Manufacturers’ instructions are crap. They are responsible for a lot of bad beer. Why?
The kindest thing I can say about manufacturers’ instructions is they are failsafe instructions. More bluntly, they are idiot instructions. The problem is that the idiots are not going to read them anyway, and when all the sensible people (like you) read and follow the instructions they get an inferior result.
The two main problems with the manufacturers’ instructions are recommended fermentation temperatures, and the instruction to brew for five days and then bottle.
Most brew kits are supplied with an ale yeast, even though it might say lager on the label. Typically ale yeasts like temperatures in the high teens or low twenties. Some will ferment at cooler temperatures, while others can handle slightly warmer temperatures. For any yeast, ale or lager, the warmer you ferment, it the more fermentation flavour you get from it.
For some beer styles, such as Bavarian wheat beers or Belgian ales, fermentation character is integral to the style. For other styles such as lagers and American ales fermentation character should generally be unnoticeable.
If fermentation temperatures get too warm the fermentation character can become increasingly fruity in a sickly way that can be simply too much like over-ripe fruit. From here the beer can start to acquire a medicinal character rather like cough syrup, and at the extreme with very warm fermentation temperatures and very fast fermentations the beer can have a solvent like character – think nail polish remover.
So why do manufacturers suggest temperature ranges ranging from 18 to 30 degrees? The first reason is they are selling the kits across the country and throughout the year. There are many places in Australia that will rarely experience temperatures less than 20 degrees.
The second reason is that warmer temperatures give a quick start, and a quick finish to fermentation. This reduces the chances of people deciding that after several hours nothing is happening, and then opening the fermenter to check and perhaps to fiddle, and thereby exposing the wort to the risk of infection.
Given that many brew kits are sold off the supermarket shelf, it is reasonable for the manufacturers to assume that the brewer concerned does not have a hydrometer. So the easiest way to determine the end of fermentation is to watch the airlock and count five days. With a warm and quick fermentation this is probably OK most of the time, but if there is cooler weather etc then this will probably cause problems later with the beer in the bottle. If the beer is not fully fermented when it is bottled, fermentation will restart in the bottle once things warm up. The result is over-gassed beer or even exploding bottles.
Of course the manufacturers have no control over who brews their beer kits, and the circumstances in which they do. And yes, they need to formulate instructions accordingly. But for the sake of all the sensible people out there, the manufacturers should provide something more than “idiot” instructions – especially when idiots won’t read the instructions anyway.