My daughter eats oatmeal for breakfast—just boiled rolled oats with some sugar and milk. She loves it, but she’s a slow eater. Like any 13-year-old, she also sits down for breakfast just a few minutes before the school bus arrives. Inevitably, there are leftovers. “I promise to eat it in the evening,” she said recently before rushing out the door. When evening arrived, we pulled her leftover oatmeal from the fridge, but the texture had changed—it looked watery. Meanwhile, a second portion of untouched extra oatmeal that I’d also saved from the pot that morning was not only still thick, it had congealed quite a bit. This seemed rather odd.

I couldn’t be the only one who’s noticed this, right? So I started looking up information about foods thinning after cooking and found several questions posted on various cooking sites over the years. In almost all cases they involved starchy or starch-thickened recipes:

• Why did my leftover chowder turn out watery?
• Why do all thick sauces and soups turn into thin liquid as I eat them?
• Why is my soup getting thin midway through serving?

People often responded with more questions: What kind of starch did you use to thicken the dish? How much did you add? How long did you cook it? The original poster’s answers tended to suggest that these lines of inquiry were dead ends.

So, why were these foods thinning? Nobody seemed to know for sure, but every once in a while someone in the discussion would float a suggestion that made everyone uncomfortable: Did you double dip into the food? 

It’s a question that comes with an inevitable “eww!” factor—double-dipping is like backwash, and most of us recoil at the thought of gross mouth germs getting mixed into whatever we have yet to consume. If we can put our gag-reflex on pause for just one second, however, there’s a lot that’s interesting about this suggestion, because what it’s hinting at reveals more than just the answer to this food-thinning mystery, it’s a great example of the important digestive powers of saliva.

Without saliva, eating and digesting would be a lot harder, and less effective. The mucus in saliva, for example, is essential to swallowing. When we chew, the mucus and water in saliva transform dry or crumbly food into a soft, sticky lump called a bolus, making it easier to swallow. This helps prevent choking and protects the esophagus from damage caused by rough food particles.

But the mucus isn’t the main point here. Saliva is mostly water—about 99%. The remaining 1% is a mix of mucus, proteins, and electrolytes. Among the proteins are enzymes that help break down food into smaller pieces, making it easier for the body to digest. The most abundant enzyme in saliva is salivary amylase, and this is the critical one to look at in this mystery of foods thinning and double-dipping.

Salivary Amylase: I Have One Job to Do

Salivary amylase’s job is to break the α-1,4 glycosidic bonds in starch molecules to form smaller units like maltose and dextrins. That it works inside the mouth isn’t surprising; that is, after all, where saliva is found. What is surprising is that it continues to work outside the mouth as well, to the potential shock and horror of oatmeal eaters, soup sippers, and party guests crowded around bowls of dip everywhere.

But this shouldn’t surprise you, because you’ve seen amylase enzymes at work outside their intended environment before. When you hydrate flour to make dough, the amylase enzyme in the flour starts breaking down starches into smaller units for the yeast to feed on, kickstarting the fermentation process and producing gasses that cause the bread to rise. Traditional alcoholic beverages like chicha beer and masato even use saliva to begin fermentation, snipping starches into smaller pieces that can then be fermented by yeasts. Enzymes take the mantra “You have one job to do” very seriously. Like robots programmed on an infinite loop, they keep doing their job until they’re finally rendered inactive, whether denatured by changes in pH or temperature, or brought to a halt by dehydration.

Testing Salivary Amylase on Starchy Foods

Now that we know amylase can continue working outside the body, the question is: What kinds of foods would it affect, and under what conditions? To test this, I needed a few starch-thickened dishes. Here are the ones I tested:

1. Oatmeal
2. Cream of Mushroom Soup
3. Cream of Potato soup
4. Cheese sauce
5. Creamy Mac and Cheese

In all these cases, the dishes were thickened partly or entirely by starch molecules, which is a simple and inexpensive method for making foods less liquid. It works by heating a liquid that contains starch; once the temperature exceeds 140°F (60°C), the starch molecules start to unravel and absorb water in a process called gelatinization. The initially cloudy mixture transforms into a translucent gel, trapping water molecules between the starches.

I compared one set of starchy foods left untouched (as a control) with another set where I licked a spoon and swirled it around the foods. I repeated the licking and swirling process three times. After waiting 15 minutes, I examined the results.

Why 15 minutes? Researchers estimate that salivary amylase remains active in the body for about 15 minutes before the food we chew and swallow gets fully mixed with gastric juices, where the acidic environment denatures the enzymes. This time also roughly matches the duration it takes to finish a meal. Therefore, whether inside or outside the body, we should see the effects within this timeframe. All the dishes were kept at room temperature, as enzymes work best around body temperature 90-98°F (32-37°C).

If the enzymes did their job, the smaller cleaved molecules would lack the thickening power of the longer starch molecules each dish started with. So, if my hypothesis was correct, the resulting foods should be thinner compared to the control version.

Results

Within 5 minutes, I began to see some thinning in the mushroom and potato soups. By 15 minutes, the results were quite significant for both the soup and the oatmeal. There was pronounced thinning and it was easy to visually tell apart the control versus the one with the amylase. Salivary amylase had a clear impact on the texture of these three foods in a brief window of time.

However, the cheese sauce and mac and cheese showed very little to no change. Why did the hypothesis fail for the thicker dishes? Most likely, because the thicker dishes depend partly on proteins in the cheese for thickening. Since saliva doesn’t contain enzymes that break down proteins, this might have limited the effect. Additionally, the increased viscosity of the cheese sauce could have slowed down the enzymes, reducing their ability to cleave as many starch molecules in the given timeframe.

Does This Mean One Should Never Double Dip?

Enzymes work and they do have an impact. But if you are cooking for your family and checking for salt levels 18 times during cooking, you do not need 18 spoons. Enzymes are incredibly efficient at what they do, but they are easily disrupted. You do this on a daily basis. For instance, when you apply lime juice to apples and avocados, you inhibit the enzyme polyphenol oxidase, which prevents browning. Similarly, blanching vegetables before freezing them uses high heat to quickly deactivate the enzymes. Refrigerating foods slows down enzyme activity as well.

So, when feeding your family, don’t worry about licking the tasting spoon. Enzymes need a Goldilocks set of requirements in pH and temperature to function optimally. The more you stray from their optimal zone, the less effective they perform. For instance, putting your spoon into boiling porridge renders them completely inactive. Similarly, tomato or other acidic soups and stews also denature the enzymes.

But if you find a suspiciously runny soup, porridge, or roux-thickened sauce or dip in the office refrigerator or at a holiday party, you might want to stay away…because the enzymes from someone’s spit have been hard at work!