For many years bakers have had to make a choice between batch and continuous dough mixing solutions. Has anything changed? Suzanne Callander reports.
The mixing stage is a critical point of quality control in the production process for commercially baked foods. For many years bakeries have needed to evaluate and choice between two different dough mixing solutions when automating the process – batch mixing or continuous mixing. Each has its advantages and disadvantages and determining the most optimal mixing solution for an application will depend on a variety of considerations.
Batch mixing can offer versatility and flexibility, while its downsides are that it is a more labour-intensive process and there can be variations between different batches. Meanwhile continuous mixing, which involves a steady flow of ingredients into the mixer, creating a continuous stream of dough, is considered to work best in high-volume production. It has lower labour requirements and offers greater product consistency. However, it offers less flexibility for product variations and comes at a higher initial cost than batch mixing solutions.
Throughput and initial cost are often the most significant considerations when choosing between a batch or continuous mixing solution. Bakers also need to consider their final baked product specifications, total costs, ease of operation, maintenance requirements, and the downstream processes.
Batch mixing remains a mainstay of the bakery industry today because of its versatility and compatibility with other processes such as ingredient delivery and pre-hydration systems. Its initial cost also makes it a popular choice. Batch mixing entails making large batches of dough, one set at a time, making it particularly suited to applications with recipe variations. At the end of the mixing process, the dough exits the machine, and the machine is ready for cleaning or for the next batch.
During a continuous mixing cycle, ingredients constantly flow through controlled feeds into the mixer, being mixed for a predetermined amount of time and before being discharged. Continuous mixing offers high throughput and low energy use when compared to mixing dough in batches.
When determining the type of dough mixer for an application, AMF Bakery suggests bakeries consider the following:
Processing time: The time it takes for the material to flow into the mixer through to the discharge outlet is known as the rate per moment (RPM). For longer RPMs, batch mixing is considered to be more appropriate because applications that rely on an even product blend need to be mixed quickly.
Productivity: Continuous mixing can offer a faster process for high volume throughput in a shorter time, although continuous mixing is less expensive than the highest production rates. Batch mixing is cheaper than continuous mixing for lower and mid-size production rates. Additionally, batch mixing requires less equipment and can be easily adapted to changing production needs.
Energy costs: Continuous mixers typically utilise less energy per throughput cycle than batch mixers as they do not require frequent starts and stops.
Labour costs: Like energy costs, with continuous mixing, the labour costs and human intervention will be lower than for batch mixers.
AMF Bakery believes that its AMF Fusion technology can improve the reliability and precision of batch mixing processes and also make it possible for bakers to convert to continuous mixing through a partnership with Exact mixing, a Reading Bakery Systems brand. AMF Fusion’s batch mixing systems are designed to optimise control over ingredients, mixing, and dough parameters. Glycol cooling jackets on bowl mixers and AMF’s proprietary Mixer Guardian technology are said to allow operators to monitor and adjust machine parameters to maintain a constant temperature and consistent dough structure. Intuitive operator controls and alarms allow more control over mixing and refrigeration times which can help reduce the training and onboarding time required for new production personnel.
Considerations
According to European Process Plant (EPP), the biggest consideration on the choice of dough mixing solution will relate to the end product and the process priorities to create this. It argues that selecting the optimal mixing method depends on several factors:
Product type: Consider the nature of the products. Delicate batters might benefit from batch mixing, while high-volume bread doughs could be better suited for continuous mixing.
Production volumes: Batch mixing is well suited to use in small to medium-scale bakeries, while continuous mixing excels in large-scale operations.
Desired consistency: If consistent product quality is paramount, continuous mixing should be the preferred choice. Batch mixing offers more flexibility but requires careful monitoring and is prone to batch differences.
Budget: Continuous mixing systems often involve a higher initial investment but can offer long-term cost savings due to increased efficiency.
Space availability: Evaluate the available space in your bakery to determine the suitability of different mixer types.
Pizza dough
When it comes to pizza dough, Sobatech argues that a continuous mixing solution can help simplify the mixing process, reduce labour, cut costs and improve product quality consistency.
Pizza dough can present some specific mixing challenges, according to the company. It is important to mix the dough just until the ingredients are combined and form a homogeneous mass. Overmixing will result in a tough and chewy crust rather than a light and airy one, while undermixing the dough can result in uneven hydration and poor gluten development, leading to a dense and crumbly crust.
The temperature of the ingredients and the environment can also impact pizza dough mixing. Cold ingredients can slow down yeast activity, while warm temperatures can speed it up, so maintaining the right temperature throughout the mixing process is essential for proper fermentation and dough development.
Sobatech has also identified that the inclusion of a short resting phase can help ease pizza dough forming as gluten retraction is prevented. To achieve this, it has added a continuous bulk fermenter to its mixing solution.
Putting a case for continuous mixing
Arguing the case for continuous mixing, Jim Warren of Exact Mixing, pointed out that continuous mixing technology has greatly advanced over the years. Today’s solutions include improved ingredient metering, metering of solid fats and particulates, enhanced controls, specialised mixer designs, larger capacity mixers, improved temperature control and more.
While all of these advances have made continuous mixing more attractive to bakeries, it is the high-capacity continuous mixers that offer the biggest benefits, according to Jim.
