煮咖啡的技巧

Espresso Science

煮咖啡不需要什么高科技,但是一项新研究表明,最好要有数学和分析化学的专业背景。研究人员在煮咖啡的过程中应用材料学和建模方面的专业知识,得出了一个反常的结论:咖啡豆少用一点,咖啡渣磨粗一点,味道更浓。这项研究发表在物质期刊上。咖啡狂热爱好者一定会发现,即便用同样的咖啡豆,同样的机器和同样的设置,煮出来的味道也是时好时坏。为了搞清楚这种水平参差的原因,研究人员建立了一个数学模型,看看咖啡是如何随着水分透过咖啡渣层而被萃取和溶化的。

我们最开始只列了一些应用到单一咖啡渣层的等式。朴茨茅斯大学的数学和物理学高级讲师杰米福斯特说,单一层不算复杂,在现实的咖啡渣层里,会有数以百万计的颗粒以非常复杂的方式堆积在一起。所以单一渣层的等式会更好处理。为了对整个咖啡渣层进行建模,福斯特和同事将等式复制了数百万次,加入了更多的数学计算,然后模拟倒入水中的结果。这个模型得到的,是所有咖啡都和水充分接触的理想条件下产生的结果。克里斯多弗亨顿是俄勒冈大学的计算化学研究员,他也参与了这项研究。

的确,只要大颗粒数量足够,水分可以在咖啡渣层之间自由流动,模拟出的结果就和不同研磨设置下的实际结果非常吻合。可一旦咖啡豆磨细了,就会发现实际操作中开始出现和模型不一样的结果。咖啡豆磨得越细,有时味道会比模型预测结果更浓烈,有时反而更清淡,这和我们的常识恰恰相反。

我们通常认为,想要咖啡浓一点,就要把咖啡豆磨细一点。因为磨得越细,咖啡渣层上的颗粒就越小,表面积就越大,表面积越大就能更快地萃取咖啡。然而研究人员发现,咖啡豆磨得过细,细微的颗粒会堵塞渣层的缝隙,导致水分无法自由流动,反而帮了倒忙,阻碍了咖啡的萃取。同样的道理,咖啡豆也不是越多越好,有些咖啡豆少,味道反而更浓。

这项研究在推特上引起巨大反响,很多人兴致勃勃地争辩和讨论。且不说这项研究能否让我们维持特浓咖啡的制作水准,研究本身已经大获成功。因为我们让数百万人读了一篇科研论文,提高了民众的科研参与度,这是科普的巨大胜利。单凭这一点,就值得我们喝一杯。

原文

Making a cup of coffee isn’t rocket science. But a new study shows that a background in math and analytical chemistry doesn’t hurt. Because researchers who applied their skills in materials science and modeling to brewing espresso have made a grounds-breaking discovery: contrary to popular belief, using fewer beans and a coarser grind will give you a more consistent shot. Their work appears in the journal Matter. If you’re a coffee aficionado, you’ve no doubt noticed that some days, you may get a great espresso; other days, not so much. Even with the same coffee, the same machine, the same settings. To understand that variability, the researchers developed a mathematical model to explore how coffee is extracted or dissolved as water passes through the bed of grounds.

“Basically, what we did was to start by writing down some equations, which apply to just a single ground.” Jamie Foster, a senior lecturer in mathematics and physics at the University of Portsmouth. “So it’s a less intimidating task, because in a real coffee bed, you’ve got millions and millions of particles that are packed together in this very complicated way. And so a more tractable problem is to write down the equations on a single ground.” To model the entire coffee bed, Foster and his colleagues copied that equation millions of times, stirred in a bit more math and then poured on the theoretical water. “The model tells us what we should expect in an ideal situation when all of the coffee is being contacted by all of the water equally.” Christopher Hendon, a computational chemist at the University of Oregon, who also took part in the study.

“And indeed, the model describes reality very well for particular grind settings, where there is a sufficient amount of large particles so the water can flow freely through that bed. But when you grind sufficiently fine, that’s when we started to see, in real life, a divergence from the expectation that the model was telling us.” With the fine grind, some shots were stronger than predicted. But some were considerably weaker, which flies in the face of conventional wisdom.

“The thinking, if you want a stronger cup of coffee, is, well, I’ll grind it finer, because by doing that, I’ll have smaller particles in my grounds, and the smaller particles will have a higher surface area. And so this high surface area allows for more rapid extraction from the grains”. But what the researchers found is “If you kind of overdo the grinding, what ends up happening is: the particles are so small, in fact, that they kind of clog up the gaps where the water’s trying to flow. And that actually hampers the extraction rather than helping it.” And the same is true for the amount of coffee you start with—so that less can, counterintuitively, end up tasting like more.

“Since this article came out, there has been a tremendous amount of activity on Twitter—arguing, discussing, just general excitement and interest in the project. And independent of whether this helps make more reproducible coffee or not, this is a tremendous success—that we have got millions of people to have read a scientific article and engage with science. That’s a great success for scientific literacy.” And we can all drink to that.

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