(76a) Impact of particles characteristics and mixing conditions on wheat flour agglomeration behaviour

Introduction - In numerous cereal product manufacturing (noodles, pasta or couscous), the water and wheat flour mixing is a critical unit operation with two main aims. The initial aspect is to ensure a uniform distribution of ingredients, particularly water, which can contain other dissolved ingredients. The second function is to ensure production of an evenly hydrated dough crumb of a consistent size for further processing. A good mixing process (with correct water content) should produce a uniformly coloured, stiff and crumbly mixture (apparent dough pieces), without any pockets of dry flour. The preferred dough crumb is usually low in diameter and should display a moderate degree of adhesiveness. The formation of small agglomerates of dough is a condition to ensure good processability of the crumbly dough obtained at the end of the mixing stage. Crumbly dough should form a ball when pressed together in the hand, and should separate into granules when gently rubbed. Unlike bread production, minimal development of the flour protein network takes place during mixing. The objective of the present paper was to study the impact of physical and biochemical characteristics of flour particles and the impact of mixing process parameters in the agglomeration behaviour of wheat flour under various water content addition levels.

Materials and methods - The impact of particles characteristics and mixing conditions on the agglomeration behaviour of wheat flour was studied using a specific 5 kg mixer equipment and 17 different wheat flours. The selected wheat flours were characterized according to physical (particle size distribution), biochemical (contents of protein, damaged starch, soluble and insoluble pentosans) and functional (farinograph, extensograph and RVA behaviour) parameters. Wheat flour and water mixing was conducted under different water contents (28 - 38%) and process conditions (water addition rate, mixing rate, filling level, and temperature). The wheat flour agglomeration properties during mixing were monitored at molecular scale, by measurements of wheat protein polymerization rates, and at macroscopic scale, by measurements of weight distribution of three classes according to particle size (isolated particles, agglomerates, and dough pieces).

Results and discussion - The contribution of physical, biochemical and functional characteristics of flour particles in the agglomeration behaviour was estimated by calculating linear regression coefficients (at 33% water addition level). We first demonstrated homogeneous agglomeration behaviour of the 15 hard wheat type flours. The physical specificities (particle size and density) of the 2 selected soft wheat type flours explained their very specific low agglomeration behaviour. The calculation of linear regression coefficient demonstrated a relatively high negative contribution (R2 from 0.58 to 0.70) of the particle size into the agglomeration behaviour of the hard wheat type flours. A significant contribution of the particle biochemical factors into the agglomeration behaviour of the wheat flours was also reported. High correlation coefficients were found between the agglomeration parameters and flour ash content (R2 from 0.37 to 0.40), damaged starch content (R2 from 0.19 to 0.45) or soluble pentosans content (R2 from 0.50 to 0.53). On the other hand, we also observe low correlation values between the 3 agglomeration parameters and protein contents (R2 from 0.11 to 0.15). The wheat flour agglomeration behaviour is also poorly dependent on the flour functionalities (i.e. flour ability to give the bread dough, dough extensibility properties and dough viscosity under thermal treatment).

The flour agglomeration behaviour and the crumbly dough formation is greatly affected by the processing conditions and wheat flours characteristics. Experimental data permit to define critical water addition level (33 g added water / 100 g flour) that strongly separates two levels of powder reactivity levels and two types of processing effects on the flour agglomeration behaviour. Below 33% water content, the crumbly dough behaves as brittle powder, characterized by low agglomeration rates, by low changes in wheat protein polymerisation levels, and by negative agglomeration effects of mechanical energy input. On the other hand above 33% water content the crumbly dough behaves as a cohesive powder, characterized by high agglomeration rates, by high changes in wheat protein polymerisation levels, and by positive agglomeration effects of mechanical energy input. It should be noticed that the critical water addition level (33%) is classically considered as optimal for some technological purposes.

The contribution of mixing processing factors into agglomeration behaviour was studied by taking into consideration external and internal mechanical factors, under a large extend of water addition level (28 to 38%). The external mechanical factors are defined by the geometrical characteristics of mixing equipment (mixer type) and by the mixing conditions (powder level, rotation rate, mixing time, mixing energy, etc?). The spontaneous internal mechanical factors are linked to the kinetic contribution of the gravitational force. They mainly depend on time and on product height inside the mixer. The parametric study of mixing unit operation demonstrated opposite (positive or negative) effects of internal and external factors due to the levels of water addition. The effects of mixing rotational rates are greatly dependent on the water content levels. Under high water contents, an increase in mixing rotation rates (from 20 to 120 rpm) induces a significantly improve the agglomeration level of the crumbly dough. On the other hand, negative effects of an increase in rotation rates were observed under low water contents. Similar opposite effects were also observed by taking into consideration the effects of mixing time (from 3 to 15 min). Positive agglomeration effects of mixing time are observed under high water content levels. The description of the effect of external mixing factors (mixing time and rotation rate) was proposed (at 33% water content) by taking into consideration two cumulative parameters: kinetic - rotation numbers (= mixing time x rotation rate) and energy factor (= mixing time x rotation rate2). Efficient description of the flour agglomeration behaviour in regards with the energy factor parameter can indicate a high contribution of external energy factor. A higher contribution of external energy factor was observed by increasing the flour filling level during processing. This effect was associated to efficiency improvement of the mixing system. On the other hand, the mixing energy factor does not contribute to biochemical changes involved in the flour agglomeration behaviour.

The kinetic factor is the main factor involved in the biochemical mechanisms. The contribution of kinetic mechanisms on the wheat flour agglomeration behaviour was evaluated by taking into consideration the resting period after the mixing. Large significant changes in particle size distribution and biochemical values were reported during the 2 hours rest. Kinetic increases in the content of large agglomerates and protein depolymerisation rate were then observed during resting (and more particularly under high water content). Mathematical monitoring of the kinetic changes was conducted by considering sequential mechanisms during agglomeration (flour particles --> agglomerates --> dough pieces) and a set of logistic equations. Several mechanisms can be supposed to explain the kinetics factors: spontaneous mechanical factors (i.e. gravitation force), water distribution homogenisation, strengthening of molecular bonds between proteins and of physical interactions between particles, or residual enzymatic reactions.

Conclusion - Description of mechanisms involved in wheat flour agglomeration demonstrated the poor dependence between the physical mechanisms involved in particle size increase mainly due to mechanical energy input, and the biochemical mechanisms involved in crumbly dough structure formation mainly due to kinetic factors. The physical mechanisms are mainly controlled by water addition levels and by external mixing energy input. The biochemical mechanisms depend on water addition and time. Water level addition can be considered as critical process parameter to manage the flour mixing process. It is then possible to describe wheat flour mixing processing as two successive mechanisms: (i) flour particle activation by water addition; (ii) structure formation mechanisms of hydrated particles due to internal kinetics factors and external energy factors.