Types of Dough Mixer

There is a surprisingly large number of manufacturers of dough mixing machines. The mixers vary in size, sophistication of control, power and overall weight. However, they can all be classified into a few types and the aim here is to indicate these giving some of the major advantages and disadvantages:

3cc-Types-of-mixer-thumbExamples of Dough Mixer (after CABATEC)

Batch mixers

These are by far the most common type.

Vertical, detachable bowl, mixers

The beaters are mounted vertically and either they and their drive mechanisms are lowered into a bowl or the bowl is raised to locate with the beaters and a lid. The beater shafts may rotate on fixed positions, in which case there are usually two or three beaters which intermesh with each other, or there is a single shaft which rotates vertically and itself is driven in a circular, planetary, manner. This action allows a single beater to reach all the dough in the bowl without merely moving it in a circular motion.

It is sometimes possible to fit exchange beaters of different shape and action and to drive them at different speeds. This allows a gentle rolling and cutting action at one extreme to a vigorous whisking action at the other.

In small types of vertical mixers the beater speeds can be fast but in larger ones, mixing up to a tonne of dough at a time, the beaters revolve slowly probably not more than 20 revs per minute. Motor powers of up to 70 hp may be used.

Advantages

  • The bowls can be filled with ingredients at various places, away from the mixer, so that loading and discharge is not a critical feature of the mixing time cycle.
  • Doughs which need to be rested, fermented or remixed can be left in a tub without additional handling.
  • Different mixing actions can be achieved either by using more than one mixer or by changing the beaters on a single mixer.
  • In many cases the mixing action and the state of the dough can be visually monitored.
  • It is easy to manually charge the bowl with awkward ingredients such as scrap dough or biscuit dust.
  • Bowls containing dough can be readily moved to different locations for tipping or storage.
  • High speed mixing with a whisk allows thin batters to be mixed and aerated.

Disadvantages

  • The mixing action is sometimes not uniform between the bottom and top of the bowl resulting in more or less development in some parts of the dough.
  • The water will always go to the bottom of the mixer before mixing starts.
  • It is difficult to maintain good temperature control of the bowls because jackets containing circulated water must be connected and disconnected.
  • Dough temperature sensors are rarely provided as these must also be connected and disconnected.
  • The bowls are heavy and not very manoeuvrable sometimes requiring mobile power units, such as forklift trucks, to move them.
  • Mixing times are often long because the beaters rotate slowly. It may take as much as 45 minutes to mix a developed dough.
  • The slow mixing action may be an advantage for short doughs as it is less easy to over mix by accident.

Horizontal high speed mixers

There is rather less variety in the design of horizontal high speed mixers. There are some where the bowl is fixed and a door at the side or in the bottom opens to allow discharge of the dough, but more usually the bowl rotates on a horizontal axis around the beaters to allow discharge of the dough. The beaters are driven horizontally within the bowl and are fixed to one or two shafts. Where only one shaft is employed the beaters are usually inclined so as to throw the dough not only upwards but also somewhat to one side and then the other during rotation. The blades may pass close to the bowl surface or at some distance. The former type ensures that material lying in the bottom of the bowl is moved but the latter is better for kneading, rolling and stretching the dough. The action whereby the dough is cut and sheared depends on the exact shape and speed of the blades, but sometimes a stator fixed to the bowl provides an additional means for cutting the dough to achieve good homogeneity. Where two shafts are employed, the bottom of the bowl is W shaped and the shafts rotate in opposite directions driving the dough towards the centre and downwards across the centre of the bowl or in the other direction. These mixers are usually known as Z blade mixers.

The beater speeds can be as high as 60 revs per minute which means that doughs can be mixed in relatively short times, possibly in 12-15 minutes for developed doughs. Such mixing actions for loads of up to a tonne require very powerful motors; up to 180 hp are not unusual.

3ccSingleShafMiixers-thumbTypical single shaft high speed mixers

3ccZBladeMixer-thumbTypical Z blade twin bladed horizontal high speed mixer

Advantages

  • These are very powerful mixers as the shafts have bearings at each end and they are, therefore, able to develop tough doughs more rapidly than most of the vertical types of mixer.
  • Provided that the discharge is efficient, it is possible to locate the mixer directly over a sheeter hopper and this obviates the need for handling the dough via a tub.
  • Tubs can, of course, be used should it be necessary to take the dough to another location.
  • There is good and accurate control of the temperature of the mixing bowl from a jacket with constantly circulating water or refrigerant. However the heat exchange possibilities are very limited.
  • Ingredients can be added through the roof of the mixer while the beaters are moving, that is, after the mixing has started.

