Alginates are carbohydrate polymers which are commonly used in food and beverages, cosmetics, wound dressings and pharmaceutical applications to provide stability and texture, to carry drugs and for protection. They have also been applied in biochemical engineering for the immobilization of enzymes, cells, as scaffolds for cell growth, in bioinks etc.
Introduction
Alginate was first isolated by Stanford in 1881 because he was looking for useful products in kelp. He developed an alkali extraction process which produced a viscous material he termed ‘algin’. This was precipitated later in his studies using mineral acids and would be termed alginic acid. A few years later, Krefting in 1896 extracted algin formally. By 1929, Kelco (California, USA) had the first production process which they sold as a boiler compound and for can-sealing. In 1934, alginate was starting to be used as an ice-cream stabilizer having received approval in foods.
Alginates are extracted commercially from brown seaweeds. The main sources are Laminaria hyperborea, Ascophyllum nodosum and Macrocystis pyrifera. These polymers are included in a group of compounds that are generally considered safe by the FDA i.e. have GRAS status. Their wide spread use occurs because of the absence of toxicity associated with them and their versatility as the ‘go to’ solution for a number of applications.
Alginate is a carbohydrate polymer, a polyelectrolyte which forms hydrocolloids. The structure is a linear copolymer of the sugars, d-mannuronic acid designated as M, and l-guluronic acid, designated as G. The linkages are all 1,4-linked.
Some soil-borne bacteria also produce alginates in their capsules – strains such as Pseudomonas and Azotobacter are grown and fermented for commercial purposes (Clementi, 1997; Stephens et al., 2006).
The application of alginates is for the most part based on three particular properties:-
- to modify viscosity because they dissolve easily in water and can thicken a solution.
- the formation of gels.
- to form films and fibres.
The properties of alginates are reviewed by Nussinovitch & Nussinovitch (1999) and Draget (2009) but there are many books devoted to all levels of properties and applications. This article only covers some pertinent aspects and it will be added to as significant information comes to hand on the subject. Alginates certainly do have ‘power’. We now delve further into their significant properties and uses.
Structure
Alginate is a linear and binary copolymer of (1→4) linked β-D-mannuronic acid (M) and α-L-guluronic acid (G) sugars. These are classified as uronic acids and their anionic nature as we see later has the most significant impact on their performance.
The M and G residues are arranged in homopolymeric sequences of either these two types of residues. This arrangement is usually in a non-regular, block-wise fashion along the chain. There are three types of blocks commonly referred to as M or G blocks, which are interspersed by MG alternating blocks. The MG alternating block is also described as a heteropolymer and contains random arranged units of M and G. The distribution of the residues as well as the proportion or ratio of M to G residues varies greatly with algae species (Fang et al., 2007). The physical properties and physiological bioactivity depends too on this ratio and the block arrangement (Morris et al., 1973).
The three types of blocks have all been characterised by partial hydrolysis with hydrochloric acid.
Pure polyguluronate, forms a highly buckled two-folded zigzag chain that is independent of charge or counterion (Atkins et al., 1973b; Mackie et al., 1983). A flat ribbon like two-fold conformation is observed for the pure polymannuronic acid, which converts to an extended three-fold ribbon upon addition of salt (Atkins et al., 1973a).
Polypropylene Glycol Alginate (PGA)
PGA was first prepared by Steiner (1947) and the process of manufacture improved by Steiner and McNeely (1950). This was to improve the versatility of alginates further by reducing their overall negative charge. It’s a common practice too in pectin production where amidation and esterification are typical reactions. PGA is prepared by partial esterification of the carboxylic groups on the uronic acid residues by reaction with propylene oxide.
PGA produces stable solutions in acidic pH especially whilst the equivalent unmodified alginate would normally precipitate. This is due to the lower fraction of carboxyl groups that can be protonated. The inability to precipitate is a positive benefit because it means it is added to products which are acidic (acidic pH) or calcium-rich that could not use sodium alginate. Adding bulky groups prevents any inter-molecular interaction.
Adding ester groups to the molecule increases its lipophilic or hydrophobic character. That alters its surface activity which makes it suitable where emulsification would normally be needed. It achieves this by reducing the surface tension of the air/water interface. This also implies it can be used to stabilize milk proteins under acidic conditions. PGA is also used for stabilising pulp and oil rings from colours and flavours in acidic beverages. When used in combination with other gums such as xanthan gum, it can also stabilise emulsions (Yilmazer et al., 1991). The modified alginate is also a suitable viscosity modifier; its addition to a model salad dressing for example reduces its viscosity.
There is also the implication that high levels of esterification produce a reduction in gelling ability. It is one of the reasons why PGA is used in particularly acidic food products: it stabilises acid emulsions such as dressings, and fruit beverages. We have noticed that the majority of the commercially available PGAs have a high degree of esterification and as a result of this, a poor gelling ability. Propylene glycol groups in G-blocks will disrupt the formation of functional crosslinking zones. It has been reasoned that at low and intermediate degrees of esterification some of the gelling potential of the PGAs will be conserved, making it possible for the polymer to simultaneously act as a surface active agent and gelling agent. There is the possibility for further work on the application of these PGAs with much lower degrees of esterification.
The viscosity of PGA is such that it shows a low degree of non-Newtonian behaviour unlike some of its counterpart gums at the same concentration such as xanthan gum (Coia and Stauffer, 1987) which show pseudoplastic behaviour. The viscosity behaviour is probably as much a function of its degree of polymerization of the alginate chain and indirectly the molecular weight. It is noticeable that when it is used to replace gums in applications, the emulsion that may be formed is more fluid-like and shows reduced viscosity (Pettitt et al., 1995). Any remaining unesterified acid groups will have an anion charge as low as pH 2.75. It means they can still participate in weak but significant cross-linking with calcium and proteins which carry a cationic (positive) charge.
For EU ingredient purposes, it has the E number 405.
