Why are potato chips so delicious? It has something to do with how they look!

It's the autumn outing season again. If you want to talk about the indispensable snacks in the school bag, it must be potato chips. The light airy feeling is wrapped under the golden and crispy skin, and the pleasant crisp sound of "crackling" makes people feel happy while enjoying the delicious food. When you open the potato chip bucket again, have you ever been curious for a moment: Why are potato chips in this shape?

Why do potato chips bend? (Photo courtesy of the author)

The question of the shape of potato chips seems to have become a popular topic on the Internet, with many discussions on the Internet [1], and the arguments are basically similar: the saddle-shaped shape has more stable performance and is more likely to remain intact during manufacturing and transportation. A few years ago, Bi Dao [2] took a different approach and did not discuss the stability of the saddle-shaped shape. Instead, he proposed the more nonsensical "tangent edible theory", which really attracted a lot of attention.

Bi Dao’s “Eatable if Cut” Theory[2]

In the article “More than 40 billion servings a year, what’s the secret of instant noodles besides being delicious?” [3], we discussed the shape of instant noodles. Their curved structure is indeed similar to an “arch”, which can disperse external loads and avoid bumps. Saddle noodles also have an “arch” shape. So in the field of potato chips, is the saddle shape really also used to make them more stable, so that they are easier to manufacture and transport?

01Are all potato chips the same shape?

Potato chips can be divided into bags and barrels according to the packaging style. Bags of potato chips are cut out of real original potato chips, while barrels of potato chips are made by pressing mashed potatoes through a mold. Therefore, the shape of barrels of potato chips is uniform, which is convenient for packaging in order. The shape of bagged potato chips is more random and can only be packed in air bags.

Barrel potato chips forming mold[1]

The shape of potato chips in a barrel depends on the mold used to form it. In theory, it can be made into any shape. However, there are actually only three common shapes of potato chips in barrels on the market: single curved surface, double curved surface (saddle surface), and wavy surface. The common original cut bag potato chips (the top one in the picture below) also have a saddle surface shape. Is this a coincidence or a necessity?

Comparison of potato chip shapes (Photo provided by the author)

In fact, in a bag of potato chips, there is more or less a "saddle noodle" shape. The potato chips expand after frying, but the expansion is uneven and warps. In addition, due to factors such as the growth of fiber in the sliced ​​potatoes, it warps in two directions, eventually forming a saddle noodle.

Uneven expansion (Photo provided by the author)

Therefore, saddle noodles are actually the naturally formed curved surface of raw potato slices after frying.

02Solid and stable saddle surface

No matter how the saddle surface is placed, it always contacts the ground at two points on both sides; when the single-curved surface opens upward, the contact surface with the ground is a "line", and when it opens downward, it is the same as the saddle surface. According to the formula in the figure below, under the same conditions, the bending moment (M1) of the dangerous section of the single-curved surface opening upward is greater than that of the saddle surface and the single-curved surface opening downward (M2). From this perspective, after the potato chips are compressed, the friction torque (fh) of the saddle surface at the contact position relieves the bending moment (M2) at the internal dangerous position, thereby improving the strength.

Comparison of bending moments of two bending shapes (Photo provided by the author)

However, for potato chips, this strength comparison is not very useful. The potato chips in the barrel are very strong and the possibility of potato chips breaking under pressure is very small. Here, the difference in the shape of the potato chips is actually not that important. It can be seen that the shape of the saddle surface is not designed simply for high strength.

Image source: Photo Network

However, potato chips in barrels do break. The chips at the two ends of the package vibrate back and forth during transportation and collide with the ends of the box. The warped potato chips are more likely to break under the impact of this collision because the bottom is empty. On the contrary, if it is made into a flat shape, the possibility of vibration and breakage caused by transportation is much smaller.

03Why are potato chips made into saddle noodles?

