Even a small trace of a non-kosher substance—as little as 1/60th (1.66 percent) of the food’s volume, and in certain cases, even less than that—will render an otherwise kosher food not kosher.
By the same token, utensils that come in contact with hot food will absorb its “taste” and subsequently impart it to other food.
For example, a loaf of bread baked in a pan smeared with shortening that contains a small percentage of lard, fruit juice pasteurized in the same machinery as non-kosher milk, or a vegetarian dish prepared in a restaurant kitchen with the same utensils in which a non-kosher dish was cooked earlier—these would all be regarded as non-kosher if the proportion of the non-kosher substance is greater than the permissible percentage.
It is for this reason that separate utensils are used for meat and milk, and that a reliable kosher certification is needed for foods processed or prepared outside the home.
Even the slightest residue or “taste” of a non-kosher substance will render a food not kosher. So it’s not enough to buy only kosher food. The kitchen, too, must be “kosher,” meaning that all cooking utensils and food preparation surfaces are used exclusively for kosher food, and that separate stoves, pots, cutlery, dishes, counter surfaces and table coverings are used for meat and dairy.
A general rule of thumb is that any time hot food comes in contact with another food or a utensil, the food or utensil will absorb its “taste.” Also cold foods and utensils will, under certain circumstances (such as when the food is spicy or salty, is cut with a knife, or it sits in the utensil for an extended period of time), transmit their “taste.”
So food prepared in a kitchen or plant in which non-kosher food is also prepared will invariably become non-kosher as well (unless the embedded taste is first extracted from the utensils in a special koshering process).