Why halving a recipe breaks the instructions
Most apps scale the ingredient list and leave the method untouched, so step four still tells you to add both eggs. Here is why that happens and what correct scaling looks like.
Halve a recipe in almost any recipe app and you get this:
Ingredients: 1 egg. Step 4: Add both eggs and whisk until pale.
The ingredient list scaled. The instructions did not. And the instructions are the part you are actually reading while you cook, with your hands full.
Why apps get this wrong
Scaling an ingredient list is easy. It is a column of numbers with units next to them; multiply and round.
Scaling the method is a different problem, because the amounts in a method are buried in prose. "Add both eggs." "Pour in half the stock." "Reserve a cup of the pasta water." "Form into twelve balls." None of those sit in a tidy field. Finding them means reading the sentence and understanding what the number refers to.
So most apps do the easy half and leave the sentences alone. Which is defensible right up until you are standing over a bowl trying to remember whether "both eggs" meant the original recipe or yours.
What has to be true for scaling to be correct
Three things, and the third is where it gets interesting.
1. The ingredient list scales. Table stakes.
2. The amounts in the instructions scale with it. If the list says one egg, step four says one egg.
3. The things that must NOT scale are left alone. This is the hard part, and the reason a naive find-and-replace is worse than doing nothing:
- Oven temperature. 350°F does not become 175°F because you halved the
batch.
- Pan and tin sizes. These change with quantity, but not proportionally,
and usually not at all for small adjustments.
- "Season to taste", "a pinch", "a splash". Already unquantified.
- Times. Halving a stew does not halve the simmer. Some times shift a
little; most do not, and none scale linearly.
- Counts that are not ingredients. "Fold in three from the top" is not an
amount.
An app that multiplied every number in the method would produce a 175°F oven and a confidently wrong recipe. That is worse than the "both eggs" problem, because it looks correct.
How A La Carte handles it
Change the serving count and it rescales the ingredient list and rewrites the amounts named in the instruction text — while deliberately leaving temperatures, times, pan sizes and unquantified amounts alone.
The rule it follows is that an amount is only rewritten when it can be anchored to an ingredient that actually scaled. "Both eggs" is anchored to the eggs in the list, so it changes. "350°F" is anchored to nothing, so it does not. That anchoring is what keeps the rewriting safe rather than clever.
Scaled servings then carry through the rest of the app: build a grocery list and it shops for the number you set; put the recipe on a meal plan and it plans for that number.
Things worth knowing about scaling anything
Independent of any app — this is cooking, not software:
- Baking resists scaling. Chemistry does not care about your convenience.
Halving works reasonably; thirds and odd multiples get unreliable, and the pan matters more than the arithmetic.
- Pan size changes cooking time, and not in proportion. Halve a tray bake
into the same tin and it will be thinner and cook faster.
- Aromatics do not need to scale linearly. Doubling a stew rarely wants
double the garlic. Season to taste, as ever.
- Eggs are lumpy. Half an egg is a real problem. Beat one and use half by
weight, or pick a batch size that lands on whole numbers.
An app can fix the arithmetic. It cannot fix the physics.
Try it
A La Carte is on iPhone and Android, with a 7-day trial that unlocks everything. A quick test: open a recipe that mentions an amount inside a step, halve the servings, and read step four.