The general components of Akamoku (Sargassum horneri) are high in carbohydrates, ash, and dietary fiber. Compared to wakame, a seaweed commonly consumed by Japanese people, Akamoku contains approximately 1.2 times more carbohydrates, 1.3 times more ash, and 1.4 times more dietary fiber. Analysis of the mineral content revealed that Akamoku had higher levels of potassium, calcium, magnesium, iron, zinc, and copper than wakame. Akamoku also contained a high amount of polyphenols, suggesting potential antioxidant properties. Akamoku was separated into male and female seaweeds, and the contents of water-soluble sugar, water-soluble protein, and polyphenols in extracts from untreated and chopped samples were examined. The results showed that finely chopping enhanced the elution of all components, with the female strain yielding higher amounts than the male strain. The molecular weight distribution of water-soluble proteins from male and female seaweeds was assessed by electrophoresis, revealing that proteins with higher molecular weights were present in the female strain compared to the male strain. It was also observed that male seaweeds had a darker color tone, whereas female seaweeds appeared brighter.

Keywords: Akamoku, Sargassum horneri, general ingredients of seaweed, characteristics of male and female seaweeds, Japan

Sargassum horneri (referred to as Akamoku) is a brown algae belonging to the Sargassaceae family that grows to about 10 m and turns reddish-brown as it matures. Akamoku is widely distributed from Hokkaido to Kyushu, excluding the northernmost part of Hokkaido, and has been consumed in the Tohoku region of Japan since ancient times. It is known by various regional names across Japan, such as "Gibasa" (Akita, Ishikawa)," "Ginbasou" (Yamagata)," "Nagamo" (Niigata)," “Nagaramo "(Toyama Prefecture)", and "Hanamatsumo" (Noto Peninsula)".^1,2 In its raw state, it is brownish, turns bright green when boiled, and has a sticky texture. Fucoidan, the viscous component of Akamoku, is known for its antitumor and immune-boosting properties^3,4 and is considered a functional ingredient. Recently, Akamoku has been commercialized for its appealing texture and health benefits, with products such as heated retort packs and soups now available. Some seaweeds in the Sargassaceae family have distinct male and female types, and Akamoku also exhibits this trait. In this study, we examined the general components, mineral content, and polyphenol levels of Akamoku, as well as the food composition and cooking characteristics of male and female Akamoku.

Sample

The Akamoku sample used in this study was cultivated on the Kamakura coast and harvested in February 2021.

Measurement of general components of Akamoku

The general components were analyzed following methods consistent with the Standard Tables of Food Composition in Japan. Moisture content was measured at 105°C using the conventional drying method,⁵ where the sample was dried to a constant weight and calculated. Protein content was determined by the Kjeldahl digestion method,⁶ fat content by the Soxhlet extraction method,⁷ and ash content by the direct ashing method.⁸ Carbohydrate content was calculated by the subtraction method.⁹

Measurement of dietary fiber content

Dietary fiber content was measured using a modified Prosky method.¹⁰ One gram of crushed Akamoku was sequentially treated with heat-resistant α-amylase, protease, and amyloglucosidase following the Prosky method.¹¹ The filtrate and residue were then separated, precipitated, washed with ethanol and acetone, dried, and weighed.

Measurement of mineral content

Mineral content was analyzed by atomic absorption spectrometry.¹² The samples were ashed and dissolved in 0.1M hydrochloric acid. A Shimadzu AA-6300 atomic absorption spectrophotometer was used to measure sodium (589.0 nm), potassium (766.5nm), calcium (422.7nm), magnesium (285.2nm), iron (248.3nm), zinc (213.9nm), and copper (324.8nm).

Measurement of water-soluble components in male and female seaweeds

Preparation of extracts of water-soluble components: Akamoku was prepared in three forms: uncut, cut to 1cm, and cut to 0.5cm. For each preparation, 50g of Akamoku was mixed with 200ml of distilled water at 95°C, stirred with a glass rod for 30s, filtered under suction, and diluted 100 times to prepare the extract samples.

Measurement of water-soluble sugar: The amount of water-soluble sugar was measured using the phenol-sulfuric acid method.¹³ To 1.0mL of the extract, 1.0mL of 5% phenol solution was added and mixed, followed by the rapid dropwise addition of 5.0mL of concentrated sulfuric acid directly onto the liquid surface, with subsequent mixing. After standing for 10min, the mixture was placed in a 30°C water bath for an additional 10min, and absorbance was then measured at 490nm. A calibration curve was generated using glucose prepared in the same manner.

