The Science Behind Freezing Natto's Live Cultures
What Happens to Bacillus Subtilis at Freezing Temperatures?
The bacteria responsible for natto's fermentation, Bacillus subtilis var. natto, is a spore-forming bacterium. This is the key to its survival. Unlike many other probiotics that are sensitive to cold and die when frozen, Bacillus subtilis can protect itself by forming a highly durable endospore. While the active, or 'vegetative,' cells may be ruptured and killed by the ice crystals that form during freezing, the robust spores remain dormant and viable.
Upon thawing, these spores can reactivate, bringing the live culture back to life. Research has shown that these reactivated bacteria can continue their beneficial work, including producing valuable enzymes and contributing to gut health. However, this reactivation does not mean the thawed natto is identical to its fresh counterpart.
Fresh vs. Frozen Natto: A Comparison
To understand the full impact, it's helpful to compare fresh natto with its frozen and thawed version. While the core nutritional value remains, some properties can be affected.
| Feature | Fresh Natto | Frozen & Thawed Natto |
|---|---|---|
| Live Culture Count | High population of active, vegetative cells and spores. | Reduced population of active, vegetative cells, but resilient spores survive and reactivate. |
| Probiotic Activity | Immediate and high. | Delayed, as dormant spores must reactivate; potentially lower initial active count. |
| Texture | Distinctly sticky, slimy, and stringy. | Can become slightly grainy, watery, or less sticky due to cell damage and moisture changes. |
| Flavor | Umami-rich with a characteristic, pungent aroma. | May have a stronger, more pronounced ammonia smell and slightly altered taste due to over-fermentation post-thaw. |
| Vitamin Content | High levels of water-soluble vitamins are preserved. | Some water-soluble vitamins may break down or be reduced. |
| Enzyme Content | High levels of nattokinase. | Nattokinase is very resilient to freezing and largely retained. |
How to Properly Freeze and Thaw Natto
Proper handling is crucial to preserve as much quality as possible. Here are some steps to follow:
- Freezing: If you buy fresh natto and don't plan to eat it all quickly, freeze it promptly. To prevent freezer burn, wrap the sealed packages in plastic or an airtight container. Portioning before freezing can make it easier to thaw only what you need. Some people freeze natto in ice cube trays for single-serving portions.
- Thawing: The best way to thaw natto is by transferring it from the freezer to the refrigerator and letting it defrost slowly overnight. Thawing at room temperature is faster but can cause a stronger, faster re-fermentation, leading to a more intense flavor. Once thawed, it should be consumed within a few days.
- Reactivation: After thawing, give the natto a good stir to help reactivate the cultures and re-establish the characteristic stickiness. The stringiness may not be as robust as fresh natto, but it should still be present.
Is Frozen Natto Still Beneficial?
Despite the potential textural and flavor changes, frozen natto is still a highly nutritious food with significant health benefits. The bacteria's hardy spores ensure that the probiotic properties are not entirely lost. Furthermore, many of natto's most praised compounds are retained even after freezing.
- Nattokinase: This potent enzyme, known for its blood-thinning properties, is highly resilient to freezing temperatures. You can still gain the cardiovascular benefits associated with nattokinase from properly thawed natto.
- Vitamin K2: Natto is one of the richest sources of Vitamin K2, a vital nutrient for bone health. Research suggests that freezing does not significantly degrade Vitamin K2 content.
- Protein and Fiber: The core nutritional components of the soybeans, such as protein and prebiotic fiber, remain intact. These provide a solid base of nutrition regardless of freezing.
Potential Downsides and How to Manage Them
While the nutritional benefits are largely retained, consumers may notice some changes. The texture can become slightly grainy, and the signature stickiness can be reduced. Upon thawing and reactivating, some batches may develop a more pungent, ammonia-like aroma, which is a sign of continued fermentation. This can be managed by consuming it sooner after thawing or mixing it with strong-flavored ingredients like mustard, green onions, and soy sauce, which is the traditional Japanese way of eating it.
Conclusion: The Final Verdict on Frozen Natto
Ultimately, freezing natto does not eliminate its live cultures. The resilient spores of Bacillus subtilis survive, go dormant, and can be reactivated when thawed. While there may be a minor loss of water-soluble vitamins and a potential alteration in texture and flavor, the major health benefits—including the potent probiotic effects, nattokinase, and Vitamin K2—are preserved. For those without access to fresh natto, the frozen variety remains an excellent, convenient, and healthy option. As long as it is stored and thawed correctly, it is a perfectly good way to enjoy this nutritional powerhouse.
WebMD provides a detailed overview of nattokinase and natto's benefits
