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What I'm asking is not chemically how they do it, but rather how do each practically thicken.

For example:

  • Thickening with wheat flour is thickened when at boiling point - cooking further does not destroy the sauce/liquid. The thickening effect is increased alot when cooling.
  • Thickening with potato flour yields a goopy, gummy liquid when boiled. Etc

What I'm mainly wondering about is:

  • Wheat flour
  • Potato flour
  • Potato starch
  • Corn flour
  • Corn starch
  • Gelatin
  • Tapioca

Of course, if any of these can be grouped together, that's great aswell!

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There is no difference between corn flour and corn starch; the former is a regional (UK) name for the same product - unless you are referring to cornmeal, which is never used as a thickener. Potato flour does technically exist but, as with corn flour, most people actually mean potato starch when they say potato flour; the most common use of real potato flour is in Passover (Jewish) cooking and it's not really worth mentioning as a thickener because it doesn't have a neutral flavour. Starches thicken; flours make dough. – Aaronut Jan 18 '12 at 23:47
There's also modified corn starch (in Canada). Thickens near the boil point but not quite. Nice advantage is that it keeps the same thickness hot or cold unlike other starches and doesn't break down when frozen. Same look and texture as regular corn starch. – Chef Flambe Jan 19 '12 at 3:47
@Aaronut, isn't the whole point of a roux to thicken? This is used in many sauces, gravies, etc. – Ray Jan 19 '12 at 17:07
@Ray: The point of a roux isn't just to thicken, it's also there for flavour - it's the base of the sauce, not an additive - flour cooks and some roux-based sauces actually involve cooking it to very dark stage. Maybe somebody with more formal training can correct me if I'm wrong, but I do not believe that wheat flour is chosen for its thickening properties. – Aaronut Jan 19 '12 at 19:35

There are a number of different ways in which gelling agents are classified. Off the top of my head:

  • Viscosity (firmness/thickness) of solution and gel forms
  • Thermoreversible/irreversible (does it "melt"?)
  • Hysteresis (water loss)
  • Hydration, melting, and setting points
  • Appearance (in particular transparency)
  • Sensitivity to heat, cold, alcohol, and pH

Starches are all very similar in this regard. They all form viscous solutions and gelatinize to a stiff and elastic consistency. They are all opaque-to-translucent, have very slow hysteresis, hydrate in cold water and gelatinize in hot water. They tend to tolerate some alcohol and a fairly low pH.

Tapioca has the lowest gelation point, and the most heat stability, and potato starch is similar. Corn starch is particularly weak against acid, heat, and cold (especially freezing), unless it's been modified. There's also arrowroot which is somewhere in between.

Wheat flour is something completely different because it has proteins which form gluten in the presence of water and heat. I wouldn't even consider comparing it to the others, it's sometimes used in gravy but generally you would only use flour to make doughs or batters.

As far as gelatin and all of the other E-numbers are concerned, you can find a lot more about their properties in Kymos' Texture and of course, Modernist Cuisine if you want to shell out the cash. There's quite a lot to read about.

Gelatin, specifically, is thermoreversible, hydrates (blooms) in cold water, disperses in hot water, sets below 15° C, melts at 25-40° C (mouth temperature), tolerates alcohol but is weak against all of the other usual inhibitors, and forms a soft, transparent gel. It's nothing like starches but is a lot like a lot of the other E numbers (Gellan, Agar, Carrageenan, Pectin, etc.) Refer to the aforementioned link for more information.

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