I want to chop onions and green peppers then freeze in a vacuum sealed bag without blanching the vegetables first.

I read the article on blanching after vacuum sealing and then blanching but to your point the heat transfer is dramatically reduced by the insulating value of the plastic and the bulk thickness of the vegetables in the bag. Any insight that you can provide would be greatly appreciated.

A timely answer would be greatly appreciated as we need to start this process as soon as possible with the utmost safety in mind.

  • 1
    I'm not sure I've ever seen a way to store raw chopped vegetables long-term, except freezing. Is this just for storing them a few days until you need to use them?
    – Cascabel
    Mar 25, 2015 at 2:36
  • No matter what you decide to do, you have to keep them out of the temperature danger zone (meaning freeze or refrigeration) no way around this, without removing all of the moisture or adding a preservative.
    – Chef_Code
    Mar 25, 2015 at 7:31
  • "Blanching" after sealing will not give you anything, not because of the insulating properties of the bag, but because the plastic bag will melt long before you have reached safe bacterial reduction. In fact, most vegetables cannot be safely canned without adding acid.
    – rumtscho
    Mar 25, 2015 at 10:40
  • The vacuum sealed chopped onions and green peppers will be frozen and kept frozen for several weeks, not long term. Thank you. Mar 25, 2015 at 20:46
  • What is the article you referred to?
    – Cascabel
    Mar 26, 2015 at 22:04

2 Answers 2


I suppose dropping the bags in liquid nitrogen for a few minutes, and then storing in the freezer might suffice.

mostly fish, but not a bad read: http://www.fda.gov/downloads/Food/GuidanceRegulation/UCM252416.pdf

Edit post clarification of the question:

Liquid nitrogen is still fun if you can find an excuse, and highly effective.

In a less drastic direction, simply doing the processing on pre-cooled (below 38F, preferably 33F) vegetables (and preferably in a walk-in cooler held below 38F) should keep things out of the botulism growth zone, and putting them in a -20F freezer for long enough (do some test runs where you actually stick a thermometer in a test bag) to get the center of the mass down below 0F before transfer to a 0F freezer (assuming that's your standard freezer temperature) would likely suffice. A cold brine step might help freezing get started faster but would be messy as compared to just using a -20F freezer (perhaps with racks to separate bags and extra air circulation, so you don't have bags packed together until after they are frozen through.) Depending on scale, maybe just size the -20F unit for a days production and clear it out at the start of each day after holding product overnight.

If processing in a warmer environment, you'll want to minimize time between "being held pre-cooled" and "being packed and frozen" - and especially "time between being vacuum-packed and being frozen." But giving the workers warm coats and hot coffee (and breaks) while working in a cooler will be safest. You will also need to sanitize any equipment on a regular schedule, which will be more often if they are not cold (and you'll need to check the temperature of the food contact parts of any machines when they have been running for a while.)

  • The OP clarified that he intends to freeze them - so the issue is basically just the time between sealing and when it actually freezes and gets out of the danger zone.
    – Cascabel
    Mar 26, 2015 at 22:09

Food preservatives such as nitrite, sorbic acid, phenolic antioxidants, polyphosphates, and ascorbates are all proven additives to help reduce the risk.

phenolic antioxidants in particular come from plant materials. If I were you, I would invest my time into researching such ingredients as bay leafs because you can buy them in bulk and because they're food (again this is what I would do, I have never done this and I don't know if this would work without further research).

Bay leaf's may although also change the flavor (but there is many more ingredients that are high in phenolic antioxidants that might not affect the flavor profile of the vacuum packed product).

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