Molecular parameters of starch with varying amylose (AM) content.
Abstract
Osmotic properties of polysaccharides’ solutions and associated biopolymer-solvent and biopolymer-biopolymer type interaction are very important from a technological point of view. The knowledge of osmotic properties of these systems provides the basis to appropriate use of polysaccharides having comply with the relevant technology functions, impart the appropriate texture and forming the sensory properties of the final product. Furthermore, an important issue is the effect of time on the osmotic properties of polysaccharides’, because with time, the aforementioned effects may change. Membrane osmometry is one of the methods used in the studies of synthetic polymers to determine their average molecular mass and the degree of interaction between a polymer and a solvent. This method is successfully applied in the case of biopolymers that include polysaccharides. The existence of the osmotic pressure, formed by diffusion of solvent molecules through a semi-permeable membrane, is the basis of this method. Analysis and interpretation of osmometric research results is based on the van’t Hoff equation dependency of the concentration. The second virial coefficient obtained based on this relation allows characterisation of biopolymer-solvent interactions, and thus biopolymer tendency to solvation. The third virial coefficient provides information on mutual interactions between the biopolymer molecules, as well as its tendency to aggregate.
Keywords
- osmotic pressure
- virial coefficient
- overlap concentration
1. Introduction
Osmotic properties are part of wider group of colligative phenomena and concern the liquid-vapour equilibrium in multi-component systems. This group of colligative properties includes depression of the freezing point (cryoscopy), boiling point elevation (ebullioscopy) and osmotic pressure in general. The osmotic pressure can be associated with water activity of various types of products. The essence of the discussed phenomena is related to the changes of pressure of the saturated vapour (Figure 1), which causes the dissolution of the non-volatile substance.
If the non-volatile substance is a low-molecular-weight chemical compound, the changes in the vapour pressure can be explained by common phenomena, such as association or solvation, which are a result of interactions between molecules. The difference of the vapour pressure would in this case be directly proportional to the molecular mass of a dissolved substance. The osmotic measurements can therefore be used to determine the molar mass, or for multi-molecular substances with significant polydispersion, to determine the average osmotic molecular mass. The osmosis process takes place between the solution and a clear solvent, or between solutions of different concentrations, provided they are separated by a membrane, which is permeable only to the solvents molecules (Figure 2). The solvent moves through the membrane from the solution with lower concentration of the dissolved substance (or from the area of clear solvent) towards the solution with higher concentration. From the point of view of the molecules present in the solution, it is a phenomenon opposite to diffusion. From the solvents perspective, it is a natural intent to balance the chemical potentials, which results in the dilution of the solution with higher concentration. If the osmotic equilibrium takes place in a solution-clear solvent system, the dilution of the solution becomes so significant that the dissolved substances presence becomes suppressed and as a result, the pressure of solvents vapours over the solution becomes equal to the vapour pressure over the clear solvent. Because of that, the osmotic pressure is considered as one of the colligative processes.
2. Measurements of osmotic properties
The measurements of osmotic pressure can be carried out using two types of osmometers: membrane and vapour (Figure 3).
A membrane osmometer consists of two chambers divided by a membrane with specific pore sizes, which allow the solvent to move. One of the chambers is filled with pure solvent, while the other with the studied solution and the difference in the hydrostatic pressure between the two chambers is measured. These devices can be used to determine the pressure up to about 0.1 mmH2O, which in practice means measuring polymer and biopolymer solutions of up to 2000 kDa. The lower measuring range depends on the membrane’s permeability. In the case of these devices, the membrane itself is the source of the main issues. The membrane’s permeability depends on its structure: whether it has a system of pores (inorganic materials) or is a molecular sieve (organic materials). Permeability is usually expressed by the lowest molecular mass of a substance that the membrane allows through (
3. Osmotic equilibrium equation
The test results obtained from the osmotic pressure measurements are analysed based on the virial equation, which is mainly associated with real gases. While it might seem surprising, the validity of this approach is based on the nature of interactions between molecules, which move chaotically and collide with each other. In the real gases, the collisions between molecules are not elastic, and as a consequence, they shape the characteristics of the gas phase, so that they diverge from the Clapeyron’s law. For liquid solutions, a similar interpretation can be applied: the polymer/biopolymer molecules are incomparably larger than the solvent molecules (colloid solutions), and therefore, the solvent becomes the `background’ for the interactions between macromolecules. For a macromolecule-solvent system, the nature of the interactions is more complex. Numerous studies on the structure of macromolecules in solutions indicate to two possible behaviour patterns, dependent on the interactions between the solvent and the polymer/biopolymer chains (Figure 4A). As a result of those interactions, the polymer chain can undergo either expansion or contraction (collapse). The scenario is decided by the affinity to the solvent. The chain’s expansion is related to the absorption of the solvents molecules in between the chain’s segments and can be caused by solvation, electrostatic (Couloumb) interactions, formation of a helical structure or the presence of a spatial hindrance in the case of branched polymers (Figure 4A). A contraction caused by low affinity to the solvent induces a collapse, which is often accompanied by the aggregation of the chains or creation of a rigid branched structure, which leads to phase separation (Figure 4A).