Extraction
Sodium alginate is commonly extracted from seaweed in the Phaeophyceae family. This polymer forms up to 40% of the dry matter. The seaweed of all types and species is cultivated globally. About 30,000 metric tons are produced every year.
The seaweed is usually mechanically collected and dried. The alga is milled to a rough particulate. The seaweed is pretreated and soaked with formaldehyde (2% v/v) and ethanol prior to extraction in half the extraction processes (Trica et al., 2019). Mild heating up to 40ºC is used for 24 hours to soften the seaweed walls, remove pigments and polyphenols. Adding formaldehyde and increasing the soaking time produces alginates with higher intrinsic viscosity which is a reason for its use (explained later). Formaldehyde also removes coloured polyphenols too. Any soaking over 24 hours brings no further reward. It is thought that the removal of phenolics and other compounds may be the reason for the increase in viscosity. Phenolics cause the carbohydrate to degrade. These bind to formaldehyde producing an insoluble polymeric solution which can be removed.
The alginate is then treated with dilute acid or alkali. A typical acid bath immersion is 0.5N sulphuric acid for several hours. This removes any neutral homopolysaccharides (fucoidans, laminarins), proteins as well as more polyphenols. It also exchanges any alkaline earth cations such magnesium and calcium (Periodic Table – Group IIA) with protons prior to alginate extraction to bring it from the alginate form into the alginic acid form. Alginic acid is very insoluble and precipitates within the cell walls of the alga.
The alkaline extraction step is the critical step of the process because it converts insoluble alginic acid (in its protonated form) to the soluble sodium alginate. It always involves sodium carbonate. The sodium carbonate is added to below pH 10. It requires several hours to reach the optimum extraction yield, depending on the seaweed species considered. Seaweed material is then separated from the sodium alginate solution using floatation and/or flocculation with ultrafiltration or by centrifugation and filtration.
Sulphuric acid or calcium chloride is then added again to precipitate alginates in their acid or calcium salt form, respectively, with the latter being easier to dewater. An alternative for similar reasons is to add ethanol (Donati & Paoletti, 2009; Chee et al., 2011) which also acts as a precipitant. That approach is much more common in pectin manufacture. It is often purified further, pressed and dried with heat.
The second type of extraction process may involve neutral conditions using calcium sequestrants such as EDTA. The alginates generated by both methods have enhanced viscosities (Wedlock & Fasihuddin, 1990).
Alginates are available commercially mostly as a sodium alginate salt or as the acid form. Different salts are produced by reacting alginic acid react with different bases. So, potassium chloride and ammonium chloride might be used.
As we alluded to earlier, the intrinsic viscosity and molecular weight of alginate samples during extraction decrease significantly with increasing pH i.e. alkali extraction, due to the interactions of phenolic compounds and with temperature. The optimal duration of processing is determined by the type of seaweed being extracted. By isolating the phenolic compounds with formaldehyde or extracting them with ethanol, degradation processes can be inhibited. This is down to preventing the polyphenols acting as free radicals which are also co-extracted with the alginate. Free radicals reduce the molecular weight of the polymer by encouraging cleaving via initiation of oxidation and reduction reactions. The reaction is worse too in highly acidic pHs, irrespective of the elevated temperatures. There may also be an involvement with microbial damage and the presence of activated endogenous alginate lyases.
Removal of colour is vital if the product is to be attractive enough for sale. The polyphenols along with other coloured material such as chlorophyll and carotenoids will produce an unpleasant dark brown discolouration during the alkaline treatment. This often leads to a dark brown powder. Formaldehyde treatment is effective for most but bleaching pretreatments can fix such coloured compounds into algal tissues to reduce the browning. They will unfortunately affect the molecular weight and M/G ratio (Mohammed et al., 2020).
Ultrasound has also been tested as a means of assisting in extraction of sodium alginate from the seaweed Ascophyllum nodosum (Montes et al., 2021). This is part of a greener strategy to encourage recovery of the polyphenols for other uses.
Solubility, Rheology And Viscosity
Alginates perform similarly to other polyelectrolytes such as pectins and carrageenan in solution. Solutions of sodium alginate have been investigated for their viscosity and osmotic pressure effects since the 1950s (Donnan & Rose, 1950). The measurement of osmotic pressure to determine the molecular weight of polymers is well established.
The rheology of alginate solutions in aqueous and model systems is measured using all the typical analytical methods including rheometers and texture analysers. Model liquid systems based on emulsions such as mayonnaise are good systems for study (Mancini et al. 2002).
The study by Mancini used a controlled stress rheometer. They did dynamic oscillatory tests in strain sweep and frequency sweep modes. The rheological parameters were storage (G′), loss (G″) and complex (G*) moduli and the strain, which determined the limit of the linear viscoelastic region (γc).
A typical viscosity can produce values as low as 10 cps for a 1% solution of a low viscosity alginate and up to 2000 cps for a high viscosity material of the same concentration. For commercial alginates, a 1% w/v aqueous solution of sodium alginate has a dynamic viscosity of between 20–550 mPa·s at 20°C.
Alginate solutions become less viscous with heating but will degrade when held at too high a temperature for long periods of cooking. They usually regain their viscosity when allowed to cool assuming heat degradation has not been allowed to occur.
The pH of the alginate solution has little impact on viscosity variation especially over the pH range of 4 to 10. Below pH 4, viscosity tends to increase because of the lower solubility of the free acid form and eventually alginate precipitates as alginic acid. As discussed earlier, propylene glycol alginate solutions are more robust especially at the lower pH range (below pH 4) because they have less tendency to precipitate or form strong gel structures.
The value for the intrinsic velocity is linearly related to the degree of polymerization as well as molecular weight. Studies have been conducted on viscosity to establish the behaviour of the polyelectrolyte when it comes to electrospinning (Dodero et al., 2019).
When low molecular weight alginates are solubilized they show Newtonian flow behaviour at low shear rates. As the degree of polymerization rises or their concentration in solution increases, the behaviour becomes more pseudoplastic, irrespective of the shear rate.
