Chocolate Refining – a review of appropriate technology
There are a number of technologies available for making chocolate (and similar vegetable fat based coatings) but they differ considerably in their application, particularly in their suitability for different scales of operation and the degree of product flexibility required. Most of the issues around scale are associated with the grinding stage of the process and this article therefore concentrates upon that stage of the process.
Choosing the “best” grinding system for a particular application really needs a professional, in depth study of the business and the opportunity – not least because the equipment being selected is expensive and has a long life, so this is a genuine long term investment in the future of your business. However there are some basic “rules” which can be used to help you to understand the choices available and therefore refine the questions you need to ask.
Manufacturing good quality chocolate is reasonably difficult and cannot be achieved with “domestic” equipment – it needs industrial or semi industrial machinery for two main reasons:-
- You have to achieve a fine particle size and a good particle size distribution.
- You have to achieve a smooth “fat continuous” product at a relatively high solids concentration
The “ancillary” processes such as weighing out and premixing ingredients, addition of flavours and emulsifiers are quite generic and as mentioned above this article focuses on particle size reduction as this is the core of the chocolate manufacturing process.
The general particle size requirement for any chocolate is that very few particles should exceed 28? (as this is the generally accepted detection threshold of the human mouth) with very little above 20?. In addition the particle size distribution should be fairly narrow. Of course many chocolates are finer than this, but if the average particle size is too low (mainly below 12-15?) the product will be cloying and unpleasant to eat. Although this sounds complex, the commercial equipment available is designed to achieve the required standards, so it should not be too much of a concern. Note also that there are many different ways of measuring and expressing the particle size of chocolate so the above figures may not apply to “your” method – perhaps the most reliable and consistent one is to taste it!
Typical Chocolate Particle Size Distribution
Chocolate is a fat continuous product – ie it is a suspension of fine solid particles in fat. As chocolate is ground, more, smaller particles are created and this represents a challenge in terms of retaining this fat continuous structure. There are two approaches to resolving this problem, either concentrating all the grinding in one process and then liquefying the resulting powder in another, or simultaneously grinding and liquefying. A variety of commercial equipment exists to carry out these processes but the most commonly encountered are:-
- Roll refining followed by liquefying
- Bead milling with simultaneous liquefaction
- Dry grinding followed by liquefying
- Impact milling with simultaneous liquefaction (these machines are often referred to as “refiner conches” or “universal machines”)
All of these have advantages and disadvantages and optimum application envelopes. In addition to these there are some less commonly encountered processes which have their own advantages and disadvantages. I am not going to discuss those here, but an in depth survey of requirements may indicate that a less common process may be suitable for a particular application.
Liquefying the powder from roll refiners or dry grinding requires a conche (these are generally batch machines of various designs) and for simultaneous liquefying with a bead mill a continuous high shear mixing system is often used in line with the mill itself. There are continuous conching systems based on extruders but they are really only suited to very large installations due to their high capacity and capital cost.
Roll Refiners are the core technology for large-scale chocolate manufacture. They are large, heavy and fairly complex machines, which require a degree of skill to operate and a maintenance infrastructure with appropriate skills and resources.
5-roll chocolate refiner (Heidenauer)
Roll refiners have to be run continuously for long periods as starting and stopping this type of equipment frequently is not good practice. Even small refiners produce in excess of 0.5 tonnes per hour, so the absolute minimum threshold is about 30 tonnes of chocolate per week for even a single small machine and in practical terms output needs to be much higher than this. Whilst refining (and subsequent conching) is an excellent technology, it is expensive in capital terms and not really suited to small to medium sized installations below about 5000 tonnes per year. Refiners themselves are easy to clean down and can thus produce a number of different recipes and although conches are less easy to clean completely, careful scheduling can allow the same machines to produce both dark and milk chocolates with minimal cross contamination. White chocolate and recipes containing incompatible fats are more difficult to accommodate and may require additional conches reserved for specific recipes.
Most modern roll refining systems use two refining stages – a two-roll pre refiner followed by a five-roll main refiner. This allows the use of granulated sugar in the initial mix rather than having to use milled sugar. In addition the two roll refiner can be manipulated to produce a paste of optimum consistency for the five roll refiner and hence better control of particle size and improved output.
Two roll chocolate pre-refiner (Buhler)
Bead Mills are manufactured by a number of companies and although designs and processes differ slightly they generally recirculate a batch of material through the mill continuously via a secondary tank and mixer until the product is fine enough. They are an excellent alternative for medium and even large-scale production, requiring less space, skills and specialist maintenance than roll refiners with their attendant conches. However they are more difficult to clean and whilst again careful scheduling can permit both milk and dark chocolate to be produced on one machine, white recipes and incompatible fats in practice require dedicated equipment. A further drawback is that bead mills are less adept at handling high viscosity chocolates and may not be able to achieve quite as low a fat content as a refiner/conch combination for a given viscosity. Bead mills are well suited to and widely used for the manufacture of cake, biscuit and ice cream coatings, all of which are relatively low in viscosity.
