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Food Science and Technology QUESTION #5947
Question 1
In spray drying, the glass transition temperature (\(T_g\)) of milk powder is critical because:
  • Below \(T_g\), the powder is in a rubbery state and flows freely
  • Above \(T_g\), amorphous lactose transitions from glassy (stable) to rubbery (sticky, prone to caking and collapse), reducing powder quality✔️
  • \(T_g\) determines the fat melting point in powder
  • \(T_g\) affects the solubility of casein in reconstituted powder
Correct Answer Explanation
Below \(T_g\), amorphous lactose is in a glassy rigid state — stable. Above \(T_g\) (lowered by moisture content), lactose enters a rubbery state: high molecular mobility, sticky surface, particle caking, and potential collapse of the powder structure. Keeping powder moisture \(<3\text{–}4\%\) maintains \(T_g\) well above ambient storage temperatures.