Alginate solubility is limited by the solvent pH. A decrease in pH below pKa 3.38–3.65 will lead to polymer precipitation. The level of solubility is maximal at 3.5% for a low-viscosity alginate which has a degree of polymerization of ca. 80. Solubility is also limited by ionic strength, and the content of “gelling ions” such as calcium.
As the heterogeneous structure based on MG-blocks increases, it becomes more soluble in acidic pH solutions compared to just alginates of mostly poly-G or poly-M. These latter more homogeneous types precipitate too often readily under such conditions.
There are odd peculiarities associated with alginates in solution. The viscosity of dilute sodium alginate aqueous solutions has been measured at various temperatures (Zhong et al., 2010). There is an upward bending phenomenon in the reduced viscosity of sodium alginate solutions in the dilute concentration region which has been linked to adsorption of the polymer to the glass capillary wall used in viscosity measurement.
The thermodynamic reasons for the high solubility of sodium alginate is because of a high entropic contribution from the free or noncondensed counterions. It also means that the presence of a high amount of supporting salt is a limiting factor in the solubilization of this polyelectrolyte (actually a polyanion). Increasing the ionic strength of a solution of alginates starts to counter the entropic gain arising from the polysaccharide counterions. If potassium chloride is added it eventually causes the alginate chains to lose solubility leading to phase separation. This is a good example of a salting out effect and is used to fractionate alginates. It is also suggested that ions have a strong negative effect on the kinetics of alginate dissolution because they serve to reduce the water chemical potential difference between the alginate polymer and the liquid medium.
The dissociation constants of mannuronic and guluronic acid are 3.38 and 3.65 respectively (Haug, 1964). So, pH has an impact as is often stated. Inorganic acids will protonate the uronic acids in alginate. When the pH of any alginate solution is lowered below the pKa of the uronic acids, phase separation and hydrogel formation is promoted. Alginates rich in MG-blocks have a reduced tendency to phase-separate at acidic pH values when compared with G- and M-rich alginates. This is probably because of the higher conformational disorder of the glycosidic bonds (Hatmann et al., 2006).
In summary, apart from molecular weight and degree of polymerization, the alginate capability of creating viscous solutions varies according to a host of factors which all play their part in selection of the right alginate for the job. You need to consider solution concentration, solvent pH-with a maximum pH reached around 3.0–3.5, temperature, and the presence of divalent ions (Draget et al., 1994).
Molecular Weights
Cook & Smith (1954) measured the molecular weights obtained from sedimentation–diffusion and sedimentation studies. The molecular weight ranged from 46,000 to 370,000 g/mol. This study also showed that sodium alginate has a relatively high extension ratio.
The intrinsic viscosity has also been used as a measure of molecular weight from about 100,000 to 270,000 g/mol – here these measurements were performed in aqueous salt solutions (Smidsrod, 1970).
The commercial alginates have an average-weight molecular weight Mw of approximately 200,000 but all those early studies indicate that molecular weights up to 400,000 and 500,00 are easily possible. The actual molecular weight of native alginate is not known because extraction always causes degradation (Donatti & Paoletti, 2009).
A polydispersity index polymer is calculated as the ratio of weight average to number average molecular weight i.e. Mw to MN. The ratio is 1 in the case of a perfectly monodisperse polymer solution such as a protein. For polydisperse solutions where Mw > MN, the index is above 1. Alginates have a polydispersity index of between 1.5 and 3 but a value of 6 has been reported (Draget et al., 2005).
The NG>1 values measure the average block length (ASTM, 2012). M/G and NG>1 values vary among species, within the different parts of an organism, life stages of seaweeds, and extraction and isolation procedures. M/G ratio, among other properties, can be used as an index for the alginate gels physical properties, for example, lower values (<1) produce stronger gels (Gomez-Ordonez & Ruperz, 2011).
Preservation
Sodium alginate solutions can be preserved using sodium benzoate and potassium sorbate in solution. Other materials include silver nanoparticles (Mohammed Fayaz et al., 2009).
There is evidence that sodium alginate in solution reduces the oxidation of vitamin C (ascorbic acid) to dehydroascorbic acid (Ericson & Gasparetto, 1953).
Film Forming Behaviour
Alginates are used as edible coatings. The films are especially useful in protecting soft fruit from degradation and from ripening. Films generally will extend shelf-life by minimising dehydration, reducing or controlling respiration and improving the mechanical properties of soft fruit in particular by reducing damage. Alginate has delayed ripening in tomato (Zapata et al., 2008).
The solubility of edible films is claimed to be one of alginates most important properties (Rhim, 2004). The effects of different ratios of mannuronic to guluronic acid has been studied in sodium alginate gels (Russo et al., 2007). The increasing degree of chain-to-chain interaction produces films of increasing brittleness.
To produce pliable films, an emulsifier and plasticizer might need to be added such as glycerol. Typical emulsifiers include Tween 80 (1%w/w).
Alginate gel (2% w/v) infused with pomegranate peel extract is effective at protecting avocado and guava which are prone to microbial damage (Nair et al., 2018).
Alginate can be conjugated with chitosan to improve adhesion to cell membranes and other organic systems which have a net negative charge. A chitosan-alginate polyelectrolyte complex (CS-AL PEC) has been formed that is water insoluble and claimed to be more effective for releasing encapsulated ingredients compared to just alginate or chitosan films on their own (Yan et al., 2002).
Chitosan is positively charged which helps in adhesion. It will also form hydrogel beads which can be used in drug release systems. It also forms scaffolding material in cartilage tissue reengineering.
Alginate-poly-l-lysine-alginate is used to prepare capsules which enclose living cells and protect them from immune rejection when transplanted into animals. This eliminates the requirement for too high a level of immunosuppressive therapy (Ma et al., 1994).
The physical performance of propylene glycol alginate with soy protein isolate has been explored (Rhim et al., 1999).
