Bioactive compounds in farm-raised sea vegetables

Final Report for GNE15-110

Project Type: Graduate Student
Funds awarded in 2015: $7,616.00
Projected End Date: 12/31/2016
Grant Recipient: University of Maine
Region: Northeast
State: Maine
Graduate Student:
Faculty Advisor:
Dr. Denise Skonberg
University of Maine
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Project Information

Summary:

Most of the research reported to date on bioactive compounds in sea vegetables has been conducted on dried, wild harvested product. Bioactive compounds are often associated with various health promoting effects including high antioxidant capacity, anti-inflammatory, anti-cancer and anti-aging.  Sea vegetables are considered to be high in bioactive compounds, and this project aims to focus specifically on total phenolic content (TPC) and antioxidant capacity of fresh farm-raised sea vegetables grown in Maine. Effects of minimal processing such as blanching and freezing on bioactive compounds in sea vegetables have been poorly investigated. Moreover, the amount of bioactive compounds present in sea vegetables can vary within different parts such as blade versus stipe. Therefore, this research project assessed the effects of minimal processing treatments (blanching/freezing) and source of edible plant tissue (blade/stipe; in species where there is a clear distinction between their parts) on sugar kelp, dulse, alaria and gracilaria.  Fresh, fresh frozen, blanched and blanched frozen samples of the aforementioned species underwent 1,1-diphenyl-2-picrylhydrazyl (DPPH), ferric reducing antioxidant power (FRAP) and TPC analyses. A small sensory panel was used to determine optimal blanching (80 ºC for 1min or 100 ºC for 5s) treatment. The panelists did not detect any significant (p<0.05) differences between the two blanching treatments. Blanched samples were evaluated immediately after processing whereas the frozen samples were stored at -20 ºC for one month.  The antioxidant capacity of blanched, frozen and blanched frozen dulse, gracilaria, sugar kelp and winged kelp were compared to fresh samples. Blanching significantly (p<0.05) decreased TPC and the antioxidant capacity of the sea vegetables, however, freezing for one month did not affect their TPC and antioxidant capacity in most cases. Overall, the brown sea vegetables had higher antioxidant capacity compared to the red sea vegetables. For sugar kelp and alaria, stipes had a higher phenolic content and antioxidant capacity compared to blades. Correlation analysis showed that TPC was strongly positively and negatively related to FRAP and DPPH, respectively

Introduction:

Efforts are in underway in the Northeast to develop a sustainable sea vegetable industry. The growing demand for local, farm-raised and fresh healthful foods (Sloan 2015) shows great potential for a fresh sea vegetables market. A lot of attention has been given to analyzing antioxidants present in sea vegetables over the past decade. Claims such as “high in antioxidants” have been shown to affect consumers’ attitudes about product quality positively, and may be beneficial in promoting farm-raised sea vegetables (Daniells 2009). In a recent article about the top ten food trends in North America, the authors cited multiple trend reports showing that 30% of consumers made a strong effort to consume more minimally processed foods (Sloan 2015). Another article reported that approximately 55-60% of consumers are likely to buy or continue purchasing a product having an antioxidant claim (Daniells 2009). Sea vegetables contain heat sensitive nutrients such as vitamins C and phenolic compounds which are likely labile to thermal processing. Sea vegetable producers in the Northeast have developed minimally processed sea vegetable products including fresh and frozen salads; ‘ready to eat/cook’ blanched and salted fronds, and frozen prepared soups (Redmond 2012). Blanching of sea vegetables results in an attractive green color of the samples, making them more attractive to American consumers. Additionally, blanching also aids in inactivation of enzymes that may lead to off-flavor development, change in nutritional quality and texture of the food (Rahman and Perera 2007). However, commonly used processing methods such as blanching and freezing may negatively affect bioactive compounds present in these value-added products. Previous research has shown that the amount of bioactive compounds found in different parts of sea vegetables, such as the blade versus stipe, may vary (Connan and others 2006). However, to date there have been no reports on the effects of selected processing treatments and source of edible tissue (blade/stipe) on antioxidant capacity of sea vegetables.

References

 

Connan S, Delisle F, Deslandes E, Lebham EAG. 2006. Intra-thallus phlorotannin content and antioxidant activity in Phaeophyceae of temperate waters. Botanica Marina 49:39-46.

