What causes that foul taste with combination of toothpaste and orange juice?
Why do juice taste awful?


Awful taste is due to sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES), or sodium lauryl sulfate (SLS)- depending on which toothpaste you use. 

SLES and SLS are surfactants(wetting agent). Both chemicals are added in toothpaste to create foam and make the paste easier to spread around your mouth.

Both chemicals they suppress the receptors on our taste buds that perceive sweetness, inhibiting our ability to pick up the sweet notes of food and drink. Also break up the phospholipids on our tongue.  These fatty molecules inhibit our receptors for bitterness and keep bitter tastes from overwhelming us, but when they're broken down by the surfactants in toothpaste, bitter tastes get enhanced.

Thus, anything you eat will be less sweeter and more bitter! 

Source: mentalfloss


  How to prevent awful taste after brushing?


After a bit research I found few ways to avoid awful taste:


Avoid brushing. (not recommended)
Wait for about 30 minutes.
Look for SLS-free toothpaste.
eat a little bit of cheese first.(suggested by repurposer)
hot water will do the trick.