The 3-D Network
In acidic conditions, alginates have a negative charge and are ideal for forming complexes with polyvalent cations including metal ions such as calcium and even aluminium. These cations serve as bridges between the anionic polymer chains. Such bridges are also termed as junction zones so that a hydrogel network can be formed (Sinjan and Robinson, 2003).
Calcium and some other polyvalent cations preferentially bind to the carboxylate groups in the G blocks rather than the M blocks although both sugars play a significant role in complexation. Typically the “egg-box” model has been used to explain the Ca-alginate junction zones; however, recent work has suggested that this is not the only possible structure for the junction zones (Li et al., 2007). We’ll note later on if we haven’t stated already, that a higher ratio of G to M produces stronger gels.
Gel Formation
Gel formation and thickening with alginates is a difficult distinction to make. A thick alginate solution is actually likely to be a very weak gel and vice versa. The benefits especially for alginates are because there is a controlled interaction between sodium alginate and calcium salts producing cold-setting gels that are shear irreversible and heat stable. Control is affected using citrate or phosphate sequestrants, or by processing at temperatures above about 70°C and cooling.
Alginates will only form gels in the presence of divalent cations such as calcium and magnesium. The gels fall apart in the presence of excess monovalent ions, when calcium-chelating agents are present and harsh chemical environments exist.
An alginate gel is heat stable. The strength of these gels is roughly independent of the molecular chain length but the chains do need to be above a certain length. The degree of polymerization should be above 200 to achieve optimal gel strength.
The formation of an alginic acid gel is formed in a similar way to a calcium alginate gel. In this gel it involves junction zones where protons in the form of added acid are part of gel structure. In an alginic acid gel, hydrogen bonds form between the homopolymeric regions of adjacent alginate molecules. There is a minimum number of consecutive intermolecular that must be formed in order to create a stable junction zone. The G-blocks as in calcium alginate are the most effective building blocks for junction formation.
Product developers need to look for alginates with a high guluronic acid (G-content) because they produce gels of greater strength than those richer in mannuronate (M-content) (Draget et al., 1994). The G-block fraction of the molecule binds calcium ions with greater preference than the mannuronate fraction. A high fraction of homopolymeric blocks seems to favour the formation of junction zones.
Alginates with a higher M content are preferred for viscosity related applications. High M/G ratios are the signature of alginate producing an elastic gel whereas low M/G ratios provide brittle gels. The length and ratios of the G blocks does not always correlate with the M/G ratio (Grant et al., 1973; Rianudo, 2007).
The alginate gel strength is measured three different ways:-
- FIRA value (ml water/30°deflection) using a FIRA Jelly Tester
- initial deformation (Youngs modulus, E) using a texture analyser
- rheometer to measure the dynamic storage modulus G′
Laminaria hyperborea stem alginate has a FIRA value of 65 to 75, the whole plant is 55- 65, the leaf is 40 to 50 and Ascophyllum nodosum has a gel strength of 25 to 35.
There are two approaches to create an alginate gel. The first method produces an externally set alginate gel. This type of gel is most commonly prepared by surrounding the alginate solution in a highly soluble calcium bath – calcium chloride (2%-5% w/v) is standard. This allows the calcium to set the alginate from the outside in, and it allows for the gel to be molded into the desired shape. The second method produces an internally set alginate gel. This relies on the calcium source being sequestered before it has a chance to mix with the alginate. The alginate needs time to hydrate first before the calcium is released from the sequestering agent. This is achieved by either choosing a minimally soluble calcium source or by adjusting the pH once the solution has hydrated. This is another advantage of sodium alginate, because gel formation can be delayed by adding a sequestrant, like phosphate.
Encapsulation
Alginates are extensively used to encapsulate a wide range of materials including cells, enzymes, nutraceuticals, etc. The gum has unique properties for an encapsulation gel because of the interaction with multivalent ions which cross-link with it to form stable hydrogels. These gels are formed independent of temperature and this property makes it stand out from other gel-forming gums such as pectin (Draget et al., 2005). We discuss the application more extensively in a separate article on cell and enzyme immobilization.
Alginate beads are commonly prepared by extruding an alginate-core material solution into a calcium chloride solution. The diameter of these beads can vary depending on the size of the needle used for extrusion and the alginate concentration.
Calcium alginate beads have a relatively small pore size, ranging from between 5 and 200 nm. This makes the hydrocolloid a suitable wall or encapsulating material for large molecules like proteins especially enzymes. Calcium-alginate beads have small pores too, which means that the release of large molecules such as protein from the capsule can be slower than with small molecules. The rate of release depends on the molecular weight of the alginate matrix.
Encapsulation, compared to gels outright, requires a liquid core surrounded by a semipermeable membrane that retains the core within it. Unlike a full gel, mass transfer is higher. Liquid starch cores with a calcium alginate membrane have been developed to deliver probiotics to the gut. Some alginate coated beads are also coated with poly-l-lysine to deliver monoclonal antibodies and baculoviruses so they are destroyed before they can deliver their load.
Alginates in Mixed Gels
Mixed gels are extremely valuable because of the synergism that can be achieved in terms of offering variety in strength and ease of preparation. They have found extensive application in food and consumer healthcare products.
Alginates also form strong complexes with other polyelectrolytes such as pectin by undergoing chain-chain association. The rheology of these particular mixed gels has been explored extensively (Thom et al., 1982). These mixed hydrogels form after the addition of divalent cations such as Ca2+ (Liu et al., 2003).
Pectin on its own too also forms a gel in the presence of calcium ions. There is a synergistic relationship between it with alginate in forming various gels (Wong et al., 2002; Jaya et al., 2009) of differing strength and functionality. The synergistic behaviour between alginates and pectin has been studied extensively using pectins with different degrees of esterification including amidated pectin and alginates of high and low M/G (mannuronic acid/guluronic acid) ratio (Walkenström et al., 2003).
