Background: What Fermentation Does
Sauerkraut is produced through lacto-fermentation. When shredded cabbage is mixed with salt, the salt draws water from the cabbage cells by osmosis, forming a natural brine. Naturally occurring bacteria on the cabbage surface — primarily Lactobacillus species — begin converting the sugars in the cabbage into lactic acid. This lactic acid creates the acidic environment that prevents spoilage organisms from growing.
The process does not require any starter culture. The bacteria are already present on fresh cabbage. The role of the cook is to create the right conditions: enough salt, enough brine to cover the cabbage, and the right temperature range.
Key point: Lactic acid fermentation is distinct from fermentation using yeast (as in bread or beer). There is no alcohol produced in sauerkraut fermentation under normal conditions. The end product is acidic, not alcoholic.
Ingredients and Ratios
The most important variable is the ratio of salt to cabbage. Traditional Polish recipes use between 1.5% and 2% salt by weight. This means:
| Cabbage weight | Salt at 1.5% | Salt at 2% |
|---|---|---|
| 1 kg | 15 g | 20 g |
| 2 kg | 30 g | 40 g |
| 5 kg | 75 g | 100 g |
Use non-iodised salt. Iodine inhibits bacterial activity and can prevent fermentation from starting correctly. Plain pickling salt, kosher salt, or coarse sea salt without anti-caking agents are all suitable.
Optional additions common in Polish recipes include:
- Caraway seeds (1–2 teaspoons per kilogram of cabbage)
- Juniper berries (4–6 per kilogram)
- Bay leaves (1–2 per kilogram)
- Grated carrot (adds sweetness and colour; about 100 g per kilogram of cabbage)
None of these are required for fermentation to succeed. They are flavour additions only.
Equipment
A ceramic crock with a water seal is traditional and still widely used in Poland. Glass jars with loose lids, wide-mouth mason jars, or food-grade plastic buckets are practical alternatives. The container must be non-reactive — no uncoated aluminium or standard steel. Stoneware, glass, and food-grade plastic are all suitable.
The cabbage must remain submerged under its brine throughout fermentation. A weight — a sealed bag of water, a small plate, or a dedicated fermentation weight — is placed on top of the cabbage to keep it below the surface.
Step-by-Step Process
- Remove the outer leaves of the cabbage and set one or two aside. Quarter the cabbage and remove the core.
- Shred the cabbage thinly — strips of 2–4 mm are typical. A mandolin, food processor with a slicing disc, or a sharp knife all work.
- Weigh the shredded cabbage. Calculate the required salt based on your chosen ratio.
- Combine the cabbage and salt in a large bowl. Massage and squeeze with clean hands for 5–10 minutes until the cabbage releases significant liquid. This step is important: the released liquid becomes the brine.
- Pack the cabbage tightly into the fermentation vessel, pressing down firmly between additions so no air pockets remain. Pour any liquid from the bowl over the cabbage.
- Press the cabbage down firmly. It should be submerged under its own brine. If the brine does not cover it, make additional brine using 1 teaspoon of salt per 240 ml of water and add as needed.
- Place a weight on top to hold the cabbage below the brine. Fold the reserved outer leaves over the surface before weighting if using a crock or wide-mouth container.
- Cover loosely — the vessel should not be airtight, as fermentation produces carbon dioxide that needs to escape. A cloth secured with a rubber band, or a jar lid set loosely without sealing, is sufficient.
Fermentation Time and Temperature
Temperature is the main factor controlling fermentation speed. The bacteria responsible for lactic acid fermentation are most active between 18°C and 22°C. At these temperatures, sauerkraut typically reaches a pleasant level of acidity after 2–4 weeks.
- At temperatures above 24°C, fermentation accelerates but the flavour is often sharper and less complex.
- At temperatures below 15°C, fermentation slows significantly. The product may take 6–8 weeks to develop, but many cooks prefer the flavour from slow cold fermentation.
- Below 10°C, fermentation largely stops. This is how fermented cabbage is stored once ready.
Check the vessel every day or two during fermentation. Push down the cabbage if it has risen above the brine. Skim off any white foam or surface film — this is normal kahm yeast and is not harmful, but it should be removed to prevent off-flavours.
Storage
Once the sauerkraut has reached your preferred level of sourness, move it to a cool location — ideally between 4°C and 10°C. A cellar, unheated pantry, or refrigerator all work. Fermentation slows but does not stop entirely at these temperatures, so the flavour will continue to develop slowly over months.
Keep the cabbage submerged under brine at all times during storage. Well-made sauerkraut stored under brine in a cool environment will remain safe and palatable for several months.
References
The science of lactic acid fermentation in vegetables is documented in food microbiology research. For background on fermentation biochemistry, the FAO's resources on fermented foods provide reliable reference material. The historical context of Polish food preservation is documented in the collections of the Museum of Agriculture in Szreniawa.