In a large pot, bring the water to a boil. Gradually stir in the maize meal, sugar, and salt. Reduce the heat to low and cook, stirring frequently, for about 10-15 minutes, until the mixture thickens.
Remove the pot from the heat and let the mixture cool to lukewarm.
Dissolve the yeast in a small amount of warm water (about 1/4 cup) and let it sit for 5 minutes, or until it becomes frothy.
Stir the dissolved yeast into the cooled maize meal mixture. If using lemon juice or vinegar, add it at this stage.
Cover the pot with a clean cloth and let the mixture ferment in a warm place for 6-12 hours, or overnight. The mixture should develop a slightly tangy flavor and a bubbly texture.
Stir the fermented Emahewu before serving. It can be enjoyed as a refreshing, slightly sour beverage. Serve chilled or at room temperature.
Enjoy your meal!