There are two main reasons why you might want a salad dressing:
To make a tasteless salad tasty.
To tone-down a too tasty salad.
As a consequence, there are two different main types of dressing:
In either type of dressing, the main taste ingredient is usually vinegar or other acidic liquid, for instance lemon juice. The other liquid ingredients chiefly dilute the acid as both vinegar and lemon juice are too powerful, undiluted.
Oil based dressings
Oil in a dressing has three main functions:
To emulsify with the aqueous components (such as vinegar) and thicken the mixture so that it sticks better to the salad.
to take some of the heat out of hot leaves, seeds and spices, such as watercress and mustard.
To alter and transport the flavour of the dressing and ingredients.
It is often said that oil is needed to help the gut absorb some of the oil-soluble vitamins and minerals If you think about this statement a bit, it is clearly untrue:
All life be it animal or vegetable, is water based. Any essential oil-soluble substances are already likely to be bound up to some agent that renders them water-soluble.
Bile, produced by the liver, is a digestive fluid whose main purpose is to break down fats and to fender them absorbable by the gut. That absorption also includes fat-soluble nutritive components.
Humans have evolved from vegetarian ancestors. As we evolved - we never evolved requiring oil-based dressings on our vegetable food!
An emulsion is a mixture of oil and water such that particle size is small enough that the two liquids behave as one.
For a dressing, this can be accomplished by shaking the oil and watery component together immediately before applying to the salad. however this emulsion is very unstable and soon separates into oil and vinegar again - but not too soon to to eaten on your salad!
The dressing will work a little better if some emulsifying agent is added to the mixture.
An emulsifying agent is one that is wetted both by oil and by water.
Emulsifying agents include
Plain English mustard powder.
Oil free dressings
Plain vinegar is too strong to be used as a base, so it needs to be diluted. You could use water, buy the dressing is much tastier made with apple juice, orange juice or some other fruit juice - or a mixture of juices.
Such a dressing won't be thick enough to adhere to the salad, it will simply run off. So it needs some thickener. You can use yoghurt. Most commercial dressings use xanthan gum - which you can get from health food shops, or on-line.
This is supplied as a powder. One level teaspoonful will thicken about a litre of liquid. I use ½ a teaspoonful to 400ml of liquid. However you cannot dollop ½ into the liquid and stir: you will end up with a very lumpy pot of gunk! There are two methods of mixing.
Mix the dry xanthan gum with a small amount of oil to make a paste about the consistency of cream.
Slowly pour into the oil/xanthan paste the liquid to be thickened, stirring all the time.
The oil stops the xanthan absorbing water until it is well dispersed in the liquid.
Sprinkle a thin layer of xanthan gum powder on the surface of the liquid to be thickened, Leave to stand until all the gum has been thoroughly wetted and taken into the liquid.
Stir or shake vigorously to homogenise the mixture.
Sprinkle another layer of xanthan gum and repeat until all the mixture is made up.
I successfully used the time method sprinkling a whole ½ teaspoonful on the top of 100ml of vinegar and leaving it 48 hours. There were some low-density clots which dispersed properly when the whole was vigorously shaken. The thinner the layer of powder with each addition, the quicker it will be absorbed.
Vinegar is probably the main active ingredient of most dressings: certainly it is the main taste in vinaigrette, with the oil there mainly to dilute it. The visual effect of the dressing implies that wine or cider or white balsamic vinegar are best. However there's no other reason why malt vinegar cannot be used.
If you do your own pickling, you will have left-over vinegar. This can be used in the dressings: the vinegar from pickled onions gives added flavour but I have used red cabbage, walnut and other pickling vinegars - or a random mixture.
Vinegar that has been used once for pickling is less acid: you will probably want to use a bit more vinegar in whatever dressing you are making.