以上是我用大白話轉述的，追求嚴謹性的知友可以看下附原文。出處是Kenneth Davids大神的烘豆科普書Home Coffee Roasting, Revised, Updated Edition: Romance and Revival （2003）。
Coffees are decaffeinated in their green state. Three principal processes are used today in specialty coffees to remove caffeine: the traditional or European process, the water- only or Swiss- water process, and the CO 2 process. All are consistently successful in removing all but a trace (2 to 3 percent) of the resident caffeine.
The European or Solvent Process.There are two variants of the solvent method. The direct solvent process opens the pores of the beans by steaming them and applies the solvent directly to the beans before removing both solvent and caffeine by further steaming. The indirect solvent process first removes virtually everything, including the caffeine, from the beans by soaking them in hot water, then separates the beans and water and strips the caffeine from the flavor- laden water by means of the caffeine- attracting solvent. The solvent- laden caffeine is then skimmed from the surface of the water, and the water, now free of both caffeine and solvent, is reunited with the beans, which soak up the flavor components again.
The Swiss Water or Water- Only Process. In this commercially successful process there are two phases. In the first, start- up phase, green beans are soaked in hot water, which removes both flavor components and caffeine from the beans. This first, start- up batch of beans is then discarded, while the caffeine is stripped from the water by means of activated charcoal filters, leaving the flavor components behind in the water and producing what the Swiss water- process people call 「flavor- charged water」— water with the flavor but without the caffeine. This special water becomes the medium for the decaffeination of subsequent batches of green beans. When soaked in the flavor- charged but caffeine- free water, new batches of beans give up their caffeine but not their flavor components, which remain more or less intact in the bean. Apparently the water is so charged with flavor components that it can absorb no more of them, whereas it can absorb the villainous caffeine. Having thus been deprived of their caffeine but not their flavor components, the beans are then dried and sold, while the flavor- charged water is cleaned of its caffeine by another run through charcoal filters and sent back to decaffeinate another batch of beans.
The CO2 Process.In the CO2 method, the green beans are bathed in highly compressed carbon dioxide (CO2 ), the same naturally occurring substance that plants consume and human beings produce. In its compressed form the carbon dioxide behaves partly like a gas and partly like a liquid, and has the ability to combine selectively with caffeine. The caffeine is stripped from the CO2 by means of activated charcoal filters.
DECAFFEINATION AND FLAVOR
However powerfully it may affect our nervous systems, caffeine has very little effect on flavor. Isolated, it is a bitter, almost tasteless white powder. Coffee without it should taste virtually the same as coffee with it. Nevertheless, soaking green coffee beans in hot liquid and drying them out again is not a gentle process. It definitely affects the flavor of the abused beans. Affects how much? Depending on how careful the decaffeination process and how attentive the subsequent roasting, from a little to a lot. If you buy decaffeinated beans to roast at home you may notice that they are no longer the common gray- green to blue- green color of unroasted coffee, but instead range from a rather sallow yellow to a light brown. This color change is due to the soaking and drying to which the beans have been subjected during decaffeination. The result for roasting purposes is delicate beans that roast much less predictably than untreated beans. The combination of the loss of some flavor agents in the soaking process with the difficulty in roasting accounts for the fact that decaffeinated coffees purchased in the store may not taste as consistently good as coffee from untreated beans. The main message for the home roaster is to buy green decaffeinated beans from a reliable source, and roast them carefully. See Chapter 5 ( here ) and the instructions following that chapter for suggestions on handling decaffeinated coffees. You might also consider making blends of decaffeinated and untreated coffees. The untreated beans bolster the taste of the decaffeinated beans, yet still limit the amount of caffeine. Remember, however, that you may need to roast the decaffeinated beans in a separate session before blending, since they typically reach the same degree of roast 15 to 25 percent faster than untreated beans.
個人體驗結果是：只要是低因的 風味跟常規的咖啡都不能比 坦白的說 我覺得是差很多 最後只能挑了一個相對來說 風味保留得相對較好的。