performance of a heating block system designed for studying the heat resistance of bacteria in foods
Choosing the right test method is important to accurately determine the heat resistance of bacteria.
Although it is a major factor affecting temperature
Tolerance of bacteria, due to the thermal properties mainly dependent on the sample, the heating rate in the sample cannot be controlled in the water or oil bath method.
Heating block system (HBS)
Designed to adjust the heating rate in liquid and semi-liquid
Solid and solid food using a temperature controller.
Distilled water, apple juice, mashed potatoes, almond powder and beef were selected to evaluate the performance of Harvard Business School through experiments and computer simulations.
The results show that the heating rate finally set to 1, 5 and 10 °c/min
Of the five selected food materials, you can easily and accurately reach the point temperature and retention time.
At different heating rates between the experiment and the simulation, the temperature distribution of the sample center is well consistent.
The results of experiments and simulations show that Harvard Business School is able to provide a sufficiently uniform heating environment in food samples.
Use Harvard Business School to evaluate the effect of heating rates on the heat resistance of bacteria.
The system can be used to quickly and accurately evaluate the heat of bacteria-tolerances.
Consists of a heating unit, a data acquisition/control unit, and a computer ().
The heating unit includes the top (
28 u2009 length u2009 × u2009 28 u2009 length u2009 × u2009 1. 6u2009cm)and bottom (
28 u2009 length u2009 × u2009 30 u2009 length u2009 × u2009 2. 4u2009cm)
Block, heating pad and 6 pull-
As mentioned earlier, push case with 6 TDT units.
The heating block is made of aluminum alloy with low thermal capacitance (903u2009J/kg°C)
High thermal conductivity (234u2009W/m°C)
Provides a smooth block temperature during heating and holding. Eight custom-
Manufacturing heating pads (250u2009W)
On the surface of the block that is glued to the top and bottom, the maximum heating flux density of 12000 w/m is provided.
T thermocouple (TMQSS-020-
Omega Engineering Limited, CT, USA)
Insert through the sensor path to monitor the temperature of the top and bottom blocks, as well as a sample temperature in the TDT battery. Heating rate (0. 1 to 13. 3u2009°C/min)and the set-
Point temperature (
Maximum 120 °C)
By Visual Basic software through solid-state relay.
Two proportionsintegral-derivative (PID)controllers (
I32 Omega Engineering Limited
United States, CT, Stamford)
Two surface temperatures are adjusted separately.
6 TDT batteries have just been installed on the pull-
A push box that is evenly distributed in the bottom block ().
The box can be easily pushed into the block to heat up and removed in ice water for quick cooling.
The TDT unit consists of a base and a threaded connection
On the lid for easy loading and unloading of samples (). A rubber o-
The ring between the two parts provides a seal to maintain a constant water content of the sample. The disk-
The cavity is 20.
With a diameter of 6mm and a height of 3mm, the sample space of 1 is provided. 00u2009ml.
The design of a TDT battery with a ratio of high heated surface area to sample volume may provide a shortcut.
Represented by distilled water, apple juice, mashed potatoes, almond powder and ground beef, half
Solid and solid food.
Dry mashed potato slices during sample preparation (
Mixed with distilled water to 15. 38% wet basis (w. b. )mashed potato.
Apricot kernel is obtained from the Almond Board in moderstowe, California, USA, and has been refrigerated before use.
The core is grinded with a grinder and passed by a number18 mesh (16 Tyler).
6% moisture content of wet base (w. b. )
According to the standards of the official association of analytical chemists, the vacuum oven method was used to determine [method 27. 005].
Ground beef (90% lean)
Obtained from the alocal store, placed in the zipper
Vacuum lock bag-
Seal, freeze at-20 °c
Distilled water of 1 ml, 1-Apple juice (ml)
100% apple juice in Yanling Huiyuan, China)or 1-
G mashed potatoes, or 0. 8-
Almond powder g, or 1. 0-
G-thawed beef is placed in TDT cells at room temperature.
For water, apple juice, and mashed potatoes, then raise the block temperature to 90 °c at 1, 5, and 10 °c/min, and hold for 1 min at that temperature, enough to kill most bacteria
For almond powder and beef, the temperature is set to 120 °c due to their stable state, and the heat treatment follows the same procedure.
The data acquisition/control unit and computer program monitor and record the sample temperature and block temperature in the TDT battery.
Repeated three times per treatment.
To further evaluate the performance of Harvard Business School, 1-
Use ml distilled water from the same TDT pool as a reference method for comparison.