He explained that, in 2000, the highest throughput continuous mixers operated in the range of 5,000kg/hr of dough produced. By 2020, continuous mixers were being manufactured in the 15,000 to 20,000kg/hr range. Even higher rates were accomplished by linking continuous mixers together. With the increased throughput of the largest continuous mixers, you would expect the additional cost of a larger mixer to magnify any resistance due to initial investment requirements. However, higher throughput mixers quickly became best sellers. Why? Because while the larger mixers had a higher initial cost, the advantages seen with the smaller continuous mixers became much more important at higher rates.
Also, as production rates increase, the cost of a batch system tends to increase linearly – double the capacity and double the cost. On the other hand, if you double the capacity when using a continuous mixing system, the cost only increases by about 25%.
According to Jim, the five primary reasons that the advantages of continuous mixing increases as throughput increases are:
Continuous mixing is more cost effective to operate than batch mixers for larger capacity lines: In both continuous mixing and batch mixing processes, four factors contribute to the required initial investment – mixer cost, bulk materials delivery, ingredient metering, and post-mixing dough handling. In the case of continuous mixing, three cost factors comprise a system. Those are metering, mixing, and post-mixing dough handling. For batch mixing, three costs also comprise a system – metering, mixing and dough handling.
The key concept to understand is that when the throughput of a continuous mixing system increases, the cost of the system changes at a much lower ratio because the ingredient metering portion of the cost is nearly constant no matter what the new throughput is.
When considering batch mixing, the cost of the model is quite different. Basically, doubling the throughput of a batch mixing system increases the cost by 80% to 100%. Both the mixer and the downstream metering system must double in capacity.
Continuous mixing is fully automated, requiring less labour than batch mixers: Does higher dough output require more personnel in the mixing area? The intuitive answer is yes. Mixers are larger. More mixers are required. Ingredients are added in larger quantities. Larger dough quantities are discharged and must be delivered to the downstream equipment. Failing to deliver dough on time results in a larger reduction in production.
With a batch mixing system, all these factors require more employees. This is not the case with continuous mixing, as the system is fully automated. First, the ingredient addition and mixing are controlled by the recipe control system. Second, the dough is automatically cut into smaller chunks by the mixer or allowed to flow as a rope. This means no additional dough sizing and handling is required. As a result, no more employees are required at higher capacities.
With any size continuous mixer, the operator has two primary responsibilities. The first is to occasionally inspect the dough quality as a final verification of the quality prior to forming. The second responsibility is to address any system alarms.
As batch sizes increase other concerns arise. Most crucial among these is training. Batch processes tend to be manual in nature, requiring operator intervention. This is even more so as the dough volume increases. This requires a great deal of training to ensure the mixing steps are completed correctly.
As the dough requirement increases with a batch system, the demands on the staff are multiplied. In the case of continuous mixing, no matter what the rate, only one operator is required and for only a small portion of the day.
Continuous mixing requires less dough handling equipment than batch mixing: The method of handling dough after mixing is completely different with batch versus continuous mixing systems. In the case of continuous mixing, the dough generally comes out of the mixer in a rope or log. A cutter at the end of the mixer allows the baker to make any size dough chunks. The chunks fall onto a conveyor belt or directly into a hopper.
In the case of batch mixing, the dough discharges from the mixer in a large mass that must be resized for downstream equipment, resulting in additional cost for this handling equipment.
As the requirement for dough increases, dough handling becomes a greater challenge with a batch mixing system. Larger chunks of dough mean more equipment, and more people. However, in the case of continuous mixing, dough handling is the same regardless of throughput.
More efficient throughput – less dough is at risk with continuous mixing compared to batch mixing during shutdowns: When a baking line is running, dough exists between the mixer and the proofing or forming equipment. This is referred to as dough-on-hand. This dough-on-hand includes dough in the forming hoppers, dough waiting to be placed in the hopper, dough being discharged from the mixer or the dough currently in the mixer. In the case of a batch mixer, this amount is typically around 2.5 to 3 mixer volumes. The issue for batch mixing is that all dough-on-hand is at risk should downstream equipment require an extended shutdown.
In the case of continuous mixing, the hopper levels can be kept as low as possible to improve piece weight control, so only a few chunks are in transit. In effect, the dough at risk for continuous mixing is basically the volume of the mixer at any capacity.
Automated continuous mixing systems eliminate variables that exist in the batch mixing process: Mixing systems inherently have variables which must be carefully controlled. Some of these include recipe, temperature, energy input, humidity, flour quality, and batch spacing. There are two steps required to control such variables – measurement and reaction to inconsistencies.
It is how variables are measured and how the system reacts that determines the success of the mixing operation and ultimately the consistency of the dough.
In the case of batch mixing, the recipe is often based on hand-measured minor ingredients. So, the operation is dependent on a person and a scale.
In the case of continuous mixing, minors are blended with the use of loss-in-weight technology and a blender on load cells. All measurements are weighed automatically, and the data is saved in the system. Dry and liquid ingredients are metered into the mixer, and automatically weighed and recorded. As the mixer throughput increases, hand measuring creates greater opportunity for errors, more employees are required to batch ingredients and the pressure to create blends quickly increases. Continuous mixing guarantees accurate measurement all day, every day.
It would appear that there is still no definitive answer to the question of whether to specify batch or continuous mixing solutions _ both still have a distinct set of pro’s and con’s and both have benefitted from technology advances. The choice, it seems, will still depend on the size of the operation, the budget available, and the product that is being produced.
Editorial contact:
Editor: Kiran Grewal kgrewal@kennedys.co.uk
