A foam is a substance that is formed by trapping many gas bubbles in a liquid or solid. It can be considered a type of colloid. Culinary foams are best associated with the Chef Ferran Adria from El Bulli Restaurant in Spain who began experimenting with foams consisting of natural flavors mixed with a gelling agent such as agar (see below). The ingredients are then placed in an espuma or thermo whip where the foam is forced out with nitrous oxide. Food foams however are not new with soufflés first being seen at restaurants in France around the late 1700's.
Producing a foam involves the generation of a protein film surrounding a gas bubble and the packing of gas bubbles into an overall structure. Destabilization of protein foams occurs due to creaming, drainage (from lamellae and plateau boarders), bubble coalescence and disproportionation
(Examples of Liquid and Solid Foams: Beaten Egg Whites, Milk Foams and Whipped Cream --gas in liquid; and, Marshmallows -- gas in solid)
BEATEN EGG WHITES
An example of a colloid foam (gas in liquid) used in cooking is egg white which is a gas dispersed or spread throughout a liquid.
Egg whites are made up of water , protein, and small amounts of minerals and sugars (see egg protein composition).. When eggs are beaten, air is added and the proteins are denatured exposing their hydrophobic (water hating) and hydrophilic (water loving) ends of the protein. The proteins align themselves between air and water forming bubbles with their hydrophilic chains pointing into the water and dangling their hydrophobic chains in the air. In addition the proteins can bond to one another side-to-side as crosslinks which add to the stability of a foam.
How can you stabilize egg white foams?
1) Copper bowl The copper in a copper bowl assists in creating a tight bond in reactive sulfur in egg white preventing the sulfurs from binding other materials. This makes it take longer to form the foam, but leads to a much more stable foam.
If the foam is overbeaten in a non-copper bowl, eventually the proteins become completely denatured and coagulate into clumps. These clumps cannot be turned back to smooth peaks. If a copper bowl is used, fewer protein molecules are free to denature and coagulate, because of the conalbumin-copper complexes. In addition to forming complexes with conalbumin, the copper may also react with sulfur-containing groups on other proteins, further stabilizing the egg white foam.
2) Cream of Tartar - (potassium bitartrate) is an acidic salt that can be used to change the pH of the egg white to an acidic range by boosting the number of free-floating hydrogen ions in the egg white. This has the effect of stabilizing the foam, and is therefore an alternative to using a copper bowl. 1/8 teaspoon/0.5g cream of tartar should be used per one egg white to create this effect. 1/2 teaspoon/2ml of lemon juice can also be used to create the same results.
3) Sugar -- Sugar is added during foam preparation because it creates smooth, stable foam one that will not collapse and drain quickly.
Why fats kill an egg white foam?
Fat molecules also have both hydrophobic and hydrophilic parts and will compete with proteins in the hydrophobic/hydrophilic environment. The difference though is unlike proteins, fats don't bond to each other side-to-side to form a reinforcing networks instead they will compete with protein molecules in forming bonds. So, the addition of any fat e.g,, egg yolks will interfere with the formation of egg white foam. Note: Once the protein complex is formed though it is safe to expose fat molecules to it.
MILK AND CREAM FOAMS
What is the best type of milk to produce a foam?
As in the case of egg white foams it is the protein molecules that are responsible for milk being able to be foamed. And, as in the case of egg whites addition of fat will minimize the formation of foam. Foam stability decreases with increasing fat reaching a minimum at about 5% and then increases rapidly as fat is increased to 10%. At this point highly stable cream type foams form. Therefore 'skim milk' will produce the greatest volume and most stable foams, unless of course you go very high fat (35%) where whipping cream will also produce a very stable foam.
Why does milk foam?
There are two different types of proteins in milk: whey proteins and caseins with caseins making up 80% of the total protein of milk. Casein imparts good surface-active properties and thus plays a role in the functional properties of whipping/foaming. Whey proteins although offering less surface activity than casein, they offer far superior foam stabilizing properties creating a more rigid film at the air/water interface of the foam.
Both proteins are stable up to approximately 140F after which they become susceptible to denaturation. At this temperature new proteins are needed for stable foam so more milk must be added.