Daniells S. 2009. The growth of brand ‘antioxidant.’ Available from:

http://www.nutraingredients.com/Suppliers2/The-growth-of-brand-antioxidant. Accessed 2015 December 10

Sloan EA. 2015. The top ten food trends. Food Technology 69(4):24-42.

Rahman MS, Perera CO. 2007. Drying and food preservation. In: Rahman MS. Handbook of food preservation. 2nd edn. New York: CRC Press. p 404-427.

Redmond S. 2012. Beyond Sushi: A review of current and potential products from marine macroalgae. The seaweed scene 2012. Available from: http://www.seagrant.umaine.edu/files/pdf-global/12beyondsushiSR.pdf. Accessed 2016 April 11.

 

Project Objectives:

Objective 1: Determine total phenolic content (TPC) and antioxidant capacity in four species of fresh farm raised sea vegetables.

Objective 2: Determine the effects of minimal processing (blanching and freezing) on the stability of TPC and antioxidant capacity of the sea vegetables.

Objective 3: To determine if any significant differences exist in TPC and antioxidant capacity between blade and stipes of the sea vegetables.

Objective 4: To compare the data for farm raised products with data for wild harvested sea vegetables in the scientific literature.

Objective 5: To communicate research results to interested members of the aquaculture and seafood communities in the Northeast region.

Objectives 1 to 4 were completed by August 2016. Although objective 5 has not been completed yet, the results of this study will be presented at national and regional conferences next year.

 

Cooperators

Click linked name(s) to expand
  • Dhriti Nayyar
  • Dr. Denise Skonberg

Research

Materials and methods:

Four processing treatments were chosen for this study for all the four sea vegetables: fresh, blanched, fresh frozen and blanched frozen. The brown sea vegetables were sorted into blades (WF) and stipes (ST) and processed similarly. The red sea vegetables were not sorted into blades and stipes since the parts were not distinguishable. Samples were blanched at 80 ºC for 1 min and the frozen treatments were stored at -20 ºC for one month. All the processing was done in triplicate (A, B, C).

Determining Blanching Parameters

            Low-temperature long-time (80 ºC for 1 min) and high-temperature short-time (100 ºC for 5 s) treatments were selected for preliminary testing. Final blanching temperature and duration were chosen based on a sensory evaluation of a gracilaria salad made with blanched gracilaria from both the treatments. Gracilaria was selected for sensory evaluation due to its availability. At test time, a triangle test followed by a preference test was conducted to assess whether panelists could differentiate between treatments, and if so, which one of the two treatments they preferred. Due to poor growth of farm-raised gracilaria, wild harvest was used instead.

Five hundred grams of gracilaria were added to the water once the desired temperature was reached, 80 ºC or 100 ºC, and kept in the water for 60 s or 5 s, respectively. An Asian salad dressing was made using ingredients from a local supermarket one day prior to the sample delivery and then refrigerated overnight (Table 1). All of the ingredients were mixed together in a salad bowl by hand using a whisk. The same dressing was used for both the treatments the next day.

Table 1. Salad dressing formulation

Ingredient

Amount (g)

% Weight

Rice Vinegar

380

36.3

Sugar

240

22.9

Soy Sauce

150

14.3

Sesame Oil

120

11.5

Lime Juice (bottled)

100

9.6

Grated Ginger (fresh)

57

5.4

Total

1047

100

 

Salads were prepared using blanched gracilaria, shredded carrots, salad dressing and toasted sesame seeds (Table 2). Both the salads were thoroughly mixed so that the ingredients were well-dispersed. The salads were allowed to chill in the refrigerator, and taken out of the refrigerator 15 minutes prior to the sensory evaluation.

Table 2. Salad formulation

Ingredient

Amount (g)

% Weight

Gracilaria

250

49.2

Shredded Carrot

127

25

Salad Dressing

125

24.6

Sesame Seeds

6

1.2

Total

508

100.0

 

            In the triangle test, panelists were presented with three samples in a randomized order, of which two samples were identical. The panelists had to choose the odd/different sample (Meilgaard and others 2006). Twelve panelists familiar with sea vegetables were recruited via word of mouth from University of Maine to participate in the test. A tray with three paper cups filled with 20 g of salad each and a paper evaluation ballot was prepared for each panelist. After panelists chose the odd sample, that sample cup was removed from the tray and the panelists were requested to continue with the preference test. The number of correct responses were counted and compared to the tables for the critical number of correct responses for statistical significance (Meilgaard and others 2006).

