= Yeast extract
<Yeast extract> is the water-soluble material obtained from broken-down <yeast> cells. In the preparation described by ATCC, <Saccharomyces cerevisiae> undergoes <autolysis>: its own <enzymes> break down cellular material, including <proteins> into <peptides> and <amino acids>. The resulting ingredient can supply nitrogen-containing <nutrients>, <vitamins> and other soluble constituents to a <culture medium>.<Mycology Culture Guide by ATCC>[printed page 15, Media Ingredients: Yeast Extract]{ref}
The word extract matters. The ingredient is a <mixture> of recovered cellular substances, rather than a count of growing <yeast> cells or one chemically defined nutrient. Adding <yeast extract> therefore changes several inputs together. Its known total mass does not specify every individual constituent's <concentration>.<Mycology Culture Guide by ATCC>[printed pages 14–15, Media Ingredients and Note 4, differing preparation compositions]{ref}
An original accounting example illustrates the distinction. Suppose an analysis of a hypothetical dry extract finds 2 milligrams of nutrient X and 1 milligram of nutrient Y per 100 milligrams of extract. A 5-milligram addition then supplies 0.10 milligrams of X and 0.05 milligrams of Y. Equal additions of total extract would supply different amounts of X if another preparation had a different measured X fraction. These values are invented and do not describe a commercial product.
A <defined medium> can vary an explicitly specified component while holding the others fixed. With <yeast extract>, interpreting an observed growth change requires considering the accompanying <mixture>. Neither the total mass nor one measured constituent establishes that all remaining nutrient inputs were matched. The distinction helps separate a convenient composite ingredient from a chemically isolated experimental variable.
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