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I was surprised to read the answers to a recent question on thickening - two of the three, including the accepted answer, suggested using cornstarch instead of flour to save calories.

I have always assumed that flour and starch are so close in binding power per weight that there is no reason to change any proportions when substituting, and also that the starch source doesn't matter. OK, I know about amylose and amylopectin being present in different amounts in different plants, but my impression is that the same amount of both gives a different thickened texture, not different thickness.

Now the answers and comments there contradict my impression. So, I would like to know: how much is the difference in thickness? If we take some reference, for example

5 g of cornstarch per 100 g water, mixed and then cooked until thickened

how much of some other starch (potato, arrowroot, whatever) and how much wheat flour (AP or 550) is needed to achieve the same thickness as the reference?

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  • For what it's worth, my comment there on the accepted answer was based on googling "cornstarch vs flour thickener" and finding a few results that said to use half as much cornstarch as flour for the same volume to thicken.
    – Cascabel ♦
    Mar 4, 2017 at 17:27

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When it comes to food 'thickness' (viscosity) is frequently discussed in 'relative terms' but not 'measurable units'. As such I'm not sure your question can be answered in 'absolute' terms.

Turning to Ratio where it discuss "Stocks and Sauces"

  • A Roux is 3 parts flour to 2 parts fat and the 'thickening ratio' is 10 parts liquid to one part roux. vs.
  • A Slurry 1 part cornstarch to 1 part water(by volume) and the 'thickening ratio' is 1 tablespoon slurry to 1 cup liquid.

Arrowroot and potato starch can be substituted, in equal volumes, to cornstarch but they have differing outcomes based on the application (stews vs pie filling) where the difference is more than just 'thickness' but the things like the dairy or acid content of the stuff to be thickened.

In the absence of an objective standard for 'what is thickness' I don't believe you can get a fully satisfactory answer to this question.

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    There is an objective standard of "what is thickness" and a whole discipline which defines it: en.wikipedia.org/wiki/Rheology. I am pretty sure that food technologists have applied it to food, including starch- and flour-thickened water. It is not in my books aimed at the home cook, so I hoped someone will have come across the more technical sources. The Ratio reference is interesting, I wonder if the output of the two is supposed to be equally thick. I made some calculations: the input is 3.4 g starch (slurry case) vs. 6 g flour (roux case) for 100 ml liquid.
    – rumtscho ♦
    Mar 4, 2017 at 16:30
  • There are many standards to measure thickness, what I should have said was there are none commonly (if it were common I suspect one of us would have come across it) in use when it comes to food. With oil viscosity is measured by flow rate at 100°C (as 10w40). With food 'ideal' temperature is dependent on the dish (custard vs. chowder) and time. My want for a more objective standard when it comes to food would need to account for all of that.
    – Cos Callis
    Mar 4, 2017 at 17:12
  • Yes, I agree that there is no widespread standard, and I appreciate your answer!
    – rumtscho ♦
    Mar 5, 2017 at 22:03

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