Several by-products are obtained after the fermentation and distillation of sugarcane molasses.
The molasses are diluted with water to 40 Brix and heated to 80°C. Sulphuric acid is added to pH 4, and after one hour the solution is centrifuged, thereby removing about 80% of the calcium. The hot clear molasses is cooled, diluted and transferred to the fermenting vessels, where yeast is added. During fermentation the sugars are converted into alcohol. After about forty hours the yeast dies and settles to the bottom of the fermenting vessel, forming rum vat yeast.
The alcoholic liquid (wash) is drawn off at a level of about 20 cm from the bottom, centrifuged and passed into the stills. The solids from the centrifuge, mainly yeast, are placed in settling tanks. Once a day the material at the top of the settling tanks is recycled through the centrifuge and the bottom sediment is dumped. This sediment is similar in composition to the rum vat yeast. The wash contains about 8% alcohol, which is distilled off, leaving a residue that is called spent wash, molasses distiller's solubles, dunder, stillage or distillery slop. The by-products are dried yeast, 1.5% of the weight of the molasses, and spent wash, four times the volume of the molasses or 28% by weight.
The yeast spoils quickly if it is not dried, preferably on roller drums, as it has been shown that rum vat yeast dried at a high temperature (130°C) is nutritionally better than that dried at lower temperatures.
The residue after distillation has a very high content of ash, especially calcium sulphate. If the residue is to be used as animal feed, the calcium must be removed from the molasses prior to fermentation by means of the Reich process.