Tempering (chocolate)

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Above: Properly tempered dark couverture. Below: the same couverture, processed at 40 ° C at a low temperature: dull surface, fat bloom

As tempering or pre-crystallising is called the thermal and mechanical treatment of molten chocolate prior to casting, molding, or coating with the aim that the finished product receives a beautiful, shiny surface and a crisp breakage and can be easily released from the mold. Chocolate that solidifies at low temperatures quickly becomes dull, breaks peaty and tends to form fat bloom . The need for tempering arises from the special crystallization behavior of the cocoa butter contained in the chocolate , which without this treatment would develop an unstable crystal form when it solidifies; the subsequent conversion into a more stable form then leads to the undesired effects described. The tempering process therefore aims in various ways to induce the cocoa butter to crystallize as largely as possible in a stable form.

Correct tempering is a basic requirement for the production of chocolate without serious quality defects. On the one hand, it is a craft technique used by confectioners and chocolatiers (and also in the household) when making pralines , chocolate figurines , etc. and coating baked goods with couverture , and on the other hand, it is a step in industrial chocolate production. You can temper chocolate by hand with simple kitchen tools , but there are also tempering machines of very different designs on a craft and industrial scale.

Fat icing containing cocoa usually does not need to be tempered, which is one of its advantages over real chocolate.

background

Cocoa butter is a mixture of different triglycerides , which can crystallize polymorphically , i.e. in different lattice structures . These crystal forms are called modifications, are designated with Roman numerals (especially in the confectionery industry) or Greek letters (in oleochemistry) and differ in their melting point , their density and the inclination of the fatty acid chains to the level of the crystal lattice. The polymorphism of cocoa butter is quite complex and remains the subject of research; However, the following is primarily decisive for understanding the temperature control process: When fully melted cocoa butter cools down quickly, it mainly crystallizes in an unstable modification with a melting point of approx. 28 ° C. In the literature, this is sometimes referred to as Form IV and sometimes as β '. Its instability manifests itself in the fact that it normally recrystallizes within a few hours to another modification, the form V or β, which has a melting point of approx. 34 ° C, is reasonably stable and is generally regarded as the desired crystal form for chocolate.

The crystal lattice of form IV is less dense than that of form V. The volume contraction of the chocolate during solidification with a high proportion of form IV is less, so that separation from molds may be more difficult. In contrast, the recrystallization in the solidified chocolate causes a further contraction, which leads to a dull surface and to the emergence and crystallization of fat on the surface in the form of fat bloom.

It is also worth mentioning that form V is not completely stable either, but recrystallises into a final form VI after long storage, with effects similar to the transition from IV to V, but much more slowly, so that chocolate can often be stored for years, before fat bloom occurs. Pre-crystallization in form VI would be desirable, but is not feasible with conventional temperature control technology.

Procedure

The aim of all tempering processes is to produce a high proportion of shape V crystal nuclei and a lower proportion of crystals of the more unstable forms in the melted chocolate . Since the melting point of Form V is higher than that of the other forms, it is generally sufficient to cool liquid chocolate to 34 ° C and stir until enough crystallization nuclei have formed to give the whole mass the desired modification in the event of rapid cooling to freeze. However, this takes far too long - days rather than hours - so that means are resorted to to speed up the crystallization. There are two main options for this:

  • On the one hand, it is possible to initiate crystallization by adding a small amount of already solid chocolate that has crystallized in the desired form. For this purpose, the fully melted chocolate is cooled to approx. 30–33 ° C (for milk chocolate approx. 3 degrees colder) and then inoculated with crushed chocolate and stirred. This procedure is particularly suitable for working with small batches and is often described in household recipes, where part of the couverture is retained from melting and grated or rubbed, but the inoculation method is also common in the pastry shop when working with temperature control and pouring machines .
  • The second possibility makes use of the knowledge that crystallization can be accelerated not only by cooling, but also by mechanical action, in particular by exerting shear forces on the chocolate mass. This is because, with the same degree of crystallization, many small, well-distributed crystals have a better effect than a few large crystallization nuclei. The shear forces break the crystals open and distribute them evenly in the mass; the effect is so pronounced that, with very heavy processing, cocoa butter can be precrystallized in 30 seconds instead of hours or days. In practice, the heating caused by stirring poses a problem, because the resulting crystals threaten to melt again. You therefore work with moderate shear forces and continue to cool the chocolate mass. A typical feature of this process is that the crystals that develop on the cold surfaces of containers and tools are repeatedly mixed with the warmer parts of the mass. This is the principle behind the manual “tableting” of chocolate, but it is also the basis for most industrial tempering machines.
Temperature profile during tempering, example for a milk chocolate (after Talbot 2009)

The usual temperature profile during tempering is shown in the diagram opposite (the temperature data are to be understood as approximate examples, the exact temperatures vary depending on the composition of the chocolate and the processing method used): First, the chocolate is heated to around 50 ° C to make it complete to melt, which means that it should not contain any crystalline fat if possible. In practice, the temperatures are typically somewhat lower, partly to save energy, partly because milk chocolate in particular should be kept at a maximum of 45 ° C in order to avoid the aggregation of milk proteins.

