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Gellan gum
9 Gellan gum G. Sworn, Danisco France SAS, France
Abstract: Gellan gum is an extracellular polysaccharide secreted by the micro-organism Sphingomonas elodea (ATCC 31461) previously referred to as Pseudomonas elodea. Commercially it is manufactured by a fermentation process. It is available in two forms, high acyl (HA) and low acyl (LA). Gellan gum forms gels at low concentrations when hot solutions are cooled in the presence of gel promoting cations. An overview of the manufacturing process and chemical structure of gellan gum is given. The functional differences between the two forms of gellan gum are reviewed in detail with regard to hydration, gelation, stability and texture. Methods for the effective preparation of gellan gum solutions and gels are described for the high and low acyl forms. The effects of food ingredients such as salts, sugars, and acids on gel properties are discussed in relation to the main applications. Finally, the main applications of gellan gum in food are described with example formulations. Key words: gel, setting temperature, texture, extracellular, fermentation, fluid gel.
9.1
Introduction
Gellan gum is an extracellular polysaccharide secreted by the micro-organism Sphingomonas elodea (ATCC 31461) previously referred to as Pseudomonas elodea. Gellan gum forms gels at low concentrations when hot solutions are cooled in the presence of gel promoting cations. It is available in a substituted or unsubtituted form. Gel properties depend on the degree of substitution with the substituted form, producing soft, elastic gels whilst the unsubstituted form produces hard, brittle gels.
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Manufacture
Commercially, gellan gum is manufactured by inoculating a fermentation medium with the micro-organism. The medium contains a carbon source, such as glucose, phosphate and nitrogen sources, and appropriate trace elements. The fermentation is carried out under sterile conditions with strict control of aeration, agitation, temperature and pH. After fermentation, the viscous broth is pasteurised to kill viable cells. The polysaccharide can then be recovered in several ways. Direct recovery by alcohol precipitation from the broth yields the substituted native, or high acyl (HA), form. Alternatively, treatment of the broth with alkali prior to alcohol precipitation results in deacylation and yields the unsubstituted, low acyl (LA) form. Gellan gum is currently sold commercially in three basic forms, namely high acyl, unclarified (KELCOGELÕ LT100), low acyl, unclarified (KELCOGELÕ LT), and low acyl, clarified (KELCOGELÕ and KELCOGELÕ F). A number of specialty grades are also available.
9.3
Structure
The primary structure of gellan gum, shown in Fig. 9.1, is composed of a linear tetrasaccharide repeat unit: !3)- -D-Glcp-(1!4)- -D-GlcpA-(1!4)- -D-Glcp(1!4)- -L-Rhap-(1!.1,2 The polymer is produced with two acyl substituents
Fig. 9.1
Primary structure of (a) low and (b) high acyl gellan gum.
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present on the 3-linked glucose, namely, L-glyceryl, positioned at O(2) and acetyl at O(6). On average there is one glycerate per repeat unit and one acetate every two repeats.3 X-ray diffraction of low acyl gellan gum has shown that the polymer exists in the solid state as a co-axial three-fold double helix.4,5 Computer modelling of the high acyl structure, based on the knowledge of the low acyl form, concluded that the acetate substituents would be positioned on the outside of the double helix.6 Subsequent X-ray diffraction data revealed that the glycerate substituent was accommodated by an approximately 14ë rotation of the carboxyl group about the C(5)-C(6) bond on the adjacent glucuronate residue.7 This was accompanied by a 17 3ë rotation of the glucuronate residue itself to maintain the required spacing between the carboxyl and glycerate groups. It is proposed that this structure results in helices stabilised by interchain associations involving the glycerate groups, with the acetyl substituents positioned on the periphery of the helix.8
9.4
Technical data
There are three steps to consider for the successful formulation of gellan gum systems: 1. dispersion 2. hydration 3. gelation. These will now be considered in turn for both the LA and HA forms of the gum. 9.4.1 Dispersion The first step in preparing any gum solution is to ensure that the gum particles are properly dispersed in the solvent and do not clump together. Poor dispersion will result in incomplete hydration and loss of gum functionality. Both forms of gellan gum are insoluble in cold water although they will tend to swell in water of low calcium content. The gum can therefore be readily dispersed in deionised water by stirring and adding the powder slowly to the vortex. As the ion concentration in the water increases, dispersion becomes even easier. For example, in moderately hard water (~180 ppm hardness expressed as CaCO3), the gum can be added to the surface of the water and then dispersed with gentle agitation. By blending the gum with dispersants such as sugar (5±10 times weight of gum) or glycerol, alcohol, or oils (3±5 times weight of gum), it is possible to add the gum directly to hot water. Both forms of gellan gum are also readily dispersible in milk and reconstituted milk systems. 9.4.2 Hydration Low acyl gellan gum The temperature at which LA gellan gum hydrates is dependent on the type and concentration of ions in solution. The presence of ions such as sodium and, in
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Table 9.1 Effect of dissolved salts on the hydration temperature of 0.25% LA gellan gum Water hardness (ppm CaCO3) 0.000 100 200 300 ± ± ± ± ± ± ±
Dissolved NaCl (%)
Hydration temperature (ëC)
± ± ± ± 0.10 0.25 0.45 0.70 0.90 1.00 1.30
75 88 > 100 > 100 50 52 60 70 82 89 > 100
particular, calcium, in solution will inhibit the hydration of LA gellan gum as shown in Table 9.1. It is therefore necessary, in most circumstances, to use a sequestrant to bind the soluble calcium and so aid hydration. Typically, between 0.1 and 0.3% of a sequestrant such as sodium citrate is sufficient to allow complete hydration at 90±95 ëC in water of up to 600 ppm as CaCO3 water hardness. Incomplete hydration will result if the sodium ion concentration exceeds 0.5% (approximately 1.3% sodium chloride). Once the gum is hydrated, additional ions can be added to the hot solution and, provided the temperature is maintained above the gelation temperature, no gel will form. Table 9.2 provides details of the effects of the sequestrant sodium citrate on the hydration temperature of LA gellan gum and shows that hydration can be achieved at temperatures ranging from room temperature to boiling point. Table 9.3 lists the relative sequestering power and effective pH ranges of a number of commonly used sequestrants. In foods containing sugar, LA gellan gum should be hydrated in water and any sugars can be added to the hot gum solution. However, LA gellan gum can be hydrated directly in sugar solutions up to 80 total soluble solids (tss) by heating to boiling. In some cases a low level of sequestrant such as sodium Table 9.2
Effect of sodium citrate on the hydration of 0.25% LA gellan gum
Water hardness (ppm CaCO3) 0 100 300 600 900
Sodium citrate (%)
Hydration temperature (ëC)
0.00 0.05 0.10 0.20 0.40
75 25 65 65 68
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Table 9.3
Relative sequestering power of sequestrants commonly used in foods
Sequestrant
Sodium hexametaphosphate Tetra sodium diphosphate Di sodium orthophosphate Tri sodium citrate dihydrate
Parts of sequestrant required to sequester 1 part of available calcium pH6
pH4
7 10 130 20
7 20 180 40
citrate (less than 0.3%) is required to bind the free calcium often present in sugar syrups. LA gellan gum will not fully hydrate below pH 3.9. In this case, the acid should be added, preferably as a concentrated solution, to the hot gum solution. Prolonged heating in acidic conditions should be avoided as this leads to some hydrolytic degradation of the gum resulting in a reduction in the quality of the final gel. However, at pH 3.5, LA gellan gum can be held for up to 1 h at 80 ëC with only a minimal loss in gel quality. In neutral conditions solutions can be held at 80 ëC for several hours. LA gellan gum is readily dispersible in milk and reconstituted milk systems and will hydrate upon heating to approximately 80 ëC without the need for a sequestrant. High acyl gellan gum The hydration of HA gellan gum is much less dependent on the concentration of ions in solution than LA gellan gum and generally heating to 85±95 ëC is sufficient to fully hydrate the gum in both water or milk systems. As a dispersion of HA gellan gum is heated, it swells rapidly at approximately 40±50 ëC to form a thick, pasty suspension. With continued heating the suspension loses viscosity suddenly at approximately 80±90 ëC signifying complete hydration. The swelling stage can be avoided by adding the gum directly to hot water (>80 ëC) with the aid of a dispersant such as sugar, oil or glycerol, as described in the previous section. The hydration of HA gellan gum is inhibited by the presence of sugars; therefore it is recommended to hydrate the gum in less than 40% tss. Additional sugar can then be added to the hot gum solution. As with LA gellan gum, HA gellan gum will not hydrate below pH 4.0. Similar care must also be taken under hot acidic conditions to avoid hydrolytic breakdown and loss of gel quality. 9.4.3 Gelation The proposed gelation mechanism of gellan gum is based on the domain model which assumes the formation of distinct junction zones and disordered flexible polymer chains connecting adjacent junction zones.9 As a hot solution cools, gellan gum undergoes a disorder±order transition. This transition has been
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attributed to a coil-helix transition.10 In the case of LA gellan gum, gel promoting cations such as sodium, potassium, calcium and magnesium promote aggregation of the gellan double helices to form a three-dimensional network and the subsequent gels are hard and brittle. Recent studies using atomic force microscopy have challenged this and proposed a fibrous model in which network structures develop through the formation of non-associated fibres or strands via either elongation or branching.11 The acyl substituents have a profound effect on the structure and rheological characteristics of gellan gum gels. The gellan gum undergoes a similar disorder to order transition as the solution is cooled but according to the domain model further aggregation of the helices is limited by the presence of the acetyl group.8 According to the fibrous model the acyl groups inhibit end-to-end type intermolecular associations through a type of steric hindrance, resulting in a decrease in the degree of continuity and homogeneity of the gelled system.11 The subsequent gels of HA gellan gum are therefore soft and elastic. Low acyl gellan gum The easiest and most common method of making LA gellan gum gels is to cool hot solutions. LA gellan gum forms gels with a wide variety of cations, notably calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+) as well as acid (H+).10,12 Divalent cations are more efficient at promoting gelation of LA gellan gum than monovalent ions. Gel strength increases with increasing ion concentration until a maximum is reached. Further addition of ions results in a reduction of gel strength due to the `over conversion' of the LA gellan gum with excess ions. Ion concentrations for optimum gelation are generally independent of gum concentration but are reduced as the level of sugar is increased. Figure 9.2 compares the effect of both mono- and divalent ions on the modulus of 0.5% LA gellan gum gels in water and in 60% sucrose. Below pH 3.0 LA gellan gum is able to form a gel without the need for mono- or divalent metal ions. Optimum gel modulus occurs for these acid gels at approximately pH 2.8±3.0 regardless of the acid used. This optimum is not affected by the presence of sugars to the same extent as ion requirements. For example, in the presence of 60% sucrose the optimum shifts to approximately pH 2.5±2.7. Acid gels are generally stronger than ion mediated gels in both water and sugar. Addition of other gelling ions, such as sodium or calcium, generally results in a reduction in gel strength of the acid gels. Gel properties at optimum conditions for LA gellan gum gels in water and 60% sucrose are summarised in Tables 9.4 and 9.5 respectively. In many instances addition of gelling ions is not necessary since there are sufficient ions present in the water or other ingredients to promote gelation of the LA gellan gum. When required, the gel promoting ions can be added to the hot gum solution and, provided the solution is kept above its setting temperature, no gel will form. This allows for the easy preparation of stock solutions (1±2% gum) which can be held at high temperature until required. LA gellan gum is often described as having a 'snap set' since gelation is very rapid once the setting temperature is reached. As with hydration temperature, setting and
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Fig. 9.2 Effect of calcium (squares), sodium (triangles) and potassium (circles) ion concentration on the modulus of 0.5% LA gellan gum gels in water (open symbols) and 60% sucrose (closed symbols). Table 9.4 Properties of 0.5% LA gellan gum gels in optimum conditions for gel formation in water Gelling ion
Ion concentration (mM)
Modulus (Ncmÿ2)
Brittleness (%)
Setting temperature (ëC)
Calcium Sodium Potassium Acid
8±10 260±300 240±260 pH2.8±3.0
19.3 12.3 12.1 20.3
28.5 28.2 30.0 27.7
42 54 59 10
Table 9.5 Properties of 0.5% LA gellan gum gels in optimum conditions for gel formation in 60% sucrose Gelling ion Calcium Sodium Potassium Acid
Ion concentration (mM) 0.5±1.0 25±35 8±10 pH2.5±2.7
Modulus (Ncmÿ2)
Brittleness (%)
Setting temperature (ëC)
1.66 3.13 1.71 7.74
61.5 54.1 65.6 43.9
38 47 43 64
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melting temperatures of the gels depend on the ion concentration in solution. The higher the ion concentration, the higher the setting and melting temperature. Significant thermal hysteresis between the setting and melting temperature is observed in LA gellan gum gels, i.e., the gels melt at a higher temperature than that at which they set.13 In most conditions LA gellan gum gels are not thermally reversible below 100 ëC. The exceptions are gels formulated with a low level of monovalent ions, particularly potassium, and milk gels. Setting time is governed by the rate at which heat is removed which, in turn, depends on the dimensions of the system being cooled. Thin films on a cold surface, for example, set almost instantaneously. Once set, gel strength does not change markedly over time. LA gellan gum is capable of forming self supporting gels at concentrations as low as 0.05% gum. LA gellan gum gels do not synerise unless cut or broken. High acyl gellan gum As with LA gellan gum the easiest way to form HA gellan gum gels is to cool hot solutions. Addition of cations is not necessary for the formation of HA gellan gum gels and their properties are much less dependent on the concentration of ions in solution. Gels typically set and melt between 70 and 80 ëC and show no thermal hysteresis, i.e., they melt at the same temperature at which they set. The setting temperature increases with increasing cation concentration. For example, the temperature increases from approximately 71 ëC to 80 ëC as the calcium increases from 2 to 80 mM. A similar increase is seen when sodium or potassium concentration is increased from 10 to 200 mM.14 HA gellan gum is capable of forming self supporting gels at concentrations above approximately 0.2% gum. HA gellan gum gels do not synerise. 9.4.4 Texture of gellan gum Texture is generally regarded as a multifarious property.15 Texture profile analysis (TPA) is a technique based on compression of free standing gels twice in succession and is capable of providing both fundamental and empirical data on the mechanical properties of gels. It has the advantage of providing data at both low and high strains allowing gels to be characterised by multiple parameters. These include: · modulus: a measure of gel firmness when lightly squeezed · hardness: a measure of the force required to rupture the gel · brittleness: a measure of how far the gel can be squeezed before it ruptures; it is important to note that the higher the brittleness value the less brittle the gel is, i.e., it has to be compressed further to break · elasticity: a measure of how far the gel springs back after the first compression cycle · cohesiveness: indicates the degree of difficulty in breaking down the gel in the mouth. Texture profile analysis has been used to characterise a diversity of foods and
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Fig. 9.3
Schematic comparison of the gel texture of HA and LA gellan gum with other common gelling agents.
