Apple seeds contain amygdalin, which decomposes into toxic benzaldehyde and prussic acid (hydrogen cyanide. However, humans are able to detoxify small amounts of these chemicals.
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Question: Do Apple Seeds Contain Poison?
Apples, along with cherries, peaches and almonds, are members of the rose family. The seeds of apples and these other fruits contain natural chemicals that are toxic to some animals. Are they poisonous to humans? Here's a look at the toxicity of apple seeds.
Apple seeds do contain a small amount of cyanide, which is a lethal poison, but you are protected from the toxin by the hard seed coating. If you eat whole apple seeds, they pass through your digestive system relatively untouched. If you chew the seeds thoroughly, you will be exposed to the chemicals inside the seeds, but the dose of toxins in an apple is small enough that your body can easily detoxify it.
Reference: Juniper BE, Mabberley DJ (2006). The Story of the Apple. Timber Press. p. 20. ISBN 0881927848.