The final step in any chocolate project is the hardening of the chocolate. While conceptually it is just a matter of letting the liquid fat system solidify, because cocoa butter is a polymorphic fat system, it must be cooled in such a way as to prevent the Form V (beta crystals) in the fat system from reverting back to lower crystalline forms. That means both the removal of the sensible heat put into the chocolate to cause it to liquify and of the latent heat of crystallization. The key to this process is to get the chocolate to the point where Form V crystals will no longer melt. This ensures a sufficient number of Form V crystals will be present at the end of the crystallization process to cause the lower melt forms of cocoa butter to transform into primarily Form V crystals over time. This is important in that too few beta crystals will cause a non-molded item to scuff easily and melt quickly upon holding in one’s hand. In addition, molded chocolate items will not be released easily from the mold.
Sensible and latent heat are the two forms of heat that must be removed during the cooling process in order to drop the chocolate being solidified to a temperature where lower Form crystals will transform into Form V, both during the cooling process and the over-the-shelf-life of the item. It is thought that items leaving a cooling tunnel will be at approximately 40% Form V crystals. Sensible heat is the heat that was added to melt the chocolate to both liquify and remove all previously formed beta crystals. Latent heat is the heat given off during the development of Form V crystals. This heat needs to be removed as the chocolate solidifies. Since this heat is developed throughout the chocolate as it is solidifying, it is very important to keep the heat moving from the center of the mass to the surface, so it can be transferred to the air. Thus, the cooling system must be able to maintain the air temperature (heat sink) at a constant, even while pulling sensible and latent heat from the product, as the product’s temperature equilibrates towards the temperature of the air of the cooling system.
Figure 1 provides a visual view of the solidification of a chocolate. The view is a picture of a solidified molded chocolate item broken into two pieces at the snap point. The piece was cooled in a refrigerator which was at a temperature around 43°F (6°C). This process was based on a simple radiation cooling system where the heat from the product was allowed to transfer to the air by simple temperature equilibration. In view A, you see a well solidified chocolate as can be determined by the tight packing of the crystals and a uniform texture. Also note the nice gloss on the inside surface of the bubble. This gloss is a sign of a well packed crystalline structure. In view B, you see a very loosely packed structure with a cottage cheese texture. This texture occurred because heat was entrapped in the chocolate during the cooling process. This caused Form V crystals to revert to lower forms as they solidified. The physical cause of this packing is due to the less tight packing of lower form crystals. The likely cause for this was that there was no fan to pull heat evenly away from the center of the chocolate item, and thus to the surface, where it is transferred to the cabinet air.
Ideal solidification of a tempered chocolate item requires two primary components. First is a heat sink to absorb the heat being given off by the chocolate due to the addition of sensible heat and the creation of latent heat in the crystallization process as the chocolate transforms from a liquid to a solid. A heat sink is simply a material that will absorb heat from a higher temperature source. Based on the second law of thermodynamics, this heat sink must be cooler than the desired end temperature as heat flows from a higher temperature item to a lower temperature environment. The desired temperature range for this system is between 70°F and 50°F (21.1-10°C), and preferably between 65°F and 55°F (18.3-12.7°C). Actual temperatures will be based on the humidity level of the sink air and the chocolate formulation.
The second component is one or more methods for pulling heat from the center of the item to the surface to prevent entrapment of heat in the center of the chocolate item. Both convection and conductive heat transfer systems can be used for this purpose. A conductive system would be a cold plate used to transfer heat out of the chocolate in a mold or a cold pan lined with a sheet of parchment paper (the chocolate item is placed on the parchment sheet for cooling). The most frequently used and easiest system is a convention system. The best example of this is cooled air blown over a mold or chocolate item to remove the heat from the area surrounding the item, and thus provide a continuous gradient to transfer heat from a warmer product to the cooler heat sink air. The goal is to have air at the surface of the item constantly being refreshed with heat sink temperature air. This has two purposes: First is the prevention of warm air being entrapped around the items being cooled. Second is the movement of air over the surface of the item to provide a continuous temperature gradient that will allow for the transfer of heat according to the second law of thermodynamics.
The simplest example of radiation cooling is setting the item on a table in an air-conditioned room at a temperature of 68°F (20°C). This system can work well for items with a thin coating of chocolate, such as deposited truffles where heat is easily transferred to the air through radiation due to the large surface of the products. The system can be made more efficient by adding a fan to blow over the items to force more interaction of the heat that needs to be removed with the heat sink air. An even more effective system would be an area isolated to maintain a specific temperature environment by having a separate cooling source for just that area (i.e., a room air conditioner). This helps maintain a constant temperature of the air being blown over the cooling products, even with the addition of latent heat.
For more information on this topic, please visit https://sweetanchors.podia.com/ to check out my new eBook on this topic.
(This article appeared in the Fall 2023 issue of Pastry Arts Magazine)
After 30 years of working in the confectionery industry as a product developer, researcher, scientist, trainer, innovator, and consultant, Dennis decided to start a confectionery coaching website aimed at transferring the science behind confectionery processes into practical skills. With his deep knowledge of cocoa butter crystallization, Dennis helps students to understand what is happening in the physical process and how to control it. He hopes that knowing the science will free the student to explore the art and that more people will be able to enjoy freshly tempered chocolate products. To learn more about Dennis’ new adventure, please visit https://sweetanchors.podia.com/.