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A review of a machine design of chocolate extrusion based co-rotating twin screw extruder

Based on innovation and competitive market for food industry, there are several food products which have been designed to attract customer. Since there is USD 39,431 millions of chocolate sales in 2018, USA [1], chocolate product shapes have been developed based on manufacturing process. This paper presents a review process of a cocoa machine design of chocolate extrusion based co-rotating twin screw extruder. A property of suitable chocolate for extruder was established. The pros and cons of machine extruder for food processing including, a screw extruder design were exposed. Since there were problems in the chocolate extruder, the process parameters such as barrel temperatures, feed rate, screw speed, motor load and melt pressure were established. These parameters would be applied to design screw extruder for chocolate processing.

Demand is growing for higher-quality chocolate, alongside more sustainable manufacturing processes. But current chocolate production faces problems keeping up. The different production stages rely on their own specific cocoa equipment, components can be difficult to clean and potentially harmful metal particles can be released into finished products.

EU support helped the BAT project optimise the chocolate manufacturing process with a versatile ‘one-stop shop’ machine which, as well as being easy to clean, avoids the need for producers to run a set of expensive, bulky and energy-intensive machines.

The new refining technology

There are typically around seven stages to chocolate manufacturing involving three to five dedicated machines. First, beans are toasted before husking to remove the shell surrounding beans. The resulting cocoa goes through a pre-refiner before passing through a refiner to create a smoother consistency. Next, the ingredients enter a separator which ensures no grains are too large before going through a further refining stage. The last stage is conching in which the acidity of the cocoa is removed.

The BAT solution combines the pre-refining, refining and conching stages into a single step, reducing the number of machines needed for the process to a single machine. Importantly, the machinery is designed to be easy to clean and does not release metal into the product.

Crucially, it can also refine both products containing water (mix hydrated) and those without anhydrous, something that is not possible with current technology. The icing on the cake is that it comes at a comparatively low purchase price.

The team built a prototype BAT refining head, to carry out tests.

“The refining head is the processing step on which our innovation is based. We had a number of challenges to overcome with these tests, such as keeping temperature and quality constant across the whole product batch. Despite the complexity we managed to solve these with our design,” says Stefano Marello, project coordinator.

Opportunities for smaller businesses

The chocolate and cocoa market is a fast-evolving market. According to recent research, the demand for chocolate in Europe is predicted to grow by a compound annual growth rate of 3 %, between 2018 and 2022. While Europe is the world’s largest producer, the bulk of this production rests with six big multinationals.

Additionally, despite advances in food production, cocoa is still one of the products that goes through a high number of stages prior to consumption. The quality of the finished product is dependent on the multiple steps and the quality of the treatment it undergoes.

The BAT system offers chocolate manufacturers a more effective solution as they refine their products in the pursuit of higher quality. Importantly, it also opens up opportunities for producers to produce chocolate directly from the bean, while avoiding the cost associated with multiple cocoa processing machine.

“BAT controls the temperature of the core of the product much better than current technologies throughout the refining cycle. This makes it possible to produce different chocolates from the same cocoa beans.

“This could lead to an increase in the quality of finished products on the market, as well as new opportunities for smaller businesses, as the barriers to entry are reduced,” says Marello.

The team are currently pursuing further funding opportunities in order to develop an industrial-scale prototype, capable of running 24 hours a day.

A lot of time has passed since the first refiner conches were built to make chocolate. At that stage all necessary processing steps were done in the same chocolate tempering machine, which sometimes took a week to get the final product. This paper is not intended to summarise all the technical developments since then as such information is available in textbooks1. Instead it aims to briefly introduce the different systems for chocolate mass production offered by various companies in order to give readers an overview on what is currently available on the market.

Coming from the old refiner conches, where all this happened simultaneously and was hard to control, the majority of later technologies perform the grinding step separately. Only few mill types are able to handle chocolate preparations, as it is initially a very sticky mass, which can transform to a sticky powder during milling, when specific surface of particles increases. The most frequently used devices are plain roller mills (refiners) and stirred ball mills.

