Why is it recommended to blanch onions before pickling them?
I was recently looking for a recipe to pickle onions, and among several others I found this one. Skimming through the comments I came across the same question as I'm asking now, but without any other answer than the musings of the author of the recipe:
Pouring water over the onions is something I've always done without quite knowing why. A quick internet research shows that most picked onion recipes call for this, but without explanation.
Hot water is often pour over onions (that aren't going to be pickled or cooked) in order to tame their sharpness and soften them a little. But a pickling brine is so intense and sharp itself, perhaps it isn't necessary.
So I started wondering myself whether or not the latter part of the quote is true; is it necessary to blanch the onions before pickling them, as the pickling solution is so sharp in taste anyway?
The recipe in question mixes:
1 medium red onion, about 5 ounces
1/2 teaspoon sugar
1/2 teaspoon salt
3/4 cup rice vinegar, white wine vinegar, or apple cider vinegar
And then let it all rest in a jar for 30 min-2hrs before it is ready. It is supposed to keep for a couple of weeks in the refrigerator. The blanching of the onions happens before it is all put in the jar.
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From indiacurry.com :
Blanching Vegetables for freezing or pickling
Vegetables have a natural enzyme that continues to effect texture, color and flavor. Blanching stops the enzyme action
The natural enzymes help vegetable to grow and mature until they are harvested. After the vegetables have been harvested, they continue to remain active even when frozen or pickled making the pickles to be tough, effect the color and flavor. Blanching stops the action of the enzymes.
The purpose of blanching is not to sterilize or pasteurize vegetables. It may kill some but not all the surface micro-organisms
Blanching is done by either scalding vegetables in rapidly boiling water, or in a steamer for a short time period. The vegetables must be scalded just long enough to stop the enzyme reaction, but not too long to make them soft and mushy by breaking up the cell-walls.
Under-blanching stimulates enzyme activity and is worse than not blanching
Over-blanching causes loss of texture, color, flavor and vitamins.
Vegetables are blanched for freezing or making Indian pickles.
It's likely a matter of taste. Why not do a little experiment? Blanch half, pickle all, keep track of which is which, taste, report back.
Blanching is a common technique for making firmer vegetables and fruits that will be later go through other preparation steps, such as being placed in a stew or pickled. But it is a tricky process, because you want to be in the temperature range where an enzyme turns on (opposite of what the quote from indiacurry in another answer states). The reason is intricate and has to do with tangling up pectins.
Pectins are longish molecules with some side branches. They are from the same family as cellulose (the woody part of plants), but unlike cellulose they are water soluble. (You may have used it in making jams.) Pectins are present in the wall of cells and in the glue that bind cells togethers. Most edible plants also produce some member of the pectinase family of enzymes (PME for onions), which help break down the pectins making the fruit of vegetable softer.
The reason to blanch is to activate the PME so that it will start breaking the pectins in the onion. When this is done in a solution containing calcium (which can come from inside the cells), the calcium gets tangled up with the pectin creating a mesh that is not as soluble. This will help prevent the onions from becoming mushy in the pickling solution. If the temperature is too high the enzyme will be deactivated, so it needs to be done at the right temperature. Work by Gonzalez et al. suggests that 70C is the best temperature and 90C is too high and 50C too low. The time will depend on the size of the onions, but of the order of half an hour.
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