Undaria pinnatifida was collected from four farms near Nelson, New Zealand on a monthly basis from June to September in 2011. The total nitrogen content and amino acid profile of the samples were determined. Seasonal and geographical variations in the total nitrogen and amino acid composition of the samples were evaluated in the blade and sporophyll.

The total nitrogen content determined by the Kjeldahl method ranged between 12.99 mg/g to 30.00 mg/g of dry weight with an average content of 21.02 mg/g. Total nitrogen content was significantly higher (p < 0.05) in June and July for both exposed site in Port Underwood. Higher value of the total nitrogen was observed in exposed farm 327 in Port Underwood and sheltered farm 233 in Pelorus Sound in the blade, and farms 327 and sheltered farm 122 in Pelorus Sound in the sporophyll at the harvest month September. The content in blade was significantly higher than the one in sporophyll in farm 327 between four months.

High performance liquid chromatography was used to determine the composition amino acids in Undaria pinnatifida. A total of 15 amino acids acids including 7 essential amino acids, 2 semi-essential amino acids and 6 non-essential amino acids were detected. Similar to total nitrogen, seasonal, geographical and plant parts affected amino acid composition. The proportion of essential amino acids to total amino acid was close to the recommended WHO/FAO standard except for sulphur containing amino acids. Glutamic acid, aspartic acid and alanine that can contribute to flavour were present at high concentrations in Undaria.

The average nitrogen-to-protein conversion factor based on nitrogen determination by Kjeldahl method and amino acid analyses was calculated to be 4.44. Hence the traditional conversion factor of 6.25 may overestimate the true protein content of Undaria in New Zealand. Further protein characterization of the Undaria species by SDS-PAGE revealed seven protein bands having molecular weight of 77 kDa, 55 kDa, 37 kDa, 26 kDa, 20 kDa, 17 kDa and 14 kDa, which has not been reported in other algae species.