Some key advantages and important points to consider:
Instead of artificially forcing exogenous CO2 into the beer, it is possible to carbonate it by refermentation in the bottle.
In this case, sugar and yeast are reintroduced into the finished beer before storing it for two to three weeks at around 25°C. The CO2 produced by the yeast during this second fermentation is trapped in the bottle which will pressurise and will dissolved into the beer.
The main advantages of this technique are the consumption of the last traces of oxygen in the beer which allows less oxidation of polyphenols, bitter compounds and thiols, and the capacity of the yeast to adsorb or reduce undesirable components (trans-2-nonenal, Strecker aldehydes, humulinones, etc.).
Yeast won’t produce esters in those conditions but are able to release pleasant fruity aromas from a non-aromatic precursor found in hops.
Some sugars/carbohydrates such as brown sugar, candi syrup will also bring flavour to the beer (and possibly colour). It must therefore be chosen according to the desired beer profile.
There is also a risk of autolysis when keeping yeast in a beer bottle. Amount many other compounds, fatty acids and enzymes can be released and be detrimental to foam and reduce some fruity esters aroma.
Studies show however that high level of autolysis is only observed in very extreme cases of bad yeast conditions, inadequate pitching rate and high alcohol content).
Slight autolysis may even be positive and bring flavours described as “toasty” or “hazelnuts”.
Bottle conditioned beers will therefore age better than regular beers and their flavour will evolve with time. This may however be considered a flaw for commercial beers where stability is a priority.
Carbonation standardisation will also be more complicated and involve more critical controls of residual sugars and dissolved CO2 to accurately calculate the amount of sugar and yeast to add in the brew. Once it is all in the bottle, there is no tweaking anymore!
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