What is the chemical formula for yeast extract?

Commercially, yeast extract is most commonly made by adding salt to a yeast suspension to make the liquid hypertonic, which causes the enzymes in the yeast cells to start breaking down its own cell walls and release the fluids contained within the cells. This is called autolyzation, or self digestion. Gently heating the liquid aids further enzymatic breakdown of complex proteins into simpler amino acids. The remaining pieces of cell walls (yeast hulls) and other residual solids are then filtered off, leaving the liquid extract. The extract can be reduced to a paste, such as found in commercial products like Marmite or Vegemite, or dried into a powder.

Another method of yeast extraction can be easily achieved at home by using a papaya enzyme (papain), or a pineApple enzyme (bromelain, found in meat tenderizers) to aid in the yeast cell autolyzation process. Both the vegetable enzymes and the yeast enzymes will be much more active if the broth is slowly heated to about 50 degrees C, and held at that temperature for a few hours.

Or you could simply warm a yeast suspension to 50 C for 24 hours or more. That would also produce a fairly high yield of yeast extract, without the high salt content of commercial products. At 55 C and higher temperatures, however, the enzymes will deactivate, stopping the autolyzation process.