Mashing is the first, and arguably most important, step in brewing. Any failings at this stage will have an impact not just on the rest of the process, but also on the final beer itself. It’s a nightmare when that happens, particularly when you know it was avoidable.
Milling grain can contaminate the brewing area
Grain is a food source. As such it is literally covered with different types of bacteria, with Lactobacillus being the most common. In the brewing process this bacteria usually does not present a problem thanks to the high temperatures used in the mashing and boiling stages. If allowed to come into contact with cooled wort, Lactobacillus produces lactic acid in the finished beer. This creates sour, unpleasant off-flavors. Milling grain usually creates a lot of dust, which in addition to being an irritant and nuisance also releases an airborne invasion of Lactobacillus into the surrounding environment. Most successful breweries move pre-mash grain processing as far away from the brewing and fermentation areas as possible, and homebrewers should do likewise. If you pre-mill your grain and need to transport it home, do so in a bag or old, unused pail, not a fermentation bucket.
Mash temperatures dramatically affect beer flavors
In all-grain beer, conversion of starches into fermentable and unfermentable sugars takes place over a wide temperature band from 145° to 165° F. However, within this range different enzymes are working at different temperatures. The work done by these enzymes has a big impact on the flavor profile of the finished product. Beta-amylase, the enzyme responsible for creating easily fermented simple sugars, works best on the lower end of this range. Alpha-amylase, the enzyme responsible for breaking down starches into unfermentable long-chain sugars, works best at the higher temperatures. Adjusting mash temperatures within this range gives you control over the finished wort. It can be very fermentable, resulting in a dry beer, or very dextrinous with a sweet, malty character. A good compromise allowing both enzymes to work relatively well is in the center of this range, 150° to 155° F
Grains should be sparged at 168° F and 170° F
The mashing process creates a sweet, sugary solution that must be lautered, or extracted to the brewpot. The sugars present in the mash react to temperature much the same way the sugars in a can of malt extract syrup do. When cold they become very thick and viscous, and when hot they tend to lose viscosity and flow more easily. Extract brewers many times will run a package of malt syrup under the hot water faucet for a few minutes to make it easier to pour into the brewpot. For all-grain brewers the goal in sparging is to use the hottest temperature possible to take advantage of this effect and improve extraction efficiency, yet not extract any of the harsh, astringent tannins that are present in the husks of grain. This occurs best at 168° to 170° F.