Canadian Grain Commission
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The implications of frequently encountered grading factors on the processing quality of durum wheat



1. Abstract

Grading factors associated with adverse growing conditions affect the processing value of durum wheat. Mycotoxins associated with fungal infections like Fusarium head blight and ergot partition themselves among milling products, and are relatively stable throughout pasta processing. Fusarium head blight also has adverse effects on semolina milling, gluten strength and pasta color. Durum wheat frequently is segregated according to protein content in recognition of the primary importance of protein content in determining cooked pasta texture. Hard vitreous kernels are a widely used durum wheat grade specification because nonvitreous kernels are lower in protein content and softer than vitreous kernels. The softness of nonvitreous kernels results in a lower yield of coarse semolina, an important factor to many durum wheat millers. Damage due to orange wheat blossom midge is tolerated in low amounts in top durum wheat milling grades because of increased specks in semolina. Other factors that cause surface discoloration such as smudge, black-point and mildew are also tolerated in low amounts to avoid specks in semolina. Severe frost damage is a serious grading factor, resulting in lower semolina yield, poor semolina refinement (high ash, dull color, specks) and poor spaghetti color. Severe frost damage also reduces gluten quality. Moderate pre-harvest sprouting has little impact on durum wheat quality providing that mildew damage is not present. Even when sprout damage is severe there is little effect on pasta cooking quality, aside from an increase in solids lost to cooking water. The starch degrading enzyme -amylase associated with pre-harvest sprouting has little opportunity to react during pasta processing because of the relatively low moisture content of pasta dough and rapid loss of moisture during early stages of pasta drying, and is quickly denatured during cooking. Heat damage due to improper use of hot air dryers harms gluten functionality and if severe can reduce pasta cooking quality. Recent advances in durum wheat milling and pasta processing such as debranning, a trend to finer semolina granulation for high capacity pasta presses, and the general acceptance of high temperature pasta dryers are significantly impacting on the relative importance of grading factors on durum wheat processing quality.