Colloid Chemistry

The American Brewer Nov. 1939
Colloid Chemistry
by Prof. J. W. McBain –  Stanford University
Presented before the Convention of the Master Brewers Association of America, San Francisco, Oct, 16, 1939

It is my pleasant task and privilege to address this Convention of Master Brewers upon a scientific subject of especial interest in the brewing industry. I shall endeavor to define the field of colloids and to indicate some part of the essential role they play in the properties of beer and to call attention to a few of the modern tools now available for their investigation and control; and include a few experimental demonstrations.

What Are Colloids
But first what are colloids? The answer may surprise some of you. Nearly everything you have ever seen except water, salt and sugar is a colloid or in the colloidal condition. Everybody has heard of molecules and atoms, the units of chemistry. The splitting of the atom into smaller particles has been insistently publicized during the past decade or so and we have even had cartoons in the papers, such as these: The professor first doing it who thought that if successful the result would be signalized by the sudden explosion of our world into a new star. A recent cartoon showed professors after a lifetime of efforts on their hands and knees searching for the little pieces of the atom somewhere on the floor.

However, for the main properties of matter all this is comparatively unimportant and even the molecule of academic chemistry has been inadequate to account for the everyday properties of most materials, the smoothness of ice cream, the skin you love to touch, the body and head of beer, the oiliness of a good lubricating oil, the setting of a jelly, the texture of soil, or the properties of textiles, or the behavior of a virus or of protoplasm. Now the important fact is that in most materials the unit is not the molecule but a larger particle consisting of many molecules—the colloidal particle. These colloidal particles are the bricks of which the varied and beautiful architecture of common materials both animate and inanimate are mainly made up. May I quote a sentence to emphasize the importance of these colloidal particles as primary units, as well as of the secondary structures that are built from them? “If you cannot see the design of a house in the pile of bricks dumped on the site, still less can you see it in the clay (molecules and atoms) from which the bricks (or colloidal particles) were originally made.”

Spontaneous Formation of Emulsions and Stable Colloid Particles
How do we find how big these bricks or colloidal particles are? They are too small to see even in the best microscopes. They are so fine they pass through the medical man’s finest bacteria-tight filters, yet they are larger than the molecules which go in orderly array to make them up. The most important may be spontaneously formed and organize themselves, especially where soaps and the enormous class of related sub- stances are concerned.

For example, I place a liquid colored with dye, letting it rest quietly upon water—spontaneously of its own free will, even against gravity, you will see the upper liquid subdivide itself into the water until a uniform colloidal solution is produced, like this soap solution which contains dye that is insoluble in water and yet has come in to join the colloidal particles of soap.

Similarly the proteins and other colloids of beer spontaneously subdivide and organize into particles so small that the liquid appears clear and brilliant. One such colloid in small amounts may induce others to do likewise, probably in joint association.

Ultrafiltration
Now here is one example of a sieve (Cellophane) which sorts out molecules from colloidal particles. These sieves may be made of any desired degree of porosity from membranes so tight that no molecules at all can pass through, to those which will separate two sizes of alcohol molecules, or salt from water, up to those which will let through anything and whose pores reach up to microscopic dimensions and through which all colloidal particles can freely pass. Some years ago we measured the size of the pores in commercial Cellophane using it to sort all molecules from colloidal particles passing the one and holding back the latter. Sometime later we heard that an. enterprising salesman had read our scientific article and immediately used it to sell some millions of packages because we had shown that the pores were too small to let either bacteria or viruses through. Needless to say, neither I nor my collaborator was offered any commission. All these sieves are called ultra-filters.

Ultracentrifuges of Different Types
Svedberg in Sweden earned the Nobel prize by setting up a machine that cost $41,000, called the ultracentrifuge. It still costs $25,000 to duplicate. It is simply a high-power centrifuge in which the liquid lies quietly, undisturbed by vibration or currents or stirring so that one can measure how fast colloidal particles are being centrifuged out. Just as you may drop a spoonful of sand into a quiet glass of water and calculate from the rate at which the sand settles, the size or sizes of the particles of sand, so in the ultracentrifuge we measure the size or sizes of the particles or even molecules.

