17. Foam The Physics of Foaming The key processes leading to the collapse of beer foam are drainage and disproportionation. The most stable foams are those with a homogeneous distribution of small bubbles. If the size population is mixed, then gas will pass from small bubbles to adjacent larger bubbles, in the process known as disproportionation, making the small bubble even smaller until it disappears and the large bubble is even larger. The consequence is fewer bubbles (i.e., less foam) and bigger, bladdery bubbles that are less attractive. The equation describing disproportionation is from De Vries: 4RTDSσ t Patm θ
ro2 – rt2 = where ro = bubble radius at t = 0 rt = bubble radius at t = t t = time R = universal gas constant T = absolute temperature D = diffusion coefficient S = solubility of gas σ = surface tension Patm = atmospheric pressure θ = film thickness between bubbles
In foams of uniform bubble size, surface collapse is more important. The speed of collapse is proportional to the rate at which successive layers of bubbles come to the surface. Thus, if the bubbles in a foam are small, there are more layers of bubbles in a given depth of foam. Liquid draining from a foam of small bubbles also has a more convoluted route to take (because of the greater surface area), and this is another reason why foams composed of smaller bubbles are more stable. The equation governing drainage is Q= where
2ρgqδ 3η
Q = flow rate (m3 s–1) η = viscosity of film liquid 161
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ρ = density q = length of plateau border (m) g = acceleration due to gravity δ = thickness of film (m) Beer Polypeptides and Foam There are two schools of thought: 1. There are certain proteins in beer that make a primary contribution to foam stability—the discrete polypeptide hypothesis. 2. The essential feature of a protein or polypeptide determining its foamability is its hydrophobicity. Individual polypeptides in beer may exist in different conformational states, and it is when they present a hydrophobic exterior that they are particularly surface active—the generalized amphipathic polypeptide hypothesis. Lipid transfer protein (LTP1) is a barley-derived protein and a significant component of beer foam. Neither malting nor mashing has any effect on the level entering wort. LTP1 is transformed (by denaturation) during wort boiling into a much more foamstabilizing form. A protein with a molecular weight of 40,000 known as Protein Z may also be important. (It is the protein that binds to β-amylase in barley.) The alternate hypothesis is that it is hydrophobicity that is the primary determinant of foam-stabilizing ability in beer polypeptides: polypeptides with increased hydrophobicity display higher foam-stabilizing capability. Both LTP1 (after boiling) and Protein Z have hydrophobicity—but so do other polypeptides. At least some of these polypeptides originate from the barley storage protein (hordein). Recent evidence suggests that these polypeptides have superior foamability but lower foam stability than the albumin-derived polypeptides (LTP1, protein Z). They outcompete LTP1 and protein Z to get into the foam, but as a result, the foam is less good than if LTP1 and Protein Z preferentially entered the foam. Lipid-binding proteins, found especially in wheat (puroindolenes) but also in barley, enhance foam by binding inhibitory lipid. The enhancement of beer foam by the use of isinglass finings probably reflects their lipid-binding capability. Foaming polypeptides are lost by precipitation throughout brewing, but particularly during wort boiling, in the yeast head on fermentation, and during filtration. Other Components of Beer Foam Melanoidins are capable of stabilizing foams. When present alone as foaming species, both proteins and surfactants (such as lipids and detergents) can give good foams, whereas when both are together simultaneously, the foam is of inferior quality because the different types of molecule stabilize foams in different ways (Figs. 104–106). The iso-α-acids cross-link polypeptides via hydrophobic and ion-dipole interactions (Appendix 1). The iso-α-acids probably interact through hydrophobic bonds with adjacent hydrophobic polypeptides, thereby “locking up” the foam surface. Divalent cations promote the interaction. The trans isomers of the individual iso-α-acids
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Fig. 104. Detergent-stabilized bubble.
Fig. 105. Protein-iso-Îą-acid stabilized foam.
Fig. 106. Presence of both detergent (or lipid) and protein means that no bubble stabilization is achieved.
