Hydrocolloids


1. Raw materials

1.7. Gums and hydrocolloids

Gums and hydrocolloids are used to create texture. These are substances that are added to foods to emulsify and create interesting mouth feel and diversity in texture. The picture below is raw, unprocessed gum arabic, for example. It is used as a thickener in many food items. Two fun properties of thickeners that are of interest to food scientists and culinarians are both shear thickening and shear thinning. Basically, a thickener that exhibits shear thickening behaviour thickens under stress. Ketchup, on the other hand, is a perfect example of a shear thinning fluid. This is the reason you beat the side of the (rigid) ketchup container with your hand to get it out - you apply stress, and the fluid thins and begins to flow.

It is obvious that this kind of substances can be used in the bakery as well. They can be used for shelf life extension of cakes and bread for instance, to control batter consistency, as partial fat replacers etc. This chapter gives an overview of the hydrocolloids used in the bakery industry. If you read carefully the label of your bread improver or your cakes stabilising system you might discover some gums and hydrocolloids in the ingredient declaration.

A hydrocolloid can simply be defined as a substance that forms a gel when it comes in contact with water. Such substances include both polysaccharides and proteins which are capable of one or more of the following:

Along with the increased interest in gums and hydrocolloids for texture modification there is a growing scepticism to use chemicals in the kitchen. Many people have come to view hydrocolloids as unnatural or unhealthy ingredients. However it is important to remember that most gums and hydrocolloids are obtained from natural sources and have a marine, plant or microbial origin. They only have been extracted and purified from these natural sources. On the other hand they allow fat and sugar reduction in many foods so they can aid to combat obesity. The hydrocolloids themselves have a low calorific value and are generally used in very small quantities.

Gums and hydrocolloids such as CMC or carrageenan are used in bread improvers for frozen dough. At the end of this chapter there is a section dedicated to the use of hydrocolloids in bread improvers. This chapter however is still under construction. First we will discuss each hydrocolloid separately.

Agar

Agar-agar, usually seen abbreviated as agar, is a gelatinous substance derived from a polysaccharide that accumulates in the cell walls of agarophyte red algae. There are a number of uses for agar. Agar is also perfectly edible, and in addition to appearing in regional cuisine, it is also used as a thickening agent in candies and many other foods. Since agar is derived from plant material, it has the advantage of being vegetarian, unlike gelatine derived from animal sources.

Chemically, agar is a polymer made up of subunits of the sugar galactose. Agar polysaccharides serve as the primary structural support for the algae's cell walls. Agar exhibits hysteresis, melting at 85C and solidifying between 32-40C. This large difference between the gelling temperature and the melting temperature is called hysteresis. This property lends a suitable balance between easy melting and good gel stability at relatively high temperatures. Since many scientific applications require incubation at temperatures close to human body temperature (37 C), agar is more appropriate than other solidifying agents that melt at this temperature, such as gelatine.

Agar-agar is a natural vegetable gelatine counterpart. White and semi-translucent, it is sold in packages as washed and dried strips or in powdered form. It can be used to make jellies, puddings, and custards. For making jelly, it is boiled in water until the solids dissolve. Sweetener, flavouring, colouring, fruit or vegetables are then added and the liquid is poured into moulds to be served as desserts. It also can be used in vegetable aspics, or incorporated with other desserts, such as a jelly layer in a cake.

Name
Agar (E406)
Origin
polysaccaharide obtained from various species of red algae
Properties
thermoreversible, heat resistant, brittle gel, high hysteresis
Clarity
clear to semi opaque
Dispersion
in cold and hot water
Hydration
heating to boil necessary for gelling (> 90°C)
pH
2,5 - 10
Setting
35 - 45°C
Specific

sorbitol and glycerol improve elasticity
prolonged heating at pH below 5,5 or above 8,0 inhibits the gelling properties

Tolerates
salt, sugar, alcohol, acids and proteases
Viscosity
low
Typical concentration
0,2 % up to 0,5 % which gives a firm jelly
Synergy
locust bean gum
Syneresis
can be prevented by adding 0,1 - 0,2 % locust bean gum

Agar can be used in donut icings. All the dry ingredients are thoroughly blended and stirred into the water at 50C. At the end the shortening is added and the mixture is blend until smooth. This yields a soft icing with minimal flow. The recipe is as follows: sugar (64 %), water (12 %), alkalised cocoa (9 %), vegetable shortening (9 %), cold soluble agar (3 %) and skimmed milk powder (3 %).

