Foams are fascinating food structures which appear in all aspects of the culinary arts. They are critical features of beer heads, meringues, coffee crema, angel cake etc.
Foams are very simply a three-dimensional structure created by trapping gas bubbles in either a liquid or a solid. In more scientific terms a foam is a biphasic system. In other words, a coarse dispersion of gas bubbles in a liquid continuous phase. Research scientists regard foams as a discipline within the general study of colloidal science (Vernon-Carter et al., 2001; Thakur et al., 2003).
Examples Of Food Foams
Beer foam is an important visual attribute when assessing this beverage’s quality. Stabilising the foam has been a common goal in studies aimed at keeping beer quality over shelf-life. Foam in this case is created by malt ingredients and the mannoproteins isolated from brewer’s yeast.
In traditional cooking, beaten egg whites are added to a liquid to create a type of whipped foam mousse and this has been a standard technique for centuries. Whipped foams such as whipped cream and meringues are usually dense and wet foams. They are produced using a whipping siphon, espuma, thermo whip or a standing mixer with a whisk attachment.
Culinary foams are also key elements of high-end cuisine. Many chefs have created foams for serving at the table. One of the first types of foam to be created was the souffle in the late 1700s. Nowadays, top chefs create signature dishes where a foam or mousse is a characteristic of their culinary expertise.
Creating foams is an important aspect of foam mat drying and offers anybody interested in the science a chance to examine other aspects of the technology.
The Structure Of Foams
As we have already stated,food foams can be considered as biphasic systems in which a gas bubble phase is dispersed in a continuous liquid phase. The food foams of greatest interest are called solid foams and they occur in two main types: open-cell or closed-cell foams.
A closed-cell foam is one where distinct bubbles of gas trapped individually. The blowing agent remains intact in the bubble over the life-time of the foam. It has good dimensional stability with a lower moisture level than its open-cell counterpart. As a structure, 90% of the foam cells are actually closed as opposed to being open. They also have a lower air or gas content.
An open-cell foam is one where there are windows between adjacent bubbles which create serpentine passages through the foam. Being open cell, the agent which blew the foam equilibrates with the air and very usually just disperses into the atmosphere. There is a rapid, almost immediate transfer of vapour and air through the ‘open’ or ‘porous’ interconnected regions.
The air content is higher than in a closed-cell foam because 50% of the cells are open.
Foam structure takes two forms:-
– a bubbly foam as in an ice-cream where it is formed when the amount of gas incorporated is low enough for the bubbles to retain a roughly spherical shape.
– a polyhedral foam. When the volume fraction of dispersed gas is high, bubbles become distorted in the form of polyhedra separated by thin liquid films called lamellae. Three adjacent films intersect in a channel called the Plateau border, and the continuous phase is interconnected through a network of Plateau borders (Wang & Narsimhan, 2004).
Foams As Thermodynamic Systems
Foams also belong to the class of thermodynamically unstable systems, so foaming agents which we reference later are added into these systems. They act as surface-active components that are required to form and stabilize foams (Brush & Roper, 2008).
Foams behave as elastic solids at small strains and flow like viscous liquids at large strains. They also show yield stress. Yield stress is the minimum stress required to initiate flow (the transition stress from solid-like to liquid-like behavior), and is related to the strength of the network structures of the material.
Foam Formation And Stability
There are two main quality parameters to foaming. One is foamability which is a measure of the extent of foam formation and the other is foam stability itself which is described as the rate of loss of foam structure once formed. The foam stabilizing effect is attributed to increased viscosity imparted by an agent such as galactomannans for example, lowering the drainage of the liquid in the foam lamella.
The foaming capacity of a protein is measured as the amount of interfacial area created by whipping the protein. Foam stability is measured as the time taken to lose either 50% of the liquid or 50% of the foam’s volume.
Various technical factors affect both characteristics. Water hardness, temperature of foaming as well type of protein affect these characteristics.
The Physical Process Of Foam Collapse
The main physical processes for foam breakdown are drainage under gravity from films, the coalescence of bubbles and a phenomenon called disproportionation which is also known as Ostwald Ripening. Look at articles by Bikerman (1973), Yung et al., (1989).
