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Our obst-entsteiner and its design

Many years ago, when Tina and I were younger, we had camping holidays in Europe.  On one of our visits to Germany we saw and bought a little kitchen gadget called an obst-entsteiner; the box still has the price (19.40 DM, as this was before the Euro became currency).  What does it do?  It removes stones from fruit, such as plums and cherries. Here it is:
Today we stripped one of our plum trees of fruit, and needed to remove the stones from 1500 grams of Mirabelle plums, ready to make jam.  (Mirabelle plums are very scarce in the UK, but they make the most delicious jam.)  However, these plums are very small - between 1 and 2 cm in diameter, the size of a large cherry.  So the stone remover came into use, for the first time in ages.

In operation, one must place the fruit in one of the two cup-rings, press the orange plunger and a prodder hits the fruit and pushes the stone through and out of the hole at the bottom.  Even for our small plums, we used the larger prodder (on the left in t…

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