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Lesson 5
Post-Harvest Handling
Back to: Harvest
How do you control kelp quality?
The moment seaweed is harvested, degradation begins. That is, kelp will always be at its highest level of quality at the moment of harvest — it will never improve from that point, only decline. The speed of degradation depends on what kind of light, temperature, moisture, air, and pressure conditions the kelp is subjected to. The more a farmer is able to control these factors, the higher the quality of kelp will be at the time of delivery to the buyer.
Factors that impact the speed of degradation
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Temperature
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Moisture
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Access to oxygen
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Pressure
Temperature
While for public health purposes, clean raw kelp could remain unrefrigerated for up to 8 hours before pathogen growth is a potential food safety hazard, the quality of raw kelp diminishes rapidly if kept out of temperature control for more than 2 hours after harvest (Concepcion et al., 2020). The possibility of shellfish bio-fouling layers on additional shellfish food safety considerations.
For this reason, it’s best to place seaweed under temperature control via ice, gel packs, or mechanical refrigeration set at less than or equal to 40F as soon as possible after harvest (Concepcion et al., 2020). Be careful not to “shock” the seaweed by putting it in direct contact with sub-zero materials; sugar kelp may start to release mucus if subjected to such conditions.
If ice is used to control temperature, a food-safe, insulated barrier (such as a plastic bag covering corrugated cardboard) should be used between the seaweed and the ice to prevent freshwater ice melt and temperature shock that might affect seaweed quality. If gel packs are used, they should be clean and sanitized between each use. Monitor temperature, ideally with a continuous temperature recorder, but otherwise at least twice daily.
Air Flow
Scientific studies conducted by QFreshLab and GreenWave in 2021 and 2022 indicated that freshly-harvested sugar kelp requires access to oxygen to maintain quality. This is because similar to land-based plants, sugar kelp switches from photosynthesis to respiration after it is cut from the holdfast. Respiration is the same activity that humans and all other animals perform day in and day out: also known as breathing!
Definition
Respiration
Glucose (sugar) is combined with oxygen and transformed into carbon dioxide, water, and energy (heat).
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Cutting
Given the known similarities to produce, it’s likely that cutting seaweed also results in a “wound response” that speeds up degradation. Therefore, cutting should be minimized as much as possible — ideally to a single harvesting cut and perhaps a tip trim — in order to maximize shelf life. However, if there is significant degradation or bio-foul on your kelp, it may be advantageous to do some additional cutting to facilitate sorting at the time of harvest. You will need to make a judgment call based on your buyer and their intended use case for your kelp.
Other Considerations
In addition to the considerations above, farmers should also take care to:
Be sure to:
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Handle the kelp as little as possible
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Keep kelp covered to avoid moisture loss and protect it from contaminants
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Store kelp out of direct sunlight, and other sources of heat and light
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Avoid freshwater contact from ice or rain, which may cause the kelp to blister
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Avoid exposing harvested kelp to freezing temperatures, which may result in water loss and texture degradation
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Use only food-grade hydraulic oil and machinery grease, to prevent contamination
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Use only materials that can be sanitized, such as plastic or stainless steel
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Clean the harvest vessel daily
Implications for Transportation, Storage, and Packaging
Transportation and Storage Guidance
Additional research is needed to understand precisely how much oxygen is required to maximize shelf life. In the meantime, our guidance is to ensure adequate airflow through one or more of the following methods during transportation and storage:
- Store sugar kelp in mesh bags in the ocean (which has oxygen dissolved in it) for as long as possible before bringing it onto the harvest vessel
- Use more, smaller containers rather than fewer, larger containers
- Use perforated or vented containers
- Keep kelp “fluffy” rather than packing it down
Packaging Guidance
If you plan to package fresh, minimally-processed sugar kelp, we recommend using Modified Atmosphere Packaging (MAP). MAP is designed to control the transmission of gasses, altering the atmosphere inside of the package through the use of specialized materials and small holes called microperforations. GreenWave worked with QFreshLab to create packaging recommendations based on sugar kelp’s specific respiration rate.
Implications for Shelf Life
Similar to quality, there is no industry standard for the shelf life of fresh sugar kelp. However, understanding the shelf life of your particular product is critical, especially if it will be sold for human consumption. Shelf life is most important in the context of your end product since consumers or buyers will expect this information.
The shelf life of fresh, minimally processed sugar kelp could be as low as 48 hours or as high as 10+ days under optimal handling conditions. Studies conducted in partnership with QFresh Labs showed a shelf life on the higher end of this range under optimal conditions. The shelf life of your product will depend on your exact handling and storage conditions, your packaging, and the product itself, including any processing. GreenWave recommends conducting shelf life testing, taking into account your entire supply chain and handling practices from harvest to sale.
Learn More
GreenWave and QFresh Lab’s 2023 Post-Harvest Handling Report
READGreenWave’s Sugar Kelp Respiration Rates and Implications for Post-Harvest Handling
READGreenWave’s Fresh Sugar Kelp Packaging Guide
READConnecticut Sea Grant’s A Guide to Understanding and Controlling Potential Food Safety Hazards
READAlaska Sea Grant’s Seaweed Handling and Processing Guidelines for Alaska
READPerennia and Ecology Action Centre’s Post-Harvest Handling Guide for Cultivated Kelp in Nova Scotia
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