舊文新讀

轉貼一篇在2004年初alt.coffee的文章，這是因為討論water debit所衍生出來的，作者是 Josuma Coffee Company的Dr. Joseph John。如果要我選出alt裡頭文章最有建設性的人，Dr. John肯定會是排名最前面的那幾個。原文如下：

1. I have been struggling to get a handle on this whole issue for some time and I am not even close. But let me share my partial qualitative understanding, as preliminary as it may be.
   
   Contrary to popular belief, a good espresso grinder should produce a certain distribution of particle sizes, both coarse and fine particles, in order to result in a quality espresso in the cup. It is this balance between the fine particles and the coarse particles that result in the necessary compromise. One desires rapid flow to limit over extraction and at the same time sufficient extraction to make it flavorful.
   
   I know that is confusing. Let me try to get at this another way. The large particles in the ground coffee allow the water to flow through the puck rapidly, and limit the contact time between water and coffee, so as to extract the good parts of coffee and leave the bad parts behind.
   
   However, if all the particles were uniformly large, and adjusted for a 30 second pour, there would not be sufficient extraction to make the espresso taste good. It would be under extracted. Those large particles would require longer contact time to get enough of it into solution.
   
   Now if a collection of fine particles be added to this assembly of coarse particles, the coarse particles will still largely control the flow time, but it is the fine particles that will control the degree of extraction. Contact time between water and coffee is still limited to 30 seconds, say.
   
   Thus, the large particles control the flow time and small particles control extraction, and one needs both to produce properly extracted espresso.
   
   With preinfusion, all the particles absorb some moisture and swell in size, with little or no relative migration of the small particles to the bottom of the basket. When full pressure is applied, some migration of the fines to the bottom is inevitable, but hopefully is kept to a minimum. Thus the fines
   throughtout the basket are properly extracted.
   
   With no pre-infusion, or with a rapid buildup of pressure, the fines all migrate to the bottom of the basket and create a sort of obstruction. The large particles no longer control the flow rate. This causes the first droplet to appear later and result in over extraction in a shorter period of time.
   
   Is that confusing?
   
   Dr. Joseph John
   
   
   
2. There in an underlying belief at work here: Approximately 30 seconds is the optimum contact time between water and ground coffee in a typical espresso extraction context. Based on solubility considerations, this provides the optimum balance between the good, sweet parts of ground coffee that are highly soluble in water and the less good, bitter parts that are less soluble in water. In practice, grinders produce some distribution of particle sizes. Because the smaller particles present a proportionately greater surface area for contact with water, one expects that a larger fraction of its weight will be extracted into solution. The opposite will be true for the larger particles. Assuming that one is optimizing the grind for the ideal 30 second extraction, then the large particles are always "under-extracted" and the small particles are always "over-extracted." That is the nature of the beast. Hopefully, this mixture of this under-extracted and over- extracted brews provides the optimum balance. An assumption here is that there is no uniform size grind that is correct for the optimum extraction. Although I am not certain if this has been, or can, be proved, I intuitively subscribe to this concept. If the particle size is correct for the water flow and contact time, there will not be sufficient fraction of coffee going into solution. Conversely, if the
   particle size is optimum forextraction of solids and emulsified oils, then
   contact time is too long. It will be over-extracted.
   
   With pre-infusion, the small particles tend to swell some and to adhere to one
   another and to larger particles and they are not as free to migrate freely; unless the pressure build up is much faster. That is what I meant by what I wrote.
   

Dr. Joseph John