Boyd’s Bible Dictionary:

Of cows, goats, camels, and sheep a favorite Oriental food
(Gen. 32:15; Deut. 32:14). Symbol of fertility (Josh. 5:6; Heb. 5:12).

Concise Bible Dictionary:

This from cows, goats, sheep and camels is plentifully used in the East. It is drunk in its natural condition, or shaken up in a skin into a sour curdled state, called in scripture “butter.” Flocks and herds were so numerous, and wild honey so plenteous, that the country was fitly called “a land flowing with milk and honey” (Gen. 18:8; Ex. 3:8, 17; Ex. 18:19; Joel 3:18). Milk is regarded as such a necessary article of sustenance that it is associated with wine to prefigure that which grace now supplies, without money and without price, and which will be supplied to Israel in a future day (Isa. 55:1). In the description of Israel’s promised glory it is said, “Thou shalt also suck the milk of the Gentiles” (Isa. 40:16). In the New Testament milk is a symbol of the sustenance of God’s word (1 Peter 2:2); and is also referred to as food for infants in contrast to the solid food used by adults (1 Cor. 3:2; Heb. 5:12-13).

From Manners and Customs of the Bible:

Genesis 49:12. His teeth white with milk.
This is meant to represent the pastoral wealth of Judah. Milk is, in the East, a very important and highly valued article of diet. In India it is sometimes said of a rich man, “He has abundance of milk.” A saying somewhat similar to this, but more closely resembling the text, is applied to one who has a plentiful supply of milk: “His mouth smells of milk.”