(585ae) Modeling and Validation of the Continuous Htst Thermal Processing of Liquid Foods With Plate Heat Exchangers

Thermal processing of liquid foods aims the
inactivation of micro-organisms and enzymes that compromise the safety or the
shelf life of the product. The design of continuous processes using heat
exchangers usually rely on simplifying assumptions that provide a wide safety
margin and facilitate calculations, such as isothermal processing only at the
holding tube at the minimum residence time. These assumptions can impair
sensory or nutritional attributes of the product because of over-processing. The
aim of this work was to develop and validate a phenomenological model to
simulate the continuous HTST pasteurization with plate heat exchangers in order
to determine the distribution of the temperature of the fluids and of the
concentration of the target micro-organism or activity of the target enzyme
throughout the process.

The considered pasteurizer comprises the
inlet tank, a plate heat exchanger (PHE) with three sections (heating, cooling
and heat regeneration), the holding tube, the tubular connections between the
exchanger and the holding tube, the hot water circuit and the cold water
circuit (Figure 1). The model includes equations for flow, heat exchange and
lethality. Main assumptions are: steady-state operation, plug-flow in the
channels of the PHE and tubes, uniform flow distribution among PHE channels,
heat exchange with ambient air only in the tubular sections, no axial diffusion
and uniform thermo-physical properties in each section.

Figure 1: Representation of the HTST thermal process unit

Each PHE channel or tube provides one
differential equation for heat transfer and one for thermal inactivation with the
corresponding boundary conditions. The sterilization value was evaluated as the
number of decimal reductions on the target micro-organism. Space time (length velocity
ratio) can be replaced in the thermal inactivation equations by the experimental
mean residence time or by the minimum residence time, depending on the desired
study (evaluation, validation, design or optimization). Model simulation
provides axial distribution of product temperature and the associated
lethality, allowing the assessment of the contribution of each section of the
process.

In order to test and validate the model, a
FT-43A laboratory scale plate pasteurizer (Armfield) was used. The sections of the
PHE were configured for series flow (one channel per pass) with 11 thermal
plates in the heating section, 7 thermal plates in the cooling section and 19 thermal
plates in the heat regeneration section. Product flow rate was 20 L/h and the
flow rate of the service fluids was 1,0 L/min (hot water and cold water). The
tested product was phosphate buffer (pH = 6.6) with alkaline phosphatase (EC
3.1.3.1) acting as an enzymatic time temperature integrator (TTI), as proposed
by Aguiar et al. (2012, DOI: 10.1016/j.lwt.2011.12.027). Enzymatic activity was
measured using a reflectometric method and the inactivation kinetic model used
was first order with two isoenzymes.

Processing temperatures were 70, 75, 80 and
85 ºC and the controller manipulated the inlet temperature of the hot water in
order to achieve the desired temperature at the end of the holding tube.
Temperatures at 12 points along the process were registered using thermocouples
and a data acquisition system (National Instruments). Samples of the TTI were
collected at the exit of the heating section of the PHE (p3 in Figure 1) and
after the cooling section (p8 in Figure 1). Samples were immersed in ice-water
bath to stop the inactivation, prior to activity assessment.

The process was simulated using the proposed
mathematical model in order to compare experimental and predicted results of
temperature and lethality. Parameters provided for model simulation were:
geometrical parameters of plates and tubes, mean thermo-physical properties,
inlet temperatures and flow rates, inlet concentration or activity of target,
kinetic parameters of thermal inactivation, heat exchange parameters and mean
residence times. The model was solved using a finite difference method in
software gPROMS 3.2 (Process System Enterprise). Discretized variables were the
temperature of the fluids and target concentration.

The mean residence time in the channels of
the PHE were obtained from Gutierrez et al. (2011, DOI:
10.1016/j.applthermaleng.2011.02.015) and the mean residence time in the
holding tube were obtained from Gutierrez et al. (2010, DOI:
10.1016/j.jfoodeng.2010.01.004). Space time was calculated for the tubular
connections. The Nusselt x Reynolds correlation for the heat transfer in the
PHE was obtained according to Gut et al. (2004, DOI: 10.1016/j.ces.2004.07.025)
for series pass arrangements. Mean overall heat transfer coefficients for the
tubular section were acquired from experimental data.

The model simulation provided the
temperature history of the product along the process and the distributed
residual activity of the enzymatic TTI. Figure 2 presents the obtained results
for temperature and residual activity of the TTI. There was an excellent
agreement between predicted and experimental temperatures at the registered
points with a mean absolute error of 1.1+?1.0 ºC for all four processing conditions.

Figure 2: Obtained experimental and simulated results for
temperature distribution and residual activity of the TTI for the four
processing temperatures

The results of temperature history and
residual activity of the enzyme showed that the inlet and outlet connections of
the holding tube play an important role on the process lethality. The lethality
associated with the heat exchangers was also significant. There was a very good
agreement between predicted and measured residual activities at the end of the
heating section because the simulated values were in the confidence interval of
the experimental values. However, the simulated values of residual activity at
the end of the process were consistently inferior to the measured values. This
deviation can be explained by the use of the space time instead of the mean
residence for the tubular connections. Since the experimental mean residence
time is shorter than the calculated space time, the predicted residual activity
was smaller.

The proposed model allows the simulation of a
continuous HTST thermal processing with plate heat exchangers in order to
determine the temperature history and process lethality. With the experimental
data used in this work it was possible to validate this model, which can be
used for analysis, design or optimization problems, depending on the residence
time used for each process step (space time, experimental mean residence time
or minimum residence time).