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concentrate in beer foam to a greater extent than the corresponding cis isomers, with especial concentration of the trans isomers of isohumulone and isoadhumulone. The reduced iso-ι-acids are substantially more foam active than their unreduced counterparts due to their increased hydrophobicity. It has been suggested that certain amino acids (the basic ones, arginine > lysine > histidine) interfere with the interaction of polypeptides and iso-ι-acids, thereby inhibiting lacing. Ethanol inhibits foam, probably by acting in a fashion analogous to that of lipids. At low concentrations, ethanol promotes foam formation by lowering surface tension. Process Effects on Foam Barley. Higher N levels will lead to higher levels of foam protein. Malting. Hydrolysis of hordein releases foam-active polypeptides. Protein Z is released from linkage to β-amylase. Wort production. Beer produced under high-temperature mashing conditions has a better foam. Acidification of the mash (to pH 5.1) also benefits foam, as does the use of malt containing high levels of melanoidins (e.g., crystal malt). Fermentation and maturation. High-gravity brewing results in beers of inferior foam performance. This is due to such beers containing a lower quantity (at least by 40%) of hydrophobic polypeptide, which is lost in increased proportions during boiling and fermentation. It also seems that less of this foam-active polypeptide is extracted during the mashing of high-gravity worts. Yields of hydrophobic polypeptide were higher through a mash filter than a lauter tun. Yeast proteinase A reduces the foam quality of beer by hydrolyzing the hydrophobic polypeptide fraction. Yeast of low vitality yields high protease levels and correspondingly poor beer foams. Yeast should be separated from the beer as soon as is practical. A slow cooling after fermentation yields a better beer foam than a fast cooling. Proteinase A release depends on the yeast strain (top-fermenting strains apparently produce more) and is increased when yeast is under stress (e.g., nitrogen starvation, high alcohol levels, high CO2, high pressure). Pasteurization destroys the proteinase, but the enzyme survives in sterile-filtered beer, which means that foam quality progressively deteriorates in the latter.
Fig. 107. Propylene glycol alginate (PGA).
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Novel Solutions for the Enhancement of Beer Foam Nitrogen gas enhances foam stability. For the re-creation of draught-style beers in small pack, brewers have linked the use of nitrogen to the widget (a plastic or metal device in cans or bottles that promotes nucleation). Alternatively, etched drinking glasses may be used to promote the replenishment of foam for any beer style. Properly configured and clean dispense conditions with respect to pumps, pipes, taps, and glasses are a key factor determining foam quality. It is only to a limited extent that protectants against lipid damage such as PGA (Fig. 107) will be able to overcome deficiencies in the trade or the home. Assessment of Beer Foam For QC/QA purposes, the most frequently used methods worldwide are the procedures of Rudin (Fig. 108), Ross and Clark (ASBC standard), and NIBEM (Fig. 109). Decisions on whether to release beer to trade are most rapidly and effectively made by little more than a shaking test. A fixed volume of finished beer is shaken in a closed cylinder and allowed to stand for a few minutes. Any beers with severe foam problems will be evident from the appearance of the head. Such a test should be applied to representative batches from all bright beer tanks and packaged beers. Methods for the assessment of cling include the lacing index technique and another procedure from NIBEM.
Fig. 108. The Rudin method for assessing foam stability. Degassed beer is added to the 10-cm mark and foamed by injecting carbon dioxide (or nitrogen) at a controlled rate and constant temperature until the foam reaches the 32.5-cm level. The gas flow is then stopped. In this way, the whole column is filled with foam. When the foam-liquid interface reaches the 5-cm mark, a stopwatch is started, and the time taken for the interface to reach the 7.5-cm mark is recorded. The longer this time, the slower the rate of liquid drainage and the more stable the foam.
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Fig. 109. The NIBEM method for assessing foam stability. Beer is introduced to the glass such as to produce a constant foam. The needles measure conductivity. When the foam collapses and contact with the needles is lost, the machine responds by lowering the needles until conductivity is once more established. The rate at which the needles are lowered is in proportion to the stability of the foam.
Further Reading Bamforth, C. W. (2004) The relative significance of physics and chemistry for beer foam excellence: Theory and practice. J. Inst. Brew. 110:259-266.