PHYSICAL APPROACHES TO IMPROVE THE QUALITY OF GLUTEN-FREE PASTA

The production of gluten-free (GF) pasta characterized bygood cooking behaviour is a challenge, above all for celiac people with a tradition of wheat-based products. GF pasta cooking quality depends on creating a network that imparts firmness and consistency to the pasta as well askeeping starch inside that matrix during cooking. The aim of this PhD thesis was to investigate physical approaches for improving the cooking quality of GF pasta. Particular attention was focused on the macromolecular organization of rice starch according to different processing conditions and on the relationships between starch structure and cooking behavior. The present PhD thesis, therefore, was divided into six chapters: 1. RECENT ADVANCES IN THE DEVELOPMENT OF GLUTEN-FREE PASTA Formulating and optimizing the conditions of the technological process to improve the nutritional and sensory characteristics of GF products still present a great challenge for researchers. In the case of dried pasta, several GF ingredients (mainly starch and/or GF cereal flours) have been used as alternatives to gluten in order to create a starchy network that would withstand the physical stresses associated with cooking and impart firmness to the product. Moreover, different variations of Oriental noodle-making technology have been proposed to simplify the artisan process based on several heating treatments and cooling steps of the dough, which are difficult to control and monitor. This section intends to overview the recent advances in the preparation of GF pasta. 2. EFFECT OF PHYSICAL TREATMENTS AND PROTEINS ADDITION ON THE QUALITY OF RICE PASTA. The production of GF pasta with cooking behavior similar to that of semolina remains a primary goal of the GF industry. In this study, the effects of physical treatments on rice kernels and the possible utilization of its corresponding flours for GF pasta-making were investigated. Three different heat-treatments were carried out on rice kernels: i) pregelatinization; ii) mild parboiling and iii) severe parboiling, obtaining PGF, MPB and SPB flours, respectively. Starch underwent new structural arrangements, the first involving macromolecular disorganisation and degradation. The heat-treatments affected the physical properties of the rice flour to a different extent (damaged starch evaluated as a-amylase susceptibility, water affinity and solubility, and pasting properties). The loss of starch granule integrity occurring during pregelatinization increased the fraction of damaged starch (54 %db), the mass of water taken up by starch (4.3 times more than native starch), the amount of soluble components leached out from the granules (3.2 %) and the viscosity at 30 °C (45.5 BU). Compared to pregelatinization, both parboiling processes promoted starch gelatinization and amylose and/or amylopectin reassociation, resulting in increased rigidity of the starch molecules (lower water absorption and solubility, lower damaged starch extent and viscosity at 95 °C). The magnitude of these changes significantly increased with the increase in parboiling treatment. Pasta from flour obtained from native milled rice (RF) was prepared by two extrusion steps, the first with steam and extrusion-cooking (140 °C) and the second with a conventional extruder (55 °C) to shape the steamed dough. On the contrary, pasta samples from heat-treated flours were prepared by using only the conventional extrusion step. The use of heat-treated flours improved the consistency of pasta (evaluated by Kramer cell test), in comparison with pasta from RF. Pasta from PGF showed the highest consistency during the compression (310 N) and shear (293 N) steps but, at the same time, an extreme elasticity (1530 Nmm). The use of SPB flour lowered the elasticity of the cooked product, keeping the consistency at high values (firmness 275 N). To lower the high cooking loss of this product (12.6%), the influence of texturing ingredients, such as PGF (50%), egg (1.5%) and whey (3-5%) proteins on pasta cooking behaviour was investigated. The best result was obtained by using 5% whey proteins, providing a positive effect on cooking loss (6.4%) and water absorption (94.5%), while maintaining the texture of the product prepared by using SPB rice flour. 3. RICE-BASED PASTA: COMPARISON BETWEEN CONVENTIONAL PASTA-MAKING AND EXTRUSION-COOKING. Good quality gluten-free products continue to be in demand among the celiac community and the production of pasta from non-conventional raw materials is a major technological challenge. In this work the effects of two different pasta-making processes (conventional and extrusion-cooking) were investigated on parboiled brown and milled rice flours. The two processes differed as to extrusion temperature (conventional extrusion: 50 °C, max; extrusion-cooking: 115 °C), whereas the drying diagram was the same. Starch modifications induced by each pasta-making process were analyzed by using a Micro-ViscoAmylograph (MVAG), Differential Scanning Calorimetry (DSC), and X-ray Diffraction. Cooking quality was evaluated by weight increase, solid loss into the cooking water, and texture analysis. Pasta obtained from milled rice using the extrusion-cooking process was characterized by the best cooking behavior. In this sample, starch presented the highest peak and final viscosities, the highest gelatinization temperature and lower enthalpy value, and the lowest crystallinity. The cooking quality of pasta obtained from brown rice appeared to be less affected by processing conditions. Therefore, the nature and intensity of starch modifications can be modulated by processing conditions which might explain the different cooking behaviour of rice pasta. 