In this case, why do some brands insist on making saddle noodles? In fact, doing so is largely a pursuit of product differentiation, which is mainly reflected in appearance and taste. In terms of appearance, the saddle noodles are obviously more artistic. In terms of taste, the saddle noodles have a stronger sense of breaking after being stressed. After the potato chips enter the mouth, the potato chips are given forces in the upper and lower directions when biting. Under the action of this pair of forces, single-curved potato chips are more likely to break into two pieces at the concave line, while saddle-shaped potato chips are equivalent to having 2 concave lines, breaking into at least 4 pieces.

Comparison of fracture lines (Photo provided by the author)

It is worth mentioning that it is not because the saddle surface is more evenly stressed that the potato chips break more. In fact, under the condition of oral occlusion, the stress inside the potato chips is uneven.

04 Saddle roof

The mechanical principle of the saddle roof is completely different from the principle of potato chips breaking. The force on potato chips is relatively random, while the force on the saddle roof is fixed, that is, the roof load is vertically downward. It is under the action of this vertical uniform load that the saddle roof can reflect its superiority.

If we cut open the saddle surface, we will find an "arch" structure. As we all know, the arch structure has a large load-bearing capacity, thanks to which the structure can convert its own weight and load into the extrusion force inside the structure, eliminating the weakness of the pulling force.

Arch force (Photo provided by the author)

The internal force of the saddle surface is similar to that of the arch structure, but there are some differences. After the saddle surface is cut along the concave line, the upper concave section (the blue section in the figure) is the compression section, and the middle concave section is similar to the symmetry surface of the arch structure. The two sides squeeze each other, and concrete is used as the pressure-bearing material. The lower concave section (the green section in the figure) is the tension section, which is similar to a catenary connecting surface and stretches each other. In this direction, steel bars must be arranged to enhance the tensile performance.

Internal force tension and compression of the section (photo provided by the author)

The tension and compression of the saddle surface can bring out the best performance of the steel bars and concrete structure of reinforced concrete[4]. This structure has many applications, such as Guangdong Xinghai Concert Hall, South China University of Technology Gymnasium, Puerto Rico Pons Stadium, Berlin World Expo Conference Hall, etc. However, the span of the concrete thin shell structure of the entire saddle surface is limited after all. If it is used in a gymnasium, it needs to be divided into four saddle surfaces like the South China University of Technology Gymnasium, and supported separately.

South China University of Technology Gymnasium (South China University of Technology official website)

The entire saddle-shaped roof structure of a large gymnasium does not use this steel-concrete thin shell structure, which is even more different from the saddle surface of potato chips. In order to reduce the deadweight, a flexible cable structure is used, such as the roof of my country's Ice Ribbon National Speed ​​Skating Hall. There is no pressure in the flexible cable structure, only tension. The entire roof structure naturally concave under the gravity of its own weight and other ancillary facilities. The designer took advantage of the situation and created such a saddle-shaped roof structure, turning mechanics into art.

The saddle-shaped roof of the National Speed ​​Skating Oval (National Speed ​​Skating Oval official website)

References

[1]. Why do potato chips often appear in a shape similar to a saddle surface (hyperbolic paraboloid)? https://www.zhihu.com/question/635339502/answer/3378790089

[2]. Bi Dao, can we still eat potato chips if they fall on the ground? I have officially overturned the “tangent edible theory” of a Tsinghua PhD!

https://mp.weixin.qq.com/s/nT9zwAtjSRToNJtSBHoFrQ

[3]. Wang Yongjian, more than 40 billion servings a year, what is the secret of instant noodles besides being delicious? https://mp.weixin.qq.com/s/UMzQxVbf_tbtLo0mLEWQGQ

[4] Liang Sheng, Research on the integral earth-covered roof system of large space "saddle-shaped" thin shell structure, South China University of Technology, 2014.

Author: Wang Yongjian, Associate Professor of Nanjing Agricultural University, Deputy Secretary-General of the Science Popularization Committee of Jiangsu Mechanics Society, Chief Science and Technology Communication Expert of Jiangsu Province

Planning & Editing: Tiara