INTRODUCTIONModern science associates many diseases withthe destructive effect of oxidants, or free radicals.Peroxidation is the most grievous consequence offree radicals entering the cell. Lipid oxidation inbiological membranes is a highly destructive processthat triggers liver damage, carcinogenesis, and aging.Lipid peroxidation is a major cause of the defective cellproliferation [1, 2]. Oxidized lipid compounds reactwith proteins, thus lowering enzymatic activity, causingcleavage of polypeptide chains, initiating DNA damage,accelerating aging, and aggravating such diseases ascancer and atherosclerosis. The aldehyde groups ofthese compounds form intermolecular crosslinks, whichviolate the structure of macromolecules and disorganizetheir functioning. By oxidizing lipids, free radicalscause glaucoma, cataracts, cirrhosis, ischemia, etc. [3].The negative effects of oxidative stress can be mitigatedby antioxidants, which neutralize potentially harmfulreactive free radicals in the body cells before theycause lipid and protein oxidation. As a result, they canreduce potential mutations and prevent cancer and heartdisease [4, 5].Plants contain effective antioxidant substances.Polyphenols or bioflavonoids of plant origin, when theywork in tandem, are extremely efficient against freeradicals. Therefore, any study of vegetables, fruits, andspices as an alternative source of antioxidants will behighly relevant, considering that synthetic antioxidantsare potentially toxic and carcinogenic. The past twodecades have seen a lot of studies in phytochemicalsfrom various types of terrestrial plant materials [6–8].In this respect, seaweed attracts much lessscientific attention than higher plants. Seaweed makesup a significant part of the marine flora. It containscompounds of various structures and bioactivities,which provide antioxidant, antibacterial, antiinflammatory,and anticarcinogenic properties [9–11].Although seaweed is rich in polysaccharides andminerals, it is seldom used in food technology. Ascience-based proof of its antioxidant activity couldincrease its value as a food or supplement and expandits market. When exposed to light and oxygen, seaweedproduces free radicals and other strong oxidants.However, the absence of oxidative damage in itsstructural components and its resistance to oxidationduring storage suggest that seaweed has a goodantioxidant system. Seaweed extracts are known to berich in such hydrophilic components as polyphenolsand soluble polysaccharides [9, 12–15]. In particular,brown seaweed is rich in natural antioxidants,e.g. such phenolic compounds as phlorotannins and suchcarotenoids as fucoxanthin and isoprenoids [16].Many seaweed extracts possess strong antioxidantproperties [17–21]. However, there have been nopublications on the antioxidant activity of seaweedfrom the Northern Coast of the Sea of Japan. Moreover,the antioxidant activity of seaweed has been studiedmainly in non-polar extracts and very seldom – inaqueous extracts [22, 23]. Seaweed owes its antioxidantproperties due to its hydrophilic and hydrophobiccompounds. Therefore, its antioxidant activity inhydrothermal extracts needs more research becausehydrothermal treatment is an integral part of processing.This study featured three species of seaweeds thatare part of human and animal diet in the PrimoryeTerritory of Russia’s Far East.Codium flagile from the family of Codiaceae is aspecies of green seaweed found in the Indo-Pacific andthe Atlantic. In Russia, it grows on the Northern Coast ofthe Primorye Territory. Its thallus is prostrate, branched,pillow-shaped or upright. It can be 5–30 cm long. InOceania and Asia, and especially in Korea and Russia’sPrimorye, C. flagile is usually served raw or boiled.C. flagile extracts are widely used in medicalcosmetology as it synthesizes water-retainingsubstances.Gracilaria verrucosa, Gracilariaceae family, isa species of red-purple seaweed found in the lowboreal-tropical Atlantic-Indo-Pacific. In Russia, thisseaweed grows in the Far East, in the Sea of Japan andthe Sea of Okhotsk. It has branched, cylindrical or flat,gristly thallus, which is purple-red or