Thermal Food Processing

136 Thermal Food Processing: New Technologies and Quality Issues
the average velocity. As such, the turbulence regime provides a more uniform
mixing condition for thermal processing. The average residence time is the length
of the pipe divided by the average velocity; thus, the microbes that travel at the
maximum velocity would not be fully processed.
This is related to aseptic processing, where food is sterilized or pasteurized
in a tubular, helical heat exchanger, scraped-surface heat exchanger, microwave,
or ohmic heater. The aseptic process has significant quality advantages over
classical thermal techniques, such as batch (canning) and semibatch operations.
In general, flow modeling has to be used to ensure that food products meet the
safety requirements. It determines the optimal lengths of the heating, holding,
and cooling sections.
5.2.2 THERMAL RESISTANCE CONSTANT Z
The thermal resistance constant Z is a parameter representing the microorganism’s
resistance to temperature rise:
5.2.3 THERMAL DEATH TIME F
(5.8)
The thermal death time F is the time required to cause a stated reduction in the
population of microorganisms or spores. This time is expressed as a multiple of
D values. A 99.99% reduction in microbial population is equivalent to 4D, for
instance. Usually, F is expressed as F z
T for a specific temperature T and a thermal
resistance constant z.
5.2.4 RELATIONSHIPS BETWEEN CHEMICAL KINETICS
AND THERMAL PROCESSING PARAMETERS
It is generally accepted that at a constant temperature, the microbial population
or number concentration (microbe· m –3 ) (N) reduces following the first-order
reaction:
Therefore, the solution for this is
T2 − T1
Z =
DT1
log
D
dN
dt
T2
=−k⋅N ln N
=−k⋅t N
o
(5.9)
(5.10)