‘Ephraim, he mixes himself among the peoples,
Ephraim is a cake not turned.'
The use of ‘Ephraim' may indicate a time when what was mainly left of Israel was that part which was on the mountain of Ephraim and its surrounds, around Samaria. The complaint here is that they have turned to the nations instead of to YHWH. They have mingled among the peoples, becoming as one of them. The result is that they are like a cake not turned over. This might mean ‘half-baked' like a piece of dough being heated on a hot stone and only baked on one side, because they are incomplete, having missed out on what is really important. Or it may mean left lying flat instead of being turned over in readiness for consumption, and thus unready and unprepared. It may include the thought that they have not made themselves into a fit state to be available to YHWH.