Have you seen a bottle of Saké that seems to contain a whitish cloud, a cloud which also seems to contain little bits of something? It contains some Saké lees, unfermented rice and koji. Theis type of Saké is known as Nigori (a Japanese term which means "cloudy") and many people refer to it as "unfiltered" Saké. But is that really accurate?
No, it isn't. There are two basic ways to make nigori, and both still require filtering. So if it is filtered, then why does it have all that whitish gunk in the bottle? First, it may have been filtered using something with large holes, so not everything actually gets filtered out. Second, the "gunk" may have been added back later, after the Saké has been filtered.
Historically, all Saké was once nigori Saké and it was not until the late Heian Era (794-1192 A.D.) that brewers began to produce clear Saké. Later, the government began to tax all filtered Saké, making nigori illegal and it took many years for that to change. There was an exception under the law, which allowed 10 Shinto shrines to produce nigori without filtering. This nigori was used in religious ceremonies.
Less than fifty years ago, the Tsukinokatsura brewery in Fushimi desired to produce nigori, and devised an ingenious way around the law. They filtered their Saké but used a large filter that allowed some of the lees into the Saké. The Tax Department decreed that this was acceptable under the law, and nigori returned to Japan.
Now nigori is fairly common but remember that it is not really unfiltered.