Buckwheat belongs to the family of Polygo-naceae but because of its functional pro-perties and usage it is also classified as a pseudocereals with Amaranth and Quinoa. There are two types of buckwheat that are being used: common buckwheat (Fago-pyrum escelentum) and tartary buckwheat (Fagopyrum tataricum). Common buck-wheat is the most prevalent type of buck-wheat and it has advantages in sweet taste, large seed size and easy dehulling process, whilst tartary buckwheat posses minor dis-advantages  that  are connected  to  a bitter taste, small seed size and tight seed coat which makes the process of dehulling more complicated (Gawlik-Dziki et al., 2009). Despite the mentioned disadvantages of tartary buckwheat it was reported that it contains more rutin in seeds than the common buckwheat (Jiang et al., 2007). Common buckwheat is generally grown in Europe, USA, Canada, Brasil, South Africa and Australia while the tartary buckwheat is generally grown in mountainous regions (southwest China, Austria, Slovenia, Italia etc.) (Bonafaccia et al., 2003). The main products of buckwheat are buckwheat flour and groats. Also pasta and other buck-wheat products are commonly used in Italy, Slovenia and Asia. Other buckwheat pro-ducts are buckwheat honey, green buck-wheat tea, buckwheat sprouts etc. It is well known that buckwheat has nutritional pro-perties and health promoting components that makes it very important supplement in various food products especially in bakery goods, most commonly in bread products.

Present phenolic compounds in buckwheat are considered to have high antioxidative activity (Halosava et al., 2002; Sensoy et al., 2006). The buckwheat seeds contain ru-tin and isovitexin while hulls contain even more phenolic compounds (rutin, orientin, vitexin, quercetin, isovitexin and isorientin) that possess antioxidative activity (Dietrych-Szostak and Oleszek, 1999). Rutin has an-tioxidant, anti-inflammatory and anti-cancer properties and also posses the ability to prevent the arteriosclerosis etc. (Sun and Ho, 2005).

It is also found that buckwheat has prebiotic properties because it could increase lactic acid bacteria in intestine (Prestamo et al., 2003). The addition of buckwheat to food products, due to its antioxidative stability, provides beneficial health effects and ma-kes the food resistant to oxidation during the food processing and storage. Very im-portant nutritional value of buckwheat also arise from its protein characteristics due to well balanced amino acid composition that is more favorable than the protein compo-sition of commonly used cereals (Pomeranz et al., 1972; Wei et al., 1995). It was confir-med that buckwheat proteins are consisted mostly of albumins and globulins (Skerrit, 1986) that is the major difference compared to wheat protein composition. Thereby buckwheat does not contain gluten proteins and thus can be used in gluten-free for-mulations (Schoenlechner et al., 2008). As it is well known gluten complex present in wheat flour significantly impacts on rheo-logical properties of wheat dough systems and on final product characteristics. It has most important role in building a protein network with well known characteristics which  are  substantial in  dough processing and ability to retain gasses formed during the fermentation process (Auerman, 1998). Also gluten complex impacts on the struc-ture and final product volume. As it was already determined by numerous different measurements, protein complex of buck-wheat flour is consisted mainly from albu-mins and globulins with minor percentage of glutelin and prolamin. This is the major difference between the protein complex of buckwheat and wheat flour which is con-sisted of mainly prolamin and glutelin (Guo et Yao, 2006).

The aim of this study was to determine the influence of addition of unhusked and hus-ked buckwheat flour on rheological proper-ties of wheat-buckwheat dough using Mixo-lab.

Wheat flour (moisture content 12.8%, pro-tein 11.8%, ash 0.56%), unhusked buck-wheat flour (moisture content 9.76%, pro-tein 12.38%, ash 2.19% cellulose 3.02%, lipid content 2.77% and starch 67.38%) and husked buckwheat flour (moisture content 10.11%, protein 7.5%, ash 1.09%, cellulose 0.43%, lipid content 1.75% and starch 68.24%) were used. Four flour mixtures were prepared containing 50% and 75% husked buckwheat flour and unhusked buckwheat flour respectively.