High efficiency bead mill for chocolate (Buhler)
Dry Grinding followed by pasting and conching is not widely used, but for the right application can be very effective and allows great flexibility in recipe and throughput. Dry grinding has potential problems with particle size control, safe operation and materials handling, but modern grinding systems have proved to be capable of providing an excellent, safe solution to these issues. Energy consumption for dry grinding can be higher than for other approaches due to the amount of air which has to be handled and cooled.
However, the grinding system is relatively easily cleaned (although filters can prove a problem) to facilitate recipe changes and the system can be adapted to a wide variety of throughputs and recipe ingredients. The feed to the milling system will generally need to be low in fat content with most fats added in the subsequent conching stage.
Equipment for the liquefying stage will be relatively similar to that for other applications and the manufacturer of at least one system promotes its improved flavour potential. Most dry grinding systems are large, continuously operated, integrated installations with sophisticated control and materials handling systems. Because of their scale and continuous operation they are highly suited to extruder based continuous conching in line with the milling system.
Dry Grinding System for Chocolate (Bauermeister)
Impact grinding is a batch process where the material is ground in a pasty state by the impact of rotor bars mounted on a central rotating shaft on the ribbed outer shell of the machine. The equipment is simple to operate and robust although process times are long, particularly with larger machines and noise levels can be high. Once a process has been identified the machine needs little more than loading with ingredients and discharging after a set time, although some recipe additions will be needed during operation.
Small Refiner Conche (Impact Milling System) (MacIntyre)
The equipment is capable of producing good quality chocolate but the technology is also used widely for reprocessing scrap material into centres and for cake and biscuit coatings. In common with most other technologies it is possible to switch between milk and dark chocolates without major cross contamination issues, but white chocolates and incompatible fats do require either a very thorough clean out or a separate machine.
The simplicity of operation, robust nature of the machinery and minimal maintenance requirements make these machines very suitable for small and new businesses in the range 5 – 20 tones per week and it is not uneconomic to have a small machine and to operate it intermittently for very small outputs of specialist or niche products. Although process times are relatively long, the machines can be automated and run unattended.
All of the approaches discussed here are capable and reasonable technologies to consider for a chocolate making application. However, it is important when considering which manufacturing technology is most appropriate for your application to consider the following key questions:-
- Existing equipment – what, if anything do you already have
- Scale – how much chocolate do you wish to make
- Flexibility – how many different recipes do you have to make
- Capability – what skills and experience does your workforce and management have
- Facilities – how much space do you have available and what type of building do you have
The existing technology in a factory is a significant consideration when planning an installation, particularly if the existing equipment is in good condition. Whilst the expansion may signal the need for a change of technology, adding capacity using existing technology, even if it not on its own the optimum approach, has the advantage of familiarity for both operating and engineering staff and of course enhances overall flexibility. Conversely, if the existing equipment is old and in poor condition then it may be better to replace all the capacity with a new technology.
Scale is one of the most important factors in deciding what technology to use. This table gives an approximate indication of the minimum and maximum tonnage for different technologies, but the final decision will be heavily influenced by the flexibility needed and other factors, as these may dictate a different approach or number of smaller capacity lines rather than a single high capacity unit.
Note that roll refining does not really have an upper capacity limit, this is more an estimate of a practical single line capacity (in this case with about five main refiners) – multiple lines can extend this to almost any capacity. Similarly with dry milling, several milling systems can be employed in parallel.
Impact grinding machines are excellent for low to medium capacity installations and can make up for the difficulty of changing recipe by relatively low cost duplication of equipment. In themselves they are extremely flexible machines although difficult to clean completely for a change, for example from milk chocolate to white chocolate. The long processing time (particularly when using granulated sugar) is on the one hand a drawback in capacity terms but may be advantageous for flavour development and volatiles removal. In addition to chocolate and coating manufacture they are widely used in non specialist environments (such as bakeries) and for primary reprocessing of scrap material.
Bead mills are very effective for quite large-scale operations and their relative simplicity of operation and maintenance and absence of the need for conches make them quite attractive. There are potential drawbacks in terms of the viscosity that can be handled and that they are essentially a closed loop system with none of the surface exposure in a solid continuous phase. This is often regarded as important for development of flavour and removal of some undesired volatiles and neither of these is likely to be as effective where cocoa particles are in a fat continuous phase. However there are “add on” processes, which can compensate for this in many cases. They are also widely used for processing chocolate type masses for bakery and ice cream coatings and should certainly be considered for any application.
Dry grinding is not widely used but has advantages which appeal to some businesses, particularly its ability to process complex recipes incorporating a variety of milk sources. It is most effective in handling low fat mixes which are then pasted and conched either in batch mixing systems supplied by the manufacturer or in specialist equipment such as extruders. Dry grinding does offer a high level of flexibility in that the grinding systems themselves are relatively straight forward to clean and liquefying systems can be duplicated where necessary. Because of the nature of the grinding system they are also likely to be more suitable for handling mixes containing “sugar free” bulking agents than other grinding systems.
Because of the airflow required by this type of mill and the associated refrigeration systems, dry grinding is unlikely to be a low energy consumption alternative, and unlike the other systems they do have to handle potentially explosive powder mixes and effective relief or suppression systems are essential to any installation. However, for large scale, continuous operation they are an interesting alternative to roll refining and well worth consideration.
