Biotechnology has been providing humanity with useful products such as beer, bread, and cheese for thousands of years. Today, the biotechnology industry has key roles to play in making our society more sustainable and in reducing global greenhouse gas emissions, as biotechnology allows us to make products from renewable feedstocks. Current biotechnology uses sugars derived from agricultural crops as the main feedstock. Although these sugars are renewable, several steps in the sugar refinery process are coupled to the release of CO2. This makes it difficult in practice to achieve net neutral or extremely low CO2 emissions with sugar-based processes. This thesis focuses on the possibility of using ethanol as main feedstock instead of sugars. Unlike sugars, ethanol can be made directly from CO2 and renewable energy, without the need for agriculture. In this PhD project, research was conducted into the physiology of yeasts when they are grown on ethanol as the only source of carbon. In particular, it was investigated how to grow yeast with a high protein content in an efficient way, to contribute to feeding a growing world population in a sustainable and healthy way.... ... Read more