every month.  
More by Jack: #1, #2, #3, #4, #5, #6, #7, #8, #9, #10, #11,
#!2, #13, #14, #15, #16