From the blog April 26, 2013 Wayt

A shocking (and hot!) tip for preserving produce

By W. WAYT GIBBS
Associated Press

Nothing is more frustrating than finding the perfect cucumber or head of lettuce at the farmers market, paying top-dollar for it, and then… tossing it out a week later when it has gone moldy or slimy in the refrigerator.

No doubt one reason so many of us eat too many convenience foods and too few fruits and vegetables is that it can be hard to get our busy schedules in sync with the produce we bring home with the best of intentions.

Food scientists, however, have discovered a remarkably effective way to extend the life of fresh-cut fruits and vegetables by days or even a week. It doesn’t involve the chlorine solutions, irradiation or peroxide baths sometimes used by produce packagers. And it’s easily done in any home by anyone.

This method, called heat-shocking, is 100 percent organic and uses just one ingredient that every cook has handy – hot water.

You may already be familiar with a related technique called blanching, a cooking method in which food is briefly dunked in boiling or very hot water. Blanching can extend the shelf life of broccoli and other plant foods, and it effectively reduces contamination by germs on the surface of the food. But blanching usually ruptures the cell walls of plants, causing color and nutrients to leach out. It also robs delicate produce of its raw taste.

Heat-shocking works differently. When the water is warm but not scalding – temperatures ranging from 105 F to 140 F (about 40 C to 60 C) work well for most fruits and vegetables – a brief plunge won’t rupture the cells. Rather, the right amount of heat alters the biochemistry of the tissue in ways that, for many kinds of produce, firm the flesh, delay browning and fading, slow wilting, and increase mold resistance.

A long list of scientific studies published during the past 15 years report success using heat-shocking to firm potatoes, tomatoes, carrots, and strawberries; to preserve the color of asparagus, broccoli, green beans, kiwi fruits, celery, and lettuce; to fend off overripe flavors in cantaloupe and other melons; and to generally add to the longevity of grapes, plums, bean sprouts and peaches, among others.

The optimum time and temperature combination for the quick dip seems to depend on many factors, but the procedure is quite simple. Just let the water run from your tap until it gets hot, then fill a large pot of water about two-thirds full, and use a thermometer to measure the temperature. It will probably be between 105 F and 140 F; if not, a few minutes on the stove should do the trick. Submerge the produce and hold it there for several minutes (the hotter the water, the less time is needed), then drain, dry and refrigerate as you normally would.

Researchers still are working out the details of how heat-shocking works, but it appears to change the food in several ways at once. Many of the fruits and vegetables you bring home from the store are still alive and respiring; the quick heat treatment tends to slow the rate at which they respire and produce ethylene, a gas that plays a crucial role in the ripening of many kinds of produce. In leafy greens, the shock of the hot water also seems to turn down production of enzymes that cause browning around wounded leaves, and to turn up the production of heat-shock proteins, which can have preservative effects.

For the home cook, the inner workings don’t really matter. The bottom line is that soaking your produce in hot water for a few minutes after you unpack it makes it cheaper and more nutritious because more fruits and veggies will end up in your family rather than in the trash.

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HEAT-SHOCKING GUIDELINES

The optimal time and temperature for heat-shocking fruits and vegetables varies in response to many factors – in particular, whether they were already treated before purchase. Use these as general guidelines.

- Asparagus: 2 to 3 minutes at 131 F (55 C)

- Broccoli: 7 to 8 minutes at 117 F (47 C)

- Cantaloupe (whole): 60 minutes at 122 F (50 C)

- Celery: 90 seconds at 122 F (50 C)

- Grapes: 8 minutes at 113 F (45 C)

- Kiwi fruit: 15 to 20 minutes at 104 F (40 C)

- Lettuce: 1 to 2 minutes at 122 F (50 C)

- Oranges (whole): 40 to 45 minutes at 113 F (45 C)

- Peaches (whole): 40 minutes at 104 F (40 C)

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Photo credit: AP Photo/Modernist Cuisine, LLC, Chris Hoover

Discussion

  1. Jenn M April 27, 2013 Reply

    Interesting. However, how does this process affect the taste and texture of fruits and vegetables. I’m particularly curious about lettuce for texture and oranges for flavour.

    • Judy April 29, 2013 Reply

      Hi Jenn,

      For citrus fruits, you can also shock the segments, not the whole fruit.

