Processing of Seaweed and the Effects on Food Quality and Safety

The potential of seaweed as a sustainable food source has gained increasing attention in recent years, driven by the fact that it can be cultivated in the sea, without taking up land areas or requiring freshwater. Additionally, seaweed can play a role in meeting the world's future need for food. However, as with most new food sources, seaweeds come with new challenges within post-harvest processing and ensuring food safety. This PhD project aims to study industrial post-harvest processes, such as drying, blanching, and washing of commercially available European seaweeds to ensure controlled, stable, and safe food products. The study has an industrial relevance, and the intention is to clarify good practices within each respective post-harvest process and species. The species in focus are sugar kelp, winged kelp, bladderwrack, sea lettuce, and dulse. The thesis is divided into four main chapters: vitamin C from seaweeds, blanching and washing of kelp, drying of bladder wrack and sea lettuce, and shelf-life of refrigerated sugar kelp.

Generalizing claims that seaweeds are rich in vitamins, are seen in both commercial promotion and scientific literature. Therefore, the vitamin C content of Northern European seaweed species are examined by a review and compared to commonly consumed foods, the recommended intake, and the possibility to claim Tance of vitamin C. Results showed that seaweed has lower vitamin C content (0.330-0.942 mg (g dw)^-1) than oysters, lettuce, potatoes, cucumber, broccoli, and rosehip (0.704-36.4 mg (g dw)^-1). This shows that seaweeds are not a rich source of vitamin C.

The presence of potential toxic elements such as arsenic, cadmium, and iodine as well as microbial load on sugar kelp and winged kelp are a matter of concern. The effects of washing and blanching on the levels of potential toxic elements are investigated, as well as their impact on desirable food qualities such as nutrient and bioactive content and sensory properties such as color, texture, and odor. Blanching (45-80 °C for 30-120 s) can reduce the initial microbial load of 3.52-4.54 log (CFU g^-1 wet weight) to 0.906-2.32 log (CFU g^-1 wet weight). It also reduces the levels of potential toxic elements such as arsenic from 47.0-66.7 mg (kg dw)^-1 to 28.5-39.7 mg (kg dw)^-1, as well as iodine. For the first time a predictive model has been made that can estimate the iodine reduction in sugar kelp depending on the process parameters: time and temperature. It predicts, as an example, that a blanching process carried out at 60 °C for 2 minutes results in an iodine content between 500-750 mg (kg dw)^-1 or blanching at 45 °C for a duration of 8 minutes will achieve a maximum iodine content of 500 mg (kg dw)^-1.

Of the more positive quality nutrients and bioactive compounds, several are reduced due to blanching, including the amino acids with umami taste, magnesium, mannitol, vitamin B9, and vitamin C. Moreover, sensory, and physico-chemical properties are also altered due to blanching. A principal component analysis reveals that blanching increases the intensity of the odors: sweet, fresh sea, umami, sour, and rubber. Additionally, the color of the kelp is found to shift from brown to an intense green during blanching at 80 °C. In conclusion, it is recommended to blanch sugar kelp and winged kelp at minimum a temperature of 45 °C for a duration of 30 seconds.

Drying preserves seaweeds by removing water and lowering the water activity with the possibility to retain the food quality. However, several drying methods exist, which each have different energy and time consumptions as well as different impacts on the food quality. Three drying methods (convective air (52 °C), freeze drying (-20 to 20 °C at 20 Pa), and microwave-vacuum drying (-40 to 40 °C at 10 Pa) are explored on the seaweed species: bladder wrack and sea lettuce. Overall, the findings are not consistent for the two species. Microwave-vacuum drying does not reduce the bioactive compounds comparably to freeze drying in any of the species. However, as a result of convection drying, only free glutamic acid decreases for bladder wrack. For sea lettuce more valuable compounds are compromised because of convection drying (free aspartic acid, free glutamic acid, the pigment lutein, and polyunsaturated fatty acids). This means that microwave-vacuum drying leads to similar chemical quality as freeze drying. For both species, the three drying methods result in products which vary in the physico-chemical and sensory qualities analyzed (color, water activity, water absorption, and water holding capacity, appearance, odor, flavor, and texture).

Nonetheless, microwave-vacuum drying gives a product closer to freeze drying, compared to the product dried by convective air. This means that microwave-vacuum drying has the potential to replace freeze drying, if a high quality product is required, since it is faster.

The pH, water activity, and salt concentration of fresh sugar kelp promote the growth of microorganisms, which can cause food spoilage. Thus, shelf-life extension of sugar kelp is important to reach a safe and shelf stable product. Washing and blanching sugar kelp in either potable tap water or seawater are investigated to understand their influence on shelf life. The shelf life for refrigerated (2-3 °C) sugar kelp (untreated, washed (4-16 °C for 5 minutes), or blanched (76-80 °C for 2 minutes)) is 7-9 days with Pseudomonas spp. as the dominant spoilage bacteria, meaning it is not treatment dependent. To predict spoilage of sugar kelp it is recommended to control the aerobic viable count (AVC) on marine agar and kept below 7 log (CFU g^-1 wet weight).

This research provides a foundation for best practice and innovation in the European seaweed industry and can help in choosing and developing post-harvest processes with food safety and quality in mind. In the future, the seaweed industry must prioritize developing end-products and establishing what the important food qualities in seaweeds are. This will be essential to be able to optimize the post-harvest processing methods for the future.