How Kimchi Ripens, and When It Is Ready

Ripe kimchi is defined by acidity rather than by a date, and the usual target band is a pH between 4.0 and 4.5. How long that takes is set almost entirely by temperature, which is why the same recipe reads ready in two days on a counter and in weeks in a fridge.

Here is the band itself, with the numbers that go with it.

In a published study of napa cabbage kimchi, the optimal fermentation period is described as the stage when pH reaches between 4.0 and 4.5, with a pH below 4.0 generally taken to indicate over-ripening or excessive fermentation; the same study measured fresh unfermented kimchi at pH 5.57 and reported that its 4 degree Celsius samples reached pH 4.32 at day 47 and fell to pH 3.98 by day 168, while its 15 degree Celsius samples reached pH 4.36 in only 3 days and pH 3.89 by day 14.¹

Three days at 15 degrees against 47 days at 4 degrees, from the same starting point. That is not a small adjustment to the schedule, it is a different unit of time.

Temperature is the whole clock

Another study ran the same comparison across more temperatures and put a multiplier on it.

In a published study, kimchi took about 20 days at 4 degrees Celsius to reach pH 4.2, about 10 days at 10 degrees and about 2 days at 23 degrees, and the same study reported the optimum 0.6 percent acidity reached at about 20 days, 5 days and 1 day respectively; a separate study concluded that kimchi stored at 15 degrees ferments approximately 15 times faster than at 4 degrees in terms of reaching the optimal pH range.²

Roughly fifteen times faster at 15 degrees than at 4. So leaving a jar out for one day on the counter is not a shortcut to a fridge week, it is closer to a fridge fortnight, and that is the lever people actually use when they want kimchi to ripen faster.

Who is doing the work, and when

The souring is not one organism grinding away from start to finish. There is a handover partway through.

In a published study of long-fermented kimchi, lactic acid bacteria succession is described as typically occurring during fermentation with Leuconostoc species dominating the early and middle stages and Lactobacillus species becoming predominant in the later stages; a separate study describes Leuconostoc as dominant during the early stage where it creates an acidic environment under anaerobic conditions, after which Lactobacillus rapidly increases under those conditions during the late stage.³

Early and late are run by different genera, which is part of why kimchi at two weeks and kimchi at two months do not taste like the same thing at different strengths. They are different fermentations.

Why the numbers you read disagree

If you have found conflicting targets online, the conflict is real and it is in the literature too.

Published kimchi studies do not agree on a single ripeness setpoint. One study names pH 4.2 with 0.6 percent acidity as the optimum values of highest-quality kimchi, a second defines the optimal band as pH 4.0 to 4.5 and treats anything below pH 4.0 as excessive fermentation, and a third sampled commercial kimchi for analysis at what it called a ripening point of pH 3.8 to 4.1. A cook reading only one of these will set a different target than a cook reading another.⁴

And the same disagreement runs at the sour end of the scale.

Published reports disagree on the acidity at which kimchi stops being acceptable. One study cites the Codex Alimentarius that acidity for fermented kimchi should not exceed 1 percent and classes samples over 1.0 percent acidity as the high ripening stage, while another states that kimchi quality is usually unacceptable when acidity is 1.5 to 2.0 percent. A third study of long-fermented mukeunji reports total acidity of 1.05 to 1.66 percent at day 14 and treats that product as desirable, which sides with the looser limit.⁵

So there is no single published number to hand you. What there is: a band most sources put around pH 4.0 to 4.5, and beyond it a stretch that some call overripe and others call mukeunji and cook with on purpose.

Read your own jar

Since the published targets disagree, your own record is worth more than any of them. pH strips cost very little and turn a guess into a log.

• Note the date you made or opened the jar, and where it sits, counter or fridge.
• Taste a small piece every day for the first three days if it is at room temperature, or every few days in the fridge.
• If you have pH strips, dip one in the brine at each check and write the number next to the date.
• Move it to the fridge when the sourness is where you like it. Cold does not stop the ferment, it slows it a lot.
• If it went too sour before you caught it, nothing is wasted. That is the stage for stews and fried rice.
• After one full jar you will have your own time-and-temperature note, which beats every number in this post.