What makes a pig not kosher?

Of the "beasts of the earth" (which basically refers to land mammals with the exception of swarming rodents), you may eat any animal that has cloven hooves and chews its cud. Lev. 11:3; Deut. 14:6. Any land mammal that does not have both of these qualities is forbidden. The Torah specifies that the camel, the rock badger, the hare and the pig are not kosher because each lacks one of these two qualifications. Cattle, sheep, goats, deer and bison are kosher.

The medrash on this pasuk says that the reason that the pig is called “chazir” is because in the future Hashem will return (lehachzir) the pig to Bnei Yisrael and permit it to be eaten. According to many Achronim the medrash is to be taken literally; pig will be kosher in the future. The Rama Mipano (Asarah Mamaros Chikor Hadin 4:13) explains that Hashem will make the pig chew its cud, thereby making it kosher.

There are currently teams of scientists working on creating a genetically-engineered pig, so that it will chew its cud, which will make it Kosher.  The problem of the split-hoof is easily fixed by using a species of hog called the 'Mulefoot Hog'.  These are an incredibly rare species, but definitely do exist.  Hopefully it won't be too long before all people observing kashrut will be able to enjoy the fine delicacy that is bacon.

 

Sources: http://www.jewfaq.org/kashrut.htm; http://en.wikipedia.org/wiki/Religious_restrictions_on_the_consumption_of_pork