Colloids, Gels and Suspensions
Emulsions are just one example of a colloid, a mixture where one substance is evenly dispersed in another. The particle size of the substance that is dispersed is too large for it to dissolve, and too small for it to settle out easily. Generally this means they are between 5 and 200 nanometers in diameter.
Fog and clouds are colloids of water in air. Smoke is a colloid of solid particles in air. As we have seen, foams are colloids of a gas in water, and emulsions are colloids of one liquid in another. Ink is a colloid of a solid pigment in water. Styrofoam is a colloid of a gas in a solid. Gels are a colloid of a liquid in a solid. There are also colloids of solids in other solids, as in some types of glass.
One type of colloid often encountered in cooking are hydrocolloids. These are gels (solid) or sols (liquid) made of particles dispersed in water. Gelatin is a sol when hot, and a gel when cooled. Other examples are jellies made from pectin, agar, carrageenan, or other gelling agents.
If the particles are too large, instead of a stable colloid, we get a suspension. A suspension is when particles are temporarily suspended in a fluid, but will eventually settle out if left undisturbed. The particle size is generally larger than a micrometer. Particles between 0.2 micrometers and 1 micrometer can form colloids or suspensions, or something that behaves somewhat like either one.
In cooking, sometimes we go to some effort to create colloids, and at other times we go to some trouble to prevent them. We add pectin to fruit juice to make jelly, but we add flocculating agents to wine to clarify it by removing fine particles. Particles in water often form colloids when they all carry the same electric charge, and thus repel one another. Adding salts to a colloid can provide charged ions that surround charged solid particles and neutralize the repulsion. This allows them to bind together into particles too large to remain as colloids, and they settle out.
A gel is formed when the particles cross-link, using chemical bonds, into a three dimensional network that behaves like a solid, even though it is mostly liquid. Cooked egg white is a gel formed when the proteins in the egg white open up from their natural folded shape, and then bind to one another in many places.
Proteins are large molecules that, when opened up and spread out, provide lots of opportunities to bind and form links to one another. Other large molecules that form gels include starches and charged polymers.
Starch is a polysaccharide, a word that simply means many sugars. It is formed when molecules of the simple sugar glucose are joined together to form long chains and branched trees. The long chain form is called amylose, and the branched form is called amylopectin. In most plants, starch grains are about one quarter amylose, and three quarters amylopectin. Animals form a more branched type of amylopectin called glycogen to store energy.
In the organism creating the starch, there are enzymes that create straight chains (amylose), and enzymes that create branched trees (amylopectin), and enzymes that convert branched tree shapes into long chains. All of these forms are used for different purposes in the cell, some structural, some related to energy storage.
In the plant, the starch is bundled into crystallized packets called starch granules. The long chains align and pack tightly, creating a dense solid.
When heated in water, these starch granules absorb water and expand, eventually separating into a loose liquid colloid. If left to cool, the chains in the starch once more align, forming a more solid gel, at the same time excluding water in a process known as syneresis.
Amylose molecules are smaller than amylopectin molecules. An amylose molecule might be composed of 250 to 2,000 glucose molecules, and have a molecular weight of 40,000 to 340,000. Amylopectin is composed of many branching chains of amylose, and can have a molecular weight of as much as 80,000,000.
Amylose often forms double helices that form long hollow tubes. In one common test for starch, iodine is added to a starch solution. The iodine fits neatly inside the helical tube, changing the way it interacts with light. The result is a blue-black color in a solution that used to be transparent.
Agar is a gelling agent that is similar to starch. Instead of using glucose molecules as a base, the simple sugar galactose is used. As with starch, there is a long chain version, called agarose, and a branched version, called agaropectin. It is the agarose that is the primary gelling agent, and purified agarose is widely used in molecular biology because of its excellent gelling characteristics, and it large pore sizes, which allow easier transport of molecules through the gel.
Agar has advantage over starch in both cooking and science. It melts at a high temperature (185° F, 85° C), but solidifies at a much lower temperature (90° F, 32° C). Water molecules bind inside the helices of agarose and stabilize the gel, creating a strong gel without losing any water. Bacteria can't eat agarose like they can starch, so it is used to culture colonies of bacteria on agar plates (which can be incubated at higher temperatures than starch could, due to the higher melting point).
In cooking, agar is used as a vegetarian gelling agent instead of gelatin.
