Resumen de Thermorheological characterization of gluten-free doughs from chestnut flour
This work of Thesis involves the systematic research of the physicochemical characterization and the mixing and thermorheological behaviour of gluten-free doughs based on chestnut flour. The absence of gluten, low proteins content and the presence of high sugars content provoke a lack of suitable flow-mechanical properties for the processing of chestnut flour doughs. This calls for the intervention in the preparation conditions (kneading and mixing) and in the presence of specific additives (hydrocolloids, fats, etc.) or other flours (cereals) in order to provide technological aptitude to these doughs, particularly in the rheological viscoelastic character. This is a scientific and technological challenge, because during the doughs processing several simultaneous or successive phenomena takes place (water absorption, proteins network weakening, gelatinization and gelling of starch, modification of amylose/amylopectin ratio, damaged starch, etc.) which make the process difficult to control.
The demand of high quality gluten-free food products has considerably increased in recent years due to an increase in celiac patients. Celiac disease is a chronic intolerance to the gluten proteins and the only treatment is to keep a gluten-free diet for all the life, since its ingestion damages the small intestine mucous, decreasing the nutrient absorption. Hence, the target operational conditions are sought to be studied in order to develop new formulations based on raw material currently underused in the industry, but with attractive physicochemical, nutritional and healthy characteristics like chestnut flour. One possibility with a great potential for increasing production and chestnut consumption (Castanea sativa Mill) consist of obtaining flours and starches to prepare the doughs and their future transformation in new products. The physicochemical and thermorheological characterization of chestnut flours and doughs and their starch in a wide range of operational conditions is a first step of great interest for this research and at industrial level.
In this work, the commercial chestnut flour was used as control sample while other commercial flours (soft, hard and whole wheat) were selected as reference systems; other gluten-free flours (rice, corn and formulations based on corn starch) were employed with comparative purposes. The preparation (30ºC) and characterization of doughs of these systems as well as the systematic study of their thermorheological behaviour were carried out in this Thesis taking into account the influence of particle size (chestnut flour) and the presence of common additives in the bakery process (sugars (sucrose: 0.6; 1.8; 3.4 and 5,0%, flour basis, f.b.), salts (sodium chloride: 0.6; 1.2 and 1.8% f.b.), olive and sunflower oils (2.0; 4.0; 6.0; 8.0 and 10%, f.b.), hydrocolloids (agar, arabic, carboxymethyl cellulose, guar, hydroxypropyl methyl cellulose, tragacanth and xanthan gums: 0.5; 1.0; 1.5 and 2,0%, f.b.) or other new sources such as the chestnut starch (5.0; 10 y 15%, f.b.) or the chia flour (2.5; 4.0; 5.0; 6.0 y 7.5%, f.b.). Likewise, the determination of the rheological properties with the aim of generate possible interactions which involve enhanced rheological phenomena under the simultaneous presence of chia flour (4.0%, f.b.) and some of the studied hydrocolloids (guar, hydroxypropyl methyl cellulose and tragacanth gums: 0.5-2.0%, f.b.), or blends with other gluten-free flours such as rice at different proportions (chestnut flour/rice flour: 100/0, 90/10, 80/20, 70/30, 60/40, 50/50 and 0/100) were also considered as objectives of this Thesis. The concentrations of the assayed additives in the systems with lower particle size were only modified in the case of oils addition (4.0-12%) and the blends of chestnut and rice flours (100/0; 30/70; 25/75; 15/85; 5.0/95 and 0/100).
For the understanding, assessment and control of the stages of mixing, kneading and doughs preparation it was necessary to determine a large number of properties by laboratory assays using several techniques and equipments. Thus, this was achieved by the chemical characterization (moisture content, amylose/amylopectin ratio, proteins, etc.), particle size characterization (equipment of sieving and laser diffraction), colour evaluation (colorimeter), morphology (microscopy), etc., that have been carried out for chestnut flour and its starch. Other devices as the laboratory kneader-mixer (Mixolab®) that allowed obtaining different parameters of mixing stage (water absorption, development time, stability, etc.) and, above all, the called mixing and complete curves as well as the controlled stress rheometer that allowed to obtain the flow curves, mechanical spectra, creep-recovery curves and temperature curves were employed for the thermorheological characterization.
