If pasta water gets poured down the drain by accident, the closest fix for thickening sauce is whisking 1 teaspoon of cornstarch into a little water and stirring it into the simmering sauce; for added flavour, chicken or vegetable stock is a good option.
How do you make a starch slurry?
Pasta water's ability to bind sauce comes from the starch that leaches out while boiling; to mimic that, whisk 1 teaspoon of cornstarch into 2-3 tablespoons of cold water until smooth, then stir it slowly into the simmering sauce. Add it while stirring continuously rather than pouring it in all at once, or it can clump.
When is chicken or vegetable stock the better choice?
In creamy or cheese-based sauces where you want depth of flavour as well as body, a few spoonfuls of vegetable or chicken stock give a richer result than pasta water would. These liquids don't contain starch though, so they bind the sauce to the pasta less effectively; a bit of starch slurry can be added alongside if needed.
Can whey be used?
The leftover liquid from making homemade yogurt or curd cheese is mildly acidic and rich in protein; stirred into creamy sauces by the spoonful, it adds both body and a light tang. Whey's saltiness varies from batch to batch, so taste the sauce's salt level before adding it.
Why does pasta water matter so much, and when do you really need it?
In minimal-ingredient dishes like cacio e pepe, where starchy water is what binds the whole sauce together, no substitute fully replaces it; for that kind of recipe, the most reliable fix is remembering to set aside a cup of water next time before draining. In tomato- or meat-based sauces that already have their own body, the lack of pasta water is barely noticeable.
Whichever alternative you use, keeping the heat low while tossing the sauce with the pasta gives the mixture extra time to come together and helps keep it from separating.
What's the most common mistake when fixing a sauce this way?
The most common mistake when using a starch slurry or another substitute is pouring it in cold straight into a simmering sauce; that sudden temperature difference can cause the sauce to clump or split. Stirring a spoonful of the hot sauce into the substitute liquid first to temper it, then adding it back in, gives a much smoother result — a difference that's especially noticeable in creamy sauces. Transferring the pasta straight into the sauce pan and tossing them together for a couple of minutes helps the sauce cling to the pasta better regardless of which liquid was used; this step alone makes up for a lot of what missing pasta water would have provided, and serving right away without letting it sit keeps the consistency from changing before it reaches the table.