• Food Hydrocolloids

One of the key components in modern baking are food hydrocolloids. These complex, non-digestible polysaccharides play crucial roles in creating the perfect texture, stability, and nutritional profile of baked goods. They are typically hydrophilic polymers that generally possess many hydroxyl groups, and are colloids meaning that they are 1-1000nm in diameter. They are also able to remain evenly distributed throughout the solution. In this article, we’ll be discussing the functional properties of food hydrocolloids and how they can be harnessed to achieve your desired baking results. Some of the gelation processes discussed in this article requires more advanced chemistry knowledge (A level chemistry knowledge).

Functional Properties

  • Viscosity

Certain food hydrocolloids can be used to thicken water, creating products with similar properties to full fat food even when oil or fat content has been eliminated or reduced through substitution with water. This makes them ideal for use in reduced fat salad dressing and low calorie table syrup.

  • Stability

Food hydrocolloids can stabilise emulsions by preventing separation and settling of insoluble particles. This is used particularly in emulsions where oil/fat is partially removed from a formulation and is replaced with thickened water. Food hydrocolloids can also control ice crystals formation in frozen food. For example, carrageenan, locust bean gum and guar gum are used to stabilise ice cream.

  • Suspension

Food hydrocolloids can create solutions with a yield point that immobilises particles in suspension. For example, xantham gum is used in salad dressing.

  • Gelation

Form strong cohesive gel through synergy such as in xantham and locust bean gum, or reversible heat-induced gel like methyl cellulose and hydroxylpropylmethyl cellulose.

  • Nutritional and nutraceutical

Food hydrocolloids can serve as sources of soluble dietary fibre, with potential benefits for cholesterol reduction and cancer risk prevention. They are also used in weight loss programmes.

Specific Hydrocolloids

  • Agar agar

Agar agar is a strong gelling hydroxolloid derived from a polysaccharide that accumulates in the cell walls of agarophyte red algae, including agar, alginates and carrageenans. Its use dated back to 1658 in Japan, and was subsequently spread to other agarophyte producing countries.

The chemical composition of agar agar is a mixture of agarose and agaropectine. Agaroses are fractions of agar that gel, with a high molecular weight of over 100,000Da (1Da = 1gmol-1) and a low sulphate content of less than 0.15%. It is consisted of repeating units of D-galactose and 3,6-anhydro-L-galactose, whose structures are demonstrated below:

Agaropectin is the rest of the fractions of agar, with a relatively low molecular weight of less than 20,000Da and a sulphate content of 5-8%. Agaropectin is studied less extensively as they lack practical application and any process used to produce them would be costly and complicated.

Physical gel is produced when polymers aggregate through hydrogen bonds. Hydrogen bonds are where lone pair of electrons in the negative dipoles of the polymers are attracted to 𝛿+ hydrogen atom in another polymer, this is an exothermic process forming strong intermolecular forces between polymers. Agar gel has several properties:

  • Hydrogen bonding leads to a large amount of water being held in the internal structure, as water molecules are also polar and can be involved in hydrogen bonding.
  • High exclusion limits: Exclusion limit is the greatest globular protein size that can transverse the gel in an aqueous solution. An exclusion limit of 30,000,000Da in 2% agarose gel suggests that all proteins are able to pass through the gelling mesh, given that there is no protein that reaches the size of 30,000,000Da (this is the size of sub-cellular particles such as ribosomes and viruses). However, the presence of agaropectin reduces the exclusion limit slightly.
  • High gelling hysteresis: Gelling hysteresis is the temperature difference between gelling and melting. In agar gel, the gelling temperature is 38°C whereas the melting temperature is 85°C. The degree of methoxylation of carbon atoms in agarose has a great impact on the gelling temperature. Methoxy group is a functional group consisting of a methyl group bonded to an oxygen, and methoxylation is when more methoxy groups are attached to an organic molecule. With a greater degree of methoxylation, the molecule would have more negative dipoles so there would be more bonding points between negative dipoles and 𝛿+ hydrogens. As more hydrogen bonds are formed and the formation of hydrogen bonds is exothermic, the gelling temperature would be reduced, increasing the gelling hysteresis. The gelling hysteresis of agar agar is significantly higher than that of carrageenans, even the hysteresis of the strongest gelling carrageenans is 12-26°C lower than that of agar.
  • Syneresis capacity: Agar gel is able to eliminate water contained in the gel mash. This ejection of aqueous solvent can be speeded by applying pressure on a properly confined gel. One of the properties of agar gel is gelling memory indicating that the gel structure can be maintained during syneresis and can recover exactly to the previous form upon rehydration.
  • Reversible: Upon cooling, agar gel can form again even after being molten by heating. This process can be repeated indefinitely in the absence of aggressive substances that could hydrolyse the agarose molecules or destroy them by oxidation. This gives agar gel an advantage as opposed to chemical gel as chemical gel is formed through the formation of covalent bonds, so they tend to be irreversible.