The mixed gels show, according to transmission electron microscopy (TEM), a coarse network of many strands and fibrils. The pores are of micron size. Whilst no comparison was made with pure alginate gels, pectin gels are composed of thinner strands with smaller pore sizes compared to the mixed network. These gels were characterised in strength by their storage modulus G’. The strongest mixed gels occur between high-methoxyl pectins (HM pectins) or at least those with a high degree of esterification, and with amidated pectins. The M/G ratio has no impact on the type of gel structure generated. The stronger gels are formed when the network of the gels according to TEM are much more highly branched. The highest storage modulus was noticed for a gel of ratio amount 1:1 for low-G alginate with HM-pectin.
One of the main benefits of an alginate-HM-methoxyl pectin gel is its thermoreversibility under acidic conditions. The pH needs to be below pH 4 and ideally around pH 3.4.
Sources of Alginate And Their Different Properties
Most of the world’s supply comes from Ascophyllum nodosum and Macrocystis pyrifera. The other to some extent is Sargassum polycystum.
The commonly used species both produce alginates where the M:G ratio is greater than 1. They are chosen for cost as well as functional property. Their gels are more pliable and elastic (Rehm & Moradali, 2017).
Ascophyllum nodosum has a an M:G ratio of 1.56. The FM is 0.61 and FG is 0.39. In sequence terms, FMM is 0.46, FGG is 0.23 and FMG,GM is 0.16.
Macrocystis pyrifera has a an M:G ratio of 1.63. The FM is 0.62 and FG is 0.38. In sequence terms, FMM is 0.42, FGG is 0.18 and FMG,GM is 0.20.
At the other end of the scale where the M:G ratio is much less than 1:-
The stipe of Laminaria hyperborea has a an M:G ratio of 0.47. The FM is 0.32 and FG is 0.68. In sequence terms, FMM is 0.20, FGG is 0.56 and FMG,GM is 0.12.
Saccharina japonica has a an M:G ratio of 1.85. The FM is 0.65 and FG is 0.35. In sequence terms, FMM is 0.48, FGG is 0.18 and FMG,GM is 0.17.
Sargassum vulgare and other species in this group produce alginates where M:G is less than one and have high homopolymeric blocks of M(η<1) (Taratra et al., 2010). It isn’t always the case though – the environment certainly has its part to play.
The alginates from telluric bacteria such as several species of the genus Pseudomonas and Azotobacter are acetylated usually on the C2 and C3 of the M units.
Uses
Alginates are highly valuable in food, cosmetic and pharmaceutical applications. You find this gum in desserts, jams (Mancini & McHugh, 2000), fruit purees and savoury sauces such as salad dressings and mayonnaise (Mancini et al., 2000; Onsøyen, 2001; Helgerud et al., 2009; Qin et al., 2018; Nordgård & Draget, 2021). It is also used as an alternative to gelatin because it can then be used in foods suitable for vegans, vegetarians, ovo-lacto vegetarians etc.
All alginates are ideal at producing cold-setting gels, freeze-thaw stable gels and non-melting gels. The formation of heat-stable gels is one of their best properties (Roopa & Bhattacharya, 2008).
Sodium alginate (E401) is described as odourless and slightly sweet, light yellow to beige powder. Crude fractions contain about 1.4% w/w crude protein with 9.9% w/w water. In general use, the amount required is anywhere between 0.2 and 1% w/w. It needs to be added in small doses to a liquid solution. Alginates will dissolve in cold water but they are best hydrated between temperatures of 155 and 160ºF (68-71ºC). The temperature of the mix can be as high as 75ºC. It helps to mix it with sugar like a dextrin to aid dispersion. More specific instructions for its application depend on which food and whether it is required to produce pearls. The grain size is variable – ranging from powder to granular and various mesh sizes (30/60/80/100/120/170/200 mesh) are available depending on how the manufacturer wishes to exploit them.
Potassium alginate (E402) can be substituted for sodium alginate in a host of applications. It’s especially useful where low sodium products are required. Calcium alginate (E404) tends to be used in ice creams and frozen bakery products. Ammonium alginate (E403) is used more in soft drinks, food colouring agents and in icing.
Generally, they are not good for particular flavour applications because they form hard-set gels that simply don’t release flavour. Shearing will break the gel and in time they do suffer with syneresis but when used in food applications, they are added to prevent this occurrence.
In the food industry, calcium-alginate gels have been used in the production of restructured food products and as edible coatings (Rhim, 2004). Alginates are also used as biodegradeable packaging film (Rihm, 2004).
Beverages
In beverages such as soft drinks, adding a gum such as an alginate helps improve mouthfeel as well as keeping materials in suspension such as fruit cells, particulates and encapsulated probiotics. They also stop layering of particulates, prevent oil separation and help avoid protein sedimentation. We tend to use alginates for non-protein based beverages which can be acidic such as fruit based drinks or in neutral pH products such as functional waters. They are used in neutral protein-based drinks which are UHT treated or just pasteurised. They are not used however with acidic protein-based drinks such as protein-rich juices and milks although there are still examples that sit on the margins when it comes to pH. Levels used are between 0.1 and 0.3%w/w. The alginate of choice has been PGA at a level of 0.1 to 0.2% w/w for acidified protein-based and non-protein based beverages.
PGA is also added at 0.1%w/v level to orange juices to stabilise the pulp without affecting flavour too much. In whipped fruit drinks, it helps to stabilise the foam and provide a smoother flavour. High shear creates a better foam.
The only other additional point is to source alginates with more M blocks because these are less likely to bind calcium and so be preferred for contributing viscosity and reduce the chances of forming gels if calcium was to be present. Its not an absolute requirement though.
Sauces
Alginates are used as delayed thickening agents or for creating a temporary thickness. As a temporary thickener, alginate addition to gravy containing meat particles prevents these from settling out before they are canned. This helps in pet food production too before canning and is similar to their role in dressings too. The alginates on heating lose their thickening capability which has become a redundant property of the alginate by this time.
