The presence of large ice crystals on frozen shrimp and its packaging during receiving indicates that the shrimp has likely undergone temperature abuse. This means the shrimp has been exposed to temperatures above the ideal storage temperature, allowing partial thawing to occur. When the shrimp is then re-frozen, large ice crystals form.

Here are the main causes of this issue:

1. Fluctuating Storage Temperatures: The shrimp may have been stored at temperatures that fluctuated above and below freezing. Each time the temperature rose, the shrimp partially thawed, and when it fell again, the water refroze, forming large ice crystals.
2. Improper Handling During Transportation: If the frozen shrimp was not kept at a consistently low temperature during transportation, partial thawing and refreezing could occur, leading to the formation of large ice crystals.
3. Inadequate Freezing Equipment: If the freezing equipment at any stage of the supply chain was not functioning properly, it could result in temperature abuse.
4. Extended Time at Ambient Temperature: If the shrimp was left out at room temperature for too long before being placed in the freezer, this could cause partial thawing. Subsequent refreezing would then lead to the formation of large ice crystals.

These large ice crystals can damage the quality of the shrimp, affecting its texture and taste. Therefore, the shrimp is rightfully rejected to ensure that the food safety and quality standards are maintained.