LETHALITY CONTRIBUTION FROM THE TUBULAR HEAT EXCHANGER DURING HIGH‐TEMPERATURE SHORT‐TIME PROCESSING OF A MODEL LIQUID FOOD

Abstract

ABSTRACT Experiments were conducted using Bacillus stearothermophilus spores in 0.5% w/w carboxymethylcellulose suspension to evaluate come‐up contributions from the tubular heat exchanger. A lab‐scale UHT/HTST simulator that allowed samples to be collected at the exit of the heat exchanger and holding tube was used with operating temperatures up to 270F. The bulk mean residence time in the heat exchanger ranged from 30 to 89 sec, while that in the holding tube ranged from 4 to 14 sec. It was observed that between 40% and 51% of the cumulative lethality ( ) at the exit of the holding tube, was contributed by come‐up in the tubular heat exchanger. This come‐up contribution was determined on the basis of having an of 8.5 min in the holding tube alone. It was evident that come‐up lethality will depend on product initial temperature, residence time and temperature history in the heat exchanger, with higher temperatures obviously contributing more lethality. Therefore, the entire aseptic system becomes even more complex since several critical parameters need to be monitored, controlled and documented. Experimental data compared favorably with computer‐simulated data using the AseptiCAL™ software, with the software package giving more conservative results. Ultimately, come‐up credit (CUC) should be tested on a pilot scale or industrial setup by way of reduced residence time (i.e. increased fluid flow rate), reduced holding tube length or temperature in order to determine if CUC can be applied towards the lethality required for the product. Monitoring and control devices become critical to ensure consistency and reproducibility in product residence time and time–temperature history, in the tubular heat exchanger.