The quality of a solvent is examined in close relation to a specific polymer/biopolymer. A good solvent (Figure 4B) causes the expansion of the chain in the solution, which in the range of concentrations
All the outlined consequences of the solvent-macromolecule and macromolecule-macromolecule interactions mean that the osmotic state equation should, as extensively as possible, take into account the interactions shaping the properties of a tested solution. Because of that, the applied virial state equation takes the following form:
Its form is analogous to the virial equation of real gas. From a mathematical point of view, it can be considered as an expansion of the function into a series, around a solution corresponding to ideal solution behaviour. In that equation,
4. Starch polysaccharides and osmotic properties of its pastes
Starch, which is produced and stored in plants seeds and bulbs, is one of the most important plant polysaccharides. It is composed of two types of biopolymers: amylose (AM) and amylopectin (AP). Amylose is a linear fraction composed of α-D-glucose units, bonded to each other through
Starch | AM, % | g | ||||
---|---|---|---|---|---|---|
Amylose* | 100 | 150–750 | 25 | – | – | – |
Potato | 24 | 51,000 | 222 | 8.96 | 7.9 | 0.166 |
Corn | 22 | 88,000 | 213 | 3.65 | 3.1 | 0.077 |
Waxy corn | 0 | 76,900 | 234 | 4.20 | 3.1 | 0.130 |
Amylstarch | 76 | 16,700 | 231 | 3.14 | 17.0 | 0.168 |
leads to a conclusion that AP chains exhibit smaller dimensions (
For linear amylose, the interactions between its chains result in a formation of cluster structures (Figure 4A). Because of that, the molecular masses determined with the use of light scattering measurements (SLS) are significantly higher than those determined chromatographically for a single chain. The level of aggregation of AM chains depends on the initial concentration, at the start of dissolution. Dissolution of AM in water requires application of high temperatures of 135°C, as well as pressure. The authors of the Ref. [2] study carried out autoclaving at 10–15 bar. These distinct conformations of AM and AP raise a question about the nature of these chains’ coexistence in water solutions: do amylose and amylopectin form separate structures and therefore are two separate components or whether they form a blend. Tests on solutions and pastes indicate the latter; however, phase separation is observed in those systems (Figure 6).
Starch solutions obtained with the use of DMSO do not exhibit incompatibility with the solvent [8]. This behaviour is a result of the fact that the chains do not exhibit a tendency to aggregate. The values of the average radii of gyration and
Starch | AM, % | |||
---|---|---|---|---|
Amylose1 | 100 | 108–235 | 24–31 | 0.87–0.94 |
Potato1 | 24 | 26,000 | 127 | 0.0148 |
Waxy potato2 | 0 | 4000 | 15 | – |
Waxy corn2 | 0 | 53,000 | 242 | – |
The mixture of DMSO and water can be used as a special solvent. These solutions exhibit a strongly non-ideal behaviour, due to the interactions between molecules. The methyl groups of DMSO may induce cooperative ordering in the system by hydrophobic hydration effects. The oxygen atom of a DMSO molecule can interact with water through H-bonding [10] with the continuum percolation transition in aqueous DMSO solutions with the percolation threshold of 12–15 wt.% of DMSO. A number of studies have been carried out over the years to understand the conformational properties of linear amylose [10–14] (Table 3) and branched amylopectin [14–17], in water and DMSO mixtures. The authors determined that the temperature and time of the dissolution have substantial influence on the weight average molecular mass, radius of gyration and the dispersion of the polysaccharide chains in the solution. With an increasing addition of water, the interactions between amylose and DMSO were reduced, leading to the conformational transition of AM from tight helical via loose helical to disordered coil [11]. The AP’s solubility is limited by the presence of water in the solvent [17]. Increasing water content not only limits the AP’s solubility but also causes an aggregation of its chains in the solution. In the case of starch solutions in binary solvents, the phenomenon of coil overlap occurs.
