Pulse-Cereal Blend Extrusion for Improving the Antioxidant Properties of a Gluten-Free Flour

Abstract

Extrusion is an interesting technological tool that facilitates pulse formulation into flour mixtures, with tailored fibre content, total antioxidant capacity (TAC) and glycemic index (GI) among other components in final formulas. The gluten-free (GF) market has significantly grown during the last years. GF products have evolved from specialty health foods to products targeted to the general population and not only associated to celiac consumers. This study evaluates how temperature, cereal base (rice/corn) and pulse concentration affect extruded flour properties and which conditions are more efficient to develop a gluten-free flour with high TAC and low GI. Additionally, it evaluated the effect of this optimal formula after the baking process. The results showed an increase of total phenol (TP) and antioxidant activity with extrusion, with a temperature-dependent effect (130 °C ≥ 120 °C ≥ 110 °C), which may imply an enhanced bioaccessibility of phenolics compounds after extraction. Extrusion increased GI in comparison to native flour; however, a dough temperature of 130 °C resulted in a significantly (p ≤ 0.05) lower GI than that observed for 110-120 °C doughs, probably associated to the pastification that occurred at higher temperatures, which would decrease the degree of gelatinization of the starches and therefore a significant (p ≤ 0.05) GI reduction. Corn-lentil flour showed higher antioxidant properties and lower GI index in comparison with rice-lentil blends. The formulation of the optimal blend flour into a baked product (muffin) resulted in a significant loss of antioxidant properties, with the exception of the reducing power (FRAP), although the final antioxidant values of the baked product were in the range of the original native flour blend before any process.

Keywords: SEM; antioxidant; corn; extrusion; glycemic index; lentil; rice.

Figure 1

Image analysis (R: red; G: green; B; brown) of native and extruded flours. Abbreviations: cf: corn flour; rf: rice flour; lf: lentil flour; ecf: extruded corn flour; erf: extruded rice flour; elcf: extruded lentil (50% or 15%)-corn flour; elrf: extruded lentil (50% or 15%)-rice flour; elcm: baked product with extruded lentil-corn flour.

Figure 2

Total phenol (TP, mg GAE 100 g⁻¹ dry matter, (I)), total antioxidant activity as ORAC, ABTS•+, DPPH ((II–V), µmol Eq. Trolox 100 g⁻¹ dry matter), and FRAP (mmol Fe²⁺ 100 g⁻¹ dry matter) of native and extruded flours. Abbreviations: cf: corn flour; rf: rice flour; lf: lentil flour; ecf: extruded corn flour; erf: extruded rice flour; elcf: extruded lentil (50%)-corn flour; elrf: extruded lentil (50%)-rice flour. Different small letters indicate differences (p ≤ 0.05) between samples.

Figure 3

PCA analysis for native flours and extruded flours at different conditions. Abbreviations: cf: corn flour; rf: rice flour; lf: lentil flour; ecf: extruded corn flour; erf: extruded rice flour; elcf: extruded lentil (50%)-corn flour; elrf: extruded lentil (50%)-rice flour.

Figure 4

Glycemic index (GI, (I)) and glucose kinetic consumption (µg mL⁻¹) (II) of native and extruded flours. Abbreviations: cf: corn flour; rf: rice flour; lf: lentil flour; ecf: extruded corn flour; erf: extruded rice flour; elcf: extruded lentil (50%)-corn flour; elrf: extruded lentil (50%)-rice flour. Different small letters indicate significant differences (p ≤ 0.05) between samples.

Figure 5

Scanning electron microscope (SEM) of native (corn and lentil) and blend lentil-corn extruded flours at different temperatures. Abbreviations: cf: corn flour; lf: lentil flour; elcf: extruded lentil (50%)–corn flour.

Figure 6

Experimental design.