“I’ve had people tell me the orangy color next to the pit in peaches and other fruit is there because it leached out of the “poisonous” pit. So what causes this color change and is it dangerous – what chemicals are involved?”
To answer the question, we’ll describe the composition of the peach seed, focusing on the compounds which could either leach into the fruit or be potentially dangerous.
Peach Stone (Pit) Composition: Lignin
The main component of peach stone is lignin; depending on the kind of fruit and its maturation stage, the lignin content can be as high as 50 % weight. Lignin is a large, complex natural molecule; it is present in various parts of plants. Wood from trees, for instance, is made of lignin.
Lignin is a chromophore, or a compound which absorbs light in the visible region and, hence, is colored. The color in lignin is due to the presence of o-quinones, compounds that absorb at about 420 nm; this corresponds to a yellowish color. Moreover, other chemicals, such as cathecol, can form complexes with iron (Fe); this strongly absorbs at about 550 nm (strong red-brown color).
In other words, it is the lignin that gives peach stones their characteristic color.
Other Peach Stone Components: Amygdalin
Peach stones contain other components; the most abundant ones are cellulose (about 20-30 %) and minerals/ashes (about 3 %).
However, peach seeds also contain amygdalin, a minor component which could potentially be dangerous if consumed. Amygdalyn is a compound made of two molecules of a sugar (glucose) linked to a benzene ring to which a cyanide group is attached (see Figure below). In the presence of some enzymes, for instance b-glucosidase, amygdalin can be decomposed into glucose and cyanidric acid (hydrogen cyanide, HCN). This last compound is very toxic, and it can be lethal.
Cyanide-containing molecules can also be found in other fruits, plants and vegetables. Almonds, cassava roots, bamboo shoots, apricot and plum stones, for instance, all contain chemical compounds which could lead to the formation of HCN.
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