Hydrocolloids Introduction
The term ‘hydrocolloids’ is commonly used to describe a range of polysaccharides and proteins that are nowadays widely used in a variety of industrial sectors to perform a number of functions including thickening and gelling aqueous solutions, stabilizing foams, emulsions and dispersions, inhibiting ice and sugar crystal formation and the controlled release of flavours,etc. The commercially important hydrocolloids and their origins are given in below.
Source of commercially important hydrocolloids
Botanical:
Trees:cellulose
Tree gum exudates: gum arabic, gum karaya, gum ghatti, gum tragacanth
Plants:starch, pectin, cellulose
Seeds:guar gum, locust bean gum, tara gum, tamarind gum
Tubers:konjac mannan
Algal:
Red seaweeds:agar, carrageenan
Brown seaweeds:alginate
Microbial:
xanthan gum, curdlan, dextran, gellan gum, cellulose
Animal:
Gelatin, caseinate, whey protein, soy protein, egg white protein, chitosan
The food industry, in particular, has seen a large increase in the use of these materials in recent years. Even though they are often present only at concentrations of less than 1% they can have a significant influence on the textural and organoleptic properties of food products.
Some typical examples of foods containing hydrocolloids are clearly demonstrating the
widespread application of these materials.
widespread application of these materials.
The specific hydrocolloids used in the production of the individual products shown are:
• baked beans contain modified corn starch as a thickener
• hoi-sin sauce contains modified corn starch as a thickener
• sweet and sour sauce contains guar gum as a thickener
• Sunny Delight fruit drink contains modified starch as an emulsifier with carboxymethyl cellulose (CMC) and xanthan gum as thickeners
• the Italian dressing includes xanthan gum as a thickener
• ‘light’ mayonnaise contains guar gum and xanthan gum as fat replacers to enhance viscosity
• the yoghurt incorporates gelatin as a thickener rather than a gelling agent
• the mousse contains modified maize starch as a thickener with guar gum, carrageenan and pectin present as ‘stabilisers’
• the Bramley apple pies contain modified maize starch with sodium alginate as gelling agent
• the fruit pie bars contain gellan gum and the blackcurrant preserve and redcurrant jelly contain pectin as gelling agents
• the trifle contains xanthan gum, sodium alginate and locust bean gum as ‘stabilisers’, modified maize starch as a thickener and pectin as a gelling agent.
The changes in modern lifestyle, the ever growing awareness of the link between diet and health and new processing technologies have led to a rapid rise in the consumption of ready-made meals, functional foods and the development of high fibre and low-fat food products. In particular, numerous hydrocolloid products have been developed specifically for use as fat replacers in food. This has consequently led to an increased demand for hydrocolloids. The world hydrocolloids market is valued at around $4.4 billion p.a. with a total volume of about 260,000 tonnes. The market has been growing at the rate of 2–3% in recent years.
Hydrocolloid selection is dictated by the functional characteristics required but is inevitably influenced by price and security of supply. It is for these reasons that starches (costing typically < US$1/kg) are the most commonly used thickening agents. It is interesting to note here, however, that xanthan gum (~US$12/kg) is becoming the thickener of choice in many applications.
This is because xanthan gum has unique rheological behaviour and its increased use has led to strong competition between supplier companies ensuring that the price has remained at reduced levels. Xanthan gum forms highly viscous, highly shear thinning solutions at very low concentrations and the viscosity is not influenced to any great extent by changes in pH, the presence of salts and temperature. The high viscosity at low shear enables the gum to prevent particle sedimentation
and droplet creaming and the shear thinning characteristics ensure that the product readily flows from the bottle after shaking. This explains its widespread application in sauces and salad dressings.
Gelatin is by far the most widely used gelling agent, although with the increasing demand for non-animal products and in particular the Bovine Spongiform Encephalopathy (BSE) outbreak in the UK, prices have increased significantly over recent years. There is currently considerable interest in
alternative sources of gelatine, notably fish skins, and in the development of gelatin replacements. The carrageenan market has also been very competitive over recent years due to the introduction of cheaper lower refined grades (Processed Euchema Seaweed, PES), which can be used effectively where gel clarity is not important. The price differential between carrageenan and PES has decreased and the use of carrageenans has increased markedly since its use in meat and poultry products was recently approved.
