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One of the advantages of all-grain brewing is the ability to use ingredients that can't be used in extract brewing. Specifically, all-grain brewing allows brewers to use starchy grains or adjuncts that would cause haze (and instability) in an extract beer. Because grain-derived enzymes in the mash (alpha and beta amylase) degrade starch into simple sugars, starchy adjuncts can be added to an all-grain mash.
In order to degrade starch in a mash, however, the starch needs to be accessible to the starch-degrading amylase enzymes. In most plants, including barley, starch is stored in granules. In these granules, starch has an organized structure. When a starchy food is soaked in cold or luke-warm water, the starch absorbs some of the water, but the granules remain essentially intact. Within a range of temperatures, however, the starch loses its structure and becomes a "net" of starch with lots of water molecules interspersed. This is called the [https://en.wikipedia.org/wiki/Starch_gelatinization gelatinization ] range. Above the gelatinization range, the starch dissolves into the water. Because the starch-degrading amylase enzymes are water soluble, they can then get to the starch and begin degrading it.
The gelatinization range varies among starches for different types of plants. Most, however, begin the range of 120–140 °F (50–60 °C). [Differences exist among plants starches because different plants have a different ratio of amylose (straight-chained starch) to amylopectin (branched starch).] An upshot of this is that many grains or other starchy adjuncts can simply be stirred into the mash. The temperatures in the saccharrification range or below are sufficient to gelatinize the starch and render it accessible to amylase enzymes. For example, unmalted wheat has a gelatinization temperature range starting between 136-147°F (58-65°C) and can, therefore, be gelatinized during a beta amalyse/maltose rest (fine milling will help efficiency) <ref>[https://onlinelibrary.wiley.com/doi/full/10.1002/jib.107 Common wheat (Triticum aestivum L.) and its use as a brewing cereal – a review. Andrea Faltermaier, Deborah Waters, Thomas Becker, Elke Arendt, Martina Gastl. 2014.]</ref>. Some starches, however, have gelatinization temperatures starting above mash temperatures. Likewise, some starchy adjuncts have traditionally been boiled to obtain a better extract or to speed up hydration. For example, Mike Karnowski reported increasing the gravity of a raw wheat and pilsner malt wort from 1.030 SG to 1.036 SG by doing a cereal mash with the raw wheat and 1/4 of the recipe's pilsner malt. Karnowski reported the best efficiency of 1.044 SG with wheat flour at a 154°F saccharrification mash rest, indicating that a fine crush might be the most efficient way to convert raw wheat starches to sugar <ref>[Mike Karnowski. "Homebrew Beyond the Basics: All-Grain Brewing & Other Next Steps". Sterling Epicure; Revised, & Expanded edition (October 2, 2018). Pg 156.]</ref>. Briggs et al (2004) recommends pre-soaking raw wheat in 185°F (85°C) water instead of boiling so as to avoid frothing, and then mashing in with room temperature water to avoid clumping. Raw wheat and other flaked/undermodified adjuncts such as flaked barley release beta-glucans into the wort that can make wort separation difficult. Breweries will sometimes add beta-glucanase enzyme to the mash to break down the beta-glucans and allow for easier lautering <ref>Dennis E. Briggs, Chris A. Boulton, Peter A. Brookes and Roger Stevens. "Brewing Science and Practice". 2004.</ref>.