Disadvantages

  • Charging with ingredients is usually a significant time period in the mixing cycle time and all ingredient feeds must be located over the mixer or be done by hand.
  • The beaters tend to throw material up to the roof of the mixer which can result in blind spots where ingredients hang up.
  • The lid totally encloses the mixer so that the progress of the mix cannot be readily observed.
  • Cleaning is a major operation and because there are some unswept areas of the bowl a "scrape down" part way through a mix may be necessary.
  • It is very inconvenient to remix a dough as charging with dough from a tub is a particularly difficult operation.
  • The beater shape is usually a compromise to allow blending, dispersion and kneading so each action may not be ideal.
  • Where a central shaft crosses the mixer there is often a severe impediment to efficient and rapid dough discharge and this shaft may prevent the free movement of the dough resulting in barrelling, that is, the dough rotates stuck to the beater assembly without much mixing action.
  • The mixer itself is heavy and the beating action can give much vibration. This makes important structural demands on the floor upon which the mixer is located, particularly if it is not on the ground floor of the building.
  • As the size of these mixers increases the efficiency in terms of kneading action tends to decline relative to the heat build up due to friction between the dough and bowl surface.
  • Large motors require heavy electrical supplies and starting under load draws very heavy currents.

Size of batch mixers

There tends to be some confusion on the capacities of various batch mixers. They may be described in terms of bowl volume or in dough weights. It is important to establish, either by experiment or by consultation with the mixer supplier, the maximum (and minimum) dough weights that may be mixed efficiently. Limitations may be related to motor power or the areas swept by the beaters. It is possible that the dough quantities will be different for different types of dough.

The capacities may be based on "sacks" of flour (280 lb or about 125 kg) but this is not very helpful where large amounts of sugar, fat and other ingredients are included in the recipes. Alternatively, capacities may be based on volume such as 100, 200 or 500 litres. As a rough guide, these volumes relate to recipes containing 32, 64 and 192 kg of flour. A common size for a batch mixer is 500 kg or dough but others up to about 1000 kg are available.

Continuous Mixers

These are generally of a rotor-within-a-barrel variety. By arranging different arms and stators along the length it is possible to alter the mixing actions within the range of blending, dispersing, aerating and kneading. Multi section water jackets allow excellent heat exchange and temperature control and by adjusting the barrel length, dough retention and mixing times can be suited to the optimum. The overall capacity of the mixer is usually flexible by adjusting the rotor and ingredient feed speeds. It is possible to feed all the ingredients at the start of the mixer or have successive ports along the barrel so that different additions can be made after appropriate intervals.

Automatic small batch mixers

As a compromise between batch mixers and continuous mixers, automatic small batch mixers may be used. These have ingredient feed systems that are totally electronically controlled and the mixing cycle is triggered to start by reference to the level of dough in a hopper directly below the mixer.

The cycle starts by calls for all of the ingredients and after the mixing periods the bowl inverts completely and discharges into a hopper below. When the dough has been used and the level drops to a pre-set level the cycle begins again. There is no human involvement, except when something goes wrong!

The size of the mixer and the quantity of dough that is mixed each time is chosen to be as small as possible so that many small batches are produced successively. In this way the age of the dough as it is processed through the baking plant is very nearly uniform.

It is normal to design in a sensing system that can be used to compare conditions in the mixer, for example temperature and or power usage related to time, against a norm. If conditions do not correspond the dough is not dropped into the hopper, an alarm is sounded and an operator must check that the dough is satisfactory to discharge normally.

Advantages

  • The mixer uses actions with which one is already familiar on standard batch mixers.
  • The sequence may be stopped at any point to allow visual inspection of the material in the mixer.
  • It is easy to reprogramme the system with a different formulation or with different mixing times and speeds.
  • Being intermittent in action, stops on the baking plant do not require special attention to the mixing operations.
  • If a fault arises, dough of substandard quality can be easily removed from the system, provided it is detected before dropping into the hopper.

Disadvantages

There are few disadvantages compared with continuous mixers.

  • The machinery is larger but not necessarily more expensive.
  • There is no provision for hand feeding ingredients. This means that there may be some limitations in formulation where special, difficult to handle materials, are used.