  2. Dave April 28, 2013 Reply

    I got this about a week ago via an email from the Washington Post & immediately tried it on some grapes with fantastic results. Is there an associated , by-item listing of the expected increased storage time?

    • Judy April 29, 2013 Reply

      Hi Dave,

      You can find a table on page 3·359 of Modernist Cuisine.

  3. Rickard April 28, 2013 Reply

    I have to toss out a lot of leafy greens like spinach and ruccola. Can they be made to last longer using this method?

    What is the best way to store leafy greens? I’ve been taught to use a non-airtight container with a dry paper towel in it to soak up moisture.

    Thanks for an informative article.

    • Judy April 29, 2013 Reply

      Hi Rickard,

      Yes, you can use this for greens. Lettuce will not start to brown for two weeks after being treated.

  4. Scott April 29, 2013 Reply

    Are there any forms of produce that should not be heat treated?

    • Judy April 30, 2013 Reply

      Hi Scott,

      Wayt answered your question below.

  5. Bill April 29, 2013 Reply

    Strawberries are mentioned in the article, but there is no time/temperature guide. They are a fruit that does not keep well, and I would love to try this technique. Any guidance on time/temp? Thanks

    • Judy April 29, 2013 Reply

      Hi Bill,

      We recommend 60 C / 140 F for 15 seconds for strawberries. You can find more guidelines on page 3·359 of Modernist Cuisine.

  6. Wayt April 29, 2013 Reply

    Re: Scott’s question: Heat-shocking works best on minimally processed produce–fruits and veggies straight from the orchard, garden, or farm. Supermarket produce that originated thousands of miles away or is packaged for long shelf life (e.g., bagged or boxed greens) most likely already received either a heat treatment or a chemical wash of some kind. Heat-shocking such items at home will probably do little to further extend their longevity.

  7. Martin May 1, 2013 Reply

    How does this treatment compare to cold shocking vegetables? I’ve watched Heston Blumenthal in one of his shows revive iceberg lettuce by putting it in ice water for 15 minutes.

    • Wayt Gibbs May 2, 2013 Reply

      That can work for certain green leafy vegetables. Heat-shocking delays wilting; it won’t rescue already wilted leaves. Heat-shocking only works if used in advance of spoilage, but it works on a wider range of produce.

  8. curtdbz May 1, 2013 Reply

    Thanks a lot for this tip! Two questions:

    1) Does this leach any nutrients out of the vegetables the way that boiling them would?
    2) Please tell me this works for bags of milk. I dread the days that come close to the expiration date. (but seriously, do you know of any tricks to extend milk’s shelf life?)

    Thanks as always.

    • Wayt Gibbs May 2, 2013 Reply

      Some nutrients (e.g., certain vitamins) are water-soluble, so any water treatment (even washing) diminishes them by a bit. But heat-shocking at the temperatures recommended in the column won’t have nearly the nutrient-altering effects that blanching or boiling does.

      As for milk, long heat treatments should effectively deepen the degree of pasteurization and thus extend the safe shelf-life. If you search in Google Scholar for ultrapasteurization protocols, you will find an extensive literature on this subject. I don’t know for certain whether this treatment, in the absence of vacuum sealing, will prevent the formation of off-flavors, however. In many cases, the bacteria that create off-flavors are different from those that cause foodborne illness. Chemical effects, such as the oxidation of milkfat leading to rancidity, can also produce off-flavors independent of biological activity.

  9. Jonathan Kaplan May 11, 2013 Reply

    Thoughts on cucumbers? Will it help? Time / temperature recommendation?

  10. Peter MacKichan May 12, 2013 Reply

    Hello
    Can you heat shock herbs?
    Peter

    • Judy May 14, 2013 Reply

      Hi Peter,

      As Wayt noted above, it will depend on whether or not they were treated before hand. Try it on herbs fresh from your garden or farmers’ market, but bagged or otherwise packaged herbs from the grocery store may have already been treated for longevity.

  11. fg June 7, 2013 Reply

    I asume these times and temps are ideal. How much can they be fudged and still be beneficial?

  12. Dan May 24, 2014 Reply

    Does heat shocking prepare the veggies for freezing, or must I blanch them

  13. Kathleen Booren June 19, 2014 Reply

    As a small Market Farmer I’m always interested in providing my produce in a way that will last longer for my customers. Would love to know more about this exciting research. Any direction you can provide would be appreciated. Love the article.

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