Lateral Thinking Riddles

Lucretia Borgia invited a prospective victim to lunch. They ate a hearty meal of roast venison, with a selection of fresh vegetables, all washed down with the finest wine imported from Bordeaux in France.

After the meal they finished off with figs and grapes freshly picked.

“Just one apple left”, said Lucretia, “I insist you have that. “No”, said the guest, “I couldn’t”. “Tell you what”, said Lucretia, “we will share it”, and promptly sliced it neatly in two with her sharpest knife. The guest and Lucretia started to eat their respective halves when suddenly the guest’s eyes rolled towards the ceiling and he keeled over backwards stone dead.

“Another victim successfully despatched,” thought Lucretia.

Why was Lucretia not poisoned after eating the apple?