Investigating the particle morphology and molecular structure of roller-milled yellow pea flours using high-resolution imaging, mid-infrared spectroscopy, and synchrotron X-rays

Abstract

Milling influences flour morphology and the molecular structure of starch. Such structure-function interactions are crucial in determining ingredient functionality to develop specific end-products. Laboratory scale X-ray instruments offer limited brightness and are time-consuming when investigating the porosity, starch lamellar structure, and crystallinity of pulse flours. Herein, synchrotron X-rays were employed along with mid-infrared spectroscopy (mid-IR) and scanning electron microscopy to non-invasively investigate the molecular changes in roller-milled yellow pea flour as influenced by milling configurations (termed as flour streams) and flour blending (termed as flour blends). Our results illustrate that flour streams have higher overall protein content, distinct flour morphology, starch lamellar structure, and starch and protein quality than flour blends. Interestingly, flour blends (21.79 to 29.27%) displayed an overall higher degree of crystallinity than flour streams (20.03 to 25.75%). The broadband absorption spectra in the 1200–900 cm⁻¹ range showed carbohydrate features, while protein spectra in the 1600–1500 cm⁻¹ range highlighted amide I (~ 1650 cm⁻¹) and amide II (~ 1540 cm⁻¹) bands. Secondary protein structures, analyzed through second derivative spectra (1700–1600 cm⁻¹), revealed consistent β-sheet (~ 1636 cm⁻¹) and α-helix (~ 1656 cm⁻¹) ratios, with absorbance changes reflecting protein content variations in flour streams and blends. Furthermore, a principal component analysis model was developed with a total variance of 95.2%, using mid-IR data to classify the samples based on their streams and blends. In conclusion, the state-of-the-art advanced techniques captured and characterized the micro and macromolecular changes in pulse flours as influenced by milling.