This year a lot of our maple syrup bulged out the bottles and almost blew the tops off when loosened. There were particles found in the syrup. What is going on??
What would cause maple syrup bottles to bulge
food-safetymaple-syrup
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Yeah, syrup can ferment + convert sugar to alcohol. It has more of a tendency to do so if the sugar content is lower -- I tend to err on the side of overconcentrating my syrup.
You can try boiling it for a while to see if the alcohol boils off + if the flavor is OK then use it... but I'd boil down a bit more first, to make sure the sugar content is back up to standards. Either boil until the boiling point is 7 degrees F higher than the boiling point of water at your altitude, or boil until the syrup "aprons" (e.g. starts to drip in a sheet rather than discrete drops; a flat edge of a metal spatula works well), with the former being more accurate if you have a good thermometer.
If the flavor remains after boiling, then try using in recipes -- perhaps in brownies/blondies or with ice cream.
I just looked it up. Maple syrup is almost pure sucrose-water syrup. (I know that the colour and flavour are very distinctive, but the molecules responsible for them are a very small percentage by weight). Which means that it behaves just like regular candy crystalization.
The bad news is that candy crystalization isn't easy. There are just too many factors which have an influence on the texture. The good news is that people have already created such recipes. Find such a recipe and follow it exactly to the letter! If it says "wooden spoon", don't use a metal spoon, etc. And don't even start without a thermometer, unless you have had years of experience to perfect your water test technique.
As I haven't ever made or seen this specific candy, I can't tell you which parameter values will give you the needed texture. But I can list the usual parameters which influence the final texture.
As for general understanding, what you have is a saturated sugar solution. When you heat it, water evaporates, making the solution more concentrated, but on the other hand, sugar is more solvable at higher temperatures, meaning that it doesn't get supersaturated when heated gently. When the concentrated syrup cools, it becomes supersaturated, because the concentration doesn't change, but the solvability does. If you give the sugar the smallest chance to form separate largish crystals during cooling, it will do it. For a consistent texture, you must watch following parameters extremely carefully.
the density of your sugar syrup. It is measured indirectly by measuring the boiling temperature of the syrup. The exact temperature at which you must stop boiling will be given in the recipe (don't forget to adjust for altitude, subtract 1°C for every 300 meters). If the recipe only tells you a syrup stage (something like "hardball"), use a chart. Allow leeway for carryover - when you remove a pan from the stove, it continues to heat its contents for a while, so remove two-three degrees early (or more, depending on the pot). You can immerse the pot immeidately in cold water if it has gotten too hot, but this isn't so good for the pot itself.
The speed of hardening. I have a vague memory that you can't use rapid cooling for a non-sandy candy texture, but don't remember it 100% correctly. Rely on your recipe, if it tells you to pour it on a marble slate, do it, or at least use a refrigerated thick metal pan or something similar which cools quickly.
Nucleation points. These are very hard to avoid, but if you need a sandy texture (what you seem to want), it is much easier. If you want to have no nucleation (for a smooth hard candy), you must do everything possible to have no sugar crystals and no impurities in your syrup and to not disturb the pot. This is the reason for most weird requirements in candy recipes, like washing and drying the termometer before each dip, or the abovementioned wooden spoon.
Stove temperature. It must be hotter than your goal temperature, but if too hot, the sugar will scorch on the bottom. Also, the evaporation rate is changed by temperature, which may contribute to supersaturation.
Chemical help. You may need to invert your sugar (use acid, cream of tartar is common - just follow the recipe) or add glucose and/or fructose. This prevents the creation of crystalization nuclei in your supersaturated syrup. You probably don't need that, because it is more important for the popsicle-style candies.
Breaking up crystals. For candies where multiple soft crystals are expected, you may need to manipulate the mass mechanically after the crystalization has started. Kneading and beating are common. They result in the breaking up and mixing the crystals, resulting in a soft mass (fondant, fudge) as opposed to big hard crystals (the ones daniel's method will create). The more you do that, the creamier your texture will get.
As you see, there are way too many variables to experiment with. You could do it, but you'll need lots of luck and patience to hit on the right combination. Pick a recipe, and follow it. If needed, adjust the result according to the guidelines I gave (e. g. kneading more for creaminess).
I just noticed, the site I linked for the chart has a page describing some maple syrup candy types, and lists the correct temperature and basic handling stages for them. Could be a good starting point. (Maple candy page)
Best Answer
Even though the risk with canning/bottling maple syrup is supposed to be virtually nonexistent, it sounds like yours was somehow contaminated. I definitely would not use them as "bulging" cans or bottles are a prominent sign of botulism spores.
See the CDC's Home Canning and Botulism:
That said, it's not necessarily botulism, that's just the worst-case scenario - but it definitely sounds like some sort of bacterial contamination which is usually due to either improper canning or improper storage. What you're seeing is the result of a pressure build-up, and the pressure has to be created by something, and that "something" is usually bacteria eating the food inside and producing gas.