hydrocolloid gels. A more detailed account of the technique can be found in a review by Pons and Fiszman.16 HA and LA gellan gum gels have very different textures that can be considered to be at opposite ends of the textural spectrum of hydrocolloid gels and Fig. 9.3 shows schematically how gellan gum gels compare with other
Fig. 9.4
Texture profile of HA (ÐÐ) and LA (-----) gellan gum gels measured on 1% gels at 70% strain.
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Fig. 9.5 Effect of HA and LA gellan gum blend ratio on the modulus and brittleness of gels prepared at 0.5% total gum concentration.
common gelling systems. LA gellan gum forms hard, non-elastic, brittle gels whereas HA gellan gum gels are soft, elastic and non-brittle. A comparison of the texture of HA and LA gellan gum gels, made using texture profile analysis, is shown in Fig. 9.4. It is immediately apparent that through blending of the two forms, a diverse range of textures can be achieved that encompass many of the textures produced by other hydrocolloids (Fig. 9.5). It has been demonstrated by differential scanning calorimetry (DSC), and rheological measurements that mixtures of the HA and LA forms exhibit two separate conformational transitions at temperatures coincident with the individual components.8,13 This is important to note since in mixtures consideration for the high setting temperature of the HA gellan gum will need to be made. No evidence for the formation of double helices involving both HA and LA molecules has been found.17 Properties of the blended system can be varied through control of the blend ratio and level of ions in the mixture.18,19 At low ionic concentrations the HA form predominates, but as the ionic concentration increases the contribution of the LA form to the texture increases.
9.5
Uses and applications
Before discussing the main applications of gellan gum, an overview of the key properties of both the HA and LA forms is given in Table 9.6. This provides a useful frame of reference from which existing applications can be understood and new opportunities visualised.
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Table 9.6
Comparison of the key properties of HA and LA gellan gum
Hydration Sequestrants Viscosity Gelling ions Setting temperature Melting Clarity Texture
Low acyl gellan gum
High acyl gellan gum
> 80 ëC Yes Low Yes (mono or divalent or acid) 25±60 ëC No (except low ionic strength and in milk) Clear Firm, brittle
> 70 ëC No High Not required 70±80 ëC Yes Opaque Soft, elastic
9.5.1 Dessert jellies Water-based dessert jellies are popular throughout the world and have a range of textures. The firm brittle texture of LA gellan gum, for example, complements the flavour of fruit juice jellies. Alternatively, combinations of HA and LA gellan gum can be used to produce jellies with a variety of textures. Products can be `ready-to-eat' (RTE) or in dry mix form. Example formulations are provided below. Formulation 9.1 is an example of a fruit juice jelly prepared with LA gellan gum. It can be made with apple, orange, grape, pineapple or grapefruit juice and will hydrate as a dry mix in a water hardness of up to 600 ppm (as CaCO3). The pH and solids vary according to the juice used, but are typically pH 3.5±3.7 and 17% total soluble solids. Alternatively, blends of LA and HA gellan gum can be used to give a range of textures. Blend ratio and gum concentration will depend on the final texture required but a 3:1 HA:LA gellan gum blend at approximately 0.3% is recommended as a starting point for textural evaluation. Formulation 9.1
Recipe for fruit juice jelly using LA gellan gum
Ingredients Water Fruit juice Sugar Citric acid anhydrous Tri sodium citrate dihydrate LA gellan gum
Weight (g)
(%)
250.0 250.0 90.0 2.4 1.8 0.9
42.00 42.00 15.15 0.40 0.30 0.15
Preparation 1. Pre-blend all the dry ingredients. 2. Heat the water to boiling and dissolve the dry ingredients in the hot water. 3. Add the fruit juice, mix and chill. The gel sets at approximately 40±45 ëC and the use of chilled fruit juice with dry-mix desserts ensures a rapid set.
Gellan gum Formulation 9.2
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Recipe for a dessert jelly using LA gellan gum and gelatin
Ingredients
Weight (g)
(%)
Water Sugar Gelatin (type B, 240 Bloom) Citric acid anhydrous Tri sodium citrate dihydrate LA gellan gum Colour and flavour
500.0 90.0 10.2 2.3 1.6 0.35 as required
82.6 15.0 1.7 0.38 0.26 0.06
Preparation 1. Blend all the dry ingredients. 2. Heat the water to boiling and dissolve blend into the hot water by stirring for 1±2 minutes. 3. Deposit and chill.
LA gellan gum can also be used to modify the properties of traditional gelatin dessert jellies and an example is given in Formulation 9.2. The LA gellan gum in this formulation raises the initial set temperature of the dessert to around 35 ëC, allowing more rapid processing of an RTE product. The formulation can also be used in dry mix composite desserts allowing further layers to be added more quickly than with gelatin alone. The time to consumption is, however, not reduced as the maturation time of the gelatin gel remains unchanged. The gellan gum also raises the melting point of the gel so that desserts maintain their shape for longer, when removed from the fridge. This approach can also be used in savory gelled products such as aspics. Gellan gum is an anionic polysaccharide (ÿve charge) whereas gelatin is a protein and as such its overall charge will be dependent on the pH of the system. Below its isoelectric point the gelatin will carry an overall positive charge and will therefore interact with the negatively charged polysaccharide. This can lead to cloudiness in the gel or even precipitation. For this reason it is recommended to use type B gelatins since these have the lowest isoelectic point (pH 4.5±5.5). The extent of the interaction will depend on the pH and the ratio of gelatin to gellan gum. 9.5.2 Suspending agent Gellan gum is commonly used as a gelling agent; however, it can be used to prepare structured liquids which are extremely efficient suspending agents. These structured liquids are gelling systems which have been subjected to shear either during or after the gelation process. The application of shear disrupts normal gelation and results, under certain conditions, in smooth homogeneous, pourable systems often referred to as `fluid gels'.20 To produce smooth homogeneous fluid gels with gellan gum, systems must be formulated to give weak gelation, either by manipulating the ion type and concentration or gellan gum concentration. The viscosity and structure of the
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system correlates with the gel strength of the unsheared gel. Therefore, the greater the gel strength of the unsheared gel, the greater the viscosity and structure will be when the system is sheared. Systems which gel too strongly, however, can give rise to a grainy appearance in the final fluid. Gellan gum fluid gels can be prepared using a variety of processes. Three potential processes are outlined schematically in Fig. 9.6. The first step in each case is to hydrate the gellan gum through a combination of heat and sequestrants. Method 1 simply involves continually stirring the solution as it cools to form the fluid gel or allowing the weak gel to form undisturbed, then shearing to form the fluid gel. Alternatively, in method 2 the hot gellan gum solution can be added to cold water whilst mixing. This results in cooling of the solution and formation of a fluid gel. In method 3 it is possible to prepare gellan gum solutions that will not gel on cooling. Addition of ions to these cold solutions results in gelation and formation of a fluid gel. The application of shear can be achieved by using stirring, homogenisation, filling or even `shake before use'. Shear can also be applied during or after gelation. UHT, HTST and processes involving scraped surface heat exchangers (i.e. for the production of custards, gravies and ketchup) are an ideal way to shear during gelation as the solution cools. Gellan gum fluid gels can be used to produce shelf stable suspensions in a variety of beverage products.21 Generally, HA gellan gum should be used at approximately 0.02±0.05% to give a smooth fluid gel. For LA gellan gum the use level is dependent on the ionic concentration in the system and a guide to the formulation of fluid gels using LA gellan gum is given in Table 9.7. Example formulations for beverages
Fig. 9.6
Outline of potential processes for the preparation of gellan gum fluid gels.
Gellan gum Table 9.7
217
Guidelines for formulation of LA gellan gum fluid gels
Ion
Concentration
Calcuim
Low (< 50ppm) Optimum (100±600ppm) High (> 600ppm) Low (< 0.25%) Optimum (0.5±2.0%) High (4.0±10%)
Sodium Milk Sugars
LA gellan gum concentration (%) 0.05±0.2 0.03±0.05 0.05±0.2 0.05±0.2 0.03±0.05 0.05±0.2 0.05±0.2 0.1±0.3
(40±60%)
using LA or HA gellan gum which can be used to suspend gelled beads or fruit pulp are given below. Formulation 9.3 provides a starting point for a beverage type fluid gel. It has a pH of 2.9 and a setting temperature of approximately 15 ëC. It can be used to suspend jelly beads and is prepared as outlined in method 1 of Fig. 9.6 by shearing after the weak gel has been allowed to form. Formulation 9.4 is an example of a fluid gel formed with HA gellan gum. HA gellan gum fluid gels are less sensitive to the ionic conditions and have longer, more elastic flow properties when compared to LA gellan gum fluid gels.