Agar is extracted from either a red algae or from a type of seaweed. Similarly, another gelling agent, carrageenan, is extracted from certain red seaweeds. Like agar, carrageenan is formed of galactose and curls into helices, making it gel nicely at room temperature. It is viscoelastic like toothpaste, meaning it can liquefy under shear stress and be pumped or extruded easily, and then regains its solid form when the stress is removed.
Pure agarose makes excellent gels. It is used in molecular biology to separate strands of DNA by their sizes. The large shaggy agaropectin molecules would get in the way, but the pure agarose allows the DNA to flow through the gel under the influence of an electric field.
Pectins are one of the soluble dietary fibers, and as such they have well known effects on human digestion, as well as lesser known effects, such as reducing the digestibility of some proteins and amino acids. The remedy Kaopectate used to be made of pectins and a fine clay called kaolin.
Where the big shaggy pectin molecules are used most is in making jams and jellies. Pectins used for making jelly come from citrus fruits and apples, but most plant cell walls contain pectins as structural elements and for lubrication. Pectins hold cell walls together, and when fruit ripens, enzymes are released that break down pectin, making the fruit get soft. The pectin in jams and jellies is for the most part indigestible, although some gut bacteria can metabolize it.
Generally, harder fruits like apples, oranges (the peel), and plums have the most pectin, while grapes, cherries, and strawberries have much less, since pectin is a structural molecule that binds the cells together. Citrus peels are about 30% pectin, and whole oranges are about 2%. While whole apples have even less than that (1%), commercial pectin is extracted from the solids left over after pressing the juice from the apples, and is thus concentrated.
Commercial pectin comes in two forms, called high methoxyl pectin (HMP), and low methoxyl pectin (LMP), and they set into gels in different ways. Traditional pectin is the high methoxyl type, and it gels only when the sugar content is very high (generally over half sugar by weight) and the acidity is high.
HMP molecules are negatively charged, and repel one another. This means they won't develop bonds between the molecules and form a solid gel. Two things are needed to overcome this. One is acid, which neutralizes the negative charges. The other is a high concentration of sugar, which binds well to water molecules, so the water binds less to the pectin. This allows the pectins more opportunities to bind together.
LMP pectins were developed to allow gelling in the absence of sugar. They are usually made by treating HMP pectins with ammonia, sodium hydroxide, or acids. Instead of requiring sugar and acid, low methoxyl pectins require a source of calcium to gel. Generally a slow dissolving calcium salt is used, such as calcium phosphate, to give the pectins time to fully dissolve before they react with the calcium.
Water soluble proteins like the albumins in egg white are generally spherical globs, with the hydrophobic amino acids tucked away inside. They can form gels and colloids in this state (as the ovomucin does in the firm gelled part of the egg white). But when they are denatured by heat or beating or acids, they unfold, and can join together to form firmer gels, or tough plastic sheets.
Gels made from proteins are very common in the kitchen. From scrambled eggs to yogurt, custard to gelatin, proteins form gels when heated or acted upon by acids or enzymes.
When proteins are heated, they open up from their natural folded state and take up more room, impeding the flow of the water they are in, causing it to thicken. With further heating, the proteins bond together, forming a firm solid gel.
The nature of the protein determines features of the gel. Egg protein has many sulfur containing amino acids, and the sulfur atoms find it easy to link together to form disulfide bonds. This cross linking firms up the gel, and is the reason why egg protein is so easy to denature into a gel that they can be cooked on a hot sidewalk in the summer.
Milk proteins are easily denatured by acids. As the bacterium Lactobacillus acidophilus oxidizes the milk sugar lactose into lactic acid, that acid unfolds the protein called casein in the milk. The proteins can then over time link together to form the soft gel we call yogurt.
Gelatin is a protein extracted from the collagen tissue that connects bones to each other and to muscle, and which makes up most of the protein in skin and bones.
Collagen is insoluble, made up of three chains of protein that twist into a rope in what is called a triple helix. When heated in water, usually with added acid or alkali, it hydrolyzes, and the helical rope unwinds, letting the chains float free individually.
When the solution cools, the chains begin to curl back up into helices. However, they are all tangled together, and as they curl, the tangling increases, like a coiled telephone cord. The chains knot together, and some of the ends of the chains form double and triple helices together.
All of this tangling forms a kind of gel that is different from what we have seen in other protein gels. Gelatin forms a thermo-reversible gel. Since the chains don't form a lot of strong cross linked bonds, the gel can be undone by reheating. This makes it similar to gels made from vegetable gums and starches.
Gelatin melts at body temperature, and this gives it special qualities in foods that melt in your mouth.