Why are baked sweet potatoes tastier than steamed ones?

As autumn approaches, there is always an irresistible aroma in some corners of the streets and alleys - the aroma of roasted sweet potatoes. This aroma seems to capture the soul of autumn and deeply attracts the attention of passers-by.

Many people can't help but wonder, why do sweet potatoes roasted on the roadside have a more tempting aroma and sweeter taste than those steamed in a steamer at home?

Sweet potatoes on the street (Source: AI generated by the author)

Just in March of this year, a research team actually studied the effects of different cooking methods on the flavor and chemical composition of sweet potatoes.

What substances change when baking sweet potatoes and steaming them?

The researchers prepared a total of 12 yellow-fleshed sweet potatoes and randomly divided them into four groups. One group served as the control group and received no treatment, while the other three groups were steamed for 40 minutes, boiled for 30 minutes, and baked in an oven at 200°C for 90 minutes.

Raw sweet potatoes, steamed sweet potatoes, boiled sweet potatoes and baked sweet potatoes (Source: Reference 1)

The researchers also invited 10 volunteers to evaluate the sweet potatoes. The baked, steamed and boiled sweet potatoes were different in appearance, texture, taste and chemical composition. The baked sweet potatoes tasted sweeter and had a stronger aroma, while the steamed and boiled sweet potatoes tasted moister but had a relatively weaker aroma.

Sensory evaluation directly reflects the differences between sweet potatoes cooked in different ways (Source: Reference 1)

Why is it sweeter? The soluble sugar content is directly related to the sweetness of cooked sweet potatoes. Baking increased the soluble sugar content of sweet potatoes from 9.12% to 36.65%. A total of five sugars (galactose, glucose, fructose, sucrose and maltose) were detected in all samples. Specifically, the maltose in baked sweet potatoes increased by 200 times, which may be the reason why baked sweet potatoes are sweeter.

Why is it more fragrant? The use of gas chromatography-mass spectrometry analysis showed that the aroma of roasted sweet potatoes is more fragrant because a large number of furans and terpenoids are produced during the roasting process. In the study, five furans were identified, including furfural, 2-furan methanol, 2-acetyl furan, 2-pentyl furan, and 2-methyl-benzofuran. In raw, steamed, and boiled sweet potato samples, only a small amount of furan compounds were detected.

What do these five furan compounds smell like?

Furfural: has an almond aroma.

2-Furylalcohol: Has a woody or burnt sugar odor.

2-Acetylfuran: The smell is similar to that of coffee or baked goods.

2-Pentylfuran: It has a fruity and leafy odor, which has been described as close to tomato or fruit in some cases.

2-Methylbenzofuran: The odor of this compound varies, generally having a more complex aroma that may include spicy, woody, or phenol-like notes.

How to produce so many furans? This depends on the Maillard reaction, which is indispensable in cooking. The Maillard reaction is a non-enzymatic, heat-induced chemical reaction that mainly occurs between reducing sugars and amino acids. This reaction was first described by French chemist Louis-Camille Maillard in 1902.

Baking sweet potatoes involves the Maillard reaction (Source: AI generated by the author)

The Maillard reaction is a very complex multi-step reaction, including various intermediates and final products. Initially, reducing sugars and amino acids (or proteins) generate stable or unstable glycosylamines under heating or some kind of catalysis. Then, these glycosylamines undergo a series of reactions such as rearrangement, oxidation-reduction, and polymerization, and ultimately generate a variety of complex molecules, including glycosylation end products.

Some classic foods produced by the Maillard reaction include roasted meats, baked bread and pastries, coffee, and chocolate.

In addition to Maillard, caramelization is also important!

The standard of a good baked sweet potato is that it should ooze out deep red sugar juice. In addition to the Maillard reaction, the reason why baked sweet potatoes are delicious is also inseparable from the caramelization reaction.