Solution H2O/DMSO | Reference | |||||
---|---|---|---|---|---|---|
100/900 | 765 | 37.5 | 272 | 2.192 | ||
AM | 200/800 | 660 | 38.8 | 276 | 1.952 | [16] |
500/500 | 555 | 34.0 | 123 | 1.304 | ||
700/300 | 506 | 26.3 | 55.6 | 1.189 | ||
100/900 | 151 | 84 | – | [13] | ||
100/900 | 15,300 | 99.8 | ||||
300/700 | 57,500 | 182.3 | [17] | |||
AP | 500/500 | 192,700 | 182.8 | |||
100/900 | 171,000 | 238 | [18] | |||
100/900 | 150,000 | 238 | 0.055 | [13] |
In the case of utilising pure DMSO or DMSO/water mixtures, it is possible to achieve a decidedly higher solubility of starch and its derivatives than for pure water—up to 50% (w/w) starch solutions in pure DMSO [20] and in binary solvents ([21], the below photo from own studies).
For amylose solutions (Figure 7), a change in character of the
Day | AM | AP | Starch native | Ac | Ph | E1404 | ||
---|---|---|---|---|---|---|---|---|
30°C | 1 | 2.05∙10−6 | 1.34∙10−5 | −1.09∙10−5 | 4.17∙10−6 | 1.06∙10−6 | −1.84∙10−6 | 7.47∙10−7 |
2 | 4.42∙10−5 | 4.17∙10−5 | ||||||
8 | 4.17∙10−5 | 4.48∙10−5 | ||||||
40°C | 1 | 4.25∙10−5 | 1.87∙10−5 | |||||
2 | 3.32∙10−5 | 2.62∙10−5 | ||||||
8 | 2.68∙10−5 | 3.07∙10−5 |
4.1. The effect of time on osmotic properties of pastes
Storage of the pastes in room temperature causes changes in the interactions between their components (Figure 8). The results of osmotic pressure measurements at 30°C carried out 24 h after the first measurements (
No qualitative changes in the course of the
The measurements of
Chemical modification of starch causes changes in the interactions between polysaccharide chains and water (Figure 10). For phosphorylated potato starch, the reduced osmotic pressure has negative values. This behaviour is different to the native potato starch, as for the tested range of concentrations (0.050–0.075 g/100 mL), the positive values of osmotic pressure were not observed. Based on that, it is possible to speculate that phosphorylation has caused the change to the overlap concentration, at which values of
Acetylation of starch, similarly to phosphorylation, causes a change to the concentration at which the values of osmotic pressure are positive. This results in a change of osmotic properties, which is in accordance to the results presented by Żmudziński et al. [22]. The values of reduced osmotic pressure are negative for concentrations in the range from 0.050 to 0.065 g/100 mL and positive from 0.070 to 0.080 g/100 mL. The correlation obtained for the tested systems is ascending for the whole range of concentrations. Positive values indicate an increase of water absorbability of the tested solutions. The second virial coefficient calculated for these systems has a positive value. An increase of the affinity of the solvent molecules to the polymer chains occurs, which means that water is a good solvent for acetylated starch.
For solutions of starch oxidised in a microwave radiation field, the obtained
The values of the reduced osmotic pressure obtained for the oxidised starch (E1404) are the highest of all tested systems; however, the obtained correlation is descending. It is worth noting that the concentration range included higher concentrations than for the other systems. A common concentration for the tested starches is 0.080 g/100 mL, for which the value of
5. Non-starch polysaccharides
The non-starch polysaccharides group includes gums of various plant origin: locust bean gum, konjac, guar gum and carrageenans or bacterial origin polysaccharide: xanthan and chemical modified cellulose: carboxylmethylcellulose.