Sample Processing for Antioxidant Assays

            Processing of all the four species took place on separate days, depending on their harvesting season. gracilaria was harvested in November, 2015 whereas dulse, winged kelp, and sugar kelp were all harvested in April, 2016. Except for gracilaria, all the other species were farm-raised. The samples were harvested from Clark Cove farm (Bristol, ME), shipped in a cooler overnight, and processed within 2 days of the harvest. Sugar kelp and winged kelp samples were cut by hand to separate the blades and stipes prior to blanching or freezing. All the blades and stipes were mixed within species to insure homogeneity. Two hundred and twenty-five grams of sample were processed for each species and plant part in triplicate for all the treatments except sugar kelp stipes. A lesser amount (150g) was used for sugar kelp stipes due to a shortage of the harvested sample.          

Fresh

            Fresh, unprocessed samples, were randomly selected prior to being weighed and packaged in pre-labelled polyethylene bags. These bags were heat sealed after pressing out the air by hand. 

Blanching

            The sample was added to the hot water at 80 C and transferred to a strainer after 60 s. The sample was then added to an ice bath, which had equal proportion of water and crushed ice (1:1) for one minute. The sample was strained again and then spun in a salad spinner for 1 minute. Blanched samples were reweighed and packaged in plastic bags. The bags were heat sealed after air was removed manually.

Blast Freezing

            All the samples were blast frozen at -30 ºC post processing for 1 h. These were then either prepared for freeze-drying or frozen storage. The frozen and blanched frozen samples were transferred to the freezer and the fresh and blanched samples were freeze-dried immediately. The freeze drying cycle was for 20 h but multiple cycles were used until the samples reached a constant weight. The freeze-dried samples were crushed and stored in whirlpack bags at -80 ºC until further analysis. One week prior to the analyses, all the samples were ground using a coffee grinder and stored at -80 ºC until extraction.

Frozen Storage

            These samples were packaged and stored at -20 ºC in a walk-in freezer) for one month. The temperature was chosen based on what industry would use to store their frozen samples.

 

 

Preparation of Sample Extract

            Ground, freeze-dried samples were extracted with 60% methanol for 24 h on an orbital shaker. The samples were centrifuged at 2100 xg for 10 minutes. The supernatant was collected and a pellet wash was performed twice by adding 10 mL of 60% methanol, shaking for 10 minutes on the shaker, and then centrifuging. All the supernatant was pooled, then brought to 50 mL with distilled water, and then vortexed for 30 s to insure to adequate mixing.

Determination of Total Phenolic Content

            Total phenolic content of the sample extract was determined according to the Folin-Ciocalteu method (Taga and others 1984, Matanjun and others 2008, Rajauria and others 2010). Varying concentrations (0-200 µg/mL) of gallic acid were used as a standard. Absorbance was measured at 725 nm using a UV-vis spectrophotometer. Total phenolic content was expressed as mg gallic acid equivalents per gram of freeze-dried sample.

2, 2-diphenyl-1-picrylhydrazyl (DPPH) Assay

DPPH radical scavenging activity of sample extracts was determined based on Blois (1958) with modifications. DPPH was prepared in ethanol. Varying volumes of sample extract (0.5-2 mL) were brought up to 2 mL with 40% methanol. Two mL of DPPH solution was added to this and incubated for 30 min in the dark. The control, 40% methanol, was treated the same way as the sample and sample blank, where either 2 mL DPPH or ethanol was added to 2 mL 40% methanol. The absorbance was all measured against 100% ethanol at 517 nm. The following formula was used to calculate % inhibition:

% DPPH inhibition =    Control Abs – (Sample Abs – Sample Blank Abs)   x 100

                                                                                 Control

The % inhibition results were plotted against varying concentrations (g/mL) of sample using MS Excel. Linearity was ensured by looking at the R2 values and EC50 was calculated using the slope and constant of the plotted line. The assay was performed in duplicate and the average was expressed as EC50 (mg/mL), the concentration of sample necessary for a 50% inhibition of DPPH activity.

Ferric Reducing Antioxidant Power (FRAP) Assay

            The antioxidant capacity was also assessed according to the method described by Benzie and Strain (1996), with some modifications. The FRAP reagent was prepared fresh daily by mxing 300 mM acetate buffer (pH 3.6), 10 mM 2,4,6-Tripyridyl-s-Triazine (TPTZ) solution and 20 mM FeCl3.6H2O solution (100:10:10). This solution was stirred and warmed to 37 ºC in a water bath. An aliquot of 3 mL FRAP reagent was added to 100 µL sample extract or varying concentrations (0-1000 µM) of the FeSO4×7H2O standard directly in the cuvette. The absorbance was measured at 593 nm after exactly 4 min. The analysis was performed in duplicate and their average was expressed in µmol ferrous sulfate equivalents per gram of freeze-dried sample.

