Preserving the harvest: which method for which crop

Preservation is a choice about what you are willing to lose. Every method trades one thing for another — heat destroys some compounds and makes others available, salt adds sodium, drying concentrates sugars and loses volatiles, freezing preserves texture poorly but nutrients well. Matching the method to the crop is the whole craft.

Drying suits leaf, flower, root, seed and anything with a low water content. It removes water and therefore stops microbial growth, and it concentrates flavour and sugars. What it loses: volatile aromatic compounds, which is why an oven-dried herb is brown and odourless and an air-dried one is not. Dry aromatic material at ambient temperature or at no more than 35-40 C, in the shade, with moving air.

Lactic fermentation is the one method that improves the nutritional value of what it preserves. Phytate binds iron, zinc, calcium and magnesium into insoluble complexes; fermentation degrades it and measurably raises mineral absorption. Because of that, fermentation is also the right answer for grain and legume, where phytate is highest. It adds salt, which matters on a sodium-restricted diet.

Boiling is the method to be careful with. Cooking measurably lowers the potassium and magnesium content of greens tested, and the minerals go into the water. This cuts both ways: for minerals, ferment or eat the plant raw and do not throw the cooking water away; for oxalate, boiling and discarding the water is exactly right. Those two pieces of advice point in opposite directions and both are correct, because they are about different compounds.

Freezing preserves vitamins well and texture poorly. What it does not do is kill microorganisms — freezing stops growth, it does not sterilise — so a frozen vegetable must still be safe before it goes in.

Root cellaring suits roots, alliums and hard-skinned squash. It is a living storage: the crop continues respiring and slowly converting starch to sugar, which is why a cellared carrot is sweeter in February than in October. It requires cool, dark, humid, ventilated conditions, and it fails through condensation and rodents more often than through temperature.

The safety line that applies to all of them. Botulism is anaerobic, which is precisely the environment that brining, oil-covering and vacuum-sealing create. The protection is acid (pH below 4.6 for safety, below 4.0 for good keeping) and salt (2.0-2.5% by total weight) for vegetables, and it is destroyed by a single mistake with either. Do not invent a canned recipe — for low-acid vegetables the tested pressure-canning times are not optional, and home-canned low-acid vegetables are a documented botulism source. Anything with fuzzy mould, a rotten or varnish smell, or a slimy softened texture is discarded, not salvaged.