Measurement of water-soluble protein: Measurements were made using the Lowry method.¹⁴ First, 0.9mL of Reagent A (1g potassium sodium tartrate and 50 g sodium hydrogen carbonate dissolved in 250mL of 1M sodium hydroxide solution) was added to 1mL of the extract and heated at 50°C for 10min. Then, 0.1mL of Reagent B (2 g potassium sodium tartrate and 1g sodium sulfate dissolved in 10mL of 1M sodium hydroxide solution) was added, followed by heating at 50°C for another 10min. Next, 3mL of Reagent C (12-fold dilution of phenol reagent) was added, and the mixture was again heated at 50°C for 10min. Absorbance was measured at 650nm. Protein concentration was calculated from a calibration curve prepared using bovine serum albumin.

Measurement of polyphenol content: Polyphenol content was measured using the Folin–Denis method.¹⁵ Specifically, 200μL of the extract was added to 3.2mL of water, followed by the addition of 200μL of Folin–Denis reagent. After stirring, 400μL of saturated sodium carbonate solution was added. The mixture was left to stand for 10 min, and absorbance was measured at 700nm. The polyphenol content was calculated from a calibration curve prepared using gallic acid.

Color measurement: Color was measured by extracting pigments from 10g of Akamoku using 90mL of 95% ethanol, followed by analysis with a colorimeter. A Konica Minolta spectrophotometer CM-600 d was used for the measurements.

Molecular weight distribution of proteins by electrophoresis: Protein molecular weight distribution was analyzed using non-denaturing polyacrylamide gel electrophoresis. The analysis was performed following the standard method¹⁶ using ATTO’s Pagel Compact Gel (10% concentration) and a mini slab electrophoresis apparatus (ATTO AE-730).

General components and dietary fiber content of Akamoku

The general components of Akamoku were measured (Table 1), revealing a moisture content of approximately 87%, which is comparable to that of common seaweeds. Compared to wakame (according to the Japanese Food Composition Tables 2020, 8th Edition), Akamoku had higher carbohydrate and ash content. Specifically, Akamoku from Kamakura contained about 1.2 times more carbohydrate and 1.3 times more ash than wakame. The elevated carbohydrate content was attributed to its high dietary fiber level. The dietary fiber content in Akamoku was 5.0%, approximately 1.4 times higher than that of wakame. These findings indicate that Akamoku is a seaweed rich in ash and dietary fiber, making it a potentially functional food. To compare with Kamakura samples, Akamoku collected from Sendai and Sadowas also analyzed. Both showed higher levels of carbohydrate, ash, and dietary fiber than wakame.

Mineral content of Akamoku

Given the confirmed high ash content of Akamoku, the mineral content was also measured. The results (Table 2) showed that, in comparison to wakame, Akamoku contained 1.6 times more potassium, 1.9 times more calcium, 1.7 times more magnesium, 14.7 times more iron, 3 times more zinc, and 2.5 times more copper (Table 2). These findings confirm that Akamoku is rich in various minerals. Since calcium, magnesium, and iron are nutrients often deficient in the Japanese diet, Akamoku is considered a valuable food source for mineral supplementation.

Polyphenol content of Akamoku

The polyphenol content of Akamoku was measured (Table 3) and found to be 95.2mg/100g. In comparison, the polyphenol content of Tsuruarame is reported to be 10mg per gram of dry mass,¹⁷ indicating that Akamoku contains higher levels of polyphenols than some other seaweeds. These results suggest that Akamoku may possess antioxidant properties.

Comparison of the characteristics and components of male and female Akamoku seaweeds

Morphological characteristics of male and female seaweeds: To examine the differences in components between male and female Akamoku, the samples were separated by sex. The male seaweeds were characterized by long, pointed receptacles, while the receptacles of the female strain were shorter and thicker, featuring small round surface protrusions—demonstrating distinct morphological differences between the strains (Figure 1). It is also known that the texture differs between the sexes, with the female strain exhibiting greater viscosity than the male strain.

Figure 1 Morphological characteristics of male and female seaweeds.

Water-soluble sugar content of male and female seaweeds: The water-soluble sugar content of untreated male and female seaweeds, as well as samples cut into 1 cm and 0.5cm pieces, was examined (Figure 2). The sugar content in female seaweeds was approximately 4 to 5 times higher than that in male seaweeds. In male seaweeds, there was little difference in sugar content between untreated and finely chopped (0.5cm) samples. However, in female seaweeds, finely chopping to 0.5cm significantly increased sugar content—approximately 1.7 times higher than in untreated samples. These results confirm that finely chopping female seaweeds allows for greater extraction of water-soluble sugar.

