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Dlaczego ciemne piwo ma białą pianę?

Oczywistym jest, że piana na piwie jest biała, prawda? Zazwyczaj pijemy, nie zastanawiając się nad tym zbyt długo. Rozsmakowujemy się tylko w białej gęstej pianie dobrze nalanego trunku. 

Czy rzeczywiście sprawa jest tak oczywista, jak wydaje się na pierwszy rzut oka? Przecież zarówno piana, jak i piwo składa się z tej samej substancji. Piwo wypełniające naszą szklankę może mieć różne kolory. Jasne piwa mienią się złotem, piwa nieklarowane mogą mieć barwę żółtą, a nawet pomarańczową. Oprócz tego istnieje też cała gama ciemnych piw. Jednak nawet na tych najciemniejszych, piana jest zawsze biała. Dlaczego?

Najpierw zastanówmy się nad tym, co nadaje kolor ciemnemu piwu. Światło padające na kufel piwa jest częściowo rozpraszane, częściowo absorbowane. Substancje zawarte w ciemnym piwie najbardziej pochłaniają światło niebieskie, rozpraszając czerwone i zielone. W konsekwencji postrzegamy ciecz jako ciemnobrązową.

A czy ten sam mechanizm działa dla piany, która jest złożona z sąsiadujących ze sobą pęcherzyków gazu, z których każdy otoczony jest cieniutką warstwą piwa? Światło przechodząc przez cienkie warstwy cieczy nie jest pochłaniane w tak dużym stopniu, jak przez pełną szklankę. Możemy się o tym przekonać rozprowadzając na ściance pustego kufla resztki piwa. Nie zaobserwujemy wtedy ciemnej barwy, piwo stanie się prawie tak przezroczyste jak szkło, na którym je rozsmarujemy. Dzieje się tak, ponieważ pochłanianie światła zależy od grubości ośrodka, przez który przechodzi. Opisuje to prawo, w sformułowaniu którego nie mały udział miał niejaki pan August Beer* (zbieg okoliczności?)

Dlaczego więc piana nie jest przezroczysta?

Mnogość bąbelków sprawia, że światło przechodzi przez wiele warstw piwnych, ułożonych pod różnymi kątami. Jest wielokrotnie załamywane i odbijane pod losowymi kątami, co skutkuje równym rozpraszaniem wszystkich kolorów we wszystkich kierunkach, co odbieramy jako kolor biały.

Oczywiście piana piwna nie jest wyjątkowa, inne spienione ciecze również rozpraszają światło w taki sposób. Warto się w tym miejscu zastanowić czy istnieje piana, która nie jest biała? Potraficie podać jakieś przykłady?

Poza kolorem, w piwnej pianie kryje się jeszcze dużo fizyki, którą będziemy odkrywać w kolejnych fiztaszkach. Wszystkie teksty z tej tematyki znajdziecie tu: fizyka piwa.


*beer - z ang. piwo

  • Obrazek użytkownika piotrowski
    O autorze:

    Marcin Piotrowski

    Wydawca portalu fiztaszki.pl, webmaster, redaktor działu fresh science.
    Fiztaszki popija zawsze yerba mate. Mimo tego, że w pracy w laboratorium ma do czynienia z ekstremalnie niskimi temperaturami, nie przepada za terrere. Oprócz kuchennego eksperymentowania, zajmuje się fiztaszkowym webmasteringiem. Aktualnie bada czy fiztaszkowa fizyka działa tak samo w Australii.

Komentarze

Obrazek użytkownika Max

Potrafimy podać przykład piany, która nie jest biała, np. piwna ;P
Tutaj link do zdjecia dla niedowiarków:
http://definitiveale.files.wordpress.com/2010/08/simple-malt-imperial-st...
Gdyby ktoś się zastanawiał, to jest Simple Malt Imperiale Stout. Swoją drogą, dużo ciężkich, ciemnych piw ma pianę brązową, a nie białą.

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Też się kiedyś nad tym zastanawiałem. Pochwalę się, że samodzielnie doszedłem do podobnych wniosków... a fizyka nie jest moją dyscypliną. Teraz, gdy moja teoria została potwierdzona przez specjalistę poczułem się nielada spryciarzem. Dzięki! :)

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W dziale "fizyka piwa" apelowaliście o zadawanie pytań piwnych. Jest jedna zagadka, której rozwiązanie chętnie poznam. Gdy mamy dwie butelki piwa i stukniemy denkiem jednej o szyjkę drugiej i na odwrót, butelka zmieni się w piwnopianowy wulkan. Dlaczego tak się dzieje?

Obrazek użytkownika mojenazwisko

Bardzo dobre pytanie! I bardzo trudne! Sprawa jest dość złożona, ale ugryziemy i ten problem, póki co prosimy o cierpliwość, kolejne piwne fiztaszki w drodze :)

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Piwa ciemne takie jak Porter, Stout, Milk Stout mają karmelową piane

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