ICAR AIEEA PG 2024 - Dairy Technology

Thermal death of bacterial vegetative cells and endospores follows logarithmic first-order reaction kinetics: $\frac{dN}{dt} = -k N$, which integrates to $\log_{10}\left(\frac{N_0}{N_t}\right) = \frac{t}{D}$, where the rate of destruction at any given moment is directly proportional to the viable population remaining.

Pasteurization regulations (FDA Grade 'A' Pasteurized Milk Ordinance, Codex, and dairy standards) define continuous HTST pasteurization ($72^\circ\text{C}$ for 15 s) to provide at least a 5-log ($5D$, i.e., $99.999\%$) to 6-log reduction of the heat-resistant zoonotic bacterium Coxiella burnetii (or Mycobacterium bovis).

Indirect heating systems transfer thermal energy across a physical barrier (metal wall or glass container) separating the heating medium (steam/hot water) from milk; this includes plate heat exchangers (A), tubular heat exchangers (B), and in-container retort sterilization (C). Direct heating (D) involves direct physical mixing of culinary steam with milk via steam injection (steam into milk) or steam infusion (milk into steam chamber).