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I'm starting here from the conservative position both the UK food standards agency and the NHS take regarding reheating food. Both bodies state that food should only be reheated once.

So the filling is cooked and left to cool. The cold filling is then added to the pastry shell which is then partially or fully baked. The consumer then reheats this filling a third time when eating it after purchase.

Assuming everything is brought up to a sufficiently high temperature and refrigerated quickly, I cannot see any problem with this process other than the potential deterioration of the quality of the filling. The pies should still be safe to eat, however.

Are there any other techniques used commercially in the preparation of pies? If not, am I correct in assuming these food safety standards are over zealous in this instance?

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    I don't think this will completely answer your question, and it is a US podcast, but they are two food scientists...its short....it is about pie safety. Some interesting points. riskyornot.co/episodes/…
    – moscafj
    Commented May 27, 2023 at 5:22
  • I'm not saying this is how it's done commercially but you can add the filling hot to the pastry immediately before baking. You can even lightly blind bake the bottom, cool a bit, add the hot filling and the top pastry, then bake, for better texture underneath. While this all requires you to work quite quickly and precisely, factory machinery is good at that.
    – Chris H
    Commented May 27, 2023 at 7:10

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The answer to this lies in why food authorities don't recommend reheating food more than once.

Simply put, it's because of the amount of time the food can spend in the "danger zone" between 5C and 60C. Consider this sequence, for example:

  1. Cook some food at 180C
  2. Let the leftovers cool on the counter for 2 hours
  3. Put them in the fridge
  4. Reheat the leftovers to comfortable eating temperature, with them only around 40C in the center
  5. Let them cool again before refrigerating the leftover leftovers
  6. Heat them again to 40-50C

By the time you reach 6, your food has spent at least four hours in the danger zone, maybe up to ten hours depending on how effective your fridge is and how long they spent on the table, without ever reaching bacteria-killing temperatures. That's what you're being warned against.

Now, here's the sequence for a savory pie (fruit pies also have acidity to protect them):

  1. Filling is cooked at 160C.
  2. Filling is cooled for 1 hour
  3. Filling is baked in the pie at 180-200C
  4. Pie is cooled for 2 hours
  5. Pie is chilled
  6. You reheat the pie to 50C

In the case above, the pie filling has not spent more than three hours in the danger zone after being heated to over 160C. Further, the fact that the filling is "sealed" inside a crust provides some additional protection from spores and the like. Food authorities would, however, caution against reheating the pie multiple times.

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  • I'm not a food scientist, but I don't think this is the whole story...bacteria isn't the only risk. Give the podcast I linked to above a listen. Water activity is a large factor for food safety. This will depend on the specific pie filling. Plus, "sealed inside the crust" really means nothing from a food safety perspective. It certainly doesn't protect from toxins produced from problematic spores that are heat resistant, and my already be present. Plus, lots of already cut pies are kept outside of refrigeration. This is not only about the danger zone. It's actually a bit more complex.
    – moscafj
    Commented May 27, 2023 at 20:25
  • @moscafj I'd expect the water activity to be pretty high in a lot of pie fillings. OTOH there's no way the filling is heated to 160C. It might be heated at 160 but if the filling reached anything like that temperature it would be inedible - and have no water left in it. A (semi-) liquid filling could be cooled far faster than the answer suggests. Even the finished pie could be in a factory (but not a little bakery).
    – Chris H
    Commented May 27, 2023 at 21:41
  • @ChrisH my point is, this is not a straightforward issue of the danger zone, and when one refers or water activity in a food safety context, it doesn’t mean eliminating all of the water. Pie filling is tricky (that is why the podcasters, again actual food scientists, wind up disagreeing.) Water activity, sugar, acid are all in play….mass produced pies almost certainly require refrigeration, freezing, and/or use preservatives. Local bakers have to consider the safety factors. …just saying, it’s a more complex issue than heat.
    – moscafj
    Commented May 28, 2023 at 4:50
  • @moscafj I agree with your conclusion, and yes, water activity isn't just water content but in the sort of pie that's meat in gravy, or some fruit pies it's never going to be low enough to preserve the contents. But it might explain why shelf stable apple pies have a filling more like a thick jam than like that of chilled ones. Of course a bakery has to ensure food safety, but an industrial scale blast chiller may not be an option (though they've become much more widely available). Same goal, fewer tools.
    – Chris H
    Commented May 28, 2023 at 7:12
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    @moscafj the OP's question was "why do food authorities recommend this" rather than "what are all the real factors we should consider". I answered on that basis.
    – FuzzyChef
    Commented May 28, 2023 at 18:27

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