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Could the foam on top of a beer be more important than what lies beneath? Dr Evan Evans from the Tasmanian Institute of Agricultural Research, University of Tasmania blows the foam off a glass...
The aesthetic of drinking beer is to an extent subliminal. The presentation of beer in a glass with its foam head, clarity and colour conjure Pavlovian anticipation for the perceptive drinker. It is said that a beer drinker drinks as much with their eyes as with their mouth.
But that foamy head confers not just a visual benefit. It also acts as an efficient gas exchange surface pitching aromas towards the drinker's olfactory sensors and so provides a drinker's first tantalising glimpse as to the quality of the beer's flavour, freshness, refreshingness and wholesomeness. Foam is also tactile to the lips and affects mouthfeel through its stability and its structure (bubble size). But what are the features of good foam quality? Typically, this is defined by a combination of its stability, quantity, lacing (glass adhesion or cling), whiteness, ‘creaminess' and strength. Here beauty is definitely in the eye of the beholder as consumers discriminate between beers based on their foam characteristics. These choices have been found to diverge between genders, race or even region. There are those consumers who like to see stable (but not excessive) head but a clean glass at the end of drinking and those who prefer to be left with a lacing pattern on the glass. More recently, it was demonstrated that men generally rate foam lacing higher than women. Foam stability is the result of the interaction between the proteins from the malt and hops. Australian varieties of malt such as Gairdner (most common malt) and the new variety, Flagship, have high levels of the protein Z4 compared to older varieties such as Schooner and Stirling. Isomerised hop-α-acids, which provide the bitter taste of beer, have also been demonstrated to be foam promoting, especially in their hydrogenated forms such as ‘Tetra' hop. The tetra hop was primarily developed to provide protection from light strike - or what the Americans call ‘skunking' - in beers sold in the less protective but visually attractive clear or green glass bottles. While contributing to a crisp and dry final product popular with many drinkers, the practice of using cane sugar in brewing dilutes out the foam proteins that contribute to a highly stable head. To counter this, many brewers can add the chemical propylene glycol alginate (PGA). Adding PGA is reputed to increase foam stability by around 5-10 per cent and many brewers rely on it to provide satisfactory foam quality for some of their products. Other strategies for promoting foam stability involve the use of devices added to the packaging or included in the glass. The most technically simple is the use of ‘nucleated' glassware such as the ‘headkeeper' style that can be found in many Australian pubs.
Through the use of microscopic abrasions on the bottom of the glass, this ‘energised' glassware produces new bubbles that replenish collapsed bubbles in the foam head. The process also results in ‘beading' which is the attractive spectacle of the bubbles rising from the beer up to the foam. While most beers are naturally and conventionally carbonated, the use of nitrogen gas also improves foam stability. Nitrogen has a lower partial pressure compared to CO2 and results in the production of smaller, more stable bubbles. Nitrogen is also less soluble in beer than CO2 so it does not ‘leak' as quickly from foam bubbles, further maintaining foam stability. The addition of nitrogen into the beer changes the foam's mouthfeel to a ‘creamy' texture, however the lower CO2 content leads to beer with less ‘prickle' (acidic feeling from bubble collapse) on the palate making the beer taste flat and watery. Still, tests have shown that the visual impact of the foam head on a glass of beer is more important than the tactile impact on perceptions of flavour and mouthfeel and the brewer's use of nitrogen depends on the beer style and consumer preferences.
The nitrogen widget was designed to emulate the draught style of UK pub presentation. Widgets work as nitrogen foam nucleating devices and in some cases the widget can also form bubbles from its nucleated surfaces. The widget also provides a degree of theatre upon package opening with a characteristic rumble of the gas being released through the widget vent to produce a slight fob at the package opening and pour associated with the rise of a multitude of tiny bubbles to a creamy head. The main downside of widgets is that they are considered by the industry to be expensive both in terms of capital and consumable costs. Foam quality, however, is not just about ‘quick' fixes such as the inclusion of widgets, ever greater levels of ‘tetra' hop or gas composition but attention to the beer making process from grass to glass (malting variety breeding to dispense). Brewers do have solid options in manipulating the quality and quantity of malt foam-positive proteins and selection of hop acids, the interaction of which provides the basis for foam stability and quality. Brewers also have a range of palliative options such as additives, gas composition, widgets and methods for dispense that can be used if suitable to the style of beer being produced. Dr Evans' brewing quality research is being applied in Australia's malting barley breeding programs (Adelaide and Perth) to provide maltsters and brewers with the best possible barley for brewing.
This is an edited extract of the chapter Beer foam: achieving a suitable head, contained in the forthcoming book Handbook of alcoholic beverages: Malting and Brewing, Bamforth, C.W., Russell, I., and Stewart, G.G., Editors. Evans, D.E., and Bamforth, C.W.,
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The use of the widget is relatively widespread in some specific styles of beer. It is either attached to the base of the package or floats within it. These characteristics reduce the appeal of the widget in the more popular bottle, presumably because the sight of a widget floating or attached to the bottom of a bottle is somewhat disconcerting for the consumer. In cans, the widget can be commonly found in the less carbonated stout and bitter ale style beers of the United Kingdom, such at the cans of Guinness, Boddingtons ale and Newcastle Brown ale, but has not found acceptance in other regions or beer styles.
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