In the natural state, agar occurs as structural carbohydrate in the cell walls of agarophytes algae, probably existing in the form of its calcium salt or a mixture of calcium and magnesium salts. It is a complex mixture of polysaccharides composed of two major fractions - agarose, a neutral polymer, and agaropectin, a charged, sulphated polymer.

Agarose, the gelling fraction, is a neutral linear molecule essentially free of sulfates, consisting of chains of repeating alternate units of -1,3-linked- D-galactose and a-1,4-linked 3,6-anhydro-L-galactose. Agaropectin, the non-gelling fraction, is a sulfated polysaccharide (3 % to 10 % sulfate), composed of agarose and varying percentages of ester sulfate, D-glucuronic acid, and small amounts of pyruvic acid. The proportion of these two polymers varies according to the species of seaweed. Agarose normally represents at least two-thirds of the natural agar-agar.

Agar-agar is insoluble in cold water, but it swells considerably, absorbing as much as twenty times its own weight of water. It dissolves readily in boiling water and sets to a firm gel at concentrations as low as 0.50 %. Powdered dry agar-agar is soluble in water and other solvents at temperatures between 95 and 100 C. Moistened agar flocculated by ethanol, 2-propanol or acetone, or salted out by high concentrations of electrolytes, is soluble in a variety of solvents at room temperature.

The gelling portion of agar-agar has a double helical structure. Double helices aggregate to form a three-dimensional structure framework which holds the water molecules within the interstices of the framework. Thus, thermo-reversible gels are formed. The gelling property of agar-agar is due to the three equatorial hydrogen atoms on the 3,6-anhydro-L-galactose residues, which constrain the molecule to form a helix. The interaction of the helixes causes the formation of the gel.

Regarding its gelling power, agar-agar is outstanding among other hydrocolloids. Agar-agar gels can be formed in very dilute solutions, containing a fraction of 0.5 % to 1.0 % of agar-agar. These gels are rigid, brittle, have well defined shapes, as well as sharp melting and gelling points. Moreover, they clearly demonstrate the interesting phenomenon of syneresis (spontaneous extrusion of water through the surface of the gel), and hysteresis (temperature interval between melting and gelling temperatures). Gelling occurs at temperatures far below the gel melting temperature. A 1.5 % solution of agar-agar forms a gel on cooling to about 32 to 45C that does not melt below 85C. This hysteresis interval is a novel property of agar-agar that finds many uses in food applications. The gel strength of the agar-agar is influenced by concentration, time, pH, and sugar content. The pH noticeably affects the strength of the agar gel; as the pH decreases, the gel strength weakens. Sugar content has also a considerable effect over agar gel. Increasing levels of sugar make gels with harder but less cohesive texture.

The viscosity of agar solutions varies widely and is markedly dependent upon the raw material source. The viscosity of an agar solution at temperatures above its gelling point is relatively constant at pHs 4.5 to 9.0, and is not greatly affected by age or ionic strength within the pH range 6.0 to 8.0. However, once gelling starts viscosity at constant temperature increases with time.

An agar-agar solution is slightly negatively charged. Its stability depends upon two factors: hydration and the electric charge. The removal of both factors result in flocculation of the agar-agar. Prolonged exposure to high temperatures can degrade solutions of agar-agar, resulting in lower gel strength after temperature decrease and gel formation. The effect is accelerated by decreasing pH. Therefore, it should be avoided to expose agar-agar solutions to high temperatures and to pHs lower than 6.0 for prolonged periods of time. Agar-agar in the dry state is not subject to contamination by microorganisms. However, agar-agar solutions and gels are fertile media for bacteria and/or moulds and appropriate precautions should be taken to avoid the growth of microorganisms.

Carrageenan

Carrageenan comes from algae or seaweed, and can be used as a thickening agent in place of animal-based products like gelatine, which is extracted from animal bones. It is usually derived from either red alga, sometimes called Irish moss. Carrageenan is a common ingredient in many foods, such as milk products like yogurt or chocolate milk.

