The Wheat Starchy Endosperm Protein Gradient as a Function of Cultivar and N-fertilization Is Reflected in Mill Stream Protein Content and Composition.

Within the wheat starchy endosperm, the protein content increases biexponentially from the inner to outer endosperm. Here, we studied how this protein gradient is reflected in mill fractions using three cultivars (Claire, Apache, and Akteur) grown without and with N-fertilization (300 kg N ha-1). The increasing protein content in successive break fractions was shown to reflect the protein gradient within the starchy endosperm. The increasing protein content in successive reduction fractions was primarily due to more aleurone contamination and protein-rich material being harder to reduce in particle size. The miller's bran fractions had the highest protein content because of their high sub-aleurone and aleurone content. Additionally, the break fractions were used to deepen our understanding of the protein composition gradient. The gradient in relative gluten content, increasing from inner to outer endosperm, was more pronounced without N-fertilization than with and reached levels up to 87.3%. Regarding the gluten composition gradient, no consistent trends were observed over cultivars when N-fertilization was applied. This could, at least partly, explain why there is no consensus on the gluten composition gradient in the literature. This study aids millers in managing fluctuations in the functionality of specific flour streams, producing specialized flours, and coping with lower-quality wheat.

The Contribution of Sub-Aleurone Cells to Wheat Endosperm Protein Content and Gradient Is Dependent on Cultivar and N-Fertilization Level.

The proteins in the starchy endosperm of wheat determine wheat quality and exhibit a quantitative gradient decreasing from the outer to inner endosperm. Here, we investigate how protein-rich sub-aleurone cells contribute to the protein content and gradient by studying three cultivars, each cultivated at three levels of nitrogen (N)-fertilization. The observed increased protein content with increased N-fertilization was cultivar-dependent. Image analysis showed that the underlying protein gradient could be described by a declining biexponential curve, with protein contents up to 32.0% in the sub-aleurone. Cultivars did not differ in protein content in the center of the cheeks and only differed in the outer endosperm when N-fertilization is applied. N-Fertilization resulted in relatively higher increases in protein content in the outer compared to inner endosperm. Hence, sub-aleurone cells could affect the classification of cultivars by baking quality. Cultivar selection and N-fertilization could furthermore be promising techniques to produce protein-rich miller's bran.