While the process is secret, the general method for making yeast extract on a commercial scale is to add salt to a suspension of yeast, making the solution hypertonic, which leads to the cells shrivelling up; this triggers "autolysis", in which the yeast self-destructs.
Today, the main ingredients of Marmite manufactured in the UK are glutamic acid-rich yeast extract, with lesser quantities of sodium chloride, vegetable extract, spice extracts and celery extracts, although the precise composition is a trade secret [REF].