Re: Why, after we brush our teeth, do some foods taste so horrible?

I found two web sites that answer your question.  The first (reference 1) 
was written by Robert Krampf, who runs a science education company in 
Florida, and is maintained by About. com and the second (reference 2) is 
from a Cyber Science journal called The Helix, which is from Csiro
Publishing.   Both of these claim that the disgusting tatse of orange juice 
that you sense just after brushing your teeth (and I presume your 
tongue)with toothpaste is the result of detergents (e.g., sodium lauryl 
sulfate), which are present in toothpaste,  blocking the functioning of 
your sweet taste buds without inhibiting (reference 1), and possibly even 
enhancing (reference 2), the functioning of your sour and bitter taste 
buds.  Oranges contain acids (malic acid and ascorbic acid- see reference 3 
from a Mad Sci site).  The pH of orange juice is 4.2 according to 
information found in the MadSci site in reference 4.  Acids 
taste sour.  In fact, the word acid comes from the Latin word acidus, which 
means sour (see MadSci site in refernce 5).  Normally, the sour (acidic) 
taste is covered (at least partially) by the sweet taste produced by the 
sugars that are present in the orange.  However, if your ability to taste 
the sweetness is blocked, then the acidic taste will predominate.  A 
similar, but less mechanistic, conclusion is found in a 1997 Mad Sci answer 
to this question (reference 6), which states that "brushing your teeth may 
stimulate the taste buds on your tongue...". 


REFERENCES:
1  http://kidshealth.about.com/kids/kidshealth/library/weekly/aa051900a.htm

2  http://www.publish.csiro.au/cyberscience/helix/TH49/TH49B1.htm

3  http://www.madsci.org/posts/archives/jun99/927818980.Sh.r.html

4  http://www.madsci.org/posts/archives/mar2000/953662129.Ch.r.html

5  http://www.madsci.org/posts/archives/oct2000/971708740.Ch.r.html

6  http://www.madsci.org/posts/archives/aug97/870196053.Ch.r.html