2.1. Mixing of the dough
2.1.1. The mixing process
The first stage in dough processing is the mixing. During the mixing both the development of the dough and the temperature of the dough are established. If either of them or both of them are not spot on the processing and the product quality will suffer.
It cannot be stressed enough that the mixing is the most important stage of the entire process. If you do it wrong, there is no possibility to correct it later. Mixing is normally the discontinuous step in an otherwise continuous process. Therefore discipline is required. I know it is not easy to repeat exactly every 12 or 15 minutes exactly the same process however it is necessary and of the utmost importance. Someone who wants to be proud of quality of the product he made, must also be proud of the fact that he is capable of repeating over and over again the same process. And that really is a challenge.
Another aspect is of course that all ingredients must be correctly weighed and that all ingredients should be added to the dough. Easy method to check whether there is yeast in the dough : put a little piece of dough in lukewarm water. After a while it should start floating, the reason being that CO2 is produced which changes the specific weight of the dough; it makes the dough lighter so it begins to float in the water. To check whether or not there is salt in the dough, one can taste a little piece of dough to establish that the salt is there. It must also be avoided that the yeast comes in direct contact with the sugar and especially with the salt. Because of its hygroscopic nature the salt will start to suck water out of the yeast cell (osmotic pressure) and the yeast will dehydrate, a process that can be compared with a grape becoming a raisin. If you put salt on the yeast, you will see that the salt will start to dissolve and that the mixture liquefies.
On top of that most characteristics of the final product are determined directly or indirectly during the mixing stage :
· if the dough is under mixed or over mixed, you will notice it in the handling properties of the dough
· if the scaling of the ingredients went wrong, the bread will show faults depending on the ingredient which was weighed wrongly
· if the temperature isn't right, the fermentation rate will be faster of slower and that will influence the volume of the bread and the colour of the crust.
· if the mixing time is not respected, the texture and the grain of the crumb will suffer
The mixing of the dough has a number of objectives :
· uniformly incorporate all ingredients
· hydrate the flour and other dry ingredients
· develop the gluten
To develop the gluten one has to put in energy and mix the water and the flour. Slowly but surely the gluten network will start to develop. The art is to develop them to the proper consistency so the dough will have an excellent machinability as well as good gas retention properties.
In conventional spiral mixer, the mixing time for a dough of about 165 kg will be around 12 minutes depending such factors as the quality of the flour and the mixing method (f.i. the moment when salt is added will influence the mixing time; delayed salt addition will shorten the mixing time). During these 12 minutes one can distinguish a number of stages :
· pick up : dough is sticky, cold and lumpy
· initial development : dough is getting warmer, smoother and drier
· clean up : dough is at maximum stiffness and comes together as one mass. The colour will also change from yellowish to more white
· final development : dough is at its correct temperature and handling quality. A gluten film can be easily obtained by extending a piece of dough
· letdown : dough is too warm and sticky, it lacks elasticity and has too much flow
· breakdown : dough is beginning to liquefy
Mixing times will be influenced by :
· speed of the mixer
· mixer design
· dough size in relation to mixer capacity
· dough temperature (efficiency of cooling systems)
· quality of the flour
· water absorption of the flour (influenced by the particle size)
· amount of shortening (more shortening will increase mixing times)
· amount and type of reducing and oxidising agents
· amount of milk solids and other ingredients 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.
There are a number of different types of mixers
· blending and dough break : dough break is still used quite often in
· spiral mixer
· fork mixer (typically used for baguettes)
· artofex mixer (mimicking human arms)
· vertical mixer (
· horizontal mixer (seldom found in
· high speed mixer (with or without vacuum)
· continuous mixer
Whatever type of mixer is used, remember that the principal aim is always the same (developing the gluten to maximum gas retention capability) but also that the type of mixer will influence – together with other mechanical actions the dough will undergo during rounding and moulding – the final structure of the crumb. And of course doughs can also be mixed by hand.
