It is accepted among Australian beer drinkers that a glass of cold draught lager holds its head for longer if the beer is pulled in two or three separate pours rather than in one continuous pull. What causes this effect and does it also apply to beers served at cellar temperature?
鈥� If you pour the beer in one pull, the foam grows under uniform conditions, producing relatively few bubbles and mostly large ones. Large bubbles pop quickly, so the head doesn鈥檛 last. By pausing during pulling, one gives the first bubbles time to grow larger and more flexible before the turbulence of the next pull shears some of them into more numerous, smaller bubbles. Furthermore, the concentration of carbon dioxide in the poured beer surrounding a large bubble has had a chance to drop, so that the bubbles stop growing so rapidly. The effect of the extra pulls is to reduce the size of the bubbles and increase their number. This means a smoother, finer, firmer froth. As smaller bubbles do not pop as easily as large ones, the finer froth lasts longer too.
Qualitatively, warmer beer behaves in much the same way, but it froths too violently and briefly, which masks the effect. Bubbles in warmer beer are larger and more fragile anyway, so the overall improvement is less worthwhile.
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Jon Richfield, Somerset West, South Africa