Playing with the air in the flesh of floating stewed pears

Playing with the air in the flesh of floating stewed pears

Not everyone can look back on a carefree Christmas celebration. There was rain, a dike was washed away, cellars were flooded and there must also have been readers who put 25 grams of flour in the Christmas pie of the NRC-Christmas menu where 35 grams was intended and who therefore served their guests a pie of the wrong consistency. They were victims of the devil.

And there was more annoying stuff. It NRC-Christmas menu also prescribed the preparation of stewed pears, eight in total, which, thinly peeled but otherwise intact, had to be simmered in white wine with sugar, cinnamon and lemon peels. At least one reader got stuck in this. She had, she informed Janneke Vreugdenhil, placed the eight pears in a shallow pan belly to belly and tried to submerge them in accordance with the instructions, but that was not possible, the pears had started to float. Extra water or wine had not helped. Now she wanted to know what was causing it, and Vreugdenhil wanted that too, because it happened often. Artichokes and the like also have it.

It is an interesting issue because it is almost always assumed, see the internet, that pears sink and apples float. Precisely for this reason, a large amount of salts such as lignosulfonate, sodium silicate or soda are dissolved in water that is used as a means of transport for the notoriously vulnerable pears. They give the pears more buoyancy. The upward force that submerged bodies experience from liquids, Archimedes already knew, is equal to the weight of the quantity of liquid displaced. The more salts or sugars there are in that displaced fluid, the higher the buoyant force.

Floating in the tap water

In that respect, the 70 grams of sugar that Janneke Vreugdenhil’s recipe prescribes could have been the culprit, as it may have brought the specific weight of the white wine to 1.05 kg/liter. (The specific gravity of the pure wine itself hardly differs from that of water.) You would say: leave out the sugar and let the pears sink.

The matter turns out to be more complicated. The zinc tendency of pears is not as great as is generally assumed. The first Doyenné du Comice, for example, turned out to be floating in Amsterdam tap water, it wouldn’t go up or down. And the raw Gieser Wildeman stewed pears, damned if it’s not true, they floated, especially if they were peeled because the peels are relatively heavy. Loose peels sink, just like loose stems.

Why do raw stewed pears float? It can’t be the tiny core with its little seeds, it’s nothing. It’s in the air in the flesh. Pear flesh contains 5 percent air, you see it come out as fine trains of bubbles when you bring the cooking water to the boil, but it has also been scientifically proven, even with the help of synchrotron radiation. The air mass is contained in the ‘intercellular space’ and forms a coherent fine network through which oxygen can penetrate to the deepest depths of the pear. And how the excess CO2 can flow off. Remember that pears are alive and the cells need to be able to breathe.

The stem and the crown

The flesh from the belly zone of the pears in particular contains a lot of air. Cubes of pears that you cut from the polar areas around the stem and crown immediately sink to the bottom in a glass of tap water, it is an illuminating experiment.

Only when peeled, intact stew pears have been soaking in boiling water for two to three hours do so much air escape from the intercellular space that they sink. (You can easily boil Wildeman pears for twelve hours before they break down.) Before that happens, they strangely end up higher and higher in the water. Even more mysterious is that the pears can be sunk within half an hour if you let them cool for a while. If you then bring them to the boil again, they will not float again, the mind stops thinking about it. The intercellular air movement under the influence of temperature change is incomprehensible to the outsider.

Spontaneous red discolouration

Anyone who does not stew stewed pears every day may be surprised by the spontaneous red discolouration of the fruits that develops after an hour or two. It turns out that there really is no need for red wine or red port at all. The red comes from the conversion of procyanidins from the cell interior into the well-known anthocyanin-like compounds that give many flowers and fruits their red color. The dye appears to mainly adhere to the cell wall. Sadly enough, red stewing pears are no longer appreciated these days, the culinary community wants their pears white. And indeed they remain white if you cook them in white wine, as Vreugdenhil’s recipe prescribes.

It is not entirely clear what it is in the white wine that suppresses or cancels out the red coloration. It’s not the alcohol because a glass of pure alcohol has hardly any effect. It is probably due to the typical sourness of the wine, but a dash of vinegar also appears to prevent the red discolouration well. Baking powder (sodium bicarbonate), which raises the pH well into the alkaline range, has a completely different effect: it causes such a heavy leak that the cooking water turns a deep pink and the pears are left vitrified.

The amateur researcher is left with a strange question. Would red wine also suppress the spontaneous red discolouration of stewed pears? Is the effect of that wine simply to replace one color with another? Is there a visible difference between pears with their own red and those with wine red? And would fellow culinary people recognize that difference? Someone should ask him.




SCIENCE