Constitutive differences between steely and mealy barley samples associated with endosperm modification

BACKGROUND

Structurally different areas may occur in the endosperm of the barley grain, and they can be visually classified as either mealy or steely. Barleys with a high proportion of grains that are mostly steely often show uneven physical-chemical modification of the endosperm during malting. To study the relationship between steeliness and endosperm modification, two samples of barley cv. Scarlett with contrasting malting quality were analysed.

RESULTS

The proportions of steely grains were 77% and 46% in the two samples, which were then defined as steely sample and mealy sample, respectively. The steely sample showed slower modification during malting (in terms of beta-glucan degradation, friability increase, and Calcofluor staining), lower hot water extract (HWE) and acrospire growth, and higher extract viscosity. Endosperm permeation to large molecules (tested with the fluorescein isothiocyanate-dextran conjugate, FITC-D) closely followed cell wall modification in the steely sample, but this was not so in the mealy sample.

CONCLUSIONS

Higher steeliness was associated with higher levels of C hordeins in the grain of barley cv. Scarlett. It is proposed that such hordeins can increase the permeability to large molecules (FITC-D) but slow modification. Like steeliness and the level of C hordeins, permeability to FITC-D appears to be more linked to environmental rather than genetic effects. Although a more general association of C hordeins with steeliness of malting barley still has to be ascertained, the negative role of C hordeins in malting quality has been confirmed.

Structural changes of malt proteins during boiling

Changes in the physicochemical properties and structure of proteins derived from two malt varieties (Baudin and Guangmai) during wort boiling were investigated by differential scanning calorimetry, SDS-PAGE, two-dimensional electrophoresis, gel filtration chromatography and circular dichroism spectroscopy. The results showed that both protein content and amino acid composition changed only slightly during boiling, and that boiling might cause a gradual unfolding of protein structures, as indicated by the decrease in surface hydrophobicity and free sulfhydryl content and enthalpy value, as well as reduced α-helix contents and markedly increased random coil contents. It was also found that major component of both worts was a boiling-resistant protein with a molecular mass of 40 kDa, and that according to the two-dimensional electrophoresis and SE-HPLC analyses, a small amount of soluble aggregates might be formed via hydrophobic interactions. It was thus concluded that changes of protein structure caused by boiling that might influence beer quality are largely independent of malt variety.