Here the onion prices shoot up to exorbitant (1000% increase) levels every year and this year it has hit a crisis level , hence it would be nice to preserve them when they are cheap. One way is to caramelize and preserve them, how long such caramalized onions would last and most importantly what are some other ways to preserve onions upto a year or so?
Preserving onions and retaining their qualities is a very difficult challenge.
Most known effective methods require freezing, which requires reliable power.
The main problem is that well tested recipes for preservation (pickling or canning) from trusted, research-based sources like government agencies or university extensions are extremely difficult to locate. You will have to assess your tolerance of risk in using recipes that you do find.
If you have reliable power to maintain a freezer, onions can be frozen. The University of Nebraska Extension recommends washing, peeling and chopping the onions into about 1/2 inch (1 cm) pieces. Freeze in single layer freezer bags, making it easy to break off a portion.
Frozen onions are best used in cooked dishes.
Should last 3-6 months (in terms of quality; they will be safe indefinitely if continuously frozen).
Onions are a low acid food, and so require pressure canning for preservation. Make sure to locate a trustworthy recipe.
Canning and Preserving for Dummies offers a detailed recipe online, but I cannot verify that it is backed by quality safety research.
Properly canned onions should last a year, and will be best suited for cooked dishes.
Caramelized or Made into Jam and Canned
Again, onions are a low acid food, safe preservation requires pressure canning and a safe recipe from a qualified source such as a university or government agency.
If you can find a recipe you trust, they should last a year, but will of course have a substantially different flavor and texture from raw onions.
The problem is that I am unable to find a single recipe whose pedigree I would trust as showing that it is backed by the proper research to indicate its safety.
It is difficult to find reliable shelf stable pickle recipes from a trustworthy source for full sized onions. Most are intended to tiny (1-2 cm) onions.
If you can locate a trustworthy pickling recipe (a true canned pickle, heat treated, or a true fermented pickle) from a reputable source like a university or government agency, this may be an option, but the onions would of course be pickled, and have a different flavor and be much more acidic.
Properly pickled onions should last a year, but the challenge is finding a safe recipe, and pressure canning is a must.
Dehydrating should be a good option in theory, but with home equipment, you are unlikely to desiccate the onion to a safe level for shelf stability. The National Onion Association recommends freezing dehydrated onion, which may or not be an option.
Cool Dry Storage
Onions can naturally be stored, if properly cured (dried and prepared for storage), for many months. Unfortunately, you may not be able to control how well they were cured after being harvested.
Properly cured onions, if kept in a cool place (around 40 F, 4 C) should last 10-12 months (which is how they are stored commercially for sale at retail year round). Again, this may not be feasible.
Here in the US, I get to my nearest India Bazaar and buy a huge bag of fried/caramelized onions. They will last me more than a year. I don't need to refrigerate them. I throw them into the soup and they expand back by absorbing the water.
Before there were India Bazaar, I had to caramelize onions myself. Which I learned from my mom. It is a slow process done on a large flat pan on a very slow fire to dry the onions but avoid cooking them as much as possible. For just an onion or two, the drying would take about two hours on the pan, with the onions spread out and a thin layer of oil.
But I read that you could sun dry the onions first.
The other way is to dry it in a baking oven at 100-120F for a day or two. http://www.ehow.com/how_3905_dry-onions.html.
I would sun dry them, if I had the place to do it.
When the onions are mostly dehydrated and crisp (a little moist and not brittle), fry them in whatever oil you are comfortable with, again with low heat. I would fry them in a mixture of sesame and olive oil. Sesame oil also has preservative effects.
I normally could not resist frying garlic slices together with the onions. You do not need to dry the garlic slices first. Also, not being experienced in caramelizing onions, I use the garlic as an indicator when to stop the heat - when the garlic starts to get brittle and toasty.
I know my technique and timing would not produce perfectly caramelized onions due my lack of experience, but the procedure is correct and simply needs further trial and error to fine-tune it.
Perhaps, you should start with a handful of sun dried onion slices first.
So it would be a two step sunning. First to dry it and then to fry it.
At the frying step, tilt the pan a little so that you could drain the oil and reuse them. You should know that you need your pan to have a surface to be as dark as possible to absorb as much heat from the sun as possible. You should also place a piece of glass over the pan to retain the heat and reduce evaporation of the oil. I am thinking that sun cooking this way, will ensure your onions will not get burnt.
Instead of the glass covering, I am wondering if Saran wrap would do. Would the plastic wrap melt in the heat?