Blanching before drying food (2024)

Most vegetables should be blanched before drying them.

Blanching means to very lightly cook something at high heat for a very short time.

This results in a higher quality dried product with better colours and storage life; recent evidence-based research has been showing that it also increases food safety. Cornell Cooperative Extension says that“blanching cleanses the surface [of the produce] of dirt andorganisms.” ^[1]Katherine J. T. Humphrey and Judy L. Price. Drying Foods in New York State. Cornell Cooperative Extension.2011. Accessed January 2018 athttps://blogs.cornell.edu/cceherkimer/files/2009/04/2011_Drying_Foods_in_New_York_State.pdf . Page 3

Blanching can be done by immersing the food items in boiling water, or, by steaming them.

The Ball Blue Book says,

Blanching (heating in steam or water for a specific time, then cooling quickly) is the most common method of pretreating vegetables. Steam blanching is preferred because more water-soluble vitamins and minerals are preserved. In steam blanching, vegetables or fruits are placed in a colander that is suspended above boiling water and heated by the steam. In water blanching, the vegetables of fruits are placed directly in the boiling water. Blanching shortens the drying and rehydration times, sets color, slows enzyme action, and kills many spoilage microorganisms.” ^[2]Ball Blue Book. Muncie, Indiana: Healthmark LLC / Jarden Home Brands. Edition 37. 2014. Page 162.

Water blanching

For water blanching, a common recommended ratio of veg to water that you’ll see in many places is 1 pound of prepared veggie per 4 quarts (1 gallon) of boiling water (½ kg per 4 litres).When water blanching, use 1 pound veg per 4 quarts boiling water (½ kg per 4 litres). Double water for leafy greens. Begin timing when water returns to a boil after adding veg. Extend both water or steam blanching times by 1 minute when over 5000 feet (1500 metres).

“Use one gallon water per pound of prepared vegetables.” ^[3]Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 277.

At that ratio, the pros advise that the water should stay boiling when the vegetables are added.

The University of Minnesota Extension Service recommends doubling the water when blanching leafy greens:

Use one (1) gallon water per pound of prepared vegetablesortwo (2) gallons water per pound leafy greens.” ^[4]Driessen, Suzanne, Ed. Vegetable blanching directions and times for home freezer storage. University of Minnesota Extension. 2015.

Here are two slightly differing suggestions of how to go about it.

Colorado State gives the following methodology:

Work with small amounts so [the] water doesn’t stop boiling. Watch closely and precook as follows:

• Fill large kettle half full with water and bring to a boil.
• Put no more than one quart of the vegetable pieces in a cheesecloth or other mesh bag. A 36-inch cloth square gathered at the corners works well. Secure ends.
• Drop vegetable bag in boiling water, making sure water covers the vegetables. Shake bag so hot water reaches all pieces.
• Start timing as soon as vegetables are in boiling water. Adjust heat to ensure continuous boiling.
• Heat for length of time [required].
• Drop bag in very cold water to cool (same time as blanched).
• Drain on paper towel or cloth.” ^[5]P. Kendall, P. DiPersio and J. Sofos. Drying Vegetables. Colorado State University Cooperative Extension. Publication no. 9.308. July 2004.

TheUniversity of Georgia Extension Service gives the following directions:

Water Blanching – Fill a large pot two-thirds full of water, cover and bring to a rolling boil. Place the vegetables in a wire basket or a colander and submerge them in the water. Cover and blanch according to directions for each vegetable. Start counting blanching time as soon as the water returns to a boil. If it takes longer than one minute for the water to come back to boiling, too many vegetables were added. Reduce the amount in the next batch.” ^[6]Harrison and Andress. Preserving Food: Drying Fruits and Vegetables. Page 346

NOTE ABOUT TIMING: Colorado says to start timing as soon as the vegetables are in the water (presuming supposedly that you’ve put a very small amount that won’t interfere with already ongoing boiling). We found it more common in literature on the topic to see explicit directions to start timing only when the water returns to a boil after the produce has been placed in the water.

