Lacto-Fermented Plums

Sourdough rises on the carbon dioxide produced as lactic acid bacteria and yeasts break down the sugars in flour. So sourdough is a kind of lactic fermentation, which is where its slight acidity and particular character come from. But then — what is fermentation, actually? Saying that microbes break down sugars is one thing; none of it is visible, and following the process properly from beginning to end takes a certain amount of imagination. What exactly makes it different from rotting? I want to understand fermentation better. If I could work it more skilfully into cooking, the range of what is possible would widen considerably.

The other day I bought a book called "The Noma Guide To Fermentation (Foundations Of Flavor) " — a book of fermentation recipes overseen by Noma, the Copenhagen restaurant often called the best in the world. The first thing in it is lacto plum, so I made that: plums salted and left to ferment. Fruit preserved in sugar is easy enough to picture, but salted fruit? I was curious enough to try. My guess was that it works like sauerkraut, where salting the cabbage draws liquid out of it and lactic fermentation happens in that.

From a quick look at the recipes, the basic rule seems to be salt at 2% of the weight of whatever vegetable or fruit you are using.

Ingredients:

• Plums, 10 (850g)
• Salt, 17g (2% of the weight of the plums once stoned)
• Storage jar
• Zip-lock bag

1. Run a knife around each plum along the stone, cutting at right angles to the seam. Hold it gently and twist the halves slightly against each other to open it.

2. Cut lightly around the stone left in one half, then twist to remove it.

3. Quarter them and sprinkle over the measured salt.

4. Put the salted plums from step 3 into a sterilised container and weight them down.

* A zip-lock bag placed inside the container and filled with water makes a good weight.

5. Within a few hours liquid comes out of the plums; once the fruit is fully submerged, leave it at room temperature for several days. In summer, lactic fermentation starts and the acidity comes through in two or three days.

6. This time it had reached a very good level of acidity by day four, so I strained it and separated the liquid from the fruit.

The liquid is red, a lovely colour. The taste is very close to umezu — plum vinegar — with something fruity about it. I dried the fruit in the sun for a while; that tastes close to umeboshi. Use the plum vinegar in place of ordinary vinegar for salad dressings or dressed vegetables and it is very good.

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Here is a dressing made with lacto plum.

Pumpkin, Summer Vegetable and Fig Salad with Lacto-Plum Dressing