Horsford added cornstarch to baking powder to stop premature reaction

Through the 1800s, bread relied on yeast for leavening, but yeast was a living organism whose potency varied unpredictably batch to batch. Chemists found in the 1840s that cream of tartar mixed with baking soda released carbon dioxide gas far more predictably, but cream of tartar was a byproduct of European winemaking that American bakers had to import at real expense — Harvard chemist Eben Horsford set out in 1854 to find a domestic substitute, and by 1856 had patented monocalcium phosphate, made from treated animal bone, as a cheaper acid replacement.

The obstacle wasn't the chemistry, it was the packaging. Monocalcium phosphate (the acid) and sodium bicarbonate (the base) start reacting and releasing their leavening gas the moment they're combined in the presence of moisture, including ordinary ambient humidity. The standard practical fix was to sell the two ingredients in separate packages and have the baker measure and combine them fresh at the moment of use, which kept the chemistry safe but pushed the work, and the risk of getting proportions wrong, onto every individual baker.

Added cornstarch as a desiccant to absorb moisture, letting acid and base sit together inert, so the factory pre-mixed exact proportions and shipped ready-to-use.

Cornstarch absorbed ambient moisture, keeping acid and base dry and unreactive in storage.

Dry ingredients could be pre-mixed at the factory, ensuring exact proportions every time.

Bakers no longer had to measure and combine two chemicals, reducing error and effort.

Consistent proportions gave predictable rising, making baking powder reliable and popular.

When two components must react later but not early, suppress the environmental trigger (moisture, heat, pH) instead of separating them physically.

Because the cornstarch buffer suppressed the premature reaction rather than trying to keep the reactive ingredients apart in separate containers, every can shipped with a predictable, factory-controlled ratio of active ingredients — bakers using the same amount of baking powder got the same rising action every time, something neither unpredictable yeast nor a baker's own hand-mixed acid-and-base combination could reliably guarantee. Modern baking powder became Rumford's primary product.