After melting, the chocolate mass is cooled to just below the melting point of crystal form V and this is followed by the phase in which crystallization is promoted. While this is happening, the temperature will continue to drop. On the one hand, this leads to the formation of crystallization nuclei of lower-melting, unstable forms, and on the other hand, the viscosity of the mass increases, i.e. it becomes thick. For further processing, be it for molding or as a coating compound, a thin-bodied compound is usually better suited. For this reason, the chocolate is reheated to a processing temperature of approx. 30 ° C or a little more. It is crucial that this temperature is above the melting point of form IV and other, even more unstable forms, but below the melting point of stable form V. This causes the unwanted unstable crystals to melt, while the mass becomes thinner at the same time.

Manual tempering

Small quantities of chocolate often have to be tempered in the household and in catering , bakery and confectionery. You work partly by hand, partly with machines that more or less automate the temperature control process.

Tempering by hand

When tempering by hand, the chocolate is first completely melted by heating it to a temperature of 45 to 50 ° C, usually in a water bath or heating cabinet or, as described below, in a microwave oven . Several methods have proven themselves for the further procedure:

  • Tabliermethode: The greater part of the liquid chocolate is cooled to a working plate of marble molded about one-third is retained. The mass is worked on the plate with a spatula until it has cooled to approx. 26–28 ° C and has become thick. Then it is returned to the liquid chocolate and mixed well. If necessary, the process can be repeated until all of the chocolate has cooled down; at the end it is reheated to 31-33 ° C.
  • Inoculation method: Crushed or grated solid chocolate or chocolate lentils (pellets) are stirred into the melted chocolate until the mass has cooled down enough that it becomes thick and the added pieces are difficult to dissolve. Then it is reheated to 31-33 ° C.
  • Tempering in a water bath: The chocolate is melted and cooled in a cold water bath while stirring to approx. 26 ° C, then reheated to 31–33 ° C.
  • Tempering in the microwave: The solid chocolate is crushed or procured in the form of pellets. The pieces are melted in the microwave oven by heating them for about 30 seconds and then stirring them through.

Casting and tempering machines

Scraper on a casting machine, on the left the clockwise rotating disc

Especially in the craft sector, work can be supported by various devices:

  • Pouring and tempering machines are mainly used to pour chocolate into molds. They consist of a heated tub into which the solid chocolate is poured and slowly melted. Before processing, chopped chocolate or chocolate lentils are added and melted. A circular disc is attached to the tub in such a way that it dips into the liquid mass in the lower area. This disc is now set in a constant rotating movement so that it lifts the adhering chocolate. In the upper area is a stripping device with a drain from which the chocolate flows back into the tub in a jet. In this way you can fill molds, but you can also catch the flowing chocolate and feed it into a coating machine, etc .; so small automatic production plants can be built.
  • Automatic temperature control machines: There are also fully automatic temperature control machines for the manual sector, into which solid chocolate can be filled and from which fully tempered liquid chocolate can be removed at the end. They work in a similar way to the temperature control vessels described below.

Temperature control machines

Temperature control tank

Temperature control boiler at Wissoll 1962

Among the simplest and oldest industrial models are the plate or bowl temperature control machines, which are basically an automation of the temperature control in the water bath described above on a larger scale. The machine consists of a double-walled kettle in which the melted chocolate is constantly mixed with a stirrer. The container can be heated and cooled with water that circulates in the jacket space. During the cooling phase, the crystals that form on the cold walls are constantly scraped off by special knives and mixed into the liquid mass.

These machines require the chocolate to dwell for a long time, normally only work in batches and require a relatively large amount of space. Because of these disadvantages, they are not well suited for large-scale chocolate production and have largely disappeared from industrial chocolate production. However, they also have some advantages; In addition to their simple structure and uncomplicated cleaning, this is above all the extraordinarily homogeneous crystallization that can be achieved with good designs and which continues to set the benchmark for more modern continuous systems.

Continuous systems

In the continuously operating systems, the liquid chocolate is conveyed through several heat exchangers arranged one behind the other , which first cool it down to the crystallization temperature and then heat it again slightly. The solidifying parts are constantly exposed to shear forces by moving parts and scraped off the surfaces of the heat exchangers. The exact construction of the cooling zones and the scraper varies greatly depending on the manufacturer and model.

Measuring the degree of temperature control

Conventional measuring method

In order to determine how far the pre-crystallization of a chocolate mass has progressed - that is, how many crystallization nuclei are present, the degree of temperature control - one can use the influence of the pre-crystallization on the course of the mass temperature during solidification. It is claimed that experienced chocolatiers have the ability to put a drop of liquid chocolate on their lip and to see from the perceived cooling effect whether it is sufficiently heated and can be processed. For quality assurance in industrial processes and for similar purposes, however, a precise recording of the cooling curve of the chocolate mass under controlled conditions (i.e. a dynamic thermal analysis ) is required . For this purpose there are electronic measuring devices, so-called temperature meters.