Roll refining is the classic system for grinding chocolate and similar masses and has a long history. Roll refining systems are unsuitable for low outputs as even relatively small machines have a high output and need to be run continuously. Conversely there is almost no upper limit to the capacity of a roll refining system as machines can be banked in parallel feeding systems of multiple batch or continuous conches. A well set up and operated roll refining system is the technology of choice for large output plants and the machines are flexible in that they can be cleaned down easily to allow a change of product.
Roll refiners are sophisticated and complex machines that need highly skilled operating and engineering support even with the assistance of manufacturers for training and support. Installations need strong and vibration resistant foundations which can bear the weight of these large and heavy machines. A prime maintenance requirement is for the regrinding of rolls which can generally only be undertaken by returning rolls to the manufacturers location, which is an expensive operation required every 2-5 years depending on the usage pattern and ingredient quality. Regrinding also requires spare roll sets if production is to be maintained.
Flexibility is an important issue when considering which technology to choose. Whilst recipe changes from dark to milk recipes can be accommodated by most technologies, changes to white recipes are extremely difficult, particularly on a regular basis. The same applies to changes to and from different fat systems because of the need to eliminate virtually all cross contamination.
When a business has to produce a wide range of different recipes with frequent changes then it is better to have a number of small units dedicated to recipe groups operating in parallel rather than to try operating frequent changes on a large throughput unit.
The optimising of product ranges, operating schedules and production plans – current and in the future – is core to determining the level of flexibility, which is actually needed in an installation and hence the most appropriate process design and equipment selection.
Capability within the business is an important factor, particularly for small and medium sized businesses and those entering the market. Some technologies – particularly roll refining and dry milling, require a high level of investment and high competence and experience both on the part of operators and maintenance teams. Although suppliers will assist with this, some experienced staff will almost certainly be needed to develop a successful and efficient manufacturing operation. With these large scale, continuous, technologies consideration has also to be given to the conching and materials feeding and transfer operations required.
Systems such as bead milling and impact milling are much simpler to operate as there are fewer variables to control and the maintenance requirements are much simpler. This makes them very suitable for smaller businesses and those which do not have a suitable technical base.
The facilities available also have a role in determining the choice of technology. Roll refiners in particular require substantial buildings not only to support the weight of the machines themselves and their associated conches but also to prevent vibration being transmitted to the refiners, which can reduce their efficiency. High efficiency bead mills in particular are lighter machines which require less robust buildings and less space than other technologies.
Large impact mills are needed to achieve high outputs because of the relatively long cycle times so although this technology is simple and easy to operate it can require strong foundations and ground floor installation.
Dry grinding systems require a carefully designed installation in many ways untypical of traditional chocolate making and are therefore a choice needing careful consideration and expert advice both in selection and in the design of the installation.
Liquefying (Conching)
Both roll refiners and dry milling systems require a conching process to convert the dry, “solid continuous” powders produced to a smooth, fat continuous chocolate or coating. This process is also important in flavour development and volatiles removal and a complete understanding requires an article of its own.
At its most basic level conching involves the input of mechanical energy to the mass. Most of that mechanical energy is converted to heat in the machine and to prevent overheating of the product that heat has to be removed. Most modern conches have designs which maximise the mechanical energy input while balancing this with very efficient heat transfer. This has allowed much shorter process times and hence fewer, smaller machines which need less space.
The process can be broken down into a series of stages:-
- Loading of the powder from the refining or milling system, perhaps with the addition of some fat to form a stiff paste.
- The paste is mixed vigorously to start to form the required structure and to release volatiles. The speed of this process is often limited by the ability of the conche to control the temperature of the mass as most of the mixing energy is converted into heat and heat transfer to the cooling jacket may be poor, particularly in older machine designs
- At an appropriate point emulsifiers and some additional fats are added and this causes the mass to liquefy into the desired fat continuous, smooth liquid chocolate. Vigorous mixing will be maintained for a period to achieve optimum properties in the chocolate or coating and flavours are most commonly added at this point
- On completion the product is discharged from the machine
Conch process times vary widely as to temperature profiles and energy input rates depending on the requirements of a particular product.
Although the majority of conching capacity is in the form of batch machines of a wide variety of ages and design, there are some continuous technologies which can be coupled to both roll refining and dry milling systems. However these continuous systems tend to involve a considerable amount of development by the user companies as the manufacturers of the basic machines (generally modified extruders) have little knowledge of chocolate.
Pair of modern high speed Batch Conches (Lipp)
Modern high speed batch conches are mechanically much simpler than the more traditional machines. They are basically single shaft horizontal mixers with sophisticated paddle designs and close wall to paddle tolerances. However they are still quite heavy machines and have high motor power.
The cost and additional complexity of conching systems should not be forgotten when considering a chocolate processing technology.
Conclusion
There are a variety of technologies available for achieving the desired particle size in chocolate manufacture, but choosing the right one for a particular application requires consideration of a number of factors, including not only the current product range, capacity and capability, but how those and other factors will develop in the future.