fading to green.G. verrucosa is 20–40 cm long, sometimes reachingup to 100 cm. It grows in the littoral and sublittoralsea zones at a depth of up to 2 m. The species prefersestuarine and sheltered coastal areas and stony, siltysandyand sandy bottom, interspersed with stonesand shell rock. Classified as a potentially commercialspecies, G. verrucosa is a fast-growing seaweed witha biomass of 4.5 kg/m2, which makes it convenient forcultivation. In Southeast Asia, it is used for food andas a tonic substance in folk medicine. G. verrucosa canserve as a raw material for agar and various food andfeed additives. It is known to contain minerals, proteinswith antibiotic properties, B vitamins, phycocolloids,etc. However, agar remains the only commercial productrelated to this raw material.Sargassum miyabei, Sargassaceae (Decne) Kutz.family, is a species of brown seaweed that thrivesin cold waters in the low-boreal-subtropical areasof Asian. Its thallus is branched, rough, and thick,2.0–2.5 m in length, olive green. It grows in the littoraland sublittoral at a depth of 11 m on rocky, stony, andsilty-sandy bottom, near semi-sheltered and open coastalareas, in separate bushes or small clusters, clusteringwith kelp seaweeds and sea grasses. It is a potentiallycommercial perennial species, with the biomass up to15 kg/m2, the mass of one thallus up to 7 kg, andpopulation density of 2–6 specimens per 2 m2. In theRussian Far East, it grows in the Sea of Japan and theSea of Okhotsk. It is used in the food industry andas a raw material for therapeutic and prophylacticmedications, food and feed additives. S. miyabei isknown to contain vitamins A, В1, В6, B12, minerals,polysaccharides with immunostimulating and antitumoractivity, etc. [24].The research objective was a comparative analysisof hydrothermal extracts of the following seaweeds:green C. flagile, brown S. miyabei, and red G. verrucosa,obtained by hot-water or autoclave methods for 30 and60 min. The extracts were compared according to thefollowing parameters: total phenols, phenolic profile, andantioxidant activity, i.e. DHHP radicals, OH• scavengingactivity, and O2•− scavenging activity.STUDY OBJECTS AND METHODSMaterials. All samples of Codium flagile, Gracilariaverrucosa, and Sargassum miyabei were harvested inthe coastal sea waters of the Sea of Japan in the northernPrimorye Territory in May-July 2018 (Table 1).Chemicals and reagents. 1,1-diphenyl-2-picrylhydrazyl (DHHP), ВНТ-2,6-di-tert-butyl-4-methylphenol (ionol), and tannic acid werepurchased from Sigma-Aldrich (USA). The Folin-Ciocalteu phenolic reagent was purchased from Fluka(Switzerland). All the other reagents were of analyticalgrade.Table 1 Place and time of harvestSeaweed Place TimeCodiumflagileOlga Bay of the Sea of Japan,Primorye Territory, RussiaMay 2018GracilariaverrucosaValentina Bay of the Sea of Japan,Primorye Territory, RussiaJune 2018SargassummiyabeiValentina Bay of the Sea of Japan,Primorye Territory, RussiaJuly 2018265Tabakaev A.V. et al. Foods and Raw Materials, 2021, vol. 9, no. 2, pp. 262–270concentrations (0.1–1.0 mg/mL) of extracts in MeOH(3.0 mL) were added to a 40 mM H2O2 solution (3.0 mL).The optical density was measured at 230 nm after10 min of incubation against an empty solution ofphosphate buffer without H2O2.The O2•− scavenging activity was determined by theformula:O2•− scavenging activity (%) = [A0−А1/A0]×100 (4)where A1 is optical density of the extract and A0 is opticaldensity of the control solution.Calculation of ЕС50 (mg/mL) was the next step.Statistical analysis. The data were obtained as meanand standard deviation (SD) and analyzed by one-wayANOVA using SPSS 11.5 for Windows. The differencein mean values was significant at P < 0.05.RESULT AND DISCUSSIONThe hot-water and autoclave hydrothermal extractsobtained from three edible seaweeds from the NorthernCoast of the Sea of Japan demonstrated different drymatter yields. Figure 1 shows the effect of processingtime and method on the extraction yield.A comparative analysis of the quantitative yieldresulted in the following descending order: Gracilariaverrucosa > Codium flagile > Sargassum