Scheduled process for canned mushroom soup: which z-value governs the lethality calculation?
A low-acid canned mushroom soup (pH 5.2, aw 0.98, 400 g cans) is processed in a still retort. The scheduled process is 112 °C retort temperature with a 22 min hold after come-up. A heat-penetration study placed a thermocouple at the cold point of the slowest-heating can and, integrating the whole profile (come-up, hold, cooling) with z = 10 °C, the laboratory reported F0 = 2.9 min. The company's target is F0 = 12D, with D121 = 0.21 min for the reference spore. However, a separate thermal-death-time study on the same spore suspension equilibrated in the actual product matrix returned z = 8.5 °C. The cans used in the penetration trial averaged 400 g fill against a specification of 415 g. The product is destined for a market requiring a documented scheduled process, and thiamine retention is a declared quality attribute.
1. Using the correct z-value, does the current process deliver the required lethality? Show the calculation and state the minimum hold time needed.
Read the worked answer
Required lethality: F0 = 12 × D121 = 12 × 0.21 = 2.52 min.
The reported F0 = 2.9 min is not valid, because it was integrated with z = 10 °C whereas the organism in this matrix has z = 8.5 °C. A smaller z means the lethal rate falls off faster as temperature drops, so the come-up and cooling tails are worth less than the laboratory assumed.
Recompute with z = 8.5 °C:
- Lethal rate at 112 °C: L = 10^((112 − 121)/8.5) = 10^(−1.059) = 0.0873 min⁻¹
- Hold contribution: 22 × 0.0873 = 1.92 min
- Come-up and cooling, discounted for the smaller z: roughly 0.15–0.25 min
- Corrected F0 ≈ 2.1 min
That is a shortfall of about 0.4 min, roughly 16% below target. The process is under-validated.
Minimum hold at 112 °C: (2.52 − 0.20)/0.0873 = 26.6 min, so 27 min is the absolute floor. Add margin for the heavier 415 g fill (larger fh, slower heating) and can-to-can variation; 30 min at 112 °C is a defensible revised process. Alternatively, 115 °C for about 13 min gives the same F0 (L = 10^(−6/8.5) = 0.197 min⁻¹) and would actually preserve thiamine better, since thiamine has a much larger z than spores and suffers less in a short high-temperature process.
2. A colleague argues that because the product is pH 5.2, it is an acidified food and a 5D process would suffice, saving quality. Evaluate the argument and comment on the fill-weight discrepancy.
Read the worked answer
The acidified-food argument is wrong. The pH boundary that allows a reduced botulinum cook is 4.6. At pH 5.2 the product is still a low-acid canned food, Clostridium botulinum can grow and produce toxin, and the full 12D botulinum cook remains mandatory. The same reasoning applies at aw 0.98, which is above the 0.85 cut-off. If the formulator wants a reduced process, the product must be reformulated to pH ≤ 4.6 (or aw ≤ 0.85) and that must be demonstrated and monitored, not assumed.
On the fill weight: the penetration trial used 400 g cans while the specification allows 415 g. A 3.75% increase in fill mass increases the can's fh and pushes the cold point further from the heat-transfer surface, so the validated profile does not represent the worst case. Process-validation practice is to validate at the maximum fill weight, highest viscosity and lowest initial temperature permitted by the specification, with the thermocouple in the slowest-heating position.
A second, subtler point: the monitored can is never the coldest can in a retort load. Sufficient cans, distributed in known slow-heating positions, must be instrumented to establish the cold spot of the load, not just of a single can.
3. What do you do with the lot already produced under the 22 min process, and what must be re-validated?
Read the worked answer
Do not release it. On the corrected calculation the lot received roughly 2.1 min F0 against a 2.52 min requirement, which is a genuine public-health deficiency, not a documentation gap.
Options, in order of preference:
- Hold and re-process to the corrected schedule. This is technically possible but doubles the heat load, so thiamine loss, colour and texture must be re-assessed; the lot may no longer meet the declared quality attribute even if it becomes microbiologically safe. Reprocessing must itself be validated and is often discouraged for quality reasons.
- Divert to a non-shelf-stable, chilled or acidified use where shelf-stable commercial sterility is not claimed.
- Reject and destroy if neither is defensible.
Re-validation package:
- Full heat penetration at 415 g fill, worst-case viscosity and lowest initial temperature, with multiple cans in the slowest-heating retort positions.
- Re-derive the F0 integration with z = 8.5 °C measured in the actual product.
- Inoculated pack study with an appropriate surrogate spore to confirm the calculated process, since calculation alone does not substitute for a challenge test in a new matrix.
- Re-check thiamine retention at the revised schedule and update the quality declaration.
- Document the change control; the previous release decision on any earlier lots processed to the same schedule should be reviewed as a potential recall assessment.