Note also that you don’t need a fresh pot of water for each round of blanching: the blanching water in the pot can be used over and over, though you may need to top it up as needed.

The same water can be reused several times forblanching; just be sure to bring it back to avigorous boil before adding vegetables.” ^[7]Mason, April C. et al. Freezing vegetables at home. Purdue Extension. CFS-134-W. December 2001. Page 2.

You will usually wish to switch to a fresh pot of water if you switch to blanching another type of vegetable, so that the taste and colour doesn’t transfer (i.e. you won’t want to blanch eggplant in water you’ve been blanching Brussels sprouts in.)

Steam Blanching

TheUniversity of Georgia Extension Service gives the following directions:

Steam Blanching – Use a deep pot with a tight fitting lid and a wire basket, colander or sieve placed so the steam will circulate freely around the vegetables. Add water to the pot and bring to a rolling boil. Place the vegetables loosely in the basket no more than 2 inches (5 cm) deep. Place the basket of vegetables in the pot, making sure the water does not come in contact with the vegetables. Cover and steam according to the directions.” ^[8]Harrison and Andress. Preserving Food: Drying Fruits and Vegetables.

Preserve It Naturally says,

Steam blanching… is preferable to water blanching because few water soluble vitamins and minerals are lost. To steam blanch, you’ll need a steamer with a lower part that holds water and a perforated upper section that allows steam to circulate. The average steamer will hold 2 cups of vegetables. Place the basket in the steamer, cover tightly, and begin timing. The basket should not touch the water, and the heat should be high enough to keep the water boiling rapidly the entire time. The water can be saved and reused for other batches.” ^[9]Excalibur. Preserve It Naturally. Sacramento, California. 4th edition, 2012. Page 24.

The authors of Putting Food By do not like steam blanching for leafy-green vegetables:

Leaf vegetables are usually better boiled than steamed, because they tend to mat in the steam basket, but can roll around if popped into water at a thrashing boil…” ^[10]Hertzberg, Ruth. Putting Food By: Fifth Edition (p. 16). Penguin Publishing Group. Kindle Edition.

To be clear, note that the recommended quantity of veg at one time for steam blanching varies depending on whether the intended purpose is freezing or drying. For freezing, the recommendation is “Put the vegetables in the basket in a single layer so that steam reaches all parts quickly.” ^[11]Andress et al, So Easy to Preserve, 2014, page 278. For drying, as seen above, the recommendation is a layer up to 2 inches (5 cm) deep.

After blanching

In cooking, and when freezing vegetables, the vegetables are almost always plunged into very cold water to immediately cool them and stop the cooking process.

When you are going to be drying food, opinion seems to vary as to what you should do afterward.

TheUniversity of Georgia Extension Service recommends a brief cold water dip in a brochure they produce on drying foods,

After blanching, dip the vegetables briefly in cold water. When they feel only slightly hot to the touch, drain the vegetables by pouring them directly onto the drying tray held over the sink. Wipe the excess water from underneath the tray and arrange the vegetables in a single layer. Then place the tray immediately in the dehydrator or oven. The heat left in the vegetables from blanching will cause the drying process to begin more quickly. Watch the vegetables closely at the end of the drying period. They dry much more quickly at the end and could scorch.” ^[12]Harrison and Andress. Preserving Food: Drying Fruits and Vegetables.

They repeat the recommendation in their book, So Easy to Preserve

After blanching, dip the vegetables briefly in cold water, only long enough to stop the cooking action. Do not cool them to room temperature. When they feel only slightly hot to the touch, they will be cooled to about 120 F [around 50C].” ^[13]So Easy to Preserve. Sixth Edition. 2014. Page 347.

