Carbon Dioxide Technology in Food Preservation: Extending Shelf Life and Ensuring Safety

• Carbon dioxide technology in food preservation lowers fruit respiration rate to maintain quality and extend marketing time.
• CO2 antimicrobial properties make food safe to meet consumer demands and protect crop yields.
• Carbon dioxide technology is useful during transportation, storage, and retailing.

The food supply chain uses many strategies to increase food production, extend fresh produce availability and shelf-life, and reduce waste. Carbon dioxide technology in food preservation is a leading postharvest method to preserve food quality and yield from the farmgate to the retailer’s shelves. Find out how carbon dioxide helps the food supply chain.

Postharvest Problems CO2 Technology Resolve

The supply chain attempts to deliver value to consumers and extend their marketing times, starting with measures that growers take. These measures must be supported by correct postharvest handling, storage, and transport to deliver quality and safe products to the retailers.

Carbon dioxide has two main functions in the postharvest phases: reducing fresh produce respiration rate and microbial and pest infestation, see Table 1. A third lesser use is for cryogenic cooling.

Table 1: Major changes due to CO2 and O2 altered levels in MAPs, Ovando-Martínez et al. (2016). (Image credits: https://doi.org/10.1007/978-3-319-23582-0_1)

Lowering Fruit Respiration Rates

Fruits and vegetables are alive and continue to respire even after harvest. Fruit respiration is necessary for ripening. However, it has a downside: It can negatively affect internal and external fruit quality attributes, leading to senescence. The higher the respiration rate, the more perishable the produce is, so high respiration rates result in less shelf-life.

During respiration, carbohydrates are consumed to produce energy, but around 90% is lost as respiration heat leads to fruit transpiration/water content loss. Reducing fruit respiration and its adverse effects is one of the main aims of postharvest handling.

The factors affecting fruit respiration rates are fruit type, maturity at harvest, storage time, wounding, and microbial infection. External factors like temperature are also crucial. For every 10°C hike, fruit biological processes rates increase 2-3 folds, within the physiological temperature range of 0°C–30°C. The upper limit is 40°C for tropical fruits. Therefore, cooling external temperatures is one of the main postharvest techniques for fruit quality preservation starting at the farm.

Increasing carbon dioxide (CO2) levels is another way to lower fruit respiration rates. Higher CO2 and lower oxygen (O2) levels can also prevent chilling damage and ethylene effects of ripening, sprouting, flower abscission, and senescence.

Standard CO2 technology involves increasing CO2 and lowering O2 levels, combined with low temperatures in a controlled atmosphere (CA) storage and modified atmosphere packaging MAP. O2 levels are kept low and close to the anaerobic compensation point (ACP). O2 levels above ACP increase respiration rate, and below ACP leads to anaerobic respiration.

High CO2 levels benefit tropical and sub-tropical fresh produce that cannot tolerate very low temperatures, like bananas, mangos, avocados, and foliage and flower plants that develop browning under cold conditions.

The ideal combination of CO2 and O2 depends on the product type, which is determined by many factors, such as the following:

• Fruit type: Nonclimacteric produce has higher respiration rates in earlier stages but declines during maturation. Climacteric produce’s respiration rate increases in later stages, where the peak triggers ethylene production and ripening.
• Stage of development: As fresh produce matures, the respiration rate falls. Produce with vegetative or floral parts like broccoli and asparagus have very high respiration rates. Similarly, immature fruits and vegetables experiencing growth have high respiration rates. Mature fruits and storage organs like tubers and nuts have low respiration rates.
• Processing: The respiration rates of cut iceberg and romaine lettuces

are 20-40% higher than intact heads. Shredded lettuce and cabbage have a respiration rate of 200-300% greater than intact heads.

Ambient air has 0.04% CO2, 21% oxygen, 78% nitrogen, and traces of other gases. CO2 can suppress or increase respiration rate in CA depending on its levels, temperature, product type, and exposure duration. For example, at 5% CO2 + 5% O2, respiration rates of fresh-cut onion, leek, and carrots are only slightly reduced, but cut potatoes’ respiration rates are increased.

