Gosh, I don't know if there would be a way to calculate the extent of food poisoning in medieval Europe. Even today it's often a guesswork self-diagnosis, except in cases of sophisticated medical tests for E. Coli or salmonella or something. In "Food, Hygiene, and the Laboratory," Anne Hardy argues that food poisoning as such became a concern for consumers and government officials alike in the 19th century. She points to two key developments: the recognition of particular bacteria causing food poisoning, and better treatment of more serious diseases allowing people to be concerned with lesser ones.

But something really interesting becomes apparent when looking at late medieval civic records (court registers, laws, chronicles, even literature). Scholars generally see the first attempts by a government authority in the Latin Middle Ages to regulate food prices and distribution during and in response to the Great Famine of 1315-1322. In England, where most of this research has focused, this primarily related to the price of bread and ale. It's interesting to note that they aimed specifically at baked bread, not raw grains or flour. Basically, the idea was the keep a loaf of bread costing one farthing, the smallest currency unit, so everyone could afford it. The size of that loaf would change based on grain prices. Bakers in towns would submit sample loaves to be weighed to make sure they were in compliance, were restricted to baking with particular types of wheat, and other fun regulations.

Naturally, bakers make frequent appearances in court, charged with selling underweight bread. The usual charge involves substituting some of the pricey flour with inedible garbage--sawdust and cobwebs are my "favorite" examples. This was perceived as a crime against the community, so often--especially for repeat offenders--humiliating public punishment was in order. Unlike cases of personal affront, where it could enhance one's status immensely to show Christian charity and forgiveness to the offender, crimes against civic society were played by authorities to fire up the zest of the mob. Offenders might be sent to the pillory for three days--erected in the middle of the marketplace, of course, so shoppers would know to avoid their fraudulent wares in the future. Or they could be sentenced to a short stay in jail, which at this time essentially meant begging for food or money to buy food from passers by on the street, from either behind bars or perhaps chained to the outside of the jail. Contemporary woodcuts depict duplicitous bakers dragged through the streets on a cart, wearing a penitential shift, no hat or shoes, with the offensive loaf hanging around their neck. Humiliation, shame, future repercussions; this was no mere fine or out of court peace agreement. Not for messing with the amount of bread in town.

But equally with the existence of bread, town authorities strove mightily to ensure its quality. Charges that bread, or the flour with which it was made, were "rotten" or "putrid" receive as much, maybe more scorn and wrath in sources than underweight loaves. After all, people were being "poisoned and choked." A pastrymaker (pasteler) whose pies looked fine from the outside might well find themselves hauled up for the insides being rotten. It seems medieval people were not only obviously aware that meat spoiled when left at room temperature too long (duh), but wary of butchers and pastelers trying to cheat them. The Host of Chaucer's Canterbury Tales accuses the Cook:

From many a pastry you have drained the juice

And you have peddled many a Jack of Dover [meat pie]

When twice already you had warmed it over.

There's many a pilgrim wishes you Christ's curse

Your parsley has them feeling all the worse

They ate it with your stubble-nourished goose

This is of course fiction with a satirical tinge, but satire must reflect social concerns back at the reader in some way to be funny/effective. Here, Chaucer has his duplicitous cook not only reheat meat pies to try to sell them a second day, but the cook also sskimps on rotten minor ingredients. Which, Chaucer recognizes, are still making people sick. The identification of the parsley is actually really interesting, though. It was medieval practice to overcook vegetables into mush-Johanna van Winters argues that fresh salad is a Renaissance Italian invention. One would imagine the overcooking would reduce vegetable-based food-bourne infections. If parsley is being used as an herb/spice, I'm honestly not sure how that would effect cooking time. More relevant, I think, is the recognition that individual parts of a dish can cause problems.

Of course, we associate rotten and moldy food with pretty clear visuals. Hiding the rotten stuff inside a pastry crust could certainly disguise it temporarily. Evidence from wine fraud cases suggests that con artist food suppliers actually got quite clever about making their spoiled or lower quality product both look and smell like the real thing (down to adding colored dye). And one case of food supply malice cited by Barbara Hanawalt offers an even bigger reason people might not have noticed: hunger. In 1351, Henry Pecche and his friends had gobbled through an entire rabbit pie and started on the second before they realized the meat was spoiled. That's how hungry they were.

And this, in the end, explains why we see government authorities so concerned with quality of food products, not just price. They made profit on bread sold anyway, and on market stall rent. They were targets of public riots when there was insufficient bread; individuals like Chaucer's Cook were the culprits with bad quality prepared food. And yet, towns punished sellers of bad food as severely as those of insufficient food. The passion is baked into the legal formulae: "a shame and a disgrace to the city;" "the manifest danger to the plaintiff"; "the scandal of the city."

Hardy sees concern with food quality as the result of better health knowledge and practice. The people of the late Middle Ages, coming off seven years of bad harvests and ensuing famine, locked in cycles of grain seizure for export to feed troops at endemic war, had different priorities. When food was so precious to everyone, even the elite, ensuring people had good food was as important as ensuring they had it at all. And so scholars speak of a medieval "moral economy": driven first by profit, of course, but also with an eye towards the broad meaning of charity.