Digestibility of seaweed protein from Ulva sp. and Saccharina latissima in rats

Seaweed is gaining attention as a sustainable source of protein. However, the nutritional protein quality needs further investigation to explore the potential of seaweed as a protein source. This study evaluates in vivo digestibility of six different seaweed protein products, four from Ulva sp. and two from Saccharina latissima. Ulva sp. was either preserved in a preservation liquid and spin flash-dried, freeze-dried without any pre-treatment or protein was extracted from the crude biomass with a screw press and isoelectrically precipitated (pH 2.5) with or without sulfite addition during extraction. Saccharina latissima was either freeze-dried or fermented with lactic acid bacteria prior to freeze-drying. The digestibility of the seaweed products was tested in a rat trial, where seaweed protein products constituted 50% of the nitrogen (N) in diets, the other 50% of N coming from whey protein concentrate. The protein concentration and proportion of essential amino acids (EAA) were increased upon protein extraction from Ulva sp. and the ash content was reduced. This led to an increased N digestibility of 55.5 ± 2.7% for the protein extract without sulfite, being 10–13%-points higher than for the other Ulva sp. products. The sulfite addition seemed to counteract the effect of extraction regarding the N digestibility. Fermentation of S. latissima increased the total amino acid content along with increased EAA proportion and lowered the ash content. However, the fermentation did not improve the organic matter, ash or N digestibility.