First there's a discussion of the vats used for ageing:
"In the first place, absolute cleanliness on the store vats, &c., must be observed and everything that will aid us in securing this cleanliness must be considered. The use of wood is not commendable for this purpose, unless the pores have all been stopped on the inside by some impermeable substance. Lager beer brewers are compelled to use pitch, or its later substitute varnish, for the interior of their vats, otherwise the pressure of the gas would force the beer into and ultimately through the wood-pores; besides which the gas itself would leak out ; but we are pleased to see that ale brewers are beginning to recognise the value of similar treatment for casks, as a material aid to cleanliness."It's fairly obvious that hygiene is pretty important if beer is going to be in a vessel for any length of time. What really intrigues me though is the mention of lining wooden casks. It was common practice on the Continent, but this is one of the first mentions I can recall of some UK brewers adopting the technique.
Holmes' Brewing Trade Gazette - Friday 01 September 1882, page 11.
This is a good summing up of why a large vat was better than a small vat for ageing:
"Then, again, the storage vats should be made as large as convenient. A more equal temperature is thereby secured, a more uniform beer is the result, and the loss from evaporation is always proportionately less. Besides, the gradual amalgamation of the prominent flavours in the beer is better accomplished in a large than in a small vat. All malt liquors darken to a greater or less extent, according to the time they are kept in stock, and where a pale beer is required, the longer it is stored the paler must it be to commence with. This is of course regulated by the temperature at which the malt is finally kilned. Loss of hop flavour always occurs in a beer, owing to the before-mentioned gradual decomposition and amalgamation of the prominent flavours, including the hop-bitter, and a beer for long storage must be more heavily hopped than one intended for immediate consumption. Then, again, beer keeps its good qualities longer in a cellar possessing a certain humidity than where it is perfectly dry. So far, what we have said applies equally well to lager beer or ale, but there are certain points to be observed that apply specially to each."Another factor in favour of a large vat is that a much smaller proportion of the beer is in contact with wood. Porter brewers twigged the advantages of large vats early on and contructed massive ones, the largest containing thousands of barrels. Though by the time this article was written the massive Porter vats had been dismantled as standard-strength Porter was no longer being ages, only some Stout. And that was brewed in much smaller quantities.
Holmes' Brewing Trade Gazette - Friday 01 September 1882, pages 11 - 12.
Does beer really darken when aged? Not totally sure about that one.
It's obvious from brewing records that beer meant to be aged was much more heavily hopped. For example, Keeping Porter had between 50% and 100% more hops than Running Porter. But I see from this description it wasn't just about the hops protecting the beer, but also about making sure the beer was sufficiently bitter when finally served.