Depending on the heat resistance in the water bath, the prepared TDT battery is heated to 70 °c (SC-
Ningbo Scientz Biotechnology Co. , Ltd. , Ltd. , China)
, Keep 2 at this temperature.
4 min, then cool immediately in ice water.
The core temperature of the sample is the use of thin pre-Calibration typeT thermocouple (TMQSS-020-
Omega Engineering Limited, CT, USA)
Record With Data Recorder (CR-
Campbell science company 1000
Logan, Utah, USA)
During the interval of 6 Pax s, the interval is the same as the one used in Harvard Business School.
For the Harvard Business School approach, the same target temperature is 70 °c and the fastest heating rate is 13.
3 °c/min for processing the same sample and the same procedure for post-processing
Heat treatment like water bath method.
The experiment was repeated three times.
Heat transfer model is mainly used to simulate the heating process of high boiling alcohol.
Harvard Business School is modeled as three
A component system consisting of two aluminum blocks and six battery samples.
Set the heating pad to a boundary heat source.
Heat is passed from the heating pad to the top and bottom blocks and then to the battery and sample via heat conduction.
The heat loss from the side wall of the heating block to the environment is estimated by the natural convection heat loss with a value of 5 w/m °c.
The model assumes that the thermal properties of aluminum blocks, air and samples and the heat transfer resistance between blocks or between heating pads and blocks are constant within the test temperature range, as listed in.
Transient heat transfer through blocks and samples is controlled by the following differential equations: where the mass density of the material is kg/m, is J/kg °C, is the thermal conductivity in W/m °C, it is the temperature in c, the time in s, or M. Equation (1)
Affected by the following initial conditions: where is the initial temperature of the material (°C). Heat flux (W/m)
Starting with the heating pad, in the vertical direction of the interface between the top and bottom blocks and the heating pad, it is described as follows: where the vertical direction outward is the surface.
Convection heat transfer perpendicular to the edge of the block on the side is given by: where, the surface heat transfer coefficient at W/m °c, and ambient air temperature at °c.
The boundary heat source from the heating pad provides different heating flux ()
Affected by the heating rate and block thickness, it can be calculated as follows: where the heating rate is (°C/min)
And is the thickness of the top (16u2009mm)
And the bottom block (24u2009mm), respectively.
Temperature uniformity index (TUI)
Is a useful parameter to evaluate the heating uniformity of the treated sample in the simulation: where, the final local temperature in c, is the average temperature in c relative to volume (, m)
And is the initial temperature before heating.
The equations of energy, momentum and transmission are solved by finite element method.
Build a physical model using Creo software (
Creo parameter 2.
0, parameter Technology Co. , Ltd.
Joseph Needham, MA, USA).
COMSOL multi-physical software (V4.
COMSOL, COMSOL, Co. , LTD.
For numerical solution of heat transfer equation (
Equations 1, 2, 3, 4, 5, 6).
A relatively fine four-sided mesh was created.
When the temperature difference at the same point between two continuous grid groups is less than 1%, the grid size is considered appropriate.
A final grid system consisting of 23,342 domain elements (tetrahedral)
7,220 boundary elements (triangular)
Edge element 1,404 (linear)
, 210 endpoint elements were used in subsequent calculations. The time-
Dependency solver is used.
The time step is set to 0.
1 min, the relative tolerance is 0. 01.
A lenovoa46 00k computer with two Dual-Core i5-2400, 3.
00 ghz Xeon processor with Windows 8 64-and 4 gb RAM
Bit operating system is used when running the software.
According to the simulation sequence and specific conditions, the total solution time varies from 5 to 10 minutes.
ATCC 25922 is chosen because it is non-pathogenic and has been proved to be log-
Linear deactivation dynamics under constant temperature conditions.
Mashed potatoes were selected as semi-solid food to eliminate heat convection and facilitate placement in TDT cells.
The strain was obtained from the School of Food Science and Engineering, Northwest Agriculture and Forestry University (Yang Ling, China). 0. 8-
Mashed potatoes and 20-
μ l bacterial suspended solids containing a 10 CFU/ml cell population are placed within each TDT cell.
The treatment temperature is 57 ℃ and the heating rate is 0. 1, 0.
Use 5, 1, 5 and 10 °c/min.
When the target temperature is reached, the test unit maintains a different time interval depending on the heating rate to achieve a minimum of 5-log reduction.
Test cells are placed on ice immediately after holding
Take a shower before further analysis.
All the experiments were repeated twice.
All statistical analyses were performed at a notability level of 5% using the Microsoft Excel variance program (
Microsoft Office excel 7).