Vol. XII · Food Safety Plate 5 of 12log reduction, D-value, z-value, thermal death, pasteurisation, Salmonella, lethality, 7-log, kill stepSH

Killing Bacteria.

Heat does not kill all the bacteria at once. It kills nine in ten every few minutes, and the cook decides how many tens to remove.

Hold a food at a lethal temperature and the bacteria in it die at a steady fractional rate: in each interval the same share of the survivors dies. The time to kill nine in ten — one log reduction — is the D-value at that temperature. Two D-values remove 99 %, three 99.9 %, and so on. There is no point at which the count becomes exactly zero; instead a process is designed to make the chance of any survivor vanishingly small, for example a 7-log reduction of Salmonella for poultry (FSIS) — from ten million to one.

The D-value shrinks steeply with temperature: it falls tenfold for every z degrees hotter, about 5–6 °C for Salmonella. So holding chicken at 60 °C for many minutes kills as surely as reaching 74 °C for a few seconds (XII.6). The workshop heats a population and counts the survivors.

Below: the surviving count, in logs, of a population of Salmonella held at the temperature you choose against minutes (microbe bench, log-linear death with the D and z fitted to the FSIS tables), and the D-values across temperatures.

What you will learn
Workshop
Log-linear death: log N falls by t / D(T), D(T) = D(60 °C) × 10^((60 − T) / z). Salmonella in beef: D(60) = 1.73 min, z = 5.6 °C; in chicken: D(60) = 3.9 min, z = 5.4 °C — course fits to the USDA FSIS lethality tables (Appendix A and the poultry tables).

Nine in ten, again and again

For Salmonella in chicken the course fit gives a D-value of 3.9 minutes at 60 °C, 0.46 at 65 °C and 0.055 at 70 °C — each five degrees about ten times faster, because z is 5.4 °C. A 7-log reduction therefore needs about 27 minutes held at 60 °C but only seconds at 70 °C. That is all the FSIS tables say: the same kill, reached by many combinations of time and temperature.

Two cautions. The numbers describe the food's coldest point, usually the centre: the surface may be far past the target while the centre has not started (II.1). And they describe growing cells; spores and some toxins are far tougher (XII.9). A thermometer in the thickest part, and a target taken from an official table, is how a cook turns this arithmetic into safe food.

At the stove

With a thermometer, poach a chicken breast in water held at 65 °C and note how long the centre takes to reach it. Then look up how long it must stay there for a 7-log kill in the FSIS poultry table.

The log-linear law of thermal death was worked out for canning around 1920 by W. D. Bigelow and colleagues at the National Canners Association; the same arithmetic now sets the times for milk, juices, eggs and sous-vide cooking.

Terms

Log reduction
A tenfold fall in a count: 1 log kills 90 %.
D-value
Minutes at a temperature for a one-log reduction.
z-value
Degrees of temperature rise that cut the D-value tenfold.
Lethality
The total log reduction a process delivers.
Heat kills a steady share: one log every D minutes, and D falls tenfold every z degrees. Safety is a log reduction at the coldest point.

Sources

  1. USDA FSIS, Appendix A to the Compliance Guidelines for Meeting Lethality Performance Standards for Certain Meat and Poultry Products (time–temperature table for 6.5- and 7-log reductions of Salmonella in cooked beef).
  2. USDA FSIS, Time-Temperature Tables for Cooking Ready-to-Eat Poultry Products (7-log Salmonella reduction in chicken, 5% fat), as republished in D. E. Baldwin, A Practical Guide to Sous Vide Cooking, and J. K. López-Alt, The Food Lab.
  3. The 12D botulinum cook: D(121.1 °C) = 0.21 min, z = 10 °C for proteolytic Clostridium botulinum spores (standard values in thermal-processing texts, e.g. Fennema's Food Chemistry and the FDA low-acid canned food regulations, 21 CFR 113).