Mushy Peas (Marrowfat Peas Collapsed with Soda)

Variations

• Without mint: the northern chip-shop version has none. Mint belongs to the southern garden-pea tradition, so whether it goes in is a regional question.
• Sweetened: a pinch of sugar, as some market stalls do it.
• With split peas: skinned, split peas collapse without any soda, but they give a smooth yellow purée with no whole peas left in it. That is closer to a different dish.
• With fish and chips: that page names this as "the classic British side" and stops there; this is the rest of it. Its job is to take the fat off the fried food, which is also why vinegar comes with it.
• Pie and mash: in London it goes with a pie and parsley liquor instead.

Tips

• What the soda dissolves is not the skin. Dal sets the pulse question out cleanly: "what holds a pulse in shape and keeps water out is its seed coat", pozole stakes a whole technique on dissolving that hull with alkali, and skinned split masoor collapses in twenty minutes. Mushy peas is a case none of those three covers. A marrowfat pea keeps its coat, and the soda is aimed somewhere else entirely: at a raised pH the pectin of the middle lamella — the glue between the cells inside the pea — dissolves, so it goes from the inside while the skin stays on. Whole peas sitting in the purée of collapsed ones, as in the photograph, is what that looks like.
• Rinse the soda off before boiling. Left on it tastes of soap, alkali destroys thiamine, and it keeps working past the point of collapse until the peas are paste. Throwing away the soaking water feels wasteful and is not optional.
• Salt at the end — though not for the reason usually given. Recipes say salt firms the skins, and soaking explains why that is the myth it looks like: sodium displaces the calcium cross-linking the pectin, so salted water softens skins rather than firming them. The reason to keep it out of the pot here is a different one — this dish wants some skins still whole, and the seasoning cannot be judged until the water has gone down and the amount is settled.
• It has to be marrowfat. Not young peas picked and frozen, but peas left to mature and dry hard on the plant — starchy and low in sugar, which is why they do not taste sweet after all that boiling and can sit next to something fried.
• The same alkali does other jobs on the pages next door. On bagels it is "a colour device", and pretzels push that as far as lye; Lanzhou lamian's kansui is there to make gluten stretch. One pH, different purposes.
• Do not blend them. A blender gives you smooth pea soup. Half-crushed is nearer the definition of the dish.