A further development has been the introduction of kappa/iota carrageenan hybrids which have potential to provide novel functionality. The gum arabic market has traditionally been erratic due to price fluctuations and security of supply and much effort has been directed at finding alternatives. A number of starch-based substitutes (for example, succinylated starch) were introduced some years ago as alternatives in the emulsification of flavour oils and recent work has been concerned with using pectin (notably sugar beet pectin) as a gum arabic replacement. It has been shown that the emulsification properties of gum arabic and pectin are due to the small amount of protein (2–3%) which is present as in integral part of their structure. This has led to significant interest in the development of polysaccharide–protein complexes (gum arabic lookalikes) which can be formed
by a mild heat treatment through the Maillard reaction and by electrostatic interaction. Gellan gum was approved for food use in Japan in 1988 but much later in the USA and Europe, and is beginning to establish its own niche markets. Regulatory aspects
Food hydrocolloids do not exist as a regulatory category in their own right,rather they are regulated either as a food additive or as a food ingredient. With the exception of gelatin, however, the vast majority of food hydrocolloids are currently regulated as food additives.
International
The most widely accepted fully international system to regulate the safety of food additives is that set up by a Joint FAO/WHO Conference on Food Additives in September 1955, which recommended that the two organisations collect and disseminate information on food additives. Since that time more than 600 substances have been evaluated and provided with specifications for purity and identity by the Joint/WHO Expert Committee on Food Additives (JECFA).
JECFA was first established in the mid-1950s by the FAO and WHO to assess chemical additives in food on an international basis. In the early 1960s the Codex Alimentarius Commission (CAC), an international inter-governmental body, was set up with the primary aims of protecting the health of the consumer and facilitating international trade in food commodities. When CAC was formed, it was decided that JECFA would provide expert advice to Codex on matters relating to food additives. A system was established whereby the Codex Committee on Food Additives and Contaminants (CCFAC), a general subcommittee, identified food additives that should receive priority attention, which were then referred to JECFA for assessment before being considered for inclusion in Codex Food Standards. Specialists invited to serve as members of JECFA are independent scientists who serve in their individual capacities as experts and not as representatives of their governments or employers. The objective is to establish safe levels of intake and to develop specifications for
identity and purity of food additives.
Thus Codex Alimentarius is a collection of internationally adopted food standards presented in a uniform manner. The food standards aim at protecting consumers’ health and ensuring fair practice in the food trade. Once accepted, an international number is allocated to the additive which is an acknowledgement of its acceptability. It must be stressed that there are food additives which stay at the JECFA specification level and that this advisory specification is the authority for its use, at the conditions given, until the full acceptance by Codex is given. Gum arabic is one such example, a food hydrocolloid which has been used for more than 2000 years but only finally gained full Codex specification in June 1999.
The European system
Clearance of food hydrocolloids by the European Commission was first introduced in 1995 under Directive 95/2/EU for Food Additives other than colours and sweeteners. This is known as the Miscellaneous Additives Directive (MAD), which provides authorisation for a large number of additives from the hydrocolloid group. The majority of these are authorised for general use in foods to Quantum Satis (QS) levels given in Annex 1 of the Directive. Starches, the vast majority of gums, alginates and celluloses enjoy this wide authorisation.
Almost immediately following adoption of the original Directive, the Commission began working on proposed amendments, largely to take account of market developments that had not been taken into account in the last stages of the lengthy and complicated legislative process. The historical development of the process must be referred to in order to understand the almost unintelligible machinery adopted by the European Commission in its work. The ground rules for food additives harmonisation were set out in the form of a framework Directive, 89/107/EEC adopted in 1988 (and amended by the European Parliament and Council Directive 94/34/EC).
It instructs the Council to adopt in subsequent follow-up Directives
• a list of additives to be authorised
• a list of foods to which the additives may be added and the levels of use, which gives delegated powers to the Commission to adopt
• specifications for each additive
• where necessary, methods of analysis and procedures for sampling.A number of general criteria for the use of additives in food are also set out.