To produce agar agar, agarophyte seaweed is used. Agar is embedded in a structure of fibres of crystallised cellulose and act as a polysaccharide reserve, with its percentage content varying depending on the season. When forming agar, an intermediate form of agar with a lower molecular weight is first deposited in the cellular wall, the agar then enzymatically polymerises and desulphates, with most of it converting into agarose and the rest remaining in the form of agaropectin.

Natural agar is used in Far East traditional kitchens, and is mainly produced using Gelidium by traditional methods. Whereas industrial food grade agar tends to have elevated melting point so that it could withstand sterilisation without melting. Examples include agar produced in Portugal by Iberagar used in Mitsumame — a Japanese dessert made with fruits, ice cream and small cubes of agar jelly. Agar gel tends to be firm and brittle and is often tasteless.

Some varieties of agar show reactivity with sugar, increasing in gelling properties when used in food with high sugar content such as in jams and jellies. Hence, agar is often premixed with a part of sugar and is added slowly to the mixture to avoid clumps forming.

  • Carrageenans and alginates

Carrageenans and alginates are linear sulfated polysaccharides extracted from seaweeds. The main varieties of carrageenan arise from variations in the number of sulfate group per disaccharide.

Carrageenans and alginates are an intermediate between physical and chemical gel, as they require the presence of cations to form gel structures. Ionic bonds are formed between the polysaccharides and the cations when a complexing agent such as ethylene diamine tetracetate (EDTA) is used, which can only be broken by eliminating the bonding cation. Irreversible gels are formed and will not melt by heating. To form gels with alginates, calcium cations are used, producing softer and more elastic gels than agar. 

As cations have a characteristic flavour, carrageenans and alginates should only be used in dishes with strong flavours, capable of masking the flavours of cations. Hence, they are typically used in dairy and meat products. To use carrageenans, add carrageenan to 30 times the amount of water and boil for 10 minutes, allowing colloid to form when the solution cools. This increases viscosity and the reaction between carrageenans and proteins allow carrageenans to act as an emulsifier, stabilising the emulsion.

  • Gelatin

Gelatin is derived from collagen through processes that break up polypeptide chains with varying degrees of hydrolysis in its backbone. Raw materials used in manufacturing includes any collagen containing tissue such as hides, skins, bones from mammalian sources and skins of fish/fowl. With the main raw material used in Europe and North America being pig skin. However, gelatins from cold water fish species have sub-optimal physical properties when compared to mammalian gelatins, limiting their application. The manufacturing process is outlined below:

