Sulfur Nutrition and Wheat Quality
Global sulfur deficiency in agricultural lands is the result of gradual replacement of sulfur-containing fertilizers with high purity nitrogen fertilizers, introduction of high yielding crop cultivars, and since 1970s strict restrictions on sulfur dioxide emissions to reduce greenhouse gases. Wheat (Triticum aestivum L.) has a low sulfur requirement, and an addition of 15 to 20 kg ha−1 is usually sufficient for optimum yield and quality. Grain sulfur content of ≥1.2 mg g−1 and a nitrogen/sulfur ratio of ≤16:1 appears to be critical for optimum quality characteristics. The endosperm storage proteins or prolamins determine the end-use quality of wheat flour. They are classified on the basis of their solubility in different solvents, structure, and molecular size. There are three main groups based on the structure, namely sulfur-rich, sulfur-poor, and high molecular weight glutenin subunits (HMW-GS). Sulfur deficiency does not affect total grain protein but does affect accumulation of different protein groups during grain development. Amounts of sulfur-poor proteins such as ω-gliadins and HMW-GS increase at the expense of sulfur-rich protein groups such as α-, β-, and γ-gliadins and low molecular weight glutenin subunits (LMW-GS). HMW- and LMW-GS are polymeric proteins held together by disulfide bonds. Sulfur deficiency reduces the accumulation of LMW-GS, total amount of glutenins, and changes HMW-GS/LMW-GS ratio, thus a shift in molecular weight distribution of gluten proteins toward higher molecular weight. These quantitative changes in protein composition modify flour functional properties. Dough made from sulfur-deficient flour is stronger, has increased mixing requirements, reduced extensibility, and loaf volume. Sulfur deficient grains show higher pearling index. Nonprotein nitrogen significantly increases because of accumulation of excessive amounts of free asparagine and glutamine. Free asparagine serves as a precursor in the formation of acrylamide, a carcinogenic neurotoxin, during baking, roasting, and frying because of the Maillard reaction. Inadequate sulfur supply promotes accumulation of increased levels of asparagine thus posing a health risk. Recent studies have shown that accumulation of free asparagine in wheat grain depends on wheat cultivar, soil type, and growing season weather conditions. Sprouting significantly increases quantity of free asparagine in the grain. Ways to reduce free asparagine in the raw materials and formation of acrylamide in the processed food are summarized.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
Copyright © 2008. American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, 5585 Guilford Road, Madison, WI 53711-5801, USA. Sulfur: A Missing Link between Soils, Crops, and Nutrition. Agronomy Monograph 50.