Molecular features of starch in rice pasta depend on processing conditions and affect cooking behavior

Although there is a broad variety of gluten-free pasta (GFP) on the market, unfortunately, most of them exhibit poor cooking quality. Currently, GFP production is mainly based on empiric approaches, in which ingredients and/or processing conditions (heating/cooling treatments) are varied without a thorough understanding of the macromolecular organization associated with good or poor cooking quality. The ability to relate process-dependent modifications in some macromolecules to product features may represent the starting point for engineering rice pasta with suitable textural and nutritional properties. The aim of this work was to study the structural changes on starch in rice pasta as a function of raw-materials and pasta-making conditions by investigating the accessibility of starch macromolecules to various specific enzymatic activities. Moreover, the impact on cooking behavior was assessed. Rice pasta was prepared from native rice flour by extrusion-cooking (pasta A) and from parboiled rice flour by conventional extrusion (pasta B) or by extrusion-cooking (pasta C), as described by Marti et al. (2010). Starch arrangement was studied by assessing starch accessibility to specific enzymes (alpha-amylase or pullulanase). Glucose release after 1h and 24h hydrolysis was measured, and the molecular properties of fragments from enzymatic action were evaluated by means of light scattering/sec. Native rice flour showed the lowest susceptibility to both alpha-amylase and pullulanase, suggesting packing of amylopectin branches into a dense structure. Parboiling increased accessibility to alpha-amylase, but amylopectin was not accessible to the action of pullulanase in flour from parboiled rice, mainly due to the reassociation of gelatinized starch granules during the cooling and drying steps. Pasta-making process induced further changes in the structural organization of polysaccharides. The amount of starch solubilised from pasta A after 24h alpha-amylase hydroysis was lower than the ones from pasta B and pasta C, suggesting that estrusion-cooking process on native flour did not allow a complete accessibility of the starch to alpha-amylase. Moreover, in pasta A, amylopectin debranching by pullulanase increased more than 10-fold after 1h hydrolysis, suggesting the presence of a high portion of -1,6 linkages in the external regions of the structure. As regards pasta from parboiled rice, the amount of soluble material released by pullulanase was higher in pasta C than in pasta B, indicating that extrusion-cooking of previously parboiled flour could generate a starch structure characterized by less organised (and therefore more accessible) regions. Regarding the nature and size of soluble products from enzymatic hydrolysis, starch in pasta A showed a greater release of di- and trisaccharides than the corresponding starting flour. Release of di- and trisaccharides from pasta C was very low, suggesting once again a significant impact of the extrusion-cooking step on the amylopectin component. This aspect, together with the limited accessibility to pullulanase, resulted in a GFP with extreme firmness after cooking. On the contrary, pasta B had satisfactory textural parameters. Amylopectin in the uncooked material was loosely structured, as indicated by its sensitivity to the action of pullulanase. From a methodological standpoint, the approach presented seems to be able to provide useful insights for improving the current molecular-level understanding of the effects of treatment conditions on starch properties and overall pasta quality. References: Marti A., Seetharaman K., And Pagani M.A. (2010): Rice-based pasta: A comparison between conventional pasta-making and extrusion- cooking. Journal of cereal science, 52, s 404-409. This work was supported in part by grants from global rice science partnership (GRISP).