Post-Harvest Processing of Cultivated Kelp for Food Production

Seaweeds are known to contain nutritional and bioactive compounds beneficial for human health. This, combined with cultivation methods that require no land area, fresh water, or fertiliser, has led to increased interest in the use of seaweeds as a sustainable food source. In Norway, seaweed cultivation is on the rise, and it is dominated by the two species Alaria esculenta and Saccharina latissima. These are brown seaweeds part of the order Laminariales, commonly called kelps. However, there are some hurdles to overcome on the road from cultivated kelp to food product. Seaweeds are known to accumulate potentially toxic elements (PTEs) such as arsenic, cadmium, mercury, lead, and iodine. Arsenic, especially in its inorganic form, cadmium, mercury, and lead can lead to adverse health effects for consumers at low levels. Meanwhile, iodine is an essential mineral in the human diet, but it can lead to adverse health effects when consumed in excess. Kelp species, and especially S. latissima, have been shown to contain high levels of iodine. Additionally, the harvesting season for cultivated kelp and its shelf life after harvesting are very short. Therefore, to produce safe and stable food products or ingredients from cultivated kelp, post-harvest processing is necessary to reduce the PTE concentrations and stabilise the biomass. This processing could alter the nutritional value and flavour of the seaweed as well as impact its environmental footprint due to energy and water usage during processing. Optimising post-harvest processing methods for kelp can aid in the sustainable production of safe, nutritious, and flavourful foods and food ingredients from cultivated kelp.

This thesis consists of three studies designed to assess the effect of different processing methods and schemes on the safety and nutritional value of A. esculenta and S. latissima. They include more traditional processing methods such as warm water treatment (WWT), freezing, drying, and fermentation, and the more innovative processing methods pulsed electric field (PEF) and ultrasound (US). The effects of the processing methods on the kelp were assessed by the determination of changes in total dry matter, PTE concentrations, nutritionally relevant minerals and trace elements, amino acid composition, and protein concentrations. The effects of pretreatment by WWT or PEF on the fermentation of S. latissima and the effects of using pretreated A. esculenta in fish patties were also investigated.

General trends seen after most processing methods were that dry matter content and minerals were lost, while total protein content was retained. Essential amino acids were even better retained than the total amino acid content. This led to increased protein concentrations after processing due to the loss of other dry matter and increases in the ratio of essential amino acids compared to total amino acids. For the PTE content, where differences from the unprocessed biomass were observed, processing was able to reduce iodine and arsenic concentrations, while cadmium, mercury, and lead concentrations were increased. Fermentation was the only processing method able to reduce cadmium concentration significantly.

Despite iodine reductions, the iodine concentration was the limiting factor when calculating safe portion sizes of all unprocessed and processed seaweed samples. The nutritional contribution of minerals from these safe portion sizes was low, implying that for the seaweed to have significant nutritional benefits, the iodine concentration needs to be further reduced. Safe portion sizes, and consequently the nutritional contribution, were higher for A. esculenta samples than for S. latissima samples due to substantially higher iodine concentrations in S. latissima. Meanwhile, safe portion sizes based on cadmium intake were much lower in the A. esculenta samples than in the S. latissima samples. This indicates that if higher iodine reduction is achieved in A. esculenta, too high cadmium concentrations could become a problem.

The results in Paper I showed that slow freezing and thawing reduced the iodine and arsenic content slightly. However, this was accompanied by a high drip loss during thawing. Quick freezing limited drip loss during thawing and was therefore recommended as the freezing method that best preserved the kelp biomass. Results on fermentation from Paper III showed that pretreatment by PEF led to significantly lower pH values after 24 h of fermentation than for WWT and directly fermented samples. However, all fermentations were successful in reducing pH sufficiently for preservation, regardless of pretreatment.

Incorporation of processed A. esculenta into fish patties led to no significant changes in the odour or flavour profile of the patties. The sensory analysis showed some variation in rubbery texture, but the texture analysis showed no significant differences between the different fish patties. The inclusion level of the seaweed was low, and iodine was the only nutrient added in nutritionally relevant amounts. The total iodine concentrations of the fish patties were substantially increased by using seaweed as an ingredient. This shows that kelp can be used for iodine fortification of food products, which can be especially relevant in a vegan diet where iodine sources are scarce.

The US treatment of A. esculenta led to the highest reductions in iodine and arsenic concentrations and the highest increases in cadmium, mercury, and lead concentrations. However, the iodine concentration was not significantly higher than in the WWT A. esculenta from the same study. The US treatment was executed at the same temperature as the WWT, but for a much longer time, indicating that neither the US treatment nor the longer treatment had a significant effect on the iodine reduction. Nevertheless, US treatment had a significant effect on the reduction in arsenic concentration. For S. latissima, PEF processing at the highest level and WWT reached similar iodine reductions, with the WWT requiring more than ten times as much energy, indicating that PEF could be a low-energy alternative to WWT. Overall results also suggested that a higher water-to-seaweed ratio can lead to higher reductions in PTE concentrations.

The results from this study can guide kelp producers in planning post-harvest processing schemes for cultivated kelp and understanding the effects the processing or preservation method has on their product. However, more research is needed on how to reduce iodine concentrations further, and techniques for cadmium reduction should be investigated. Optimal processing conditions need to be determined to limit the use of energy and water while maximising the reduction of PTE concentrations. Legislation on PTE concentrations in seaweed meant for human consumption would make the use of seaweed as a food or food ingredient easier and aid in the establishment of a strong future seaweed industry.