How to preserve vegetables

In this brief guide, we are going to answer the question “how to preserve vegetables”, and discuss the methods to preserve both raw and cooked vegetables. We are also going to discuss the advantages and disadvantages of preserving vegetables.

How to preserve vegetables 

Most fresh vegetables arrive to the consumer several days to weeks after it is harvested. During this time, cellular respiration and oxidation can cause substantial nutrient degradation. To halt spoilage and eliminate pathogens, food processing methods such as blanching and freezing have been developed (1).

Vegetables can be preserved by :

• Refrigerating
• Freezing

• Using techniques such as pickling, dehydration, and canning for long term storage
• Using proper storage conditions

How to refrigerate vegetables for preservation

Refrigerating is the most common method to preserve vegetables for a few days. Storing fruits and vegetables at low temperatures can maintain their tissue cells in a living state without postharvest chilling damage and the destruction of cell structures (which can occur by freezing), ensuring food quality. Moreover, these low temperatures minimize the respiratory rate and deterioration reactions in organisms, and the growth of pathogenic microorganisms (2).

Most vegetables preserve better in cold, moist surroundings. Refrigerating also halts the ripening of vegetables.

The best place in the fridge for vegetables is the crisper drawer. The crisper drawer has the correct humidity and temperature to preserve vegetables.

When refrigerating, store ethylene producing and ethylene absorbing vegetables separately. For example bananas, apples and melons are ethylene producers while cabbage, broccoli, and cauliflower are ethylene absorbers.

How to freeze vegetables for preservation

Freezing is the most widely used method to preserve vegetables since it can be done easily at home by anyone. Vegetables can be frozen either raw or after being blanched. Blanching is a crucial processing step in the industrial production of frozen vegetables; this treatment inactivates the enzymes responsible for color and texture deterioration and reduces microbial load (3).

Freezing food reduces the enzyme activity in food that causes spoilage. Freezing also crystallizes the water in the food so microorganisms cannot use it for growth (3).

Freezing itself does not significantly reduce the nutrient content of food. However blanching before freezing and defrosting will cause the loss of some nutrients, mainly water-soluble nutrients such as vitamin C. However, blanching can be done in the microwave. In a comparison of vitamin C content in tomatoes, a study found that the content of this vitamin decreased after boiling-water blanching (65% retention), but there was only 10% loss after the microwave blanching method (3).

Tips for freezing vegetables:

• Wash and dry the vegetables before freezing. Any moisture will cause spoilage.
• It is better to peel and slice the vegetables before freezing.
• Store frozen vegetables in a ziplock bag or an airtight container.
• Maintain the freezer temperature at -18 °C or slightly below. Fluctuations in temperature will cause the food to spoil faster.
• Wherever possible blanch the vegetables before freezing. Blanching deactivates the enzymes that cause a loss in flavor, color, and nutrient content. Blanching temperature and time depends on the vegetable, but common practices are >90°C for 2-3 minutes (1).
• Store vegetables in vacuum packaging if possible. Vacuum packaging will create an oxygen-free environment that reduces the formation of ice crystals in the vegetables.

How to dehydrate vegetables for preservation

Dehydrating is one of the oldest methods of food preservation. Dehydrating works by removing the water from food, thereby reducing the growth of microbes that cause spoilage. Dehydrated food has a long shelf-life and can be stored for up to 5 years under the proper conditions.

For dehydration, the moisture can be removed by several methods such as sun-drying, heating at a low-temperature setting in an oven, or using a food dehydrator. The moisture content of the food is reduced to 10-15%, thus, the present microorganisms can be inhibited and become inactive (4).

Dehydration requires little to no preparation time. Dehydrated vegetables retain most of the nutrients and there are no added preservatives. However, studies show that the antioxidant content in fruits and vegetables may decrease due to oxidative damage caused by the drying of various plant materials because drying damages phenolic compounds. In other cases, the generation and accumulation of Maillard-derived melanoidins with various degrees of antioxidant activity also enhance antioxidant properties at high temperatures (5). 

