Stabilising the Foam

Stabilising the foam is the more difficult part of the process. Some systems such as egg albumen will foam easily and set on cooking, which are desirable characteristics in the food industry. 

The important issue is to produce a layer between the bubbles that is sufficiently stable that the bubbles do not burst either before the food is cooked or served. In bakery products the foam has to be stable until the surrounding proteins have been denatured and the starch gelatinised. 

Some factors such as the viscosity of the medium can make the foam harder to aerate but when aerated they work to resist the collapse. Other factors normally only work in one direction. 

A stable foam is likely to have ingredients that are in a low energy state at the air-liquid interface. Substances that fit this description include proteins, emulsifiers some fats and fat components such as diglycerides monoglycerides and fatty acids. Food law uses the term emulsifier and stabiliser to cover the situation where the ingredient is stabilising an emulsion rather than helping to form it. 

The layers between bubbles can be as thin as one molecule thick. Some systems, usually those involving eggs, are fairly robust. Others can easily be caused to collapse. Some years ago purified lactalbumin was offered as a substitute for egg albumin. The purified lactalbumin would foam up when whisked but the foam would collapse if more than a trace of fat was present. While there might be food systems where this would be acceptable it is not generally acceptable as a general replacement for egg albumen. 

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Surface active agents are important components of foam formulations. They decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic resin. In addition they can aid nucleation, stabilise the foam and control cell structure. A wide range of such agents, both ionic and non-ionic, has been used at various times but the success of the one-shot process has been due in no small measure to the development of the water-soluble polyether siloxanes. These are either block or graft copolymers of a polydimethylsiloxane with a polyalkylene oxide (the latter usually an ethylene oxide-propylene oxide copolymer). Since these materials are susceptible to hydrolysis they should be used within a few days of mixing with water. 

Fig. 11.4 shows separately curve 1 from Fig. 11.3 which is the dependence of W on the DPPC concentrations in the AF. The W(C) curves allow to determine the threshold concentration C i.e. the minimum phospholipid concentration at which there is a 100% probability of observation of black films (see Eq. (3.130)). At concentrations lower than C, NBFs are no more observed, since W sharply decreases to zero (films rupture). At concentrations higher than C, (W = 1), NBFs always form. Special studies with phospholipid analysis of amniotic fluid indicate that of all phospholipids in the AF, it is the DPPC that stabilises the foam bilayers. This analysis gives grounds to conclude that the concentration of each phospholipid (except DPPC) in the native AF is of an order lower than the corresponding... 

The latter, however, do not have the capacity, once adsorbed, to stabilise the foam, it is well established that pure liquids do not foam. Transient foams are obtained with solutes such as short-chain aliphatic alcohols or acids which lower the surface tension moderately really persistent foams arise only with solutes that lower the surface tension strongly in dilute solution - the highly surface-active materials such as detergents and proteins. The physical chemistry of the surface layers of the solutions is what determines the stability of the system. 

Expandable graphite acts mainly in the condensed phase as a smoke suppressant and an insulator. Where the expanded carbon layers are too unstable, it is possible to combine the graphite product with other FRs, such as those mentioned above, to form intumescent layers to stabilise the foam. 

TTiis is particularly the case with nonionic surfactants that produce a lamellar liquid crystalline structure in the film between the bubbles [24, 25]. These liquid crystals reduce film drainage as a result of the increase in viscosity of the film. In addition, the liquid crystals act as a reservoir of surfactant of optimal composition to stabilise the foam. 

Iso-a-acids are moderately strong acids with values around 3 (beers of Pilsner type have pH around 5.0-5.2), more soluble than the corresponding humulones (piC values are around 5.5) and have an intensely bitter taste. According to the older classification, iso-a-acids are a-resupones, the most important bitter constituents of beer, stabilising the foam and exhibiting bacteriostatic effects. 

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