The physicochemical characterization of the assayed chestnut flour (particle size, morphological, colorimetric and compositional properties (mainly starch and proteins)) which was employed as control sample was carried out. Similar studies with chestnut starch isolated from different sources under different processing conditions (drying/milling) were performed. The basic properties of the rest of the assayed flours (wheat, rice and corn) and their starches were also determined with comparative purposes. The commercial chestnut flour showed high average particle size values (varying from 146.5 to 190.0 ¿m depending on the employed experimental technique) and the lowest moisture content (9.1%) when compared with the assayed cereal flours. A bimodal particle size distribution was obtained for chestnut flour and the studied starches (similar to the typical particle size distribution of the commercial wheat flour). The particle size of chestnut starches decreased significantly depending on the previous processing of initial raw material. The oval shape and brownish colour were predominant in the chestnut flour grains used as control sample. Peculiar shapes were also identified in its starch granules. The chemical composition analysis showed that chestnut flour presented high sugar content (23.7%) and low protein content (6.3%) and damaged starch (3.1%) corresponding to typical values of flours used in baking.
Water adsorption/desorption isotherms for chestnut, wheat and rice flours at different temperatures (range from 20 to 65ºC) in a wide water activity range (0.09-0.91) were experimentally determined (gravimetric method). The chestnut flour was less hygroscopic than wheat or rice flours. Using the same experimental conditions water adsorption isotherms for some of the main components (chia flour and hydrocolloids: guar gum, tragacanth, carboxymethyl cellulose and xanthan gum), used in this study as additives on chestnut flour doughs, were also obtained. The obtained curves for all cases were type II according to the Brunauer-Emmett-Teller (BET) classification. The experimental results were fitted by means of different mathematical models found in the literature, usually applied to this kind of systems. The best results were achieved using the model proposed by Guggenheim-Anderson-de Boer (GAB). Additionally, this model provides the advantage that its parameters show physical meaning. A basic thermodynamic analysis of the sorption data was also developed. The net isosteric sorption heat variation with equilibrium moisture content was assessed using the Clausius-Clapeyron equation. In the case of the studied hydrocolloids, a cross point between differential and integral entropy was observed at a moisture content value where the maximum of integral entropy was achieved, that, in turn it is close to the values of monolayer moisture content obtained by the GAB model and thus constituting the maximum stability point for these systems.
In this sense, a prediction model of hygrometric equilibrium was proposed and developed in order to obtain water desorption isotherms applicable to flour and a wide range of foods (fruits, vegetables and legumes), based on the chemical composition of the main components (glucose, fructose, sucrose, salt, starch, protein and fibre) in the studied temperatures range (25-65ºC). In this way, the performed experimentation was intended to be useful as experimental reference in order to validate the developed prediction model. Due to the scarce literature data in order to develop such model, it was necessary obtaining experimentally water desorption isotherms of different components (sugars and chestnut starch) in the temperature ranges of interest. Particularly, sugars water desorption isotherms, conversely to the rest of assayed systems, showed desorption isotherms of type III and were satisfactorily fitted with the water activity and the temperature using the Henderson model.
Once the physicochemical properties of the assayed systems were characterized, the preparation and characterization of chestnut flour doughs using Mixolab®, as well as other cereal flours or formulations employed as reference systems were performed. In this apparatus the torque variation generated by dough with the mixing time at pre-fixed speed (80 rpm) depending on the temperature was registered. The first assays (mixing curves) for each one of the studied doughs were carried out at constant temperature (30ºC) during 30 min, according to the official protocols, in order to determine the water absorption level necessary to obtain the dough target consistency (1.10 Nm), as well as the rest of the mixing characteristics parameters. This type of assays allowed determining the great influence of the type of flour (presence/absence of gluten), particle size distribution of flour, assayed additives (type and concentration) and blends of flours (chestnut/rice) about the mixing and thermal properties of studied doughs. In this sense, the tested chestnut flour as control sample showed low values of water absorption, (increased up to 35% with decreasing average particle size) and increased with the presence of the most of studied additives at high concentrations. The reduction of particle size involves a positive reduction in the development time, yet doughs stability loss is produced, hence the presence of additives (chia flour, hydrocolloids and oils) that improve this behaviour is essential in this type of systems. In the case of chestnut flour blends with different average particle size with rice flour, it was found that the obtained parameters followed a mixing rule in this type of assays, which were assessed taking into account the proportions of chestnut flour in the blends. It is noteworthy that the studied chestnut flour doughs (different particle size, presence or absence of additives and blends with rice flour in different proportions) were analyzed by means of the mixing tests.
