Palmaria palmata (dulse) is an edible red alga constituting a potential protein source in human diet. However, previous studies showed that the digestibility of dulse proteins is bad because of the cell-wall encapsulating cytoplasmic proteins and the presence of fibers. The water-soluble xylan, present in high proportions in dulse, could be involved to explain the weak digestibility of proteins. To limit the influence of fibers and to improve the nutritional quality of these proteins, we have treated dulse by physical processes or by fermentation by moulds. After a 30 min predigestion by pepsin followed by a 6 h digestion into a cell dialysis containing porcine pancreatin, the corrected in vitro digestibility of crude dulse was very low (about 1.5% after correction by digestibility blank). The in vitro protein digestibility was estimated to 58% of that of casein for dulse samples obtained after washing in demineralized water and grinding in liquid nitrogen. The in vitro protein digestibility of fermented samples was 45%-65% of that of casein. After physical treatment, the digestibility improvement was related to the elimination of soluble molecules such as xylan and mineral salts. The improvement observed after fermentations seemed due to the degradation of insoluble fibers.

Palmaria palmata and Gracilaria verrucosa are edible red seaweeds and potential protein sources for human or animal nutrition, so studies were conducted on their in vitro protein digestibility. After 30 min predigestion by pepsin followed by 6 h digestion into a cell dialysis containing porcine pancreatin, the in vitro protein digestibility of P. palmata and G. verrucosa, expressed in regard to casein digestibility, was 4.9% and 42.1%, respectively. The level of protein digestibility seems to be related to the amount of soluble fibre, which was 45.3% and 30.5%, respectively.

Plant hydrocolloids used in the food industry to improve texture and stability of food, such as dairy products, can reduce protein digestibility and, consequently, modify the bioavailability of amino acids. We studied the in vitro hydrolysis at 37 degrees C of beta-lactoglobulin (beta-lg) in mixed dispersions containing either gum arabic or low-methylated pectin or xylan at levels of 0, 1, 10, 20, 30, and 50% weight. Proteolysis used either pepsin alone by progressive reduction of pH during proteolysis or pepsin followed by trypsin and chymotrypsin in two different dialysis bags with a molecular weight (MW) cutoff of 1000 or 8000 Da. Results showed that beta-lg was almost resistant to pepsin digestion and that the three plant hydrocolloids inhibited significantly beta-lg digestibility as determined using dialysis bag with a 1000-Da MW cutoff. Among the three polysaccharides used, xylan showed a digestibility decrease greater than that obtained with gum arabic and low-methylated pectin. On the other hand, no significant effect of polysaccharides on the in vitro beta-lg digestibility was detected using the dialysis bag with an 8000 Da MW cutoff. This mainly suggests that peptides with MW in the range 1000 to 8000 Da may interact with polysaccharides more than peptides and proteins with a greater molecular weight to decrease the protein digestibility, and that the nature of the polysaccharides plays a role in the interaction.