Rheology is established as the science of the deformation and flow of matter. It is the study of the manner in which materials respond to applied stress or strain. Stress is defined as a measure of internal forces acting within a deformable material. A stressed material often undergoes deformation (change from original dimension) and the rheological term associated with deformation is strain, a measure of relative displacement between the particles of a material.
Basic rheological properties of any material are: strength (solidity), viscosity, elasticity, plasticity. Viscosity and elasticity are the most important characteristics for describing a material and for its behaviour in the process. The ideal elastic materials have the ability to recover to its original shape upon the removal of stress while the stress acted on the ideal viscous materials caused them to deform and it is non-recoverable. By combining both, the ideal elastic and viscous behaviors, the viscoelastic materials, such as a biscuit dough, exhibit behavior in recovering some of its original shape by storing the energy.
Biscuit dough is viscoelastic system consists of complex structures of protein and carbohydrate cross links and due to this many studies had been reported on their rheological properties. Rheological behaviour of biscuit doughs is complex and influenced by numerous factors, such as:
1. Flour composition (low or high protein content, protein quality),
2. Processing parameters (energy applied during mixing, sheeting/gauging, mixing time, temperature, resting time)
3. Ingredients (sugar, fat, water, salt, yeast, SMS, enzymes)
Wheat flour is the basic ingredient of nearly all biscuit doughs. Flour constituents are carbohydrates (starch), protein (gluten), fat, fiber, ash and moisture. It is generally accepted the gluten is responsible for the rheological properties of dough. Flours with higher protein content require more water for absorption, and increased viscosity and relaxation time of the dough. Main biscuit flour characteristics are given in Table 1.
|Flour/dough treatment||Reducing agents|
As flour quality fluctuates, monitoring rheology and making corrections to dough, to maintain consistent product quality, takes on added importance. Testing flour rheological characteristics in the laboratory using various equipment (e.g. farinograph, extensograph) and in process, by sensory evaluation of the mixing operator, are key for adjusting and smoothening processing of dough in the plant.
There are basically two types of biscuit dough, hard and soft. The difference is determined by the amount of water required to make the dough. When there is a high water content (and therefore a relatively low level of fat) blending with the flour during mixing process involves the formation of gluten from the hydrated proteins. The formation of gluten requires mechanical work, kneading, and thus we say that hard doughs are ‘developed’ doughs, referring to the mechanical action.
Manufacturing and processing of ‘developed’ biscuit dough is characterized by interplay between tension (generating elasticity) and relaxation (dough resting, Picture 1). Dough becomes more elastic after each mechanical processing step. Therefore, dough resting periods are essential to achieve relaxation and ´machinability´ of the dough.
Picture 1: Impact of processing on biscuit dough
Mixing is one of the key stages in biscuit making. The gluten network formed during this stage is responsible for viscoelastic properties of the dough and determines its machinability as well as final quality of the biscuit. For transformation of ingredients into a cohesive, extensible, machinable dough the amount of energy transferred to the dough appears to be an essential element of mixing, as its influence is determinant of the biscuits dimension (length and thickness). Depends on product, mixer type, flour characteristics and other factors affecting dough development, an energy threshold value for optimal dough consistency should be identified by mixing operator. Usually it is done by combining mixing time and speed with other important factors for dough mixing process (amount of added water, temperature, SMS, enzymes).
In practice, the freshly mixed dough is allowed to rest for up to 45 min prior to sheeting/gauging, cutting and baking in order to allow partial dispersion of the stresses generated in the dough during mixing. If the dough is not rested the rheological properties of the dough may change with time, thereby causing differences in processing performance between the start and end of a single dough batch. The dough sheeting/gauging operation causes the gluten network in the dough to be aligned in the direction of machining. Following sheeting/gauging the aligned gluten proteins start to relax. In order to reduce dough stress and to prevent disruption of dough´s structure, reduction of thickness after each gauge roll should be about 2:1.
Ingredients, too, can influence dough’s rheological characteristics. In hard biscuit dough, in addition to wheat flour, the main components are fat, sugar, and water.
Sugar causes a softening of the dough, fall in viscosity and a great reduction in relaxation time, with increase in length, and reduction of the thickness and weight of the biscuit.
Fat (or shortening) contributes to the plasticity of the dough and acts as a lubricant. When mixed with the flour before its hydration, the fat prevents the formation of a gluten network and produces a less elastic dough. A highly elastic dough is not desirable in hard biscuit making, since it shrinks after sheeting/gauging and cutting. Adding fat softens the dough, and causes a reduction in viscosity and relaxation time. Fat also contributes to an increase in length, and to a reduction in the thickness and weight of the biscuits, which is characterized by a friable structure easy to break.
Increasing of water quantity leads to a reduction of consistency, an increase in fluidity and in adhesiveness of dough. Increase in water content decrease in viscosity and decrease in relaxation time. Water contributes to a reduction in dough elasticity. The quantity of water affects the behavior of the dough after baking. A slight increase in biscuit length can be noticed when the water content is increased. In addition, the thickness and weight of biscuits are reduced when the water concentration is increased.
Reducing agents also impact rheology and fall into the dough conditioner category. These ingredients: L-cysteine, sorbic acid, and sodium metabisulfite (SMS), can reduce mixing time and help develop more pliable, extensible doughs. Reducing agents reversibly break down the gluten and, once they are used up, the gluten reforms, similar to the action of a mixer. They are typically used with a combination of high-strength flour and high-speed processes to improve machinability, but they also help reduce mixing time and energy input. Reducing agents can also decrease the elasticity that can cause dough shrinkage during cutting and baking.
Rheological properties are important in determining the behavior of biscuit doughs during mechanical processing in addition to their influence on the quality of the finished product. Knowledge of rheological behavior and dough properties is becoming more important as the biscuit industry becomes more automated. Proper use of all available information, empirical, from sensory evaluation of mixing operator, and fundamental, from laboratory testing using various equipment, and adjustments of process parameters related to that information, are crucial for reliable biscuit making process and high quality of final products.
Steffe, J.F. 1996. Rheological Methods in Food Process Engineering, second edition. Freeman Press, East Lansing, MI, USA
D.N. Abang Zaidel, N.L. Chin and Y.A. Yusof, 2010. A Review on Rheological Properties and Measurements of Dough and Gluten. Journal of Applied Sciences, 10: 2478-2490.
Prof. Dr. Klaus Lösche, Bremerhaven Institute for Food Technology and Bioprocess Engineering (BILB), Influencing the rheological properties of wheat dough – part 1
Zoulikha Maache-Rezzoug, Jean-Marie Bouvier, Karim Alla & Christian Patras, 1998. Effect of Principal Ingredients on Rheological Behaviour of Biscuit Dough and on Quality of Biscuits. Journal of Food Engineering 35 (1998) 23-42,
A. S. Contamine, J. Abecassis, M.-H. Morel, B. Vergnes, and A. Verel, 1995. Effect of Mixing Conditions on the Quality of Dough and Biscuits. Cereal Chem 72:516-522
Lene Pedersen, Karl Kaack, Merete N. Bergsøe, Jens Adler-Nissen, 2005. Effects of Chemical and Enzymatic Modification on Dough Rheology and Biscuit Characteristics. Journal of Food Science, Volume 70, Issue 2, March 2005, E152-E158