Dunking dumplings
Whenever I am preparing Italian potato dumplings, or gnocchi, I notice that they behave strangely. When I put the frozen gnocchi into lightly salted boiling water, they immediately sink to the bottom. But the main ingredient of frozen gnocchi is frozen water, whose density is about 0.92 kilograms per litre, and the density of boiling water is 0.97 kilograms per litre, so shouldn鈥檛 the gnocchi stay afloat until the ice melts and then sink to the bottom? Instead, they rise to the surface after 2 minutes and all float when cooked, when they should be heavier than water. What is going on?
鈥� When the frozen gnocchi are placed in hot water the combined density of all the ingredients is greater than the density of boiling water, and therefore the gnocchi sink. As the gnocchi warm up it鈥檚 a bit like inflating a rubber dinghy at the bottom of a swimming pool. The air trapped in the dough expands and the combined density of all the ingredients becomes less than the density of boiling water, causing the gnocchi to rise to the top.
Martin Garrod
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Portsmouth, UK
鈥� I鈥檝e been making gnocchi for a long time and, as I had some frozen ones, I decided to do some rudimentary measurements in my kitchen.
First, my frozen gnocchi had a density of 1.1 grams/millilitre and they duly sank in plain boiling water. When they came to the surface and were well cooked, I scooped them up, drained them on a towel and took the same measurements again. This stage was very fiddly so the results must be taken with a pinch of salt. There was a 14 per cent increase in volume, an 8 per cent increase in weight and their density was reduced by 5.5 per cent. Curiously, when I placed them in cold tap water to measure the volume, my cooked gnocchi sank.
Gnocchi sink because they are denser than water. However, the dough of well-made gnocchi has many small bubbles of air which stay there and expand when placed in boiling water, so they come to the surface. My cooked gnocchi sank in cold water because the air trapped inside contracted slightly.
Maria Fremlin
Colchester, UK
Lump life
I have a photo of a phenomenon that has me baffled. These gelatinous lumps turn up about this time each year, around the perimeter of the pools in my garden. Although they appear at around the time frogs are breeding, they do not seem to be spawn because they don鈥檛 appear to contain any eggs. The photo shows lumps that have been subject to rain: when fresh they are more angular in shape. They remain in the grass for up to a week, and then disappear as quickly as they appeared.
As the answers below show, anyone encountering one of these lumps will have to decide if it is natural or man made 鈥� Ed.
鈥� These gelatinous lumps are sometimes known as 鈥渟tar spawn鈥� and were reputed to be what was left of a shooting star when it fell to Earth. They are common near herons鈥� nests, which provides a clue to their real nature.
Frog spawn does not swell to its full size until it is laid and is in an aqueous environment, otherwise imagine how big a female frog would be in spring. Herons are extremely partial to frogs, and mating frogs are very easy targets.
There are two possibilities for what happens next and, as a heron鈥檚 mode of feeding tends to be grab-toss-swallow, I favour the first one. As the unfortunate frog is digested by the heron, its ovaries encounter an aqueous environment. They absorb water from the bird and begin to swell. Again, imagine how big a frog鈥檚 worth of spawn is, and the result if it swelled in the heron鈥檚 gizzard. So the bird must regurgitate it or risk ending up as big as Mr Creosote. The alternative is that the heron picks out the ovaries before swallowing the frog. They then swell where they lie.
Adrian Gray
Wolvercote, Oxford, UK
鈥� The lumps of jelly are from frogs, presumably squeezed out of the victim when gobbled by a fox. I find them in various places, some not very near water, and the earliest two finds last season were before Christmas. Most do not show signs of eggs or spawn, but some do. In the photograph below, black eggs are clearly visible.
I. C. N. Alcock
Aboyne, Aberdeenshire, UK
鈥� The questioner was right to suspect frogs. The jelly is the water-saturated gelatinous mucilage from their oviduct linings. These organs are rejected by the birds, commonly herons and crows, that feed on the amphibians. The swollen jelly often persists for a long time after the tissue residues have decayed. Here in Pembrokeshire it was called pwdre ser鈥� (pronounced 鈥減oodra sair鈥�) meaning 鈥渟tar-rot鈥�, referring to the ancient belief that it fell from shooting stars (Nature, vol 118, p 552).
No eggs are present unless the frog was eaten whilst ovipositing. This is because the eggs only receive the jelly coating as they pass down the oviducts just before discharge.
John Etherington
Llanhowell, Pembrokeshire, UK
鈥� Certain types of compost for flower tubs and baskets contain 鈥渨ater-storing granules that retain hundreds of times their own weight in water鈥�. They allow the compost to stay moist for longer, so that plants need watering less frequently.
I used some surplus compost in my strawberry tubs, and after several days of heavy rain angular lumps of 鈥渏elly鈥� worked their way to the soil鈥檚 surface. The longer the lump survived, the more rounded and blob-like it became.
David Williams
Bournville, Birmingham, UK
This week鈥檚 question
Congeners and congeniality
I recently picked up a leaflet published by Health Scotland which said the darker the colour of my alcoholic drink, the worse my hangover would be. Whisky, red wine and brandy would lead to a worse morning after than would vodka or white wine, because the darker drinks contained something called congeners. After experimenting, I have to say this seems to be the case. But is it the congeners? If so what are they and what do they do?
Richard King
Glasgow, UK