Extraction technology impacts on the structure-function relationship between sodium alginate extracts and their in vitro prebiotic activity
Introduction
Alginates are a group of commercially valuable food hydrocolloids derived from some brown seaweeds, with a wide range of industrial and pharmaceutical applications. Alginates are derivatives of alginic acids that are localized in the cell wall of brown seaweeds. Alginic acids are industrially extracted from brown seaweeds in their salt forms, i.e., sodium, potassium, and calcium alginates. Alginates can also be obtained from some bacteria as capsular polysaccharides (Draget & Taylor, 2011). Structurally, alginates are linear co-polymers of β-D-mannuronic acids (M) and α-L-guluronic acids (G) linked together with 1 → 4 linkages. The arrangements of these uronic acids in various alginates are either homo-polymeric (MM and GG) or hetero-polymeric (MG), and these may vary in their proportions, depending on certain conditions such as seaweed source, harvest season, geographical location, climatic factors, and extraction method/conditions (Dettmar, Strugala, & Craig Richardson, 2011; Rhein-Knudsen, Ale, Ajalloueian, & Meyer, 2017; Youssouf et al., 2017). The arrangement of these uronic acids in various alginate polymers corresponds to specific gelling properties and their functionality. For instance, a high guluronic acid provides various alginates with their gel-forming ability, which are used in the food and textile industries as stabilizers, thickeners, gel-formers and film-formers (Dettmar et al., 2011; Youssouf et al., 2017). On the other hand, mannuronate-rich alginates have been reported to give an immune response through their interactions with pattern-recognition receptors (PRR) (Espevik et al., 2009). Other potential biomedical applications of various alginates include drug delivery, immobilization of cells, and modulation of appetite (Draget & Taylor, 2011). The application of different processing methods, with potentially increased efficiencies, may significantly impact yield. This may be economically desirable considering the costs of raw materials/extraction, time, labour, etc. Alginate structure and composition could also be modified to obtain specific products for specific applications. The use of different extraction methods may have an important role in modifying the composition of various alginates, particularly de-polymerization of the complex matrix of alginate polymers.
Current methods that may be used include microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), and enzyme-assisted extraction (EAE). These techniques all use different mechanisms for the extraction of compounds from raw materials. The fast, microwave heating results in the splitting of the cell wall, and subsequent release of its content into an extraction medium (Rodriguez-Jasso et al., 2011; Yuan & Macquarrie, 2015a). Acoustic cavitation with ultrasound leads to intense formation and collapse of cavitation bubbles within the liquid medium, leading to intense stresses and irreversible chain splitting (Yan, Wang, Ma, & Wang, 2016). The enzymatic technique uses cell-wall degrading enzymes, such as cellulase, to rupture thick, seaweed cell walls and release their contents (Charoensiddhi, Conlon, Franco, & Zhang, 2017). The hydrolytic effect of these processes may influence the structural and functional properties of the alginate extracts.
A prospective application of various alginate extracts is their use as a prebiotic in functional foods and/or nutraceuticals. Alginates are non-hydrolysable by the human gut digestive enzymes, but they are often fermented by members of the intestinal microbiome (Ramnani et al., 2012). Alginates can act as a carbon source for the growth and stimulation of beneficial populations of the gut microbiota, in particular members of the genera Lactobacilli and Bifidobacteria, hence the prebiotic potential of seaweed-derived fibres (Ramnani et al., 2012; Wang, Han, Hu, Li, & Yu, 2006). Lactobacillus casei and L. delbruecki ssp bulgaricus are two pro-biogenic bacteria that are commonly observed in the human gut and have also been used as commercial probiotics in dairy products (Cats et al., 2003). Preliminary evidence supported in vitro prebiotic activity of most seaweed polysaccharides (Okolie et al., 2017). However, there is a lack of information on the prebiotic capacity of isolated alginate extracts from A. nodosum, and the role of extraction processes on important structural parameters that may be responsible for this activity.
The objective of this study was therefore to investigate the impacts of different extraction methods on the structure-function relationship between sodium alginate extracts from A. nodosum and in vitro prebiotic activity.
Section snippets
Seaweed sample
Air-dried wild Ascophyllum nodosum seaweed was provided by Acadian Seaplants Ltd. (Dartmouth, NS, Canada). The whole harvested seaweed was chopped and ground into a fine powder with a Black and Decker FP2500C food processor (Townson, MD, USA) for ~5 min at full speed, using a stainless-steel chopping blade (~14 cm diameter), prior to pre-extraction.
Chemicals
All chemicals used in this study were of analytical grade (Sigma-Aldrich, (St. Louis, MO, USA); BDH VWR Analytical (Radnor, PA, USA); Thermo Fisher
Extract yield
Extract yield is often used as a measure for the efficiency of an extraction. The yields of sodium alginate from the 4 extractions, CCE, MAE, UAE, and EAE/CCE were compared. From the results shown in Table 1, the Alg-MAE extract was significantly lower when compared to the Alg-CCE and Alg-EAE/CCE extracts. The study also showed that the Alg-EAE/CCE had the highest yield which was significantly (p < 0.05) higher than that of the Alg-MAE or Alg-UAE extracts. Whilst the yields may be expected to
Conclusion
Sodium alginate extracts may have prebiotic potential. From the results, Alg-UAE showed a significant advantage in the structural properties compared to the others. However, the alginate extracts from the MAE method had higher growth rates than the other extracts. The hydrolytic effects of the microwave method may have resulted in the production of shorter chain oligosaccharides. This may have increased the bio-accessibility of its carbon molecules, as nutrients for bacterial growth. Despite
CRediT authorship contribution statement
Chigozie Louis Okolie: Conceptualization, Investigation, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Beth Mason: Conceptualization, Supervision, Funding acquisition, Writing - review & editing. Aishwarya Mohan: Writing - review & editing, Project administration. Nancy Pitts: Supervision, Writing - review & editing. Chibuike C. Udenigwe: Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that there is no conflict of interest.
Acknowledgements
Financial support for this project was provided by Mitacs, a Canadian non-profit organization promoting industrial/government/academic cooperation (Ottawa, ON, Canada) and Acadian Seaplants Ltd. The authors are grateful to the Verschuren Centre research team and the Department of Chemistry, Cape Breton University for their assistance with laboratory resources and data interpretation. Special appreciation is given to Dr. Alan T. Critchley for his expert contribution in reviewing this document.
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