The subsequent characterization of chestnut flour doughs with or without additives previously prepared in the mixing tests was carried out at 30ºC. The apparent viscosity behaviour and the corresponding dependence on the shear rate in the range from 0.01 to 10 s-1 (flow curves) were determined according to the particle size. Likewise, the presence of additives (type and concentration) and the blend proportions with other gluten-free flours were studied. The correlations between these variables using the Cross model were established.
It was observed that the flow curves obtained for all studied doughs showed a pseudoplastic behaviour region at intermediate-high shear rates and other region of behaviour close to newtonian behaviour (plateau) at low shear rates. The shear rate range where the newtonian plateau was observed, decreased depending on the type and additive concentration employed. Chestnut flour doughs for the control sample showed excessively highly apparent viscosity values whose magnitude decreased with reduction of average particle size. The presence and concentration of additives showed different effects. Particularly, the reduction of apparent viscosity of control sample at each shear rate was observed with the addition of chia flour, oils, hydroxypropyl methyl cellulose, agar and chestnut starch in all studied concentration range, as well as with the presence of guar gum below 1.0% and tragacanth at 0.5%. The experimental data corresponding to all chestnut flour doughs were satisfactorily fitted using the Cross model taking into account the shear rate. The presence of additives in chestnut flour doughs with lower average particle size showed a similar effect on the flow curves behaviour. The magnitudes of the apparent viscosity obtained in this case were lower for the same additive at the same concentration.
The apparent viscosity determination at the same conditions exposed in the previous paragraph was carried out for control sample (chestnut flour doughs) with the combined effect of chia flour (4.0%, f.b.) and some hydrocolloids (0.5-2.0%, f.b.) which provided the best results when were added separately (guar gum and hydroxypropyl methyl cellulose) or due to the novelty of its study in flour doughs as it is the case of tragacanth gum was interesting to perform a more detailed study of its effects. The found results showed the same flow curves shape and trends as when these additives were added separately. Nevertheless, the Newtonian plateau was lower than for the control sample and the reduction of apparent viscosity was enhanced. Once more, the Cross model was satisfactorily applied to correlate the apparent viscosity with shear rate.
The apparent viscosity of blends of chestnut flour with different particle size and rice flour at the proportions previously exposed was studied. Again, the presence of a Newtonian plateau and subsequent pseudoplastic behaviour was observed. The different studied systems were satisfactorily correlated by the Cross model. In a similar way, the obtained parameters with this model presented values within the range of those obtained for chestnut flour doughs (different particle size) and rice. Thus, the parameters were linearly correlated with the chestnut flour fraction, satisfying the mixing rule with acceptable reproduction of experimental apparent viscosity values.
The viscoelastic behaviour for chestnut flour doughs (moduli of storage, G', and loss, G") was studied with the angular frequency in the range from 1 to 100 rad s-1 (mechanical spectra) at 30ºC. The oscillatory assays were carried out for the same systems and operating conditions than those used in the steady-shear flow assays. Previously, stress/strain assays were developed in order to establish the linear viscoelaticity range of doughs at a strain value below 1.0%. The storage and loss moduli were satisfactorily correlated with angular frequency (from 1 to 70 rad s-1), particularly using a power model.
The mechanical spectra of all assayed doughs showed that the storage and loss moduli increased with increasing angular frequency for each studied systems in the assayed angular frequencies range. In all cases the elastic behaviour was higher than viscous behaviour in all observed angular frequency range. The high values found for both moduli decreased at each angular frequency with the average particle size reduction of chestnut flour. The influence of the studied additives (type and concentration) on the storage and loss moduli showed a similar trend to that previously detailed for the additives influence on the apparent viscosity. In similar fashion, the tan ¿ values were obtained and their dependence on the angular frequency was also analyzed. Concerning the relative behaviour of both moduli, a preponderance of elastic modulus above viscous modulus was observed for all studied systems. This behaviour was enhanced with the presence of the most used additives. The observed trends for all chestnut flour doughs with lower average particle size were analogous to the aforementioned ones for the control sample. The G' and G" moduli were satisfactorily correlated with the angular frequency (from 1 to 70 rad s-1) using a power model. The correspondence of the apparent viscosity data (steady-shear flow) and complex viscosity (oscillatory flow) did not follow the Cox-Merz rule, as usual in this type of systems.