Dough rheological investigations were performed by Mixolab (Chopin, Tripette et Renaud, Paris, France) which simultaneo-usly determinates dough characteristics du-ring the process of mixing at constant tem-perature, as well as during the period of constant heating and cooling. Required amount of flour for analysis was calculated by Mixolab software according to input va-lues of flour moisture as well as water ab-sorption. All the measurements were per-formed using the Mixolab ´Chopin +´ pro-tocol which parameters are presented in Table 1.

In order to make this parameters more un-derstandable, a common Mixolab profile is shown in the Figure 1:

Water absorption (%) represents the amount of water that is required to produce a dough with consistency of 1.1 ± 0.07 Nm (point C1). This value is equivalent to dough consistency of 500FU obtained by Brabender Farinograph. The dough deve-lopment time is the time measured from initial dough mixing until the C1 point is reached. The stronger the flour is, the dough development time is longer. Stability (min) represents the resistance of dough to applied mixing forces (time during the mea-surement when the dough consistency is not lower than 11% of value of torque at C1 point). If the resistance is higher, the dough is stronger. Amplitude (Nm) can be inter-preted as curve width at C1 point and it rep-resents dough elasticity. By increasing the value of amplitude, dough elasticity incre-ases too. The slope α can be observed as the rate of weakening of the protein struc-ture due to the effects of temperature in-crease and applied forces during the dough mixing. The value C2 represents the mini-mum torque recorded during the period of mixing and increasing the system tempe-rature. This value is also dependent on pro-tein structure characteristics. The rate of starch gelatinization can be observed as the slope β, while the value of maximum vi-scosity correspondents to the value of the maximum torque at the point C3 repre-senting gelling ability of starch. Enzymatic degradation of starch and its rate can be indicated by the slope γ and the value of the second minimum in the recorded curve, C4 point, showing the stability of hot paste. Starch retrogadation is measured at the end of the cooling period at point at point C5 (Mixolab Chopin, 2006).

The obtained Mixolab profiles of wheat flour as well as the prepared flour mixtures (wheat flour/unhusked buckwheat flour) are presented in Figure 2:

By observing the first part of the curve, which mainly depends on the physical cha-racteristics of protein matrix, it can be seen that the increasing amount of unhusked buckwheat flour in the mixture resulted in weaker protein network in comparison to the characteristics of system containing wheat flour solely. Thereby, by increasing the amount of the unhusked buckwheat flour, dough stability of tested system de-creased (Table 2). On the contrary the dough elasticity expressed by the values of amplitude (Nm) were higher with larger amounts of unhusked buckwheat flour in mixtures due to higher content of hydro-colloids in unhusked buckwheat flour in comparison to wheat flour. It is observed that addition of unhusked buckwheat flour lead to the increase of water absorption as a consequence of the higher cellulose con-tent and the composition and the nature of the buckwheat flour, as well. Also dough development time increased in mixtures containing larger amounts of unhusked buckwheat flour due to different water ab-sorption capacities of the present com-ponents of the buckwheat flour (mainly ce-llulose and hydrocolloids).

Mixture containing 50% of unhusked buck-wheat flour expressed significantly lower value of minimum torque C2 compared to pure wheat flour system. This was the con-sequence of dilution of wheat gluten complex and inability of buckwheat flour to form dough with similar physical charac-teristics to one obtained using the wheat flour solely due to the different protein com-positions of these two types of flours.

The increasing amount of unhusked buck-wheat flour to 75% did not have impact on further weakening of protein complex com-pared to mixture containing 50% of unhus-ked buckwheat flour.

Second part of the curve follows the chan-ges in dough structure caused by increa-sing temperatures and mechanical forces of mixing. These changes are mainly influen-ced by starch and enzymatic characteristics present in flour system. From the resulted slope β it can be concluded that highest rate of the starch gelatinization showed wheat flour dough which also had the maximum value of torque at the point C3 (Table 2). Measurements performed by Bra-bender Amylograph (ICC, 1996) sho-wed that tested wheat flour possess low enzymatic activity and relatively high value of peak viscosity (810 AU). The increase of amount of unhusked buckwheat flour in mixtures resulted in decrease of the gela-tinization rate and in lower value of ma-ximum torque at point C3. Lower value of torque at point C3 can be consequence of starch nature present in buckwheat flour as well as of milling process which can be responsible for causing larger amount of damaged starch having poorer pasting properties. Also it can be observed that the time needed to reach the value of maximum torque (C3 point) was longer for system containing wheat flour compared to flour mixtures with buckwheat flour. So it can be concluded that starch present in buckwheat flour has lower resistance to increasing temperatures than in the wheat dough system.