Foam drainage occurs as water drins from the foam. It drains along the lamellae to the curved junction of the thin lamellae called plateau borders where the interfacial tension is lower.
Coalescence is the fusion of foam bubbles.
Disproportionation is situation in a foam where there is diffusion of gas from small bubbles into big bubbles It is the most important form of instability in a foam. If a stabilizing film of polymer molecules is missing, disproportionation occurs extremely quickly.
Egg Foams And Meringues
Egg foams made from egg white are used in the preparation of angel cakes, sponge cakes, souffles etc. The classic example of a food foam is that of the noble meringue. These are essentially produced from a batter which is made by whipping egg white whilst gradually adding sugar. This batter can be shaped in a variety of forms and is then baked. The meringue batter before cooking may also serve as a culinary ‘scaffold’ in for example the production of angel food cakes.
Several studies have dealt with meringues or similar systems. The proteins such as ovalbumin (conalbumin) in egg white not only lower the surface tension of the meringue foam but some are denatured at the surface. This denaturation means there is coagulation of the proteins at the gas-liquid interface. They form a network that lends rigidity to the foam.
At the molecular level, the egg white proteins diffuse to the lamellae where they are adsorbed. Generally, these are glycoproteins because they contain carbohydrate moieties on their structure. At the interface, the proteins unfold because the carbohydrates portions are hydrophilic and orientate themselves towards the aqueous region. The protein unfolds and their hydrophobic regions are exposed to the oil or fat phase. The carbohydrate portion also hydrogen bonds with water forming a loose shell and this too increases the viscosity of the water phase in the lamella. It also means less drainage.
As well as ovalbumin, a number of globulins are present which appear to be very good foaming molecules. They produce small gas bubbles with a large volume ratio. Ovomucin is not as good a foaming agent but stabilises the foam because it is easily denatured or insolubilised at the bubble interface.
Duck eggs do not always contain globulin and will not form a foam.
The Effect Of Heating
Heating globular proteins helps to partially denature them which improves their foaming properties. The reason is that heating encourages further unfolding of the tertiary structure and exposes more hydrophobic sites for binding especially when they are at the interface. It also appears to improve their adsorption to air-water interfaces and lowers their interfacial tension which increases their foaming capacity. The extra protein at the interface increases film thickness enhancing foam properties further (Kinsella, 1979).
Various seed proteins have potential as foaming agents. Those proteins which are highly coiled and compact are especially effective.
Overdoing the level of heat denaturation of a protein decreases its ability to form a foam.
Alternatives To Egg Whites
There is a growing interest if not demand from many consumers for plant protein substitutes for the equivalent animal protein in a food system. Typically, hen egg white has been the standard foaming ingredient because it has excellent foaming properties but there are some new ones available.
Enzymatically hydrolyzed wheat gluten as well as wheat gluten itself all work to some extent. The modified wheat gluten has better solubility and foaming capability than wheat gluten itself. Aquafaba is the new vegan alternative.
Pea protein has reasonable foaming properties. When it is treated with high-pressure supercritical CO2 treatment (HPT-scCO2) then its foaming properties are better (Saldana do Carmo et al., 2016).
Fats Destroy Foams
Fats and lipids are well known for wrecking (destabilizing) a foam. Brewers, pubkeepers and wine drinkers are all to aware of the issues of how fat will collapse a foam. Lipids in lipstick for example left on beer or wine glasses will suddenly cause any foam to disappear much to the consternation of the brewer.
The fat or lipid molecule has both a hydrophobic and hydrophilic part which competes with proteins that have similar parts. Proteins usually aggregate together to form a reinforcing network. Unfortunately, lipids will migrate to the air-water interface before the protein molecules can. This inhibits the unfolding of the protein which prevents the foam forming.
Egg yolks will often disrupt an egg white foam for example. This is partly due to the presence of lecithin which binds to the egg white protein to form micelles. It prevents the proteins associating at the air-water interface and competitively displaces them from the interface.
However once the protein complex is created, it is safe to add fat because the competition for loose bonding spots have been taken.
Increasing the amount of protein increases the level of foamability and its stability. This is because of a viscosity effect which produces a thicker lamella film.
The Addition Of Salt
If you add a salt to egg white it lowers both the volume and stability of the foam because it becomes less elastic. To overcome this requires increasing the mixing or beating time.
