At Maruboshi Vinegar Company, the vinegar-making process begins at local sake shops with a thick, primitive-type sake made from only two ingredients: brown rice and spring water. The sake-maker steams brown rice, sprinkles it with spores from an Aspergillus culture, and sets it to incubate in a warm, humid room. The spores germinate and the culture begins to produce digestive enzymes using the brown rice as a nutrient source. After two days, the fermented rice and Aspergillus become koji (the ubiquitous starter used in most Japanese fermented foods). The sake-maker next combines the koji with water and cooked brown rice, then pours the mixture into 100-gallon wooden casks.

Gradually, the enzymes in the koji convert the proteins, carbohydrates, and fats of the brown rice into amino acids, simple sugars, and fatty acids. Next, naturally occurring yeast converts the sugars to ethyl alcohol.

After about eight weeks, the thick, heady brown rice sake automatically stops fermenting when its alcohol content reaches about 20 percent, which inhibits yeast growth. It is then delivered to the vinegar shop, where it is mixed with spring water and seed vinegar (good vinegar from a previous batch). Finally, the liquid is poured into partly buried crocks that are sealed with thick, natural-fiber paper and wooden or ceramic lids.

The crocks, many over 100 years old, are arranged in rows going north to south; this is so each crock receives maximum warmth as the sun travels from east to west. In the summer, when warm temperatures could cause the vinegar to overheat, the grass is left to grow tall around the crocks to provide cooling shade. In winter, the grass is cut short to expose the upper third of the crocks to the warming sun.

Within two to four months, depending on the season, the acetic-acid bacteria transforms the ethyl alcohol of the brown rice sake into dark, rich vinegar. The vinegar-maker pumps the vinegar from the crocks, dilutes it with water to reduce its acidity, and puts it in large tanks where it is left to age and mellow for about ten months. Once aged, the vinegar is filtered through cotton, flash-pasteurized, and bottled. The Kyushu vinegar process takes over a year to complete.

Kyushu brown rice vinegar is unique among natural rice vinegars because it is fermented outdoors. For centuries, Kyushu vinegar-makers have buried their brown, twenty-five gallon, glazed crocks about two-thirds in the ground. Maruboshi brewmaster Tetsunori Ezaki says, "This technique keeps the vinegar temperature constant over a narrow range. Daily and seasonal temperature fluctuations are very important because they give the vinegar deep character and richness," but, Ezaki warns, "temperature extremes can destroy the vinegar completely."