The American Brewer June 1935
Colloidal Material Affects Head Formation Of Beers
By T. R. BOLAM, D. Sc.
EDITOR‘S NOTE——In view of the increasing significance of physical and colloidal chemistry in the study of brewing processes, we take the liberty of reprinting. in somewhat condensed form, this interesting article on Colloids by Dr. Bolam, from the March issue of the Journal of the Institute of Brewing, 1935, p. 102-107.
COLLOIDAL science embraces such a diversity of phenomena that is worthwhile, in the first place, to recall the origin of the term “colloid.” For this we must go back to the middle of last century, when Thomas Graham carried out many experiments on the rate at which dissolved substances diffuse through a membrane of parchment paper. It was found that whereas certain materials, for example, sugar and salt, easily pass through the membrane, many substances, including gelatin, albumin, starch, dextrin and tannin, show relatively little tendency to do so. Graham thought that there might be some connection between the diffusibility of a substance and the nature of its solid state, since sugar and salt readily crystallize, while gelatin and the like normally are non-crystalline. He, therefore, conceived the idea of two fundamentally different kinds of matter, the one crystalline and diffusible, the other non-crystalline and non-diffusible. To these he gave the names “crystalloid” and “colloid” respectively, deriving the word “colloid” from the Greek, kolla, glue, and eidos, form. Further investigation, however, showed, as Graham himself had suspected, that substances could not be classified in the manner indicated. Nevertheless, for the purpose of distinguishing between systems (solutions, or apparent solutions) containing rapidly diffusing and those containing slowly diffusing matter, the terms have continued in use.
The simplest explanation of the difference in the rates of diffusion of, say albumin and salt, is that the pr0tein particles are much larger than the sugar molecules. Since Graham’s time, many attempts have been made to obtain an exact idea of the size of the particles in solutions of proteins and complex carbohydrates. Most of the reliable data in this field, however, have been obtained only within the last decade, mainly by means of an ingenious method developed by Professor Svedberg of Upsala. In Svedberg’s experiments, the protein solution is subjected to the action of an extremely powerful centrifugal force (up to 400,000 times the force of gravity). Depending upon conditions, the protein either sediments out completely or reaches a steady state of distribution. The weight of the particles can be calculated in the one case from the rate of sedimentation, and in the other from the variation in concentration of the protein from point to point in the solution. Investigation of a large number of native proteins has shown that, for the range of pH of importance in brewing, all the particles of any one protein have the same weight. Svedberg ﬁnds that the particles of the vegetable proteins, edestin (hempseed), amandin (almonds), excelsin (Brazil nuts), and legumin (vetch, ﬂour) are 210,000 (± 7,000) times as heavy as the hydrogen atom. Hence, if the protein particle is a single chemical molecule, the above figure represents the molecular weight of each of these proteins. Egg albumin has a molecular weight of 34,500, which is the smallest value given by any of the proteins examined. Since the molecular weight of cane sugar is 342, it is evident that the protein particles must be very much larger than sugar molecules. According to Svedberg’s calculations, the particles of egg albumin are spheres of diameter 4.34 milli-microns (one milli-micron is a millionth of a milli-meter), and are therefore too small to be made visible by means of simple magnification.
The word “colloid” is nowadays used to denote a certain range of subdivision of matter. Liquid systems are called “colloids” when they contain particles which are too small to be visible under the microscope, but yet are so large that they diffuse only slowly. Roughly speaking, the diameters of such particles, when spherical, lie between 1.0 milli-microns and 200 milli-microns. It should be noted that liquid colloidal systems, or sols, resemble solutions of simple substances, such as salt and sugar, in that they are transparent and pass unchanged through the finest filter paper.
Since each sol has its own peculiarities, depending upon the nature of the particles and that of the liquid throughout which they are distributed, or dispersed, any classification of colloidal systems is somewhat arbitrary. The classification which appears to possess fewest disadvantages is based on the behavior of the sol on the addition of salts of the alkali metals. Provided certain precautions are taken, when a sol of native egg albumin is boiled the protein remains in the colloidal condition. The derived sol, however, differs from the original, since ﬂocculation of the protein is produced by much smaller concentrations of the above-mentioned salts. In all probability the change is due to the breaking down, by the heating, of an intimate association of the native protein with water. Hence the sol of native protein is described as lyophilic (associated with the solvent), and that of the denatured substance as lyophobic (antagonistic to the solvent).
