Mixing of Doughs

To run efficiently the baking plants require a continuous supply of dough which is of uniform quality. Stops in production cause many problems and are therefore costly. Stops may occur if the dough supply runs out, even temporarily, or the consistency of the dough is unsuitable for the dough piece forming machines. It is therefore a key requirement of the mixing operator to provide dough as required, neither too soon nor too late, and to ensure that the dough consistency is similar and satisfactory from one batch to the next.

Dough consistency

The term "consistency" covers all those aspects of a dough that can be felt in a dough such as resistance to deformation and stickiness. Characters such as softness, plasticity, elasticity, stickiness and pourability can all be assessed when a lump of dough is squeezed or pulled. In common with most other materials, as the temperature of dough is increased it becomes softer. Thus, temperature is another character that can be felt and measured in a dough and which is used in an assessment of consistency.

Biscuit doughs are very complex being made up of a liquid phase that will be of fat and water and a solid phase that includes starch, protein, sugar and often many other materials. Some of the fat may be solid and some of the solids may be dissolved in the water. Furthermore, doughs change on standing. In some cases the water is absorbed slowly by an ingredient after the end of the mixing period (oat flakes are a good example of this). This results in a tightening or hardening of the consistency. In other cases, the elasticity of the hydrated and mechanically developed flour protein, then known as gluten, becomes less extensible on standing and this results in a significant change in the feel and behaviour of the dough. In general, dough that has just been worked or moved either by the mixer or by handling has a softer consistency than one which has been standing. This property is known as thixotropy (a well known example of thixotropy is in "non drip" paints which are thick as they are taken from the container and become more liquid as they are worked on a surface with a paint brush).

It is very difficult to measure dough consistency and to give it a value. This is principally because of the effect of handling the dough before the measurement is made. Just the act of putting a sample of dough into an instrument can have a significant affect unless very strict procedures are followed. Furthermore, most instruments that can be used to measure consistency are too delicate to use in the rugged environment of the biscuit mixing department. For this reason it is almost impossible to give a specification for the consistency of any particular dough and considerable reliance is made on the ability and experience of the mixer operator to "know" his dough and to be able to detect differences and spot changes and faults.

However, the consistency of the dough is of very great importance to the smooth running of dough piece forming machines. These machines press and roll the dough continuously so changes in the consistency and stickiness have significant effects on performance. Smooth running of the forming machinery depends on a uniform quality of dough.

Hard doughs

Typically hard doughs (cracker doughs and semisweet types) are mixed after all the ingredients have been loaded into the mixer. Mixing is by time or to a final dough temperature. Typically the mix time is in excess of 15 minutes by which time all the sugar will have dissolved, the flour will have hydrated and the gluten will have been mechanically kneaded into a more or less extensible state. The dough will be warm. The consistency is closely related to the dough temperature so the mixer should be stopped when a predetermined temperature of about 40°C is reached.

The dough should be used as soon as possible after the completion of mixing.

Puff doughs

Puff doughs are of two types. Those where pieces of fat are incorporated into the dough and those where fat is introduced between layers of dough after sheeting. "All in" puff doughs require very gentle mixing to ensure that the pieces (lumps or flakes) of fat are not melted into the general dough. The other sort of dough for puff is mixed as a developed dough. In all cases great efforts must be made to have the puff dough as cold as possible.

Short doughs

A short dough requires good dispersion of fat and sugar over flour without the development of any appreciable amount of gluten. Thus short doughs are usually mixed in two or more stages.

The most critical aspects of mixing short doughs are firstly to form a good emulsion of water and fat and then, in the second stage of the mixing sequence to limit the formation of gluten. Firstly the fat, sugar, and aeration chemicals are blended together (known as the cream up stage). This stage allows as much of the sugar as possible to dissolve in the dough water and for an emulsion of fat and sugar syrup to be formed. The mixing speed of this stage is usually slow to reduce splashing and the time is not critical.

If the fat is hard (usually because it is too cold) it will be difficult to disperse and emulsify during the first stage of the mixing. If lumps persist after the "cream up" mixing stage and are only broken up during the final stage of the mixing it is probable that the dough quality will be unsatisfactory. One should aim to have a smooth, creamy and homogeneous mixture before adding the flour.

Fat that is hard can be "improved" by plasticising though a machine that works it but the best solution is to store the fat at a specific temperature so that it is soft at the time it is needed. Unfortunately, warming a block of cold fat takes a long time. If the fat is heated quickly so that part or all of it becomes liquid the quality of a dough made with it will not be satisfactory.

After the homogeneous emulsion has been formed the flour is added and blended in for a minimum time, about 1 minute should be enough. However if the blending efficiency of the mixer is not good this stage could be longer. It is possible to adjust the amount of sugar that is dissolved by adding at least some of it with the flour rather than at the first stage of mixing. The proportion of sugar that is dissolved in the dough has an effect on the texture and shape of the baked biscuit.

Overmixed short doughs may give dough piece forming problems and harder eating baked biscuits.

The best quality biscuits will always be made from dough that has been made by at least a two stage mixing process and has had a minimum of mixing after the flour has been added, if too much mixing is given here the usual results are that the biscuits do not develop well in the oven or may show shrinkage in one axis after baking. This is because too much gluten development has occurred. This type of problem is likely to be seen mostly in short doughs with relatively low fat levels.

The converse problem is sometimes encountered. Namely that there is too little mixing at the final stage. The result is that the dough is very short and will not make a cohesive sheet or will not extract satisfactorily from a rotary moulder. Also, during baking there may be too much spread and not enough thickness development. A little more mixing at the final stage will help the dough to become more cohesive. This type of problem is likely to be seen mostly in short doughs with relatively high fat levels where the water level is low.

The ideal temperature for most short doughs is 18-22°C. The mixing procedure for short doughs means that the dough temperature is largely determined by the temperature of the ingredients. Thus in warm weather less water may be required to give a desired consistency.

Soft doughs and sponge batters

The consistency of these doughs is very soft and more or less pourable. Mixing is made in a vertical mixer which in the case of sponge batters must give high shear to develop a foam structure.