Parents know that there is really only one acceptable morning routine. Drink your orange juice then brush your teeth. Never in the reverse. The reason is simple — orange juice tastes utterly disgusting after you have brushed your teeth. Adults might weather the torment, but order of operations matters for kid, who are more likely to skip either the citrus or the bristles in order to avoid an unfortunate taste.
But why? What poison is in our toothpaste, and what does it have against Tropicana? Guy Crosby, a nutrition professor at the Harvard T.H. Chan School of Public Health, recently told Live Science that he has a compelling theory: Sodium lauryl sulfate, a detergent found in toothpaste, that makes sweet flavors less sweet and bitter flavors more bitter.
SLS is found in foam or bubble-related products, such as dish soap and toothpaste. It’s why toothpaste turns into a foamy lather, and sneaks into those hard-to-reach places. But a 1980 study in the journal Chemical Senses found an unexpected side-effect to SLS. According to Live Science it “reduces the sweetness of sucrose (essentially sugar), the saltiness of sodium chloride (salt) and the bitterness of quinine (the flavoring used in tonic water) but increases the bitterness of citric acid (typically found in fruits like limes and oranges).”
The study itself never spelled out the connection, but it’s hard to miss. SLS reduces how our taste buds process sweet flavors, and increases how they process the bitter flavors that are specifically found in oranges. Furthermore, SLS removes phospholipids in the mouth, which are compounds that normally make bitter flavors taste less bitter. It’s a perfect storm and, at the center of it, an unsweetened, exceptionally bitter take on an otherwise delightful beverage.
This is, obviously, not a public health concern. SLS wears off quickly, and even a few minutes after brushing your teeth, you will be able to enjoy orange juice once more. But discussing the mechanism by which toothpaste ruins orange juice could make breakfast more interesting. Besides, it never hurts to throw a little science into your morning routine.