From THE BREWER’S JOURNAL July 1917
Beers which have been fully matured in bottle or cask undoubtedly hold their gas in a different manner from those in which the condition has been artificially induced by saturation under pressure. In perfect natural maturation the gas seems to have become part of the “soul” of the beer, while carbonation under pressure seems to thrust a somewhat false brilliancy upon the liquid. Some chemists assert that the difference is due to the formation of real orthocarbonic acid in the natural beer, while the carbonated beer is a simple solution of carbon dioxide. There is no proof that this theory is correct, although the experience of mineral water manufacturers seems to favor it. Freshly made minerals do not hold their gas as well as those a little older in which the gas may have had time to pass from the state of simple solution to that of orthocarbonic acid.
Some of the recent discoveries of physiologists with respect to the carrying of carbon dioxide from the tissues to the lungs in the blood for respiration may possibly afford a clue to the undoubted difference between natural and artificial condition of beer. According to the latest work carbon dioxide forms a dissociable compound with protein. There is a strong possibility that such a combination occurs in a naturally conditioned beer which is often fairly rich in nitrogenous bodies. On the other hand, chilling followed by filtration has the deliberate effect of removing such bodies, and this removal may affect the manner in which the carbon dioxide is held in the beer. – (London “Brewing Trade Review”)
Update – Science has progressed a bit since 1917 and this phenomenon is better understood. Carbonating under pressure provides a solution of gas and liquid. CO2 (carbon dioxide) when mixed with the water in the beer converts to H2CO3 (carbonic acid). Many commercial brewers use forced carbonation in a Brite tank. Most home brewers use forced carbonation in kegs. The beer clears and the CO2 converts simultaneously.
The same process occurs when “natural” carbonation is applied. The krausen or priming sugars re-ferment to produce CO2 which converts in the same way. Most, if not all home brewers have noticed this conversion in their beer which has been bottle conditioned. After a week, the beer seems carbonated but doesn’t produce a good stand of foam. After 2½ to 3 weeks, however, the same batch of beer exhibits a nice head (assuming the beer is properly made.) This is due to the conversion of carbon dioxide to carbonic acid. The speed at which this occurs is a bit dependent upon the pH level of the beer.
The orthocarbonic acid (H4CO4) mentioned in the above article is a bit of a misnomer as this acid converts to carbonic acid almost instantly and does so spontaneously. Maybe this is a bit more than any of us really need to know but this explanation is in order to dispel possible confusion.