  1. Raw material is washed to remove impurities. Bones are treated by maceration which is when crushed bone chips are exposed to acidic condition, leading to minerals such as Ca₅(PO₄)₃(OH) and CaCO₃ being removed resulting in a sponge-like material called ossein.
  2. The raw materials then undergo preliminary treatment; there are two types of pre-treatments including acid pre-treatment and alkali pre-treatment. In acid pre-treatment, hydrated raw material is immersed in cold dilute mineral acid with a pH of 1.5-3.0 for 18-24 hours. It is then washed in running water and is neutralised so that the extraction pH is reached. This yields type A gelatins. In alkaline pre-treatment, raw material is placed in vats with sufficient hydrated calcium hydroxide that has a pH of 12.0. Limed material is then washed with water until approximately neutral conditions are obtained. This is followed by treatment with dilute acid such as hydrochloric acid to obtain the extraction pH. The temperature required for this process is below 24°C, yielding type B gelatins.
  3. Gelatin is extracted through consecutive hot water treatment, resulting in a polydisperse protein mixture consisting of proteins of different chain types with varying molecular weights. Three dominating protein fragments present in the mixture includes 𝛼-chains, 𝛽-chains and 𝛾-chains. 𝛼-chains can be further depolymerised into sub-𝛼-chains. If present at a high percentage, the gelatin mixture would have a low viscosity forming a slow-setting sticky gel. 𝛽-chains are formed when 2 𝛼-chains bond together covalently. The presence of 𝛽-chains increases gel strength and viscosity. 𝛾-chains are formed when 3 𝛼-chains are covalently bonded together. If the percentage of 𝛾-chains present in a mixture is too high, a fast setting and viscous solution would form. However, if the percentage of 𝛾-chains is too low, the gelatin would set too slowly and would fail to peel adequately from the gel spreading drum.
  4. The aqueous gelatin solution undergo evaporation, increasing its concentration. As the molecular weight of 𝛾-chains is higher than that of 𝛽-chains which is in turn higher than that of 𝛼-chains, and the increased viscosity being related to the increased molecular weight, a concentration of 20-25% can be obtained in high molecular weight gelatins whereas a concentration of more than 40% can be obtained in low molecular weight gelatins. The gelatin solution continues to undergo evaporation until the increased viscosity make it impossible to further increase the concentration.
  5. Concentrated gelatin solution is sterilised and cooled, allowing the solution to gel. To form powder gelatins, gels are extruded into noodles and are fed onto conveyor belts for drying. The gelatin noodles are then crushed and milled into powders containing particles ranging from 0.1-10mm in diameter. The moisture content of commercial gelatins is typically between 8-12%.

The general amino acid sequence of gelatin is Gly-X-Y where Gly is glycine, X is often proline and Y is often hydroxyproline. About one-third of all residues in collagen and gelatin are glycine whereas sulphur-containing amino acids are virtually absent. The solubility of collagen decreases as mammals age. This is because more intramolecular and intermolecular covalent linkages are formed in the source tissue. Type A gelatins have similar amino acid compositions as their parent collagen, with an average isoelectric point of 7-9.4. Isoelectric point is the pH at which a molecule carries no electrical charge. Type B gelatins lack many non-ionisable amino acids and are generally more acidic due to deamidation, with an isoelectric point of 4.8-5.5. Non-polar regions are rich in proline and hydroxyproline, interspersed with polar regions. The presence of polar regions makes gelatin more soluble in water. However, the rigidity of a gelatin gel measured under standard conditions — bloom strength — shows no correlation with average molecular weight or viscosity values.

The thermal stability of gelatin gel is dependent on hydroxyproline content, with lower stability in cold-water fish gelatins compared to mammalian gelatins and warm-water fish gelatins. This is because hydroxyproline located at the third position of the amino acids triplet has hydrogen bonding ability, making it the major stabiliser. 𝛼-chains in gelatin forms a network of triple helices upon cooling, creating a thermoreversible viscoelastic gel. However when the gelatin gel is heated above the helix-to-coil temperature, the intramolecular and intermolecular hydrogen bonds that stabilise the triple helical structures are broken, unfolding the triple helices. The helix-to-coil temperature is 15-20°C for cold-water fish gelatin whilst it is 36°C for mammalian gelatin.

Reference
  • Imeson, A. (2010). Food Stabilisers, Thickeners and Gelling Agents. Chichester: John Wiley & Sons.
  • Philips, G.O., Williams, P.A. (2009) Handbook of Hydrocolloids. 2nd ed. Cambridge: Woodhead Publishing Limited.
  • Science of Cooking. (N.D.) Science of Hydrocolloids in Cooking. [online] Available at: https://www.scienceofcooking.com/science_of_hydrocolloids_in_cooking.htm [Accessed 6th February, 2024]