Millet-, soybean-, and rice bran-based gluten-free breads formulated with Type I sourdough: effect on quality and in vitro glycaemic index

Gluten-free breads (GFBs) are typically formulated with refined flours and starches, and are characterised by the absence of the viscoelastic gluten network, which compromises the structural integrity and promotes rapid staling and starch digestibility. The aim of this study was to evaluate GFB formulated with Type I sourdough (SD) from millet (M), rice bran (RB), or extruded-expelled soybean (SEE), on fermentation parameters, technological properties, and estimated glycaemic index (eGI). GFB were prepared with 30.0 SD-M (GFB-M), 37.5 SD-RB (GFB-RB), or 52.5 SD-SEE (GFB-SEE) g.100g^-1 (d.b.). The control formulation without sourdough (GFB-O) was prepared with dehydrated baker's yeast and contained 15.0 M, 15.0 RB and 15.0 SEE g.100g⁻¹ (d.b.) Total titratable acidity (TTA) was determined by titration with 0.1 M NaOH to pH 8.3. Specific volume (SV, cm³·g⁻¹) was measured using the rapeseed displacement method (AACC 10-05.01) and eGI was determined using an in vitro methodology. GFB-O showed the highest pH and lowest TTA (6.32 ± 0.09; 0.25 ± 0.07 mL NaOH), while GFB-M, GFB-RB and GFB-SEE presented significantly lower pH and higher TTA than GFB-O (p ≤ 0.05). SV ranged from 2.2 to 4.3 cm³/g, with GFB-SEE and GFB-RB showing the lowest values and GFB-O the highest (p ≤ 0.05). GFBs with SD addition had a significantly reduced eGI, with a maximum reduction of 12.3% compared to GFB-O (p ≤ 0.05). Overall, the use of Type I sourdough process modified the fermentation dynamics and quality characteristics of GFBs, thereby affecting both technological and nutritional properties in the formulations.