I'm making my first batch of homemade Kimchi today using a recipe from Jamie Oliver. As I also have a mature sourdough starter in the fridge, I'm thinking about adding a bit of sourdough (a few grams) to the Kimchi in order to start fermentation.
Is this a good idea, or useless, or even dangerous?
It is not dangerous, but it is not necessary.
Sourdough starter is not going to ruin it, but it is not going to be REAL kimchi. Kimchi is really easy to make plus it tastes better when naturally fermented. Some people even prefer to ferment it in the fridge. It is going to take longer, but it's going to taste differently.
The is nothing unnatural about the SS, but SS is not natural for fermentation of kimchi. It is not used traditionally. It also might alter the kind of yeast and bacteria that are present in kimchi. The microorganisms present in naturally fermented kimchi are: Bacillus mycoides, B. pseudomycoides, B. subtilis, Lactobacillus brevis, Lb. curvatus, Lb. parabrevis, Lb. pentosus, Lb. plantarum, Lb. sakei, Lb. spicheri, Lactococcus carnosum, Lc. gelidum, Lc. lactis, Leuconostoc carnosum, Ln. citreum, Ln. gasicomitatum, Ln. gelidum, Ln. holzapfelii, Ln. inhae, Ln. kimchii, Lactobacillus kimchii, Ln. lactis, Ln. mesenteroides, Serratia marcescens., Weissella cibaria, W. confusa, W. kandleri, W. koreensis, and W. soli. The bacteria and yeast present in SS are somewhat different.
If you are going to add some sourdough starter, it will still come out well, but it's not going to be authentic. My question is: do you want to make REAL kimchi (김치) or do you want to make your own version of kimchi?
As an avid supporter of Korean cuisine, I want to take it one step further: James Oliver's recipe doesn't sound very Korean (sorry). He omits Asian pears and adds daikon which is predominantly used in radish kimchi (깍두기).
From my personal experience, kimchi tastes best when it is naturally fermented, no starter needed.
I also recommend an authentic Korean recipe which is surprisingly easy to make, here it is:
https://www.youtube.com/watch?v=fznTL6TzsqI
The only thing that I personally do differently, I don't add sugar at all, just more Asian pears. It still comes out nice.
The main issue here is what microorganisms you'll be introducing via the sourdough starter, and whether you want them in your fermented vegetables.
The sourdough has lactobacilli, which chow sugar and produce lactic acid. This is perfect, since that's same category of bacteria that you want to encourage in your kimchi. I'm sure you're aware: the acid is preservative in that it deters the growth of other, harmful, bacteria. And it tastes good. While I don't know if there's a difference in the particular species that are happy living in dough versus those found on vegetables, I expect this would work out well, and it's probably the reason you thought of this.
The other main microorganism your starter has is yeast, and I think this will get you into trouble. Yeast has a large part in giving bread its special savor. But those flavors are not what you want in a vegetable ferment -- they come off as "boozy" or "bready". Indeed, the presence of wild yeast -- that bready smell, an overabundance of gas -- is usually a bad sign when fermenting. This is not just because of the flavor, but also because the yeast are directly competing with the lactobacilli for food.
The success of a fermentation from a preservation standpoint depends on your encouraging the lactobacilli to dominate the environment and make it acidic enough quickly enough that harmful things (other bacteria, mold) can't get a foothold. If they have to share their food with a large thriving population of yeast this is more difficult. That is less of a concern if you're not canning the result, just storing it in the fridge and consuming it reasonably quickly, but if you're looking for true preservation adding yeast is not going to be a benefit.
On balance, while it's a good thought, I'd suggest not using the sourdough starter for fermented vegetables. The impact of the yeast is likely to outweigh the benefit of jumpstarting the lactobacilli.
That said, maybe partition off a small part of your second or third batch and see what happens? I say "second or third" so that you can see what the results are like without it, for a clearer comparison.
I've kept a sourdough starter for some years and make kimchi once or twice a year, as well as various other vegetable ferments. I think that adding sourdough starter would be safe but not useful, as kimchi behind fermenting on its own very quickly.
Additionally, kimchi (and many other vegetable ferments) has a flavor that develops over time, and sampling the flavor at different times is interesting, informative (on how the fermentation is going), and helps you find how funky you like your kimchi.