There are several varieties of carrageen used in cooking and baking. Kappa-carrageenan is used mostly in breading and batter due to its gelling nature. Lambda carrageenan is a non-gelling variety that assists in binding, retaining moisture and in contributing to viscosity in sweet doughs. Iota carrageenan is used primarily in fruit applications and requires calcium ions to develop a heat-reversible and flexible gel.

Carrageenans are large, highly flexible molecules that curl forming helical structures. This gives them the ability to form a variety of different gels at room temperature. They are widely used in the food and other industries as thickening and stabilizing agents. A particular advantage is that they are pseudo-plasticthey thin under shear stress and recover their viscosity once the stress is removed. This means that they are easy to pump but stiffen again afterwards.

There are three main commercial classes of carrageenan:


Eucheuma cottonii

The primary differences that influence the properties of kappa, iota, and lambda carrageenan are the number and position of the ester sulfate groups on the repeating galactose units. Higher levels of ester sulfate lower the solubility temperature of the carrageenan and produce lower strength gels, or contribute to gel inhibition (lambda carrageenan).

All are soluble in hot water, but, in cold water, only the lambda forms (and the sodium salts of the other two) are soluble.

When used in food products, carrageenan has the EU additive E-number E407 or E407a when present as "processed eucheuma seaweed", and is commonly used as an emulsifier. It is known as Carrageen Moss it is boiled in milk and strained, before sugar and other flavourings such as vanilla, cinnamon, brandy, or whisky are added. The end-product is a kind of jelly similar to panna cotta, tapioca, or blancmange. When iota carrageenan is combined with sodium stearoyl lactylate (SSL), a synergistic effect is created, allowing for stabilizing/emulsifying not obtained with any other type of carrageenan (kappa/lambda) or with other emulsifiers (mono and diglycerides, etc.). SSL combined with iota carrageenan is capable of producing emulsions under both hot and cold conditions using either vegetable or animal fat.

The basic structure of carrageenan is a linear polysaccharide made up of a repeating disaccharide sequence of a-D-galactopyranose linked 1,3 called the A residue and -D-galactopyranose residues linked through positions 1,4 (B residues). Carrageenans are distinguished from agars in that the B units in carrageenan are in the D form whereas they are in the L form in agar's.

Carrageenan has to be dispersed well before its solubilisation to avoid the formation of lumps and to obtain its complete functionality. Carrageenan should be premixed with other dry ingredients such as sugar or salts, adding the product slowly into cold liquid with agitation.

All carrageenan are dispersible in cold water, and when heated above 80C they are completely dissolved. During cooling process Kappa and Iota carrageenan form double helix molecular structures cross-linked by potassium and calcium ions, forming a tridimensional gel-type network.

Sodium and potassium salts of polyphosphates and citrates enhance solubility of carrageenan in cold and hot solutions and reduce their viscosity due to divalent cations chelation.

Synergism with starch: iota carrageenan increases the viscosity of starch systems by as much as 10 times the viscosity of the starch alone. When kappa carrageenan is added to starch systems no increase is noted. Carrageenan has a strong functional synergism with starches and can be used in starch-based foods to retain moisture. Mixed carrageenan/starch systems have unique properties which are a cost-effective answer to improving the quality of high starch formulations. The strong functional interaction between starch and carrageenan allows the starch content of soups, pie fillings and pudding to be reduced whilst improving the organoleptic properties of the system. Additionally, starch/carrageenan combinations offer resistance to shear degradation and low processing viscosity while maintaining excellent stability during thermal cycling