Locust bean gum (carob gum) is obtained by the milling of endosperm of the seeds from the carob tree pods (
Guar gum is a non-ionic polysaccharide obtained from the endosperm of guar seeds (
Konjac gum is obtained from the tubers of the
Gum arabic is a natural plant sap excretion harvested from acacia trees (
Xanthan gum is an extra-cellular polysaccharide produced by the
Carboxymethylcellulose (CMC) is a half-synthetic anionic polysaccharide obtained by chemical modification of cellulose. During the process, a partial substitution by carboxymethyl group occurs of the second, third and sixth hydroxyl groups in cellulose. The linear chains of CMC are formed of glucose units joined by β-(1,4)glycosidic linkage. Average substitution CMC level is defined as an average number of carboxymethyl groups repeating unit. This parameter is defined in the range 0.4–1.5. This polymer is usually available in the form of sodium salt, and the product soluble in water is characterised by the substitution level above 0.5. Molar mass of this polymer is varied, for example, it can be in the range 2.5∙10−5–7.0∙10−5 g mol−1. CMC is widely used in the food, cosmetic, paper, textile and drilling industries [76, 77].
Carrageenans are a group of polysaccharides, in which 15 types can be distinguished, differing between each other based on structure. These polysaccharides are obtained by alkaline extraction of red edible seaweeds. The ι-, κ-, λ-carrageenan are mainly used in practical application. The main native polysaccharide chain of these three fractions is formed by a repeated poly-(1,3)-[4-sulphate-β-D-galactopyranose-(1,4)-3,6-anhydro-2-sulphate-α-D-galactopyranose]. κ-Carrageenan poly-(1,3)-[4-sulphate-β-D-galactopyranose-(1,4)-3,6-anhydro-α-D-galactopyranose]. λ-Carrageenan poly-(1,3)-[2-sulphate-β-D-galactopyranose-(1,4)-2,6-anhydro-α-D-galactopyranose. κ- and ι-Carrageenans, as opposed to the lambda fraction, form thermo-reversible gels. Above the melting temperature, the chains of this polymer occur in the form of coils, and during the lowering of temperature, the coils change the conformation to double helices and form larger aggregates and thus form a spatial gel network structure [78, 79].
5.1. Osmotic properties of nonstarch polysaccharides’ solutions
The tests on the osmotic properties of the polysaccharides water solutions were carried out using a membrane osmometer at two temperatures: 30 and 40°C. For the solutions of locust bean gum (LbG), the relation of the reduced osmotic pressure (
Reference | 30°C | 40°C |
|
|||
---|---|---|---|---|---|---|
KG | 105–106 | 0.08 | [87, 89] | −2.52∙10−6 | −8.14∙10−7 | 0.24∙105 |
GG | 9.1∙105 | 0.13 | ||||
7.3∙105 | 0.28 | −2.43∙10−7 | −5.40∙10−7 | 1.6∙105 | ||
4.0∙105 | 0.45 | [70] | ||||
2.7∙105 | 0.45 | |||||
LbG | 0.697∙106–0.94∙106 | 0.4 | [71] | −2.02∙10−7 | −2.20∙10−7 | 1.4∙105 |
1.94∙106–2.29∙106 | 0.33 | [72] | ||||
XG | 0.03 | [72, 84] | −1.36∙10−4 | −8.02∙10−5 | ||
CMC | 3.5∙104–1.7∙106 | – | [68] | −9.84∙10−5 | −1.58∙10−4 | 2.57∙106 |
AG | 5∙105–6.5∙105 | – | [74] | 2.80∙10−7 | 1.07∙106 | |
CA | 6.6∙103–1.2∙106 | – | [75] | −8.41∙10−6 | 0.08∙105 |
In the case of guar gum solutions (GG), the measurements were done in the same range of concentrations as for LbG, both <
Xanthan gum solutions (XG) were tested in the range of concentrations below
The correlation of
In the case of carboxymethylcellulose (CMC) (Table 5) significantly different behaviours are observed, than in the above-mentioned examples, as the obtained correlation is ascending in the whole range of concentrations (0.005–0.02 g/100 mL) and temperatures (Figure 14). Slow increase of the reduced osmotic pressure is a result of high viscosity of the solution, which hinders the migration of the solvent in the solution. High values of
For gum arabic, the relation of
The
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