The whistling sound you hear in traditional stovetop pressure cookers is a result of the pressure regulator or pressure relief valve releasing excess steam. Here's how it works:
Pressure Buildup: When you use a pressure cooker on high heat, the liquid inside (usually water or broth) starts to boil and produces steam. As the pot heats up, the pressure inside the sealed cooker begins to increase.
Pressure Regulator/Relief Valve: Pressure cookers are designed with a pressure regulator or a pressure relief valve. This component helps regulate the pressure inside the cooker to maintain a safe level. When the pressure inside the cooker exceeds a certain threshold, the pressure regulator or relief valve starts to release steam.
Whistling Sound: As the steam is released through the pressure regulator or relief valve, it passes through a small narrow vent. This vent is designed to create resistance to the escaping steam, causing it to exit in bursts rather than a continuous flow. These bursts of steam cause the characteristic whistling sound that you hear.
Maintaining Pressure: As the heat continues, the pressure inside the cooker fluctuates between the safety threshold and the pressure at which the regulator releases steam. This cycling of pressure release and steam production maintains a constant pressure level inside the cooker, allowing it to cook food quickly and evenly.
The whistling sound serves as an audible indicator that the pressure cooker has reached the desired pressure and is actively cooking the food. It also alerts you if the heat needs to be adjusted to maintain a consistent pressure level.
Newer electric pressure cookers, like the Instant Pot, also use pressure regulators to maintain pressure, but they often have electronic controls and digital displays, so they don't produce the same audible whistling sound as traditional stovetop pressure cookers. Instead, they may display information about the cooking process on their screens.