Statistical Analysis

            Data were analyzed using JMP 12.2 (SAS Software, Cary, NC). Shapiro-Wilk’s normality test and Levene equality of variances were used to assess data prior to further analyses. One-way analysis of variance (ANOVA) was selected to find treatment differences. Tukey’s Honest Significant Difference (HSD) test was selected for post-hoc analyses. A significance level of p<0.05 was chosen for all statistical analyses. Pearson correlation between phenolic content and the antioxidant assays was determined to understand their relationships.

Meilgaard MC, Carr TB, Civelle GV. 2006. Sensory evaluation techniques. 4th edn. New York: CRC Press. p 65

Matanjun P, Mohamed S, Mustapha NM, Muhammad K, Ming CH. 2008. Antioxidant activities and phenolics content of eight species of seaweeds from north Borneo. J Appl Phycol 20:367–373.

Rajauria G, Jaiswal AK, Abu-Ghannam N, Gupta S. 2010. Effect of hydrothermal processing on colour, antioxidant and free radical scavenging capacities of edible Irish brown seaweeds. Int J Food Sci Technol 45:2485–2493.

Taga MS, Miller EE, Pratt DE. 1984. Chia seeds as a source of natural lipid antioxidants. J Am Oil Chem Soc 61:928–931.

Research results and discussion:

While processing, it was observed that the stipes of both the kelps were dissimilar, with sugar kelp stipes being hollow and light-weight whereas winged kelp stipes were solid and thick. However, the inside of both the kelp stipes had a lighter color than the outside, browner color. All the four sea vegetables, irrespective of whether they were red or brown sea vegetables, instantly changed color to green upon blanching. Immediately after blanching, they gave off a distinct odor, however, the odor faded as the sample bags were being prepared. After freeze drying, it was observed that the blanched treatment whole fronds were less dense, and absorbed extraction solvent completely, making them more viscous, compared to the non-blanched samples.  The extract color differed depending on species and treatment, with paler colors for blanched treatments.

With regard to the gracilaria sensory test, the panelists could not significantly differentiate between the two blanching treatments during the triangle test, based on the critical number of correct responses required according to Meilgaard and others (2006). The blanching treatment at 80 ºC for 1 min was selected based on two considerations; the sensory evaluation showed us that there were no detectable differences between the two treatments and because this specific treatment has been used previously to blanch sea vegetables (McHugh 2003, Boulom and others 2014).

Total Phenolic Content (TPC)

            The results of the TPC assays indicate that the blanched samples had significantly (p<0.01) lower total phenolic content compared to the fresh and fresh frozen samples for all of the species and plant parts. The TPC ranged from 1.42 to 17.44 mg GAE/g sample for the fresh and fresh frozen samples and from 0.77 to 7.44 mg GAE/g sample for blanched and the blanched frozen samples, indicating that blanching reduced the TPC by approximately half.

            Although blanching reduced the TPC in gracilaria and dulse, the effect was larger in gracilaria  (p<0.0001). In the kelp species, all samples were equally affected by blanching, except for the frozen SK blades, which did not significantly drop in response to blanching. The observed decreases in the phenolic content as a result of blanching were likely due to the loss of the highly water soluble phenolic compounds (Cheynier 2012) present in sea vegetables (Sabeena Farvin and Jacobsen 2013). Moreover, blanching may have caused cellular damage or disruption, leading the more complex polyphenols to be released to the blanch water. However, Rajauria and others (2010) reported a 75.6 % increase in TPC in sugar kelp that was hydrothermally processed at 95 ºC for 15 min, explaining that the high temperature and duration could have released previously bound phenolic compounds. In the current study, we found that blanching caused TPC in fresh SK stipes to plummet by over 70%, the highest drop for any of the species and tissues evaluated.

            In contrast to the effects of blanching, there were negligible differences in TPC due to frozen storage in gracilaria, dulse, and sugar kelp and winged kelp blades.  It is interesting to note, however, that freezing (and frozen storage) significantly (p<0.0001) reduced the TPC in the stipes of the brown sea vegetable species (sugar kelp and winged kelp), which was unexpected.

