Post-harvest and post-milling changes in wheat grain and flour quality characteristics

Authors: Baik, Byung-Kee; Donelson, Thomas

Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2017
Publication Date: 1/11/2018
Publication URL: https://handle.nal.usda.gov/10113/6542232
Citation: Baik, B-K., Donelson, T.S. 2018. Post-harvest and post-milling changes in wheat grain and flour quality characteristics. Cereal Chemistry. 95:141-148.

Interpretive Summary: The reliable and reproducible determination of wheat grain and flour characteristics is crucial for the screening of breeding lines in the successful development of wheat varieties possessing improved milling and baking quality, for the procurement of grain and the marketing of flour by milling companies, and for product quality control by baking companies. After harvest, wheat grain goes through slow but steady biological and compositional changes via a series of chemical reactions, which affect seed longevity as well as nutritional and processing quality. Similarly, milling also exposes wheat flour to numerous chemical reactions, resulting in considerable modifications in its compositional, nutritional and functional properties during storage, which can ultimately affect food product quality. These changes pose a great challenge to the reliable and reproducible evaluation of wheat grain and flour samples, especially when many samples need to be tested over a period of several months. We examined the influence of post-harvest and post-milling storage time on the typical quality evaluation test results of soft red winter wheat grain and flour, and identified the optimal time frame to assess wheat quality. Significant post-harvest and post-milling fluctuations in both wheat grain and flour quality characteristics were observed in the first four weeks of storage; however, this variability decreased with further storage. Falling number (an estimate of pre-harvest sprouting) of grain and flour exhibited a continuous and substantial increase during post-harvest and post-milling storage, respectively. The pH and protein strength measurements of wheat flour showed small but continuous decreases during post-milling storage for 26 weeks. Storage of grain after harvest and of refined flour after milling for four weeks seems to minimize the variation in milling and baking quality test results for all quality measures except falling number. Ensuring a standard post-harvest or post-milling storage time is necessary for the reliable determination of wheat grain and flour falling number. The results obtained in this study will: (1) serve as a scheduling guide for establishing a comprehensive quality evaluation protocol of wheat grain and flour, (2) allow grain inspectors to better estimate quality potential, (3) assist wheat breeders in the accurate screening of wheat breeding lines, (4) assist millers in the procurement of grain of appropriate quality specifications, and (5) enable food manufacturers to more precisely control the quality of raw ingredients.

Technical Abstract: Soft red winter (SRW) wheat grain immediately after harvest and flour after milling were stored for 26 weeks and analyzed for comprehensive milling and baking quality characteristics at different time points to examine the consistency of the quality test results. Increases in falling number (FN) of grain during post-harvest storage were observed for nine out of ten varieties, in which FN ranged from 62 to 264 sec at two weeks after harvest and increased by 18-109 sec during storage for 21 weeks at 23°C. Wheat grain exhibited evident fluctuations in refined flour characteristics during the first four weeks of storage, and then lesser changes with further storage. The storage time of grain did not significantly influence the milling yield, FN and water SRC of refined flour. For refined flour during post-milling storage, storage time significantly affected all the flour characteristics, which showed evident fluctuations especially in the first four weeks of storage. The refined flour FNs of all four tested varieties increased from 373-405 to 377-417 during the first eight weeks, and then sharply to 427-474 with further storage for 26 weeks. Flour pH decreased slightly for the first two weeks after milling, was stable for the next eight weeks, and then decreased again to 5.74-5.90 at 26 weeks. Small but steady decreases in both lactic acid SRC and SDS sedimentation volume were also observed for refined flour during storage. Post-milling storage induced relatively small changes in the solvent retention capacity and cookie diameter of flour. Post-harvest and post-milling changes must be considered to achieve a reliable determination of the FN of wheat grain and flour, and pH, SDS sedimentation volume and lactic SRC of flour. A storage time of four weeks would reduce the variation in wheat grain and flour quality test results, allowing reliable and reproducible evaluation.