Effectiveness of rice bran in delaying starch retrogradation during ageing

The high amount of starch contained in gluten-free baked products is largely responsible for their low nutritional value and fast quality decay during ageing. Starch retrogradation, in fact, is the major phenomenon involved in bread staling. The aim of this research was to evaluate the rheological properties of gels obtained from corn starch (CS), rice flour (RF) and their mixture (50CS-50RF), and the capability of rice bran (RB) to reduce starch retrogradation when added at different ratios (25% and 50%), in order to increase the shelf-life of starch-containing foods. Gels were prepared by means of the Brabender® Micro-Visco-Amylograph (MVA) (Mariotti et al., 2005) and their rheological properties, during a 7 days storage period at 4°C, were evaluated both through compression test (TA-HDplus Texture Analyzer) up to 30% deformation, and frequency sweep test (Physica MCR 300 Rheometer) performed over the range 0.1-10Hz at 1% strain. During ageing, gels consistency increased at different extents and rates. Samples containing CS showed the highest stiffness (CS: 3.34+/-0.11N/mm, 50CS-50RF: 3.33+/-0.03N/mm) and the highest storage modulus (G’; CS=1393+/-25Pa, 50CS-50RF=972+/-8Pa at 1Hz), after 7 days of storage at 4°C. On the contrary, RB did not form a gel in the experimental conditions adopted for the MVA test. If added at 25% or 50% to CS or RF as well to their mixture, RB originated weak gels and resulted very effective in delaying starchy gels hardening. Gels obtained from 50CS-50RB and 50RF-50RB, in fact, remained rather soft at the end of the storage period, exhibiting stiffness values equal to 0.90+/-0.01N/mm and 0.60+/-0.03N/mm, respectively. Furthermore, the presence of RB, at 25% and 50% substitution levels, strongly reduced the G’ values of the gels containing CS or RF (e.g. 50CS-50RB: G’=128+/-6Pa, 50RF-50RB: G’=76+/-1Pa, at 1Hz) and, for the same gels, it caused the overlapping of G’ curves up to 7 days storage, indicating very slow hardening kinetics. The results obtained thus highlight the possibility to enhance both the shelf-life of gluten-free baked products and their nutritional value by including RB among the other ingredients. The aim of the future research is to better identify the proper amount of RB to be used, in order to allow an appropriate handling of the dough and to enhance the maintenance of gluten-free baked goods softness during storage.