If I eat cookies and subsequently brush my teeth such that there is still small amounts of cookie material stuck to my teeth, the toothpaste will not foam as well as usually, making the process less enjoyable. This effect is consistently reproducible with most (sweet) cookies but any not other foods I have tried so far. The toothpaste contains typical foaming agents, such as sodium lauryl sulfate. What could be the mechanism behind this effect?
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$\begingroup$ You might want to try with other fat-rich foods (say, buttery seared fish?) to verify @iad22agp 's answer and advance your observations. $\endgroup$– ditsukeNov 29, 2017 at 7:39
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My first take on this is that the cookies contain fat, which "consumes" the surfactants in the toothpaste (like sodium lauryl sulfate) so it does not foam up the way it normally would. You might want to see what Wikipedia says about micelles and how they interact with fats. A similar effect can be seen when washing really greasy dishes with an insufficient amount of dishwashing liquid - in which case no suds will form. In your case I don't understand why the effect would not also be observed with other fatty foods such as chocolate or popcorn.
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1$\begingroup$ I tried this with chocolate today and it definitely seemed to reduce the foaming, so your hypothesis seems plausible. Maybe it is the fattiness combined with the tendency to stick to teeth that makes the effect so prominent with cookies. $\endgroup$– ntoskrnlNov 28, 2016 at 15:33