According to the criteria, food additives may be authorised only if
• a reasonable technological need can be demonstrated
• they present no hazard to health at the levels proposed
• they do not mislead the consumer.
Evidence of the need for an additive which, incidentally, plays no part in approvals in the USA, must be provided by the user of the additive, that is the food manufacturer, not the supplier or manufacturer of the additive. The criteria also stipulate that all food additives must be kept under continuous observation and re-evaluated whenever necessary in the light of changing conditions of use and new scientific observation.
The EC process acknowledges the JECFA system and in the most unintelligible legal language adopted by the Commission adds the following:
• Whereas Directive 78/663/EEC should be repealed accordingly:
• Whereas it is necessary to take into account the specification and analytical techniques for food additives as set out in the Codex Alimentarius as drafted by JECFA:
• Whereas food additives, if prepared by production methods or starting materials significantly different from those included in the evaluation of the Scientific Committee for Food, or if different from those mentioned in this Directive, should be submitted for evaluation by the Scientific Committee for Food for the purposes of a full evaluation with emphasis on the purity criteria.
It is surprising that any progress was made at all with all the accompanying bureaucracy.
After a most highly political first amendment to the MAD intended to clear Processed Eucheuma Seaweed (E407a) (see Directive 96/85/EC of 19 December 1996), a second amendment was introduced which affected many hydrocolloids. Member States were required to implement the provisions of this Directive in the year 2000. Thus at this time the new authorisation introduced by the current Directive 98/72/EC came into force in all EU Member States.
This Directive provides for E401 Sodium alginate, E402 Potassium alginate and E407 Carrageenan, E440 Pectin, E425 Konjac and E412 Guar. Each has controlling conditions associated with the approval.
Other trade blocks
While the Codex Alimentarius Commission is the ultimate specification and can provide for approval throughout the world, each country (outside the EU) is free to adopt its own standards. In the USA, for example, the United States Food Chemicals Codex (FCC) also has currency. The FCC is an activity of the Food and Nutrition Board of the Institute of Medicine that is sponsored by the UnitedStates Food and Drug Administration (FDA). The current specification of hydrocolloids are to be found in the Fourth Edition (1996). Japan too has its own specifications which include many of the food additives particular to Japan.
The international numbering system for food additives (INS)
INS has been prepared by the Codex Committee on Food Additives in order to be able to identify food additives in ingredient lists as an alternative to the declaration of the specific name. The INS is intended as an identification system for food additives approved for use in one or more member countries.
It does not imply toxicological approval by Codex. There is an equivalence with the EU system of E numbers, albeit that the EU system is more restricted. Where both INS and E numbers are available they are interchangeable.
INS numbers for modified starches(Modified starch* ,INS number):
Dextrin (roasted starch) 1400
Acid treated starch 1401
Alkali treated starch 1402
Bleached starch 1403
Oxidised starch 1404
Monostarch phosphate 1410
Distarch phosphate 1412
Phosphated distarch 1413
Acetylated starch 1414
Starch acetate 1420
Acetylated distarch adipate 1422
Hydroxypropyl starch 1440
Hydroxypropyl distarch phosphate 1442
Starch sodium octenyl succinate 1450
Starch, enzyme treated 1405
Acid treated starch 1401
Alkali treated starch 1402
Bleached starch 1403
Oxidised starch 1404
Monostarch phosphate 1410
Distarch phosphate 1412
Phosphated distarch 1413
Acetylated starch 1414
Starch acetate 1420
Acetylated distarch adipate 1422
Hydroxypropyl starch 1440
Hydroxypropyl distarch phosphate 1442
Starch sodium octenyl succinate 1450
Starch, enzyme treated 1405
*The Codex General Standard for labelling of pre-packaged foods specifies that modified starches
may be declared as such in a list of ingredients. However, certain countries require specific
identification and use these numbers.
may be declared as such in a list of ingredients. However, certain countries require specific
identification and use these numbers.
