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Many companies offer broth in a box carton with a twist-off cap. In the US, these are usually 32 oz cartons but may be other sizes as well. Twisting the cap breaks a foil seal underneath, allowing one to pour the stock. There is no way to remove the cap without breaking the foil seal, meaning there is no way to know if the foil seal was intact before it was broken as part of opening the carton. There may be some external signs that the seal is not intact or the product is otherwise not fresh (leaking, bulging, etc), but how can I tell that the broth inside is safe to use?

Larger cartons sometimes have a flip-open top and one must pull up the foil seal separately. Smaller containers are usually in cans. Neither have this problem.

Example container from Swanson: Chicken broth

Image taken from Campbell's website

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  • In Australia, these types of boxes are also used for shelf-stable (prior to opening) juices/fruit drinks and long-life milk.
    – nick012000
    Jan 27 at 12:29

5 Answers 5

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After several edits to incorporate suggestions from comments [now gone]
The thing to take away from this answer is that the seal itelf is not the primary indicator of tampering. Because there are many methods of exposing the seal, some of which will actually break the seal by the simple action of trying to check its integrity, then your indicator is the tamper-evident lid… not the seal itself.


In some designs…The lid when new is not fully closed [see above]. You must close it to puncture the foil seal using a 'blade' mechanism in the lid itself. This also breaks the plastic tamper-proof seal which is an integral part of the lid. There are variants on this design [see above] including ones that break the seal as you first unscrew the lid, but all have the tamper-evident lid & some mechanism to break the seal on first opening.
This plastic tamper-proof device is your evidence the seal may be compromised. No other evidence can be known at this point.

Even if someone just removed the lid, though they may not have broken the food seal, you could see they'd broken the plastic lid.

You don't have to check for pressure or any other signs of spoilage, just look at the tamper device on the lid. The carton re-seals perfectly with the lid fully tightened by the consumer after purchase, so pressure, leakage etc is not the true sign is has been tampered with, merely additional tell-tales.

The old method of having a separate pull-off seal underneath wasn't really liked by either consumer or manufacturer. For the consumer it meant a two-step process; those seals can be annoying if you don't have full mobility [& even if you do]. For the manufacturer, it meant they had to employ a secondary piece, manufactured & adhered separately; more room for error & more expensive.
The new design means the seal is part of the inner box; card, foil, plastic. All they need to do is leave a hole in the card before laminating the rest. The lid protects it before sale & contains the mechanism to puncture it before use.
Easier, cheaper, greener.

Here's one major manufacturer's version of this lid design, TetraPak - https://www.tetrapak.com/en-us/solutions/packaging/packaging-material/helicap--23

Tamper-proof seal, unbroken

enter image description here

If broken, the ring where those small gaps are will be separated from the rest of the lid & loose.

BTW, in the UK at least, even the flip-top containers now have this single 'open to puncture' mechanism. The flip top is a lever mechanism which itself penetrates the seal on first opening, then clips shut again. Except for bottled [as opposed to carton, UHT] milk, I think most of the old pull-top type seals are gone.

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    Great answer. I worry sometimes not about tampering per se, but that the foil is broken for some production issue, so the external cap+seal would be intact, that's what got me interested in the question.
    – Luciano
    Jan 16, 2023 at 13:29
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Give it a slight squeeze. If it’s letting air out, you’ll be able to continuously squeeze until liquid comes out. If it is sealed it will resist further squeezing after only a slight squeeze.

Additionally, the seal is under a sealed plastic twist top, which is not actually in the “closed position” at the time of purchase. Hence, you need to break the outer plastic seal (which is itself a tamperproof design) of the cap and twist it closed to break the inner seal to open it initially. If the inner seal was already broken or compromised, even with the cap plastic seal intact, it would simply pour out of the container when you lift it up.

Another precaution is if you see a carton that seems overly full (or inflated), that to me is a sign that the contents have likely spoiled.

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I don't know for sure but I think the internally sealed ones are built that way in the factory as one piece, with the lid piercing an internal wall of the container rather than having a separate hole to open. In this design I think there are typically 3 layers - outer cardboard, internal foil and innermost plastic. The lid rips open the foil and plastic layers.

I suspect that this makes manufacturing the carton easier and allows greater reliability of the packaging. With cartons where the lid doesn't pierce, and you need to peel off a seal below the lid adds an extra point of failure and possibly manufacturing complexity ...cost.

There is no way to tell whether the carton has been opened before you reach it, other than to inspect the lid and look for signs that it has been loosened. In cases where it has been opened, I would expect the contents to spoil within 24h at room temperature, which would be obvious by bulging or shrinking of the packaging (depending on what sort of bacteria contaminating and whether they metabolize to produce gas or not).

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To prevent spoilage, these sorts of cartons often will contain no air in them when sealed. By opening the package you allow air to enter the carton - which will then cause spoilage over time.

Next time you go to open a carton, feel for the difference before and after opening the carton with how the liquid moves when sloshed or shaken. There is a distinct difference with and without the additional air!

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    From admittedly a sample of only one, [I only have one carton in the house right now which is unopened], there is air in the carton. It is presumably sterile [BB date is 9 months], but it is definitely present. I think to attempt to remove all air would mean more spillage, which would then require better contamination controls outside the carton area & some cleanup of each carton before finally packing to ship. I don't think this would be a practicable solution.
    – Tetsujin
    Jan 13, 2023 at 17:17
  • Interesting! I haven't ever handled an unopened carton with air, although admittedly I predominantly handle soy milk cartons. I wonder if that's a difference of preservatives etc. It would be interesting to know if the ones with air still feel different when unopened vs opened
    – ljden
    Jan 14, 2023 at 22:04
  • @Tetsujin but how can you tell that air is "definitely present"? I am genuinely curious - in my mind distinguishing between air & vacuum would not be so easy. Or is the working assumption that a carton cannot withstand any vacuum, and therefore sploshing implies air presence? Jan 15, 2023 at 7:57
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    If there was no air there would be no vacuum, the carton is simply not strong enough to support one.
    – Tetsujin
    Jan 15, 2023 at 8:20
  • I had a shake of a few cartons at the shops yesterday, some definitely have air in them. Not able to compare it before and after opening but I'm curious if there's a difference
    – ljden
    Jan 16, 2023 at 9:12
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What I do is I shake the bottle before opening it. After carefully opening it, if the underside of the cap is wet, the seal was broken.

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