Extruded floursapplications and new functionalities

Abstract

¿ Abstract Among cereals, wheat, rice and maize, and especially their starchy fraction, are an important part of the human diet. Starch can be degraded to glucose, which provides the body with an important source of energy. In addition, starch from these flours is important because of its effect on the physical properties of many of our foods. However, continuous innovation in foodstuff and their higher quality requirements force to adapt those commodities to the emerging needs in terms of functionality and nutrition. The modification of the functionality of starchy ingredients by hydrothermal treatments is becoming of great interest, among other things due to the absence of artificial ingredients and chemicals. Among those treatments, extrusion is one of the most versatile alternatives to industrially achieve starch gelatinization. The first objective of this doctoral thesis consisted on the study of the influence of the different extrusion conditions (barrel temperature, feed rate and feed moisture content) on the physicochemical and digestive properties of gluten-containing (wheat) and gluten-free (rice) flours. An increase in the extrusion severity caused a higher amount of gelatinized starch. This gelatinised starch had a great water absorption capacity, enhancing significantly the hydration properties of flours (5-fold water binding capacity and 9-fold swelling compared to the untreated wheat flour). This hydration ability leads to higher viscosity in cold solution, which might be very interesting for some food applications. Added to that, the susceptibility to enzymatic hydrolysis increased, which could indicate that these flours can be good raw materials for the starch conversion industry in the production of maltose or glucose. Nevertheless, this high susceptibility also affected negatively the starch digestion properties, decreasing the amount of resistant starch (RS) as the extrusion severity increased in both rice and wheat extruded flours. The different physicochemical properties of extruded flours were also exploited to study the effect of their incorporation in intermediate products during processing, such as batters for coating, whose most crucial parameters is their viscosity and rheological properties. Extruded flour substitution increased the consistency coefficient and pseudoplasticity of batters, indicating an important increase of their apparent viscosity. Those changes were attributed to the diminution of the water content as a consequence of the high water absorption capacity of gelatinised starch and of the unfolded proteins of extruded flour. Results suggested that a 15% of wheat flour replacement by wheat flour subjected to different extrusion treatments would offer batters with different viscosities and viscoelastic properties increasing the range of batters coatings available in the market. Sometimes native or extruded flours can need further supplementation with other ingredients or additives for their use in certain products. In this way, extruded and native wheat flours were combined with agars (Gracilaria and Gelidium) and carrageenans (k-carrageenan and i-carrageenan), the marine hydrocolloids most commonly used in the food industry, to modify their paste and gel properties. Pasting properties of native flour combinations showed that all hydrocolloids produced an increase in peak viscosity (particularly Gelidium agar), but only agars decreased the onset temperature of gelatinization and increased breakdown, indicating a different mechanism of action. In general, carrageenans, besides producing stiffer and more stable pastes, gave rise to harder and clearer gels compared to those made with agar, for both their combination with extruded and native wheat flours. Meanwhile, pastes made with combinations of extruded flours and agar showed higher thixotropy, which could be beneficial in sauce-making. The effect of the addition of flour subjected to different extrusion conditions on gluten containing and gluten-free breads was also investigated. In wheat breads, results suggested that it is possible to obtain adequate dough and bread characteristics using dough with 5% extruded wheat flour. The addition of 5% extruded wheat flour allows the quantity of water in the formulation to be increased; the more intense the extrusion treatment, the greater the increase in the quantity of water that can be added. This would increase the bread output. In gluten-free breads, the substitution of 10% of rice flour by extruded rice flour also increased the bakery yield. However, their addition also reduced the specific volume of breads and increased hardness. Nevertheless, these effects were minimized by using the coarse flour fractions (132-200µm), which also reduced the rate of staling. In another study to delay the bread staling in gluten-free breads, the addition of lipase and extruded flour increased bread volume and reduced the initial firmness and hardening of breads. With the aim of exploiting the high susceptibility of extruded flours to the enzyme catalysis, the influence of an enzymatic amylolysis with ¿-amylase and glucoamylase on native and extruded wheat flours was studied. 300% and 500% increases of glucose and maltose contents, respectively, in extruded flours compared to their native counterparts were achieved. Thereby, results suggested that extruded flours are good substrates for fermentative microflora and the main reactants that participate in Maillard and caramelisation thermal reactions. From a nutritional point of view, that high susceptibility to the enzymatic hydrolysis also increases the action of the pancreatic ¿-amylase, declining the digestion properties of those flours. In this way, branching enzyme (B) and maltogenic ¿-amylase (MA) were studied to attenuate the starch digestion properties of maize extruded flours. The enzymatic treatment itself affected the formation of resistant supramolecular structures on extruded flours by increasing: 1) the long linear amylose chains susceptible to retrograde; 2) the level of short-range molecular order, typical from conformational ordering of amylose or amylopectin side chains; 3) and the amylose-lipid complexes, structures related with RS. Moreover, the combination of B and MA on extruded flours increased: 1) the number of branching points and the ratio of short chains to longer chains in amylopectin, associated with slowly digestible starch (SDS); 2) and the amount of panose and isomaltotriose, categorized as prebiotic.