A heat exchanger is a process device designed to transfer thermal energy between two fluids without mixing them. Depending on the processing stage, heat exchangers (such as plate, tubular, or scraped-surface types) are used either to heat the product (e.g., pasteurizing, sterilizing) or to cool the product (e.g., chilling milk or wort).

Thermal death kinetics and nutrient retention:

• High-Temperature Short-Time (HTST) thermal processing exploits the vast difference in temperature sensitivity ($z$-value) between vegetative bacterial spores and chemical nutrients.
• Bacterial death has a low $z$-value ($z \approx 8 - 10^\circ\text{C}$ for spores, $4 - 6^\circ\text{C}$ for vegetative cells), meaning a slight temperature increase drastically accelerates spore destruction rates.
• In contrast, nutrient/vitamin degradation and browning reactions have much higher $z$-values ($z \approx 25 - 35^\circ\text{C}$), meaning their reaction rate increases much less rapidly with temperature.
• Thus, high temperature for short duration achieves rapid microbial sterilization with minimal nutrient destruction.

Biological indicators for validating high-temperature thermal sterilization:

• Bacillus subtilis spores are used to validate low-temperature decontamination or dry heat/radiation processes.
• Micrococcus flavus is a non-spore-forming coccus sensitive to heat.
• Geobacillus stearothermophilus (formerly Bacillus stearothermophilus) produces extremely heat-resistant thermophilic endospores ($D_{121} \approx 4\text{--}5\text{ min}$, $z \approx 10^\circ\text{C}$), making them the standard biological indicator for measuring thermal sterilization values and evaluating continuous aseptic holding tube performance in viscous liquid foods.
• Clostridium botulinum is the target public health organism ($D_{121} = 0.21\text{ min}$), but its dangerous neurotoxin makes it unsuitable for direct continuous pilot-plant validation assays.