Which off-flavor is seldom found except in pasteurized milk that has been stored too long?

The off-flavor that is seldom found except in pasteurized milk that has been stored too long is the unclean flavor. This particular flavor often arises from the growth of bacteria during prolonged storage. Even though pasteurization is meant to kill harmful microorganisms, if the milk is not handled or stored properly afterward, it can become tainted by bacteria which can lead to the development of off-flavors, notably the unclean taste.

This unclean flavor is primarily associated with the breakdown of milk components by bacteria, leading to undesirable tastes and odors that can resemble spoiled milk. It's a key indicator of milk that has been stored for too long or has not been kept at the proper temperature, making it crucial for milk processors and retailers to be vigilant about storage times and conditions.

The other off-flavors listed can occur under different circumstances and are not solely linked to the storage duration of pasteurized milk. Metallic flavors might stem from equipment or container materials, rancid flavors are often due to the oxidation of fat, and fruity flavors can arise from specific bacterial activity but can also be present in fresh milk under certain conditions.