Now at Stanford, we have developed this very simple equipment here which is as powerful as the Svedberg machine, and is even more universal and versatile. It takes 80 hours of a mechanic’s time to make one by hand, and of course in mass production it could cost much less. It is just being put on the market by the Research Corporation.  

The principle is very simple, taking advantage of a Belgian invention which I will now demonstrate. The record speed of 1,200,000 r.p.m. It is by far the fastest rotating mechanism known. The record centrifugal force so produced at the periphery of this air-driven top was six million times gravity.

We have invented several embodiments shown here. In two of them, streamlined design avoids convection currents, through quartz or glass windows they may be watched and photographed while spinning. In the others we simply prevent convection by having different forms of mechanical baffles like these piles of discs or these nests of perforated rings which stop all undesired convection or stirring, or even let us take the whole thing apart to analyze quantitatively what has happened during the time of the run and what are the constituents of each set of particles. This latter possibility is an important advance. (For references see Chem. Rev. 1939, 24, 289; Science, 1939, 89, 611)

Protein in Beer
Svedberg two years ago made a beginning in the application of the ultracentrifuge to the materials of brewing (Comte rendus du Lab. Carlsberg, Ser. Chim., vol. 22, 1939, P- 44; Volume jubilaire en l’honneur du Sorensen). He found that the protein constituents of malt, wort and beer are still analogous to those originally present in the barley, in spite of partial removal by boiling and hops and enzymatic splitting into polypeptides and amino acids. In the barley flour many sizes of particles or protein molecules or fragments are present, ranging from a few thousand up to about 100,000 in apparent molecular weight, and differing from edestin which is another and bigger particle. Now with the analytical ultracentrifuges just shown here, it would be possible further to characterize the protein fraction by micro-analysis of the main amino acids and also to deter mine the carbohydrates associated with each or existing indepentently. Furthermore, it is possible to determine the amount of actually bound water.

Froth
At last, and finally, I come to the topic with which I’m sure your technical experts expected me chiefly to deal; namely, the surface properties and head of beer (For reference see Bolam Journ. Inst. of Brewing 1935, 41, 105; new series vol. 32). There is only time to mention a few of the newer methods of approach to such surface problems.

A very great deal has recently been learned of the surface properties of solutions and the new results differ greatly from the conceptions previously taken for granted. Surfaces of ordinary solutions are ever so much more complex and highly organized than had been suspected. Some of the newer information has been obtained by studies of surface tension by different methods, especially taking into account influences of time. Other quite unexpected information has been given by applying the firm balance developed by Miss Pockels, Langmuis and Adam for insoluble films on water, but now used on ordinary solutions without a contaminating film.

Some solutions are known to cover themselves by a surface film of high plasticity and stability but it is an interesting new discovery that even some very simple solutions when their surfaces are suitably set in motion or contracted develop a stable semi-insoluble pellicle (See McBain and Perry, Ind. Eng. Chem. 1939, 31, 35). This is being further studied by means of the PLAWM trough where two compartments in the film balance are completely separated from each other by a flexible partition.

Surface films can be studied by a number of new methods of great interest and power (For review see Rideal, Science 1939, 90, 217). Miss Blodgett and Langmuir have developed techniques for transferring surface films to metal surfaces, laying one upon another until they become accessible to quantitative study. The most striking example of this was obtained by one investigator who superimposed several thousand such films and then measured the total thickness with an ordinary micrometer gauge. These methods are particularly applicable to the protein films that are essentially responsible for the frothing of beer.

Having reminded you that colloids are the determining factors in the properties of beer, we have described or shown a number of the more modern tools which are capable of giving essential information both as regards the beer itself and its foam. It will be noted that just enough has been done with each of these methods to show that it is a practical tool for the investigation of beer, but in no case has any of these new methods been adequately exploited in its potential application to the problems of the brewing industry. We may therefore expect much new knowledge to be forthcoming when these are systematically applied.