数据分析咖啡研磨的有趣实验

咖啡的研磨(grinding coffee)充满了神秘和神话。最近,一群臭气相投的朋友着手于一个项目,试图进一步了解咖啡研磨,并且得出了一些有趣的实验结果,我们决定把这些写成文章,以把我们的研究方法交给同行评议,然后经过严格的流程出版。上周,这篇论文作为开放获取在自然科学报告上发表。这意味着任何人都可以阅读并且受益于这些研究结果。

grind_hero-1

        像这样的科学论文必须用特定的方式书写以至于难以消化。我在此乐于把这篇论文拆分;描述我们做了什么,以及这代表的意义。下面附上这篇论文的原文。
生豆产区和温度对研磨的影响

简介

下面是关于咖啡和这次实验发生了什么的最基本纲要(rundown)。不是每个同行和你一样了解咖啡,所以我们思考的时候包括了一些相关的细节:

1.生豆有两个主要的参数:品种和处理法。
2.烘焙对咖啡所表现的味道影响很大。
3.大多数烘焙产生的化合物是由于梅纳反应,但也有其他很多反应随之进行。
4.我们用了4种不同的咖啡做这次实验:
coffee-list
最后,这次实验的主要考察的是生豆的产区、处理法、烘焙程度和温度是怎样影响咖啡研磨的。

实验方法

咖啡极其复杂,我们几乎无法建立一个表现它研磨时粉碎 (fractures) 的状态的电脑模型,所以只能以实验的方式来研究它。
实验中我们主要做了两件事:
我们用相同的参数研磨不同产区、烘焙度和处理法(origins/roasts/processes)的咖啡并测出研磨样本的值。
我们研磨4个不同层次的温度的咖啡豆,并测出研磨样本的值。
为了测量研磨颗粒我们用了激光衍射粒度分析仪
Laser diffraction particle size analysis
我们假定测量研磨的咖啡粉最重要的度量标准是径粒分布,即是测量样本里每个咖啡粉的大小(直径)。是的,风味很重要,但是风味 (flavour) 更难用准确的精确的方式来测量。
测量咖啡径粒分布的是一台激光衍射粒度分析仪(LSA)。本质上,它把磨成粉的咖啡吸入一个管道,照亮它们通过的路径并测量单个咖啡粉粒投射到监视器上的“投影”。实际上发生的要比这个复杂多了,但是这样描述也够了。我的脑袋也没法探究得更加深入 (delve further)。这机器难以置信的精确敏感所以需要经常校准。下面这张图展示了LSA内部的工作原理。
laser-diffraction-particle-size-detektor

研磨

我们使用 EK43做这次研究,因为它研磨后残留在通道里的残粉量几乎可以忽略不计。这次研究依赖于在特定温度下研磨的咖啡豆,并且没有任何交叉污染。我们不允许任何咖啡豆被研磨前,在豆仓、磨豆机通道或者刀盘上停留可以感知的时间。
手头上有3台 EK43,我们选了一台产出最佳风味咖啡的来做这些样本,这台 EK43装载了土耳其式刀盘。EK43有潜在的巨大的刀盘错位,也许选用的这台的错位最小吧。
我们保持每次研磨样本时磨豆机的设定精确地相同。刻度是2.7,如果你想知道的话。同时我们也让磨豆机在每次研磨样本时冷却到室温以免摩擦静电的热量成为实验变量。
特定温度的样本放在纸杯里,分别保存在4种不同的环境:室温(20℃)、冷冻(-19℃)、干冰(-79℃)和液态氮(-196℃)。样本在被从这些环境中取出的一秒内研磨并且没有水冷凝。我们对每个数据集取3个样本,并且执行两遍。所以每种温度的咖啡有6个数据集。这些数据集的结果也通过方差分析(ANOVA),以确保他们近似,从而可以认为数据是准确的。
ek43-vs-k30-vs-anfim-vs-robur-size-graphs_2x-1

生豆的差异会影响最终的研磨吗?

