2000年的3月，一切都因爲那時還很菜的Mark Prince轉貼了一篇報章花絮開始，討論從意大利espresso的標準定義，到Mark的意大利見聞，最後有人搬出Schomer書中的章節，話題突然轉到ristretto這個名詞上……正當大家開始有點霧煞煞，懷疑東懷疑西的時候，Al Critzer(曾任職illy USA、Cimbali)加入了這個討論串，他提供了一個經過業者大量測試的歸納結果，並且爲ristretto做了批註。對他而言可能不感覺到什麼特殊意義，因爲一般相信這並非他個人獨到的見解(他自己也說This info was passed on to me by Dr. illy and his disciples, so I guess it is an Italian way of thinking as well as the single most respected scientific mind in the field of espresso. )，他可能只是一個傳播信息的好心人而已，但絕對是當時網絡論壇的經典大事件。他告訴了一大堆有研究咖啡熱情，卻缺乏正確理論基礎的人，一個可以朗朗上口的口訣。他所講的那幾段話，之後這些年經過千錘百煉，不但是歐美最有共識的espresso觀念，甚至後來幾乎所有能在網絡上看到的萃取文獻都以此爲藍本，被稱爲：Al's Rule，『艾爾鐵則：甜蜜點公式』。

原文如下：

It seems that everyone's close, but pretty much all around it. As I was trained, the perfect espresso is 30ml in 30sec. That's assuming that all other factors are in line. For longer extractions, the contact time of water to coffee would have to be lessened so that undesirable elements associated with overextraction wouldn't be present in the cup. With shorter extractions, the water to coffee contact time would have to be increased to ensure that underextraction doesn't occur. This is accomplished by grind adjustment. A good rule of thumb I have developed is this: For every 5ml of espresso above 30ml, subtract 1 sec of extract time. By this formula 1 1/2oz (45ml) would require 27sec extraction, 2oz (60ml) would require 24sec, etc. The same holds true in reverse. This will find the sweet spot in espresso regardless of volume in the cup (within reasonable limits of .75oz to 2.5oz), as you are optimizing extraction to the desirable elements, maintaining the balance between under- and overextraction. For some reason, grinder adjustment and it's crucial impact on espresso is the most difficult concept to explain and grasp in all the trainings I do.

大意是

以30cc/ 30sec得到一杯完美的Espresso的結果爲出發點。這二個數值當然視豆子而定，烘焙方式、烘焙度、配方比例的差異自然會導致基準點的變動。

萃取量與萃取時間互爲反比：

當萃取量越多的時候，咖啡粉與熱水接觸的時間必須越短(避免過度萃取)

反之，當萃取量縮減，咖啡粉與水接觸時間要增加(避免萃取不足)

Al公式：每增加5cc的萃取量，萃取時間要減去1sec。反之，則要加上1sec。

例如，萃取量增加到45cc時，30-(45-30)÷5=27，萃取時間縮減爲27sec。

萃取量60cc時，萃取時間變成24sec。

這個公式在0.75-2.5oz的Espresso合理限定內，都能迅速正確的獵取甜蜜點(Sweet Spot)，有效地抓到令人滿意的成分，並維持過度萃取與萃取不足之間的恐怖平衡。控制的方式以調整研磨粗細爲主要手段。

至於那個年代也有爭議的ristretto定義，Al Critzer也順便做了他的解釋：

As to ristretto. This a different drink altogether. If your grinder is set for regular espresso, and you choose to stop the pour at 25ml, this is simply an underextracted espresso. You haven't hit the sweet spot yet. The " restricted" part referred to as ristretto is not so much related to volume as it is to water flow through the coffee puck. The ristretto that has been used for cuppings is a 25ml cup in 30-35sec. This extraction intensifies the organoleptic perceptions of the eyes, nose, taste buds and upper pallate to better isolate the positive attributes of a given blend. Where the positives are accentuated, the negatives are exacerbated as well. The extractions are characterized by a very thin mouse's tail with rich, dark brown crema. Most people don't drink this as their everyday drink, but it is helpful in developing blends. If you customarily drink 25ml (or 50ml doubles), your grinder should be adjusted accordingly. Damn, I did it again….

ristretto是一種跟正常espresso截然不同的飲料

如果研磨不變，用正常的espresso流速，提早結束萃取，比如說原先的30sec/ 30cc流速下，只取25cc，這就是一杯萃取不足的成果，因爲還沒到達甜蜜點

所謂的ristretto就是受限定的意思(restricted)，習慣作法是25cc/ 30-35sec

所有感官上能察覺的感受，包括眼睛看到的、鼻子聞到的、舌頭嚐到的，不論是好的味道，或是壞的味道，都被加重呈現

ristretto的表徵是老鼠尾巴特別細，加上更深褐色的crema

大多數人不會每天喝這種東西，但是有利於仍在發展中的blend

如果你就是習慣要喝ristretto的人，不管是25cc或是50cc，那你的研磨就要去對應調整。怎麼調整?一樣啊，Al' Rule重新來過吧

Al Critzer當時在討論串裏與知名業者Dr. Joseph John(Josuma Coffee Company)的另一段對話也很經典：

Dr John,

you didn't mention if this was done with the single filter or double. I'm hoping it was with the single. I personally am of the opinion that any espresso (single) should not be less than 25ml. A 1/2 oz (15ml) espresso has proved impossible for me to make in any drinkable state.

這裏Al等於是重申了他的公式是在setting work的最初，採用single basket衝煮的數據來作爲推算的依歸。也就是縱然你的店裏平時出杯都是使用double濾器，single濾器仍然是設定的基準。而他個人經驗看法中，任何espresso(指single)都不應該少於25cc。一杯15cc的espresso對他而言，是無法達到『可以喝』的狀態(drinkable這個字離好喝還很遙遠) ，這也等於對後來一些喜歡30cc double ristretto的人提前做出了忠告。