Apart from this, since that starch is a major component in chestnut flour doughs, pregelatinized chestnut starch dispersions (95ºC) with several starch concentrations (4.0, 5.0, 6.0 and 7.0%) were prepared and the rheological behaviour at different temperatures were studied by assays of steady-shear flow (shear rate: 1-500 s-1) and mechanical spectra (angular frequency: 2-100 rad/s). The employed chestnut starch was isolated from flours of raw materials with different levels of processing (fresh chestnut and commercial flour). In both systems the apparent viscosity and its dependence on starch concentration and temperature was studied, showing a clear pseudoplastic behaviour in all cases. A mathematical model was established in order to correlate the apparent viscosity with the concentration, temperature and up/down cycles of shear rate based on Herschel-Bulkley model. Similarly, the experimental data obtained by the storage and loss moduli in the mechanical spectra indicated gel-like behaviour in the samples and were satisfactorily correlated by a power model in the range of angular frequencies from 2 to 30 rad/s. Additionally, the Cox-Merz rule was not completely satisfied for the different systems of chestnut starch, especially at low shear rates / frequencies by gelling starch characteristics.
The creep-recovery curves at 30ºC were studied for all assayed chestnut flour doughs which were previously analyzed using steady-shear flow and oscillatory tests. These assays consisted of applying a constant stress of 50 Pa during 60 seconds on the dough (creep), and subsequently the load was removed allowing dough recovery during 180 seconds (recovery). The obtained experimental data by means of creep-recovery assays were satisfactorily fitted using the Burgers model for all assayed systems.
The creep-recovery assays performed in chestnut flour doughs used as control sample exhibit an important lack of elastic properties. This lack was improved by the presence of assayed additives, especially chia flour and the most of studied hydrocolloids. Therefore, the combined effect of both systems involves the enhancement of unrecoverable fraction during the end stage of creep-recovery tests. Similarly, the presence of additives into chestnut flour doughs with lower average particle size involves higher increase in the elastic properties. The creep-recovery curves experimental data for all assayed chestnut flour doughs were satisfactorily fitted using the Burgers model. Particularly, the obtained parameters of such model for chestnut and rice flour blends were correlated linearly with the chestnut flour fraction, satisfying again the mixing rule. A good reproduction of creep-recovery curves was obtained by means of such rule.
The rheological characterization was also carried out according to a temperature ranging from 30 to 100ºC (temperature sweeps). The assays of temperature curves performed in the rheometer allowed to obtain the gelatinization temperatures of the control sample as well as from other tested chestnut flour doughs systems. Such temperatures were delayed with the presence of sucrose, chia flour and hydrocolloids. A more detailed thermal analysis was carried out using the second type of assays (complete curves) of Mixolab® allowing to assess the effect of temperature variation in the doughs behaviour. Initially, a mixing stage (8 min) at the same aforementioned conditions for the first type of assays was programmed. Subsequently, stages of heating (4ºC/min up to 90ºC), stabilization at this temperature (5 min), cooling (4ºC/min up to 50ºC) and, finally, stabilization at 50ºC during 5 min, were performed as the official protocols establish. The temperature tests reveal that the control sample presents similar values to those identified for soft wheat flour in the heating stage and values between hard and soft wheat flours in the cooling stage. Apart from this, the control sample behaviour is not consisted with the rice flour one. The cooking stability and the enzymatic activity rate for chestnut flour doughs with the combined effect of chia flour (4.0%) and guar gum, hydroxypropyl methyl cellulose or tragacanth gum at 1.0% shows close values to wheat flours. In the chestnut flour doughs with lower average particle size, both parameters show similar trends when compared with wheat.
Additionally, the thermal properties of chestnut flour and its starch at different water content (40-95%) have been determined by means of differential scanning calorimetry. The influence of guar gum (0.5-2.0%, f.b.) on chestnut flour and its starch has been also studied. It has been found that gelatinization temperatures obtained by rheometry and Mixolab® are within the range of those values achieved using this calorimetric technique.
Finally, a model for obtaining some of the main properties determined in chestnut flours and doughs (colour, apparent viscosity, viscoelaticity and torque) has been established for the control sample and the corresponding ones to the weight sum of fractions taking into account their average particle size (A: > 250 ¿m, B: 250-125 ¿m and C:< 125 ¿m), finding a mixing effect which was successfully assessed using the weight percentage of each particle size fraction.
