Natto is sticky because of a polymer called poly-gamma-glutamic acid, or PGA, produced by bacteria during fermentation. This substance forms the long, stretchy strings that pull away from the beans when you lift them with chopsticks. It’s not a byproduct or residue. It’s the signature output of the fermentation process itself.
What Creates the Sticky Strings
The stickiness comes from a specific bacterium, Bacillus subtilis natto, which is added to cooked soybeans and allowed to ferment for roughly 24 hours. During that time, the bacteria break down soybean proteins and use the resulting amino acids as raw materials. One amino acid in particular, glutamic acid, serves as the building block for the sticky polymer PGA.
PGA is a biopolymer, a long chain of glutamic acid molecules linked together. These chains form the visible strands you see stretching between beans. Alongside PGA, the bacteria also produce fructan, a type of sugar polymer. Together, these two substances make up the bulk of what’s called natto mucilage. The mucilage is what gives natto its characteristic slimy, viscous coating and those threads that can stretch several inches before snapping.
How the Bacteria Build the Polymer
The process starts inside the bacterial cell’s energy cycle. Bacillus subtilis natto pulls a compound called alpha-ketoglutarate out of its normal metabolic pathway and converts it into glutamic acid using a specific enzyme (glutamate dehydrogenase). That glutamic acid then gets assembled into long PGA chains by a set of four genes working together: two handle the actual assembly, and two are responsible for transporting the finished polymer outside the cell, where it accumulates as the sticky coating on the beans.
The bacteria actively shift their metabolism to favor this process. Enzymes that would normally break down PGA get dialed down, while enzymes that produce glutamic acid get ramped up. The bacteria are, in effect, prioritizing stickiness. This is why fermentation conditions matter so much. Temperature, humidity, and time all influence how aggressively the bacteria produce PGA.
Stickiness as a Quality Marker
In natto production, more stickiness generally means better natto. Sensory evaluation standards used in food science score natto on a 25-point scale for tissue quality, and the highest marks go to natto with “a lot of mucus, much stretching, slender strands, and good viscosity.” Natto that produces only short, thick strands or very little mucus scores poorly. The length and thinness of the strings are direct indicators of how well fermentation proceeded.
Mucilage yield varies significantly depending on the bean variety and bacterial strain used. Studies testing different kidney bean varieties found mucilage yields ranging from about 5% to over 20% of the finished product’s weight. The varieties with the highest mucilage yield also tended to have the highest activity of nattokinase, the enzyme natto is famous for. This correlation makes sense: both PGA and nattokinase are produced by the same bacterium under similar conditions, so a vigorous fermentation tends to produce more of both.
Why Stirring Makes It Stickier
If you’ve ever stirred natto and noticed it getting progressively stickier, you’re not imagining it. Mixing the beans mechanically stretches and aligns the PGA strands, increasing the apparent viscosity of the mucilage. The polymer chains are already present on the surface of the beans, but agitation draws them out and causes them to interlink more extensively. This is why many Japanese natto enthusiasts stir their natto dozens of times before eating. The texture becomes creamier and the strings become finer and more abundant.
How Stickiness Shapes Flavor and Texture
The sticky coating does more than create a visual spectacle. It fundamentally transforms how natto feels and tastes. During fermentation, the bacteria break down the soybeans’ protein network, shifting the texture from hard and crisp to soft, sticky, and smooth. Cohesion drops dramatically as the tight protein structure loosens into what food scientists describe as a “loose viscous” network, created by the combined effect of protein degradation and the lubricating polysaccharides.
The same process that builds the sticky polymer also releases large quantities of free amino acids, particularly glutamic acid, aspartic acid, leucine, and lysine. Glutamic acid and aspartic acid are the two amino acids most responsible for umami flavor. So the stickiness and the savory taste of natto share the same biochemical origin. The more PGA the bacteria produce, the more glutamic acid was available during fermentation, and the stronger the umami character of the finished product. As fermentation continues, the aroma also develops complexity through esters and other volatile compounds.
A Nutritional Role for the Mucilage
The sticky PGA in natto isn’t just a texture feature. It appears to enhance calcium absorption. A clinical study in postmenopausal women found that a single dose of PGA increased intestinal calcium absorption from about 34.6% to 39.1%, a statistically significant improvement. The effect was especially pronounced in women who had lower baseline absorption capacity. This is one reason natto is sometimes recommended alongside calcium-rich foods in Japanese dietary guidance, and it connects natto’s stickiness to its reputation as a bone-health food.
PGA also acts as a biosorbent, meaning it binds to metal ions efficiently. While this property is being explored for industrial applications like water treatment, in the gut it may help with mineral bioavailability more broadly. The sticky substance you see stretching between your chopsticks is, in a very literal sense, functional food chemistry in action.