This section relates to the mashing and sparging process: extracting sugars from grain and collecting it as wort in the kettle. The mash is performed in a mash tun and the sparge is performed in an lauter tun. Those vessels can be combined into a single mash/lauter tun.
The starches in the grains are broken into fermentable sugars by amylase enzymes during the mash. The simpler carbohydrate chains are broken down by beta amylase enzymes, and the more complex carbohydrate trees are broken down by alpha amylase enzymes.
The temperature of the mash is one of the most critical elements of brewing. A higher temperature leads to a less fermentable wort, resulting in a sweet final beer. A lower temperature leads to a more fermentable wort, resulting in a dryer beer. Too high or too low of a temperature can cause off flavors or poor efficiency. The most common range for a single infusion mash is 148F - 158F.
Amylase enzymes cannot act on the starches until they gelatinize, which happens in barley at 140F. Starch conversion ends if the mash temperature falls below 140F.
Beta amylase enzymes are most active in the 140-150F temperature range. Above 160F, beta amylase enzymes deactivate.
Alpha amylase enzymes are most active in the 162F - 167F range. Above 176F, alpha amylase enzymes deactivate.
Reference: The Theory of Mashing
The ratio of water to grain is important when mashing. Typical range is 1.0 - 2.5 quarts of water per pound of grain. A thicker mash (<1.25) tends to finish quicker, with less fermentable sugars. A thin mash (>2) can take longer to finish, but results in more fermentable sugars. Most brewers aim for the middle ground, a 1.25 to 1.50 ratio. Modern malts, however, show little difference in fermentability based on mash thickness, but speed of completion is still an issue. 1)
A sour mash is when the wort temperature is allowed to fall into the 90F - 120F temperature range, where bacteria and wild yeast are active. This can either be during the mash or after collecting the wort. The infection causes the mash or wort to taste and smell sour. The wort can then be boiled, killing the infection to prevent further souring. This method is used to create sour style beers in a controlled and comparatively quick manner, thought the sourness is different from that of a sour fermented beer.
A common technique is extracting 20% of the wort after the mash, souring it, and adding it back to the normal wort after a few days to revitalize fermentation and mellow the sourness.
Eff = 100 * (gravity points of wort * wort volume) / (grain weight * grain extract potential) 2)