Formulation 9.3
Recipe for a fluid gel for beverages using LA gellan gum
Ingredients Part 1 Sucrose Tri sodium citrate dihydrate LA gellan gum Sodium benzoate Deionised water Part 2 Citric acid Calcium lactate Deionised water
Weight (g)
(%)
112.0 0.60 0.28 0.20 862.0
11.25 0.06 0.028 0.02 86.60
5.00 0.25 15.00
0.50 0.025 1.517
Preparation 1. Blend the sucrose, tri sodium citrate dihydrate, LA gellan gum and sodium benzoate and disperse in the deionised water of Part 1. 2. Heat the dispersion to 70±80 ëC to hydrate. 3. Dissolve the citric acid and calcium lactate in the deionised water of Part 2 and add to the hot gum solution. 4. Cool the sample to below 15 ëC undisturbed. 5. Gently agitate the sample to form a fluid gel.
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Formulation 9.4
Recipe for a pulp suspension beverage using HA gellan gum
Ingredients Water Fruit juice Sugar HA gellan gum Tri sodium citrate dihydrate Citric acid anhydrous Potassium citrate
Weight (g)
(%)
338.10 100.0 60.0 0.25 0.25 0.9 0.5
67.62 20.0 12.0 0.05 0.05 0.18 0.1
Preparation 1. Blend the HA gellan gum with the tri sodium citrate dihydrate and disperse in the water. 2. Heat the dispersion to 90 ëC to hydrate the gum. 3. At 90 ëC add the remaining dry ingredients and the fruit juice. 4. Cool to room temperature whilst mixing to form the fluid gel.
9.5.3 Dairy Unlike water systems, much of the calcium in milk is associated with the milk proteins. During heating any remaining free calcium is also bound by the proteins and therefore does not interfere with the hydration of the gellan gum. Because of this, both HA and LA gellan gum will hydrate in milk above approximately 80 ëC without the need for a sequestrant. Milk also contains sodium and potassium ions and it is therefore not usually necessary to add additional gelling ions to milk systems. Because the LA gellan gum is gelled with a low level of monovalent ions (predominantly K+), milk gels are thermally reversible, melting at approximately 95 ëC. Thermal stability of LA gellan gum milk gels can be improved by the addition of calcium. Care must be taken when adding calcium to hot milk since it can result in precipitation of the milk proteins if added above 70 ëC. It is recommended to cool the gellan/milk mixture to between 55 and 65 ëC before adding the calcium. This temperature range is above the gelation temperature of the LA gellan gum but below the temperature at which milk protein precipitation occurs. In many dairy systems milk powders are used. These powders are natural sequesterants and will bind calcium from the water used for reconstitution. Therefore, it is not usually necessary to add a sequestrant when water of hardness up to 400 ppm (as CaCO3) is used for reconstitution. Some sequestrant may be required if less than 2% milk powder is being used or harder water is used to reconstitute the milk powder. Milk beverages As described in Section 9.5.2, HA gellan gum at low concentrations is able to form a very weak gel network often referred to as a fluid gel. These fluid gels have extremely good suspension properties and can be used in a range of neutral dairy and soya based products such as chocolate milk. Development of this application was initially limited due to creation of off-flavours as a result of
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residual enzyme activity in the native, HA gellan gum acting on the milk. The off-flavour, which is reminiscent of cleaning chemicals, renders the product unpalatable and is linked to the development of para-cresol. Development of a process in which the HA gellan gum is pre-treated with a denaturing agent that is thought to act on the residual enzymes in the gellan gum has led to a new grade of gellan gum for this application.22 KELCOGELÕ HM-B is a standardised product containing the pre-treated HA gellan gum and can be used at 0.1±0.12% for stabilisation of cocoa in chocolate milk. The HA gellan gum is tolerant to a wide range of UHT conditions and products can be filled at higher temperatures than with carrageenan, the traditional hydrocolloid for this application. It provides excellent suspension of cocoa and long-term stability. It is functional in reconstituted milk, fresh milk, whey substituted beverages and low protein milk beverages. Yogurt The standard yogurt process can be followed when using LA gellan gum for both set and stirred yogurt. There are various ways in which yogurt containing LA gellan gum can be made depending on the usual manufacturing process and on the desired properties of the final yogurt. In all cases the initial steps are the same: the LA gellan gum should be blended with skimmed milk powder and other stabilisers (if required) and dispersed in cold milk before heating, homogenising and pasteurising. Generally, the fermentation time is not affected by the presence of LA gellan gum. The important factor to remember is that the setting temperature of LA gellan gum in skimmed milk is about 41 ëC. If shear is applied through the setting temperature a fluid gel will be formed. This acts like a gel under static conditions, but flows like a liquid when shear is applied. If, however, shear is applied below the setting temperature in yogurt systems, a broken gel may result which can lead to unsatisfactory lumps in the final product. Typical use levels for LA gellan gum in yogurt are 0.04%. It can be used in combination with other stabilisers such as starch depending on the final texture required. 9.5.4 Sugar confectionery One of the fundamental techniques for the manipulation of the texture of sugar confectionery is to use combinations of a variety of sugars such as sucrose, glucose, fructose and various corn syrups. Combinations of sugars produce desirable textures, as well as preventing crystallisation of individual sugars. Another critical ingredient is the hydrocolloid. These impart structure to the product and provide the characteristic jelly texture. Before describing how to make confectionery jellies with gellan gum, it is worth discussing the influence of sugars on the properties of gellan gum. Effect of sugars on LA gellan gum The presence of sugars has two major effects on the properties of LA gellan gum gels. Firstly, the ion requirements for optimum gel properties are reduced. The
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Fig. 9.7
Effect of sucrose concentration on the modulus ( ), hardness (ú) and brittleness (n) of 0.5% LA gellan gum gels.
presence of 40% w/w sugar approximately halves the calcium required for maximum gel modulus, from 8±10 mM in water gels to 4±5 mM in the sugar gels. Addition of 60% w/w sugar results in an approximately ten-fold reduction in the requirement for calcium, with only 0.5±1.0 mM added calcium required for maximum gel modulus. Similar reduction in the requirements for sodium and potassium are also seen (Fig. 9.2). Secondly above approximately 40% sugar gels become less firm and less brittle, i.e., softer and more elastic (Fig. 9.7). These effects are believed to be the result of the sugars inhibiting the aggregation step of the gelation process.23,24 These effects are also influenced by the type of sugar. Sucrose has a greater inhibitory effect than glucose, fructose or corn syrups (Table 9.8).25 The differences observed between sugars mean that texture can, to some degree, be varied by manipulating the sugar composition of the system. For example, partial replacement of sucrose with fructose or corn syrup, a common practice to control crystalisation in confectionery manufacture, results in firmer, more brittle gels.26 Effect of sugars on HA gellan gum Less is known about the specific effects of sugars on HA gellan gum. However, addition of sugars to HA gellan gum gels generally results in an increase in the force required to break the gel. Setting and melting temperature also increase with increasing sugar concentration. In the presence of high levels of sugar (70±
Gellan gum
221
Table 9.8 Effect of sugars at 40% and 60% w/w on the textural properties of 0.5% w/w LA gellan gum gels prepared at ion concentrations giving maximum gel modulus Sugar
Fructose Glucose Sucrose Maltose 42DE corn syrup 14DE maltodextrin
Modulus (Ncmÿ2)
Brittleness (%)
40% w/w
60% w/w
40% w/w
60% w/w
14.1 12.9 13.5 15.5 19.0 16.1
3.70 2.17 1.60 3.83 5.06 5.88
31.3 36.7 30.2 30.7 27.9 24.1
53.6 62.9 63.3 51.4 53.0 43.1
80%), HA gellan gum has very high viscosity even when hot. This can make processes such as mixing and depositing difficult. This is often compounded by the high setting temperature which can result in pre-gelation, i.e., gel formation prior to deposition of the confectionery mix. However, incorporation of low levels of HA gellan gum into confections made with LA gellan gum will increase the chewiness of the jellies. Preparation of confectionery jellies LA gellan gum may be used alone or in combination with other gelling agents to produce jelly confectionery by traditional processes. Examples are provided in Formulations 9.5 and 9.6. When prepared with LA gellan gum as the sole gelling agent (Formulation 9.5), the jellies are firm with a short, clean bite and flavour. The jellies can be removed from the starch moulds after about 2 h but are usually stoved for up to 72 h before demoulding. Addition of a thin boiling starch as outlined in Formulation 9.6 results in a chewier texture. Combinations of LA gellan gum with carrageenan can be used to produce gelatin-free confectionery which is suitable for halal. Pre-gelation is the premature gelation of the confectionery mix prior to, or during, depositing. This makes depositing difficult and results in a weaker gel structure and grainy texture. Table 9.9 provides a guide to preventing pregelation in gellan gum confections. 9.5.5 Fruit preparations This application covers a wide variety of systems from 30 to 75% total soluble solids. Much of the understanding of the effects of sugars described in confectionery applications can be applied to these systems. In addition, the type of fruit used in the formulation is a key consideration when using LA gellan gum since the ion content and pH will vary. Fruit composition may also vary during the season. Table 9.10 shows that the ionic composition varies considerably between different fruits with most fruits containing significant levels of potassium ions.27 These ionic concentrations become increasingly significant in
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Formulation 9.5
Recipe for jelly sweets using LA gellan gum
Ingredients
Weight (g)
(%)
Sucrose Glucose syrup (42DE) Water Citric acid anhydrous Tri sodium citrate dihydrate KELCOGELÕ F gellan gum Calcium hydrogen orthophosphate Flavour and colour
159.0 159.0 120.0 5.00 5.00 3.75 0.20 as required
35.20 35.20 26.51 1.11 1.11 0.83 0.04
Preparation 1. Blend the LA gellan gum and calcium hydrogen orthophosphate with 1.0 g of tri sodium citrate dihydrate and 40 g of sucrose and disperse in the water. 2. Heat to boiling to hydrate the gellan gum then add the remainder of the sugar while continuing to boil. 3. Add pre-warmed glucose syrup while maintaining the temperature above 90 ëC. 4. Cook the liquor to 80±82% total solids then cool to 90 ëC. 5. Dissolve the citric acid and remainder of the tri sodium citrate dihydrate, colour and flavour in 20 cm3 of water and stir into the liquor. 6. Deposit at 76±78% total solids into starch moulds. 7. Stove to final solids as required.
Formulation 9.6 Recipe for jelly sweets using LA gellan gum and thin boiling starch Ingredients
Weight (g)
(%)
Water Glucose syrup (42DE) Sugar Thin boiling starch (FLOGEL 60) LA gellan gum Tri sodium citrate dihydrate Citric acid anhydrous Calcium hydrogen orthophosphate Flavour and colour
220.0 159.0 148.5 18.8 3.5 1.8 1.8 0.2 as required
39.0 28.2 28.1 3.4 0.62 0.32 0.32 0.04
Preparation 1. Slurry the starch in 50 g of water. 2. Blend the LA gellan gum, calcium hydrogen orthophosphate and tri sodium citrate dihydrate with 40 g of sugar and disperse in the remainder of the water. 3. Heat the dispersion to boiling to hydrate the gellan gum then add the remaining sugar and continue to boil. 4. Add pre-warmed glucose syrup and cook to boiling. 5. Add the starch slurry, breaking the boiling point and continue to cook to 78% total solids. 6. Add colour, flavour and citric acid pre-dissolved in a small amount of water. 7. Deposit into starch moulds at 74% total solids and stove to final solids as required.