这部分很难理解。或许你对下面这张径粒分布图有些熟悉:
X 轴是以微米为单位的研磨径粒尺寸,1微米等于1/1000毫米。该轴以对数标度表示,1到10和100到1000相隔的距离相同。这是因为咖啡粉的样本覆盖了跨度巨大的三个数量级(0~1000微米),我们用这个方法适配它,就不会在较小的径粒尺寸上损失太多的分辨率。
Y轴是咖啡粉径粒体积所占的百分比,这个容易理解:曲线峰值越高,代表这个体积的咖啡粉径粒越多。
举个例子,看X轴上400微米的垂直上方的棕色线,这个大小的径粒占样本体积的8.5%(不是重量!)
count-and-integral
我们采取这种风格的数据进一步呈现这个实验。
首先,来看看径粒的数量。不用体积,我们直接地用图表示每种体积的径粒数量有多少(蓝线)。最明显的一件事,在每个样本中有多得难以置信的细粉。99%的径粒直径分布在70微米(0.07毫米)之内。这意味着有一个直径大于100微米的咖啡粉,就对应着有100万个直径小于100微米的咖啡粉。
当我们分析咖啡粉时,很重要的一个点是它们的表面积。它们的表面积越大,水就越快越容易萃取出它的风味。为了将上面的数据变成近似于表面积的东西,我们首先假设每个咖啡粉都是一个球体(这是相当普遍的做法,对于粗糙、表面不平坦的咖啡粉,这是一种“保守估计”)。然后,我们取每种径粒大小的咖啡粉并计算他们的球体表面积。
这给了我们一组非常酷的数据!
下图的实现是“计数”,这与上面的蓝线相同。下面的虚线是相对应的“表面积贡献”,也就是,由每种径粒大小提供的表面积的比例。再次,一些事情相当明显,细粉贡献了绝大多数的总表面积(70%)。
origins-graph
这张图还表示出了实验中使用的各种咖啡的径粒分布。你可以注意到它们非常的相似。坦桑尼亚、埃塞俄比亚、萨尔瓦多和危地马拉的概况分别以黑色、紫色、红色和蓝色表示。
结果表明产区、处理法和烘焙对于PSD的影响要远远小于我之前的设想。这可能是一件好事:我们需要关注的变量少了一个!
下一个围绕你头脑的是那些贡献了70%表面积的细粉。是的,水通过咖啡粉内部萃取可溶解物,但是水流从进入粉内部萃取再到冲煮出来的时间可是比通过表面萃取要长指数级的时间。细粉是我们的朋友!

不同温度的咖啡豆是否影响最终研磨?

这是真正有趣的地方。
咖啡是不定型的 (amorphous) ——它是由数不清的不同分子结合成的不规则的植物结构。
婚戒上的钻石是相同的碳原子结构的结晶。
当你改变无定型物体的温度时,它们有事会经历“玻璃化转变”。
也就是说,它们很迅速地从橡胶似的软性物质变成硬似玻璃的物质。一些物质还经历过破碎转变,而且倾向于变成更多更小的粒子。这也是我们在讨论咖啡是如何在磨豆机中研磨时破碎的关键点。
temperatures-graph
我们发现在不同温度下研磨同一种咖啡的径粒分布有非常显著的差异。我们先认准一下每种颜色的线和它代表的温度。你会注意到,PSD模型的峰值随着温度的下降而变小(在4个样本中下降了31%)。随着咖啡豆变冷,它也变得更脆,所以在研磨时产生多得多的小径粒的细粉,也比温度高的咖啡样本不容易有大尺寸的径粒逃脱磨盘(即更容易粉碎)。
差距最大的是20℃和-19℃的样本。咖啡豆可能在这两种温度之间经历了某种破碎和玻璃化的转变。
我们还确认了这种转变是可逆的,所以你不用担心咖啡豆因为温度升高而不能变回好喝的状态。
temperatures-data
b图模型表示数量最多的径粒,即每条线的峰值。当咖啡豆温度越高时,数量越多。
c图斜度是数据集中不对称性的的数值表示。温度低的样本歪斜较少,因为它们包含更多的小径粒和更少的大径粒咖啡粉,所以减少了峰值右侧线内的面积。
d图均值就是平均值,你会看见冷冻的样本(-19℃/粉色)的平均值比室温的样本(20℃/红色)来得高。这是因为室温样本在3~5微米的范围内有更多数量的径粒,但是在8~30微米范围内则更少。
这个温度范围内发生的转变对未来研磨技术的发展可能是一个很重要的发现。

