Which of the following is a sign of food spoilage?

Food spoilage is indicated by changes that compromise its safety and quality. An unusual odor is a primary sign of spoilage as it suggests the presence of bacteria or mold that can produce off-smells due to the breakdown of the food's natural components. As food deteriorates, it undergoes chemical changes that often result in foul or sour smells, indicating it is no longer safe to consume.

In contrast, bright colors, smooth textures, and fresh appearances generally signal that food is still in good condition. Bright colors can indicate freshness in fruits and vegetables, smooth textures are typically a sign of well-prepared and preserved food, and a fresh appearance is essential in determining the quality of food items, especially meats and produce. Therefore, these characteristics do not typically relate to spoilage.