The need for gel formation however in cans is feasible. When cans are retorted, the property of forming a gel during heating is also possible using alginate and a calcium salt that dissolves slowly. The gelling only occurs once the calcium salt has fully dissolved which is helped by the heat that also encourages gel formation. It means that there is a quick heat transfer when the sauce viscosity is at its lowest and so the heating time is lower than if the product was a gel.
In salad dressings, the thickening and stabilising due to the emulsion benefits that prevents phase separation with the alginate concentrating in the aqueous phase (see later section on salad dressings).
Spreads
In food, they offer different textures in low fat (40%) spreads. The viscosity is generally described as medium when added during manufacturing of a product. All applications show that spreads created with the gum are spreadable without gritty particles and do not show syneresis. The polyelectrolyte also stabilises ice-cream.
Alginates are used in bakery fillings because they prevent or at least reduce the effects of syneresis in jelly. Sour cherry pie fillings have been stabilised using this hydrocolloid (Strachan et al., 1960). This also stopped the fruit filling losing too much water. The thickening effect also reduces the tendency for the fruit filling to soften and soak the pastry. It can also be added to the dough batter to thicken it up so as to produce a short flow and obtain good control of the dough.
For water-based dessert gels, alginates will form a strong and stable gel that also allows for flavour release. No heat is needed and the alginate works at any pH value and solids content. The usage level is 0.8% or less in these products.
In icing and frostings for ambient and frozen-stored cakes, alginates work well enough at levels between 0.05% and 0.2%.
Some beers, ciders and lagers which are poor at forming a stable head sometimes have polypropylene glycol alginate (PGA) added at very low concentrations to stabilize the foam formed on pouring. It also protects the beer foam from collapsing at dispense when poured into dirty mugs and glasses. It also counters those beers that suffer with a high amount of foaming inhibitors and improves beer cling to glass. A beer head is a very critical visual cue of quality.
In the wine industry, alginates are added to clarify them and reduce particulates and tone down the colour. It is also used in milk-based and egg-based liqueurs such as Advocaat for stabilizing fine particulates.
In its film-forming capacity, it can be a useful material for preserving cream in sandwich cakes. Some candy and confectionary producers use it to prevent the product adhering to candy wrappers.
A typical instant bakery filling is formulated accordingly:-
roll-dried acetylated waxy maize starch adipate (100g), icing sugar (212g), whole-milk solids (80g), sodium alginate (7g), colour, flavour (1g).
Calcium alginate coatings are used for food preservation of meat and for reducing shrinkage loss due to moisture removal either through drying and weeping. A variety of meat pieces are treated this way including chicken (Mountney and Winter, 1961) , pork and beef, lamb, fish, shrimps. The reverse is true: protecting frozen meat and fish by blocking water penetration of the flesh during storage. Alginates can be sprayed onto the surface of carcasses in a whole body coating prior to storage or butchery (West et al., 1975; Lazarus et al., 1975).
Some researchers have tried adding antimicrobials into the alginate coatings. We mentioned poly-l-lysine but various organic acids can be used which prevent microbes from penetrating the coating. They can also diffuse into the surface of the meat. The presence of acid could cause the coating to gel too much so a careful balance with added calcium ions might be justified to serve as an acid inhibitor. We might consider nisin, pediocin and various other bacteriocins, benzoate and sorbate anions.
Other applications include pimiento and anchovy filling for olives, coating fish, meat and poultry, cocktail cherries, low-sugar jams, apple pieces for pie fillings, and meat chunks for pet foods.
Reconstituted onion rings are made with onion and alginate! The food is manufactured by mixing flour with salt, water, chopped onion and sodium alginate.
Notable suppliers and brands include Algaia®(Saint Lo, France) who produce Satialgine™ LSP 263 (E402) which is a fine grade product. The coarser grade is Satialgine™ LSP 1300 (E401). Alginates are also available from the JRS Rettenmaier Group who supply pure and buffered versions and pectin-alginate blends. TIC Gums offer a range of alginates. Many suppliers, especially in China offer different grades.
Alginates in Dairy and Yogurts
Alginates work very well with dairy. With skimmed milk which has a neutral pH, they will contribute a smooth, pleasant and fat-like feel to counter the loss of fats. They will form a gel because of the calcium which can be adjusted to varying degrees depending on the amount added. The implication is that emulsifiers and calcium sequestrants need to be added. A good example is tetrasodium pyrophosphate. Alginate can be added alone above 75ºC with good agitation and if dispersed with other ingredients, to milk but there is a slow and steady precipitation with calcium over time and if processing is poor, lumps are irreversibly formed.
One other approach to sequester calcium is to heat the milk. The milk proteins bind calcium more tightly as the temperature rises. This means the alginate molecules cannot align because of kinetic interactions with the calcium ions to form the gel network and so prevent junction formation. In so doing, the calcium ions bind with greater force to the casein in the micelles. The alginate then hydrates without interaction with calcium. However, gelation will occur but to a lesser extent.
In acidic dairy systems such as fermented foods, the story is different. Yogurt is a great probiotic source of lactic acid bacteria and of significant nutritional benefit. Alginate acts as a stabiliser here because it is effective in the range of pH 3.9-4.9. Alginate will precipitates below its pKa of 3.5 so it cannot be used except sparingly in very acidic dairy beverages.
Propylene glycol alginate is preferred here although it is not wholly successful because it will produce a gritty mouthfeel as it precipitates when too much calcium is present. Product developers of whey-based drinks, kefir and drinking yogurts will try out different hydrocolloids to see which is best. Carboxymethylcellulose (CMC) and high-methoxyl pectin have always been popular choices because they can create a better thickness than alginates (Gallardo-Escamilla et al., 2007).
However, a good example of stabilization of frozen buttermilk is with PGA which helps smooth out the texture, minimises stickiness and contributes to the sensory impact when it is stirred as well as mouthfeel. It also prevents viscosity drops during pasteurisation and sterilization. One other use is to minimise syneresis. Under other circumstances, whey-based liquids have a watery mouthfeel and need gums/hydrocolloids to thicken them.
