Large starch granules, prepared from a range of barley samples, show considerable variation in their susceptibility to attack by a-amylase at 65°C. Susceptibility is not correlated with their protein content, amylose/amylopectin ratio or gelatinisation temperature as measured by loss of bire fringence. It does correlate with gelatinisation as measured by swelling of the granules or their staining with Congo Red. When granular expansion is taken as the measure of gelatinisation, the more resistant starches show gelatinisation curves displaced to higher temperatures.Key words: a-amylolysis, barley, gelatinisation, mashing, protein, starch.
IntroductionMost or the extract in a mash is derived From starch. Low mashing temperatures (50°C and below) give very poor yields or soluble extract. The yield increases considerably between 50cC and 60°C and is fairly constant between 60°C and 65°C. The increased extractability is due to the gelatinisation of malt starch which occurs between SO and 60°temperatures.Gelatinisation makes the starch more susceptible to attack by a-amylasc.Barley starch gelatinises at a higher temperature than docs malt starch and is therefore attacked less readily under normal infusion mashing conditions. This is one reason why grists containing finely ground raw barley flour give less extract than expected from their starch content even when there is more than enough a-amylasc in the malt fraction to cope with this additional starch.There is some evidence in the literature that barley starches vary from one sample to another, e.g. in amylose content,"' gelatinisation temperature7-13-14 and lipid content"-"-". In the current work, a study has been made of the percentage gelatinisation at various temperatures and the susceptibility to a-amylase of large starch granules prepared from a range of commercial barley samples of different varieties and with various nitrogen contents.
ExperimentalTotal Nitrogen (N) values were determined for each of the barley batches used.'Preparation of starch granules.-Eight separate batches of barley (Table I) were dchusked with 50% sulphuric acid8 and large starch granules were prepared by the sedimentation technique described previously.2* During preparation, starch granules from different barley varieties showed different rates of sedimentation. This might be connected with the effect which hordein associated with starch granules" is known to have on sedimentation rates in other solvents.21 Absence of other particles in the large granule preparations was checked by microscopic observation, they were analysed for protein content* and their 'Blue Values' were determined.4Hydrolysis of starch with