Effect of extraction conditions on the yield and purity of ulvan extracted from Ulva lactuca

Abstract

A study of the influence of extraction conditions (pH: 1.5 and 2; temperature: 80 °C and 90 °C; extraction time: 1–3 h), on the yield, chemical composition and purity of the sulphated cell wall polysaccharides ulvan, extracted from the green seaweed Ulva lactuca and precipitated by alcohol is carried out. The alcohol precipitate yields varied from 21.68% to 32.67% (%dw/dw) depending on the pH. At pH 2, the alcohol precipitate yields and the uronic acid recovery from extract juice are higher than those obtained at pH 1.5. Other compounds than ulvan such as cellulose, hemicellulose, proteins and ash are solubilized from the cell walls of Ulva lactuca at both pH, and they are precipitated with alcohol. The alcohol precipitate obtained from different extraction conditions has high uronic acid (20.37%–23.60%) and neutral sugar content (20.09%–29.12%), especially when the conditions (pH, temperature) are drastic. It contains rhamnose (13.35%–15.59%), glucose (2.90%–10.97%), and xylose (2.36%–2.73%). A decrease in the molecular weight of ulvan was observed at acid pH, and for long extraction times. The presence of proteins (1.94%–2.32%) and inorganic material (33.36%–47.15%) in alcohol precipitate prove the lower purity of ulvan extracted and shows that ulvan precipitation with ethanol is not specific.

Introduction

Green seaweed Ulva lactuca represents an important biomass available from proliferating algae in eutrophicated coastal water between the littoral of Taboulba and Sayada in Tunisia. It is characterized by a very interesting chemical composition and especially by its high content of cell wall polysaccharides (54% on the dry weight basis). However, the bioaccumulation of minerals and more especially the heavy metals (manganese, lead, copper and cadmium) in this algae, don't allow this seaweed to be used for human consumption or as an ingredient in some food preparations (Yaich et al., 2011). For this reason, the valorisation of this alga can be made only by the extraction of its different components such as the water soluble Ulvan. This polysaccharide displays physico-chemical and biological features of potential interest for diverse applications (Lahaye & Robic, 2007). Notably it forms unusual soft gels in the presence of divalent cations and borate (Lahaye & Axelos, 1993). Ulvan is mainly built on disaccharides repeating sequences composed of sulphated rhamnose and glucuronic acid, iduronic acid or xylose. The two major repeating disaccharides are aldobiuronic acids designated as type A: ulvanobiuronic acid 3-sulphate (A_3S) and type B: ulvanobiuronic acid 3-sulphate (B_3S). Partially sulphated xylose residues at O-2 can also occur in place of uronic acids. In addition, glucuronic acid can branch at O-2 of rhamnose 3-sulphate. Low proportions of galactose, glucose and protein are also generally found in ulvan (Quemener, Lahaye, & Bobin Dubigeon, 1997; Robic, Rondeau-Mouro, Sassi, Leart, & Lahaye, 2009).

Based on literature, ulvan is extracted by chemical methods using hot water often containing a calcium chelating agent such as sodium oxalate (Lahaye & Robic, 2007; Robic, Sassi, Dion, Leart, & Lahaye, 2009) or acidified ammonium oxalate (Robic, Rondeau-Mouro, et al., 2009) or alkali (Ray & Lahaye, 1995) or sodium chlorite or DMSO or phenol/acetic acid/water or acid (HCl) in very determined conditions without studying the influence of the variation of the extraction parameters such as pH, time and temperature in the same extraction conditions on the physicochemical characteristics of ulvan extracted. According to Robic, Rondeau-Mouro, et al. (2009), Robic, Sassi, et al. (2009), the sodium oxalate (50 mM, pH 6.5, 2 h, 85 °C), the acidified ammonium oxalate (20 mM, pH 4.6, 1 h, 85 °C) and acid extraction (50 mM HCl, 0.5 h or 1 h at 85 °C) are characterized by higher extraction yields and more ulvan extraction efficiency (percentage recovery of the initial rhamnose content in raw material) than other extractions using others solvents. Moreover, the uronic acid, the neutral sugars content and the molecular weight distribution of the ulvan are depends strongly on the extraction conditions.

In this context and in order to identify the conditions improving ulvan extraction without degradation, we study the influence of the acid extraction conditions by varying the pH (1.5 and 2), the temperature (80 °C and 90 °C) and the time (1 h, 2 h and 3 h) on the extraction yield and on the molecular weight distribution of ulvan extracted from U.lactuca.