There could be several reasons why your beef cubes turned out chewy after pressure cooking:
Cooking Time: Eight minutes of pressure cooking time may not have been sufficient to fully tenderize the beef cubes. Tough cuts of meat, such as stewing beef, generally require longer cooking times to break down the collagen and connective tissues. You might consider increasing the cooking time in future attempts to achieve a more tender result. Experiment with longer cooking times and monitor the meat's tenderness to find the optimal duration.
Quality of Beef: The quality of the beef itself can also play a significant role in its tenderness. Tough or low-quality cuts of beef may not become as tender as desired, regardless of the cooking method. Consider using cuts of beef specifically recommended for slow cooking or stewing, such as chuck roast or brisket, as they tend to have more connective tissue that breaks down during prolonged cooking.
Size of Beef Cubes: The size of the beef cubes can impact their tenderness. Larger cubes may require additional cooking time to become tender. If the cubes were relatively large, you could try cutting them into smaller pieces to promote more even cooking and tenderness throughout.
Natural Release Time: After the pressure cooking cycle is complete, it's crucial to allow the pressure to release naturally for a few minutes before manually releasing any remaining pressure. This natural release time helps the meat to relax and become more tender. If you released the pressure immediately after the cooking time was complete, it might have contributed to the meat's chewiness.
Resting Time: Allowing the cooked beef to rest for a few minutes after the pressure is released can also help to enhance its tenderness. During this resting period, the meat fibers relax and reabsorb some of the juices, resulting in improved texture.
Considering these factors, you could try increasing the cooking time, selecting better cuts of beef, cutting the cubes into smaller pieces, ensuring a natural pressure release, and allowing the meat to rest before serving. Through experimentation and adjustments, you should be able to achieve a more tender and flavorful result with your pressure-cooked beef cubes.