2.1.2. Temperature control
During the mixing process the temperature of the dough will rise due to :
a) heat generated by the frictional forces
b) heat of hydration of the flour
The frictional heat is the result of the mechanical energy one has to put into the dough in order to overcome internal and external (dough in contact with the side of the mixer bowl) friction that is caused by the dough mixing process.
The amount of friction to be overcome is related to the water absorption and to the gluten development. As mixing time is changed, the friction factor changes as well.
The heat of hydration is the energy which gets liberated when a substance absorbs water. The amount of heat liberated varies with the degree to which water is absorbed. In the case of soluble substances, energy will be needed to dissolve them so the change in energy level is of a negative nature. So amounts of heat are withdrawn from the system.
The temperature of the dough is also influenced by other factors such as :
· temperature of ingredients
· size and type of mixing equipment (artofex mixer compared to a high speed mixer for instance)
· batch size (too small batches in too big mixers)
· mixing procedures (time, speed)
· room temperature
To cool down the dough and to remove the excess heat generated during the mixing process, the baker can use one of the following methods :
a) add ice to the dough
b) use chilled water to make the dough
c) refrigerate the mixing bowl (mainly done in horizontal mixers)
d) use a saturated salt solution which can be cooled down to below 0°C instead of granulated salt
e) cool down the ingredients : mainly flour which can be easily cooled down with the injection of liquid CO2 during the pneumatic transport.
Each mixer is different and each mixer will heat up a given recipe to a greater or to a lesser degree. The friction factor is defined as the value used to compensate for the temperature increase of the dough during mixing. The friction factor has to be determined experimentally i.e. a dough is made with ingredients of which the temperature is known. After the mixing process the temperature of the dough is noted. The friction factor is then calculated as follows :
3 x t°Cdough – (t°Croom + t°Cflour + t°Cwater) = friction factor
Imagine the temperature of the dough after mixing was 26°C, the room temperature 23°C, flour temperature 33°C and the water temperature was 12°C. Then the friction factor equals :
3 x 26°C – (23°C + 33°C + 12°C) = 81°C – 68°C = 10°C
Now, once the friction factor has been determined it is easy to calculate the temperature of the water needed in order to get a predetermined dough temperature. If we want to make a dough of 25°C in a bakery where the temperature is 24°C with flour which has a temperature of 30°C, we will need water of :
(3 x 25°C) – 30°C – 24°C – 10°C = 75°C – 64°C = 11°C
The general formula is :
3 x t°Cdough – (t°Croom + t°Cflour + friction factor) = t°Cwater
Imagine we have to make a dough of 24°C with flour of 35°C in a bakery where the temperature is 28°C. What would the temperature of the water be ?
(3 x 24°C) – (35°C + 28°C + 10°C) = 72°C – 73°C = - 1°C
Now water of – 1°C doesn't come through the water mains because ice is a solid. To transform water into ice we have to put in extra energy (latent heat). The change from liquid state to solid state requires quite some energy. This negative energy is liberated when we go from the solid state to the liquid state. We will use this latent heat to cool down the doughs. So the question becomes how much water do we have to replace by how much ice ? Don't forget that 1 kg of ice = 1 kg of water. To calculate this, one has to use the following formula :
[kg of H20 (t°C water – calculated water temperature)]/( t°C water + 80)
Example
We have to make a dough which contains 100 kg of flour, 62 kg of water, 2 kg of yeast and 1,8 kg of salt. The temperature of the bakery is 28°C, the temperature of the flour is 35°C and the temperature of the available water is 6°C. The desired dough temperature is 25°C. The friction factor for that particular mixer equals 18°C. How much ice would be need to achieve this temperature ?
First we have the calculate the desired water temperature :
(3 x 25°C) – (28°C – 35°C – 18°C) = 75°C – 81°C = - 6°C
The quantity of ice we need equals
62 kg x [6°C – (- 6°C)] /(6°C + 80°C) = 8,65 kg
We will need 53,3 kg of water and 8,7 kg of ice. The ice should be flaked ice in order to increase the contact surface and to facilitate the liberation of the latent heat.
