The Kitchen Adventures of "Mr. Bubble" - Why is there a "bubble party" when frying?

In the kitchen, we often encounter some mysterious phenomena in cooking.
for example:

Why did white bubbles appear when the oil was heated this time?

When you fry an egg or deep-fry chicken wings, what is that little white bubble in the pan?

Coconut oil, in particular, seems to love to "bubble". Why is that?

Now let’s start from the kitchen and step into the scientific world of oils and fats, and by the way, talk about the reasons for the "bubbling" and health secrets of these common cooking oils, so that science can be both interesting and practical, and can also give us more "confidence in data" next time we cook!

1. Start by frying eggs: The culprit of "white foam" or "bubbling"

In the morning, take out the eggs from the refrigerator, heat the pan, pour in the oil, and as the oil temperature rises, tiny white bubbles begin to appear in the pan, even with a little "crackling" sound. Do you think: "Is the oil bad this time? Is it bad?" In fact, the appearance of this white foam "bubbling" is a common physical and chemical phenomenon, which mainly comes from the "water" and "impurities" content in the oil.

In all edible oils, whether it is soybean oil, peanut oil, olive oil, or coconut oil, a trace amount of water will remain during the production process. Even for refined edible oils, the water content can usually reach 0.1%-0.3% (don't underestimate this little bit of water, it can quickly "mess up" your oil pan at a critical moment-a lot of "bubbling"). We know that when the oil is heated to above 100°C, the water will reach the boiling point-quickly vaporize and turn into steam and run away. In this process, these small water droplets will quickly expand and burst in the oil, and this process is the white foam, "bubbling" and "crackling" we see. Scientists call this "the vaporization of water and the interface reaction of oil and fat". Does it sound instantly high-end? In fact, the water "explodes" and "runs away" in the high-temperature oil.

Now we also know that in addition to water, cooking oil may also contain other "impurities", such as residual proteins, sugars or free fatty acids in the production process. These components from the raw materials for oil extraction will also decompose, volatilize, and even oxidize at high temperatures, producing gas or foam and "bubbling". Especially some unrefined crude oils (such as some homemade lard or beef tallow and mutton fat), there will be more "impurities" and the white foam will naturally "bubble" more lively.

2. Why is coconut oil so bubbly?

At this point, you may have noticed that different cooking oils seem to have different degrees of foaming. Indeed, vegetable oils such as soybean oil, peanut oil, sunflower oil, rapeseed oil, and rugelach tend to be more "low-key" and less likely to "foam". Lard and butter are sometimes a little more "lively". In comparison, coconut oil is definitely the dancing champion in this "foam party". Why is this?

The answer lies in the "chemical composition" of cooking oil. The biggest difference between coconut oil and other cooking oils lies in its fatty acid composition. Coconut oil contains up to 90% saturated fatty acids, mainly medium-chain fatty acids (such as lauric acid, accounting for about 50%). These medium-chain fatty acids are small molecules, easy to volatilize, and more likely to react with water or "impurities" at high temperatures to release white bubbles. In contrast, soybean oil, peanut oil and other oils rich in unsaturated fatty acids are not as volatile, so there is relatively less white foam.

There is another interesting phenomenon: if coconut oil is not deeply refined, the water content will be higher, and the water vaporization will be more intense when heated, so there will be more white foam. Studies have also shown that the degree of oil refining will directly affect its "foaming behavior". In an SCI paper published in Food Chemistry, scientists found through experiments that the amount of foam generated by unrefined edible oil at high temperature is 30%-50% higher than that of refined oil, and oil with a high proportion of saturated fatty acids is more likely to foam. This also explains the "foaming talent" of coconut oil!

3. The “bubbling rankings” of different oils and the scientific reasons

Let's take stock of the "bubbling degree" of common cooking oils when frying, and talk about the reasons behind it:

1. Coconut oil: The best at bubbling. It has a lot of saturated fatty acids and small molecules. Water and impurities evaporate quickly at high temperatures, and the foam is abundant and obvious. Put a little more in the oil for frying, otherwise it will easily overflow from the pan.

2. Lard and butter: mid-to-upper-level players. Animal fats contain a small amount of protein residue and water, which decompose and volatilize when heated. They "bubble" slightly more than vegetable oils, but milder than coconut oil.

3. Rapeseed oil: above average. It contains a certain amount of monounsaturated fatty acids, and the "bubbling" varies significantly with the degree of refining. It is relatively quiet when frying at low temperature, but a bit "lively" when deep-frying at high temperature.

