Larch Arabinogalactan (AG) is a fiber supplement derived from the wood of the Larch tree. AG is a water-soluble polysaccharide that has tremendous technological importance in the fields of lubrication, colloidal science, and biofouling [1]. AG solubility is of great interest for many applications such as coatings, pharmaceutics, personal care, foods, and drinks, which incorporate polysaccharides dissolved in aqueous and/or organic solvents [2]. The dissolution of AG strongly depends on the solvent used and its conformations depend on the interactions with its local environment [3,4]. The choice of the appropriate solvents is an important preliminary step for any industrial or pharmaceutical process. Therefore, various approaches have been proposed to estimate the solvent power and predict the polymer solubility [2,3-5]. Aqueous two-phase systems provide a mild and selective method for the extraction of biological materials that have found extensive use in the laboratory. Understanding the solubility and the behavior of polysaccharides in the aqueous two-phase medium is extremely important for their food applications as most functions of polysaccharides include stability, emulsifying property, drug delivery, and membrane-forming properties. In this work, we aim specifically to study the interactions formed between the studied polysaccharide and the local environment and the mechanism of polysaccharides' solubility from the molecular level. General understandings of the solubility including definition, testing methods, and solution behaviors were provided; the relationships between polysaccharide solubility and the structural features in terms of molecular weight, degree of branching, charging properties, chain flexibility, and the special groups were all discussed. With all the information provided, molecular modification and further applications of polysaccharides in both food and non-food areas could be promoted.

Here, larch arabinogalactan (AG) is a long-chain polysaccharide, densely branched, and high molecular weight polysaccharide mainly composed of galactose and arabinose residues in a 6:1 molar ratio with small amounts of glucuronic acid [6-8]. This polysaccharide which is significantly present in Larch trees (Larix occidentalis) has potential therapeutic benefits as an immune-stimulating agent and cancer protocol adjunct [9,10]. Arabinogalactan is approved by

the US Food and Drug Administration (FDA) as an excellent source of dietary fiber and for use in food products [11,12]. Figure 1, illustrates the molecular structure of AG.

Figure 1: Molecular structure of Larch Arabinogalactan.

Arabinogalactan from larch wood was purchased from Sigma-Aldrich (Switzerland) and used without further purification. Its weight average molar masse M_w was 47 KDa. Sodium hydroxide (NaOH, 98-100.5%, pellets) and HCl were purchased from Sigma-Aldrich and used to prepare the pH solutions. The de-ionized water was previously treated with a Millipore-Q water purification system. Methanol from Sigma Aldrich was used as received.

Herein, we will be interested in the determination of the influence of various parameters such as; the concentration of methanol, the pH, and salt concentration, for a concentration of AG of 1 mg/ml. The Physical properties of AG were characterized based on the DLS, turbidity, and fluorescence techniques. These techniques give more precious information regarding the size and interactions of AG chains.

larch tree. The sample solutions at a concentration of 1mg/ml were dissolved in water solutions with different percentages of methanol. These solutions of AG were filtered through a PTFE filter with 0.45 µm pores. The polymers were dissolved in solvent filtered through a polyamide filter with 0.22 µm pores. Since the solutions were dilute, this was enough for the preparation of the dust-free samples. To vary the pH, the arabinogalactan solutions were adjusted by the addition of a strong base sodium hydroxide (NaOH) or a strong acid hydrogen chloride (HCl). All pH measurements were made at room temperature using a consort C862 pH meter (Belgium). This pH-meter offers a resolution of up to 0.001 pH unit and a pH electrode type SP10T, 3 M KCl. Temperature compensation (0 °C to 80 °C) was immersed for 10 h more in 3 M of KCl solution and it was calibrated at 3 points (pH = 4, pH = 7, and pH = 10) [13]. After mixing, the ionic strength was adjusted by adding NaCl. All measurements were measured after 24 h with the aim that all the prepared solutions reached their stability. The constancy of the pH of the solutions was verified by measuring it before and after the fluorescence measurements. The measurements were repeated after one week to ensure the stability of the solutions. All solutions were stored at 4 °C before any measurements.