An old enemy of Sake and the sake brewer is Hiochi-kin or hiochi bacteria that spoils sake as it grows and reproduces. Unlike most bacteria (bugs for short) hiochi-kin does not mind alcohol and some even like alcohol. So as most Lactobacilli, which are heavy lifters in Kimoto and Yamahai moto styles of sake, will die off as the alcohol levels increase hiochi-kin does not. The other factor that usually keeps the bugs out is low pH levels like those found in sake which are a result of acids created in or added to the moto. However these hiochi-kin also like low pH, highly acidic environments.
As it turns out, hiochi-kin is a lactic acid bug or more specifically it is one of two bugs Lactobacillus (L.) homohiochi and L. fructivorans (a.k.a. L. heterohiochi). These are the “true” or “obligate” hiochi-kin and need hiochic Acid (more commonly known as mevalonic acid) for their growth. Other hiochi-kin or sake spoilage bugs are referred to as “facultative” and include: L. fermentum, L. hilgardii, L. casei, L. paracasei and L. rhamnosus.
Koji produces the needed hiochic acid. The fact that “true” hiochi-kin need hiochic acid implies that these bugs are highly adapted to sake production. Thus they have few other environments in which they can grow. Hiochic acid is an intermediate compound in the biosynthesis of many other compounds so it is not normally seen in a stable state (i.e., not part of a biosynthesis process). The chemical makeup of hiochic acid follows.
Hiochi-kin causes sake to become turbid and acidic. The main acid created is, of course lactic acid, but some of these bugs also produce diacetyl (smells like nasty rubber, butter or butterscotch and may also seem like a slickness on the tongue). When diacetyl rises above our detection threshold in sake the aroma is called Tsuwari-ka.
To defeat these hiochi-kin there have been three methods used. The first method was low temperature “pasteurization” used with an estimated temperature of about 120F to 130F. In fact, this process goes back to at least 1568. While the Chinese had been using a similar method for at least 400 years by this time, it is unclear if the Japanese began based on the Chinese method. However, these low temperatures were not quite hot enough to do the complete job. In addition, after the sake was treated it was place back into wooden containers which, as we now know harbored the bugs even after they were thoroughly cleaned. So, while this low temperature pasteurization helped tremendously, faults remained.
Starting in the late 1800s, Salicylic acid was added to sake as a preservative. Salicylic acid worked so well that its use continued into the early 1970s when it was proven that pasteurization temperatures in use, 140F for 10 minutes, where enough to kill all the hiochi-kin. Given this it was now safe to stop adding the salicylic acid. No sakes contain salicylic acid today.
One other method is now possible but it is expensive so it is not used by many brewers. This final method is fine filtering using a membrane ultra-filtration process. All the bugs and enzymes are removed from the sake. With this process even namazake can be stored for maturation over the summer without refrigeration and without deterioration.
All in all there have been great strides taken toward victory over hiochi-kin!