sashum voshum, the symbiotic bacterium which allows those born to the sealines to digest their food properly, despite an intestinal tract which has been shortened to make way for gills and water-lung
The taste of yeast was a sign of the activity of that bacterium known to Chalakanesian science as sashum voshum: a symbiotic organism which lived in the gut of all those who were descended from the sealines. Those who owed genes to the sea had intestinal tracts which were shorter than the human norm, since their water-lungs took up space which would otherwise have been occupied by intestinal coils. To extract the full food-value from what they ate, they needed the fermentative powers of the sashum voshum bacterium to accelerate the digestion of their food.
(For this reason, those bred from the Mer could not take antibiotics, since any antibiotic would kill off the sashum voshum, leaving the antibiotic-taker without the means to properly digest food, the aftermath of such a disaster being either grave sickness or death).
Now, after the vast expenditure of energy needed to work underwater, to fight against the cold of the sea, and to supply nutrition to the adaptive skin which had mated with him, Heineman was possessed of a furious hunger. He could eat and eat and eat and eat. Attuned to the needs of his body, the sashum voshum bacteria in his gut were working furiously, hence the strength of the taste of yeast in his mouth. [EoH Ch3]