If you add salt to whole egg before mixing it produces a much smaller foam.
Foams are stabilized using lecithins or Versawhip in the liquid to be foamed. Lecithins from either soy or egg are ingredients used as emulsifiers. Versawhip is a soy protein that replaces egg whites or gelatin to stabilize whipped foams.
Use of Copper Bowls
Preparing a foam in a copper bowl helps in creating bonds between the reactive sulphur in particular amino-acids in egg white. Egg white is primarily a protein of ovalbumin which is then promoted to interact with itself rather than other compounds. Using a copper bowl may mean the formation of the foam takes longer but a more stable foam is created.
If other metal bowls are used, the proteins denature and coagulate into lumps. Once formed that cannot be returned into foams.
Cream Of Tartar
Potassium bitartrate is a traditional ingredient for helping to stabilize egg white especially with heating. It is a weak base of an acid which dissolved in water converts to tartaric acid. The acid of whatever type is best added during the first portion of the beating period.
The acid alters the pH of the egg white by raising the number of protons (hydrogen ions). This helps stabilize the foam because the drop in pH to say 6.5 makes the foam proteins less susceptible to over-coagulation.
The effect is similar to adding lemon juice or vinegar to help precipitate egg proteins when producing a poached egg. Citric acid as a single pure ingredient has also been tried with mixed results.
Nonfat Dry Milk
Nonfat dry milk (NFDM) is a common protein material that enhances viscosity, emulsion stability and foams. Dried milk generally is added to yogurt, processed cheese, bread machine mixes and ice cream to help stabilise the food structure. NFDM is mainly composed of phospholipids.
Sugars like sucrose (and even polysaccharides) enhance foam stability by increasing the viscosity of lamellar fluid which reduces the drainage rate.
Lau and Dickinson reported in (2005) that decreasing the invert sugar content from 82% to less than 60% solids in a system containing 2% to 6% egg white albumin led to an increase in foam overrun, but also to an increase in the rate of destabilization.
Typical polysaccharides extracted from roasted coffee beans generate stable foams. The key polysaccharides include galactomannans and arabinogalactans.
Foam inducers and stabilizers also include modified soybean protein (D-100) and glycerol monostearate (GMS) (Bates, 1964) for stabilizing fruit pulp foams.
Yeast And Barley Foams
In beer, barley-derived proteins, such as LTP1, protein Z, and hordein-derived polypeptides, are even more important in this respect than mannoproteins when creating a foam head.
An exceptionally good foaming agent. A warm gelatin sol can be whipped to three times its original volume. When cooled the gelatin solidifies or forms a gel which traps the air bubbles and stabilises the foam. Marshmallows are gelatin foams.
Agar fluid gels also stabilise foams and have been used as gelled particles to mimic fat droplets as well. Agar fluid gels function by localised jamming of the interstitial fluid in foam channels which considerably slows down drainage (Ellis et al., 2017). Gelled particles potentially stabilise foams by not only adsorbing at the air-water interface, but also by increasing local viscosity in the foam channels (Plateau borders and nodes) and preventing liquid drainage (Lazidis et al., 2016).
Solid particles of the appropriate size and wetting characteristics can also be extremely effective stabilizers of foams and emulsions. The underlying mechanism of stabilization is somewhat different in this case however.
Researchers at the Foods Colloids Group in the Procter Department of Food Science, University of Leeds have comprehensively investigated the phenomenon of foam stabilization. They have for example examined the combination of active polymers and surface active particles especially mixtures of proteins with novel food-compatible surface active particles (Murray et al., 2011).
Lupin Protein Isolates
Have good foaming capabilities. Some research has been conducted on lupin protein isolates but the fraction has yet to be produced on a large scale and there are issues with allergenicity.
The gene CFG1 – the Carlsbergensis foaming gene has been isolated from Saccharomyces pastorianus. This gene encodes the cell wall protein Cfg1p and which is homologous to the cell wall proteins of Saccharomyces cerevisiae. These particular proteins are vital for foam quality. The gene CFG1 has been cloned and may well be used in further biotechnology studies to understand the componentry which influences foam formation (Blasco et al., 2012).
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