The degree of starch retrogradation, which can be expressed as a difference between measured torques at C4 and C5, decreased by increasing the amount of unhusked buckwheat flour. That value for wheat dough was 0.58, while for dough containing 50% and 75% of unhusked buckwheat flour were 0.39 and 0.32 respectively. This could have the positive effect on bread stailing effect which is proved to be due to a starch retrogradation. It is assumed that the reason for such behaviour arise from different structure of amylose and amilo-pectin fraction of buckwheat starch in comparison to wheat starch (different ratio between amylose and amilopectin, bran-ching of these polymers and their molecular weight). husked buckwheat flour. Further-more, the increasing amount of buckwheat flour in tested mixtures did not have signi-ficant changes in dough development time. flour.

Characteristics of wheat flour and husked buckwheat flour mixtures in two different ratios (50:50 and 25:75) are presented in Figure 3:

The profile of the first part of the recorded curve was similar to the profile of the curve obtained with the mixture containing un-husked buckwheat flour. Dough develop- ment time i.e. time required until the first maximum value is reached (C1) was also longer for mixtures containing buckwheat flour compared to wheat systems.

This was due to higher content of cellulose (0.43%) and nature of used Dough stability also decreased with increasing the amount of husked buckwheat flour. It can be obser-ved that the addition of buckwheat flour caused weakening of protein structure (lo-wer value of slope α and of the measured torque at the minimum, point C2) due to dilution of gluten complex of wheat flour. Water absorption did not change im-portantly by addition and further increasing of husked buckwheat flour due to the ab-sence of component which could increase water absorption, in contrast to unhusked buckwheat During the heating period of the system with 75% of husked buckwheat flour due to starch gelatinization of buckwheat starch and its specific characteristics re-sulted in dough stickiness on mixing element so further measurement of resulted torque could not be performed i.e. mea-sured value of torque was 0. So the both mixing elements were covered by gela-tinized flour and there was no dough bet-ween the mixing elements which is in a fact responsible for registration of obtained tor-que. In order to overcome the incurred pro-blem it was necessary to increase the flour mass i.e. the dough mass so the measure-ments would have been performed accor-ding to the modification of standardised Chopin+ protocol.

Consequently, further comparisons were performed analysing the wheat dough system and the dough that contained 50% of husked buckwheat flour. Dough prepared with 50% of husked buck-wheat flour had also lower value of ma-ximum torque at point C3 (Table 3), but the difference was smaller in comparison to mixture that contained 50% unhusked buck-wheat flour.

This was due to larger starch content in husked buckwheat flour than in unhusked buckwheat flour. Also the investigated rate of gelatinization was higher observing the dough prepared with 50% husked buck-wheat flour compared to wheat dough. Sys-tem containing 50% of husked buckwheat flour had higher values of the degree of re-trogadation (0.47) expressed as a differ-rence of the torque values at C5 and C4 than in unhusked buckwheat flour (0.39) because of the higher starch content of husked buckwheat flour. However those va-lues were still lower than in wheat dough system.

The addition of husked buckwheat flour and unhusked buckwheat flour respectively changed investigated rheological parame-ters significantly. In both cases the addition of buckwheat flour caused weakening of protein structure due to incapability of buckwheat flour protein to form a network in dough system like gluten does in wheat dough systems. Also the parameters which describe the starch characteristics changed too. The addition of both types of buck-wheat flour resulted in lower values of ma-ximum torque which is related to starch viscosity i.e. gelling ability. The addition of buckwheat flour caused the decrease in degree of retrogradation which could be beneficial for the production of bread and bakery products with lower stailing effect.

This work was supported by Serbian Mi-nistry of Science (number of the project 20068).