Metering to the mixer


A biscuit is made from a dough with a specific composition. This is known as the recipe or formula. Typically the dough is mixed in a batch mixer and the size of the recipe is adjusted to suit the mixing machine and the frequency with which the doughs are to be made.

The formula can be adjusted only to a very limited extent otherwise the quality of the baked biscuit will be changed. These adjustments are usually limited to the water level (which affects the dough consistency) and the aerating chemicals (which affect the development of the biscuit thickness during baking).

Accurate weighing or metering of the ingredients for a dough is the key to much of the success of the biscuit making operation.

Handling ingredients

Manual weighing

In many factories some or all of the ingredients are carried to the mixer and weighed by hand. This is laborious and slow but with care is the most accurate and reliable method for assembling all the components of the dough. Weighing scales can be used that are appropriate to the size of the quantity to be weighed and each item can be ticked off as it is put into the mixer bowl. Those ingredients that must be sieved, to remove lumps, or dispersed in water before adding to the mixer, can be done easily and reliably.

The disadvantages of manual loading of a mixer include,

  • the operations are slow and very labour intensive
  •  it is hard work to lift large quantities of ingredients onto a weigher and then into the mixer bowl, especially if the mixer bowls are large and high off the ground.
  • it is potentially unhygienic to tip ingredients from bags or boxes as dirt that is picked up on the outsides of the containers may fall into the dough
  • there is a risk of contamination of the dough with string, plastic and paper from the containers of the ingredients

Bulk handling of dry materials

In order to speed up the process of loading a mixer and also to reduce the hard work involved, many factories have introduced bulk handling systems for at least some of the ingredients needed for doughs.

Bulk handling means that the ingredient is conveyed from a large container (a silo) and metered as required directly to the mixer. There are usually several mixers fed by the system which is electronically sequenced and controlled. Dry materials such as flour and sugar are usually conveyed by a stream of air through pipes, known as pneumatic conveying. The feed into the air stream has to be controlled to avoid blockages and the removal from the air is usually by means of a cyclone.

There are three common methods of weighing dry materials in a bulk handling system,

a, Dedicated metering by weighing in

The cyclone is an integral part of the weigher which is directly above the mixer where the material is to be used. When the weight is nearly made, the flow of material from the silo is stopped and just before the weight is made the diverter valve into the weigh hopper closes. Any material remaining in the pipeline is returned to the silo from whence it came (in the case of flour) or is sent to another bin (for example, in the case of sugar). The precision of weighing is affected by the rate at which the material is conveyed because at the moment that the valve on the top of the weigher closes there will always be some material still in flight which will decide exactly how much arrives in the weigher. The rate at which the material is conveyed from the silo is set to suit the system and the number of calls per hour that will be made from all the mixers.

The materials are weighed for the next mix while the later stages of the mixing sequence are proceeding. They are then ready to be dropped by gravity directly into the mixer when required by opening a valve. There is sometimes a problem that not all of the material held in the weigher falls into the mixer. A visual display or computer print out of the weight in the weigh hopper should be available for the mixer operator so that he can check that discharge has been complete.

b, Central metering by weighing in.

The sequence is similar to that described above except that the weighing occurs in a centrally placed weigh hopper and this is discharged pneumatically to one of several mixers. The advantage of such a system is that there is less equipment to buy and maintain. The disadvantage is that it is slower to feed more than one mixer and there is a possibility that all of the flour leaving the weigh hopper does not reach the desired mixer, because it is held in the pipe work, and what remains goes by mistake to the next mixer to call thereby causing a weighing error.

c. Loss in weight metering (weighing out)

This system involves a feed hopper above the mixer which is filled periodically from the silo in a non critical manner. As required, material drops into the mixer beneath and the quantity is controlled by loss in weight.

Any number of silos or tanks can be arranged over the mixer, each mounted on load cells, and many ingredients can be metered into the mixer simultaneously. The loss in weight system affords great accuracy but is handicapped by the need and expense of a great amount of dedicated equipment. Mixers filled by loss in weight systems can be loaded much more quickly than weighing-in systems.

Bulk handling of liquid materials

Bulk handling of liquids and semi solids (like plasticised fat) is somewhat easier than dry materials because they can be conveyed and metered by pumping.

Pipe work can be connected directly to the top of the mixer. In the case of fats and syrups the pipe work must be maintained at an appropriate temperature by jacketing.

It will be appreciated that liquids can also be metered by the loss in weight system if they have been held in containers above the mixer. Such a system also lends itself to solutions of ingredients (premixes) that have to be prepared in relatively small quantities.

Weighing the mixer

This is a special case of the weighing in system. The whole mixer is mounted on load cells or a weigh frame and acts as the weigh hopper for material conveyed to it. This means that there is no error between the weighing operation and the dropping of that material into the mixer.

The advantages of this system are that weight and time records can be kept of all ingredients delivered to the mixer whether added automatically or by hand. It is also possible to know how much dough was discharged from the mixer after completion of the mix. No other metering arrangements are required. However, the disadvantages are that separate holding bins and tanks are required above the mixer for each ingredient, similar to the arrangement for loss-in-weight. The weigh system must be robust to withstand a massive tare weight of the mixer and vibration when it is running yet sensitive enough to record precisely small ingredient additions. Operators must not touch the mixer while it is weighing. There is an "in flight" problem as described above and only one ingredient can be metered at once. This prolongs the total time needed to load all the ingredients for a dough.

Only this system allows automatic recording of how much recycled dough was used in a mix.

Metering of small ingredients

In order to speed up the metering and to maintain accuracy, most manufacturers dispense the small ingredients like salt, baking chemicals and flavours, into bags or containers enough for all the mixings of a shift. The mixing operator then has merely to use one container of each for each mixing. It is helpful to use different coloured containers for the various ingredients to aid identification and to reduce mistakes.

If small quantities of liquids are to be handled remember to use only plastic or metal containers never glass in the factory environment.

The action of adding small ingredients to a mix requires some care and attention. Just in case the blending action of the mixer is not ideal or if there is only a short mixing time after the material has been added, it is best to spread the ingredient across the width of the mixer bowl.

Continuous metering

A continuous mixer obviously needs to have all the ingredients of the dough supplied to it continuously and uniformly. This is a serious engineering challenge and is one of the reasons why this method of dough preparation is not commonly used. All the ingredients must be metered continuously and a combination of screw and belt feeders and pumps is used. By the use of loss in weight electronics the performance of these metering instruments can be controlled more precisely. It is usual to improve the metering of small ingredients by either diluting them with a filler like flour or dextrose or dissolving them to make a solution that can be more easily handled. To reduce the number of metering machines needed it is also common to make batches of premixes which combine a number of ingredients in the correct ratio.

It is particularly difficult or impossible to feed scrap dough as an ingredient into a continuous mixer.