Pascalle J.M. Pelgrom

Thermomechanical Morphology of Peas and Its Relation to Fracture Behaviour

Milling and subsequent air classification can be exploited for production of functional protein-enriched fractions from legumes and grains. Fracture behaviour is of large relevance to optimal disentanglement of protein and starch and is determined by the thermomechanical morphology of the seeds. Thermomechanical properties of peas were explored as a function of temperature and moisture content. Differential scanning calorimetry and thermal mechanical compression tests were carried out on pea protein and starch isolates yielding similar glass transition temperatures. Glass transition lines were successfully constructed using the Gordon–Taylor equation. Subsequently, three regions were identified in the state diagram; starch in the glassy and protein in the rubbery state, both components in the glassy state, and both components in the rubbery state. From single pea fracture experiments, it was found that the completely glassy peas fractured at a smaller critical compression distance compared to the peas in the other two regions. This can be explained by the elastic behaviour of the rubbery protein network, having a detrimental effect on the energy efficiency of milling processes. However, from scanning electron microscopy, it appeared that in rough fracture planes, visible when the protein was in the rubbery state, starch granules were present as more separate identities, suggesting increased disentanglement. Disentanglement of protein and starch by milling would then be optimal when protein is in the rubbery state. The latter can be achieved by milling at increased temperature and/or moisture content, which would be an attractive alternative.

Method Development to Increase Protein Enrichment During Dry Fractionation of Starch-Rich Legumes

A facile method was developed to establish milling settings that optimally separate starch granules from protein bodies and cell wall fibres for starch-rich legumes. Optimal separation was obtained for pea, bean, lentil and chickpea when the particle size distribution curve of flour and isolated starch granules overlap maximally. This outcome was based on scanning electron microscopy, protein content of the fine fraction and particle size distribution curves. Milling settings differed between legumes due to variances in seed hardness and starch granule size. The protein content of the fine fraction was legume specific as well and could be explained by differences in particle density, seed hardness, starch granule size, fat content and flour dispersibility.