Step 9: Make brine solution. I ended up using a solution of 3 liters (or 3000 g) of water with 200 grams of salt. This is a 6.7% salt solution*.
Step 10: Ferment soy chunks in brine for a few weeks. My fermentation took place in a food-grade plastic bucket, covered loosely by cling wrap held in place with an elastic. It is placed in a large west-facing window in a room that heats up considerably in the afternoon**. I try to stir it once a day. I have noticed that the soy chunks stayed whole for a few days, before breaking down, although a few are still fairly whole. Initially the fermentation produced some bubbles, but that stopped after the first few days.
The fermentation does not have a strong smell as it is partially covered. Even when open, during the first week it had only a mild, fungal smell like freshly cut button mushrooms. After about a week the fermentation formed a pellicle – a bacterial/fungal mat covering the surface (see photos). As well, when the cling wrap covering is removed a somewhat stronger smell is apparent. However, the smell is not unpleasant, being quite yeasty, somewhat like beer fermentation, but more earthy (and obviously without hop aroma). Directly smelling the spoon used to stir the brine brings out again the fungal smells, but also a sour smell – probably lactic acid bacteria. Haven’t had the courage to taste it yet though!
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Dry soy chunks just added to brine
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After a day of brining…
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Brining after about a week…covered in a pellicle (bacterial/yeast scum)
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Close up of the pellicle – worth looking at the full size image.
* In this case ‘canucklehead‘ was a bit vague about the amount and strength of the brine. At first he suggests: “the fermented disks are soaked in a brine that contains 8 oz of salt” and later “start out with 4lbs of water with 4 oz of salt”. That works out to about a 6% salt solution, by weight. I checked the label of my ‘naturally brewed’ Kikkoman soy sauce, and the salt concentration is about 7% by weight. So that seemed to be a pretty close match.
I also came across (have lost the link) a Google book about food safety in food processing . They talk about brining and different strengths and uses of brining. They said the minimum safe concentration of salt is about 10%; however later when discussing fermentation (of sauerkraut or pickles) they suggest concentrations of around 8% in order to allow the lactobacillus to thrive. This makes sense, as above 10% the fermenting organisms themselves are inhibited.
Canucklehead’s 4 lbs of water is about 2 liters, so I may be making a more dilute soy sauce (i.e. more brine per soy), but he says initially that his brine contains 8 oz of salt, which means that he would have needed about 4 liters of water. In that case, my soy sauce would be more concentrated. As with brine strength, I have gone with a middle estimate.
Subsequently on the eGullet forum on which canucklehead has been posting, “inigoaguirre” posted a link to his blog (unfortunately for me, in Spanish) showing some pictures of his soy experiment. As well, he states that:
“the brine should have around 25% salt (you should heat at least a part of the water in order to dissolve the salt otherwise you would end up with all the crystals in the bottom). Kikkoman uses for their classic soyzu 23% salt; you can also find soy sauce with low salt content, but the proportion should still be quite high”
Not sure about this – on one hand, it looks like his soy sauce fermentation has succeeded (see his blog). Salt concentrations of 25% or so would definitely need boiling water to create the solution. However this concentration is well above canucklehead’s suggestion, and does not correspond to what my Kikkoman soy sauce (not low-sodium) bottle’s label states (you can calculate it yourself: per 15 mL serving there is 920 g sodium). What’s going on?
One idea is that Inigo and I are calculating salt concentrations differently (I’m fairly seat-of-my-pants on this). Although his statement about heating the water suggests that his salt concentration is indeed higher than mine (which just required a bit of stirring to dissolve). Also, the Kikkoman label deals with sodium, not salt…so the chloride part of salt is not included. As I am too lazy to revisit my high-school chemistry on molar weights, I have not bothered to figure out how much this affects my salt concentration calculations. My guess is not enough to bring my figure up to 25% though – chlorine is much smaller than sodium, and even if they were the same size, it would double the salt concentration to around 14%, still far from 25%.
Another hypothesis is that Inigo’s fermentation may not be via active bacterial/yeast culture. Rather, leftover enzymes (from the molds on the soy patties) that are not denatured in the saline environment may be causing the soy sauce to ferment. As well, if these enzymes produce acids, or if the mold produced acids in the soy patties, and Inigo has placed his fermentation jar out in the hot Spanish sun, he may be creating low-level acid hydrolysis reactions. Acid hydrolysis is how a lot of cheap ‘soy sauce’ is produced – although under industrial conditions using heavy duty acids and high heat and pressure. This is pure conjecture!!!
** Canucklehead says:
“Large containers sitting out in the sun (under plexiglass). I actually don’t understand why this needs to be done – but my uncle says that my grandmother would always let the sun cook out the water – sometimes for a whole month. Perhaps this was a way to remove impurities – when tap water was not so safe”
I don’t really agree with his theory – based on what I have read about the manufacture of soy sauce, it is more likely that this practice was to warm up the fermentation, speeding it up, and creating more intense flavours. In Japan, before WW2, soy sauce was usually brewed during spring or fall specifically to achieve a mellower flavour from cool temperature fermentation. This also corresponds to beer brewing – fermenting at higher temperatures creates a lot of extra flavours that are not usually desirable in beer.
As this soy sauce recipe appears to be of Chinese origin, I have put my fermentation bucket in a warm spot in a very sunny west facing window.
Canucklehead goes on to discuss something I have read fairly frequently elsewhere on soy sauce fermentation:
“I mentioned that they looked awfully pale to make a dark soy sauce – but he said that the brew will darken as it sits in the sun. “Just like how people get tanned.” Huh? I don’t understand how it can get darker – the brine is very salty – so I assumed that there would be no further fungal growth – but how can it get darker?”
Based on what I have read of Japanese soy sauce manufacture they ferment and age their soy sauce in the dark, and their soy sauce turns quite dark without exposure to the sun. The soy sauce is made dark likely by using roasted wheat, by pigments from the molds, by reactions during fermentation, and perhaps due to oxidation (like how an apple’s flesh turns brown with exposure to the air). On the other hand, it really may be that the microorganisms ‘get tanned’ somewhat to protect themselves from UV radiation by producing some sort of pigments.
And just to clarify (“I assumed that there would be no further fungal growth“), there is definitely active growth by microorganisms in the brine – from what I have read this includes most of the organisms involved in brewing beer: yeasts (kloeckera, saccharomyces, brettanomyces and probably others), and bacteria (enterobacteria, pediococcus, lactobacillus, maybe the clostridium and probably others). I doubt the molds themselves are able to grow so canucklehead is partly right – they are likely inhibited by the anaerobic and saline environment of the brine. So I believe their primary purpose is to pre-digest the soy beans and provide extra enzymes for the fermentation. They are themselves probably consumed by the fermentation yeasts and bacteria.