ThePacific Northwest Extension service recommends skipping the cold water dip step altogether:

After Blanching. Drain vegetables by pouring them directly on the drying trays. If you plan to reuse the water, place a large pan under the trays. Wipe the bottom of the drying tray with a clean towel to remove excess water. Draining the vegetables on one tray and then transferring them to the drying tray results in unnecessary handling. Immediately transfer the blanched vegetables into the dehydrator so drying can begin while the vegetables are still warm.” ^[14]Swanson, Marilyn A. et al. Drying Fruits and Vegetables. Pacific Northwest Extension. PNW 397. Third edition, 2009. Page 10.

Penn State Extension also makes no mention of a cold water dip for veggies intended for drying:

Blanched vegetables should be drained and placed on dryer trays. The heat from blanching will give them a head start in the drying process.” ^[15]Luke LaBorde. Drying Fruits and Vegetables

A University of Illinois Extension slideshow on the topic says.

• Dip briefly in cold water only long enough to stop cooking;
• Cool till they feel only slightly hot to touch…about 120ºF / 50 C;
• Wipe vegetables and place in dehydrator.

Ed: We’re not sure how you’d measure the temperature; it’s not clear how they picture the veg being wiped. ^[16]Finck, Jananne. University of Illinois Extension Springfield Center. Slideshow presentation. 2002. Page Slide 26. Accessed January 2018 at https://nchfp.uga.edu/multimedia/slide_shows/IllinoisDryingFoods.ppt

Why blanch

Dr Patricia Kendall from Colorado State Extension Services writes,

Blanching helps slow or stop the enzyme activity that can cause undesirable changes in flavor and texture during storage. Blanching also relaxes tissues so pieces dry faster, helps protect the products vitamins and color and reduces the time needed to refresh vegetables before cooking.” ^[17]P. Kendall. Drying Vegetables.

Dr Elizabeth Andress from the University of Georgia Extension Service writes,

Blanching is a necessary step in preparing vegetables for drying. By definition, blanching is the process of heating vegetables to a temperature high enough to destroy enzymes present in tissue. Blanching stops the enzyme action which could cause loss of color and flavor during drying and storage. It also shortens the drying and rehydration time by relaxing the tissue walls so moisture can escape and later re-enter more rapidly.” ^[18]Harrison, Judy A. and Elizabeth L. Andress. Preserving Food: Drying Fruits and Vegetables. University of Georgia Cooperative Extension Service. FDNS-E-43-10. July 2000.

There are just a few exceptions, which include onions, garlic, peppers, herbs and mushrooms:“Onions, garlic, peppers, and herbs do not need blanching.” ^[19]LaBorde, Luke and Martha Zepp. Drying Fruits and Vegetables (Dehydration). PennState Extension. Code EE0065. 2017. [Ed: some sources suggest blanching mushrooms and sweet peppers anyway.]

What is better: water or steam blanching

Blanching can be done by immersing the food items in boiling water, or, by steaming them. There are pros and cons to each method; neither is right or wrong, and at present, different authorities hold differing views.

Patricia Kendall prefers water blanching outright:

Water blanching is recommended over steam blanching … because water blanching achieves a more even heat penetration…” ^[20]P. Kendall. Drying Vegetables.

North Dakota State prefers water blanching as well:

Water blanching achieves a more even heatpenetration than steam blanching….” ^[21]Garden-Robinson, Julie. Drying Vegetables. North Dakota State University Extension Service. FN1588. 2012.

Elizabeth Andress seems to be fine with either method:

Vegetables can be water blanched or steam blanched. Water blanching usually results in a greater loss of nutrients, but it takes less time than steam blanching.” ^[22]Harrison and Andress. Preserving Food: Drying Fruits and Vegetables.

For some, like the University of Missouri extension, the preference varies by item:

Steam blanching: heating in steam is the recommended method for grated summer squash and sprouts. For broccoli, pumpkin, sweet potatoes and winter squash, both steaming and boiling are acceptable methods.” ^[23]Mills-Gray, Susan. Quality for Keeps: Freezing Vegetables. University of Missouri Extension. GH1503. May 2015.