miyabei.The hot-water extract of the red-purple seaweedG. verrucosa had the highest yield – 15.90%, whichwas 1.17–1.58 times higher than that in the autoclavesamples. The extraction time increased the yield by55–57% for hot-water extracts and by 16.3–47.5% forautoclave extracts. The hot-water G. verrucosa extractand the autoclave C. flagile and S. miyabei extractsexperienced the greatest effect of extraction time.However, a high yield did not equal high antioxidantactivity, which depends on strong antioxidants,e.g. polyphenols.Figure 2 illustrates the content of phenols in theseaweed extracts.A comparative analysis of the total phenols showedthat S. miyabei had the hot-water extract with thehighest phenolic level, about 3–4 times higher thanthe other hot-water extracts. The autoclave extract ofS. miyabei had similar level of total phenols. Theextract of C. flagile had the lowest total phenols amongthe hot-water extracts, while G. verrucosa had thelowest total phenols among the autoclave samples. Thefact that brown seaweeds proved richer in fenols thangreen and red seaweeds confirms the data obtainedby other scientists [31]. However, the methods wereslightly different. The total phenol content in theautoclave C. flagile extract was twice as high as in thehot-water extract, while in the autoclave extract ofS. miyabei it was lower than in the respective hot-waterextracts (Fig. 2).Such hydrophilic polyphenols as phlorotanins,which are bipolar and occur in brown seaweeds, can actas antioxidant components and thus help the seaweedovercome oxidative stress [32, 33].Identification of phenolic compounds. Individualphenolic compounds were tested using HPLC in extractsobtained after 60 min of hydrothermal treatment becausea longer processing time provided a higher level of totalphenols. Table 2 shows the content of the main phenoliccompounds in the extracts.Table 2 demonstrates 10 phenolic compounds inthe composition of edible seaweed extracts from theNorthern Coast of the Sea of Japan. Chlorogenic acidwas the first of the phenolic compounds to elute; it wasnot detected in the hot-water C. flagile extract. Similarly,2,5-dihydroxybenzoic acid was not detected in bothextracts of G. verrucosa and the autoclave extract ofC. flagile. Caffeic, coumaric, ferulic, salicylic, and* Significant difference Codium flagilevs. Sargassum miyabei; Р < 0.05; n = 3** Significant difference Codium flagilevs. Gracilaria verrucosa; Р < 0.05; n = 3*** Significant difference Sargassum miyabeivs. Gracilaria verrucosa; Р < 0.05; n = 3Figure 1 Yield of seaweed extracts from the Northern Coastof the Sea of Japan* Significant difference Codium flagile vs. Sargassum miyabei; Р < 0.05; n = 3** Significant difference Codium flagile vs. Gracilaria verrucosa;Р < 0.05; n = 3*** Significant difference Sargassum miyabei vs. Gracilaria verrucosa;Р < 0.05; n = 3Figure 2 Total phenols in the seaweed extracts fromthe Northern Coast of the Sea of Japan********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минHot-water extract, 30 min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минHot-water extract, 60 min********C. flagile G.verrucosa S.miyabeiSeaweeds of the Sea of Japanмин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extract G.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **C. flagileG.verrucosa S.miyabeiSeaweeds of the Sea of Japanмин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минAutoclave extract, min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минAutoclave extract, 60 min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин 30 мин АК **** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин0500010000150002000025000Total phenol content mg tannicacid/ 100g dry extractЭК 30 мин **** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин05101520253035ЕС50, mg/mLЭК 30 Hot-water extract, 30 min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минTotal phenol content mg tannic**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минHot-water extract, 60 min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин****0500010000150002000025000C. flagile Total phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea ЭК 30 мин ЭК 60 мин АК 30 **** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*05101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the ЭК 30 мин ЭК 60 мин АК 30 Autoclave extract, min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*0500010000150002000025000C. flagile Total phenol content mg tannicacid/ 100g dry extractSeaweeds ЭК 30 мин ЭК 60 мин **** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин05101520253035C. flagileЕС50, mg/mLSeaweeds ЭК 30 мин ЭК 60 мин Autoclave extract, 60 min266Tabakaev A.V. et al. Foods and Raw Materials, 2021, vol. 9, no. 2, pp. 262–270syringic acids, epigallocatechin gallate, epicatechingallate, as well as epicatechin were registered in all thesamples. Syringic acid and epicatechin appeared to bethe major phenolic compounds for all the extracts.All the autoclave extracts showed higher levels ofchlorogenic and syringic acids compared to the hotwatersamples, which had a higher content of ferulic acidand epigallocatechin gallate.A comparative analysis of individual phenoliccompounds showed that S. miyabei extracts had thehighest content of epicatechin, which was maximalin the hot-water sample, and syringic acid, which wasmaximal in the autoclave sample. The C. flagile extractsdemonstrated the highest content of ferulic acid andepigallocatechin gallate, which were the most abundantin the hot-water sample. The G. verrucosa extracts hadthe highest content of chlorogenic acid, especially inthe autoclave sample, coumaric acid (hot-water sample),salicylic acid (autoclave sample), and epicatechingallate (autoclave sample). Thus, the phenolic profileof the extracts depended on the type of seaweed andthe method of hydrothermal treatment. Phenolic acids,which are the main class of phenolic compounds, arefound in seaweed in significant quantities. Typicalphenols are known to possess antioxidant properties.Other publications prove that seaweed is rich in phenoliccompounds, e.g. catechin, caffeic acid, epicatechin,epicatechin gallate, etc. [34, 35].Antioxidant activity. Radical scavengingproperties. DPPH possesses a nitrogen free radicalTable 2 The main phenolic compounds in the seaweed extracts from the Northern Coast of the Sea of JapanCompound Retentiontime,minContent, mg/gSargassum miyabei Codium flagile Gracilaria verrucosaHot-waterextractAutoclaveextractHot-waterextractAutoclaveextractHot-waterextractAutoclaveextract324 nmChlorogenic Acid 8.12 2.48 ± 0.01 4.12 ± 0.02 n.d. 0.15 ± 0.00 6.78 ± 0.23 10.82 ± 0.33Caffeic Acid 10.49 1.69 ± 0.05 2.80 ± 0.12 2.69 ± 0.01 5.09 ± 0.02 4.51 ± 0.14 3.09 ± 0.092,5-dihydroxybenzoic Acid 17.43 0.12 ± 0.01 0.45 ± 0.02 0.85 ± 0.00 n.d. n.d. n.d.Coumaric Acid 20.56 6.02 ± 0.21 8.71 ± 0.29 10.28 ± 0.32 9.04 ± 0.03 11.64 ± 0.48 8.47 ± 0.29Ferulic Acid 24.19 8.83 ± 0.43 7.12 ± 0.30 15.21 ± 0.56 10.27 ± 0.40 3.15 ± 0.09 2.70 ± 0.08Salicylic Acid 44.92 11.05 ± 0.42 10.58 ± 0.37 6.87 ± 0.02 7.18 ± 0.03 8.48 ± 0.30 12.48 ± 0.55277 nmEpigallocatechin Gallate 8.13 15.19 ± 0.59 10.48 ± 0.43 35.89 ± 0.97 20.17 ± 0.79 18.29 ± 0.75 14.50 ± 0.59Epicatechin 10.11 33.52 ± 0.98 31.84 ± 0.83 20.15 ± 0.86 24.76 ± 0.95 30.48 ± 1.07 26.53 ± 1.08Epicatechin Gallate 13.00 4.07 ± 0.15 2.11 ± 0.07 8.97 ± 0.25 9.15 ± 0.38 12.07 ± 0.43 14.20 ± 0.56Syringic Acid 14.78 57.40 ± 2.14 63.18 ± 2.46 17.67 ± 0.54 28.94 ± 1.05 25.76 ± 0.90 30.87 ± 1.14n.d. – not detectedTable 3 Antiradical properties of the seaweed extracts from the Northern Coast of the Sea of JapanSeaweed Antiradical propertiesRadical scavengingactivity,%ЕС50, μg/mL , min Antiradical efficacy, μg/L·c30 min 60 min 30 min 60 min 30 min 60 min 30 min 60 minHot-water extractCodium flagile 58.9 ± 2.8 72.8 ± 3.5 25.69 ± 1.20 20.04 ± 0.67* 22.01 ± 1.00 17.89 ± 0.80 0.0018 ± 0.0000 0.0028 ± 0.0001Gracilariaverrucosa48.2 ± 2.3 76.1 ± 3.7** 28.06 ± 1.30 21.29 ± 0.96 36.21 ± 1.70 27.12 ± 1.30 0.0010 ± 0.0000 0.0017 ± 0.0001Sargassummiyabei72.6 ± 3.5 88.9 ± 4.4 14.53 ± 0.60 10.39 ± 0.09 10.59 ± 0.50 8.09 ± 0.40 0.0065 ± 0.0003 0.0120 ± 0.0005***Hot-water extractCodium flagile 65.3 ± 3.2 80.6 ± 4.0 14.59 ± 0.21 11.23 ± 0.36 17.23 ± 0.36 15.52 ± 0.60 0.0040 ± 0.0002 0.0057 ± 0.0003Gracilariaverrucosa50.1 ± 2.5 78.4 ± 3.8** 30.83 ± 0.58 22.36 ± 1.47 32.36 ± 1.47 20.04 ± 1.00 0.0010 ± 0.0000 0.0022 ± 0.0006Sargassummiyabei75.2 ± 3.6*** 85.6 ± 4.1 12.12 ± 0.15* 11.15 ± 0.08 12.30 ± 0.08 9.08 ± 0.40 0.0067 ± 0.0003 0.0099 ± 0.0005Ionol 94.0 ± 4.5 8.75 ± 0.3 8.0 ± 0.4 0.0160 ± 0.0700* Significant difference Codium flagile vs. Sargassum miyabei; Р < 0.05; n = 3** Significant difference Codium flagile vs. Gracilaria verrucosa; Р < 0.05; n = 3*** Significant difference Sargassum miyabei vs. Gracilaria verrucosa; Р < 0.05; n = 3267Tabakaev A.V. et al. Foods and Raw Materials, 2021, vol. 9, no. 2, pp. 262–270and is easily destroyed by a free radical scavenger. TheDPPH radical assay was used to test the ability of theantioxidant compounds found in the extracts to act asproton radical scavengers or hydrogen donors. Table 3demonstrates the antiradical properties of the extracts.The autoclave extracts appeared to be more activethan the hot-water extracts. The radical scavengingactivity varied within wide limits of 39.6–88.9%.The maximal value was observed in the hot-waterS. miyabei extract, and its radical scavengingactivity was 12.8–28.4% higher than that of theother extracts and only 10.4% lower than that ofionol. The autoclave extracts of S. miyabei alsohad a high radical scavenging activity, whichexceeded that of the other extracts by 5.0–13.3%.A comprehensive assessment of the antiradicalactivity showed that the hot-water (30 min) extract ofG. verrucosa had the lowest values. The hot-waterextract of S. miyabei had the most pronouncedantiradical properties. Its EС50 concentration was1.89–2.41 times lower than the other extracts, itstime was 2.21–9.79 times shorter, and its antiradicalefficiency was 4.29–24.00 times higher.The extract of S. miyabei, which had the highesttotal phenol content, also had a much lower effectiveconcentration of ЕС50. The autoclave extracts demonstrateda strong correlation between the level of totalphenols (Fig. 2) and the effective concentration of EС50.When the total phenol level was high, the effective EС50concentration was low. Therefore, the polyphenoliccomponents in the seaweed extracts were capable ofacting as free radical scavengers. For hot-water extracts,however, this ratio was weak. Since the DPPH radicalanalysis was not specific for any particular antioxidants,the antiradical activity of these extracts result not onlyfrom phenols, but also from various other water-solubleantioxidants, e.g. polysaccharides, folic acid, thiamine,and ascorbic acid [36, 37].The OH• scavenging activity and absorptionof superoxide radicals. Antioxidant activity wasdetermined according to the ability of the antioxidantcomponents to inhibit the oxidation of deoxyriboseby the reactive hydroxyl-ion radical (OH•), formed as aresult of the Fenton-type reaction. The reaction involvestwo antioxidant defense mechanisms: suppression of thegeneration of OH• from H2O2 by binding with metal ionsand the direct transfer of one electron to the generatedradical. Figure 3 illustrates the OH• scavenging activityof the seaweed extracts.Both autoclave and hot-water S. miyabei extractsdemonstrated the highest OH• scavenging activity,while that of the other extracts was significantly lower.A comparative analysis showed similar EС50 values andranking. A longer extraction time increased the OH•scavenging activity, but the increase was insignificant.Brown seaweeds are known to contain floratanins,which chelate heavy metals [32, 38]. Probably, thehigh content of floratanins predetermined the high OH•scavenging activity of S. miyabei extracts. However,seaweeds have a complex composition, and its phenolsare not the only compounds that exhibit antioxidantactivity, which is also typical of carotenoids andpolysaccharides [39]. Hydrothermal extracts ofsome green and brown seaweeds contain sulfatedheteropolysaccharides, which strongly inhibit OH•activity in vitro [40–42].Figure 4 demonstrates the O2•− scavenging activity ofthe extracts.Although O2•− is a relatively weak oxidizing agent,it decomposes into stronger oxidative forms, suchas singlet oxygen and OH•. The autoclave extract ofG. verrucosa was the only sample to demonstrateO2•− scavenging activity. All hot-water extracts showedO2•− inhibitory activity. Figure 4 shows that themaximal inhibitory activity belonged to the C. flagileextract. Surprisingly, S. miyabei revealed no signs ofO2•− scavenging. The extraction time only slightlyincreased the O2•− absorption.