Intermediate length amylose increases the crumb hardness of rice flour gluten-free breads

Consumers of Gluten Free Bread (GFB) seek soft, cohesive and resilient crumbs. However, the combined effect of amylose (AM) content and chain length, as opposed to only AM content, on crumb hardness have not been studied, neither the synergies of multiple starch molecular features (i.e., amylopectin, AP, and AM fine structures) are understood. In this work, kernels from 5 different rice varieties (Basmati, Thai, Waxy, Sushi and Bomba) were milled after tempering to result in rice flours with less than 8% starch damage varying in AM and AP fine structure, as measured by HPSEC. Dynamic rheology of rice flour gels revealed that Basmati flour exhibited a six-fold higher propensity to form internal Physical Junction Zones (PJZ), those that contribute to the food mechanical properties, than the rest of the starches. Frequency and temperature sweeps revealed AM-AM and AM-AP interactions, respectively, as the internal PJZ. Since Basmati and Bomba exhibited similar AM content and AP fine structure, the intermediate length amylose (699 DP) of Basmati is proved to be the responsible for a greater amount of AM-AM and AM-AP internal PJZ. Interestingly, GFB crumbs made with Bomba flour were 20% softer than those made with Basmati flour. For the first time, AM length is suggested to be a crucial parameter to attain breads with softer and more cohesive crumbs. In fact, results provide strong evidence that the AM length could play a major role than its content in the texture of GFBs crumbs.