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Emulsifiers are made up of molecules that have a non-polar fat loving end which carries no charge and has an affinity for oil and a polar water-loving end which carries a charge and has an affinity for water.
Such a molecule can situate itself at the interface between oil and water and allow the two to mix. The polar end will immerse itself in the aqueous phase and the non-polar end will immerse itself in the lipid phase and prevent the separation of the oil droplets. This helps the two phases to stay intimately mixed and form a stable emulsion.
Emulsions may be characterized in two ways. Oil in Water Emulsion or Water in Oil
WATER IN OIL EMULSION
Water droplets may be dispersed in the oil leading to a water in Oil emulsion.
The affinity of emulsifiers for either oil or water is measured by the HLB scale. If the HLB of an emulsifier system is between 3-6, it has an oil affinity and such an emulsifier will optimally stabilize a water in oil emulsion.
OIL IN WATER EMULSION
Oil droplets may be dispersed in the water leading to an oil in water emulsion.
The affinity of emulsifiers for either oil or water is measured by the HLB scale. If the HLB of an emulsifier system is between 9-18, it has a water affinity and such an emulsifier will optimally stabilize an oil in water emulsion.
Emulsifiers in Cake Batter:
The main functions of emulsifiers in cake baking are:
- Optimizing the distribution of oil in water dispersions and stabilizing the resultant emulsion.
- Optimizing distribution of air and stabilizing the foam
- Optimizing internal characteristics.
- Improving shelf life
- Optimizing use of liquid oils
Long chained, saturated fatty acids have the best starch complexing ability (IV <4). To increase the shelf life of baked goods Glyceryl Mono Stearate (GMS) is one of the best choices for starch complexing and thus shelf life.
Monoglycerides are polymorphic since they can exist in 4 crystalline forms- alpha, alpha prime, beta and beta prime.
Beta Crystal Form
Powdered monoglycerides exist in the beta crystalline form- very stable and moderately functional. The beta crystal is the most rigid and stable at low temperatures. Crystalline emulsifier molecules are oriented with the hydrocarbon chains in dense parallel layers. When heated in the presence of water, the hydrocarbons change from a solid to a liquid state with the water penetrating in between the polar groups of the bi-layers. On cooling the hydrocarbon chains crystallize, forming a sandwich structure of alternating surfactant and aqueous layers giving an alpha
crystalline gel phase.
Alpha Crystal Form (unrelated to the term alpha monoglyceride)
The alpha crystal form is known to be much more functional than the beta crystal. However, this alpha gel is unstable and has a tendency to revert back to the beta crystalline form. The liquid alpha crystalline form is more flexible than the rigid beta crystalline form and rearranges more easily during heating cycles. Thus, alpha crystalline emulsifiers are able to form flexible and more protective films around the dispersed-phase be it oil, air or water and preserve the emulsion.
A traditional method for the optimum utilization of Monoglycerides has been through hydration. Such GMS hydrates helped to provide both aeration and emulsification. However on aging these monoglyceride hydrates had reduced functionality. But since this crystal form is unstable it tends to change on storage to the more stable beta crystalline form which is less effective. The common practice of improving the functionality of powdered emulsifiers through hydration is a very imprecise way of forming the comparatively unstable but more functional alpha crystalline form.
Alpha crystalline emulsifiers In CAKES
There is a class of emulsifiers are known as alpha tending which when hydrated convert to the alpha crystalline form. Some examples of alpha tending emulsifiers include PGMS, Polyglycerol Esters, SSL, and DATEM.
PURPOSE OF EMULSIFIERS IN CAKES
- A cake batter is a complex foam/emulsion. Cake quality depends on the aeration and on the optimum number and size of the bubbles which define texture.
- Alpha crystalline emulsifiers are outstanding in their aeration potential in single and/or two-stage mixing.
- The stability of the bubbles and uniform expansion during baking helps optimize the volume. Maintenance of emulsion stability and batter viscosity during the baking process is improved by the proper alpha stable emulsifier gels.
- The shelf life of a cake is influenced by the staling cycle. The shelf life depends both on starch retrogradation effect, but also on moisture retention and on optimum internal characteristics.
Propylene Glycol Monostearate (PGMS OR PGME)
PGMS is the most common emulsifier used in American cake baking to help with aeration and shelf life.
Propylene Glycol Monostearate (PGMS) is a type of Propylene Glycol monoester (PGME is an entire class of chemicals). This emulsifier is an ester where propylene glycol is combined with a fatty acid. The resultant product consists of both mono and di-esters. The monoester component (e.g. PGMS) is an alpha tending emulsifier particularly functional as an aerating emulsifier in combination with monoglycerides. Purification through molecular distillation is critical in optimizing such a blend.
Polyglycerol Esters (PGE)
Another derivative emulsifier is Polyglycerol Esters, and it is very commonly used by European cake bakers but to date has been sparingly used by the North American cake baker.
Polyglycerol esters are emulsifiers in which the fatty acid is combined with polyglycerol. The HLB of Polyglycerol Esters depends on the degree of polymerization of the glycerol moiety and have wide acceptance and use in the European sweet goods industry.
Polyglycerol Esters are a class of enzymes which have different HLB values resulting from different polyglycerol backbone length as well as amount and type of fatty acid esters. Specific functionality will vary with different polyglycerol esters.
Polyglycerol Ester/monoglyceride combinations
An alpha stable gel made up of alpha tending monoglycerides and Polyglycerol Ester has a superior effect on a cake. Long chain fully saturated monoglycerides have the best starch complexing ability. Polyglycerol Esters is alpha tending and increases batter viscosity, thus contributing to bubble stability.
Polyglycerol esters function as both a water in oil and as an oil in water emulsifier. In combination with monoglycerides, Polyglycerol Esters can stabilize the oil-water cake batter emulsion at various stages of the baking cycle and during shelf life. The proportion of the monoglycerides to Polyglycerol Esters can be modified to suit high or low-fat batters. In low-fat cakes, Polyglycerol Esters can help in direct aeration of the aqueous stage and stabilize the aqueous foam by forming an interfacial film around the air bubbles. In low-fat batters alpha, stable gels high in Polyglycerol Esters can also help by giving a smooth feel and a thick batter viscosity typically seen in high-fat batters.