Gellan gum Table 9.9
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A guide to the prevention of pre-gelation in confectionery mixes
Problem
Possible causes
Solution
Pre-gelation when acid added
Hard water
Add sodium hexameta phosphate
Depositing soluble solids too high
Lower depositing solids
pH too low
Add sodium citrate with citric acid
Hard water
Increase sequestrant level
Soluble solids too high
Add water to lower soluble solids
Pre-gelation before acid added
Table 9.10 Fruit Apple Blackcurrant Raspberry Strawberry Apricot Peach
Ionic composition of raw fruits27 Ca++ (mg/100g)
Mg++ (mg/100g)
Na+ (mg/100g)
K+ (mg/100g)
4 60 25 16 15 7
3 17 19 10 11 9
2 3 3 6 2 1
88 370 170 160 270 160
medium to high solids systems where gellan gum ion requirements are greatly reduced. Therefore, formulations optimised for one fruit will often need modification to accommodate different fruits. Formulation 9.7 is an example of a low solids (36%) jam which can be formulated to give a range of textures. HA gellan gum provides a soft, spreadable jam with excellent sheen. The addition of a proportion of LA gellan gum may be used where a firmer textured jam is required. LA gellan gum alone can be used to give a more bake stable jam. Various fruits can be used, including strawberries, raspberries and blackcurrants. Formulation 9.8 is slightly higher in solids than Formulation 9.7 and produces a lightly gelled yogfruit with evenly suspended fruit pieces. The gelled structure can be broken down by pumping to give a smooth, viscous yogfruit (pH 3.9, soluble solids 40%). Finally, Formulation 9.9 with 55% fruit and no added water demonstrates the properties of a gellan gum and starch-based preparation. The LA gellan gum filling has a glossy appearance, good flavour release and excellent bake stability (pH 3.4, tss 56%).
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Formulation 9.7
Recipe for a reduced sugar jam using HA or LA gellan gum blend
Ingredients Frozen strawberries Sugar Water Gellan gum* Tri sodium citrate dihydrate Potassium sorbate Citric acid solution (50% w/w)
Weight (g)
(%)
450.0 283.5 260.0 2.5 0.5 1.0 2.5
45.0 28.35 26.0 0.25 0.05 0.10 0.25
* High acyl and/or low acyl gellan gum can be used depending on desired final texture.
Preparation 1. Dry blend the gellan gum, tri sodium citrate dihydrate and potassium sorbate with the sugar and disperse into the water. 2. Add the fruit and heat to boiling. Cook for 1±2 minutes to ensure hydration of the gellan gum. Check the soluble solids. 3. Remove from the heat and add the citric acid solution. Fill into jars and cap immediately. Formulation 9.8
Recipe for a peach yogfruit using LA gellan gum
Ingredients Peach pureÂe Diced peach Glucose syrup LA gellan gum Tri sodium citrate dihydrate Sodium benzoate
Weight (g)
(%)
200.0 200.0 300.0 0.35 1.80 0.25
28.50 28.50 42.65 0.05 0.26 0.04
Preparation 1. Combine the fruit and glucose syrup. 2. Add the tri sodium citrate dihydrate, LA gellan gum and sodium benzoate and heat to 90 ëC with constant stirring. 3. Hold for 1 minute then cool, with stirring, to 60 ëC. 4. Deposit and allow to cool undisturbed. Note: The tri sodium citrate dihydrate in the formulation is added to give a final pH of 3.9. The addition level may be varied depending on the fruit used, and the final pH required.
9.5.6 Other applications Gellan gum forms films and coatings that can be used in breadings and batters. Films offer several advantages, particularly their ability to reduce oil absorption by providing an effective barrier. Films can be prepared by applying a hot solution of gellan gum on to the surface of the food product, by spraying or dipping, and allowing to cool. Alternatively, in the case of LA gellan gum, the food can be dipped into a cold solution of the gum, allowing ions to diffuse into
Gellan gum Formulation 9.9
225
Recipe for a bake-stable fruit preparation using LA gellan gum
Ingredients Apple (thawed) Sucrose Modified starch (THERMFLO) LA gellan gum Citric acid solution (50% w/w) Tri sodium citrate dihydrate
Weight (g)
(%)
210.0 160.8 8.00 0.32 0.80 0.88
55.2 42.2 2.10 0.12 0.20 0.18
Preparation 1. Pre-blend the dry ingredients, add to the apple and heat with stirring to boiling. 2. Remove from heat, add the citric acid solution, mix well and deposit. 3. Leave to gel before use. Shear, and use as required.
the solution, resulting in gelation or film formation. LA gellan gum can also be used to produce fat free adhesion systems. Spraying of a cold solution of LA gellan gum onto the surface of products such as nuts, crisps and pretzels forms an instant thin layer of gel when it reacts with the salt thus facilitating adhesion of spice, flavour or sweetener blends.
9.6 Regulatory status In Japan, gellan gum has been considered a `natural' food additive since 1988. It is now approved for food use in the USA and the European Union as well as Canada, South Africa, Australia, most of South East Asia and Latin America. Gellan gum appears as E418 in the European Community Directive EC/95/2 in Annex 1. Both the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and the European Community Scientific Committee for Food has given gellan gum an Acceptable Daily Intake (ADI) of `not specified'. Combinations of HA and LA gellan gum have one name. A manufacturer may label a product made with a combination of both types of gellan gum simply, `E418' or `gellan gum'.
9.7
Future trends
The current commercial HA and LA gellan gum products can be considered as being at opposing ends of the textural spectrum available with hydrocolloid gelling agents (Fig. 9.3). Blends of the two types enable some intermediate textures to be created but the mixed system retains the high setting temperature of the HA product and the ion sensitivity of the LA product. Far more interesting are gellan gums of intermediate acyl content as these show much more variation in texture and a single homogeneous setting behaviour.28,29 Methods for creating these partially acetylated products exist but they are yet to be produced on a commercial scale.30 Realisation of this control over the degree of acylation of gellan gum could lead to a truly universal gelling agent.
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9.8
Sources of further information and advice
www.CPKelco.com Imeson, A. (1999) Thickening and Gelling Agents for Food, 2nd edn. Aspen Publishers Inc., Gaithersburg, MD.
9.9 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11.
References and MORRIS, V.J. (1983) `Structure of the acidic extracellular gelling polysaccharide produced by Pseudomonas elodea', Carbohydr. Res., 124, 123±33. JANSON, P.-E., LINDBURG, B. and SANDFORD, P.A. (1983) `Structural studies of gellan gum, an extracellular polysaccharide elaborated by Pseudomonas elodea', Carbohydr. Res., 124, 135±9. KUO, M.-S., MORT, A.J. and DELL, A. (1986) `Identification and location of L-glycerate, an unusual acyl substituent in gellan gum', Carbohydr. Res., 156, 173±87. CHANDRASEKARAN, R., MILLANE, R.P., ARNOTT, S. and ATKINS, E.D.T. (1988) `The crystal structure of gellan', Carbohydr. Res., 175, 1±15. CHANDRASEKARAN, R., PUIGJANER, L.C., JOYCE, K.L. and ARNOTT, S. (1988) `Cation interactions in gellan: an X-ray study of the potassium salt', Carbohydr. Res., 181, 23±40. CHANDRASEKARAN, R. and THIALAMBAL, V.G. (1990) `The influence of calcium ions, acetate and L-glycerate groups on the gellan double helix', Carbohydr. Polym., 12, 431±442. CHANDRASEKARAN, R., LEE, E.J., RADHA, A. and THAILAMBAL, V.G. (1992) `Correlation of molecular architectures with physical properties of gellan related polymers', in Frontiers in Carbohydrate Research ± 2, ed. R. Chandrasekaran. Elsivier Applied Science, New York, pp. 65±84. MORRIS, E.R., GOTHARD, M.G.E., HEMBER, M.W.N., MANNING, C.D. and ROBINSON, G. (1996) `Conformational and rheological transitions of welan, rhamsan and acylated gellan' Carbohydr. Polym., 30, 165±75. MORRIS, E.R., REES, D.A. and ROBINSON, G. (1980) `Cation-specific aggregation of carrageenan helices: domain model of polymer gel structure', J. Mol. Biol., 138, 349. GRAZDALEN, H. and SMIDSRéD, O. (1987) `Gelation of gellan gum', Carbohydr. Polym., 7, 371±93. O'NEILL, M.A., SELVENDRAN, R.R.
NODA. S., FUNAMI, T., NAKAUMA, M., ASAI, I., TAKAHASHI, R., AL-ASSAF, A., IKEDA, S.,
and PHILLIPS, G.O. (2008) `Molecular structures of gellan gum imaged with atomic force microscopy in relation to the rheological behaviour in aqueous systems. 1. Gellan gum with various acyl contents in the presence and absence of potassium', Food Hydrocolloids, 22, 1148±59. SANDERSON, G.R. and CLARK, R.C. (1984) `Gellan gum, a new gelling polysaccharide', in Gums and Stabilisers for the Food Industry 2, eds. G.O. Phillips, D.J. Wedlock, and P.A. Williams. Pergamon Press, Oxford, pp. 201±10. NISHINARI, K.
12. 13.
KASAPIS, S., GIANNOULI, P., HEMBER, M.W.N., EVAGELIOU, V., POULARD, C., TORT-
and SWORN, G. (1999) `Structural aspects and phase behaviour in deacylated and high acyl gellan systems', Carbohydr. Polym., 38 145±54.
BOURGEOIS, B.
Gellan gum 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
29. 30.