The authors of Putting Food By say,

Leaf vegetables are usually better boiled than steamed, because they tend to mat in the steam basket, but can roll around if popped into water at a thrashing boil and quickly fished out with a skimmer.” ^[24]Hertzberg, Ruth. Putting Food By: Fifth Edition (p. 16). Penguin Publishing Group. Kindle Edition.

Blanching increases safety

Many nasties can often survive both the mild heat of drying, and the low-water activity environment that drying results in.

In 1993, an outbreak of salmonellosis was traced to contaminated paprika and paprika-powdered potato chips. The infective dose was estimated at 4–45 cells, proving that even extremely low numbers of salmonellae adapted to the dry state may be able to cause illness (Lehmacher et al., 1995).” ^[25]Influence of modified blanching treatments on inactivation of Salmonella during drying and storage of carrot slices. Patricia A. DiPersio, Patricia A. Kendall, Yohan Yoon, John N. Sofos. Food Microbiology 24 (2007) 500–507.

Patricia Kendall writes,

…research studies have shown that pretreating vegetables by blanching in water … enhances the destruction of potentially harmful bacteria during drying, including Escherichia coli O157:H7, Salmonella species and Listeria monocytogenes.” ^[26]P. Kendall. Drying Vegetables.

Citric acid might enhance safety

A Colorado State University study (results published in 2005 / 2006), using carrot slices inoculated with salmonella bacteria, found that blanching, by water or steam, helps lower the survival rate of the bacteria after drying. But, water-blanching the carrot slices in water acidulated with citric acid (at current Cooperative Extension Services recommended ratios to reduce darkening) caused an even lower bacterial survival rate.

The report concluded that further studies would be needed to develop acidulation guidelines including ratios, and to determine impact on quality / desirability from a consumer point of view.

In general, blanching carrot slices in citric acid solutions before dehydration improved inactivation of Salmonella during dehydration compared to all other treatments. Similarly, Calicioglu et al. (2003) reported that a 5% acetic acid dip (pH 2.50), applied before marination of inoculated beef strips, improved inactivation of Salmonella during dehydration compared to controls…. Steam and water blanching enhanced inactivation of Salmonella during dehydration and storage of carrot slices; however, populations were still detectable by direct plating after 30 days of storage. In contrast, populations were undetectable by direct plating on carrot slices blanched in citric acid and stored for 30 days … Compared to traditional home-drying methods, the blanching treatments used in the current study were more effective in reducing Salmonella populations on inoculated carrot slices immediately after application, and throughout dehydration and storage. More research is needed to understand Salmonella survival and growth in dehydrated vegetables, and to develop pathogen reduction guidelines for dehydrated produce. Furthermore, research is needed to evaluate the sensory characteristics (color, taste) of vegetables blanched in organic acid solutions before dehydration.” ^[27]Influence of modified blanching treatments. Page 505 – 506

We found some 2009 notes from Pacific Northwest Extension on impact of citric acid on quality:

Citric acid added to the blanching water…. will help destroy harmful bacteria but will result in a somewhat olive-green color in dried green vegetables. A citric acid blanch used with potato slices produces a superior product compared with a plain water blanch. If you plan to cook your dried vegetables before eating them, for example by adding them to soups or stews, then a citric acid treatment is not necessary.” ^[28]Swanson, Marilyn A. et al. Drying Fruits and Vegetables. 2009. Page 10.

How much citric acid

We did some searching, to see if someone had come up with a recommended ratio of citric acid to water for blanching vegetables.

We found a 2009 recommendation from Pacific Northwest Extension saying ¼ teaspoon per quart / litre of water:

Citric acid added to the blanching water (¼ teaspoon per quart of water) will help destroy harmful bacteria…” ^[29]Swanson, Marilyn A. et al. Drying Fruits and Vegetables. 2009. Page 10.

