Fractionation methods and post-heat treatment shape in vitro protein and starch digestibility of oat protein ingredients

The digestibility of plant protein ingredients is influenced by processing, yet the roles of fractionation and post-heat treatment in oat ingredients remain poorly understood. This study investigated how dry fractionation, alkaline extraction–isoelectric precipitation (AE-IP), and water-only extraction influence composition, particle morphology, solubility, and in vitro protein and starch digestibility of oat protein ingredients. Oat protein concentrate produced by dry fractionation (OPC), oat protein isolate produced by AE-IP (OPI), and water-only extracted oat protein isolate (OPIW) showed distinct compositional and morphological characteristics. Protein digestibility was lowest for OPIW (26.3%), followed by OPI (45.3%), and highest for oat flour (66.2%) and OPC (61.4%). Despite similar protein profiles and evidence that proteins were not fully denatured prior to heating, OPIW exhibited low digestibility together with low solubility and sponge-like particle morphology. In contrast, starch digestibility showed limited variation among ingredients. Heating hydrated samples at 70 °C for 5 min induced starch gelatinization but did not affect protein digestibility, whereas heating at 110 °C for 5 min reduced protein digestibility by approximately 30% in flour and 21% in OPC, consistent with the loss of protein denaturation transitions measured by DSC. The results suggest that differences in digestibility among oat protein ingredients cannot be explained by composition alone and are associated with processing-induced changes during production. The findings further indicate that thermal effects on protein digestibility depend on the fractionation method, which may be relevant for selecting oat ingredients for thermally processed foods. Understanding how these processing-dependent differences translate to digestion kinetics and postprandial responses is an important next step.