Barley Starch for Brewers (IV: Granules)


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Starch granules contain alternating layers of less dense, amorphous starch (light blue) interspersed with more tightly packed, semi-crystalline regions of starch (dark blue). When exposed to water, the less dense regions swell, disrupting the internal structure of the granule.

Starch is composed of amylose and amylopectin. In barley malt, however, starch does not exist as a pure mixture of these two molecules, contained by the husk. Instead, Amylose and amylopectin are associated with other molecules, and packed into to tiny granules.


In barley starch, there are two types of granules — large and small. The large granules are 15-25 micrometers in length and lenticular (lens-like) in shape. The small granules are irregularly shaped and typically under 10 micrometers. Although small granules greatly outnumber the large by count, they only account for 10–15% of the weight of the starch.

Small granules contain more amylopectin (75–77%) by weight than large granules (which contain 71–76% amylopectin). Smaller granules also have more protein on the outside of the granule, require higher temperatures to gelatinize, and are less digestible by amylase enzymes, compared to larger granules.


Granules Have a Layered Structure

Granules have a layered structure with alternating regions of amorphous structure and a more compact, crystalline form. The less dense, amorphous region is rich in amylopectin, and quickly swells when hot water comes in contact with it. The denser, semicrystalline regions contain mostly amylose, which is often associated with lipids.

Starch granules also have proteins with in them, including a protein called friabilin, which influences whether the starch is mealy or steely. Granules also contain surface proteins, which limit the ability of amylase enzymes to attack the starch before the granule is fully ruptured and the starch fully gelatinized.


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