Name
iota carrageenan (E407)
kappa carrageenan (E407)
Origin
polysaccharide obtained from red seaweed
polysaccharide obtained from red seaweed
Properties
thermoreversible, soft, shear-thinning, elastic gel with calcium
thermoreversible, firm brittle gel with potassium
Clarity
clear
clear to slightly turbid
Dispersion
in cold, dispersion improves by adding sugar (3 - 5 times the amount of carrageenan) or small amounts of alcohol
in cold, dispersion improves by adding sugar (3 - 5 times the amount of carrageenan) or small amounts of alcohol
Hydration
above 70C. For high sugar concentratiions add the sugar after hydration
above 70C.
pH
4 - 10
4 - 10
Setting
40 - 70°C
30 - 60°C
Melting
5 - 10°C above setting temperature unless mixed with starch
10 - 20°C above setting temperature unless mixed with certain proteins
Tolerates
salt
-
Promoter
Calcium yields soft and elastic gels
potassium, milk proteins
Inhibitor
hydrolysis of solution at low pH with prolonged heating; gels are stable
salts
Viscosity
medium
low
Typical concentration
1,0 % up to 1,5 % for gels
1,5 % for gels
Synergy
starch
locust bean gum: increases elasticity and improves clarity; milk protein
Syneresis
no
yes

Starches

Like gums, starch molecules are polysaccharides. This means that they are large, complex carbohydrate molecules made of many sugar units bonded one to the next. In the case of starch, the sugar units are glucose molecules.


basic structure of starch

Not all starch molecules are alike. Glucose units in starch can be arranged in one of two ways: either as long straight chains or as short but highly branched ones. Straight chain starch molecules are called amylose while the larger, branched starch molecules are called amylopectin. Although amylose is a straight chain, the chain twists into a helical shape, while amylopectin. with its many branches, looks like a flat coral fan.

Starch granules are small, gritty particles that are found in the endosperm of cereal grains, such as wheat and corn. Starch granules are also found in potatoes, manioc, rice, tapioca etc. Starch granules vary in size and shape depending on the source. For example, potato starch granules are relatively large and oval in shape, while corn starch granules are much smaller and more angular. Starch granules also grow larger over time, forming rings of starch molecules, much as growth rings form on a tree as it grows older.


potato starch

wheat starch

Tapioca starch granules vary in diameter from 5 to 35 microns, potato starch from 15 to 100 microns, maize from 5 to 25 microns, while rice starch granules are only about 3 to 8 microns in diameter. The shapes vary from near perfect spheres to flattened ovoids, elongated disks, polygons and many others. By observing the size and shape of the granule, the plant source of a starch can be identified even in mixtures of dry starch. Variations in starch granules from different plants may be observed microscopically.

Different types of starches have their own unique properties. Some of the differences have to do with the size, or with the ratio amylose - amylopectin. The following table summarises the major differences between the different starches.

starch high in amylose starch high in amylopectin
cloudy when cooled relatively high clarity
forms a firm gel when cooled thickens but does not gel
gel tightens and weeps over time much less likely to weep over time
not freeze stable, weeps on defrosting weeps less when thawed
much thicker cold then hot essentially the same thickness hot or cold
tends to mask flavours does mask flavours much less

It would lead us too far to discuss all the different types of starches and their functionality. Before making a comparison between the properties and uses of different starches, a word about modified and/or instant starches. Starches can be modified by various processes. This is done to enhance certain characteristics of a particular starch. Modification can be done in 3 ways:

Modified starches are designer starches i.e. they have been modified to have certain desirable features. While any native starch can be modified, most modified food starches are either made from waxy maize starch or from rice starch. Waxy maize starch is unique in as far that it made up of 95 to 99 % of amylopectin. This gives it an advantage over regular corn starch with regards to clarity and neutral taste. Starches are modified to increase their stability against excessive heat, acid conditions or freezing.

Some modified starches are precooked in order to become instant starches. Instant starches are also called pregelatinised or cold-swelling starches. To make a starch instant, the manufacturer either precooks and then dries the starch or makes some other change to the starch so that the granules absorb water without heating.

Starch
Properties
Usage in
Corn starch
cloudy when cooled
heavy body
not stable to excessive heat, acids, freezing, high shear
gel weeps over time
masks flavours
high gelatinisation temperature
puddings
Arrowroot
moderate to high clarity
soft gel
relatively stable to acids, heating, freezing
relatively low gelatinisation temperature
clean flavour
fruit fillings
sauces
Tapioca
moderate to high clarity
soft gel
relatively stable to acids, heating, freezing
relatively low gelatinisation temperature
clean flavour
fruit fillings
sauces
tapioca pudding
Waxy maize
moderate to high clarity
thickens, does not gel
relativily stable to acids, heating, mixing and freezing
clean flavour
basis for production of modified starches



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