The Hebrew word kashrut means to be proper or correct. An observer will be alert to kosher food labels with the letters kshr on them. Israelites were to not eat foods such as pork, shrimp, and squid. Why were such foods unclean (not kosher)? We can answer this question by seeing the world through the lens of an Israelite. Two possible scenarios can be deduced.
Angle 1: Creation
Genesis 1 divides animals into 3 Spheres: Animals that walk on the land, animals that swim in water and animals that fly in the air.
Leviticus 11 lists as unclean certain animals that are connected to land (vv. 2-8), water (vv. 9-12), and air (vv- 13-25). As seen, this animal symbolism mixing or blurring of categories. In contrast, the clean animal has all the defining features of its class given its creation. So animals that "transgressed" boundaries or overlapped spheres were to be avoided as unclean.
Water : To be clean, aquatic animals must have scales and fins (Lev. 11:10; Deut. 14:10); so eels and shellfish, which don't fit this category, are unclean and thus prohibited.
Land : Clean animals are four-footed ones that hop, walk, or jump. A clear indication of a land animal's operating according to its sphere is that it both (1) has split hoofs and (2) is a cud-chewer. These two features make obvious that an animal belongs to the land sphere (e.g., sheep and goat). Camels. hares, coneys (which chew the cud but don't have divided hoofs), and pigs (which have divided hoofs but don't chew the cud) are borderline cases; so they're excluded as appropriate land animals to eat.
Air : Birds have two wings for flying. birds like pelicans and gulls inhabit both water and sky, which makes them unclean. Insects that fly but have many legs are unclean; they operate in two spheres - land and air. However, insects with four feet, two of which are jointed for hopping on the ground, are considered clean (Deut. 11:21-23). These insects - the locust, katydid, cricket and grasshopper are like birds of the air, which hop on the ground with two legs. Therefore they're clean.
Unclean animals symbolised what Israel was to avoid: mixing in with the unclean beliefs and practices of the surrounding nations. Israel was to be like the clean animals - distinct, in their own category, and not having mixed features. After all, the Israelites were God's set-apart people who were to reject the religion and practices of surrounding nations.
But wasn't everything that God created "very good" (Genesis 1:31)? If so, doesn't this mean that no animal is inherently and unclean or inferior? Yes, Jesus affirms this in Mark 7: 19 (all foods are clean), and is employed in Acts 10:10-16 (Peter's vision). However, as the people of God, the Israelites were reminded that holiness requires people to conform to their class as God set-apart people.
So what the Israelites did in their everyday lives, even down to their eating habits, was to signal that they were God's chosen people who work to live lives distinct from the surrounding nations. Every meal was to remind them of their redemption. Their diet, which was limited to certain meats, imitated the action of God, who limited himself to Israel for among the nations, choosing them as the means of blessing the world.
So no religious overlap, blurring distinctions, or compromise could exist between Israel and its neighbours. Israel was called to integrity and purity of life, to avoid what would restrict or inhibit drawing near to God. Holiness involved conformity to God's order of things. Just as clean animals belong to their distinct spheres; they weren't to mix their religion with surrounding pagan nations or intermarry with those who rejected the God of Israel (cf. Ezra 9:1-4; Neh. 13:23-30). Holiness wasn't merely a matter of eating and drinking but a life devoted to God in every area. The New Testament says the same thing: while all foods are ultimately clean (Mark 7:19), our eating and drinking matter to God, who is Lord of all (1 Corinthians 10:31). Yet food matters shouldn't disrupt the church's joy and peace in the spirit (Romans 14:17).
Angle 2: The Fall, Death, and Abnormality
We can look at clean and unclean foods from another angle: that of curse and death. This connection with the fall is reinforced by the repetition of God's command in Genesis 2-3, "you may eat" (2:16;3:2) or "you shall not eat"(2:17; 3:1, 3), in Leviticus 11 (vv. 2,3,9,11,21,22).
Furthermore, the kinds of animals that were permitted and forbidden in the Israelites diet were linked to the kind of people God wanted them to be. They wouldn't be predators in human relationships. Just as discharged blood and semen symbolised death and therefore uncleanliness, so did predatory animals: "do not eat the meat of any animal torn by wild beasts" (Exodus 22:31).
A further aspect of cleanliness and uncleanliness seems to be an animal's appearance. An animal with either an odd-looking or abnormal appearance/features or one that is weak and defenceless falls into the unclean category as well.
More specifically, Mary Douglas shows the connection between the kind of animals that are permitted/forbidden to be eaten and the kind of people God wants Israel to be in its relationships. The theme of (un)cleanness in Leviticus and Deuteronomy symbolises creation's orderliness and everything in its own sphere. (so unclean animals represent a lack of wholeness or integrity in not belonging to the on sphere.) Yet something more is going on: animals that are unclean appear to be either:
Vulnerable animals (defective in appearance or characteristics)
This has a parallel to human relationships.
With regards to the predatory aspect, animals of the air (hawks, and carrion-eaters such as vultures) are forbidden in Israel's diet because they themselves have consumed blood; they're predatory. Remember the prohibition against eating blood in Genesis 9:4, suggesting respect for life, which is in blood: "the life of all flesh is its blood" (Lev. 17:14).
As for land animals, quadruped plant-eaters - rather than carnivores - may be eaten (once the blood has been drained). The fact that they (1) chew the cud and (2) have split hoofs (whether domestic or wild) are clear indications that they never eat blood and that is not predatory (Lev. 11:3). The borderline cases - the pig, the camel, the hair, and the coney -forbidden because they fit one but not both criteria. So land animals that are predators must be avoided because of their contact with blood. In a symbolic way, "they break the law."
Some scholars point out another symbolic feature. Besides unclean animals that represent predation, there are others that represent victims of tradition. For instance, prohibited aquatic animals (without scales and fins) symbolically lack something they "need"; this is a picture of vulnerability. This distinction between clean and unclean animals also serve as a picture of justice and injustice in personal relationships. Let me quote Douglas at length:
The forbidden animal species exemplify the predators, on the one hand, that is those who eat blood, and on the other, the sufferer from injustice. Consider the list, especially the swarming insects, the chameleon with its lumpy face, the high humped tortoise and beetle, and the ants labouring under their huge loads. Think of the blindness of worms and bats, the vulnerability of the fish without scales. Think of the human parallels, the labourers, the beggars, the orphans, and the defenseless widows. Not themselves but the behavior that reduces them to this state is an abomination.
No wonder the Lord made the crawling things and found them good (Gen. 1:31). It is not in the grand style of Leviticus to take time off from cosmic themes to teach that these pathetic creatures are to be shunned because their bodies are disgusting, vile, bad, and more than it is consistent with its theme of justice to teach that poor are to be shunned. Shunning is not the issue. Predation is wrong, eating is a form of predation, and the poor are not to be prey.
(Forbidden Animals in Leviticus pg.22)
What's most clear in all of this is that holiness and predatory behaviour don't mix. Holiness represents respect for human life, and the eating of blood (symbolising violent death) represent predatory activity. Clean animals don't represent virtues in their own bodies, just as unclean animals' bodies don't represent vices. They just follow the "rule" of avoiding blood.
If scholars who claim that certain unclean animals symbolise vulnerability and defencelessness are correct, then this representation of the oppressed the alien, the widow, the orphan (Deut. 14:29; 16:11; cf. Isa. 1:17) would serve as a reminder that it ought to be respected.
Israel's entire way of life down to the very food they ate (or didn't eat) mattered to God. The diet served as a reminder of the holy and the unholy: Israelites were to avoid the unholy activity of preying upon the vulnerable in society.