Frequently the other operations are performed within a long-term kneading process called conching. Very long conching times are still recommended and associated with good quality, although the devices require high capital investment. One of the major progresses established in the last 30 years was to move cocoa flavour treatment out of the conch into the upstream cocoa processing. Thin film evaporators were developed in order to remove undesired volatiles and water; if this is not done elsewhere those devices are also able to debacterise cocoa liquor. Unfortunately the very popular Petzomat is not built any more, but alternatives from other companies are available. Nowadays chocolate producers can strongly reduce conching times if they insist on using pre-treated cocoa liquor of high flavour quality. Untreated cocoa is also still used, which then requires extra conching, like in former times.

Similar principles are followed for milk chocolates by developing milk powder pre-treatment procedures. For example it was proposed to dry skimmed milk powder to below one per cent water and to coat it with fat, which allows us to perform a very short liquefaction process instead of classical conching2.

Crumb is an ingredient made by drying milk together with sugar and cocoa liquor. Originally this was done for preservation of the milk, but nowadays it is performed in order to create the strong caramel flavour preferred in some countries. For downstream mass production the same technologies can be used, as with other chocolate types.

If cocoa butter is replaced by another fat, the product is usually called compound and not chocolate. Technologically most compounds are close to chocolate mass and similar equipment can be used to make it. The largest difference is rather an economical one, as very expensive cocoa butter is replaced by relatively inexpensive alternative fats.

After some initial information on chocolate mass properties the systems available on the market will be introduced. For that purpose information was obtained from various manufacturers, followed by questions and discussions on aspects such as:

Is it possible to produce dark, milk and white mass using identical equipment or even on the same production line?

What are the main advantages of the process for larger and also for smaller chocolate producers and what is the minimum size of an industrial production line?

How much energy does the process require?

What is the approximate capital investment necessary for a production line?

Of course not all questions could be answered. In particular the last point, as process equipment is usually designed individually by chocolate tempering equipment manufacturers for their clients. So in practice, chocolate makers will always have to negotiate individually with suppliers. This paper will provide an introduction to the possibilities on the market.

Chocolate mass properties

Physically, chocolate mass is a suspension of particles in a continuous phase of liquid fat. Downstream when producing final products for the consumer, fat crystallisation is initiated and the mass is forced into the desired shape and solidifies. These steps are not considered here, although many properties of the final product can be predicted by measurable properties of the still liquid chocolate mass. Therefore flow properties are usually measured at a temperature of 40°C, which is close to the temperature that chocolate melts in our mouths. So texture sensations like a smooth melt or a sticky behaviour are usually correlated to flow properties.

As chocolate mass is a non-Newtonian fluid we have to measure its shear stress at different shear rates, which results in a flow curve. Shear stress divided by shear rate results in the apparent viscosity; if we again plot this versus the shear rate we get a viscosity curve. Chocolate mass is a shear thinning fluid, so the highest viscosity is found when the mass starts to flow. Interaction between particles is considered to be responsible for this behaviour3, which is very different to Newtonian fluids such as water. So one important part of the flow curve is at very low shear. The yield value defines the shear stress, when the mass starts to move. As a minimum shear rate is necessary for the measurement, usually the yield value has to be extrapolated from the flow curve according to model equations, like the ones developed by Casson and Windhab1. Yield values or measurements at low shear stress also have a great practical importance, as many industrial operations are carried out with masses flowing slowly, for example the equal distribution of still liquid mass in a mould.

On the other hand side some processing is done under high shear, e.g. when pumping or spraying masses. This is best described by the other end of the flow curve. So usually it is extrapolated to infinite shear, the result is then called Casson or Windhab infinite viscosity. Naturally, fat content, emulsifiers and ingredient properties have the largest influence on viscosity. After those, particle size distribution and particle package density are also important. Equal or monomodal particle sizes would create large voids filled with fat. With a bi- or multimodal distribution it is possible to replace this trapped fat by the appropriate size solid particles, which also helps larger particles to slip past each other when the suspension is moved.