Kosher foods are those that do not contain any forbidden ingredients (such as pork or shellfish) or mixtures (such as dairy and meat). Some foods, like unprocessed fruits and vegetables, are always kosher and do not require any certification. Processed foods are considered kosher if their production has been supervised to ensure they contain no forbidden ingredients or mixtures. These foods are usually marked with a symbol to indicate that they are kosher. Meat, in particular, requires special handling — meat is only kosher if it comes from a permitted species (such as chicken or beef, but not pork), if the blood has been completely drained from it, and if it has been butchered in accordance with the laws of Jewish slaughter.
Many Jews offer a blessing before eating food (and afterward, too). Food blessings are meant to thank God for the food, but do not affect that kosher status of the food itself. There is no blessing that must be said to make food kosher, nor can any blessing convert a previously un-kosher food into one that is kosher.