However, exposure to high levels of CO2 and low O2 can reduce fruit quality due to the following reasons:

• Anaerobic respiration brought on by less O2 causes controlled decay and browning.
• High CO2 levels lead to fermentation by accumulating compounds like ethanol, methanol, acetaldehyde, and succinate, for example, in apples, and cause off-odors.
• Succinate accumulation keeps fruits like strawberries firm but adversely affects internal color as alkalinity increases because of a reduction in malic and citric acids.

To prevent CO2-caused quality damage, one of the recent alterations has been to add high O2 and CO2 levels. For example, high CO2/O2 levels lower respiration rates in cut potatoes stored at 4°C for 14 days than those under low O2 and high CO2.

Preventing Microbial and Pest Infestations

CO2 also has antimicrobial properties that are used in postharvest management. High CO2 also makes the environment less favorable for all pests.

High CO2 levels of 5-10% keep microbe numbers down through two effects:

• CO2 lowers cellular pH and impacts microbe metabolism.
• Indirectly controls microbial growth by limiting tissue deterioration, such as composition changes and softening.

However, CO2 effects will depend on microbe types:

• CO2 can control mold, as fungi are sensitive to CO2.
• Gram-negative bacteria are also susceptible to CO2.
• The microbes resistant to CO2 are yeasts, anaerobic bacteria, and lactic acid bacteria.

Exposure to CO2 with heat treatment for fixed durations is a method for controlling insects. For example, coddling moth infestation in cherry is prevented by exposure to 15% CO2, 1% O2, and 44°C for 44 minutes.

CO2 for Cryogenic Cooling

CO2 is also used to a lesser extent for cryogenic cooling, such as solid CO2 or dry ice, to maintain low temperatures for fresh produce. Its use was more prevalent before refrigeration became standard for transportation. Its advantage over regular ice is that it turns to gas at higher temperatures, not liquid. If dry ice is used in CA storage and transport, proper ventilation is required to remove the gas. Nowadays, it is not used in CA to avoid unintended modification of atmospheres. Dry ice use is restricted for local and short delivery trips, air transport, and frozen food. It is lighter than regular ice, making it more suitable for air transport. The use of dry ice must have proper and transparent labeling, as excess CO2 harms people and animals.

The use of high CO2 levels is standard in CA for bulk and in MAP for food packaging.

CO2 in a Controlled Atmosphere

Controlled atmosphere (CA) units are characterized by increasing CO2 and lowering O2 levels in the atmosphere where the produce is stored to reduce physiological processes. CA is used for storage and transport.

Controlled Atmosphere Storage: It is considered one of the most successful technologies to improve the fresh produce supply chain, combined with refrigeration. CA technology is sophisticated and allows for varying the CA conditions as needed. CA can increase storage life by 30% for produce. However, since CA for storage is expensive, they are used for produce that can last a long time, like climacteric fruits, such as apples, pears, and kiwifruits. It is used less for produce like cabbages, avocados, persimmons, sweet onions, vegetables, nuts, etc.

Controlled Atmosphere Transport: Marine containers for long-distance transport often have CA atmospheres. CA is used for transporting apples, bananas, mangoes, avocados, asparagus, broccoli, berries, cherries, strawberries, figs, melons, nectarines, plums, and peaches.

Crucial elements in both applications are establishing, maintaining, and monitoring CA with testing systems for quality control. Technology is continuously developed to make CA more cost-effective and increase the benefit-to-cost ratio.

The use of high CO2 in CA units during storage or transport provides the following advantages:

• Extend seasonal availability of produce.
• Enable long-distance transport in global chains so producers can take advantage of better prices.
• Maintain the physicochemical quality and function of food.
• Limit storage and transport waste by reducing chilling injury.
• CO2 is an alternative to chemical treatment for quality maintenance and shelf-life extension.
• Reduce cost for consumers.

MAP

Modified Atmosphere Packaging (MAP) is also a controlled atmosphere application. MAP

involves creating a gas-tight system where the gaseous atmosphere of nitrogen (N2), CO2, and O2 is changed from the ambient concentrations and controlled throughout storage. O2 is removed and replaced by N2 to prevent food spoilage.

