Condensed and evaporated milk are valued for their long shelf life, yet several defects may compromise their quality if proper processing and storage are not maintained.

Thickening or Gelling: This defect arises from excessive heat treatment during sterilization, high protein stability, or protease activity from psychrotrophic bacteria in raw milk. High storage temperatures accelerate gelling. Prevention: Apply only the necessary heat treatment, ensure milk of low bacterial count, preheat adequately, and store products at cool temperatures.

Fat Separation/Churning (Cream Plug): Caused by inadequate homogenization (especially in evaporated milk), poor fat globule stabilization, or prolonged agitation during handling. Higher fat levels aggravate separation. Prevention: Use efficient high-pressure homogenization, stabilizers like disodium phosphate, and careful handling to avoid excessive vibration.

Sandiness/Sugar Crystallization (Condensed Milk): Large, gritty lactose crystals form when concentration exceeds solubility, often due to poor seeding or cooling. Prevention: Seed with fine lactose crystals during cooling and ensure rapid, uniform cooling to produce tiny, imperceptible crystals.

Browning (Maillard Reaction): Heat-driven interaction between amino acids and lactose leads to discoloration and flavor changes, worsened during storage. Prevention: Minimize heat load, adopt the lowest effective sterilization temperature, store at cool conditions, and use fresh milk with less protein degradation.

Microbial Spoilage: Results from inadequate sterilization, contamination, or insufficient sugar content in condensed milk. Spore-formers are common culprits. Prevention: Strict sterilization, proper sealing, high hygiene, and adequate sugar concentration.

Metallic/Stale Flavor: Caused by metal contact or fat oxidation during storage. Prevention: Use properly lined cans, inert equipment, and oxygen-limiting packaging.