Total extraction refers to the portion of the durum wheat kernel that can be milled into flour and semolina.
Semolina extraction is only that portion of the wheat kernel that is milled into semolina. When milling durum, most companies try to maximize the yield of semolina and minimize flour production because semolina commands a higher price.
Ash content in the endosperm of durum is inherently higher than in the endosperm of other hard wheats, but can be used as a relative measure of bran. The ash in commercial semolina of about 65 percent extraction (wheat basis) normally ranges form 0.55 to 0.75 percent (on a 14 percent moisture basis). If the ash content is about 0.5 percent or lower, the milled product most likely contains some non-durum flour.
Specks appear in semolina when small particles of bran or other material escape the cleaning and purifying process. Speck concentration is usually determined by counting the number of brown or black particles in a given area on the surface of yellow semolina.
If present in the finished pasta, consumers sometimes perceive specks to be contaminants. Millers can control speck count by selecting durum that is free of disease and foreign material, thoroughly cleaning the durum, properly tempering and conditioning the wheat before milling, and by using purifiers to remove small bran particles from the semolina.
Protein content in semolina is important for both nutritional and functional reasons. Within the United States, the U.S. Department of Agriculture requires enriched pasta products to contain a minimum 12.1 percent protein, which means that millers generally want durum with at least 13.0 percent protein. Irrespective of nutrition, protein content in semolina has a high correlation with gluten content and, in turn, mechanical strength and cooking quality.
Wet gluten is the material remaining after the starch and soluble material are washed from a dough ball. Gluten is comprised of several proteins. The quantity of protein contained in flour is important, but perhaps even more critical to the mixing and baking process is the quality of the protein.
Mixogram or farinogram curves reveal important information about the gluten quality of semolina, and ultimately for cooked firmness of the pasta. The instruments that draw these curves measure dough development, resistance of dough to mixing and the tolerance of dough to extended mixing. The instruments mix semolina and water in a cup. They have an arm with a pen that traces out the amount of water required before the dough reached a definite consistency. The curves also indicate the amount of torque, or strength, required to mix the dough to a definite consistency and the length of time the torque stays steady. Mixograms and farinograms are rated on a scale of 1 to 8, with the higher values indicating strong mixing characteristics.
A mixograph requires a smaller sample size and is quicker than the farinograph, but the mixograph has poor temperature control and is not a standardized instrument.
Definitions Courtesy the North Dakota Wheat Commission.