4. CHARACTERIZING STARCH STRUCTURE IN A GLUTEN-FREE PASTA BY USING IODINE VAPOR AS A TOOL. The suitability of starch-iodine complex to highlight differences in chain mobility and cristallinity of starch in rice pasta was investigated. Two pasta samples were produced starting from the same rice flour (RF) and using a conventional extrusion process without (Process A) and with (Process B) a preliminary extrusion-cooking step. Based on K/S spectra (obtained after equilibration above K2SO4 and exposure to iodine vapor), Pasta A showed a behavior similar to RF. Process B promoted a greater iodine binding capacity related to a higher starch chain mobility. Moreover, the extrusion-cooking conditions seem to favor the loss of starch crystallinity and the formation of larger amorphous regions. The starch organization observed in Pasta B could account for its higher capacity to water absorption during cooking. 5. UNDERSTANDING STARCH ORGANISATION IN GLUTEN-FREE PASTA FROM RICE FLOUR. Starches extracted from parboiled rice flour and pasta samples produced by two extrusion processes - a conventional one carried out at 50 °C and an extrusion-cooking process at 115 °C - were evaluated by DSC and SEC analysis. Molecular changes induced by both pasta-making process and the following cooking in boiling water were also investigated using iodine absorption properties of samples (expressed as the ratio of reflection spectra; K/S) and X-ray diffraction. A decrease in polymer chain mobility and iodine binding capacity were observed after pasta-making process. Even keeping the characteristic A-type crystalline pattern, the exposure to iodine vapor changed the peak intensity of starch samples, especially at 0.97 aw. The higher melting temperature of pasta samples in comparison with parboiled rice flour reflected the decrease in mobility of the amorphous regions detected by K/S spectral analysis. The pasta making-process also affected the molecular size distribution of starch samples. In particular, the elution peak shifted toward lower fraction numbers with increasing extrusion temperature, showing a higher molecular size for starch after the extrusion-cooking. All the differences detected between starch samples according to extrusion conditions were deleted during cooking. Compared to the uncooked samples, starch from cooked pasta showed higher K/S value at any wavelength, highlighting the increase in mobility of the amorphous region. Moreover, beside the increase in melting temperature, a decrease in endothermic entalphy were detected, confirming the important loss of order observed by X-ray diffraction. 6. INVESTIGATING THE STRUCTURAL ORGANIZATION OF PROTEINS AND POLYSACCHARIDES IN RICE PASTA BY THE AID OF ENZYMES. The consumption of rice pasta in Europe is showing a sharp increase. Processes that allow production of rice pasta with improved sensory and texture properties are expected to expand this market further. In this frame, processes for rice pasta were developed, that use pre-treated grains and result in products with the desired features. In this study, the structural organization of macropolymers in various types of starting flour (native and parboiled rice flour) and in the resulting rice pasta was investigated. Process-dependent changes in inter-protein interaction were assessed by evaluating their solubility in the presence/absence of chaotropes, the accessibility of reactive aminoacidic sidechains, and their surface hydrophobicity. Pre-treatment mainly affected the interactions among proteins, that were anymore so evidenced after pasta-making process (conventional extrusion or extrusion-cooking). The overall structure of the polysaccharide components was assessed by studying their accessibility to specific hydrolytic enzymes (a-amylase and pullulanase), and by evaluating the nature and size of the resulting hydrolytic fragments by means of Light Scattering/SEC. Grain pre-treatment increased accessibility to hydrolytic enzymes. The pasta-making process induced further changes in the structural organization of polysaccharides, that were comparatively most evident in the sample made form untreated rice. The amount of fragments generated by the action of amylase and pullulanase on various samples was consistent with the accessibility data, suggesting that differences in starch arrangements were likely related to the amylopectin fractions. Nevertheless, the development of pasta with a low glycemic index (GI) requires further studies based on the modification of amylopectin fine structure during parboiling process.