Ice-Cream
Alginates are commonly used in the manufacture of ice cream to improve its texture, stability, and mouthfeel (Onsøyen, 1997). It was the first application for this hydrocolloid in the food industry. Alginate-based ingredients are primarily employed as stabilizers and emulsifiers, helping to prevent ice crystal formation, or certainly reducing the size of the crystal formed and so maintaining a smooth and creamy texture. Alginates also prevent syneresis and prolong melting so the ice-cream retains its structural shape. The properties are further explained here:
Stabilization of Emulsions
Ice cream is an emulsion of fat globules dispersed in a water-based matrix. Alginates act as emulsifiers, stabilizing the emulsion and preventing fat separation during storage and freezing. By forming a protective layer around fat globules, alginates contribute to the overall stability of the ice cream mixture.
Ice-cream mixes of 0.2% sodium alginate or xanthan gum produce the greatest viscosity (Soukoulis et al., 2008).
Control of Ice Crystal Growth
One of the challenges in ice cream production is to control the growth of ice crystals during freezing and storage. Large ice crystals can result in a gritty texture and affect the smoothness of the final product. Alginate-based stabilizers inhibit ice crystal formation by creating a network structure that hinders the movement of water molecules, reducing the formation of large ice crystals and promoting a smoother texture.
Viscosity Control
Alginates are hydrocolloids that can modify the viscosity and rheological properties of ice cream mixtures. By increasing the viscosity, alginates contribute to better suspension and distribution of air bubbles, improving the overall stability of the product and imparting a smoother mouthfeel.
Syneresis Prevention
Syneresis refers to the release of water from ice cream during storage, resulting in the formation of ice crystals and a loss of texture and quality. Alginates can help reduce syneresis by binding water molecules and maintaining the moisture content within the ice cream matrix.
Improved Melting Resistance
Alginate-based stabilizers can enhance the melting resistance of ice cream, allowing it to maintain its shape and texture for a longer time. This property is particularly desirable in frozen desserts, as it improves the consumer experience and reduces product waste.
It’s worth noting that the specific type and concentration of alginates used in ice cream formulations can vary depending on the desired characteristics of the final product. Manufacturers often conduct experiments and optimize the composition to achieve the desired texture, stability, and sensory attributes.
Sodium alginate is better than other gums because of the mouthfeel it contributes and it also helps to prevent ice crystal formation or at least keep the ice crystals relatively small. It also helps prevent separation and syneresis in ice-cream mixes. Because it forms a gel with milk due to the calcium content, just enough is needed to stiffen the ice-cream mix. This behaviour is ideal with low-fat ice creams where the mouthfeel contribution from the fat is missing and replaced to some extent with the alginate.
In most cases, alginate-phosphate mixes are needed to prevent precipitation of milk proteins that can create a gritty feel because of the high calcium content which interacts unfavourably with the casein proteins and sometimes gels the alginate. The ideal addition level of alginate is between 0.3 and 0.4% in combination with a glyceryl monostearate emulsifier. Too much sodium alginate and it will form rigid, brittle gels. There are very many patents exploiting alginate in various dairy applications – too numerous to cover here.
Overall, alginates play a vital role in ice cream manufacture by improving its texture, stability, and mouthfeel. They help prevent ice crystal formation, enhance emulsion stability, control viscosity, reduce syneresis, and improve melting resistance, contributing to a high-quality and enjoyable ice cream product.
Milk Drinks – Chocolate Milks
In other dairy products, between 0.25% and 2 % w/v alginate is added to milk so that it can be stored at higher temperatures in when it has been effectively sterilized so that the texture and flavour does not alter.
Alginate is still added to chocolate milk, to cheese spreads and cream cheese. With chocolate milk, enough alginate is added to keep the chocolate and cocoa particles in suspension. That is the case too with any dairy product where the viscosity is too low to maintain particles suspended. To some extent alginates are slowly being replaced with kappa-carrageenan because it has a more favourable interaction with the casein micelles.
In processed cheese, alginate stops serum drainage whilst offering a desirable mouthfeel.
One of the earliest patents using alginates in chocolate milk and ice-cream is in 1937 to Kelco (Lucas, H.J. US Patent 73353034A). In this composition, a 50/50 mix of sodium alginate with sugar was developed in applications. This relied on the sugar to help disperse the alginate and in standardizing the stabilization. A small amount of water-soluble phosphate was added to ensure the alginate was ‘compatible’ with the calcium slats present in milk.
The process of application with chocolate milk is straightforward – the sodium alginate is made into a paste and then stirred into the milk before the cocoa, sugar and other ingredients are added. It is pasteurised before sale.
In some instances, success has been achieved with a phosphate-alginate at a level of 0.15% w/w for a chocolate milk.
Incidentally, Patent US2485934A (Steiner; Kelco Co. 1949 granted) defines how alginates work in ice cream.
When it is used in whipped cream and cream toppings, and synthetic creams, it is added to 0.15% w/w. In the case of synthetic creams, 0.5%w/w methylcellulose is added or some other foam-forming colloid which produces a better, more rapid whip. It also helps prevent overwhipping, reduces overrun and stops synereses. The alternative is carrageenan and Locust bean gum.
Salad Dressing
Alginates are also valuable for stabilizing all sorts of emulsions and suspended solid particles especially in creamy salad dressings. Without a thickener, a salad dressing or indeed any oil and water dressing would soon separate into its original oil and water phases.
Here PGA is preferred because it offers the fat mimicking properties. The effect is similar in low-fat dressings where it replaces the fat imparting mouthfeel by raising viscosity. PGA is excellent because it works so well in acidic systems which some dressings have when acetic acid (vinegar) is part of the mix.
A number of businesses sell specific mixes called alginate ice cream stabiliser. JRS Rettenmaier (ex. DuPont alginate business) offer Lactogel DE32 stabilizer for this purpose. In EU terms, it is a mix of ingredients, of glyceryl monostearate (E471), sodium alginate (E401), disodium phosphate (E399(ii)), sodium carboxymethyl cellulose (E466). This particular dry mix has an extremely long shelf-life of 456 days from manufacture.
