Genesis 7 orders Noah to take "clean" and "unclean" animals with him into the Ark, but this is way before people received kosher laws on what animals could be eaten. So what is meant here by clean and unclean animals, especially since people were still vegetarian before the flood? God only gave permission to eat animals after the flood.
It is the generally accepted view in Midrashic literature that God communicated to the Patriarchs which animals were clean (Heb.טָהוֹר) and which were unclean (Heb.טָמֵא) for the purpose of sacrifice.
Evidence to support this is that the Patriarchs only sacrificed clean animals before (and after) the time of Noah. For example, in Genesis 4:4 Abel brings a sacrifice from the firstborn of his flock, which is usually considered to be sheep. In Genesis 22:13 Abraham sacrifices a ram from the thicket in place of his son Isaac.
When sacrifices began being consumed, the rule for consumption was the same for sacrifice. Specifically, if it were permissible to sacrifice the animal then it were permissible to eat it.
Those same phrases of clean and unclean (in Hebrew, טהור and טמא, Vulgate: mundus and inmundus) appear in Leviticus chapter 11, with lists and guidelines regarding certain animals. For example, "Every animal that parts the hoof but is not cloven-footed or does not chew the cud is unclean" (Lev. 11:26 ESV). The camel, hyrax (or rock badger), hare, and pig are all "unlcean" (11:4-8). "And these are unclean to you among the swarming things that swarm on the ground: the mole rat, the mouse, the great lizard of any kind, 30 the gecko, the monitor lizard, the lizard, the sand lizard, and the chameleon (11:29-30).
The context is a list of animals that may or may not be eaten, as well as what "should be detestable to you", i.e. not to be touched (See Leviticus beginning of chapter 5 and chapter 7:19-21). However, these phrases are never used in the context of what may or may not be offered to God as a sacrifice.
The LORD saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. The LORD regretted that he had made human beings on the earth, and his heart was deeply troubled. So the LORD said, "I will wipe from the face of the earth the human race I have created —and with them the animals, the birds and the creatures that move along the ground —for I regret that I have made them." But Noah found favor in the eyes of the LORD. This is the account of Noah and his family. Noah was a righteous man, blameless among the people of his time, and he walked faithfully with God. Noah had three sons: Shem, Ham and Japheth. Now the earth was corrupt in God's sight and was full of violence.
Pure animals compares to tranquility, subtlety and dignity, and non-pure is a sign of rudeness, perversity, and an aggressive attitude.
"the death industry", based on violence and corruption, was the parameter of choice for "pure" and "impure".
Did I express myself well in English?