Well. Think about it. What food was considered 'unclean' when the bible was written? Keep in mind this was in the era of 0050 to 0300 AD, when (last time I checked) there was NO way to refrigerate food, so the fastest spoiling foods were considered 'bad' Also, the 'dirtiest' animals were considered bad. Of course this must be taken with a grain of salt, since man knew nothing about general health, sanitation, boiling water to prevent intestinal illness, and other generally ignorant views were prevalent then which are STILL being countered to this day.
God's distinction between clean and unclean animals was about more than one's diet. Many of God's regulations were to remind His people, Israel, to be holy and set apart to worship the one, true God. The original audience of Genesis 7, during Moses' day, would have associated the reference to clean animals with the animals God had set apart for their diet as well as what were used for sacrifice. Read more: http://www.gotquestions.org/animals-clean-unclean.html#ixzz2fRF8KKm7
In the old testament it was any food God said they couldnt eat. This was mainly for health reasons to prevent food born illness and disease. Things like pork and shell fish. Today there are no dietary restriction since the new covenant under Christ.
Unclean food can be recognized by one very significant trait, besides the cloven hoof and eating it's own cud. What does IT eat? Scavenger animals, the ones that eat anything, like, pigs, vultures, lobsters and so forth should not be eaten. Common sense can be used with not eating something found dead. You don't know what killed it or how long it's been there. The bible described that as "died of itself". :-)
That is called the laws of kashrut. They are very long and pretty complicated. Some basic rules are not to mix milk and meat, land animals must be herbivores, chew their cud and have split hooves. Fish must have find and scales. For birds there is just a list of the birds we are allowed to eat.
Jesus declared all foods "clean" in Mark 7:19. The Old Testament food laws were specific to the Jews and were temporary, from a Christian Biblical view. In Acts 10:15, God said, "What God has made clean, do not call common." For the Christian, one can eat or refrain from eating certain foods. It is up to the individual. But there is no spiritual merit either way,
The unclean food of the Bible are scavengers and not fit for humans to eat. Look at what these animals eat including disease. Mans original diet was vegetarian and it wasn't until after the flood that God allowed the eating of clean animals, fowl, and fish.