Acidifying it, and which acid
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Buy to a stated strength
Vinegar sold for food use carries a stated acidity. Buy to it, buy by the case from a consistent supplier, and do not substitute without recalculating. A vinegar of unknown or lower strength is the most common cause of an under-acid finished product, and it is entirely avoidable by how you purchase.
They are not interchangeable
- Vinegar brings acidity and a lot of flavor, and it brings water, which thins the sauce.
- Lemon juice is used where the published guidance specifies it, particularly for tomatoes, and bottled is specified over fresh because fresh varies.
- Citric acid is dry, so it acidifies without thinning, which matters in a product whose viscosity is part of its process.
That last point is worth sitting with. In a thick sauce the choice of acid changes the viscosity, and viscosity changes the heat penetration, and heat penetration is the process. The acids are not swappable even when the finished pH comes out the same.
Dilution counts
Water added anywhere is water added to the acid. A recipe that reaches its figure before a late addition of stock does not necessarily reach it afterwards, and the measurement that matters is on the finished settled product rather than on the pot at the moment you were paying attention.
Fresh lemon is not a substitute for bottled
The published guidance specifies bottled lemon juice where lemon juice is the acidulant, because its acidity is standardized and a fresh lemon is not. This is one of the few places where the less appealing ingredient is the correct one, and it is worth knowing before somebody suggests fresh would taste better.
Acidify at a point you can control
Adding acid to a hot kettle at the end, off the heat, or before reduction are three different processes with three different finished figures, because reduction concentrates and heat drives off some of what you added. Pick the point, write it into the specification, and do it the same way every time. A batch acidified at a different moment is not the same product even when the recipe sheet matches.