Figure 2 Water-soluble sugar content of male and female seaweeds.

Water-soluble protein amount in male and female seaweeds: Water-soluble protein content was also measured in untreated male and female seaweeds and samples chopped to 1 cm and 0.5 cm (Figure 3). Similar to the sugar content, female seaweeds contained about 1.5 to 5 times more water-soluble protein than male seaweeds. Unlike the trend observed in sugar content, chopping increased protein extraction in both male and female seaweeds. Specifically, more protein was extracted from samples chopped to 0.5cm than from untreated samples, with female seaweeds showing particularly high extraction levels when finely chopped.

Figure 3 Water-soluble protein amount in male and female seaweeds.

Polyphenol content of male and female seaweeds: The polyphenol content of untreated male and female seaweeds, as well as samples cut into 1 cm and 0.5cm pieces, was examined (Figure 4). Polyphenol content was higher in both male and female seaweeds when cut into smaller pieces compared to untreated samples. Additionally, female seaweeds contained approximately 1.5 to 5 times more polyphenols than male seaweeds. From these results, it was expected that the antioxidant function would be enhanced by cutting into finer pieces.

Figure 4 Polyphenol content of male and female seaweeds.

Color of male and female seaweeds: Akamoku is brownish in its raw state, but when soaked in ethanol, both male and female seaweeds turn bright green. As shown in Table 4, the lightness (L*) value was higher in female seaweeds than in male seaweeds, indicating that female seaweeds were brighter, while male seaweeds appeared darker. Furthermore, male seaweeds showed stronger red and blue hues, whereas female seaweeds exhibited more yellow and green tones. These color differences are attributed to the presence of chlorophyll and fucoxanthin,¹⁸ compounds known to have potential antitumor effects. Although not detailed in this study, Akamoku from Kamakura was observed to be darker in color than that from Sado, suggesting higher polyphenol content and, consequently, greater antioxidant potential.

Molecular weight distribution of water-soluble proteins in male and female seaweeds by electrophoresis: The molecular weight distribution of extracts obtained from male and female seaweeds chopped into 0.5 cm pieces was analyzed using electrophoresis (Figure 5). Proteins with higher molecular weights were detected in female seaweeds compared to male seaweeds. Specifically, the main molecular weight range was 25,000 to 14,000 for male seaweeds and 66,000 to 14,000 for female seaweeds. Female seaweeds were also found to contain more mucilage, along with greater amounts of water-soluble sugar and protein, suggesting the presence of glycoproteins. This component warrants further investigation in future studies.

Figure 5 Molecular weight distribution of water-soluble proteins in male and female seaweeds by electrophoresis.

The general composition analysis of Akamoku revealed high levels of carbohydrates, ash, and dietary fiber, with 1.2 times more carbohydrates, 1.3 times more ash, and 1.4 times more dietary fiber than wakame. Mineral analysis showed that Akamoku contained higher levels of potassium, sodium, calcium, magnesium, iron, zinc, and copper compared to wakame. The polyphenol content of Akamoku was 95.2mg/100g, which is estimated to be higher than in other seaweeds. Male and female Akamoku seaweeds were separated, and the contents of water-soluble sugar, water-soluble protein, and polyphenols in extracts from untreated and chopped samples were examined. It was confirmed that finely chopped female seaweeds released more of each component than male seaweeds. These results indicate that nutrient elution varies depending on the processing method, which may be beneficial in food manufacturing.

Additionally, electrophoresis revealed that female seaweeds contained proteins with higher molecular weights than those in male seaweeds. The difference in texture—female seaweeds being more viscous—appears to be related to both the quantity of soluble components and the molecular weight of the proteins.

The composition and other properties of Akamoku are considered subjects for future investigation. In terms of color, male seaweeds were darker, while female seaweeds appeared brighter and more luminous. The results of this study revealed differences in the amounts of extracted components between male and female seaweeds. Therefore, we plan to further modify the elution conditions and examine the physical properties to enhance Akamoku’s potential as a food ingredient.

Currently, the most common method of preparing Akamoku is boiling and consuming it as is. However, we also plan to explore additional cooking and processing methods. By leveraging its natural stickiness, we aim to develop products that are easy to consume across all age groups—from infants to the elderly—ultimately creating a more nutritious and versatile food product from Akamoku.

None.

The authors declare that there are no conflicts of interest.

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