We are using walnuts in our gateau buttercream filling and find that it turns black It does not appear to be mould What is the cause of this discoloration

You are quite right that the problem is not one of mould growth. The most likely cause is a reaction of the tannin in the walnut pieces with any traces of iron that may be present in the cream, perhaps picked up from the mixing utensils that you have used. The colour may take several days to appear. Sometimes the colour may be purple rather than black, depending on the product pH.

We suggest that you use stainless steel utensils and particularly avoid using any iron utensils that are scratched or damaged. You should also try to ensure that any cut cake surfaces do not come into contact with iron surfaces.

5.15 What is the role of emulsifier in the production of sponge cake products?

The key role for the emulsifier added to sponge cake batters is to assist in the incorporation and stabilising of the air bubbles mixed into the cake batter. The most common emulsifier used in sponge cakemaking is glycerol monostearate (GMS), though polyglycerol esters are also used. In both cases the emulsifier can be considered as a molecule which has a hydrophobic (water-hating) head and a hydrophilic (water-loving) tail. Thus when the emulsifier is used in a cake batter the hydrophobic head aligns itself at the interface of the liquid and air phases while the hydrophilic tail is located in the liquid phase. These actions confer stability to the air bubbles incorporated during mixing.

Sponge cakes may be made without any emulsifier and in this case the egg proteins play the bubble stabilising role. However, if any fat or oil is present in the formulation then the egg proteins cannot maintain gas bubble stability during baking and the cake may exhibit collapse and core formation (see 10.33). To overcome this problem when using fat or oil it is necessary to add an emulsifier. In this case the emulsifier takes over the gas bubble stabilising role of the egg proteins. It is important to ensure that sufficient emulsifier is added to maintain gas bubble stability in both the cold batter and during baking. Cauvain and Cyster (1996) showed how core formation was made worse when a low level of GMS was added to the batter but was eliminated as the level progressively increased (see Fig. 14).

Gas bubble size and therefore sponge cake cell structure is directly affected by the level of emulsifier. Cauvain and Cyster (1996) studied the effects of GMS

Fig. 14 Effect of level of emulsifier on bubble size in sponge cake batter.

Fig. 14 Effect of level of emulsifier on bubble size in sponge cake batter.

and found that the minimum gas bubble size, and therefore finest cell structure, was obtained at about 0.6% batter weight with their formulation. Increasing the level of added GMS had no effect on gas bubble size but at 1.5% batter weight the cake was unacceptable, having a loose crust, many surface blisters and a very close cell structure. This can be interpreted as over-stabilisation of the batter with a layer of GMS so thick around the gas bubbles that they were unable to rupture (i.e. convert from foam to sponge) at the correct moment during baking.

The volume of gas that can be incorporated into the batter is also related to the level of emulsifier used. The higher the level of emulsifier, the greater the volume of air that can be incorporated and so the lower the batter relative density can be achieved. This in turn can lead to greater product volume provided that sufficient emulsifier is present to ensure bubble stability during baking.

In order to ensure that the emulsifier is effective as a bubble stabiliser it is important to use it in its most appropriate form. In the case of GMS there are a number of different forms in which it can exist, depending on its concentration in water and the temperature of the preparation. Krog and Larsen (1968) studied the phase diagram for GMS and water and showed that the most effective form for cakemaking occurred over a limited range of concentrations and temperatures. The most appropriate form is often called the 'alpha gel' form. Commonly cake emulsifiers are used in a ready-prepared gel form and may contain a 'co-emulsifier' to prevent GMS reversion when the mixture is cooled.

References

CAUVAIN, S.P. and CYSTER, J.A. (1996) Sponge cake technology. CCFRA Review

No. 2. CCFRA, Chipping Campden, UK. KROG, N. and LARSEN, K. (1968) Phase behaviour and rheological properties of aqueous systems of industrial monoglycerides. Chemistry and Physics of Lipids, 2, 129-135.

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