227
and SWANSON, B.G. (2004) `Gelling temperatures of high acyl gellan as affected by monovalent and divalent cations with dynamic rheological analysis', Carbohydr. Polym., 56 27±33. BOURNE, M.C. (1978) `Texture profile analysis', Food Technology, 32, 67±72. PONS, M. and FISZMAN, S.M. (1996) `Instrumental texture profile analysis with particular reference to gelled systems', Journal of Texture Studies, 27, 597±624. MATSUKAWA, S. and WATANABE, T. (2007) `Gelation mechanism and network structure of mixed solution of low- and high-acyl gellan studied by dynamic viscoelasticity, CD and NMR measurements', Food Hydrocolloids, 21, 1355±61. MAO, R., TANG, J. and SWANSON, B.G. (2000) `Texture properties of high and low acyl mixed gellan gels', Carbohydr. Polym., 41, 331±8. HUANG, Y., TANG, J., SWANSON, B.G. and RASCO, B.A. (2003) `Effect of calcium concentration on the textural properties of high and low acyl mixed gellan gels', Carbohydr. Polym., 54, 516±22. SWORN, G., SANDERSON, G.R. and GIBSON, W. (1995) `Gellan gum fluid gels' Food Hydrocolloids, 9, 265±71. VALLI, R. and JACKSON, P.H. (2004) `Shelf stable suspensions: gellan gum blend forms fluid gels in beverages', Food and Beverage Asia, April, pp. 54±57. VALLI, R.C. and MORRISON, N.A. (2002) `Methods of making sterilized milk compositions comprising native gellan gum'. Patent No. PCT/WO 02/060268 A2. SWORN, G. (1996) `Gelation of gellan gum in confectionery systems' in Gums and Stabilisers for the Food Industry 8, eds. G.O. Phillips, P.A. Williams and D.J. Wedlock. IRL Press, Oxford, pp. 341±9. SWORN, G. and KASAPIS, S. (1998) `The use of Arrhenius and WLF kinetics to rationalise the mechanical spectrum in high sugar gellan systems', Carbohydr. Res. 309, 353±61. SWORN, G. and KASAPIS, S. (1998) `Effect of conformation and molecular weight of co-solute on the mechanical properties of gellan gum gels', Food Hydrocolloids, 12, 283±90. GIBSON, W. (1992) `Gellan gum', in Thickening and Gelling Agents for Food, ed. A. Imeson. Blackie Academic and Professional, Glasgow, pp. 227±49. MCCANCE and WIDDOWSON'S The Composition of Foods, 5th Edition, RSC and MAFF, 1991. BAIRD, J.K., TALASHEK, T.A. and CHANG, H. (1992) `Gellan gum: Effect of composition on gel properties', in Gums and Stabilisers for the Food Industry 6, eds. G.O. Phillips, P.A. Williams and D.J. Wedlock. IRL Press, Oxford, pp. 479± 87. MORRISON, N.A., SWORN, G. CLARK, R.C., TALASHEK, T. and CHEN, Y-L. (2002) `New textures with high acyl gellan gum', in Gums and Stabilizers for the Food Industry 11, eds. G.O. Phillips and P.A. Williams. IRL Press, Oxford, pp. 297±305. SWORN, G., CHEN, Y-L., MORRISON, N.A., TALASHEK, T. and CLARK, R. (1999) `Modified gellan gum composition process for preparation of same and use thereof'. Patent No. PCT/WO 99/64468. HUANG, Y., SINGH, P., TANG, J.
Abstract: Gellan gum is an extracellular polysaccharide secreted by the micro-organism Sphingomonas elodea (ATCC 31461) previously referred to as Pseudomonas elodea. Commercially it is manufactured by a fermentation process. It is available in two forms, high acyl (HA) and low acyl (LA). Gellan gum forms gels at low concentrations when hot solutions are cooled in the presence of gel promoting cations. An overview of the manufacturing process and chemical structure of gellan gum is given. The functional differences between the two forms of gellan gum are reviewed in detail with regard to hydration, gelation, stability and texture. Methods for the effective preparation of gellan gum solutions and gels are described for the high and low acyl forms. The effects of food ingredients such as salts, sugars, and acids on gel properties are discussed in relation to the main applications. Finally, the main applications of gellan gum in food are described with example formulations. Key words: gel, setting temperature, texture, extracellular, fermentation, fluid gel.
9.1
Introduction
Gellan gum is an extracellular polysaccharide secreted by the micro-organism Sphingomonas elodea (ATCC 31461) previously referred to as Pseudomonas elodea. Gellan gum forms gels at low concentrations when hot solutions are cooled in the presence of gel promoting cations. It is available in a substituted or unsubtituted form. Gel properties depend on the degree of substitution with the substituted form, producing soft, elastic gels whilst the unsubstituted form produces hard, brittle gels.
Gellan gum
9.2
205
Manufacture
Commercially, gellan gum is manufactured by inoculating a fermentation medium with the micro-organism. The medium contains a carbon source, such as glucose, phosphate and nitrogen sources, and appropriate trace elements. The fermentation is carried out under sterile conditions with strict control of aeration, agitation, temperature and pH. After fermentation, the viscous broth is pasteurised to kill viable cells. The polysaccharide can then be recovered in several ways. Direct recovery by alcohol precipitation from the broth yields the substituted native, or high acyl (HA), form. Alternatively, treatment of the broth with alkali prior to alcohol precipitation results in deacylation and yields the unsubstituted, low acyl (LA) form. Gellan gum is currently sold commercially in three basic forms, namely high acyl, unclarified (KELCOGELÕ LT100), low acyl, unclarified (KELCOGELÕ LT), and low acyl, clarified (KELCOGELÕ and KELCOGELÕ F). A number of specialty grades are also available.
9.3
Structure
The primary structure of gellan gum, shown in Fig. 9.1, is composed of a linear tetrasaccharide repeat unit: !3)- -D-Glcp-(1!4)- -D-GlcpA-(1!4)- -D-Glcp(1!4)- -L-Rhap-(1!.1,2 The polymer is produced with two acyl substituents
Fig. 9.1
Primary structure of (a) low and (b) high acyl gellan gum.
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present on the 3-linked glucose, namely, L-glyceryl, positioned at O(2) and acetyl at O(6). On average there is one glycerate per repeat unit and one acetate every two repeats.3 X-ray diffraction of low acyl gellan gum has shown that the polymer exists in the solid state as a co-axial three-fold double helix.4,5 Computer modelling of the high acyl structure, based on the knowledge of the low acyl form, concluded that the acetate substituents would be positioned on the outside of the double helix.6 Subsequent X-ray diffraction data revealed that the glycerate substituent was accommodated by an approximately 14ë rotation of the carboxyl group about the C(5)-C(6) bond on the adjacent glucuronate residue.7 This was accompanied by a 17 3ë rotation of the glucuronate residue itself to maintain the required spacing between the carboxyl and glycerate groups. It is proposed that this structure results in helices stabilised by interchain associations involving the glycerate groups, with the acetyl substituents positioned on the periphery of the helix.8
9.4
Technical data
There are three steps to consider for the successful formulation of gellan gum systems: 1. dispersion 2. hydration 3. gelation. These will now be considered in turn for both the LA and HA forms of the gum. 9.4.1 Dispersion The first step in preparing any gum solution is to ensure that the gum particles are properly dispersed in the solvent and do not clump together. Poor dispersion will result in incomplete hydration and loss of gum functionality. Both forms of gellan gum are insoluble in cold water although they will tend to swell in water of low calcium content. The gum can therefore be readily dispersed in deionised water by stirring and adding the powder slowly to the vortex. As the ion concentration in the water increases, dispersion becomes even easier. For example, in moderately hard water (~180 ppm hardness expressed as CaCO3), the gum can be added to the surface of the water and then dispersed with gentle agitation. By blending the gum with dispersants such as sugar (5±10 times weight of gum) or glycerol, alcohol, or oils (3±5 times weight of gum), it is possible to add the gum directly to hot water. Both forms of gellan gum are also readily dispersible in milk and reconstituted milk systems. 9.4.2 Hydration Low acyl gellan gum The temperature at which LA gellan gum hydrates is dependent on the type and concentration of ions in solution. The presence of ions such as sodium and, in
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207
Table 9.1 Effect of dissolved salts on the hydration temperature of 0.25% LA gellan gum Water hardness (ppm CaCO3) 0.000 100 200 300 ± ± ± ± ± ± ±
Dissolved NaCl (%)
Hydration temperature (ëC)
± ± ± ± 0.10 0.25 0.45 0.70 0.90 1.00 1.30
75 88 > 100 > 100 50 52 60 70 82 89 > 100
particular, calcium, in solution will inhibit the hydration of LA gellan gum as shown in Table 9.1. It is therefore necessary, in most circumstances, to use a sequestrant to bind the soluble calcium and so aid hydration. Typically, between 0.1 and 0.3% of a sequestrant such as sodium citrate is sufficient to allow complete hydration at 90±95 ëC in water of up to 600 ppm as CaCO3 water hardness. Incomplete hydration will result if the sodium ion concentration exceeds 0.5% (approximately 1.3% sodium chloride). Once the gum is hydrated, additional ions can be added to the hot solution and, provided the temperature is maintained above the gelation temperature, no gel will form. Table 9.2 provides details of the effects of the sequestrant sodium citrate on the hydration temperature of LA gellan gum and shows that hydration can be achieved at temperatures ranging from room temperature to boiling point. Table 9.3 lists the relative sequestering power and effective pH ranges of a number of commonly used sequestrants. In foods containing sugar, LA gellan gum should be hydrated in water and any sugars can be added to the hot gum solution. However, LA gellan gum can be hydrated directly in sugar solutions up to 80 total soluble solids (tss) by heating to boiling. In some cases a low level of sequestrant such as sodium Table 9.2
Effect of sodium citrate on the hydration of 0.25% LA gellan gum
Water hardness (ppm CaCO3) 0 100 300 600 900
Sodium citrate (%)
Hydration temperature (ëC)
0.00 0.05 0.10 0.20 0.40
75 25 65 65 68
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Table 9.3
Relative sequestering power of sequestrants commonly used in foods
Sequestrant
Sodium hexametaphosphate Tetra sodium diphosphate Di sodium orthophosphate Tri sodium citrate dihydrate
Parts of sequestrant required to sequester 1 part of available calcium pH6
pH4
7 10 130 20
7 20 180 40
citrate (less than 0.3%) is required to bind the free calcium often present in sugar syrups. LA gellan gum will not fully hydrate below pH 3.9. In this case, the acid should be added, preferably as a concentrated solution, to the hot gum solution. Prolonged heating in acidic conditions should be avoided as this leads to some hydrolytic degradation of the gum resulting in a reduction in the quality of the final gel. However, at pH 3.5, LA gellan gum can be held for up to 1 h at 80 ëC with only a minimal loss in gel quality. In neutral conditions solutions can be held at 80 ëC for several hours. LA gellan gum is readily dispersible in milk and reconstituted milk systems and will hydrate upon heating to approximately 80 ëC without the need for a sequestrant. High acyl gellan gum The hydration of HA gellan gum is much less dependent on the concentration of ions in solution than LA gellan gum and generally heating to 85±95 ëC is sufficient to fully hydrate the gum in both water or milk systems. As a dispersion of HA gellan gum is heated, it swells rapidly at approximately 40±50 ëC to form a thick, pasty suspension. With continued heating the suspension loses viscosity suddenly at approximately 80±90 ëC signifying complete hydration. The swelling stage can be avoided by adding the gum directly to hot water (>80 ëC) with the aid of a dispersant such as sugar, oil or glycerol, as described in the previous section. The hydration of HA gellan gum is inhibited by the presence of sugars; therefore it is recommended to hydrate the gum in less than 40% tss. Additional sugar can then be added to the hot gum solution. As with LA gellan gum, HA gellan gum will not hydrate below pH 4.0. Similar care must also be taken under hot acidic conditions to avoid hydrolytic breakdown and loss of gel quality. 9.4.3 Gelation The proposed gelation mechanism of gellan gum is based on the domain model which assumes the formation of distinct junction zones and disordered flexible polymer chains connecting adjacent junction zones.9 As a hot solution cools, gellan gum undergoes a disorder±order transition. This transition has been
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209