            Brown sea vegetables contain a group of polyphenols called phlorotannins that contribute largely to their high antioxidant capacity (Wang and others 2009). Their absence in red sea vegetables often results in low antioxidant activity in comparison to brown sea vegetables. In the current study, red sea vegetables (gracilaria  and dulse) had lower TPC compared to the brown sea vegetables. Other authors (Jiménez-Escrig and 2001, García-Casal and others 2009) have reported similar trends when comparing TPC in red and brown sea vegetables.

            More recently, researchers have been interested in intra-thallus TPC, comparing variation in different parts of selected sea vegetables species. Thallus refers to the algal body which is not differentiated in stem, leaves and roots like terrestrial plants. The current study focused on comparing blade and stipes of the brown sea vegetables, sugar kelp and winged kelp, because they are already being sold as distinct products by some producers in the northeast. For sugar kelp, the fresh and fresh frozen stipes contained 2.6 and 2.1 times more phenolics, respectively, compared to the blades. On the contrary, lower phenolic content in stipes compared to blade was reported by Connan and others (2006) in wild harvest of Laminaria hyperborea and L. digita, both belonging to the same genus as sugar kelp. Fresh winged kelp stipes were about the same in comparison to the blades whereas the fresh frozen stipes had lower phenolic content than the blades of the same treatment. Schmid and Stengel (2015) reported concentrations of pigments chlorophyll a, chlorophyll c, fucoxanthin and ß-carotene to be significantly (p<0.01) lower in stipes compared to basal and tip parts of wildly harvested winged kelp blades. The same authors also reported no significant variability in pigment levels in different plant tissue for sugar kelp but mentioned this species as having a lower concentration of pigments than winged kelp.

DPPH Radical Scavenging Activity

            DPPH results are reported as EC50 (mg/mL), which is the concentration of dried seaweed sample in the extraction solvent needed to inhibit 50% of the DPPH free radicals. The lower the EC50 of the sample, the higher its antioxidant capacity. The effects of blanching were quite evident since the EC50 levels were significantly (p<0.05) higher in the blanched samples compared to the fresh. The EC50 ranged from 0.9 to 26.2 mg/mL in fresh and fresh frozen treatments and from 1.7 to 133.7 mg/mL in blanched and blanched frozen treatments. Specifically, for gracilaria, the EC50 of blanched treatments was significantly (p<0.0001) higher than for fresh or fresh frozen treatments, indicating lower antioxidant capacity due to blanching. A similar trend was observed in dulse, where blanching increased the EC50 in fresh and frozen samples significantly (p=0.0001) compared to non-blanched samples, resulting in approximately 75% loss of the antioxidant capacity. Although the fresh sugar kelp blades had significantly lower EC50 than blanched and blanched frozen, the fresh frozen treatment was not found to be statistically different from them. For sugar kelp stipes, blanching significantly decreased DPPH antioxidant capacity by 50% compared to the fresh and fresh frozen treatments. Gupta and others (2011) reported reduction of TPC and an increase in EC50 of oven dried (varying temperatures), wild H. elongata, compared to the fresh samples, indicating that heat contributed to a reduction in antioxidant activity of the sea vegetables. In the current study, both the kelps had higher antioxidant capacity, with the winged kelp having an EC50 approximately 20-40 times lower than the red sea vegetables.

            Some differences in fresh versus fresh frozen treatments were expected since any native enzymes present, such as polyphenol oxidase, catalase and lipoxygenase (Nakano and others 1995), commonly found in vegetables, were not blanched and may have retained some activity during frozen storage. However, no significant effects of freezing and frozen storage (one month) on DPPH antioxidant capacity were observed in the species under investigation with the exception of winged kelp. In winged kelp blades, the EC50 for blanched frozen samples was significantly (p=0.0001) higher than blanched samples, indicating a negative effect of blanching combined with frozen storage. It is important to note that most vegetables are targeted to be frozen for up to 6 months to a year, however, in the current study only effects of immediate freezing were determined.

