Abstract

Dehulled yellow pea flour (48.2% starch, 23.4% proteins, d.b.), was processed by a twin-screw extruder at various moisture contents MC (18–35% w.b.), product temperature T (115–165 °C), and specific mechanical energy SME (50–1200 kJ/kg). Structural changes of extruded pea flour were determined at different scales by measurements of density (expansion), crystallinity (X-ray diffraction), gelatinisation enthalpy (DSC), starch solubility in water and protein solubility in SDS and DTE (SE-HPLC). Foam density dropped from 820 to 85 kg/m3 with increase in SME and T (R2 ≥ 0.78). DSC and XRD results showed that starch was amorphous whatever extrusion conditions. Its solubility in water augmented up to 50%. Increasing temperature from 115 to 165 °C decreased proteins soluble in SDS from 95 to 35% (R2 = 0.83) of total proteins, whereas the proteins soluble in DTE increased from 5 to 45% (R2 = 0.75) of total proteins. These trends could be described by sigmoid models, which allowed determining onset temperatures for changes of protein solubility in the interval [125, 146 °C], whatever moisture content. The SME impact on protein solubility followed similar trends. These results suggest the creation of protein network by SS bonds, implicating larger SDS-insoluble protein aggregates, as a result of increasing T and SME, accompanied by creation of covalent bonds other than SS ones. CSLM images suggested that extruded pea flour had a composite morphology that changed from dispersed small protein aggregates to a bi-continuous matrix of large protein aggregates and amorphous starch. This morphology would govern the expansion of pea flour by extrusion.

Highlights

  • Pulse legumes are excellent source of proteins (20-30% dry basis), dietary fibers (10-30%), starch (40-55%), vitamins and minerals, and they are low in sugar, sodium and fat (El-Adawy, Rahma, El-Bedawey, & El-Beltagy, 2003)

  • The most severe extrusion conditions values corresponding to highest temperature and specific mechanical energy (SME) were obtained for sample C1 with low moisture content (21.4%)

  • The large domain of hydro-thermo-mechanical conditions of pea flour extrusion led to a large panel of starch and protein chemical changes, and of morphologies of the foams cell wall material

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Summary

Introduction

Pulse legumes (pea, lentil, faba bean) are excellent source of proteins (20-30% dry basis), dietary fibers (10-30%), starch (40-55%), vitamins and minerals, and they are low in sugar, sodium and fat (El-Adawy, Rahma, El-Bedawey, & El-Beltagy, 2003). Texture properties of extruded products, envisioned like solid foam, depend on density and cellular structure and on the mechanical properties of the material which constitutes the cell walls of the foam. These properties depend on the morphology of the cell wall material created during extrusion under high temperature and shear. These conditions lead to many biopolymer transformations: starch melting, depolymerisation, protein denaturation, browning, and destruction of anti-nutritional components. Moisture content probably increases the mobility of proteins and their cross-linking (Holay & Harper, 1982)

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