Protein features of gluten-free pasta made from proso millet (Panicum miliaceum) and their relation to functional and nutritional characteristics

1. Justification To explore the potential of proso millet, a gluten-free cereal of low glycemic index, as ingredient in pasta, three varieties were obtained from two locations and used to produce pasta of which compositional, functional and sensory properties were previously reported. However, little is known about structural changes of proso millet protein attributes upon processing. 2. Objective This study’s objective was assessment of pasta protein characteristics over processing. 3. Methods Protein solubility, content of accessible thiols, and protein secondary structures were measured in dough, raw pasta after sheeting, and cooked pasta. To assess the influence of protein interaction types on solubility, three solvents were used: phosphate buffer containing 0.1 M sodium chloride, the same buffer supplemented with 4M urea, or with 4M urea and dithiothreitol. Protein secondary structures were analyzed via attenuated total reflectance Fourier transform infrared spectroscopy. Results were compared to those from commercial fresh gluten-free and wheat fettuccine. 4. Results Solubility of proso millet proteins in pure buffer was low, only marginally enhanced by urea or dithiothreitol, and considerably declined after cooking. In dough and raw pasta, β-sheets represented the major secondary structure (55-66%) and further increased after cooking. However, while proteins in cooked millet pasta were almost exclusively aligned in β-sheets (93.9-99.9%), commercial wheat and gluten-free pasta retained some random and α-helical structures. Solubility positively correlated with cooking loss and protein digestibility, -helices positively correlated with solubility and cooking loss; random structures positively correlated with solubility, cooking loss and protein digestibility. In contrast, -sheets negatively correlated with solubility, cooking loss and digestibility. 5. Significance of your research to the food science field. Protein attributes were shown to affect product functionality in proso millet pasta. Moreover, our data provide an explanation for the low digestibility and solubility observed for proso millet in other studies. The correlation of protein structure to quality parameters can form the basis for recipe and processing optimization.