http://www.straightdope.com/columns/080704.html
Even in the old days, how many poisons worked so fast that a food taster would fall ill in time to prevent the king from eating it?
Are there documented cases where involvement by food tasters actually saved a monarch’s life?
It’s simpler than that. Poisons generally taste bad. So bad that a food laced with enough poison to kill someone would probably taste foul at the first bite.
A “yucchh” and a spitting out are less picturesque than someone dropping dead face-first into the soup, Hollywood-style, but it was a more likely scenario.