What spoilage looks like — and why it usually looks like nothing

Two kinds of spoilage, and only one of them announces itself

Most food that goes wrong goes wrong loudly. Moulds fur the surface, yeasts fizz and push the lid, bacteria produce smells that clear a room. Those organisms need oxygen, or tolerate acid, or both, and when one of them gets into a jar you will know. That is what the inspection above is built to catch, and it catches it well.

Clostridium botulinum is the other kind. It grows only where there is almost no oxygen [F-1.4], which means it grows in exactly the place your inspection cannot reach and the competing organisms cannot follow. It does not have to fur, fizz or stink to do what it does. Sometimes a jar shows gas and a broken seal and you get a warning. Sometimes it shows nothing, which is what the guide means by "very little evidence" [F-11.7].

So the inspection is a filter, not a verdict. It removes the jars with obvious problems. What is left is a set of jars with no obvious problems, which is a different thing from a set of jars that are fine.

The sealed jar is the habitat

The uncomfortable part of the four conditions is how ordinary they are [F-1.4].

Moist low-acid food is most of a vegetable garden — beans, corn, carrots, greens, plain tomatoes, meat, stock. A temperature between 40° and 120°F [F-1.4] is every cupboard, pantry and basement shelf in the country. Less than 2 percent oxygen is the vacuum, the thing that means the canning worked.

Read as a set, that is a description of a correctly sealed jar of home-canned vegetables sitting where you meant to put it. The three environmental conditions are all satisfied by success, not by failure.

Which is why none of this is about storage, cleanliness, or how careful you are with your kitchen. The spores are on the food when it arrives and washing barely touches them [F-1.4]. The only step in the entire process that deals with them is the heat, at a temperature and for a time the food actually reached. That step happens once, in the canner, and nothing afterwards substitutes for it.

What the inspection is genuinely good for

It would be a mistake to read all this and stop inspecting. Do it every time you take a jar off the shelf, in the order the guide gives [F-11.7]. Three reasons.

Seals fail. Not often, but they do, and a jar that lost its seal in month four is a jar that has been open to the air since month four. The concave lid and the streaks down the glass catch that.

Ordinary spoilage is common. Under-acidified pickles, a jam that grew mould around a nick in the rim, a jar that was underfilled. These are frequent, findable, and the inspection finds them.

A bulging lid or a spurt of liquid is a real signal. When you do get one of these, it is information, and the handling rule follows immediately: a suspect container of low-acid food is treated as having produced toxin [F-11.7]. Not opened further, not sniffed at length, not tasted.

What the inspection is not good for is issuing an all-clear, and the guide never claims it is.

Where the confidence actually comes from

If not from looking at the jar, then from what happened before it was a jar.

Low-acid food, processed in a pressure canner, on a current tested schedule for that food, jar size and pack style, with an accurate gauge and the altitude adjustment applied. That is the same list as the first four of USDA's eight conditions [F-11.3], and it is not a coincidence — those four are the process, and the process is the only thing that ever addressed the spores.

Confidence is built at the canner and read from your notes. It cannot be recovered later by staring at the glass, which is why a page telling you exactly what spoilage looks like has to also tell you that it frequently looks like nothing at all.

If a jar is under suspicion

Stop, and do three things in order.

Do not taste it. Not a drop, not on a fingertip, not after boiling it briefly to check the smell [F-11.7].

Do not open it further if it is sealed. A sealed suspect jar is a contained problem and the disposal procedure for a sealed jar is simpler than for an open one [F-11.8].

Follow the disposal procedure. It is quoted in full on Botulism: what to actually do, along with what to do about the surfaces it touched. That page also covers the part this one deliberately does not: what happens if somebody has already eaten from the jar.