SOP: Conversion of Thermal Death Time Data to Classical F and D Values

GIFSQ — Global Institute for Food Safety & Quality

Principle

F values reported in the literature are often based on different initial numbers of cells and are therefore not comparable; D values are independent of initial numbers and can be compared directly. Because Esty and Meyer’s classical data for Cl. botulinum (a preparation containing 6 × 10¹⁰ spores killed at 250 °F in 2.78 min, 2.45 min by Townsend’s corrected value) were based on complete destruction, TDT data obtained in a product are converted to the classical resistance so that process calculations rest on a common basis. It is customary to assume that their TDT curve reduced 1 × 10¹⁰ spores to one spore — that is, F = 12 D.

Materials required

• TDT data for Cl. botulinum spores obtained in the test product and in phosphate buffer, at z = 18
• The classical resistance of Cl. botulinum in phosphate buffer = 2.45 min (corrected value of Townsend et al.)

Procedure

Carry out the TDT runs with Cl. botulinum spores in the test product and, in parallel, in neutral phosphate buffer (z = 18)

↓

Convert the product data to the classical resistance using the formula below

↓

Convert to the classical D value using the formula below

↓

Interconvert D and F values using Schmidt’s formula where required

Calculation — Classical F value

  Classical resistance of Cl. botulinum in phosphate buffer (= 2.45) × Resistance of Cl. botulinum obtained in the product in the TDT run

F = ——————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————————
Resistance of Cl. botulinum obtained in phosphate buffer in the TDT run

Where,

• F = classical F (i.e., classical thermal death time) of Cl. botulinum

Note: the printed layout of this fraction is garbled in the available copy; the form above is reconstructed from the book’s words, and should be checked against the printed book.

Calculation — Classical D value

      0.204 × D₁₈ obtained in the test product

D = ———————————————————————————————————————————————————————————
D₁₈ obtained in the test run using phosphate buffer

Where,

• D = classical D₂₅₀ for Cl. botulinum spores heated in the product (0.204 min is the classical D₂₅₀ in phosphate buffer, obtained as 2.45/12 — the divisor 12 comes from the F = 12 D convention, i.e., the assumed reduction of 1 × 10¹⁰ spores to one spore stated in the Principle, not from the logarithmic formula)
• D₁₈ = D value determined at 250 °F with z = 18

Note: the printed layout of this fraction is garbled in the available copy; the form above is reconstructed from the book’s words, and should be checked against the printed book.

Calculation — Interconversion of D and F (Schmidt)

       F
  D = —————————      or      F = D(log a + 2)
      log a + 2

Where,

• D = decimal reduction time in min
• F = sterilizing value (lethality) in min at 250 °F
• a = initial number of spores

Note: the printed layout of this formula is garbled in the available copy; the form above is reconstructed from the book’s words, and should be checked against the printed book.

Precautions

• Since the death rate of microorganisms is assumed to be logarithmic, it is theoretically impossible to obtain complete destruction — accept for the F value at a given temperature a heating time that results in a definite proportion of survivors: usually one surviving spore in 100,000 (5 D) for FS 1518 and PA 3679, and one in 1 × 10¹² (12 D) for Cl. botulinum spores.

References

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

Esty, J.R. & K.F. Meyer, J. Infect. Dis., 31, 650 (1922); Townsend, C.T., J.R. Esty & F.C. Baselt, Food Res., 3, 323 (1938); Schmidt, C.F., J. Bact., 59, 433 (1950) — cited in the book.