During the malting process, degradation of the endosperm starch is limited to preserve extract potential. Approximately 10% of the barley starch is hydrolysed to provide nutrition for the embryo. The majority of this degraded starch is the small B-Type granules(1 – 5 µm) that degrade more rapidly than the larger A-Type (10 – 25 µm) , due to their increased surface area to volume ratio.
Although this degradation of the small starch granules contributes towards malting loss, it is somewhat fortuitous, as the small starch granules have higher gelatinisation temperatures than the large starch granules. During mashing ungelatinized starch will generate viscous wort that are difficult to separate, whilst also contributing to carbohydrate haze formation.
There are four major starch-degrading enzymes present in germinating barley.
Barley contains no á-amylase prior to germination and its synthesis and release from the aleurone layer is dependent upon GA stimulation during steeping. â-Amylase is present in barley within the endosperm and exists bound to the proteins of the endosperm matrix. This latent â-amylase is activated on release from the matrix by proteases/carboxypeptidases.
a-Amylase slowly attacks ungelatinized starch during germination, but b-amylase cannot. a-Amylase breaks the starch down to smaller polymers (liquefaction), whilst the b-amylase cleaves individual sugar monomers (saccharification). This combined action is most effective during mashing and will be dealt with in greater detail later on in the course.
Limit dextrinase, existing in both bound and free forms, degrades branched dextrin (partially degraded starch granules) to glucose. a-Glucosidase degrades maltose to glucose providing immediate metabolic substrates for the embryo. Limit dextrinase and á-glucosidase are heat-labile and rarely survive kilning to be of benefit in the mash. a-Amylase and b-amylase are to some extent heat-labile, with a proportion lost during kilning. b-Amylase is affected by heat more than α-Amylase.