What is Mixing?
Mixing is one of the most critical and important operations in a bakery. The mixing stage allows “inert” dry and liquid ingredients to create a very reactive and dynamic system that can be then processed and transformed into value-added products.
Foams, cake batter emulsions, colloidal suspensions and doughs have all one thing in common: they are homogeneous mixtures that have a continuous phase and a discontinuous (dispersed) phase. Such systems can only be obtained by mixing and bringing their components together.
How does it work?
Mixing in the baking industry serves many purposes, such as:1,2
- Blend and disperse liquid and dry ingredients to form a homogeneous mass with a continuous phase (protein in dough and foams, and water in batters) and a discontinuous phase (fat).
- Hydrate flour (starch, gluten-forming proteins and arabinoxylans) and other dry ingredients.
- Incorporate and entrap air in the form of discontinuous gas bubbles within the dough or batter to provide gas nuclei for the carbon dioxide generated from yeast fermentation or chemical leavening. This is vital for volume expansion and texture of the finished product.
- In bread dough, this process helps develop gluten strands (binding gliadins and glutenins) by kneading for optimum dough cohesiveness and handling properties.
- Redistribution, subdivision and size reduction of air cells within the dough or batter.
It is important to note that this serves the same purpose no matter the bakery product to be produced. What changes the mixing conditions and mode of mixing is the type of product being processed. A bread dough cannot be mixed exactly the same way as a cake batter or a pie dough.
The following table summarizes key consideration according to the type of product being processed:1,2
|Lean dough (yeast-leavened)||Crackers||Rich dough, Pastry||Chemically-leavened Pastry|
|Objective||Full gluten development for target dough handling properties||Partial hydration and gluten development||Full gluten development (except in laminated pastry and pie crust)
Mild blending for pie crust
|Aeration of batter (cakes)
Incorporation and hydration of dry ingredients (cookies)
|Consistency of mixed product||Visco-elastic dough||Visco-elastic dough (stiffer than lean doughs given low hydration)||Visco-elastic dough (except pie dough)||Oil/Water fluid aerated emulsion (batter cakes)
Viscous mixture (cookies)
|Mixing time||9–15 min||5–10 min||Longer than 12 min (non-laminated)
Less than 7 min (laminated)
|4–20 min (depending on the cake mixing method)|
|Mechanical energy required to achieve goal||Highest||Medium||Low (laminated)
|High-speed mixing equipment||Continuous
|Continuous (batter cakes)
Vertical (batter cakes and cookies)
|Final dough/batter temperature||76–82°F (25–28°C)||76–82°F (25–28°C)||Laminated pastry:
76–85º F (24–29ºC)
|Batter and chiffon cakes:
68–75º F (20–24ºC)
|End point||Gluten film test||Tearing test||Window test||Specific gravity (cakes)|
Factors that affect mixing time
- Mixer speed: the higher the speed, the more blending and work is applied to the system, and the shorter the mix time needed to obtain the desired result.
- Mixer design: different arms or attachments provide different mixing patterns that can provide more or less work, shear, strain and stress forces to the mass.
- Dough size / batter volume in relation to mixer capacity: mixers are designed to operate below 100% of their capacity but above a certain minimum limit; this is to optimize processing performance.
- Dough / batter temperature: mixing implies dissipation or production of heat from the friction between product and mixer wall (especially true with dough processing). The higher the product temperature, the shorter the mixing time should be.
- Quality of flour: the stronger the gluten-forming proteins and the greater the protein content, the more mixing time is needed to develop gluten to make it extensible.
- Particle size of flour: the smaller the particle size, the greater the surface area that can be wetted and become hydrated, and the shorter the mix time required for such particles to form a homogeneous mixture.
- Water absorption of the flour: excessive water extends clean-up time in dough mixing.
- Amount and type of reducing and oxidizing agents: reducing agents decrease the mixing time required to achieve optimum handling properties of bread and bun doughs.
- Amount of salt, sugar, milk solids, egg solids that compete for the water: the higher their concentration is, the longer the mixing time will be because there is less water available for gluten development.
- Degree of aeration: as cake batter is mixed, more air is incorporated into the emulsified slurry, causing its specific gravity (SG) to decrease. A cake batter takes a certain time to reach a target SG value.
Types of mixing machines
There are the following common machines used in bakery industry:
- Spiral mixer: in which a spiral-shaped mixing tool rotates on a vertical axis.
- High speed and twin spiral mixer: where a high level of work can be inputed to the dough in a short time.
- Horizontal mixer: where the beaters are driven horizontally within the bowl and fixed to one or two shafts.
- Low speed: where mixing is carried out over an extended period of time and the commonly used slow mixing system includes twin reciprocating arm mixer and oblique axis fork mixer.
- Continuous mixing: where the ingredients are incorporated at one end of the extruder, and the dough leaves the mixer at the other end in a continuous flow.
- Haegens, N. “Mixing, Dough Making, and Dough Make-up.” Bakery Products Science and Technology, 2nd edition, John Wiley & Sons, Inc., 2014, pp. 309–320.
- Sumnu, S.G., Sahin, S. Food Engineering Aspects of Baking Sweet Goods, CRC Press, Taylor & Francis Group, LLC, 2008, pp. 81–172.