            The effects of blanching on antioxidant activity were more pronounced in red sea vegetables, compared to brown. One possible explanation could be that levels of non-water soluble pigments found in brown sea vegetables such as carotenoids and xanthophylls including abundantly present fucoxanthin (Yan and others 1999, Bocanegra and others 2009, Fung and others 2013), were higher compared to the levels in red sea vegetables. Both, sugar kelp and winged kelp stipes showed the lowest loss of DPPH scavenging activity as a result of blanching compared to all other species and product forms. This could be due to the fact that the stipes are narrower with less surface area in comparison to the flatter blades, reducing the loss during blanching of compounds that contribute to antioxidant capacity. It is interesting to note that even though both kelp species were harvested only one week apart, winged kelp showed higher radical scavenging activity compared to sugar kelp, indicating that genetic variation among kelp species plays an important role in their antioxidant activity. 

Ferric Reducing Antioxidant Power (FRAP) Assay

            The FRAP assay is based on a single electron transfer mechanism, and assesses the ability of antioxidants in the sample to reduce ferric ion to ferrous ion (Benzie and Strain 1996). The underlying mechanism for FRAP is not different from DPPH, as both work as electron donors. However, FRAP only uses a single electron transfer (SET) mechanism whereas DPPH uses SET and hydrogen atom transfer (HAT) mechanism to some extent (Prior and others 2005). It was important to perform both the assays to characterize the extent of both mechanisms while looking at antioxidant capacity of the seaweed samples.

            Overall, the FRAP values ranged from 3.9-41.0 μmol FeSO4.7H2O equivalents (FSE) per g dried sample for fresh samples versus merely 1.9-17.0 μmol FSE/g for blanched samples. Significant (p<0.05) effects of blanching were observed in FRAP values with decreased values in blanched samples compared to fresh for all species except for dulse. In gracilaria, blanching resulted in cutting the FRAP values in half, from 3.9 μmol FSE/g for fresh and fresh frozen sample to 1.8 μmol FSE/g for blanched treatments. The same change was observed in TPC of gracilaria samples. For both kelps, fresh and fresh frozen blades and stipes were significantly higher in FRAP when compared to blanched and blanched frozen samples, indicating loss of compounds with reducing power due to blanching. For dulse, only blanching in addition to frozen storage led to significant decrease in FRAP. Frozen and blanched frozen storage of winged kelp stipes resulted in significantly (p<0.0001) lower FRAP values in comparison to the fresh samples. However, in all other species and product forms freezing at -20 ºC for one month did not affect the FRAP value significantly. 

            The highest FRAP value measured for red sea vegetables was 4.4 μmol FSE/g and for brown it was 41.0 μmol FSE/g. These differences indicate that the kelps evaluated in this study had higher ability to reduce the ferric ions to ferrous compared to red sea vegetables.  Ferraces-Casais and others (2012) reported FRAP of fresh, wild Laminaria spp. to be 6.90 μmol Trolox/g sample, which is much lower than values obtained for both the kelps in the current study. However, direct comparisons cannot be made due to different standards used in the two studies. The winged kelp samples showed higher FRAP compared to sugar kelp samples. In addition to genetic variation, the presence of a tough midrib in the winged kelp blades may have protected them against antioxidants loss during blanching.             

Correlations among TPC, DPPH and FRAP

In sea vegetables, high antioxidant activity has often been attributed to the presence of abundant phenolics (Chew and others 2008, Wijesekara and others 2011, Fernandes de Oliveira and others 2012). Their ability to play multiple roles as reducing agents, free radical scavengers, hydrogen donors and metal chelators adds to their  considerable antioxidant capacity (Jiménez-Escrig and others 2001, Wang and others 2009). Correlations between TPC and the antioxidant assays were investigated for each species to determine the strength and direction of their relationship. Table 3 provides the Pearson’s r values (p<0.05) for each species, treatment, and product form. For gracilaria, TPC and FRAP showed a strong positive correlation (0.9676) whereas TPC and DPPH showed a strong negative correlation (-0.927), indicating that the antioxidant activity in this red sea vegetable was likely largely due to its phenolics content. Here, the negative correlation with DPPH makes sense because the results were expressed as EC50, where a lower concentration indicates higher antioxidant capacity. The FRAP and DPPH values also had a strong negative correlation (-0.96), indicating consistency among assay results. Although a strong negative correlation (-0.8447) was found between TPC and DPPH for dulse, there was a positive but moderate correlation between TPC and FRAP (0.6192). This shows that there may be other antioxidants such as selected proteins or small polysaccharides that contributed to their reducing power along with polyphenols. For sugar kelp blades and stipes, strong and positive correlations (0.8525 and 0.8707, respectively) were observed between TPC and FRAP whereas strong negative correlations were observed between TPC and DPPH (-0.798 and -0.8617, respectively). Winged kelp followed a similar trend to sugar kelp, exhibiting strong positive correlation between TPC and FRAP and negative between TPC and DPPH. These results agree with previously reported strong correlations between TPC and antioxidant assays, suggesting that polyphenols are large contributors to the antioxidant capacity in sea vegetables (Gupta and Abu-Ghannam 2011, Ferraces-Casais and others 2012, Chan and others 2013).