An alternative alginate is available from SKW Biosystems in France who offer Satialgine ABN 50.
Alginates and Probiotics
Alginates have been exploited with probiotics on a regular basis. Normally the probiotic bacteria is suspended in a solution of sodium alginate which is then cast as beads into a strong calcium chloride solution (4-6% w/v).
Probiotic bacteria, even when encapsulated struggle to survive in a highly acidic system such as the gut because alginate beads are extremely porous. To get round this, other materials have been added to improve their robustness and improve transit through the gut. Reference can be made to the following:-
- alginate and starch mixes (Sultana et al., 2000)
- whey
- gelatin
- chitosan
- pectin
Some dietetic foods rely on the addition of gums to make them palatable.
Some notable examples of encapsulated probiotics include that of Lactobacillus acidophilus in a liquid starch (low substituted hydroxypropyl ammonium starch) (Jankowski et al., 1997).
Alginate As a Dietary Fibre
Alginate can serve as a dietary fibre because it is very slowly hydrolysed in the human intestine. Only a few bacteria species that can live in the colon have developed lyases that degrade alginate down to single sugar units. Alginate then can function as a carbon source (Draget et al, 2005; Li et al., 2016).
Sodium alginate is added to chocolate milk as a means of minimising sugar spiking, helping in the regulation of insulin as well. El Khoury et al., (2014) studied sodium alginate in chocolate milk with its effects on these outcomes in 24 healthy men. In this study, the alginate was added at two levels at 1.25% and 2.5% w/w. the alginate used had a ratio of 0.78:1 of mannuronic acid (M) to guluronic acid (G) residues, and was block distributed.
Alginate can also suppress hunger to some extent as was shown in a study where a beverage was drunk (Peters et al., 2011).
Pectin and sodium alginate have been added to carbohydrate beverages. When they are exposed to the low pH environment in the stomach, these pectin-alginate polysaccharides encapsulate the hydrogel. This allows the carbohydrate to be transported through the stomach to the duodenum but the glucose receptors are not activated (Sutehall et al., 2018).
Alginates In Infectious Disease
The gelation of alginate by bacteria is important where infectious diseases are concerned. Chronic pulmonary infections of patients with the genetic disorder, cystic fibrosis (CF) is caused by Pseudomonas aeruginosa. The issue is the change in some strains of P. aeruginosa which turn to overproduction of alginate caused by a mucoid phenotype. Whenever alginate comes into contact with calcium ions, rigid gels develop. Gel formation is at the heart of chronic infection so it is important to understand how these rigid gels form in the lungs. Gel formation could play a role in chronic infections by contributing to the protection of the bacterial cells from the host immune response. It is thought in this situation that the presence of O-acetyl groups on the alginate provides the bacteria with greater resistance to opsonic phagocytosis (Pier et al., 2001). The lack of O-acetyl groups has been shown to produce a less effective gel in this instance.
Analysis
Alginates need to be checked for in terms of presence in foods. Methodologies are described for their amount, structure and their derivatives (Usov, 1999).
Kennedy and Bradshaw (1984) developed a method for assaying alginates in solution using the polymetric cation poly(hexamethylenebiguandinium chloride) [PHMBH+Cl−]. The concentration range was 0.01 to 0.5 per cent w/w. The structure of the alginate had no impact on the quantity detected.
The AOAC early on registered methods for various foods such as salad dressings and mayonnaise (Wiger, 1963). The method requires the precipitation of the alginate with acetonealcohol, removal of oil with dioxane, decolourization with a clay and the development of colour with ferric hydroxide in sulphuric acid. The method sees recovery at the 0.5% level for mayonnaise and salad dressing but is not suitable for french dressing.
A more comprehensive method has been developed for dairy foods in general to deal with the presence of other polymers such as pectin in dairy foods (Graham, 1968). In the method, alginate was recovered from milk, chocolate milk, ice cream, cheese and cream cheese spreads and other products by predigesting the mixture with the protease papain. The digest was clarified with Celite 535 and charcoal and filtered. Pectin, pectinic acid, and pectic acid was converted to galacturonic acid by pectin methyl esterase (pectinesterase) and polygalacturonase. Calcium chloride was added to the filtrate to precipitate the alginate and this calcium salt washed free of any sugars. It was dispersed by conversion to the sodium alginate (salt) using sodium hexametaphosphate. The alginate was determined in solution using phenol-sulphuric acid. The recoveries were between 92 and 96% for milk, 90 to 95% for ice cream and 85 to 95% for cheese spreads etc.
Beer may contain a variety of polymers. Beers brewed that use copper finings as well as other finings rely on carbohydrates for dispersion. Buckee et al., (1976) showed that alginates are generally in low amounts (less than 1 mg/L) in beer.
Regulatory Status
The Joint FAO/WHO Expert Committee on Food Additives (JECFA) evaluated the safety of alginic acid and alginate salts of ammonium, calcium, potassium and sodium at the 39th meeting in 1992. They gave it an ADI which was ‘not specified’. In 1993, JEFCA gave propylene glycol alginate and ADI of 0 -70 mg/kg at the 41st meeting.
In the EU, alginates are classified in Annex I of the EU directive 95/2 from 1995. It can be used under the quantum satis rule relatively freely. Propylene glycol alginate (PGA) is inscribed in Annex IV for ‘other permitted additives’ with a maximum level ranging from 0.1 mg/l to 10 g/l depending on the type of food product.
Sodium alginate is E401: Cas#9005-38-3. In the USA, it is covered under 21CFR184.1724; FEMA 2015.
The only issue is that too much alginate in the diet might impair iron uptake because it binds this metal. That is the case however for those who are also seeking a high enough calcium content in the diet. It is a matter though of not treating alginate irresponsibly.
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