After moromi is complete we have only a few more steps to go in our process. These are: secondary ferment, racking, fining, pasteurization, amelioration and bottling. Conditioning and maturation are also terms for the secondary ferment. For the most part the secondary ferment begins after the sake has been pressed out of the lees. At this stage the sake can be anywhere from milky white to relatively clear. However, in all but unusual cases, more, finer lees will settle to the bottom as the sake completes its ferment and rests.
As the ferment completes, alcohol production ceases but the yeast are still active. During the early stages acetylaldehyde, diacetyl and esters are produced and cleaned up, however the clean up follows production by a good amount of time so when there is no more alcohol to produce there is still a sizable amount of these compounds remaining. At this stage the yeast complete their work and clean up remaining levels. It is also at this time that the sake flavors start to come together for a more integrated taste.
Once the lees have settled as much as they are going to it is time to rack the sake to a newly sanitized conditioning container. A fining agent, like bentonite, can be used to help pull the particulates out of solution and fall to the bottom. In general it will be about 10 days for the lees to drop. Racking can be done ether by decanting or by siphoning. It is easier to siphon from one secondary container to the next without mixing in the very light sediment from the bottom than it is to decant without mixing the sediments.
As long as the sake is kept at 45-50F the racking process can be repeated several times, with each time removing more of the particulates and leaving a clearer sake. However, with each time some sake is also lost. During one of the rackings, the sake should be pasteurized.
To pasteurize, place the freshly racked sake in a water bath while monitoring the temperature of the sake. Once the sake temperature reaches 140F take it out of the water bath. This is a pretty gentle pasteurization. As the sake heats up, CO2 is driven off. This can look like it is boiling but it is only the CO2 coming out of solution.
At this point there are only amelioration and bottling left. Amelioration, for those who choose to do it is similar to the method used for wine and consists of adding a sugar solution (sake sugar syrup) to lower the SMV value to the desired range. Unless the ferment is stopped early, which can result in unwanted by products of the ferment remaining, or a yodan addition is made, the sake will have a SMV as high as +12 to +16. Most like sake with no more than an SMV of +10; even this is pretty dry sake. To lower the SMV by one point for one US Gallon it takes 0.23oz. of sucrose (table sugar). Once the SMV is where you want it and you are happy with the taste, it is time for the final step, bottling.
To bottle, rack the sake into its final resting place; these should be freshly sanitized bottles that can be filled to limit the amount of air space in the bottle. Place the filled bottles in a water bath as before and bring the sake to 140F. Once it reaches 140F take the bottle out of the water bath and put the lid on. As the sake cools a slight vacuum will form which will help to keep the bottle sealed.