Depending on the produce, O2 may be removed completely to prevent oxidation and rancidity of oils and fats, ripening, senescence, color changes, and microbial spoilage. Small amounts of O2 are included in the package to allow for aerobic respiration in other produce.

Technology like films, semipermeable membranes, and gas and chemical scavengers extends CA to many crop and food types.

CO2 is initially introduced, as product respiration is slow at lower storage temperatures and doesn’t produce enough gas to achieve the high concentrations necessary in CA. Later, excess CO2 is removed through circulation, absorption by special filters made of CO2 scrubbing materials like activated charcoal, lime, and water, or chemical CO2 scavengers like sodium carbonate and calcium hydroxide. It can reduce browning, senescence, and mold decay.

Table 2: The quality problems of typical fresh-cut produce and the optimum CO2 (and O2) levels to resolve them, Cantwell & Suslow, (1999). (Credits: https://ucanr.edu/datastoreFiles/608-357.pdf)

MAP is used for bulk packaging during storage and individual packing for retailing.

The optimum levels of each gas for each fresh produce are distinct. The initial gas combination added can change over time due to the product’s respiration and ethylene production or absorption.

• Active Packaging leverages this phenomenon by using biological processes to achieve the optimum storage gas combinations.
• In passive MAPs, chemical scavengers are added to remove excess CO2 or perforated to allow for gas escape. Meanwhile, CO2 emitters, like ferrous carbonate in pads or boxes, raise the gas levels.

MAP must also provide the correct atmosphere for fresh-cut vegetables, fruits, and intact food. The higher respiration rates of cut and processed produce cause quicker loss of quality, acids, sugars, flavor, and nutritional value. Low temperatures alone are insufficient for damage control, and MAP technology is necessary. The CO2 and O2 concentrations needed to address quality issues of typical produce are given in Table 2.

Measuring CO2 Levels

Continuous monitoring of CO2 and O2 levels is vital for CA storage, transport, and retailing MAP. Precision infrared gas analyzers track CO2 levels in rooms and MAP headspaces. Felix Instruments Applied Food Science has portable tools for measuring CO2, O2, and ethylene levels. Using carbon dioxide technology in food preservation is crucial for maintaining quality and preventing waste.

• The F-920 Check It! Gas Analyzer rapidly analyzes the gas composition of CO2 and O2 levels in the MAP headspace.
• For monitoring in CA, the F-940 Store It! Gas Analyzer and F-901 AccuRipe & AccuStore can be used.

The user-friendly precision tools that give real-time readings are an asset for monitoring CO2 levels to safeguard fresh produce quality and prevent waste.

Sources

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Beaudry, R. M. (1999). Effect of O2 and CO2 partial pressure on selected phenomena affecting fruit and vegetable quality. Postharvest Biology and Technology, 15(3), 293-303.

Cantwell, M., & Suslow, T. (1999). Fresh-cut fruits and vegetables: aspects of physiology, preparation and handling that affect quality. In Annual Workshop Fresh-Cut Products: Maintaining Quality and Safety (Vol. 5, pp. 1-2). Davis, CA, USA: University of California. Retrieved from https://ucanr.edu/datastoreFiles/608-357.pdf.

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Gross, K.C., Wang, C.Y., & Saltveit, M. (2016). The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. Agriculture Handbook Number 66. Retrieved from https://www.ars.usda.gov/arsuserfiles/oc/np/commercialstorage/commercialstorage.pdf

Ovando-Martínez, M., Ruiz-Pardo, C.A., Quirós-Sauceda, A.E., et al. (2016). Oxygen, Carbon Dioxide, and Nitrogen. In: Siddiqui, M., Ayala Zavala, J., Hwang, CA. (eds) Postharvest Management Approaches for Maintaining Quality of Fresh Produce. Springer, Cham. https://doi.org/10.1007/978-3-319-23582-0_1

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horticultural produce. In Benkeblia, N (Eds.), Postharvest Technologies for Horticultural Crops (Vol 2, pp.1-24.) ISBN: 978-81-308-0356-2. Retrieved from https://ucanr.edu/datastoreFiles/234-1291.pdf