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I made several glasses of whole strawberries in syrup. Normally I do this:

  1. sterilize the glass and the screw cap with boiling water
  2. carefully wash and sort strawberries
  3. make a syrup 300g of sugar to 0.5l of boiling water
  4. put strawberries into glass and pour the syrup on them leaving approx 1cm from the cap
  5. put the glass to the electric oven - 100°C for 30 minutes

Then I store them in the refrigerator and usually they are OK.

However this time the first 2 glasses were "strange" - when I open them the gas comes out and the syrup is sparkling. Also the strawberries are sparkling inside. The taste is very good, like sparkling water with strawberry syrup. No sour or bitter.

I think some fermentation processes started. I read the similar question Lemon liqueur has gas? and I think that the case is similar. However I used no alcohol.

The question(s):

  1. Are the strawberries and syrup safe?
  2. Can you speculate the reasons why the fermentation process started and suggest what to do to prevent such process in the future?
Tom HANAX
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    Hi Tom, you are asking a food safety question. I know that many people come to the site with the idea of food safety as if common sense can predict what will happen when somebody eats a given batch of food. This is not so; there are official food safety guidelines, which are unambiguous, but more restrictive than most people expect, and there is everybody's personal gut feeling. The second kind of safety is off topic here. That's why I had to remove the part in your question asking for consequences, and the answer which did not use official rules. See the tag wiki on food-safety for details. – rumtscho Aug 27 '21 at 10:50
  • OK understand, I am only interested in knowing about potential serious problem like a methanol poisoning in alcohol or that kind of things. I am OK to risk some minor inconveniences like going on toilet more often ;) – Tom HANAX Aug 27 '21 at 10:59
  • On a purely personal level, I understand why you want that. But this is not something we can provide. Food safety regulations only distinguish between "safe" (= the officials promise you that nothing at all will happen) and "unsafe" (= they are not willing to give you that promise, regardless of actual outcome). If a food of yours is unsafe, no further information can be provided about the type of risk or the likelihood of different outcomes. If that's enough fro you, then your question is fine here. If not, then our site is not the right place for you. – rumtscho Aug 27 '21 at 11:08
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    OK I read the wiki as you suggested and understand better now that safety is about strict binary yes/no. I will wait if someone has this strict answer for me, I am curious. – Tom HANAX Aug 27 '21 at 11:22

1 Answers1

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There’s a clear answer on the food safety part:

Not safe.

All unintended fermentation is potentially bad - you don’t know what exactly started to grow in your jars, just that something did - so the food safety verdict must be “not safe”.

If you want to prevent such an event in the future, stick to approved recipes and follow proper cleaning and preparation procedures. It’s hard to say what exactly went wrong, just that the problem was caused either by the parameters of the canning step (failing to reduce the existing pathogens) or by an improper seal (meaning bacteria reentering into the jars).

Home canning per the guidelines published by government agencies or other authoritative sources should give you a safe product, but the jar design will serve as a security mechanism: if the seal doesn’t work, the product is not safe. Failure of the seal right after cooling indicates a more mechanical problem (damaged jars, lids, or rubber bands, or a dirty rim), during storage is likely an indication of unwanted growths. Industrial canning also relies on such indicators, e.g. the “plopping” screw tops.

Further details as requested:

While oven-canning has been often declared unsafe by government agencies1 (although it was often enough done nevertheless), the German government has published instructions including a method for oven canning.

I am paraphrasing and translating the key parameters of the instructions into English below, please consider it as supplemental information only, not a full set of instructions. If you or future readers are inexperienced in canning, please do some further reading to learn about the basic principles before starting to preserve food.

For fruit:

Use a canning liquid of either

  • 1/2 liter of water and vinegar plus 600g sugar or
  • 250-500g sugar per liter water.

(I would personally choose the latter.)

Fill the clean jars with clean and prepared fruit or fruit pieces, do not use overripe, mushy or moldy fruit. Top with liquid, covering the fruit. Close jars, depending on chosen canning system.

Place the jars in a deep pan filled with water, the jars should not touch.

Heat the jars at 150-160°C fan-assisted or 150-180°C top/bottom heat until you see bubbles rising in the jars. (Personal remark: Make sure that what you see is truly steam, not just dislocated trapped air. If you see steady bubbling you should be fine.) Turn off your oven and leave jars in the closed oven for 25-30 minutes.

Note that the instructions above are for fruit, not for vegetables and not for meat.

Properly canned fruit should be good at room temperature for at least a year.

1 An - admittedly somewhat cursory - search on the website of the National Center for Home Food Preservation, set up by the USDA and others, yielded no definitive stance on the oven canning method listed above (with liquid covering the food in the jars and the jars standing in a deep pan or dish with water). There are warnings against “dry canning”, i.e. without liquid in the jars, because of improper heat transfer to the inside, and against heating the jars in a dry oven as not all types of jars can handle the temperature fluctuations and may break.

Stephie
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  • Which guidelines make it safe? The American guidelines do not allow canning in the oven. It may be that some European guidelines allow it, but a 100 C oven is suspiciously low. – rumtscho Aug 28 '21 at 07:38
  • @rumtscho “not in the oven” used to be the standard advice. However, while there have been non-authoritative recipes around for decades, now [the method is apparently ok as per the German Bundeszentrum für Ernährung](https://www.bzfe.de/nachhaltiger-konsum/haltbarmachen/einkochen/). – Stephie Aug 28 '21 at 09:49
  • The asker‘s temperature however is way too low for safe canning, otoh they are not aiming for long-term-at-room-temperature storage. Reading the description in the question, we’re in uncharted territory anyway and if we are very strict, the “cooked food in the refrigerator” time must be applied when determining food safety. Hence my „follow reliable guidelines“ - and that can include using a water bath instead of the oven depending on the source. – Stephie Aug 28 '21 at 09:50
  • I think the seal is good, because the jars are now at the pressure - filled with the gas and are holding it well. I use new lids each time. As you noted, it may be due to insufficient canning temperature. I very much like use oven... I am not good at German, so are there any English guidelines that states correct temperature and time? What about long term storage by room temperature, is the usage of oven possible? Or should I abandon the idea and use water bath? – Tom HANAX Aug 28 '21 at 10:50
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    @TomHANAX I paraphrased the key parameters from the given link. That should give you a shelf-stable product, no need for refrigeration. If you find that the results are e.g. too mushy and want to scale back on temperature and time, the conclusion is that after a few days in the refrigerator your jars are technically unsafe. As you experienced already, that doesn’t necessarily mean that the contents are actually spoiled, but they could be. Also note that not all microbiological growth can be detected by sensory checking. – Stephie Aug 28 '21 at 13:25