Structure of a temper meter (according to Windhab 2009)

Tempermeters cool the chocolate mass at as constant an ambient temperature as possible ( isothermal ) until it has solidified, and during this time continuously measure the mass temperature. The basic structure of the device is shown in the adjacent sketch: A sample container made of thermally conductive material such as copper or aluminum is filled with the liquid chocolate mass and sealed. A temperature sensor such as a platinum measuring resistor protrudes into the sample through the closure . The sample container is then immersed in an outer container with ice water or inserted into a cooling block made of Peltier elements at a constant temperature of 0 to 10 ° C. This causes the chocolate to set within about 4 minutes; meanwhile, their temperature is measured by the temperature sensor and printed out on paper (for older devices) or displayed on the screen. With some specialist knowledge, the operator can use the course of the cooling curve to determine whether the chocolate has the desired temperature control. There are also microprocessor-controlled temper meters which directly output a representative value such as "chocolate temper units". For process control, temperature meters are also available, which continuously monitor the degree of temperature control of the chocolate mass during the production process, fully automatically.

Cooling curves of chocolate at different degrees of temperature control (according to Windhab 2009)

The illustration opposite shows three different typical temperature curves. The temperature development is influenced by the fact that during the crystallization the fat molecules change into a state of lower energy and therefore give off energy in the form of crystallization heat.

  • The red curve shows a chocolate mass that can be described as under-tempered . It contains only a few crystal nuclei and therefore has a lot of latent heat that has been absorbed as heat of fusion and not yet given off again. The cooling is fast and the crystallization starts at a relatively low temperature, but leads to such a pronounced release of heat that the cooling is more than compensated and the mass is briefly warmed up again before it cools down again. The tangent of the curve at the turning point has a positive slope.
  • The yellow curve shows a chocolate mass that can be described as ideally tempered (correctly tempered or similar). Many crystals are already present here, so that the latent heat is lower and, if it is released, the cooling temporarily compensates, but does not lead to a significant heating of the mass. The turning tangent has a slope of approximately 0, that is, it is horizontal.
  • The blue curve shows a chocolate mass that can be described as over- tempered . So many crystallization nuclei have already formed here that only a relatively small amount of heat is released, which slows down the cooling process, but does not stop it. The mass cools down slowly but continuously, the tangent tangent has a negative slope.

It should be noted that, despite the evaluative designations, it cannot be said with absolute certainty which curve is best for a specific manufacturing process - this always depends on the specific circumstances, in particular the temperature control machine used and the recipe for the chocolate. The desired temperature profile - for example the optimal tangent gradient - must be determined in each case.

Further procedures

In addition to the technically relatively simple recording of the cooling curve, further processes have been developed, especially on a laboratory scale:

  • Dynamic differential calorimetry works in a similar way to conventional temperature meters in that the mass is also cooled under isothermal conditions. It allows a more precise measurement of the specific heat flow that occurs and thus provides information about the crystal content as well as the quantity distribution of the modifications present.
  • Thermorheometry is a supplementary investigation method that can imitate the conditions in a temperature control machine. It measures the development of the apparent viscosity of the mass over time at a constant shear rate.
  • Nuclear magnetic resonance spectroscopy to determine the solid fat content (SFC) can also be used to measure the degree of temperature control.

Individual evidence

  1. ↑ a b c Heinrich Fincke : Handbook of cocoa products . Ed .: Albrecht Fincke. 2nd Edition. 1965, p. 231 .
  2. ^ A b c Stephen T. Beckett: The Science of Chocolate . 2nd Edition. Royal Society of Chemistry, Cambridge 2008, ISBN 978-0-85404-970-7 , Crystallizing the Fat in Chocolate, pp. 103 .
  3. ↑ a b c d Geoff Talbot: Chocolate Temper . In: Stephen T. Beckett (Ed.): Industrial Chocolate Manufacture and Use . 4th edition. Wiley-Blackwell, Oxford 2009, ISBN 978-1-4051-3949-6 , pp. 261 ff .
  4. ↑ a b c d e f g Erich J. Windhab : Tempering . In: Stephen T. Beckett (Ed.): Industrial Chocolate Manufacture and Use . 4th edition. Wiley-Blackwell, Oxford 2009, ISBN 978-1-4051-3949-6 .
  5. ↑ a b c d e Stephen T. Beckett: The Science of Chocolate . 2nd Edition. Royal Society of Chemistry, Cambridge 2008, ISBN 978-0-85404-970-7 , Manufacturing Chocolate Products, pp. 125 .
  6. ↑ a b c d e Friedrich Holtz u. a .: Textbook of the pastry shop . 5th edition. Trauner, Linz 2009, ISBN 978-3-85499-367-4 , pp. 372-373 .
  7. ↑ a b Friedrich Holtz u. a .: Textbook of the pastry shop . 5th edition. Trauner, Linz 2009, ISBN 978-3-85499-367-4 , pp. 227 .