结语和想法

产区和处理法
所有做拼配咖啡的人都会高兴地注意到,不同咖啡豆的表面积在固定研磨尺寸下几乎是相同的。这意味着拼配的时候主要考虑的是各种配方豆有相同的可溶解度。换句话说,它们在相同的萃取时间下有相同等级的萃取率。烘焙师们,拿出你们的干涉仪吧。
温度
你记得那些倒了20杯才做出一杯完美咖啡的早晨吗?罪魁祸首不是磨豆机刀盘因为发热膨胀;而是因为咖啡豆在研磨之前就吸收了磨豆机内部的热量。这些热量使它们更不脆,即使你没有改变研磨设置它也会产生更多的粗粉。
下面是一些问题几乎都是相同的罪魁祸首。有没有想过为什么……
天气热的时候咖啡喝起来好像不如平时好喝?
你的磨豆机在一段时间内的出品非常不一致?
在出shots的时候流速越来越快?
这些情况在使用EK43的时候并没有出现?
当磨豆机调得更细使得萃取时间相同时,浓缩的味道并不像早晨早些时候的出品?
这些问题的原因都是:咖啡豆在磨豆机内部受热使得研磨产生了差异。
咖啡豆在磨豆机中停留的时间越短,它们被其中的热量影响就越小。在磨豆机通道内制造一个研磨前均匀加热或冷却咖啡豆的装置是非常困难和昂贵的。由于这个原因,我坚信唯一的解决方法就是研磨前磨豆机不装载咖啡豆。
我从来都不是 Mythos 磨豆机加热特性的粉丝,这个实验结果就是对为何如此的最佳解释。为了加热咖啡豆以实现一致性,而把咖啡豆推向了粉粹转变温度之上,这会显著地降低咖啡粉的总表面积(越来越低的萃取率)。
温度越低=越细的径粒=越大的表面积=更高的萃取率。
月越低的温度也意味着芳香化合物更少的蒸发和升华(香气流失)。
把你预先称量的咖啡豆保存在冰箱里能得到更高味道更好的萃取率(但是确保他们密封以免有太多的水气和氧气)。

细粉

哇哦。回到2012年我获得世界冲煮大赛冠军的时候,清除细粉还是流程中的重点。结果证明,过筛没有真的去除掉小的径粒,仍然有数百万的细粉围绕着一个比它大的颗粒。当咖啡在磨豆机中分裂的时候,留下了许多正负电荷在咖啡粉上吸引了细粉。结论是:过筛对于颗粒的分离和测试没有什么作用。

现在外卖知道冲煮过程中有多少是细粉萃取出来的,很明显的,细粉不是坏家伙;否则每一杯咖啡都会过萃得可怕。从这里可得出的想法是:风味萃取的上限是由萃取最多的颗粒决定的。这就是那些最小的颗粒。所以你要确保没有任何部分的咖啡粉萃取过度。
这也取决于萃取不足时你减少的咖啡豆的量。最简单的方法,像我经常说的,使用那些研磨比较均匀的磨豆机。这一点没有任何新意。
如果我知道了如何适当地萃取细粉,我会回到冲煮大赛。
一个美好的日子,期待大家的意见、建议和更正。
英文原地址: https://baristahustle.com/grinder-paper-explained/

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