Volatile Components in Seaweeds

Seaweeds are a group of photosynthetic plant-like organisms that generally thrive attached to rock or other hard substrata in coastal areas [1-5]. They have high amounts of macronutrients (proteins, carbohydrates and lipids) and thus characterize for important nutritional value [6,7]. Seaweeds are also sources of bioactive and antioxidant compounds, besides their extracts can have antihypertensive and anti-inflammatory properties [8-11]. Another important characteristic of seaweeds is their volatile compounds profile, which plays an important role in marine ecosystems, including in the chemical communication. Volatile compounds in seaweeds act as sexual pheromones, chemical defenses against herbivores and inhibitors of bacteria and fungi [12-14]. Some volatile compounds present in seaweeds such as ammonia, methylamine, and trimethylamine are generated by microorganisms or by chemical reactions. Processes like roasting are responsible for the formation of volatile compounds, especially dimethyl sulfide which stimulates the aroma of edible green and red seaweeds, which are very much appreciated by Japanese. Other volatile compounds could act like a protection for the seaweeds, for example, the diterpene holohedral, which defends seaweeds against herbivorous fishes and invertebrates [15].

The volatile compounds of some edible seaweeds have been analyzed by GC-MS systems and it showed that volatile compounds depend on the species, its geographical origin, processing method and environmental conditions [16,17]. In relation to the species, some studies revealed that green seaweed produces mainly cymopole, a product of condensation of reaction between bromophenols and monoterpenes. Brown seaweeds produce sulfur compounds, C11-hydrocarbons and diterpenes while red seaweeds are known to produce halogenated compounds containing bromine and iodine, which contribute to a sweet note in the aroma of this specie of seaweeds. Organic compounds present in seaweeds also function as therapeutic drugs, flavoring agents, odorants in perfume industry and indicator of water pollutants. For these reasons, many studies were performed with the aim of determining the volatiles from seaweeds using different analytical methods as shown in the Table 1 which cites the volatile compounds identified in brown, green and red seaweeds [18,19]. Some compounds have great impact on the aroma of seaweeds and are considered “character-impact compounds”, and these include iodoethane, iodopentane and isoprene (2-methyl-1-3-butadiene) present especially in red seaweeds [20]. Volatile halogenated compounds such as dibromoethane, bromoform, dibromochloromethane, bromodichloromethane and chloroiodomethane contribute to the characteristic aroma of the seaweeds with notes like marine, crustacean and green. Moreover, these compounds play an important role in chemical defense against predators [21,22].

The profile of volatile compounds can be very diverse between the various types of seaweeds [23]. This diversity is occasioned due to different species that can be capable of producing a variety of volatile compounds. The detailed identification of such compounds is very important to elucidate their aroma properties of the final product [24,25]. In the green seaweeds, a higher number of compounds have been identified in comparison to brown and red seaweeds (Table 1). The diversity of volatiles compounds presents in seaweeds produce different odors which involve main notes defined like “marine” and “seafood”. Showed that other odor notes like “fatty”, “fish”, “licorice”, “spices”, “green”, “honey” were detected in species of green, red and brown seaweeds. In these species, carboxylic acids were related to odor notes such as of licorice, honey and spices, while esters and Sulphur compounds were related to green odor. Notes like fish odor, marine odor, fatty odor, seafood odor are related to presence of amines and pyridines. Important volatiles responsible for the odor of green marine algae [26]. In a recent study it is reported that compounds when present in higher concentrations contribute to fruity notes in aroma characteristics of some seaweeds [27]. Another class of compounds commonly found in the aroma composition of seaweeds are the ketones. Ketones, such as β-ionone and 6-methyl-5-hepten-2-one are formed by the oxidative cleavage of carotenoids such as lycopene and phytoene [28]. These compounds are formed from the pigments contained in the seaweeds and are metabolized while capturing light and scavenging radicals [29]. β-ionone is a potent odorant in seafood and has been found in all three types of seaweeds. Other ketone frequently found in aroma of seaweeds which have an odor impact is 6-methyl-5-hepten-2-one. This ketone is also found in other foods, such as tomato as a metabolite of lycopene which results in compounds having more pleasant odors [30-32].

Table 1:

A large quantity of compounds present in the aroma of seaweeds is due to the significant contribution of lipid-derived volatiles, such as aldehydes and alcohols, which relate to the non-appreciable aroma characteristics of microalgae. It is for this reason that prevention and control of lipid oxidation is recommended to ensure good flavor quality of the product [33]. In some species of seaweed it is possible to find compounds which belong to furans. This class consists of a family of important chemicals in the formation of crab flavor and are sourced from the oxidative degradation of linolenic acid [34]. The aroma diversity of seaweeds is a result of the variety and concentration range of the aroma compounds. Since microalgae could possess important odor characteristics, it can be used in food and feed formulations. Some of these aroma characteristics may be desirable in some applications but unwanted in others. For example, while sulfur-containing compounds are highly appreciated in certain foods such as oysters, roasted coffee, fried meat, toasted bread or cheddar cheese, these can lead to very irritating offflavors, for example, in milk [35,36]. This review reveals that the odor characteristics of seaweeds can be very distinct, and it varies between the three types: red, brown and green. Ulva sp., a specie of green seaweed, received notes like seaweed odor, marine odor and seafood odor. However, Laminaria digitate, brown seaweed, showed stronger seaweed aroma and milder honey aroma [37] and the red seaweeds received the highest scores for the seaweed odor attribute.

The volatile profiles of brown, green and red microalgae have been presented in this review on seaweeds. Volatile compounds in seaweeds also act as sexual pheromones and inhibitors of bacteria and fungi. Moreover, the specific aroma notes of these seaweeds as reported in publications are also detailed. Among the three types of seaweeds, the green has been studied much more and relatively a large number of volatile compounds have been identified in this seaweed as compared to brown and red types. The characteristic marine odor is much more appreciated in food preparations and is very prominent in green seaweed.

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