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About 10 days ago I made some elderflower syrup. I used:

  • 350g of elderflower
  • 6 liters of water
  • 8kg of sugar
  • 160g of citric acid
  • 12 lemons cut in pieces

I left everything macerate for 2 days and then filtered it. I didn't boil the syrup, I simply put it into bottles (I followed the recipe given me from a friend who said boiling wasn't required in the end). I've noticed that in the last few days the remaining bottles (about 5 liters) have started fermenting and I can smell some alcohol from them.

Now I wonder, is it possible to obtain some alcoholic beverage from such bottles of syrup? Which steps should I take to avoid throwing the syrup in the trash? (Note: I have zero experience in brewing)

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  • What type of smell do you get? The aroma could indicate something bacterial as well as fermentation. – jonpd Jun 22 '15 at 15:22
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The best solution would be to try and direct the wild fermentation into a controlled one. The strange smell ("smells like goat" or garlic or bad eggs or rotten meat, depending on whom you ask) that you have may very well be what's called "Böckser" in German and "goût de bock" in French. I'm not aware of an English translation other than off-flavour. It happens often with wild fermentation.
You can normally get rid of a "Böckser" by oxygenating the young wine and then directing the fermentation into a controlled one.

You might pasteurize the half-fermented sirup, which will prevent further progression of the fermentation, but it will obviously not remove the change in flavor or remove the alcohol.

Better though, store your half-fermented syrup in the refridgerator (or as cold as you are able to, anyway) to slow down/inhibit further fermentation while you're unable to do anything better, and get some selected yeast which you can use to start a controlled fermentation (depending on where you live, you may find it in a gardening or home improvement center, otherwise by mail order).

Now, not everybody is a good neighbour, some will peacefully live side by side, and some won't.
It's the same with yeasts (and bacteria). Killer yeast [example] is sold for exactly this reason (and purpose). You can use any kind of selected yeast, it needs not be "killer". These yeasts usually give a somewhat better taste and they only cost about half as much, too (and are more widely available) but they don't produce yeast-killing toxins.

Preferrably, you will want to use dried yeast since it's both cheaper and higher quality than the liquid stuff. Let a teaspoon of yeast rehydrate in about half a liter of natural cloudy apple juice with a teaspoon of sugar (best and cheapest recipe to make yeast feel comfortable, and virtually doesn't affect taste) and let it in there for 3-4 hours. If you have ammoniumsulfate, you can add a teaspoon of that as well (yeast needs this for reproduction, but if you add none it can usually do with what's in the apple juice, anyway).
Meanwhile, take the cooled syrup out of the refridgerator so it will have a somewhat normal temperature when you put the yeast in (yeast, just like most people as well, doesn't like plunging into the cold).

After those 3-4 hours, you should see a considerable amount of foam on the surface and small bubbles everywhere (otherwise, get refunded, and buy a new package).

Now pour all your syrup into a container with an airlock (glass balloon is ideal, but plastic will do too) and add the yeast. Good luck!

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  • To retain some sweetness, the gravity may have to be monitored closely, as the amount of sugar in this brew is considerable. What are your thoughts on the effect this may have on the flavour and aroma of the elderflower, bearing in mind that the action of the wild yeast has already produced some undesirable smells? – jonpd Jun 23 '15 at 14:27
  • @jonpd: Elderflower wine should have a little sweetness, yes. Sugar can be replenished at the end, the sugar in the syrup is of artificial origin anyway, so no worries about that. I would however use fructose rather than saccharose since it will remain stable in the bottle. Saccharose ("ordinary sugar") tends to decay into glucose and fructose over time, which means the wine will lose sweetness inside the bottle. That is undesirable since it is hard to predict. During fermentation, the syrup may even have too much sugar, it may even be a good idea to dilute the syrup with filtered water. – Damon Jun 23 '15 at 14:35
  • (cont.) In any case, I would always be rather careful with adding sugar -- what's inside once can't be taken out any more, and "more sweet" is not "more good". Also, too much sugar inhibits both yeasts and their alcohol-producing enzymes, so one indeed doesn't usually want to have a too much "syrupy" liquid. Regarding the undesirable smells, those can usually be oxygenated (read as: open balloon and let fresh air in) away. If the selected yeast does its job after that (i.e. it ferments), the bad smells won't come back. At the end, test before adding even more sugar. Less is more, sometimes. – Damon Jun 23 '15 at 14:39
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Wild yeast must have gotten into the bottles, and yeast will ferment any simple sugar solution. Depending how long fermentation has been going you might already have some weird wild yeast-derived off-flavors, but hopefully you're catching this early enough to save the batch

I would pasteurize them in the bottles to kill off the yeast:

Place all of the bottles into a large kettle of water that comes up to the bottom of the neck of the bottles. (Do this in batches if you don't have a large enough kettle.)

Open one bottle and place a clean thermometer / thermometer probe into it.

Start heating the water, until the thermometer reads 72ºC / 160ºF.

Hold at that temperature for a couple of minutes, then turn off the heat and let the bottles come back to room temp. (Reseal the opened bottle as soon as you can, and I would use that bottle before the others since it's been opened.)

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  • I'm afraid this has been going on for some days. I tried tasting one bottle and the off-flavors are already strong. I wouldn't try this simply because it's probably wasted time. I'm more asking about whether there exist some way to make the fermentation produce something good than on how to kill the fermentation. Anyway thanks for your answer. – Bakuriu Jun 21 '15 at 16:04
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You can use Campden tablets (Potassium or Sodium Metabisulfite) or equivalent product available from your local home brew store, dissolved in a little water (and a bit added to each bottle), to inhibit wild yeast if you don't want any alcohol produced. I can't tell you the exact quantity you would need however.

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  • Don't. This will also inhibit any yeasts that you may want to add later. Also, in order to kill yeasts (not inhibit them) you need huge quantities of sulfite, which render the resulting beverage pretty much undrinkable (Château Headache). – Damon Jun 23 '15 at 11:46
  • Sure you wouldn't want to add too much (inhibiting the yeast would probably be sufficient), but my angle on this was to try to maintain the original drink, which was a non-alcoholic syrup, rather than completely transform it into something else. – jonpd Jun 23 '15 at 14:19
  • Fair enough, so that would be around 0.2g per liter (to kill them you would need ca. 10-15 times as much but then you're in headache land). I usually add 1g per 10l (0.1g/l) when the fermentation is nearly done, and the same amount again after filtering since you expose oxidative stress to the wine during that. So, 0.1g/l should actually suffice, but 0.2g/l is on the safe side. Commercially sold wines often have 2-3 times as much, so no problem. – Damon Jun 23 '15 at 14:28

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