A sol of native egg albumin shows a definite Tyndall effect, that is the sol appears turbid when placed in the path of a beam of light passing at right angles to the line of observation. The opalescence is, however, markedly greater in the sol produced by heating. Moreover, When the sols are viewed through a powerful microscope arranged at right angles to the incident beam (as in the ultra-microscope), the field is seen to be uniformly illuminated in the case of the lyophilic sol, whereas the lyophobic sol gives way to a large number of separate points of light. It appears probable that the alteration in appearance is due partly to increase in the size of the particles, and partly to in- crease in the difference between the refractive index of the particles and that of water. The first of these results from the linking up of the protein molecules to form larger units, and the second from some kind of dehydration of the Protein molecules.
The particles of lyophobic egg albumin carry electric charges, and hence migrate when placed in an electrical field. If the pH of the sol is greater than about 5.0, movement is towards the positive pole; and if less, towards the negative pole. In the neighborhood of pH : 5.0 the sol is most readily flocculated by salts. Moreover, a certain parallelism may be traced between the flocculating powers of salt and their influence on the migration velocity of the particles. In view of these facts it is usual to assume that the particles in the stable sol are kept apart by the mutual repulsion of their charges, and that flocculation is due to the neutralization of these charges by the oppositely charged ions of the salt.
The flocculation of lyophilic sols by salts of the alkali metals, which occurs only at high salt concentrations, is a complex process involving dehydration of the particles. The exact manner in which the water is held by the particles is still a matter for investigation.
The characteristic behavior of colloidal systems is generally to be traced to forces which operate at the interface between the disperse phase (the particles) and the continuous phase (the liquid medium). On this account colloid chemistry is often taken to include capillary or surface Chemistry, i.e., the study of surfaces in general. Of outstanding importance is the phenomenon of absorption, the accumulation of material at an interface.
Head On Beer
The formation of a good head on beer is undoubtedly due to the properties of the colloidal material present. There appears, however, to be considerable uncertainty with regard to the exact nature of the factors involved, as also to the relative importance of the individual colloidal constituents. The lyophilic constituents may lower the surface tension, but this does not in itself ensure bubble stability, i.e. head retention, though it theoretically helps in the formation of a copious foam, since the lower the surface tension the less the energy required to increase the gas-beer interface. Pure liquids do not form a stable foam, however low the surface tension.
Decrease in the surface tension of a liquid by dissolved of. dispersed material is indirectly significant, since it implies that such material is adsorbed at the gas-liquid interface. It might be thought that bubble stability resulted from the existence of a high viscosity in the surface ﬁlm formed by the accumulated material. Actually a high superficial viscosity appears to accompany persistent foaming, but it is not of the ordinary liquid type, and cannot be produced by the addition of glycerine, or similar viscous liquids, to the solution or sol.
The surface tension of beer is naturally lower than that of water, but the values recorded in the literature show considerable variation. The chief carbohydrates in beer are not surface active, and it is usual to assume that the lowering of surface tension is in the main due either to some form of protein or to the hop-bitter acids. Emslander states that beer contains, in suspension, very finely divided protein (Suspensionseweiss), which has the property of diminishing the surface tension. Presumably this is colloidal denatured protein produced during boiling. It may be questioned whether the protein in this form has any appreciable effect, since in general the surface tension of lyophobic sols is practically that of water. Peard and Johnson found that the surface tension of certain beers varied quite markedly with change in pH. One difficulty in interpreting their results is that the surface tensions of sols of both native proteins and hop-bitter acids depend upon the reaction.
Some investigators are of the opinion that there is an optimum size of the particles in beer for favoring foam retention. For example, Luers, Geyes and Baumann consider that chilling will frequently produce a poor head because the particles of nitrogenous matter have coagulated and passed the optimal size, and Geyes believes that long contact of beer with yeast is detrimental to the frothing because the yeast enzymes reduce the size of the particles below the optimum value. As far, at least, as the lyophobic constituents of the beer are concerned, this view would appear to be supported by the experiments of Bartsch. In the case of the lyophilic constituents, however, it is improbable that only change in the dimensions of the particles is involved.
Windisch and Bermann suggest that while the proteins in wort are very surface-active, and hence cause good foam formation, the stability of the foam is due to the action of non-proteins, particularly barley gum, which are only slightly surface active. These workers believe that there is an optimum ratio of protein to non-protein for the production of a good head.
Dextrin, a semi-colloid (particles intermediate, in respect to dimensions, between colloidal and crystalloidal units), is supposed by Emslander and others to assist in the strengthening of the surface films, but the mechanism of such action is obscure. In any case, the investigation of Ramsden, Metcalf, Hughes and others indicate that the lyophilic proteins, and their primary decomposition products (semi-colloids) are capable of forming films of sufficient rigidity to resist premature collapse of the head.