Danish butter cookies are a Christmas classic that is conveniently sold to consumers who are usually unaware of the rheology involved with their processing. Homemade Danish butter cookies shaped by a cookie press simulate the industrial manufacturing while elucidating the challenges of extruding dough through a die.
A hobbyist cookie press consists of the extrusion cannister, die, and plunger that is either hand-cranked or driven by a small electric motor (Figure 1). To estimate the shear stresses involved with extruding a Danish butter cookie, the formula is quite simple:
Shear stress = Force / Area
Force applied to the plunger divided by the surface area in contact with the plunger gives the approximate shear stress for extruding the cookie dough. Modern cookie presses with electrical motors typically rotate the plunger so that as it extends down the extrusion canister, the dough in contact is sheared both at the plunger and the inner cylinder wall of the canister. For a plunger with a diameter of 55 mm and an applied force of 0.5 N from the plunger, the approximate shear stress at the surface of the plunger is 52 Pa.
The impact of different applied stresses in the extrusion process can
be simulated with an applied shear stress test. Also referred to as a creep test, this method has been used to observe the changes in strain after larger deformation or stress. (1) A shear stress of 50 Pa for one minute was applied by Lazaridou et al. followed by zero stress to track the strain behavior as a way to distinguish the elasticity of doughs with various additives. (2) An example creep test is shown in Figure 2. The first interval displays the time needed for the strain to reach a plateau, followed by the reaction of the sample to a zero-stress condition. Doughs that can respond quickly when strain is applied and removed would be ideal for extrusion in order to regain structure after deposition onto the cookie sheet.
Creep studies have not always been applied for dough studies. Amplitude sweeps have traditionally been the rheometry method for assessing dough strength, but recent literature has declared that practice to be inefficient for correlating dough behavior to the mechanical properties during processing (2) or of the final product. (1)
Cookie dough temperature plays a big role in the ease of extrusion with the cookie press. As the dough is sheared in the tube, heat builds up due to the friction. This causes a viscosity decrease that may even result in some separation of the butter or egg-rich parts of the dough, which makes the dough inhomogeneous. In rheology, this phenomenon is called “shear heating” - to mitigate shear heating while making Danish butter cookies, refrigerate the dough before extruding. Cookies extruded with shear-heated dough are also likely to have greater volume expansion after extrusion. (3)
Butter-rich doughs such as Danish butter cookie dough also may have difficulty being extruded due to butter lubricating the extrusion cannister. The butter layer along the walls can cause a low viscosity region of extra-buttery dough while the bulk of the dough forms a plug that is not easily moved. This behavior may be studied with rheo-tracking techniques such as Rheo-USV or Rheo-MRI. During extrusion, the butter may hinder starch breakdown that otherwise would occur under shear. (3) This effect of changing structure due to the high shear stress may also contribute to the difficulties of extruding Danish butter cookies. Given the circumstances of cookie press extrusion, it is no wonder that dough recipes designed for use in a cookie press are different from other cookie recipes.
Danish butter cookies are just one example of many products that are manufactured by extrusion. The general rheology behind the dough behavior in a cookie press may be applied to polymer melts and resins to help improve product formulation and quality control.
To learn more about applying rheometry methods for processing, contact us for a free consultation.
Dobraszczyk, B., and Morgenstern, M., “Rheology and the breadmaking process”, Journal of Cereal Science, 2003, 38, 229.
Lazaridou, A., et al., “Effects of hydrocolloids on dough rheology and bread quality parameters in gluten-free formulations”, Journal of Food Engineering, 2007, 79, 1033.
Moraru, C., and Kokini, J., “Nucleation and Expansion During Extrusion and Microwave Heating of Cereal Foods”, Comprehensive Reviews in Food Science and Food Safety, 2003, 2, 147.