attributed to a coil-helix transition.10 In the case of LA gellan gum, gel promoting cations such as sodium, potassium, calcium and magnesium promote aggregation of the gellan double helices to form a three-dimensional network and the subsequent gels are hard and brittle. Recent studies using atomic force microscopy have challenged this and proposed a fibrous model in which network structures develop through the formation of non-associated fibres or strands via either elongation or branching.11 The acyl substituents have a profound effect on the structure and rheological characteristics of gellan gum gels. The gellan gum undergoes a similar disorder to order transition as the solution is cooled but according to the domain model further aggregation of the helices is limited by the presence of the acetyl group.8 According to the fibrous model the acyl groups inhibit end-to-end type intermolecular associations through a type of steric hindrance, resulting in a decrease in the degree of continuity and homogeneity of the gelled system.11 The subsequent gels of HA gellan gum are therefore soft and elastic. Low acyl gellan gum The easiest and most common method of making LA gellan gum gels is to cool hot solutions. LA gellan gum forms gels with a wide variety of cations, notably calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+) as well as acid (H+).10,12 Divalent cations are more efficient at promoting gelation of LA gellan gum than monovalent ions. Gel strength increases with increasing ion concentration until a maximum is reached. Further addition of ions results in a reduction of gel strength due to the `over conversion' of the LA gellan gum with excess ions. Ion concentrations for optimum gelation are generally independent of gum concentration but are reduced as the level of sugar is increased. Figure 9.2 compares the effect of both mono- and divalent ions on the modulus of 0.5% LA gellan gum gels in water and in 60% sucrose. Below pH 3.0 LA gellan gum is able to form a gel without the need for mono- or divalent metal ions. Optimum gel modulus occurs for these acid gels at approximately pH 2.8±3.0 regardless of the acid used. This optimum is not affected by the presence of sugars to the same extent as ion requirements. For example, in the presence of 60% sucrose the optimum shifts to approximately pH 2.5±2.7. Acid gels are generally stronger than ion mediated gels in both water and sugar. Addition of other gelling ions, such as sodium or calcium, generally results in a reduction in gel strength of the acid gels. Gel properties at optimum conditions for LA gellan gum gels in water and 60% sucrose are summarised in Tables 9.4 and 9.5 respectively. In many instances addition of gelling ions is not necessary since there are sufficient ions present in the water or other ingredients to promote gelation of the LA gellan gum. When required, the gel promoting ions can be added to the hot gum solution and, provided the solution is kept above its setting temperature, no gel will form. This allows for the easy preparation of stock solutions (1±2% gum) which can be held at high temperature until required. LA gellan gum is often described as having a 'snap set' since gelation is very rapid once the setting temperature is reached. As with hydration temperature, setting and
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Fig. 9.2 Effect of calcium (squares), sodium (triangles) and potassium (circles) ion concentration on the modulus of 0.5% LA gellan gum gels in water (open symbols) and 60% sucrose (closed symbols). Table 9.4 Properties of 0.5% LA gellan gum gels in optimum conditions for gel formation in water Gelling ion
Ion concentration (mM)
Modulus (Ncmÿ2)
Brittleness (%)
Setting temperature (ëC)
Calcium Sodium Potassium Acid
8±10 260±300 240±260 pH2.8±3.0
19.3 12.3 12.1 20.3
28.5 28.2 30.0 27.7
42 54 59 10
Table 9.5 Properties of 0.5% LA gellan gum gels in optimum conditions for gel formation in 60% sucrose Gelling ion Calcium Sodium Potassium Acid
Ion concentration (mM) 0.5±1.0 25±35 8±10 pH2.5±2.7
Modulus (Ncmÿ2)
Brittleness (%)
Setting temperature (ëC)
1.66 3.13 1.71 7.74
61.5 54.1 65.6 43.9
38 47 43 64
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211
melting temperatures of the gels depend on the ion concentration in solution. The higher the ion concentration, the higher the setting and melting temperature. Significant thermal hysteresis between the setting and melting temperature is observed in LA gellan gum gels, i.e., the gels melt at a higher temperature than that at which they set.13 In most conditions LA gellan gum gels are not thermally reversible below 100 ëC. The exceptions are gels formulated with a low level of monovalent ions, particularly potassium, and milk gels. Setting time is governed by the rate at which heat is removed which, in turn, depends on the dimensions of the system being cooled. Thin films on a cold surface, for example, set almost instantaneously. Once set, gel strength does not change markedly over time. LA gellan gum is capable of forming self supporting gels at concentrations as low as 0.05% gum. LA gellan gum gels do not synerise unless cut or broken. High acyl gellan gum As with LA gellan gum the easiest way to form HA gellan gum gels is to cool hot solutions. Addition of cations is not necessary for the formation of HA gellan gum gels and their properties are much less dependent on the concentration of ions in solution. Gels typically set and melt between 70 and 80 ëC and show no thermal hysteresis, i.e., they melt at the same temperature at which they set. The setting temperature increases with increasing cation concentration. For example, the temperature increases from approximately 71 ëC to 80 ëC as the calcium increases from 2 to 80 mM. A similar increase is seen when sodium or potassium concentration is increased from 10 to 200 mM.14 HA gellan gum is capable of forming self supporting gels at concentrations above approximately 0.2% gum. HA gellan gum gels do not synerise. 9.4.4 Texture of gellan gum Texture is generally regarded as a multifarious property.15 Texture profile analysis (TPA) is a technique based on compression of free standing gels twice in succession and is capable of providing both fundamental and empirical data on the mechanical properties of gels. It has the advantage of providing data at both low and high strains allowing gels to be characterised by multiple parameters. These include: · modulus: a measure of gel firmness when lightly squeezed · hardness: a measure of the force required to rupture the gel · brittleness: a measure of how far the gel can be squeezed before it ruptures; it is important to note that the higher the brittleness value the less brittle the gel is, i.e., it has to be compressed further to break · elasticity: a measure of how far the gel springs back after the first compression cycle · cohesiveness: indicates the degree of difficulty in breaking down the gel in the mouth. Texture profile analysis has been used to characterise a diversity of foods and
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Fig. 9.3
Schematic comparison of the gel texture of HA and LA gellan gum with other common gelling agents.
hydrocolloid gels. A more detailed account of the technique can be found in a review by Pons and Fiszman.16 HA and LA gellan gum gels have very different textures that can be considered to be at opposite ends of the textural spectrum of hydrocolloid gels and Fig. 9.3 shows schematically how gellan gum gels compare with other
Fig. 9.4
Texture profile of HA (ÐÐ) and LA (-----) gellan gum gels measured on 1% gels at 70% strain.
Gellan gum
213
Fig. 9.5 Effect of HA and LA gellan gum blend ratio on the modulus and brittleness of gels prepared at 0.5% total gum concentration.
common gelling systems. LA gellan gum forms hard, non-elastic, brittle gels whereas HA gellan gum gels are soft, elastic and non-brittle. A comparison of the texture of HA and LA gellan gum gels, made using texture profile analysis, is shown in Fig. 9.4. It is immediately apparent that through blending of the two forms, a diverse range of textures can be achieved that encompass many of the textures produced by other hydrocolloids (Fig. 9.5). It has been demonstrated by differential scanning calorimetry (DSC), and rheological measurements that mixtures of the HA and LA forms exhibit two separate conformational transitions at temperatures coincident with the individual components.8,13 This is important to note since in mixtures consideration for the high setting temperature of the HA gellan gum will need to be made. No evidence for the formation of double helices involving both HA and LA molecules has been found.17 Properties of the blended system can be varied through control of the blend ratio and level of ions in the mixture.18,19 At low ionic concentrations the HA form predominates, but as the ionic concentration increases the contribution of the LA form to the texture increases.
9.5
Uses and applications
Before discussing the main applications of gellan gum, an overview of the key properties of both the HA and LA forms is given in Table 9.6. This provides a useful frame of reference from which existing applications can be understood and new opportunities visualised.
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Table 9.6
Comparison of the key properties of HA and LA gellan gum
Hydration Sequestrants Viscosity Gelling ions Setting temperature Melting Clarity Texture
Low acyl gellan gum
High acyl gellan gum
> 80 ëC Yes Low Yes (mono or divalent or acid) 25±60 ëC No (except low ionic strength and in milk) Clear Firm, brittle
> 70 ëC No High Not required 70±80 ëC Yes Opaque Soft, elastic
9.5.1 Dessert jellies Water-based dessert jellies are popular throughout the world and have a range of textures. The firm brittle texture of LA gellan gum, for example, complements the flavour of fruit juice jellies. Alternatively, combinations of HA and LA gellan gum can be used to produce jellies with a variety of textures. Products can be `ready-to-eat' (RTE) or in dry mix form. Example formulations are provided below. Formulation 9.1 is an example of a fruit juice jelly prepared with LA gellan gum. It can be made with apple, orange, grape, pineapple or grapefruit juice and will hydrate as a dry mix in a water hardness of up to 600 ppm (as CaCO3). The pH and solids vary according to the juice used, but are typically pH 3.5±3.7 and 17% total soluble solids. Alternatively, blends of LA and HA gellan gum can be used to give a range of textures. Blend ratio and gum concentration will depend on the final texture required but a 3:1 HA:LA gellan gum blend at approximately 0.3% is recommended as a starting point for textural evaluation. Formulation 9.1
Recipe for fruit juice jelly using LA gellan gum
Ingredients Water Fruit juice Sugar Citric acid anhydrous Tri sodium citrate dihydrate LA gellan gum
Weight (g)
(%)
250.0 250.0 90.0 2.4 1.8 0.9
42.00 42.00 15.15 0.40 0.30 0.15
Preparation 1. Pre-blend all the dry ingredients. 2. Heat the water to boiling and dissolve the dry ingredients in the hot water. 3. Add the fruit juice, mix and chill. The gel sets at approximately 40±45 ëC and the use of chilled fruit juice with dry-mix desserts ensures a rapid set.
Gellan gum Formulation 9.2
215
Recipe for a dessert jelly using LA gellan gum and gelatin
Ingredients
Weight (g)
(%)
Water Sugar Gelatin (type B, 240 Bloom) Citric acid anhydrous Tri sodium citrate dihydrate LA gellan gum Colour and flavour
500.0 90.0 10.2 2.3 1.6 0.35 as required
82.6 15.0 1.7 0.38 0.26 0.06
Preparation 1. Blend all the dry ingredients. 2. Heat the water to boiling and dissolve blend into the hot water by stirring for 1±2 minutes. 3. Deposit and chill.
LA gellan gum can also be used to modify the properties of traditional gelatin dessert jellies and an example is given in Formulation 9.2. The LA gellan gum in this formulation raises the initial set temperature of the dessert to around 35 ëC, allowing more rapid processing of an RTE product. The formulation can also be used in dry mix composite desserts allowing further layers to be added more quickly than with gelatin alone. The time to consumption is, however, not reduced as the maturation time of the gelatin gel remains unchanged. The gellan gum also raises the melting point of the gel so that desserts maintain their shape for longer, when removed from the fridge. This approach can also be used in savory gelled products such as aspics. Gellan gum is an anionic polysaccharide (ÿve charge) whereas gelatin is a protein and as such its overall charge will be dependent on the pH of the system. Below its isoelectric point the gelatin will carry an overall positive charge and will therefore interact with the negatively charged polysaccharide. This can lead to cloudiness in the gel or even precipitation. For this reason it is recommended to use type B gelatins since these have the lowest isoelectic point (pH 4.5±5.5). The extent of the interaction will depend on the pH and the ratio of gelatin to gellan gum. 9.5.2 Suspending agent Gellan gum is commonly used as a gelling agent; however, it can be used to prepare structured liquids which are extremely efficient suspending agents. These structured liquids are gelling systems which have been subjected to shear either during or after the gelation process. The application of shear disrupts normal gelation and results, under certain conditions, in smooth homogeneous, pourable systems often referred to as `fluid gels'.20 To produce smooth homogeneous fluid gels with gellan gum, systems must be formulated to give weak gelation, either by manipulating the ion type and concentration or gellan gum concentration. The viscosity and structure of the
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system correlates with the gel strength of the unsheared gel. Therefore, the greater the gel strength of the unsheared gel, the greater the viscosity and structure will be when the system is sheared. Systems which gel too strongly, however, can give rise to a grainy appearance in the final fluid. Gellan gum fluid gels can be prepared using a variety of processes. Three potential processes are outlined schematically in Fig. 9.6. The first step in each case is to hydrate the gellan gum through a combination of heat and sequestrants. Method 1 simply involves continually stirring the solution as it cools to form the fluid gel or allowing the weak gel to form undisturbed, then shearing to form the fluid gel. Alternatively, in method 2 the hot gellan gum solution can be added to cold water whilst mixing. This results in cooling of the solution and formation of a fluid gel. In method 3 it is possible to prepare gellan gum solutions that will not gel on cooling. Addition of ions to these cold solutions results in gelation and formation of a fluid gel. The application of shear can be achieved by using stirring, homogenisation, filling or even `shake before use'. Shear can also be applied during or after gelation. UHT, HTST and processes involving scraped surface heat exchangers (i.e. for the production of custards, gravies and ketchup) are an ideal way to shear during gelation as the solution cools. Gellan gum fluid gels can be used to produce shelf stable suspensions in a variety of beverage products.21 Generally, HA gellan gum should be used at approximately 0.02±0.05% to give a smooth fluid gel. For LA gellan gum the use level is dependent on the ionic concentration in the system and a guide to the formulation of fluid gels using LA gellan gum is given in Table 9.7. Example formulations for beverages
Fig. 9.6
Outline of potential processes for the preparation of gellan gum fluid gels.