4. Peanut oil, sunflower oil, soybean oil: low-key. Polyunsaturated fatty acids dominate, the refining process is mature, and there is relatively less water and "impurities", and relatively less "bubbling".

5. Olive oil: quiet and noble. Especially extra virgin olive oil, which is mainly composed of monounsaturated fatty acids. Although the smoke point is not high, it is well refined and does not "bubble" much.

Now, we also know that the "bubbling" of cooking oils is related to the "smoke point" of the oil in addition to the fatty acid composition. The smoke point is the temperature at which the oil begins to smoke and decompose. Oils with low smoke points (such as unrefined coconut oil or lard) are more likely to decompose impurities and produce foam at high temperatures. Oils with high smoke points, such as peanut oil and sunflower oil, can withstand higher test temperatures before they begin to "bubble".

4. From "bubbling" and "smoke point" to health: the nutritional secrets of fats and oils

Speaking of the "character" of oil, let's take a look at the nutritional value and health effects of these oils. The "Dietary Guidelines for Chinese Residents (2022)" issued by the Chinese Nutrition Society mentioned that edible oil is a source of essential fatty acids and energy for the human body, but choosing the right oil and using it correctly can make it both delicious and healthy.

1. Soybean oil: Rich in linoleic acid (a polyunsaturated fatty acid), affordable, and suitable for cooking. But the smoke point is not high, so the temperature should not be too high when used for frying.

2. Peanut oil: It contains a balance of monounsaturated and polyunsaturated fatty acids, has a strong aroma, can be used for frying, and has a higher smoke point.

3. Sunflower oil: Mainly composed of polyunsaturated fatty acids, it contains vitamin E. It is suitable for cold dishes or low-temperature cooking. It will oxidize quickly when fried at high temperatures.

4. Rapeseed oil: contains erucic acid (low erucic acid varieties tend to be healthier), rich in monounsaturated fatty acids, OK for frying, and has a medium smoke point.

5. Lard and butter: high in saturated fatty acids, delicious taste, strong sense of fullness, suitable for frying, stir-frying, and not easily oxidized.

6. Olive oil: a representative of monounsaturated fatty acids, it has strong antioxidant properties and is suitable for cold dishes or low-temperature frying. It may be a bit of a "waste" to fry things.

7. Coconut oil: It contains a lot of medium-chain fatty acids, digests quickly, is high in saturated fatty acids, and is relatively not easy to oxidize when fried.

5. Tips: How to reduce the "bubbling" and cook healthily?

A few practical suggestions can not only reduce the "foam party" in the frying pan, but also make the dishes healthier:

①. Optional refined oil: Oil with a high degree of refinement has less water and "impurities", and naturally less white foam.

②. Control the fire and temperature well: the oil temperature should not exceed the smoke point. For example, use medium heat when frying eggs, and do not wait until the oil starts to smoke before frying.

③. Oil-water separation: Try to control the moisture of the ingredients before putting them into the pot. Wet chicken wings will also be the "accomplice" of "bubbling".

④. Moderate and varied consumption: The Dietary Guidelines for Chinese Residents recommends that the daily intake of edible oil should be 25-30 grams. You can also eat a variety of oils in rotation, which can sometimes provide a more balanced nutrition.

Finally: Like a scientific adventure in the kitchen

Next time, when we fry eggs and see the bubbling in the pan, will we be more calm? It turns out that it is not the oil that is "bad", but the "little dance" of water and other ingredients from food under high temperature. From the "bubbling carnival" of coconut oil to the "low-key and restrained" of peanut oil, each oil seems to have its own temperament and health attributes. Today, this journey from the kitchen to science has not only increased our knowledge, but also given us more fun and concentration when cooking. Sometimes, life is like a scientific journey through meals!

References

1. X. Wang, et al., "Effect of refining on the physicochemical properties of edible oils," Food Science and Technology International, vol. 25, pp. 321-329, 2019.

2. AG Marangoni, et al., "Structure and properties of fat crystal networks," Food Chemistry, vol. 139, pp. 1-12, 2013. (Chemical mechanism of high temperature foaming of fats and oils)

3. MA Rogers, "Novel structuring strategies for unsaturated fats," Journal of the American Oil Chemists' Society, vol. 87, pp. 747-756, 2010. (Relationship between fatty acid composition and foaming)

4. Chinese Nutrition Society, Dietary Guidelines for Chinese Residents (2022), Beijing: People's Medical Publishing House, 2022.