The importance of proper tamping techniques

首先，保证细度研磨的espresso咖啡粉既不被过度萃取，也不会萃取不足，这一点至关重要。要做到这点，需要咖啡粉保持恰当的致密程度——不太松也不过紧——水流通过紧实致密的咖啡粉块时要比通过松弛的咖啡粉块更为缓慢。其次，如果咖啡粉块致密程度不够均匀的话，（水泵压力推动下的）水流（在萃取时），将会去找寻一条阻力最小的通道，其结果是：松弛部分被过度萃取，而紧实部分则萃取不够。

It is important not to over-extract or under-extract the finely ground espresso. This can only be acheived by not too much or not too little compaction of the ground espresso. Water flows easier through loosely packed coffee and flows slower through tightly packed coffee.Another problem is that if the pack is un-even, the water will find the path of least-resistance and over-extract the low points and under-extract the high points.