There are two principal types of batch mixers in the biscuit industry, the Vertical Spindle Mixer and the Horizontal High Speed Mixer. The other systems employed are Single Screw and Twin-Screw Continuous Mixers.
The vertical mixer is the most common type of mixer, used for the sponge and dough process used in crackers, but it can also be used for short and hard dough. Example: rotary moulded; wire-cut and semi-sweet (hard) dough.
The spindle mixer has two or three shafts with vertical mixing paddles, and operates at two speeds of 15 and 55 r p m’s. The mixing paddles mix, cut and tear through the dough.
The mixers can also be used for developed dough, such as chemically modified crackers and semi-sweet dough, but due to their relatively slow rpm’s, they can take over sixty minutes to achieve the desired final dough temperature for semi-sweet dough.
The horizontal mixer has single or double arm or Z-blade beaters operating in two or three speed modes, and has a cooling water jacket. It provides good side to side mixing of the ingredients, as well as in the direction of rotation of the mixing arms.
High speed horizontal mixers should be equipped with a two speed motor for the mixing arms, run on slow at approximately 35 rpm and fast at approx. 70 rpm.
These mixers are versatile for the preparation of a wide range of dough, from soft (short) and high fat to a hard dough, and are most influential where gluten development is required. This will be necessary for developing the gluten for products like Petit Beurre and Marie dough.
The mixer capacity should be sufficient the ingredient dispensing time and process line throughput and dough resting time, influencing changes in dough rheology. It is essential not to under-load or over-load the mixer, otherwise its action may be impaired leading to variations in dough consistency.
To be efficient in mixing at slow and high speed to produce a homogenous dough in a relatively short mixing time.
Twin Helical beater and static sprag option mixing cycles can be reduces, effective mixing can be achieved down to 70% of the batch capacity, less work is input is induced into the dough, resulting in reduced gluten development etc.
The sprag configuration allows rapid incorporation of particulate ingredients e.g. chocolate chip, dried fruit, etc without degenerating the additives. I.e. melting, damaged fruit.
Both these mixers are capable of producing ‘mechanical emulsification’ in the first stage of mixing, which is discussed, later in this document.
Watt hour energy meter graph
Mixing to time, or temperature, or revs per minute; or watt hour energy?
The implications of mixing to time, or final dough temperature, or number of revolutions of the beater, often result in variations in dough consistency, but by mixing to an energy value, it reproduces dough consistency and in one location it also resulted in a 50% reduction in mixing time.
The illustrations are of significant value to the operator, in understanding the interaction of the ingredients, such as flour: water adsorption, and influence of chemicals such as enzymes, at relatively high temperatures.
This was demonstrated to me in a ‘Greenfield Site’ project, where the operators were new to the industry.
The Oakes Single-Screw Continuous Mixer consists of a feed-end stator, a rotor and a delivery end stator.
In this system the loss-in-weight dry ingredients are screw fed and the liquid ingredients pumped into a port and are conveyed through the continuous mixer.
Its prime function is a shearing action whilst blending and mixing the dough, whilst a water jacket maintains a constant temperature. In this system, the desired proportions of ingredients are conveyed through the continuous mixer. Its prime function is a shearing action whilst blending and mixing the dough. Whilst a water cooling jacket maintains a constant temperature of the dough.
Continuous mixing of short dough
The Twin-Screw Continuous Mixer consists of a co-rotating set of twin screws, where the flights are of different designs to allow for changing the transportation and kneading of the dough, along the length of the barrel.
Continuous mixing is the logical progression from batch mixing, benefitting from improved process control, increased yield, controlled dough homogeneity and rheology, increased product volume, and the potential for reducing the fat content of short dough biscuits and cookies.
Continuous mixing of biscuit dough, can yield a reduction in of up to 10% reduction in fat content, and also a reduction in dough specific gravity, yielding a light texture and homogenous dough, due to the efficiency and mixing action of the in the barrel.
Dough rheology and temperature are constant; improving process efficiency compared with the Batch process method of mixing, illuminating the requirement for ‘dough standing time’ (lay time). Continuous mixing also reduces labour costs.
Disadvantage of batch mixing
Batch to batch variations, each mix is of a different consistency, as measured by the operator’s subjective judgement and confirmed by monitoring with the aid of testing with a Texture Analyser.
Beginning and end of mix syndrome: i.e. the dough is soft and the beginning of the process but firms-up at the latter stages, contributing to process issues. Variations in baked product quality. Product waste due to inferior or damaged products and dough losses.
Disadvantage of continuous mixing
The capitol cost of both batch mixing and continuous mixing similar, but the additional on cost is often related to the ingredient feed system.
This can be minimised by having one screw feed arrangement feeding the flour directly into the feed-end of the extruder, followed by installing a high speedhigh shear mixer to mechanically emulsify the other ingredients, from which this system which is on load cells, offers a loss-in-weight continuous feed system directly into the flights of the extrude.