How to preserve leaves

In this brief guide, we are going to answer the question “how to preserve leaves” and discuss the methods used to preserve leaves.

How to preserve leaves

The average daily vegetable consumption of adults was 51% in 28 EU countries in 2014. More women eat vegetables daily than men and older adults eat more fruits and vegetables daily than younger people do. About 12% of adults in 28 EU countries have reported a daily consumption of at least five fruits and vegetables (1).

Leaves can be preserved by:

  • Refrigerating 
  • Freezing 
  • Drying
  • Canning

Leaves are an excellent source of nutrition with few calories. Kale, cabbage, spinach, lettuce, collard greens and watercress are some common leafy greens. 

Unfortunately, green leaves have a very short shelf-life at room temperature and are susceptible to drying, wilting and yellowing. 

How to preserve leaves by refrigerating

Changes in nutrient composition of vegetables occur postharvest due to its high water content (70-90%) and increased respiration rate resulting in qualitative losses such as loss in edibility, nutritional value and consumer acceptability. The rate of biological and biochemical degradation depends on both postharvest handling and on storage conditions (2).

Refrigerating is the best short-term preservation method for leafy greens. 

Most leaves can be refrigerated for about 1-2 weeks, depending on the vegetable. Iceberg lettuce can be stored longer (2 weeks) than spinach (3-7 days), according to the US Department of Agriculture. Refrigerating is the easiest way to preserve leaves, and the flavor and texture are retained throughout. It is recommended to store green leaves in the back of the refrigerator where the temperature is most stable.

To refrigerate green leaves:

  • Rinse the leaves and pick out any yellow or damaged leaves.
  • Dry the leaves with a paper towel or a salad spinner. Make sure that the leaves are completely dry as even a little moisture can cause spoilage.
  • Wrap the leaves with a paper towel or line an airtight container with paper towels.
  • Place the leaves in a sealable bag or inside the airtight container.
  • Seal, label and refrigerate for up to 1-2 weeks.

Green leaves can also be placed upright in a glass of water and then refrigerated. This method is useful for hardier leaves such as kale and it will retain freshness for a longer time.

The vitamin C content of the selected vegetables subsequently decreased during storage period and is minimal after seven days. However, the level of degradation depended on the vegetable species: fragile and perishable vegetables, expressing great sensitivity to manipulation and processing as spinach and broccoli were more affected regarding the vitamin C content (2).

How to preserve leaves by freezing

To prevent microbial growth and enzymatic deterioration in leaves, thus extending their shelf life, freezing can be applied. Raw fruits and vegetables contain large quantities of water in proportion to their weight and, consequently, the water phase change occurring in freezing makes these products more susceptible to ice crystal formation and thawing than other types of food (3).

Most leaves are frozen whole. Leaves can also be frozen as a puree. Freezing whole leaves will retain the quality as well as the nutrient content. Some freezing methods recommend blanching before freezing and some say it is best to freeze without blanching. 

Leafy greens such as kale, collard greens and spinach can be frozen for up to 9 months without blanching. Some endogenous enzymes are responsible for undesirable changes, such as off-flavors and odors, and color and nutritive alterations, during frozen storage. These deteriorative enzymes may have to be inactivated by applying particular treatments before the freezing step. Therefore, blanching is the most important pre-freezing treatment for vegetable tissues stabilization (3).

If the leaves are blanched before freezing, they can be preserved for about 1 year. This is because blanching reduces enzyme activities in the leaves which cause decomposition. Through blanching, the color, texture and flavor can be preserved for a longer time.

Blanching also removes oxalic acid in green leaves. A high concentration of oxalic acid is usually associated with kidney stones. During blanching, oxalic acid is released into the water (4).

How to preserve leaves by drying

Drying of fruits and vegetables is one of the oldest procedures for food preservation known to man and is the most important process for preserving food since it has a great effect on the quality of the dried products. The main objective in drying agricultural products is the reduction of the moisture content to a level which allows safe storage over an extended period. Also, it brings about substantial reduction in weight and volume, minimizing packaging, storage and transportation costs (5).

Drying is usually used to preserve herbs that are used for flavor and seasoning. Green leaves such as kale can also be dried and then powdered. The powdered leaves can be added to juices, smoothies and soups for an extra boost of nutrition.

Drying will preserve leaves for about a year. However, all the texture and some flavor and aroma will be lost by drying. 

Dried leaves must be stored in an airtight container in a cool, dark place away from moisture. Dried leaves can also be frozen for about a year and a half but once again, there will be a loss of flavor.

Some herbs and leaves preserved by drying include parsley, basil, kale and spinach.

There are several methods to dry leaves, such as (5,6):

  • Air-drying
  • Sun-drying
  • Using a food dehydrator
  • Using an oven
  • Using a microwave

Using a food dehydrator is the easiest and most efficient method. 

To dry leaves using a food dehydrator :

  • Wash and dry leaves using paper towels or a salad spinner.
  • Trim off the stems.
  • Arrange the leaves on the dehydrator tray.
  • Dry according to the temperature suggested in the dehydrator manual.
  • Store the dried leaves in an airtight container. The leaves can also be ground to powder in a food processor at this stage.

How to preserve leaves by canning

Canning Is the method of preserving food in which the food contents are processed and sealed in an airtight container. It is a two-step process, first the food is prepared by being packed into containers which are then sealed, then containers are sterilized or heated to ensure that all microorganisms are destroyed. The heating process in canned products kills many common pathogenic microorganisms (7).

Canning is not commonly used to preserve leaves. However, some leafy greens such as spinach, kale and chard preserve well by canning. If a pressure canner is used, leaves can be canned and stored for a long period.

A major drawback of canning includes changes in taste and technical difficulties. Canning green leaves require a pressure canner and it is also a time-consuming method.

To can leaves:

  • Rinse leaves and discard any damaged and yellowed leaves.
  • Blanch the leaves until they are just wilted.
  • Pack the blacked leaves into hot jars. Salt can be added at this stage, but it is optional.
  • Top off the jars with boiling water leaving about 1 inch of headspace.
  • Wipe off the rims of the jars and place the lids on the jars.
  • Process the jars using a pressure canner.
  • Allow the jars to cool.
  • Label and store the jars in a cool dark place.

More about canning leaves can be found here.

Conclusion

In this brief guide, we answered the question “how to preserve leaves” and discussed in depth the main methods used to preserve leaves. 

If you have any questions or comments, please let us know.

References

  1. Hong, Jungha, and Nazim S. Gruda. The potential of introduction of Asian vegetables in Europe. Horticulturae, 2020, 6, 38.
  2. Balan, Daniela, et al. Changes in the nutrients content of some green vegetables during storage and thermal processing. Roman Biotechnol Lett, 2016, 21, 11857-11865.
  3. Evans, Judith A., ed. Frozen food science and technology. John Wiley & Sons, 2009.
  4. Bevilacqua, Maurizio, Antonio D’Amore, and Fabio Polonara. A multi-criteria decision approach to choosing the optimal blanching–freezing system. J Food Eng, 2004, 63, 253-263.
  5. Sobukola, O. P., et al. Thin layer drying process of some leafy vegetables under open sun. Food sci technol int, 2007, 13, 35-40.  
  6. Kendall, Pat, et al. Drying vegetables. Colorado State University 2004, 9.308.
  7. Sigaqa, Hombisa Tozi. Effects of canning on the nutritional composition and consumer acceptance of African leafy vegetables. Diss. 2016. University of KwaZulu Natal