Caramelized baked sweet potato (Source: Photo by the author)

Caramelization is different from the Maillard reaction because it does not involve amino acids or proteins. It is a heat-induced chemical reaction that mainly involves the decomposition and recombination of sugars when heated. This process is usually carried out at relatively high temperatures, usually between 160°C and 210°C, depending on the type of sugar and the environmental conditions. Caramelization is a complex multi-step reaction involving the following main stages: Sugar molecules first undergo a dehydration reaction to form some low molecular weight compounds. These low molecular weight compounds may undergo structural rearrangements. As the reaction proceeds, the resulting low molecular weight compounds polymerize to form larger and more complex molecules. Ultimately, these compounds form a variety of different types of caramels with different colors and flavors. Like the Maillard reaction, caramelization is a highly complex and varied reaction that produces a wide variety of compounds, including polycyclic compounds, carbonyl compounds, and various organic acids. In life, we can often experience the effects of caramelization in caramel candies, baked goods, grilled and fried foods, and various desserts and specialty drinks. Whether it’s hard caramel, crème brûlée, roasted meats or specialty coffee, caramelization is one of the key factors in giving food its attractive color and delicious taste.

Caramel pudding Image source: AI drawing by the author

Steamed sweet potatoes are healthier

Or are baked sweet potatoes healthier?

When it comes to healthy ways to eat sweet potatoes, both baking and steaming sweet potatoes have their own advantages. From a nutritional point of view, studies have shown that steaming sweet potatoes has advantages in retaining carotenoids. Carotenoids are powerful antioxidants, especially beta-carotene, which not only provides rich vitamin A, but also plays a significant role in maintaining vision, strengthening the immune system and promoting skin health.

From the perspective of the glycemic index (GI), there is a clear difference in the sugar content of baked and steamed sweet potatoes. The glycemic index is an indicator that measures the impact of carbohydrates in food on blood sugar levels, with lower numbers indicating less impact on blood sugar.

Here's a comparison of the glycemic index of baked and steamed sweet potatoes: Steamed sweet potatoes have a lower glycemic index (GI of 44), while baked sweet potatoes have a higher glycemic index (GI of 94), meaning they break down into sugars quickly and may cause a short-term spike in blood sugar levels.

However, the differences in protein and fat content between baked and steamed sweet potatoes were not significant, so if those are your concerns, both cooking methods are acceptable.

Of course, eating sweet potatoes is not just about nutrition, taste and personal preference are also important considerations. Baked sweet potatoes usually have a richer flavor and better taste, which is why most people prefer baked sweet potatoes over steamed ones.

When choosing how to cook sweet potatoes, the most important thing is to follow your own taste and needs.

References

[1] Zhang, Rong, etal. "Impact of different cooking methodsson the flavor and chemical profile of yellow-fleshedtable-stocksweetpotatoes(IpomoeabatatasL.)." FoodChemistry:X 17(2023):100542.

[2] Alam, Mohammad Khairul. "Acomprehensive review of sweet potato (Ipomoeabatatas [L.] Lam): Revisiting the associated health benefits." Trends in Food Science & Technology 115 (2021): 512-529.

[3] Kourouma, Vamougne, et al. "Effects of cooking process on carotenoids and antioxidant activity of orange-fleshed sweet potato." Lwt104 (2019): 134-141.

[4] Abeynayake, Shamila Weerakoon, et al. "Biosynthesis of proanthocyanidins in white clover flowers: crosstalk within the flavonoid pathway." Plantphysiology 158.2 (2012): 666-678.

[5] Kidmose, Ulla, et al. "Effect of home preparation practices on the content of provitamin Acarotenoidsincoloured sweet potato varieties (Ipomoea batatas Lam.) from Kenya." Innovative Food Science & Emerging Technologies 8.3 (2007): 399-406.

Author: Denovo team

Review | Ruan Guangfeng, Deputy Director of Kexin Food and Health Information Exchange Center