Which condition could affect milk's quality and lead to a bitter taste when consumed?

The condition that can significantly affect milk's quality and lead to a bitter taste when consumed is milk from cows with mastitis. Mastitis is an infection of the udder that can cause various changes in the milk's composition, including an increase in somatic cells and enzymes. One particularly important enzyme is lipase, which can break down fats in the milk, leading to off-flavors, including a bitter taste, once the milk is pasteurized or stored. The presence of these enzymes in milk can spoil its taste and make it unpalatable.

While improper refrigeration, contamination with foreign substances, and milk stored for over a week can also lead to quality deterioration, they do not specifically result in a distinctly bitter flavor attributed directly to the biological processes occurring in milk from mastitic cows. Proper handling, refrigeration, and storage are crucial for maintaining the overall quality of milk, but the specific issue of bitterness arising from mastitis directly ties into the enzymatic and microbial changes occurring within the milk itself.