The main disadvantage of dehydration is the long time taken for the dehydration process.

Dehydrating vegetables will significantly alter their texture and flavor. 

How to preserve vegetables by pickling

Pickling is a traditional method of preserving food by immersing in a solution of acidic brine or vinegar. The vinegar will lower the pH value of the food and hinder microbial growth. Salt is also a natural preservative of food, thus it decreases the water available for microorganisms and enzymatic activity. Pickles are used as side dishes and garnish in many cuisines.

For making pickles, fruits and vegetables are added with 8 to 10 % of salt and 3% of vinegar. After that, they will be kept for 6 to 8 days under the sun with the temperature of 29°C to 30°C for fermentation with the assistance of lactic acid bacteria. In the last step, it will be blended together with coarsely powdered spices and then, it will be covered completely with moisture-free edible oil (5).

Properly produced pickles can be stored for about a year. However, most pickles must be refrigerated after being opened.

Pickled vegetables have a distinct flavor that is more appealing. They stimulate the production of gastric juice, which helps in food digestion. The high content of salt may increase blood pressure, so pickles must be eaten in moderate amounts.

How to preserve vegetables by canning

Canning preserves food by killing the microorganisms that cause spoilage. Canning can be done at home using the correct equipment and proper sterilization of jars but it is time-consuming and needs practice. 

This thermal process has extensively been used for the production of jams and jellies, canned and bottled fruits and vegetables. In general, fruits and vegetables in cans and bottles produced under commercial sterility can be used up to 2 years and longer. Thermal processing is the process of either heating unsterile foods in containers such as canning, or heating food products, prior to packaging or before they are put under sterile states such as aseptic handling (5).

Canned foods are shelf-stable for years and do not need refrigeration until they are opened. However, the heating process during canning reduces the nutrient content of food.

The disadvantages of canning include loss of taste, loss of nutrients, and technical difficulties. Vitamins and polyphenols are lost during this process (4).

You can find some great canning recipes here.

How to properly store vegetables so they don’t go bad 

We discussed many methods to preserve vegetables, but few vegetables cannot be preserved using these methods. A good example is raw potatoes.

Raw potatoes preserve well at room temperature. Raw potatoes, when stored properly, can last for about 8 months. Tubers such as sweet potatoes and manioc also keep well at room temperature.

Tips to store raw vegetables:

• Do not wash vegetables before storing.
• Store them in a cool, dry place.
• Store them away from sunlight. Most vegetables prefer dark places such as the basement or a cellar.
• Make sure there is enough ventilation. Do not store the vegetables in closed containers. Potatoes and sweet potatoes prefer plastic crates or wicker bags.
• Make sure that the vegetables are stored away from insects and rodents.
• Check the vegetables regularly for signs of mold and spoilage. Immediately discard spoiled vegetables.

Conclusion

In this brief guide, we answered the question “how to preserve vegetables ”, and discussed the methods to preserve both raw and cooked vegetables. We also discussed the advantages and disadvantages of preserving vegetables.

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

References

1. Bouzari, Ali, Dirk Holstege, and Diane M. Barrett. Vitamin retention in eight fruits and vegetables: a comparison of refrigerated and frozen storage. J agri food chem, 2015, 63, 957-962.
2. Liu, De-Kun, et al. Sub-zero temperature preservation of fruits and vegetables: A review. J Food Eng, 2020, 275, 109881.
3. Barrett, Diane M., and Beate Lloyd. Advanced preservation methods and nutrient retention in fruits and vegetables. J Sci Food Agri, 2012, 92, 7-22.
4. Al-Juhaimi, Fahad, et al. Effect of various food processing and handling methods on preservation of natural antioxidants in fruits and vegetables. J food sci technol, 2018, 55, 3872-3880.
5. Sharif, Z. I. M., et al. Review on methods for preservation and natural preservatives for extending the food longevity. Chem Eng Res Bull, 2017, 145-153.