 

 

Table 3. Correlations among TPC, DPPH and FRAP

Pearson’s r (p<0.05)

Gracilaria  

 

TPC

DPPH

FRAP

TPC

1

   

DPPH

-0.927****

1

 

FRAP

   0.9676****

-0.96****

1

Dulse

 

TPC

DPPH

FRAP

TPC

1

   

DPPH

-0.8447***

1

 

FRAP

    0.6192*

-0.7451**

1

Sugar kelp

 

TPC

DPPH

FRAP

 

TPC

1

   

Blades

DPPH

   -0.798**

1

 
 

FRAP

 0.8525***

-0.8498***

1

 

 

TPC

DPPH

FRAP

 

TPC

1

   

Stipes

DPPH

-0.8617***

1

 

 

FRAP

 0.8707***

-0.8982****

1

Winged kelp

 

TPC

DPPH

FRAP

 

TPC

1

   

Blades

DPPH

-0.8576***

1

 
 

FRAP

  0.9819****

-0.8234***

1

 

 

TPC

DPPH

FRAP

 

TPC

1

   

Stipes

DPPH

   -0.7201**

1

 
 

FRAP

   0.9639****

-0.7422**

1

    *=p<0.05, **=p<0.01, ***=p<0.001, ****=p<0.0001

 

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Research conclusions:

The results of this study will benefit the aquaculture industry and farmers involved with sea vegetable production and processing. They can use this information to market minimally processed sea vegetables to consumers through multiple channels, including restaurants and retail. Selected farmers in the Northeast area have already started to use this information while distributing their blanched or frozen samples. The results of this study contribute to the developing sea vegetable industry in the Northeast.

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary

Education/outreach description:

The results of this study will be presented at the Institute of Food Technologists Annual Meeting in June 2017, which is attended by researchers and food industry professionals. The abstract for the poster presentation will be submitted in December 2016. The findings of this research will also be shared at the 3rd Annual Maine Aquaculture R&D & Education Summit in Feb/March 2017. This summit is targeted toward business professionals, researchers, farmers and students who are interested in aquaculture, particularly in the Northeast. This project will also yield a research paper in a peer-reviewed journal. Although we were supposed share the results at the Maine Seaweed Festival in 2016, the event was cancelled.

Project Outcomes

Project outcomes:

It is difficult to estimate the economic aspect of this project since an economic evaluation was not part of this research. However, the results of this study can benefit the farmers and companies focused on minimally processing farm-raised sea vegetables.

Farmer Adoption

As of now, these results have been shared with a few sea vegetable farmers in Maine. They expressed great interest in this work and the outcomes as they diversify their sea vegetable products to sell in retail. Maine Fresh Sea Farms, a company that supported this project, is trying to sell the stipes portion of sugar kelp along with the blades to food service and was very happy to know that the results of this study show that stipes have more antioxidants than the blades. This could help them better market their products. They mentioned that antioxidant is still a buzz word that consumers focus on and they would like any information that can help them optimize their processing parameters, especially for blanching. In the future, these results will be shared with other farmers and people interested in aquaculture in the Northeast region.

Assessment of Project Approach and Areas of Further Study:

Areas needing additional study

The current study was an excellent start to study how processing might affect antioxidant capacity of sea vegetables. However, in addition to these results, testing antioxidant capacity utilizing methods other than DPPH and FRAP will deepen our knowledge of the mechanisms of these antioxidants. Moreover, since it is clear that phenolic compounds contribute greatly to the antioxidant capacity of these sea vegetables, characterizing specific phenolic compounds present will provide further insights about the functionality of these antioxidants. As different processes including cooking, drying and canning may alter the chemical constitution of foods differently, a future focus on assessing differences in antioxidant capacity due to processing methods will help provide a more complete picture to the sea vegetable industry. Additionally, investigating effects of processing on other nutritional benefits associated with sea vegetables including anti-inflammatory, anti-diabetic and anti-microbial properties would enhance our knowledge further.  

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.