Process to produce a fermentation product

A process to produce a fermentation product from a ligno-cellulosic feedstock hydrolyzate slurry comprising water, water soluble glucose and water soluble xylose and water insoluble pretreated lignocellulosic feedstock, said process comprising the steps of:

• creating a first conversion medium comprising a first portion of the ligno-cellulosic feedstock hydrolyzate slurry and a yeast capable of converting glucose and xylose to a propagating yeast and a fermentation product;
• allowing the yeast to convert at least 50% of the water soluble glucose and less than 20% of the water soluble xylose of the first conversion medium to a first propagated yeast and a first portion of the fermentation product in a first conversion step having a first sugar-to-cells conversion ratio in a range of from 5% to 25%, wherein the first sugar-to-cells conversion ratio is a percent weight ratio of the glucose and xylose converted to the propagated yeast in the first conversion step to the water soluble glucose and xylose totally converted in the first conversion medium, on a carbon basis;
• creating a second conversion medium comprising at least a portion of the first propagated yeast and a second portion of the ligno-cellulosic feedstock hydrolyzate slurry;
• allowing the yeast to convert at least a portion of the water soluble glucose and xylose in the second conversion medium to at least a second propagated yeast and a second portion of the fermentation product in a second conversion step having a second sugar-to-cells conversion ratio which is less than the first sugar-to-cells conversion ratio, wherein the second sugar-to-cells conversion ratio is a percent weight ratio of the water soluble glucose and xylose converted to the propagated yeast in the second conversion step to totally converted water soluble glucose and xylose in the second conversion medium, on a carbon basis.