They set the balance. Acid cuts fat, reins in sweetness, denatures protein and triggers leaveners. When a dish tastes like something is missing, it is usually acid rather than salt.
Heat drives the aroma out of acid. Squeeze the lemon at the table, not into the pot, and add vinegar at the end.
Add it off the heat: boiling drives the aroma off and leaves nothing but sharpness.
Sealed and out of the light it keeps for years; cloudiness and a mother forming at the bottom are not spoilage.
Grape, apple and spirit vinegars are gluten-free. Malt vinegar is the exception: it is made from barley and carries gluten.
This ingredient carries no meaningful carbohydrate, so it has no glycaemic index. On its own it will not move blood sugar; what you eat alongside it decides that.
"They go well together" is not an explanation. Every pairing below has a chemical or structural mechanism under it.
Olive oil
→
Acetic acid lowers interfacial tension and mechanical agitation forms a temporary emulsion. It is unstable because vinegar brings no emulsifier, which is why mustard or tahini is added in practice.
Sugar
→
Sucrose competitively suppresses sour perception at the receptor, which is the entire basis of a sweet-sour pickling brine.
Lentil
Acid during cooking keeps pectin insoluble, so pulses simmered in vinegar or tomato stay stubbornly hard.
Acetic acid produces a small but reproducible reduction in postprandial glycaemia, plausibly by slowing gastric emptying. Real, but modest.
LemonpH 2.2 · 5% citric acid
Yo
YoghurtpH 4.4 · splits above 85 °C
Pm
PomegranatepH 3.0 · anthocyanin turns blue in base
Tp
Tomato pastepH 4.3 · glutamate 250 mg/100 g
Ne
Pomegranate molasses21.6% water · 308 kcal
Lt
Citric acid (food grade)?
Bi
Red pepper paste?
S2
Pickled cucumber94.3% water · 12 kcal
Nothing matches that.
Garlic 
Aubergine 
Chickpea 
Salt 
Water