What Effect does Temperature Have on Foaming Ability?
Low fat milk foams best at low temperatures. This also applies to both whole milk and cream, although to a lesser extent. At temperatures about 100F, on up through to 160F the trend is reversed with the higher fat dairy products consistently exhibiting a greater volume of foam being produced at any given point. In general temperature trumps the influence of the fat on foaming.
Why make a culinary Foam?
1- Foams can produce a lighter feel than a to a thick sauce. 2- They can provide both a tactile and textural aspects. 3- They can provide a visual aspect to a dish.
How are Foams made?
You can "make" foam with a stick blender, but for it to hold, you need a stabilizer such as agar, gelatin, or lecithin. Many chefs are currently using the ISI Canisters to produce a foam. This also requires the presence of a foaming agent (see below).
A foaming agents is a surfactant, which when present in small amounts, facilitates the formation of a foam, or enhances its stability by inhibiting the coalescence of bubbles (see Foaming Agents -- Wikipedia)
A foam stabilizer prevents or retards the coalescence of gas bubbles.
Gelatin as a foaming agent
Gelatin is a very efficient foam stabilizer and this property is exploited in the manufacture of marshmallows. Different gelatins have different foam stabilizing properties and gelatin for this use needs to be carefully selected.
Lecithin as a foaming agent
Lecithin is classified as an amphoteric surfactant in that it can react with either an acid or a base. It is ideal for converting juices and watery liquids to airs and foams. To produce a stable foam, start with a ration of .6% of lecithin.
Agar or agar agar as a foaming agent
Chemically, agar is a polymer made up of subunits of the sugar galactose. Agar polysaccharides serve as the primary structural support for the algae's cell walls.Agar is a gelatinous substance derived from seaweed. Historically and in a modern context, it is chiefly used as an ingredient in desserts throughout Japan, but in the past century has found extensive use as a solid substrate to contain culture medium for microbiological work. The gelling agent is an unbranched polysaccharide obtained from the cell membranes of some species of red algae, primarily from the genera Gelidium and Gracilaria, or seaweed (Sphaerococcus euchema). Commercially it is derived primarily from Gelidium amansii.
class II hydrophobin (HFBII)
Interest in Culinary Foams has stimulated interest in researching new substances that would create more stable foams. The class II hydrophobin (HFBII) is an ultra low molecular weight protein that is a highly surface-active protein. It has unique functions as both superior adsorption on a solid surface and ability to spread on hydrophobic surface. It has been shown to be exceptionally stable in food foams compared to present stabilizers (Cox et.al., 2008)
The paper reports the use of a sucrose surfactant which forms a coating around the air bubbles, but Bee added that others are being investigated. "The surfactant has to be able to form a crystalline layer that adsorbs to the surface," he said.
Interfacial Polygonal Nanopatterning of Stable Microbubbles Emilie Dressaire,1 Rodney Bee,2 David C. Bell,1 Alex Lips,2 Howard A. Stone1* Micrometer-sized bubbles are unstable and therefore difficult to make and store for substantial lengths of time. Short-term stabilization is achieved by the addition of amphiphilic molecules, which reduce the driving force for dissolution. When these molecules crystallize on the air/liquid interface, the lifetime of individual bubbles may extend over a few months. We demonstrated low gas-fraction dispersions with mean bubble radii of less than 1 micrometer and stability lasting more than a year. An insoluble, self-assembled surfactant layer covers the surface of the microbubbles, which can result in nanometer-scale hexagonal patterning that we explain with thermodynamic and molecular models. The elastic response of the interface arrests the shrinkage of the bubbles. Our study identifies a route to fabricate highly stable dispersions of microbubbles.
Andrew R. Cox, Deborah L. Aldreda and Andrew B. Russell. Exceptional stability of food foamsnext term using class II hydrophobin HFBII , Food Hydrocolloids Volume 23, Issue 2, March 2009, Pages 366-376
Microbubbles could extend shelf-life on food foams -- Beveragedaily.com -- Interfacial polygonal nanopatterning of stable microbubbles
How to froth milk -- From Coffee geeks site.