The consequence of the polymorphic nature of cocoa butter is that, when used in chocolate, the chocolate needs to be tempered in order to crystallise the cocoa butter in one of the stable forms – bV or bVI. It is not possible for the cocoa butter to crystallise directly into the bVI form which is its most stable form so a well-tempered chocolate will contain cocoa butter in the bV form. Over time the bV form will gradually transform into the bVI form. This change is often accompanied by the formation of fat bloom.It also occurs in the presence of liquid oils There are three ways in which the liquid oil content of chocolate is increased: by storing it at a higher temperature; by deliberate addition of liquid oils to the chocolate; as a result of migration of liquid oils from a centre filling into the chocolate.
Bars of tempered cocoa butter containing different levels of hazelnut oil (up to 20percent) were stored at 20°C and 25°C for 10 weeks. The relative levels of bV and bVI crystals were determined by X-ray diffraction. This allowed us to calculate the amount of bVI as a per centage of the total solid phase as a function of storage time and temperature (Fig.1). At both temperatures the rates of transformation increased as the amount of hazelnut oil increased. After five weeks at 25°C pure cocoa butter contained just over 28percent bVI, while cocoa butter containing 20percent hazelnut oil was almost fully transformed. As little as 1percent added oil showed a significant increase in the rate of change.
The transformation rates were also higher at higher storage temperatures. Although liquid oil content was often higher at elevated temperatures, this was not the only reason for this effect. A blend of 80percent cocoa butter and 20percent hazelnut oil contains 61percent solid fat at 20°C; a blend of 90percent cocoa butter and 10percent hazelnut oil at 25°C contains 62.8percent solid fat. The rate of polymorphic change is higher with the 25°C sample than the 20°C sample – even though the liquid oil in the two samples is about the same.
Filled chocolates generally have a filling which is softer and therefore richer in liquid oil than the coating. On storage, a proportion of this liquid oil can migrate into the coating. The rate and extent of this migration is dependent on the storage temperature. Increasing levels of liquid oil in chocolate does accelerate polymorphic change. But does liquid oil that has migrated into the chocolate also accelerate change?
We stored hazelnut oil-rich fillings in contact with a 2.5mm deep layer of chocolate at 20°C, 25°C and 28°C for 10 weeks. We were able to sample the chocolate layer at five depths, each of 0.5mm to determine (a) how the filling fat migrated through the coating and (b) the effect that this had on polymorphic change.
Fig.2 shows that after 10 weeks the amount of filling fat in the chocolate layer next to the filling is about 35percent, irrespective of storage temperature. In the system stored at 28°C, this level of filling fat is found throughout the whole chocolate layer. At lower storage temperatures, the level of filling fat in the chocolate layer decreases in parts of the chocolate farther away from the filling until, at the surface of the chocolate (2.5mm from the filling) there is less than 10percent filling fat at 20°C and 25°C.
Storage temperature also has an effect on the rate of polymorphic change (Fig.3). In comparison to the systems stored at 20°C in which only 50percent of the fat was in the bVI form after 10 weeks, systems stored at 25°C and 28°C had almost completely transformed. Indeed, the system stored at 28°C was 80percent transformed from bV to bVI after only two weeks.
There is thus a clear link between the amount of liquid oil migrating into cocoa butter from a nut oil-rich filling and the rate of transformation of cocoa butter from bV to bVI.
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Geoff Talbot, The Fat Consultant, Bedford, UK; Kevin Smith, Unilever Research, Sharnbrook, UK?; Imro 't Zand is with Loders Croklaan, Wormerveer, The Netherlands. www.croklaan.com