SOP: Computation of High-Temperature Short-Time Processes by the Alignment Chart

GIFSQ — Global Institute for Food Safety & Quality

Principle

When high-temperature (270–290 °F) processes are used, the holding time can be reduced to a few seconds. The alignment chart (Fig. 25.25 of the book) computes the sterilization value of thermal processes for liquid and semi-liquid food products in the temperature range 260–300 °F, expressing the relationship between the sterilization value F₀, the holding time S, the process temperature T and the TDT-curve slope z.

Materials required

• Fig. 25.25 of the book (alignment chart for short-time high-temperature processes)

Procedure

Note the z value — it represents the °F by which the process temperature must be increased to compensate for a 90% reduction of holding time while maintaining the same sterilization value; it is usually taken as 18 for preliminary calculation (values of 16 to 19 are most commonly used; z varies with the type of food and the spoilage organism)

↓

If three of the four factors S, F₀, T and z are known, find the fourth: connect the known values with two straight lines intersecting on the X axis, and read the unknown value on the appropriate axis (key: S − F₀ and T − z meet on X)

↓

Multiply F₀ and S by equal factors to handle values beyond the chart range

Calculation

  F₀ = S × 10^((T − 250)/z)

Where,

• F₀ = sterilization value
• S = time in seconds in the holding section
• T = temperature (°F) in the holding section
• z = slope of the thermal death time curve (°F)

Note: the printed layout of this equation is garbled in the available copy; the form above is reconstructed from the book’s symbol definitions and verified against all three of its worked examples (Example 1: T = 286 °F, z = 18, S = 3 s gives F₀ = 3 × 10² = 300 s = 5, as printed; Example 2: S = 15 s gives F₀ = 1500 s = 25, as printed; Example 3: F₀ = 10 min = 600 s, S = 9 s, z = 17 gives T = 281 °F, as printed), and should be checked against the printed book.

Precautions

• The sterilization value required for low- and medium-acid foods (pH greater than 4.5) usually falls within the range F₀ = 4 to F₀ = 20.
• In Example 2, 15 s is off the scale — use S = 1.5, read F₀ = 2.5 for the 1.5 s holding time, then scale up for the prescribed 15 s to get F₀ = 25.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Computing High-Temperature-Short-Time Processes, James Dole Engineering Co., California, USA — cited in the book.