Rheological characterisation of long- and short-mixing flours based on stress-relaxation

Abstract

Mixing characteristics, descriptive rheological measurements, and stress relaxation behaviour of flour-water doughs from a diverse range of Canadian hard common wheat cultivars were investigated. When mixed in a mixograph, flours from two varieties and two breeding lines in the Canada Western Extra Strong (ES) breeding trials required longer time (6-8 min) and higher work input (270-350 Arbitrary Units) to mix to peak dough resistance (PDR) than moderately strong to strong bread wheats (MS) (2·5 min and 110-120 AU). Extensigraph maximum resistance to extension (R(max)/E ratio) and alveograph P/L (tenacity to length ratio) values were higher for doughs from ES cultivars than for MS cultivars. Flour-water doughs from ES cultivars exhibited higher G' and G'', and lower tan δ values than those from MS cultivars at all frequencies. Doughs from ES cultivars exhibited slower relaxation rates than exhibited by MS. Doughs from ES cultivars exhibited characteristic bimodal relaxation spectra, which appeared to represent two discrete spectra separated by time. In contrast, doughs from MS cultivars exhibited only one prominent peak at about 0·1 s, with the second peak reduced to a shoulder. Adding cysteine (30 ppm) to flour-water mixtures of two ES cultivars reduced mixograph mixing times to 2·5 min. The ES doughs with added cysteine exhibited relaxation behaviour similar to that of MS doughs without added cysteine. This suggests that high molecular weight glutenins are primarily responsible for the longer mixing times of ES cultivars, and the characteristic second peak in their relaxation spectra. Relaxation behaviour of all doughs examined was positively correlated with mixograph mixing time, extensigraph R(max)/E, alveograph P/L, and mixing energy and mixing time obtained by a long and a short bread-making process. However, the stress relaxation data demonstrated no simple correlation to loaf volume, because all of the cultivars had sufficient strength to produce high quality bread when dough was optimally developed. The stress relaxation measurements differentiated between overly strong (ES) cultivars, and cultivars that have mixing requirements that are more suitable for overall bread-making performance (MS). Stress relaxation results also appeared to reflect expected qualitative differences in the underlying molecular weight distribution of glutenin polymers which relate to dough, strength. (C) 2000 Academic Press.