Have you ever noticed that soft, minty freshness of a lively new sauerkraut ferment that's only about a week or two old? It's a fruity, apple-like tang that gives a faint cooling sensation, a lot like mint. It's one of my favorite things about a kraut, and there are a few ways to highlight this flavor profile so it plays a more prominent role in your final product.

What is Mannitol?

Mannitol is a naturally occurring sugar alcohol (polyol) that shows up early in many vegetable ferments, especially in sauerkraut and kimchi.

It’s made when early-stage, heterofermentative lactic acid bacteria (most famously Leuconostoc species) convert part of the vegetable's fructose into mannitol.

Why do they do this?

Why Do Some Lactic Acid Bacteria Make Mannitol?

If we can understand the motive and mechanism then we might be able to affect the outcome.

Cabbages (many vegetables, in fact) contain both glucose and fructose, and the heterofermentative bacteria (like Leuconostoc) prefer glucose for energy. Without getting too much into the specifics of the Krebs cycle, it's enough to know that as they use the glucose they also reduce a coenzyme, NAD, from its oxidized form (NAD⁺) into its reduced form (NADH), and the bacteria need a way to restore that coenzyme to its oxidized form so that metabolism can continue and the bacteria can, you know, keep living.

When fructose is around, some lactic-acid bacteria can use it for this reoxidative purpose, as long as they can produce an enzyme called mannitol dehydrogenase (MDH). The species Leuconostoc mesenteroides, which I mentioned earlier, is known for this ability. Essentially, NADH and fructose (in the presence of the enzyme MDH) will create mannitol and reoxidize the NADH into NAD⁺.

So, in short, the bacteria make mannitol as part of a balancing act that keeps glucose metabolism running.

It's Not Enough to Just Give More Fructose

Of course, ensuring the ferment has a supply of fructose in the first place is a good first step. In fact, cabbage contains both glucose and fructose in roughly equal ratios. But is this the best ratio? 1:1 does sound balanced. As it turns out, a higher ratio of fructose, 2:1 with glucose, is more optimal for mannitol production.[1]

So, in the case of a sauerkraut, it could be beneficial to add a source of fructose, but only to an extent. There are some practical limitations: we don't want to increase the sugar content so much that we encourage yeast or push excessive CO₂ buildup. We also risk getting a softer texture over time if the ferment runs too warm or too fast, and we can skew the flavor toward alcohol if the heterofermenters can’t keep up.

Furthermore, we'd also need a substance where fructose dominates, which means table sugar is out. Even high fructose corn syrup usually has a ratio around 1:1 glucose to fructose.

So what can we use instead?

Sweeteners: Crystalline Fructose and Agave Syrup

Don't get me wrong, we still have the "add more fructose" option open to us, it's just going to take some niche ingredients and careful measurement.

To get a sense of the baseline, cabbage, when fresh, is up to about 6% total sugars by weight. Fructose and glucose are each about 2.5% the total weight, and sucrose makes up the rest.

Crystalline Fructose

Crystalline fructose is a highly pure solid substance, often exceeding 98% fructose by weight. It's made by taking high-fructose corn syrup and further processing it until it becomes nearly 100% fructose.

You can get it at baking supply shops and home-brewing suppliers. It's often listed simply as “fructose” or "fruit sugar".

I recommend starting with a modest dose, around 0.3% by total weight of the ferment. We're really just looking to tip the balance to reinforce the fructose metabolism and mannitol production, we don't actually want to increase the fructose:glucose ratio all the way to 2:1. That would be quite a lot of sweet ener to add, which would make for an extremely active batch.

If you do want to push a bit further, I wouldn’t exceed ~5 g/kg (0.5%) unless you’re using a true airlock and you’re comfortable managing a livelier ferment.

Agave Syrup

Agave syrup (also called agave nectar) is a liquid sweetener made from the sap of agave plants (often Blue Weber Agave tequilana, also A. salmiana). It’s typically high in fructose, roughly ~65–75% fructose with the rest mostly glucose. It tastes a bit sweeter than sugar and pours like thin honey.

You can usually find it in grocerie stores in the baking or sweetener aisle.

Like with the crystalline fructose, I recommend staying between 3-5 g/kg, about 0.3-0.5% by weight of the total ferment. Even though this product contains less fructose, it is still entirely sugar (it also contains glucose), and we want to limit the sugar we add for the reasons stated earlier. The warnings about livelier ferments apply equally here.

Practical Tips

Whether you go with the crystalline fructose or the agave syrup, I recommend adding it at same time you add the salt. For a sauerkraut this would be as you dry brine the cabbage. For a kimchi you might add it to the sauce. However you apply the fructose, you just need to ensure it gets evenly distributed.

I think it may be beneficial, for clarity's sake, to state that the additional sugar does not replace any of a recipe's salt.

Non-Sweeteners: Sea Salt and Solar Salt

What if we want to avoid adding sugar to our ferment? Are there other ways to increase mannitol production?

Sea Salt, Solar Salt

As it turns out, the Leuconostoc species responsible for the mannitol are sensitive to minerals such as Magnesium and Calcium, and tend to thrive in their presence.[2]

Sea salt, also called "solar salt" in the study I linked, contains a lot more of these minerals than table salt.

This approach works by improving the environment for the Leuconostoc species. By replacing the table salt with sea salt we increase their access to minerals, which allows their metabolism to accelerate. This leads to more output from these bacteria, mannitol being among their products.

Conclusion

If there’s a single takeaway, it’s this: mannitol is the signature of a healthy early phase. When heterofermentative LAB, especially Leuconostoc, have glucose for energy and a little extra fructose to reoxidize NAD, they briefly paint your ferment with a light sweetness and a faint cooling finish. That moment also nudges the balance toward a brighter, appley tang (more acetate, less ethanol) before the baton passes to sturdier acid producers like L. plantarum.

If you want to dial up that early mannitol mintiness, tip the balance of sugar gently toward fructose (about 1–3 g per kg of vegetable) using crystalline fructose or a small splash of a fructose-forward sweetener like light agave or apple/pear juice, dissolved into the rest of your brine.

Restraint matters. Push fructose too far and you’ll invite yeasts to the party: more fizz, brine creep, and surface films that steal the spotlight. If you notice that, cool things down, don’t stir, and let the LAB retake control.

Let taste be your guide. Sample from day 3 onward: enjoy the fruity lift and soft sweetness while it lasts, then decide when the batch is perfect for you. If the early “sparkle” is your favorite part, move it to cold storage to pause the show. If you want deeper, rounder acidity, give it time. Either way, you’re working with the grain of the microbes, not against them, nudging conditions so their natural choreography makes something memorable and delicious.

References

  1. T. Rice, A. W. Sahin, et. al., Application of mannitol producing Leuconostoc citreum TR116 to reduce sugar content of barley, oat and wheat malt-based worts, Food Microbiology, Volume 90, 2020, 103464, ISSN 0740-0020, https://doi.org/10.1016/j.fm.2020.103464.
  2. Lee, Mi-Ai et al. Effects of salt type on the metabolites and microbial community in kimchi fermentation. Heliyon vol. 8,11 e11360. 2 Nov. 2022, doi:10.1016/j.heliyon.2022.e11360