Biotransformation of gluten-free composite flour mediated by lactic acid bacteria via solid-state fermentation process conducted under different moisture contents

Over the years, the development of staple food products from composite flour is considered a low-cost approach to address the burden of protein-energy malnutrition in developing countries. However, despite the numerous advantages of composite flour, the high amount of anti-nutritional factors typical to food derived from plant origin usually impede nutrient bioavailability. Five strains of lactic acid bacteria (LAB) isolated from Malaysian foods that include Lactiplantibacillus plantarum RG-14, L. plantarum RI-11, L. plantarum RS5, L. plantarum IUL-4, and Pediococcus pentosaceus UP-2 have been reported for their capabilities to produce various extracellular hydrolytic enzymes via solidstate fermentation (SSF) which can breakdown complex food matrix into smaller absorbable forms and reduced antinutrients. Therefore, the LAB strains were employed in this study to biotransform the gluten-free composite flour derived from rice, sorghum, and soybean. The SSF process was performed under 30- 60% moisture content for 7 days, where samples were withdrawn at 24 h intervals for various analyses such as LAB cell viability, pH, total titratable acidity, extracellular protease activity, soluble protein concentration, crude protein content, and in vitro protein digestibility. The pH of the biotransformed composite flour showed a significant reduction from the initial range of pH 5.98 - 6.67 to the final pH of 4.36 - 3.65, corresponding to the increase in the percentage of total titratable acidity in the range of 0.28 - 0.47% to 1.07 - 1.65% from Day 0-4 and remained stable till Day 7 of the SSF process. The LAB strains exhibited high extracellular proteolytic activity (0.63 - 1.35 U/mg to 4.21 - 5.13 U/mg) from Day 0-7. In addition, the treated composite flour soluble protein increased significantly (p≤0.05) (0.58 - 0.60 mg/mL to 0.72 - 0.79 mg/mL) from Day 0-7, crude protein content (12.00 - 12.18% to 13.04 - 14.39%) and protein digestibility (70.05 - 70.72% to 78.46 - 79.95%) from Day 0-4 of SSF. In addition, the antinutritional factors of the biotransformed composite flour showed a significant reduction (p≤0.05) in the phytic acid (127.11 – 137.73 mg/100 g to 124.84 - 120.24 mg/100 g) and tannin content (89.48 – 93.92 mg/100 g to 63.51 – 39.84 mg/100g). Since lower moisture content promotes flour quality, 50% moisture was selected as the most suitable moisture content to have effectively biotransformed the composite flour, even though a comparable result was observed at 60% moisture content. Overall, Lactiplantibacillus plantarum RG-14 was ranked the best strain attributed to the general improvement in composite flour's pH, TTA, protein quality, and antinutritional properties.

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