The obvious answer to this question is “rice” as all sake is made from rice and, in actual fact, Sake brewer’s rice can often be seen to have a white spot at its core where all of the starch is centred and this is called the “Shinpaku” which is the “white heart” of the grain of rice.
However, there are many things which could be considered to be at the “heart” of sake. For instance, how can you ferment a drink from rice when it doesn’t have any available sugars? Rice is made up of starch which is not fermentable, but, by the use of a wonderful fungus called “Koji”, the long strands of starch are broken down into the simpler, smaller sugars which the yeast can eat and, by virtue of a chemical reaction, produce alcohol with carbon dioxide as a by-product. So, Koji is essential to the production of Sake. There is even a saying in Japanese about three essential parts of the Sake making process; ichi koji, ni moto, san tsukuri. This means that, first of all, you need the Koji, secondly the “moto” which is the yeast starter and thirdly you need to make the sake which is the main fermentation process and, for this you need the people.
I truly believe that it is the people who are at the heart of sake. You could say that it’s the farmer who grows the rice who, in rural areas such as Akita prefecture, then becomes the Toji master brewer in the winter. Or perhaps it is the other kurabito (people who work in the brewery) who are often charged with being responsible for one certain aspect of the sake making process and take great pride in their responsibility. Then there is the owner who then goes out and promotes the sake which has been produced. Without all of these people sake would not be of the quality which it is nowadays with such a wide variety of flavours, aromas and styles.