To achieve this, the red seaweed Chondrus crispus was dehydrated by freeze-drying, conventional oven drying and microwave gravity hydrodiffusion (MHG). The environmental impact was determined by considering drying kinetics, energy consumption and treatment efficiency. At the same time, the various physical, chemical, and nutritional characteristics of the seaweed were studied, including changes in colour and microstructure, nutritional value, and chemical properties, as well as antioxidant and antimicrobial activities.
Regarding the technical performance of this process, it has been shown that MHG enables much faster dehydration than other technologies, with significantly lower energy consumption, which gives it the lowest environmental impact (72g of CO2 released into the atmosphere, compared with 180 to 200g for other technologies).
Changes in seaweed colour after dehydration show a greater difference from fresh seaweed using MHG technology than oven drying, but less than freeze drying. In terms of microstructure, MHG is more degrading than freeze-drying but equivalent to conventional oven drying.
The chemical composition and nutritional value of seaweed dehydrated by MHG is relatively close to those obtained by the other technologies, except for the higher fat content in freeze-dried seaweed. The calcium content of the freeze-dried algae was also twice that of the other two. In terms of heavy metals, algae dehydrated in conventional ovens appear to be the least contaminated, followed by those obtained by MHG and then freeze-drying. However, all these differences remain insignificant.
Antioxidant and antimicrobial activities were assessed on algae dehydrated by the 3 technologies. Seaweed dehydrated in a conventional oven showed an antioxidant activity 1.2 times higher than those obtained by freeze-drying and MHG. In terms of antimicrobial activity, dehydrated seaweed showed strong bacterial and fungal inhibitory potential, regardless of the technology, although the results varied depending on the bacterial strains tested. The algae dehydrated by MHG was the only one to show antibacterial activity in the presence of S. typhimurium bacteria.
Another advantage of MHG technology over the other two technologies is that it allows the recovery of dehydration water, which is rich in antioxidant compounds.
In conclusion, the new microwave gravity hydrodiffusion (MHG) method applied to dehydrate Chondrus crispus seaweed offers technological advantages from an industrial point of view compared with conventional freeze-drying and oven-drying procedures. The MHG technique saved drying time (34 min), energy consumption (324 kJ) and reduced environmental impact (72 g CO2); without compromising the colour and microstructural characteristics of the MHG compared with other traditional drying methods.
The interest was also confirmed by the nutritional and chemical characteristics of the dried algae obtained, as well as their antioxidant and antimicrobial activities. Compared with other technologies, the MHG process offers additional value, as it recovers some of the bioactive compounds in the red algae present in the aqueous phase collected during drying.
The results obtained in this study could be extended to other red algae and show that MHG is an interesting alternative for dehydrating algae. Future research should focus on the application of MHG on an industrial scale in order to extend its use as a new dehydration technology and for the potential applications of the bioactive liquid extracts obtained.
Référence : López-Hortas, L.; Caleja, C.; Pinela, J.; Petrović, J.; Soković, M.; Ferreira, I. C. F. R.; Torres, M. D.; Domínguez, H.; Pereira, E.; Barros, L. Comparative Evaluation of Physicochemical Profile and Bioactive Properties of Red Edible Seaweed Chondrus Crispus Subjected to Different Drying Methods. Food Chem.2022, 383, 132450.