Why street-cooked sweet potatoes taste sweeter than baked ones

Author: Song Yixuan, a popular science writer and member of the Chinese Society of Education; Yang Chao, the popular science supervisor at Shenzhen University of Technology and a member of the China Association of Science Writers. Review | Shao Wenya, Associate Professor at Fujian Medical University.

Winter has arrived, which means it's time to enjoy baked sweet potatoes. Have you ever wondered why sweet potatoes and regular potatoes both contain a lot of starch, yet baked sweet potatoes are much sweeter than baked potatoes, sometimes even yielding syrup? Why is it that sweet potatoes baked in a home oven are not as sweet as the ones you can buy on the street?

Both are primarily composed of starch, yet they present entirely different taste experiences after heating. This difference is the result of the interaction between the internal enzyme activity of the ingredients and the heating process. Understanding this process not only explains why sweet potatoes become sweet but also reveals the differences in heating mechanisms between home baking and street-side baking.

The reason why roasted sweet potatoes are sweeter than roasted regular potatoes is due to their higher natural sugar content. Sweet potatoes contain more complex carbohydrates that convert to sugars during the cooking process, enhancing their sweetness. In contrast, regular potatoes have a starchier composition, which results in a more neutral flavor when baked.

The reason sweet potatoes become sweeter the more they are roasted lies in the presence of amylase.

Sweet potatoes and potatoes both primarily derive carbohydrates from starch, but they exhibit distinctly different chemical changes during heating. Sweet potatoes contain a naturally active starch enzyme system, particularly α-amylase and β-amylase. Under suitable temperatures, these two types of enzymes can break down high molecular weight starch into smaller sugars like maltose and glucose, resulting in a noticeable sweetness developing in sweet potatoes as they are heated.

During the initial stage of baking, the internal temperature of the sweet potato gradually rises from room temperature to between 40 and 70°C, which is the optimal range for the activity of amylase. At this point, some starch granules inside the sweet potato absorb water, swell, and gelatinize. The enzymes begin to work, gradually breaking down the starch chains into shorter chains of dextrins, disaccharides, and monosaccharides. This process not only alters the texture of the sweet potato, making it softer, but also leads to a rapid increase in the soluble sugar content. Research has shown that the amount of reducing sugars in sweet potatoes can increase several times during baking, primarily originating from the maltose generated through the catalytic action of β-amylase.

Enzymatic saccharification and non-enzymatic browning together shape the sweet aroma of baked sweet potatoes.

When the core temperature of the sweet potato continues to rise above 80°C, the amylase loses its activity due to the heat, and the saccharification process consequently halts. At this point, there is already a sufficient accumulation of sugars inside the sweet potato. Continued heating will trigger a caramelization reaction of the sugars, producing brownish-yellow substances and aromatic compounds that give the surface a familiar golden color and a slightly charred flavor. Throughout this process, enzymatic saccharification and non-enzymatic browning work together to create the sweet and fragrant characteristics of baked sweet potatoes.

In contrast, potatoes lack active β-amylase, so their starch primarily undergoes gelatinization and moisture evaporation when heated, producing almost no maltose or glucose. Although the texture of the potato becomes soft after heating, the soluble sugar content changes very little, resulting in a relatively bland taste. It is this difference in enzyme activity that causes the two starchy ingredients to present entirely different taste experiences under similar heating conditions.

Why are sweet potatoes baked in home ovens not as sweet as those from street vendors?

Many people find that the sweet aroma of sweet potatoes baked in a home oven is far less intense than that of those sold by street vendors. The key difference lies in the speed of heating and the distribution of internal temperature.

Street-side roasted sweet potatoes are typically cooked using charcoal or iron barrel stoves. Although the heat source temperature is high, the conduction process is relatively slow. The outer layer of the sweet potato heats up first, while the internal temperature remains between 50 to 70°C for an extended period. This range is precisely where β-amylase in sweet potatoes is most active, allowing the enzyme to continuously break down starch into maltose, resulting in the accumulation of sweetness. Only when the core temperature of the sweet potato gradually exceeds 80°C does the enzyme begin to inactivate, completing the saccharification process. This 'saccharification first, then baking' method gives street-side roasted sweet potatoes their soft texture and pronounced sweetness.

The heating mechanism of home ovens varies. After preheating, the internal temperature of the oven is usually above 180℃, causing the surface of the sweet potato to heat up quickly and dry out, while the internal temperature exceeds the active range of the amylase in a short period. The enzymes are destroyed by the high temperature before they can fully complete the starch breakdown, prematurely halting the saccharification process. As a result, the starch in the sweet potato mainly undergoes gelatinization and moisture evaporation, leading to a reduced amount of sugar production and a noticeable decrease in sweetness.

How to roast sweet potatoes at home in a way that mimics the texture of those sold by street vendors.

In addition, home baking often lacks a slow heat dissipation phase. Street-side ovens can maintain a moderate temperature for an extended period even after the charcoal fire has diminished, which helps remaining enzymes further convert leftover starches into sugars. However, in home baking, items are often taken out of the oven to cool immediately after baking, missing this natural continuation of the saccharification process.

If you want to achieve a street-style flavor at home, you can try heating in stages: start by baking at a low temperature of 60 to 80°C for about 1 hour to allow the enzymatic reactions to fully occur, then increase the temperature to around 180°C for cooking and browning. This method of 'low-temperature saccharification followed by high-temperature setting' can significantly enhance the sweetness.

The ending.

The sweetness of roasted sweet potatoes comes from the action of amylase at suitable temperatures. When the roasting temperature rises slowly, the enzyme can break down starch into maltose over an extended period, enhancing the sweetness continuously. In contrast, rapid heating at high temperatures will inactivate the enzymes quickly, preventing sufficient saccharification from occurring. Potatoes lack active amylase, so even when heated under the same conditions, there will be no noticeable saccharification process. The difference between sweetness and lack of sweetness essentially lies in the timing of biochemical reactions and the enzyme system involved. Understanding these principles not only explains the delightful sweetness of roasted sweet potatoes but also allows us to better grasp the scientific rules of temperature and time in our everyday cooking.