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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.

  • What method of pickling do we talk about here? I think there is a major difference whether we are talking about refrigerator pickles vs. serious canning, at least with regard to food safety. – Stephie Aug 17 '15 at 10:37
  • @Stephie in what way would pouring hot water over onions matter for food safety? Refrigerator pickles would be refrigerated and used short term, plus are typically acidic (that appears to be the recipe quoted). By "serious canning" I assume you mean the jars will be pasteurized...so no safety issue. Finally, a lacto-fermented pickle, would also be rendered safe, from salt and acid. I can't see how this is a safety concern, but I am happy to be corrected if I am wrong. – moscafj Aug 17 '15 at 10:52
  • @moscafj There is always botulism to consider - Which we can ignore if sufficient heat and acidity are involved. But I have seen people do really weird things (ok, and live to tell...) with veggies and a refrigerator and wanted to be sure. I agree that most methods are fine, but wanted to narrow down the question a bit. (Party because I didn't want to write a long "covers all bases" answer ^_^) And some blanch by submerging in boiling water, the term is not fixed for "pouring hot water over the onions". – Stephie Aug 17 '15 at 11:01
  • Added a condensed version of the recipe to the question, for a more comprehensive version there is a link to the website it's somewhere towards the start @Stephie – eirikdaude Aug 17 '15 at 11:07
  • @Stephie We can cover the botulism risk with sufficient acid or heating to 85C for 5 minutes. In the recipe provided a rinse of onion is clearly not for safety reasons. – moscafj Aug 17 '15 at 11:30
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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.

  • Nice! Texture...good point. – moscafj Aug 17 '15 at 11:34
  • Blanching stops the enzymes by denaturing them (mangling the 3D structure of the proteins). But shouldn't pickling do exactly the same thing? – David Richerby Aug 17 '15 at 12:48
  • @DavidRicherby, enzymes are globular proteins and are unlikely to be affected by the pickling process. – Mr. Mascaro Aug 17 '15 at 13:50
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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.

  • While the book is a good find, I think you quoted the wrong part of it - having a look at the section about onions (p. 311), I found this quote, which seems to line up with the accepted answer. The section you refer to seems to deal mainly with root vegetables. Otherwise great answer and an interesting read :) +1! – eirikdaude Aug 18 '15 at 10:05
  • @eirikdaude you are been very kind. I just ignored the flavor modifying role of heat and maybe I shouldn't. I often eat freshly sliced red onions soaked in lime juice with my Indian food, so wanting to make the flavor milder did not occur to me. I was thinking more of pearl onions rather than the particular example given by the OP. This may require some experimentation, as in the OP's recipe the blanching seems too fast. – papin Aug 18 '15 at 11:43
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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.

  • So you believe there will be a difference in taste even when the onions are mixed with a "sharp pickling solution"? I actually don't think I'll bother doing it with mine, since they are fresh and shouldn't have developed too many sulfurous compounds yet, anyway. The question was mostly out of curiosity, to see what people here thought / had experienced, especially considering that it didn't seem the commments-section of the other webpage had managed to get to a definite answer through googling. – eirikdaude Aug 17 '15 at 11:11
  • @eirikdaude I don't know, but since it appears to be a common practice for pickled onion I thought it would be worthy of an experiment. Sometimes practices develop over time as "tradition" but have no real impact...other times, there is a reason for the practice. Sounds like an interesting test to me. – moscafj Aug 17 '15 at 11:32
  • 1
    This looks more than a common than an answer. – Jay Aug 17 '15 at 13:19

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