Amino acids, fatty acids, and dietary fibre in edible seaweed products

Abstract

The nutritional compositions of 34 edible seaweed products of the Laminaria sp., Undaria pinnatifida, Hizikia fusiforme and Porphyra sp. varieties were analyzed.

This study determined amino acid and fatty acid (FA) distributions and contents of protein, fat, and total fibre of these seaweed varieties. In general, the marine macroalgae varieties tested demonstrated low lipid contents with 2.3 ± 1.6 g/100 g semi-dry sample weight (s.w.) and proved to be a rich source of dietary fibre (46.2 ± 8.0 g/100 g s.w). The pure protein content of seaweed products varied widely (26.6 ± 6.3 g/100 g s.w. in red algae varieties and 12.9 ± 6.2 g/100 g s.w. in brown algae varieties). All essential amino acids were detected in the seaweed species tested and red algae species featured uniquely high concentrations of taurine when compared to brown algae varieties. Interestingly, the FA distribution of seaweed products showed high levels of n-3 FA and demonstrated a nutritionally ideal n-6/n-3 FA ratio. The predominante FA in various seaweed products was eicosapentaenoic acid (C20:5, n-3) which was at concentrations as high as 50% of total FA content.

Introduction

Commercially available varieties of marine macroalgae are commonly referred to as “seaweeds”. Macroalgae can be classified as red algae (Rhodophyta), brown algae (Phaeophyta) or green algae (Chlorophyta), depending on their nutrient and chemical composition. Red and brown algae are mainly used as human food sources.

The protein content of seaweed varieties varies greatly and demonstrates a dependence on such factors as season and environmental growth conditions. For example, the protein content of brown algae species, e.g., Laminaria japonica, Hizikia fusiforme or Undaria pinnatifida, is relatively low with 7–16 g/100 g dry weight (d.w.) (Jurković et al., 1995, Kolb et al., 1999, Rupérez and Saura-Calixto, 2001). In contrast, red algae, e.g., Palmaria palmata (Dulse) and Porphyra tenera contain 21–47 g protein/100 g d.w. (Fleurence, 1999, Rupérez and Saura-Calixto, 2001).

The protein in algae contains all essential amino acids (EAA) and all EAA are available throughout the year although seasonal variations in their concentrations are known to occur (Galland-Irmouli et al., 1999). For example, the proportion of EAA is 45–49% in Hizikia sp. and Eisenia bicyclis (Arame). In both these brown algae varieties, Trp is the first limiting EAA, followed by Lys (Kolb et al., 1999). The EAA contents of some species (e.g., Porphyra sp.) can be compared with those of soy and egg protein (Fleurence, 1999, Galland-Irmouli et al., 1999). In addition, high concentrations of Arg, Asp and Glu are found in many seaweed species (Fleurence, 1999).

The fat content of marine macroalgae accounts for 1–6 g/100 g d.w. (Fleurence et al., 1994, Jurković et al., 1995, Herbreteau et al., 1997). In some brown algae varieties, such as Hizikia sp. and Arame, only 0.7–0.9 g/100 g d.w. of fat content were found (Kolb et al., 1999).

Red algae (e.g., Porphyra sp.) have high concentrations of eicosapentaenoic acid (C20:5, n-3, EPA), with 48.0–51.0% of total FAME, and marginal concentrations of arachidonic acid (C20:4, n-6), with 2.1–10.9% of total FAME and, linoleic acid (C18:2, n-6, LA), with 1.3–2.5% of total FAME (Fleurence et al., 1994, Takagi et al., 1985). In contrast, brown algae (e.g., Laminaria sp., Undaria sp., Hizikia sp.) have high concentrations of oleic acid (C18:1, n-9) with 4.1–20.9% of total FAME, LA with 4.0–7.3% of total FAME as well as α-linolenic acid (C18:3, n-3, ALA) with 3.6–13.8% of total FAME but low concentrations of EPA with 5.9–13.6% of total FAME (Fleurence et al., 1994, Takagi et al., 1985). Interestingly, in Porphyra sp., Laminaria sp., and Undaria sp., the concentrations of docosahexaenoic acid (C22:6, n-3, DHA) and docosapentaenoic acid (C22:5, n-3) were below the detection limit (less than 0.1% of total FAME) (Fleurence et al., 1994, Takagi et al., 1985).

The types and abundance of carbohydrates vary strongly between algae species. Typical carbohydrates in red algae varieties consist of floridean starch (α-1.4-binding glucan), cellulose, xylan, and mannan. The water-soluble fibre fraction is formed by sulfur-containing galactans, e.g., agar and carrageen (Jiménez-Escrig and Sánchez-Muniz, 2000, Van den Hoek et al., 1993). The typical carbohydrates in brown algae varieties consist of fucoidan, laminaran (β-1.3-glucan), cellulose, alginates, and mannitol. Brown algae fibres are mainly cellulose and insoluble alginates. Alginates are Ca, Mg, or Na salts of alginic acid (1.4-linked polymer of β-d-mannuronic acid and α-l-guluronic acid). The amorphous, slimy fraction of brown algae fibres consists mainly of water-soluble alginates and/or fucoidan. Main reserve polysaccharides of Phaeophyta are laminaran (β-1.3-glucan) and mannitol (Kolb et al., 1999, Van den Hoek et al., 1993). The typical algae carbohydrates are not digestible by the human gastrointestinal tract and, therefore, they are dietary fibres. The content of total dietary fibre ranges from 33–50 g/100 g d.w. (Jiménez-Escrig and Cambrodon, 1999, Lahaye, 1991, Rupérez and Saura-Calixto, 2001). Accordingly, the fibre content of seaweed varieties is higher than those found in most fruits and vegetables. The human consumption of algal fibre has been proven to be health-promoting and it benefits are well documented in the scientific literature. The consumption of this dietary fibre has been related to the following health promoting effects: (1) its consumption promotes the growth and protection of the beneficial intestinal flora (Fujii et al., 1992, Goni et al., 2001, Kuda et al., 1997, Kuda et al., 1998a, Kuda et al., 1998b), (2) its consumption, in combination with high glycemic load foods, reduces the overall glycemic response, seaweed fibre acts as a hypoglycaemic (Goni, Valdivieso, & Garcia-Alonso, 2000), (3) its consumption greatly increases stool volume (Jiménez-Escrig & Sánchez-Muniz, 2000) and (4) its consumption reduces the risk of colon cancer (Guidel-Urbano & Goni, 2002).

In addition, seaweed varieties are rich sources of vitamin C, vitamin B-complex, e.g., folic acid and B12, and vitamin A precursors, such as β-carotene (McDermid and Stuercke, 2003, Takenaka et al., 2001, Watanabe et al., 1999, Watanabe et al., 2002, Yamada et al., 1999, Yon and Hyun, 2003).

Because seaweed species are rich in beneficial nutrients, in countries such as China, Japan, and Korea, they have been commonly utilised in human alimentation (since ancient times) (Lahaye, 1991). For example, Japanese people consume more than 1.6 kg algae (d.w.) per year per capita (Fleurence, 1999). In addition to their importance as traditional Asian foods, seaweed species are utilised industrially as a source of hydrocolloids, such as agar, carrageen, and alginate (Jiménez-Escrig & Sánchez-Muniz, 2000).

Over the past few decades, the consumption of seaweed products has increased in European countries. Currently, approximately 15–20 edible algae strains are being commonly marketed for consumption in Europe. These seaweed varieties differ greatly in their quality, colour, consistency, and nutrient content. Therefore, this investigation evaluates and compares the nutrient and chemical contents of 34 commercially available seaweed products which were locally purchased in German food stores and speciality shops.