The Intriguing Physics of Cooking Xiao Long Bao: A Deeper Look into the Chinese Steamed Bun


Xiao Long Bao, also known as the Chinese soup dumpling, is a delicacy that originated in Shanghai and has since become popular worldwide. These delicate steamed buns consist of a thin dough skin filled with savory ground meat and a gelatin-rich broth that melts into a soup when cooked. The cooking process of Xiao Long Bao is deeply intertwined with physics, which determines the transformation of ingredients into a delightful, soupy treasure.

This article delves into the fascinating physics behind the cooking of Xiao Long Bao, discussing the role of heat, pressure, and steam in the process.

  1. The Role of Gelatin and Its Melting Point

The foundation of Xiao Long Bao’s delightful soup lies in the use of gelatin. Before the buns are steamed, gelatin-rich broth is prepared by simmering bones and meat, which solidifies when cooled. The gelatin-rich mass is then cut into small cubes and combined with the meat filling.

Gelatin’s melting point plays a crucial role in the transformation of Xiao Long Bao during cooking. It melts at a temperature range of 77-95°F (25-35°C), which is below the boiling point of water (212°F/100°C). This means that when the buns are exposed to heat, the gelatin melts, transforming the solid mass into a liquid broth within the confines of the dough.

  1. Heat Transfer and Steam Production

Steaming is the primary method used to cook Xiao Long Bao, and this process involves the transfer of heat from the water to the buns via steam. When water is heated to its boiling point, it undergoes a phase change from liquid to vapor, resulting in steam production. The heat transfer occurs through a process called convection, in which the steam molecules transfer energy to the Xiao Long Bao’s dough, cooking both the dough and the filling inside.

  1. Steam Expansion and the Elasticity of Dough

As the steam cooks the Xiao Long Bao, it also causes the buns to expand due to the increased pressure from the steam inside. The elasticity of the dough is essential to withstand this expansion, allowing the filling to transform from solid to liquid without breaking the delicate skin.

The dough is made from wheat flour, water, and sometimes yeast. Gluten, a protein found in wheat, is responsible for the dough’s elasticity. When the dough is kneaded, gluten proteins form a network that can stretch and trap gas, providing the necessary elasticity for the buns to expand without rupturing.

  1. The Importance of Proper Timing

The cooking process of Xiao Long Bao requires precise timing to ensure the perfect balance between fully cooked dough and filling and avoiding overcooked or burst buns. Typically, steaming takes about 8-10 minutes, during which time the dough becomes cooked and translucent, the gelatin melts and forms the soup, and the meat filling becomes tender and juicy.

Overcooking can result in a tough, chewy dough or even cause the buns to rupture, releasing the soup and ruining the delicate balance that makes Xiao Long Bao so unique.