Gellan gum Table 9.7
217
Guidelines for formulation of LA gellan gum fluid gels
Ion
Concentration
Calcuim
Low (< 50ppm) Optimum (100±600ppm) High (> 600ppm) Low (< 0.25%) Optimum (0.5±2.0%) High (4.0±10%)
Sodium Milk Sugars
LA gellan gum concentration (%) 0.05±0.2 0.03±0.05 0.05±0.2 0.05±0.2 0.03±0.05 0.05±0.2 0.05±0.2 0.1±0.3
(40±60%)
using LA or HA gellan gum which can be used to suspend gelled beads or fruit pulp are given below. Formulation 9.3 provides a starting point for a beverage type fluid gel. It has a pH of 2.9 and a setting temperature of approximately 15 ëC. It can be used to suspend jelly beads and is prepared as outlined in method 1 of Fig. 9.6 by shearing after the weak gel has been allowed to form. Formulation 9.4 is an example of a fluid gel formed with HA gellan gum. HA gellan gum fluid gels are less sensitive to the ionic conditions and have longer, more elastic flow properties when compared to LA gellan gum fluid gels.
Formulation 9.3
Recipe for a fluid gel for beverages using LA gellan gum
Ingredients Part 1 Sucrose Tri sodium citrate dihydrate LA gellan gum Sodium benzoate Deionised water Part 2 Citric acid Calcium lactate Deionised water
Weight (g)
(%)
112.0 0.60 0.28 0.20 862.0
11.25 0.06 0.028 0.02 86.60
5.00 0.25 15.00
0.50 0.025 1.517
Preparation 1. Blend the sucrose, tri sodium citrate dihydrate, LA gellan gum and sodium benzoate and disperse in the deionised water of Part 1. 2. Heat the dispersion to 70±80 ëC to hydrate. 3. Dissolve the citric acid and calcium lactate in the deionised water of Part 2 and add to the hot gum solution. 4. Cool the sample to below 15 ëC undisturbed. 5. Gently agitate the sample to form a fluid gel.
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Formulation 9.4
Recipe for a pulp suspension beverage using HA gellan gum
Ingredients Water Fruit juice Sugar HA gellan gum Tri sodium citrate dihydrate Citric acid anhydrous Potassium citrate
Weight (g)
(%)
338.10 100.0 60.0 0.25 0.25 0.9 0.5
67.62 20.0 12.0 0.05 0.05 0.18 0.1
Preparation 1. Blend the HA gellan gum with the tri sodium citrate dihydrate and disperse in the water. 2. Heat the dispersion to 90 ëC to hydrate the gum. 3. At 90 ëC add the remaining dry ingredients and the fruit juice. 4. Cool to room temperature whilst mixing to form the fluid gel.
9.5.3 Dairy Unlike water systems, much of the calcium in milk is associated with the milk proteins. During heating any remaining free calcium is also bound by the proteins and therefore does not interfere with the hydration of the gellan gum. Because of this, both HA and LA gellan gum will hydrate in milk above approximately 80 ëC without the need for a sequestrant. Milk also contains sodium and potassium ions and it is therefore not usually necessary to add additional gelling ions to milk systems. Because the LA gellan gum is gelled with a low level of monovalent ions (predominantly K+), milk gels are thermally reversible, melting at approximately 95 ëC. Thermal stability of LA gellan gum milk gels can be improved by the addition of calcium. Care must be taken when adding calcium to hot milk since it can result in precipitation of the milk proteins if added above 70 ëC. It is recommended to cool the gellan/milk mixture to between 55 and 65 ëC before adding the calcium. This temperature range is above the gelation temperature of the LA gellan gum but below the temperature at which milk protein precipitation occurs. In many dairy systems milk powders are used. These powders are natural sequesterants and will bind calcium from the water used for reconstitution. Therefore, it is not usually necessary to add a sequestrant when water of hardness up to 400 ppm (as CaCO3) is used for reconstitution. Some sequestrant may be required if less than 2% milk powder is being used or harder water is used to reconstitute the milk powder. Milk beverages As described in Section 9.5.2, HA gellan gum at low concentrations is able to form a very weak gel network often referred to as a fluid gel. These fluid gels have extremely good suspension properties and can be used in a range of neutral dairy and soya based products such as chocolate milk. Development of this application was initially limited due to creation of off-flavours as a result of
Gellan gum
219
residual enzyme activity in the native, HA gellan gum acting on the milk. The off-flavour, which is reminiscent of cleaning chemicals, renders the product unpalatable and is linked to the development of para-cresol. Development of a process in which the HA gellan gum is pre-treated with a denaturing agent that is thought to act on the residual enzymes in the gellan gum has led to a new grade of gellan gum for this application.22 KELCOGELÕ HM-B is a standardised product containing the pre-treated HA gellan gum and can be used at 0.1±0.12% for stabilisation of cocoa in chocolate milk. The HA gellan gum is tolerant to a wide range of UHT conditions and products can be filled at higher temperatures than with carrageenan, the traditional hydrocolloid for this application. It provides excellent suspension of cocoa and long-term stability. It is functional in reconstituted milk, fresh milk, whey substituted beverages and low protein milk beverages. Yogurt The standard yogurt process can be followed when using LA gellan gum for both set and stirred yogurt. There are various ways in which yogurt containing LA gellan gum can be made depending on the usual manufacturing process and on the desired properties of the final yogurt. In all cases the initial steps are the same: the LA gellan gum should be blended with skimmed milk powder and other stabilisers (if required) and dispersed in cold milk before heating, homogenising and pasteurising. Generally, the fermentation time is not affected by the presence of LA gellan gum. The important factor to remember is that the setting temperature of LA gellan gum in skimmed milk is about 41 ëC. If shear is applied through the setting temperature a fluid gel will be formed. This acts like a gel under static conditions, but flows like a liquid when shear is applied. If, however, shear is applied below the setting temperature in yogurt systems, a broken gel may result which can lead to unsatisfactory lumps in the final product. Typical use levels for LA gellan gum in yogurt are 0.04%. It can be used in combination with other stabilisers such as starch depending on the final texture required. 9.5.4 Sugar confectionery One of the fundamental techniques for the manipulation of the texture of sugar confectionery is to use combinations of a variety of sugars such as sucrose, glucose, fructose and various corn syrups. Combinations of sugars produce desirable textures, as well as preventing crystallisation of individual sugars. Another critical ingredient is the hydrocolloid. These impart structure to the product and provide the characteristic jelly texture. Before describing how to make confectionery jellies with gellan gum, it is worth discussing the influence of sugars on the properties of gellan gum. Effect of sugars on LA gellan gum The presence of sugars has two major effects on the properties of LA gellan gum gels. Firstly, the ion requirements for optimum gel properties are reduced. The
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Fig. 9.7
Effect of sucrose concentration on the modulus ( ), hardness (ú) and brittleness (n) of 0.5% LA gellan gum gels.
presence of 40% w/w sugar approximately halves the calcium required for maximum gel modulus, from 8±10 mM in water gels to 4±5 mM in the sugar gels. Addition of 60% w/w sugar results in an approximately ten-fold reduction in the requirement for calcium, with only 0.5±1.0 mM added calcium required for maximum gel modulus. Similar reduction in the requirements for sodium and potassium are also seen (Fig. 9.2). Secondly above approximately 40% sugar gels become less firm and less brittle, i.e., softer and more elastic (Fig. 9.7). These effects are believed to be the result of the sugars inhibiting the aggregation step of the gelation process.23,24 These effects are also influenced by the type of sugar. Sucrose has a greater inhibitory effect than glucose, fructose or corn syrups (Table 9.8).25 The differences observed between sugars mean that texture can, to some degree, be varied by manipulating the sugar composition of the system. For example, partial replacement of sucrose with fructose or corn syrup, a common practice to control crystalisation in confectionery manufacture, results in firmer, more brittle gels.26 Effect of sugars on HA gellan gum Less is known about the specific effects of sugars on HA gellan gum. However, addition of sugars to HA gellan gum gels generally results in an increase in the force required to break the gel. Setting and melting temperature also increase with increasing sugar concentration. In the presence of high levels of sugar (70±
Gellan gum
221
Table 9.8 Effect of sugars at 40% and 60% w/w on the textural properties of 0.5% w/w LA gellan gum gels prepared at ion concentrations giving maximum gel modulus Sugar
Fructose Glucose Sucrose Maltose 42DE corn syrup 14DE maltodextrin
Modulus (Ncmÿ2)
Brittleness (%)
40% w/w
60% w/w
40% w/w
60% w/w
14.1 12.9 13.5 15.5 19.0 16.1
3.70 2.17 1.60 3.83 5.06 5.88
31.3 36.7 30.2 30.7 27.9 24.1
53.6 62.9 63.3 51.4 53.0 43.1
80%), HA gellan gum has very high viscosity even when hot. This can make processes such as mixing and depositing difficult. This is often compounded by the high setting temperature which can result in pre-gelation, i.e., gel formation prior to deposition of the confectionery mix. However, incorporation of low levels of HA gellan gum into confections made with LA gellan gum will increase the chewiness of the jellies. Preparation of confectionery jellies LA gellan gum may be used alone or in combination with other gelling agents to produce jelly confectionery by traditional processes. Examples are provided in Formulations 9.5 and 9.6. When prepared with LA gellan gum as the sole gelling agent (Formulation 9.5), the jellies are firm with a short, clean bite and flavour. The jellies can be removed from the starch moulds after about 2 h but are usually stoved for up to 72 h before demoulding. Addition of a thin boiling starch as outlined in Formulation 9.6 results in a chewier texture. Combinations of LA gellan gum with carrageenan can be used to produce gelatin-free confectionery which is suitable for halal. Pre-gelation is the premature gelation of the confectionery mix prior to, or during, depositing. This makes depositing difficult and results in a weaker gel structure and grainy texture. Table 9.9 provides a guide to preventing pregelation in gellan gum confections. 9.5.5 Fruit preparations This application covers a wide variety of systems from 30 to 75% total soluble solids. Much of the understanding of the effects of sugars described in confectionery applications can be applied to these systems. In addition, the type of fruit used in the formulation is a key consideration when using LA gellan gum since the ion content and pH will vary. Fruit composition may also vary during the season. Table 9.10 shows that the ionic composition varies considerably between different fruits with most fruits containing significant levels of potassium ions.27 These ionic concentrations become increasingly significant in
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Formulation 9.5
Recipe for jelly sweets using LA gellan gum
Ingredients
Weight (g)
(%)
Sucrose Glucose syrup (42DE) Water Citric acid anhydrous Tri sodium citrate dihydrate KELCOGELÕ F gellan gum Calcium hydrogen orthophosphate Flavour and colour
159.0 159.0 120.0 5.00 5.00 3.75 0.20 as required
35.20 35.20 26.51 1.11 1.11 0.83 0.04
Preparation 1. Blend the LA gellan gum and calcium hydrogen orthophosphate with 1.0 g of tri sodium citrate dihydrate and 40 g of sucrose and disperse in the water. 2. Heat to boiling to hydrate the gellan gum then add the remainder of the sugar while continuing to boil. 3. Add pre-warmed glucose syrup while maintaining the temperature above 90 ëC. 4. Cook the liquor to 80±82% total solids then cool to 90 ëC. 5. Dissolve the citric acid and remainder of the tri sodium citrate dihydrate, colour and flavour in 20 cm3 of water and stir into the liquor. 6. Deposit at 76±78% total solids into starch moulds. 7. Stove to final solids as required.