We found a later 2017 recommendation from Penn State Extension saying ½ teaspoon per quart / litre of water:

Blanching in a solution that contains ½ teaspoon of citric acid per quart of water is recommended for most vegetables. This enhances the destruction of potentially harmful microorganisms and slows the enzyme reactions that will continue during drying and storage.” ^[30]Luke LaBorde. Drying Fruits and Vegetables

The National Center for Home Food Preservation doesn’t yet seem to have a published position on this (as of 2017).

Note: for fruit,Pacific Northwest Extension recommends a a full teaspoon per quart / litre of water:

Citric acid does not prevent browning as well as ascorbic acid, but it’s more effective at destroying harmful bacteria. For enhanced pathogen destruction, prepare a citric acid solution by stirring 1 teaspoon citric acid crystals into 1 quart of cold water.” ^[31]Swanson, Marilyn A. et al. Drying Fruits and Vegetables. 2009. Page 8.

Canned and frozen vegetables don’t need blanching

If you are going to dry fruit or vegetables that were previously canned or frozen, they don’t need blanching: it will already have been done to them as part of their initial preservation process.

Using canned fruits is a quick way to prepare fruit for drying. Drain the syrup, rinse the fruit, and cut it into ½ inch slices, if desired, then dry as usual. Drying times will be longer than for fresh fruit because the canned fruit will contain absorbed syrup. Dried canned fruit resembles candied fruit and can be used in similar ways. Likewise, frozen vegetables can be thawed, drained, and dried. Blanching was taken care of before freezing.” ^[32]Swanson, Marilyn A. et al. Drying Fruits and Vegetables. 2009. Page 11.

Adjust blanching times for altitude

Blanching times must be adjusted for altitude.

Blanching times vary with altitude (higher altitudes require longer blanching times).” ^[33]Swanson, Marilyn A. et al. Drying Fruits and Vegetables. 2009. Page 9

The exact adjustments suggested have varied a bit since the 1990s; the most recent recommendations we have found are from 2013. They come from Colorado, which is considered an expert in high elevation cooking and food preserving.

The recommendation is:

Add 1 minute to water or steam blanching times when you are over 5000 feet (1500 metres).

“At 5,000 feet elevation or higher, heat 1 minute longer than the blanching time given for sea level. ” ^[34]Kendall, Patricia. High Altitude Food Preparation. Colorado State University Extension. 2013. Page 3. https://extension.colostate.edu/topic-areas/nutrition-food-safety-health/high-altitude-food-preparation-p41/

The authors of Putting Food By say they have consulted with the people at Colorado, and that the extra 1 minute applies to both water and steam: “As in the altitude note added to the boiling-water blanch, add 1 minute to steaming time…” ^[35]Hertzberg, Ruth. Putting Food By: Fifth Edition (p. 235). Penguin Publishing Group. Kindle Edition.

Note: Healthy Canning is aware that older advice from other Extension Services recommended more incremental adjustments starting at lower altitudes ^[36]For example, 1994 advice from Oregon State was above 1000 feet (300 metres, for each increment of additional 1000 feet (300 metres) to add 30 seconds for water blanching, 60 for steam blanching. Driscoll, D. Minar. Preparing Food at High Altitude. EC 1248. 1994. Page 1. , but publications containing that advice are now marked “archived”, and the other Extensions seem to be quoting Colorado’s more simpler recommendation.

Microwave blanching

Sadly, though microwave blanching initially promised to save energy and some nutrients, it appears to actually been proving less efficient (or less consistent) at doing a good enough job, so reputable sources currently (2017) seem either wary of it, or, to recommend against it outright. We’ll examine this topic more in depth another time, but one of the reasons might be that every single microwave in the whole world varies in capacity, strength, etc. — they can’t even agree where to put the popcorn button — so it is very hard to create standard guidelines that will work for everyone.