Fermentation Control in Sake Brewing: Understanding Sweetness, Dryness, and the Balance of Light and Rich Flavors Through the BMD Curve

Sake brewing is a true "living art," combining the power of microorganisms with the years of experience of the toji (master brewer). Among the various stages, the fermentation process, known as moromi, plays a crucial role in shaping the final flavor of sake.

In this article, we’ll focus on the "BMD value," a number used to manage the moromi fermentation process, and explore how it helps control the distinct flavor profiles of sake, from sweet and dry to light and rich.

What is Moromi Fermentation and the BMD Value?

In sake brewing, two processes occur simultaneously:

Saccharification: The enzymes in the koji mold break down the starches in the rice into sugar (glucose).
Fermentation: The yeast consumes this sugar and produces alcohol and carbon dioxide.

This dual process, known as "parallel fermentation," is what gives sake its unique aroma and flavor.

To monitor the progress of fermentation, the toji measures the bomei-do (specific gravity). When there is more sugar, the liquid becomes heavier, increasing the bomei-do. As the sugar is converted into alcohol, the specific gravity decreases.

The BMD value is calculated by multiplying the number of days since the start of fermentation by the bomei-do.BMD Value = Number of days from the start × Bomei-do

When this value is plotted on a graph, it forms the BMD Curve, which reveals the progress of fermentation, helping the toji determine the optimal timing for pressing (jōsō).

Can the BMD Curve Determine Sweetness or Dryness?

The sweetness or dryness of sake depends on how much sugar remains at the end of fermentation. The BMD curve shows how sugar levels change during fermentation, providing clues for controlling the direction of the final taste.

Making Dry Sake

To make dry sake, the goal is to ensure that all the sugar is consumed by the yeast, leaving little to no residual sugar. This results in a low BMD value at the end of fermentation.

Accelerating fermentation: Temperature management is key to keeping the yeast active. For example, using hard water, which contains minerals like calcium, can promote yeast activity, leading to smoother fermentation. In contrast, soft water slows down fermentation, requiring careful planning.
Choosing active yeast: Yeasts with high fermentation power, such as Kyōkai No. 9, Kyōkai No. 1801, or Kyōkai No. 1901, are often selected because they have strong alcohol tolerance and can efficiently break down sugar.
Adjusting ingredient ratios: Reducing the amount of koji rice (typically below 15%) or adjusting the water-to-rice ratio can help control the sugar content.
Timing the pressing: The pressing is done when the BMD curve reaches a point that matches the patterns of past dry sake. Using more active yeast will speed up fermentation, requiring earlier pressing. In contrast, slower fermentation might necessitate a longer fermentation period before pressing.

Making Sweet Sake

To make sweet sake, some sugar must be left behind. The fermentation is therefore managed gently to end with a higher BMD value.

Slowing down fermentation: The fermentation temperature is lowered to keep the yeast activity in check, ensuring that sugar remains. The ideal temperature for this is around 10–12°C, where yeast activity is slowed, but fermentation still progresses, preserving sweetness and creating a smoother, richer flavor.
Choosing gentle yeast: Yeasts with lower fermentation activity, such as Kyōkai No. 7 and Kyōkai No. 10, are selected because they ferment more slowly and create softer, rounder flavors ideal for sweeter sake.
Adjusting ingredient ratios: Increasing the amount of koji rice (above 20%) or using highly polished koji rice can increase enzyme activity and enhance sugar production.
Timing the pressing: Sweet sake is typically pressed later than dry sake to allow the sugar to remain. The BMD curve must show that fermentation is sufficiently advanced, but not too much.

BMD Curve and the Difference Between Light and Rich Sake

Light = Crisp, CleanRich = Full-bodied, Complex

The BMD curve can also provide insights into whether the sake will be light and crisp or rich and full-bodied.

Light sake: These types of sake have fewer umami components like sugars and amino acids and have a clean, crisp taste. The fermentation is smooth, and the BMD value typically ends lower.
Rich sake: These have higher levels of sugars and amino acids, offering a rounder, more complex taste. The fermentation is slower, and the BMD value tends to end higher.

However, the "light vs. rich" distinction cannot be judged purely by the BMD curve alone. Factors like rice polishing ratio, yeast selection, and fermentation methods also influence the final outcome.

Summary: BMD Curve as the Toji’s "Visualization Tool"

The BMD curve is a valuable tool for "visualizing" the progress of fermentation, helping the toji make adjustments to achieve the desired sweetness, dryness, and body of the sake. By understanding how the BMD curve works, you can gain a deeper appreciation of the complex and thoughtful process behind every bottle of sake.

Next time you enjoy a glass of sake, consider the hidden drama of fermentation happening behind the scenes and the skilled judgment of the toji. It will make your sake experience even more fascinating.