A pennyworth of barm

Text 1: Baking days, by William Gedge

An original autobiography written in a nineteenth-century voice; a period simulation, not an archival document.

On baking days my mother sent me to the brewery yard behind the Wheatsheaf for a pennyworth of barm, the frothy yeast skimmed from the top of the vats. The brewer’s man, a giant called Noakes with forearms like hams, would ladle it into my little stone jug and stop the neck with a twist of cabbage leaf. The barm was alive, he said, and must not be shaken or chilled, or the bread would sit in the tin as flat as a slate. So I carried the jug home against my chest with both hands, walking as slowly as a funeral, while the other boys raced past me to school. My mother never once thanked me, but she always gave me the first crust.

Give one phrase that shows the narrator was careful not to shake the barm on the way home.