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What determines the melting point of chocolate?

The type of chocolate and its ingredients will have an effect on the heat resistance and melting of the finished product. Melting is important for the mouthfeel and taste of the chocolate. In chocolate the fat element forms the continuous phase in which all other ingredients are embedded. Therefore, the melting characteristics of the fat used are of importance to the stability of the chocolate in a tropical climate. The supplier of the chocolate should be able to provide information on its melting point and solid fat contents over a range of temperatures.

Melting point

This should be at or just above 36 °C to ensure that the product melts in the mouth. If the melting point is too high it will result in a waxy mouthfeel and not melt entirely. If the melting point is too low the product may not be stable during storage in the summer or in hot climates. Fats melt over a temperature range and so at any temperature below the melting point the fat is partly in solid form and partly liquid.

Crystallisation

Cocoa butter is polymorphic and has more than 6 crystal forms, of which only form V or beta form has the necessary heat resistance and melting properties. To ensure that only the stable beta crystals are formed during confectionery production the chocolate mass needs to be tempered prior to solidification. Correct tempering ensures a stable shelf life.

Milk fat

The addition of milk fat to make milk chocolate can soften the product and make it less heat resistant. Milk fat has a different crystalline form compared to cocoa butter, and the resulting incompatibility can lead to a destabilisation of the product. The fat element of the cocoa butter can be replaced wholly or in part with vegetable fats in order to improve heat resistance.

Cocoa butter equivalents

The replacement of part of the cocoa butter content of chocolate with cocoa butter equivalents (CBEs) improves the heat stability of the chocolate. In countries with a warmer climate the addition of CBEs may significantly improve the shelf life of a chocolate product. CBEs are vegetable fats derived from palm and shea oils, which are chemically and physically very close to cocoa butter. CBEs can be made with heat resistant properties superior to cocoa butter.

Cocoa butter replacers

The replacement of most of the cocoa butter with cocoa butter replacers (CBRs) can improve heat resistance. CBRs are derived from oils such as soybean, cottonseed or coconut oil. They have a very dissimilar composition to cocoa butter. The melting point of CBRs tends to be higher than that of cocoa butter thus providing greater heat resistance. Products containing CBRs cannot always be called chocolate.

The tempering and cooling process is also important in improving the stability of cocoa butter and CBEs and hence improving heat resistance.

Tempering and cooling

Tempering is required to ensure that the chocolate has the necessary shelf life. Tempering is conducted in a continuous process whereby the chocolate is cooled and the cocoa butter forms stable and unstable seed crystals. To keep only the desired beta crystals the chocolate is warmed to a temperature that is intermediate between the melting points of the two forms, 31-32 °C for milk chocolate and 32-33 °C for semi-sweet chocolate. Milk chocolate is tempered at lower temperatures because the milk fat suppresses the formation of seed crystals. The temperatures will vary depending on the ingredients and the quality of the raw materials. At this point most of the unstable crystals are melted out, but some seed crystals will remain. Tempering time is also important, as seed crystals need time to grow in size and mature. The chocolate should have some 'residence' time before use in the enrobing or moulding plant. The length of time will vary depending on the intended use of the chocolate.

Tempered chocolate needs to be cooled under gentle conditions to promote the preferential growth of stable crystals. The best temperature for this process is 13-15 °C. Moderate wind speed is recommended in the cooling tunnel to remove the heat of crystallisation. The temperature should be raised gradually to room temperature towards the end of the tunnel.

References:

J. Kristott Confectionery fats - physics matters. Confection, p27-31, July 1998
Leissner, R. Reducing Costs with Cocoa Butter Equivalents. Chocolate & Confectionery International, 2 (6): 24-25, November 1998
Weyland, M. Shelf life of chocolate and compound coatings. The Manufacturing Confectioner, 78 (9): 121-140, September 1998
Duurland, F. Attributes of speciality fats. Chocolate & Confectionery International, 1 (3): 22-25, July 1997