Formulation 9.6 Recipe for jelly sweets using LA gellan gum and thin boiling starch Ingredients
Weight (g)
(%)
Water Glucose syrup (42DE) Sugar Thin boiling starch (FLOGEL 60) LA gellan gum Tri sodium citrate dihydrate Citric acid anhydrous Calcium hydrogen orthophosphate Flavour and colour
220.0 159.0 148.5 18.8 3.5 1.8 1.8 0.2 as required
39.0 28.2 28.1 3.4 0.62 0.32 0.32 0.04
Preparation 1. Slurry the starch in 50 g of water. 2. Blend the LA gellan gum, calcium hydrogen orthophosphate and tri sodium citrate dihydrate with 40 g of sugar and disperse in the remainder of the water. 3. Heat the dispersion to boiling to hydrate the gellan gum then add the remaining sugar and continue to boil. 4. Add pre-warmed glucose syrup and cook to boiling. 5. Add the starch slurry, breaking the boiling point and continue to cook to 78% total solids. 6. Add colour, flavour and citric acid pre-dissolved in a small amount of water. 7. Deposit into starch moulds at 74% total solids and stove to final solids as required.
Gellan gum Table 9.9
223
A guide to the prevention of pre-gelation in confectionery mixes
Problem
Possible causes
Solution
Pre-gelation when acid added
Hard water
Add sodium hexameta phosphate
Depositing soluble solids too high
Lower depositing solids
pH too low
Add sodium citrate with citric acid
Hard water
Increase sequestrant level
Soluble solids too high
Add water to lower soluble solids
Pre-gelation before acid added
Table 9.10 Fruit Apple Blackcurrant Raspberry Strawberry Apricot Peach
Ionic composition of raw fruits27 Ca++ (mg/100g)
Mg++ (mg/100g)
Na+ (mg/100g)
K+ (mg/100g)
4 60 25 16 15 7
3 17 19 10 11 9
2 3 3 6 2 1
88 370 170 160 270 160
medium to high solids systems where gellan gum ion requirements are greatly reduced. Therefore, formulations optimised for one fruit will often need modification to accommodate different fruits. Formulation 9.7 is an example of a low solids (36%) jam which can be formulated to give a range of textures. HA gellan gum provides a soft, spreadable jam with excellent sheen. The addition of a proportion of LA gellan gum may be used where a firmer textured jam is required. LA gellan gum alone can be used to give a more bake stable jam. Various fruits can be used, including strawberries, raspberries and blackcurrants. Formulation 9.8 is slightly higher in solids than Formulation 9.7 and produces a lightly gelled yogfruit with evenly suspended fruit pieces. The gelled structure can be broken down by pumping to give a smooth, viscous yogfruit (pH 3.9, soluble solids 40%). Finally, Formulation 9.9 with 55% fruit and no added water demonstrates the properties of a gellan gum and starch-based preparation. The LA gellan gum filling has a glossy appearance, good flavour release and excellent bake stability (pH 3.4, tss 56%).
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Formulation 9.7
Recipe for a reduced sugar jam using HA or LA gellan gum blend
Ingredients Frozen strawberries Sugar Water Gellan gum* Tri sodium citrate dihydrate Potassium sorbate Citric acid solution (50% w/w)
Weight (g)
(%)
450.0 283.5 260.0 2.5 0.5 1.0 2.5
45.0 28.35 26.0 0.25 0.05 0.10 0.25
* High acyl and/or low acyl gellan gum can be used depending on desired final texture.
Preparation 1. Dry blend the gellan gum, tri sodium citrate dihydrate and potassium sorbate with the sugar and disperse into the water. 2. Add the fruit and heat to boiling. Cook for 1±2 minutes to ensure hydration of the gellan gum. Check the soluble solids. 3. Remove from the heat and add the citric acid solution. Fill into jars and cap immediately. Formulation 9.8
Recipe for a peach yogfruit using LA gellan gum
Ingredients Peach pureÂe Diced peach Glucose syrup LA gellan gum Tri sodium citrate dihydrate Sodium benzoate
Weight (g)
(%)
200.0 200.0 300.0 0.35 1.80 0.25
28.50 28.50 42.65 0.05 0.26 0.04
Preparation 1. Combine the fruit and glucose syrup. 2. Add the tri sodium citrate dihydrate, LA gellan gum and sodium benzoate and heat to 90 ëC with constant stirring. 3. Hold for 1 minute then cool, with stirring, to 60 ëC. 4. Deposit and allow to cool undisturbed. Note: The tri sodium citrate dihydrate in the formulation is added to give a final pH of 3.9. The addition level may be varied depending on the fruit used, and the final pH required.
9.5.6 Other applications Gellan gum forms films and coatings that can be used in breadings and batters. Films offer several advantages, particularly their ability to reduce oil absorption by providing an effective barrier. Films can be prepared by applying a hot solution of gellan gum on to the surface of the food product, by spraying or dipping, and allowing to cool. Alternatively, in the case of LA gellan gum, the food can be dipped into a cold solution of the gum, allowing ions to diffuse into
Gellan gum Formulation 9.9
225
Recipe for a bake-stable fruit preparation using LA gellan gum
Ingredients Apple (thawed) Sucrose Modified starch (THERMFLO) LA gellan gum Citric acid solution (50% w/w) Tri sodium citrate dihydrate
Weight (g)
(%)
210.0 160.8 8.00 0.32 0.80 0.88
55.2 42.2 2.10 0.12 0.20 0.18
Preparation 1. Pre-blend the dry ingredients, add to the apple and heat with stirring to boiling. 2. Remove from heat, add the citric acid solution, mix well and deposit. 3. Leave to gel before use. Shear, and use as required.
the solution, resulting in gelation or film formation. LA gellan gum can also be used to produce fat free adhesion systems. Spraying of a cold solution of LA gellan gum onto the surface of products such as nuts, crisps and pretzels forms an instant thin layer of gel when it reacts with the salt thus facilitating adhesion of spice, flavour or sweetener blends.
9.6 Regulatory status In Japan, gellan gum has been considered a `natural' food additive since 1988. It is now approved for food use in the USA and the European Union as well as Canada, South Africa, Australia, most of South East Asia and Latin America. Gellan gum appears as E418 in the European Community Directive EC/95/2 in Annex 1. Both the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and the European Community Scientific Committee for Food has given gellan gum an Acceptable Daily Intake (ADI) of `not specified'. Combinations of HA and LA gellan gum have one name. A manufacturer may label a product made with a combination of both types of gellan gum simply, `E418' or `gellan gum'.
9.7
Future trends
The current commercial HA and LA gellan gum products can be considered as being at opposing ends of the textural spectrum available with hydrocolloid gelling agents (Fig. 9.3). Blends of the two types enable some intermediate textures to be created but the mixed system retains the high setting temperature of the HA product and the ion sensitivity of the LA product. Far more interesting are gellan gums of intermediate acyl content as these show much more variation in texture and a single homogeneous setting behaviour.28,29 Methods for creating these partially acetylated products exist but they are yet to be produced on a commercial scale.30 Realisation of this control over the degree of acylation of gellan gum could lead to a truly universal gelling agent.
226
Handbook of hydrocolloids
9.8
Sources of further information and advice
www.CPKelco.com Imeson, A. (1999) Thickening and Gelling Agents for Food, 2nd edn. Aspen Publishers Inc., Gaithersburg, MD.
9.9 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11.
References and MORRIS, V.J. (1983) `Structure of the acidic extracellular gelling polysaccharide produced by Pseudomonas elodea', Carbohydr. Res., 124, 123±33. JANSON, P.-E., LINDBURG, B. and SANDFORD, P.A. (1983) `Structural studies of gellan gum, an extracellular polysaccharide elaborated by Pseudomonas elodea', Carbohydr. Res., 124, 135±9. KUO, M.-S., MORT, A.J. and DELL, A. (1986) `Identification and location of L-glycerate, an unusual acyl substituent in gellan gum', Carbohydr. Res., 156, 173±87. CHANDRASEKARAN, R., MILLANE, R.P., ARNOTT, S. and ATKINS, E.D.T. (1988) `The crystal structure of gellan', Carbohydr. Res., 175, 1±15. CHANDRASEKARAN, R., PUIGJANER, L.C., JOYCE, K.L. and ARNOTT, S. (1988) `Cation interactions in gellan: an X-ray study of the potassium salt', Carbohydr. Res., 181, 23±40. CHANDRASEKARAN, R. and THIALAMBAL, V.G. (1990) `The influence of calcium ions, acetate and L-glycerate groups on the gellan double helix', Carbohydr. Polym., 12, 431±442. CHANDRASEKARAN, R., LEE, E.J., RADHA, A. and THAILAMBAL, V.G. (1992) `Correlation of molecular architectures with physical properties of gellan related polymers', in Frontiers in Carbohydrate Research ± 2, ed. R. Chandrasekaran. Elsivier Applied Science, New York, pp. 65±84. MORRIS, E.R., GOTHARD, M.G.E., HEMBER, M.W.N., MANNING, C.D. and ROBINSON, G. (1996) `Conformational and rheological transitions of welan, rhamsan and acylated gellan' Carbohydr. Polym., 30, 165±75. MORRIS, E.R., REES, D.A. and ROBINSON, G. (1980) `Cation-specific aggregation of carrageenan helices: domain model of polymer gel structure', J. Mol. Biol., 138, 349. GRAZDALEN, H. and SMIDSRéD, O. (1987) `Gelation of gellan gum', Carbohydr. Polym., 7, 371±93. O'NEILL, M.A., SELVENDRAN, R.R.
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and PHILLIPS, G.O. (2008) `Molecular structures of gellan gum imaged with atomic force microscopy in relation to the rheological behaviour in aqueous systems. 1. Gellan gum with various acyl contents in the presence and absence of potassium', Food Hydrocolloids, 22, 1148±59. SANDERSON, G.R. and CLARK, R.C. (1984) `Gellan gum, a new gelling polysaccharide', in Gums and Stabilisers for the Food Industry 2, eds. G.O. Phillips, D.J. Wedlock, and P.A. Williams. Pergamon Press, Oxford, pp. 201±10. NISHINARI, K.
12. 13.
KASAPIS, S., GIANNOULI, P., HEMBER, M.W.N., EVAGELIOU, V., POULARD, C., TORT-
and SWORN, G. (1999) `Structural aspects and phase behaviour in deacylated and high acyl gellan systems', Carbohydr. Polym., 38 145±54.
BOURGEOIS, B.
Gellan gum 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
29. 30.
227
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