Goal: When you have finished this laboratory exercise you will understand

  • the role of microbial lethality in designing food canning processes

and you will learn

  • how to determine F-value in a can for different microorganisms
  • how heating rate at different locations within a can may influence F-value
  • the role of retort temperature on the lethality of a canning process
  • In the food canning industry, both batch and continuous retorts are used. Retorts operate under atomospheric or higher pressures depending upon the process requirements.

    In a batch retort, carts loaded with canned food are pushed into the retort. The retort lid is closed and steam is turned on. After a desired heating period, the steam is turned off, cooling water is introduced to cool the cans. The retort lid is opened and carts are pulled out of the retort.

  • An animation of a batch retort is shown below. Note that steam is introduced into the retort from steam spargers located in the bottom of the retort.

  • Atmospheric Retort: The following slide shows atmospheric retorts used in canning tomato products.

  • Typically, the atmospheric retort is half filled with hot water (95-98°C). The head space contains steam vapors at atmosphric pressure. Cans enter and exit the retort in a continuous manner.

  • Pressure Retort:
    In a pressure retort, steam at a pressure, higher than the atmospheric pressure, is used to process low-acid foods.

  • In a pressure retort, the entire vessel is filled with steam at high pressure. Cans enter and exit the retort in a continuous manner.

  • A continuous retort system with a water cooler.

  • Cross-sectional view of a continuous retort system with a water cooler.

  • In a laboratory experiment, we install a thermocouple in the center of a can of food. The can is then placed in a retort and heated with steam while the temperature of the food is recorded. The composition of the nectarines used in the experiments is given as follows:

  • 1) To install a thermocouple in the center of a can, a hole is punched in the lid of the can, and special fittings are used to prevent leakage of food during the heating process. the fittings are shown below.

  • 2) The thermocouple fittings are threaded through the lid.

  • 3) A can lid fitted with a thermocouple connector is shown below.

  • 4) Using a canning machine, the lid, containing the thermocouple-connector, is attached to the can.

  • 5) The can, containing the thermocouple, is placed in the retort. Care is taken to prevent the thermcouple wire from entangling with the rotor.

  • 6) The retort lid is closed and steam is turned on. A data acquisition system is used to record the temperature of the steam and center temperature of the can.

  • In a virtual experiment, you first select the can size from the three given choices. Set the desired retort temperature, cooling medium temperature, heating time, and initial temperature. The results will include the temperature plot. The recorded data will include temperature at three locations, can center, near the can wall, and a location midway between the center and the can wall.

  • The primary objective of thermal processing of foods is to ensure the safety of the product. The lethality of microorganisms obtained during canning depends on the heating conditions, the heat resistance of the microorganisms and the heating characteristics of a food product. The decimal reduction time, or D-value, is the time required to reduce the population of a microorganisms by one log-cycle (or by 90%).

    The effect of temperature on the D-value is called the thermal resistance factor known as z-value, which is unique for each microorganism depending on the surrounding medium. It may be defined as the temperature change required to reduce the D-value by 90%.

  • The total time required to accomplish a certain reduction in the number of microorganisms is called the thermal death time (F-value)

    The F-value of an applied thermal process is often required, since it indicates the number of log cycles of microbial reduction.

  • The F-value is generally calculated based on knowning the temperature history of the product at its coldest point

    where Tc(t) is the change in slowest heating point temperature with time, Tr is the reference temperature (121.1°C), z is the z-value of the microorganism, and t is the processing time.

Retort Settings


Can Size

t = (min)
  • 1) Use the time and temperature column for the slowest heating point from the spreadsheet obtained in the experiment. Copy these columns into a new work sheet as will be shown in the next slide.

  • 2) Prepare a new column for lethality, L, using the formula for lethality as shown. Note that this formula assumes that z = 20.

  • 3) The lethality curve is shown in the following XY Scatter plot prepared using lethality (L) values vs. time. This plot shows the trapezoidal values used in the integration.

    The change in the L-value at two successive times is the longer (a) and shorter (c) dimensions of the trapezoid, the difference between two successive times is the height, h, of the trapezoid.
    The area of a trapezoid is given by:

  • 4) Create column D to calculate integration of lethality using the trapezoidal rule.

  • 5) Determine the sum of all the integrated values. In this example, data were contained in rows 4 and 104. The summation gives the F-value.

  1. How does the F-value change by retort temperature? Does it change by location in the can?
  2. What is the effect of z-value on F-value? Does F-value increase with an increase in z-value?
  3. Determine the accomplished log cycle reductions in your trials for a D-value of 2 min?
  • Karel, M., and Lund, D.B. (2003). "Physical Principles of Food Preservation," 2nd ed., Marcel Dekker, Inc., New York.
  • Singh, R.P. and Heldman, D.R. (2009). "Introduction to Food Engineering," 4th ed., Academic Press, London.
  • Stumbo, C.R.(1973). "Thermobacteriology in Food Processing," Academic Press, New York.
  • Toledo, R.T. (1994). "Fundamentals of Food Process Enginering," 2nd ed., Chapman and Hall, New York.