After all these basic sauce recipes that will allow you to master the different techniques used in sauce making, let's have a closer look at the influence of the thickeners on the sauce flavour elements. It is now, understood that the thicker the sauce is, the least flavour it will have.
The first explanation is that as thickeners do not or have very little flavour of their own (excepted brown roux), they only dilute the flavour already in the sauce.
The second one, is that thickening agents decrease the aptitude of the sauce to deliver its flavour components. When the thickeners bind with all the elements of the sauce, they also trap some of the flavour molecules in a way that our palate never senses them. By thickening the liquid phase of the sauce, it reduces its fluidity (the speed it moves). It also slows the capacity of the sauce to release its flavour molecule so that our taste buds and nasal passages miss them as they go by.
The third reason why a sauce may feel less flavourful, is that tasty molecules tend to be more fat soluble than water soluble. So, the fat globules dispersed in the liquid phase will catch aroma molecules therefore decreasing the sauce aromatic intensity. Amylose starch molecule, also bind aroma molecules. The aroma molecules in turn make the starch molecules more likely to bond with each other into a grainy milky-coloured particles.
Finally, wheat flour has a tendency to bind more sodium than the pure starches. Long-chain carbohydrates, including starch, reduce the apparent saltiness of the sauce either by binding some of the sodium ions to themselves or by adding another sensation (viscosity) for the brain to attend to.
In general, thin sauces have a more intense and immediate flavour than the same sauce with added thickeners. In the other hand a thickened sauce will release its flavour more slowly and persistently. Each effects has its uses.
Many sauces are thickened by reduction, not by adding thickeners. It removes some of the continuous phase by boiling the water off. This technique does not decrease the flavour because whatever flavour the sauce's particles or molecules can bind have already been bound. In result, the thickeners already present in the sauce as well as the flavour molecules become more concentrated, thickening the sauce and intensifying its aroma. It is this principles that are used in tomato fondues, jus, coulis and cream sauces.
And a last little thing to conclude this post, all the sauce thickened with a roux or beurre manié (handled butter) require more added salt. It may be good thing to take this into account if suffering from high blood pressure.