Physical modification of gluten-free flours by ultrasound treatments. Application to the development of new products suitable for the celiac population

Gluten-free market has gained a greater presence in the food industry in recent years due to increasing population being diagnosed with celiac disease or gluten intolerance, and consumers who decide to follow a gluten-free diet for considering it as a healthier alternative, who demand a wider range of good quality products. Gluten removal, however, represents a technical challenge in the development of products, due to the structural role of gluten in most of these products. The natural characteristics of gluten-free flours (and their starch) do not always meet specific industry requirements due to processing limitations, such as low shear resistance, thermal resistance, and high tendency towards retrogradation. Different modification techniques (chemical, enzymatic, genetic, mechanical, and physical) are applied to starches and flours with the aim of increasing their range of industrial applicability, being physical modifications the best perceived for involving more environmentally friendly methods, and for requiring, in general, shorter and simpler processing. Ultrasound treatments are physical modification methods that have demonstrated significant improvements in the modification of starches from different botanical origins. However, the modification of gluten-free flours with ultrasounds has not been greatly studied despite being an important ingredient in the food industry. The objective of this doctoral thesis was to get deeper knowledge about the physical modification of gluten-free flours by ultrasound treatments, to stablish the impact of key treatment parameters on the degree of modification achieved, and to use ultrasonicated flour in the development of gluten-free breads to determine the effect that the modification has on the quality of the final products. Ultrasound treatments were performed at a frequency of 24 kHz, a maximum power of 180 W, and a on/off cycle of 80 %, and were applied to flours of different nature: rice, tef, corn, and quinoa. Ultrasound treatments are always performed in excess water so that the acoustic waves generate a homogeneous modification in the studied sample. The study included an initial phase to determine the effect that three important parameters of ultrasound treatments had on the modification generated to rice flour. These parameters were treatment time (between 2 and 60 min), flour concentration in the treated dispersion [between5 % and 30 % (w/w)], and treatment temperature (between 20 °C and 60 °C). The research began with rice flour since it is the most commonly used ingredient in gluten-free formulations. The results obtained indicated that the modifications were significant starting from short times, and that they could be observed even at high concentrations. Treatment temperature was the parameter that influenced the most the modification caused to the treated flours. Because of this, in the next phase of the study two varieties of tef were investigated, brown and white, applying treatments of 10 min, at a concentration of 25 % (w/w), and temperatures between 20 °C and 55 °C. The modifications caused to rice and tef flours were quantified in their morphological, techno-functional, hydration, structural, thermal, and pasting properties, as well as in the rheological properties of the gels made with them. The results obtained showed that ultrasound treatments led to an overall particle size reduction of the flours, which increased their interaction with water, demonstrated by a marked swelling power increase in ultrasonicated flours. Size exclusion chromatography indicated that ultrasound treatments generated a higher proportion of intermediate length amylose chains (degree of polymerization 300 – 1600), as result of starch chains fragmentation, which took place mostly on α-(1,4) bonds rather than α-(1,6) bonds, as demonstrated by proton nuclear magnetic resonance. Thermal properties indicated a reduction of gelatinization enthalpy, as well as a reduction of the gelatinization temperature range, mainly when treatments were performed at higher temperatures.