* Significant difference Codium flagile vs. Sargassum miyabei; Р < 0.05; n = 3** Significant difference Codium flagile vs. Gracilaria verrucosa;Р < 0.05; n = 3*** Significant difference Sargassum miyabei vs. Gracilaria verrucosa;Р < 0.05; n = 3Figure 3 OH• scavenging activity of the seaweed extracts fromthe Northern Coast of the Sea of Japan* Significant difference Codium flagile vs. Gracilaria verrucosa;Р < 0.05; n = 3Figure 4 O2•− scavenging activity of the seaweed extractsfrom the Northern Coast of the Sea of Japan********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*S.miyabeiJapanмин АК 60 мин****0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минS.miyabeiof Japanмин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минHot-water extract, 30 min*******verrucosa S.miyabeiof the Sea of JapanАК 30 мин АК 60 мин****0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*****G.verrucosa S.miyabeiSeaweeds of the Sea of JapanАК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минHot-water extract, 60 min****C. flagile S.miyabeiG.verrucosaSeaweeds of the Sea of Japanмин ЭК 60 мин АК 30 мин АК 60 мин*0C. flagileG.verrucosaSeaweeds of the Sea of Japanмин ЭК 60 мин АК 30 мин АК 60 минAutoclave extract, 30 min****0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 минAutoclave extract, 60 min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of Japan*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanHot-water extract, 30 min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of Japan*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanHot-water extract, 60 min********C. flagile G.verrucosa S.miyabeiSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileG.verrucosa S.miyabeiSeaweeds of the Sea of Japan*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanAutoclave extract, 30 min********0481216C. flagile G.verrucosa S.miyabeiYield, %Seaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин******0500010000150002000025000C. flagile S.miyabeiTotal phenol content mg tannicacid/ 100g dry extractG.verrucosaSeaweeds of the Sea of JapanЭК 30 мин ЭК 60 мин АК 30 мин АК 60 мин**** **050100150200C. flagileЕС50, mg/mLG.verrucosa S.miyabeiSeaweeds of the Sea of Japan*005101520253035C. flagileЕС50, mg/mLG.verrucosaSeaweeds of the Sea of JapanAutoclave extract, 60 min268Tabakaev A.V. et al. Foods and Raw Materials, 2021, vol. 9, no. 2, pp. 262–270The established difference in the O2•− absorptionbetween the hot-water and autoclave extracts provedthat the two methods extracted different types ofantioxidants. The lack of correlation between thetotal content of phenols and О2•− inhibition means thatsuch activity demonstrated by all three species couldnot be explained solely by the presence of phenolicantioxidants. Sulfated polysaccharides obtained fromthe hot-water extracts of green and brown seaweedsinhibited the activity of О2•− in vitro [40–42].CONCLUSIONThe hot-water extraction proved more effectivethan the autoclave extraction in terms of OH• a nd О 2•−scavenging activity. However, the autoclave extracts hadbetter antiradical properties. The extracts of the brownseaweed Sargassum miyabei showed significantly higherantiradical and antioxidant activity than the extracts ofthe red seaweed Gracilaria verrucosa and the greenseaweed Codium flagile. The relationship between OH•scavenging and antioxidant activities in these samplesindicated that it came from the hydrophilic polyphenolicantioxidants. The hydrothermal extracts of the seaweedsproved to be a promising source of antioxidants forhuman consumption.CONTRIBUTIONThe authors contributed equally to the manuscriptand are equally responsible for any potentialplagiarism.CONFLICT OF INTERESTThe authors declare that there is no conflict