INS numbers for hydrocolloids(Polysaccharide ,INS number,Function):
Alginic acid ,400 ,Thickening agent, stabiliser
Sodium alginate,401 ,Thickening agent, stabiliser, gelling agent
Potassium alginate ,402 ,Thickening agent, stabiliser
Ammonium alginate ,403, Thickening agent, stabiliser
Calcium alginate ,404, Thickening agent, stabiliser, gelling agent, antifoaming agent
Propylene glycol alginate (propane-1,2-diol alginate),405 ,Thickener, emulsifier, stabiliser
Agar,406 ,Thickener, stabiliser, gelling agent
Carrageenan (including furcelleran) ,407, Thickener, gelling agent, stabiliser,emulsifier
Processed Euchema Seaweed ,407a ,Thickener, stabiliser
Bakers yeast glycan,408, Thickener, gelling agent, stabiliser
Arabinogalactan,409 ,Thickener, gelling agent, stabiliser
Locust bean gum ,410 ,Thickener, gelling agent
Oat gum ,411, Thickener, stabiliser
Guar gum ,412 ,Thickener, stabiliser and emulsifier
Tragacanth gum ,413 ,Emulsifier, stabiliser, thickening agent
Gum arabic (Acacia gum) ,414 ,Emulsifier, stabiliser, thickener
Xanthan gum ,415 ,Thickener, stabiliser, emulsifier, foaming agent
Karaya gum ,416 ,Emulsifier, stabiliser and thickening agent
Tara gum ,417, Thickener, stabiliser
Gellan gum ,418, Thickener, gelling agent and stabiliser
Gum ghatti ,419 ,Thickener, stabiliser, emulsifier
Curdlan gum ,424 ,Thickener, stabiliser
Konjac flour ,425, Thickener
Soybean hemicellulose ,426, Emulsifier, thickener, stabiliser, anticaking agent
Sodium alginate,401 ,Thickening agent, stabiliser, gelling agent
Potassium alginate ,402 ,Thickening agent, stabiliser
Ammonium alginate ,403, Thickening agent, stabiliser
Calcium alginate ,404, Thickening agent, stabiliser, gelling agent, antifoaming agent
Propylene glycol alginate (propane-1,2-diol alginate),405 ,Thickener, emulsifier, stabiliser
Agar,406 ,Thickener, stabiliser, gelling agent
Carrageenan (including furcelleran) ,407, Thickener, gelling agent, stabiliser,emulsifier
Processed Euchema Seaweed ,407a ,Thickener, stabiliser
Bakers yeast glycan,408, Thickener, gelling agent, stabiliser
Arabinogalactan,409 ,Thickener, gelling agent, stabiliser
Locust bean gum ,410 ,Thickener, gelling agent
Oat gum ,411, Thickener, stabiliser
Guar gum ,412 ,Thickener, stabiliser and emulsifier
Tragacanth gum ,413 ,Emulsifier, stabiliser, thickening agent
Gum arabic (Acacia gum) ,414 ,Emulsifier, stabiliser, thickener
Xanthan gum ,415 ,Thickener, stabiliser, emulsifier, foaming agent
Karaya gum ,416 ,Emulsifier, stabiliser and thickening agent
Tara gum ,417, Thickener, stabiliser
Gellan gum ,418, Thickener, gelling agent and stabiliser
Gum ghatti ,419 ,Thickener, stabiliser, emulsifier
Curdlan gum ,424 ,Thickener, stabiliser
Konjac flour ,425, Thickener
Soybean hemicellulose ,426, Emulsifier, thickener, stabiliser, anticaking agent
Pectin ,440, Thickener, stabiliser, gelling agent, emulsifier
Cellulose ,460 ,Emulsifier, anticaking agent, texturiser,dispersing agent
Microcrystalline cellulose ,460 (i) ,Emulsifier, anticaking agent, texturiser,dispersing agent
Powdered cellulose ,460(ii), Anticaking agent, emulsifier, stabiliser and,dispersing agent
Methyl cellulose ,461, Thickener, emulsifier, stabiliser
Ethyl cellulose ,462, Binder, filler
Hydroxypropyl cellulose ,463, Thickener, emulsifier stabiliser
Hydroxypropyl methyl cellulose ,464, Thickener, emulsifier stabiliser
Methyl ethyl cellulose ,465, Thickener, emulsifier, stabiliser, foaming agent
Sodium carboxymethyl cellulose ,466, Thickener, stabiliser, emulsifier
Ethyl hydroxyethyl cellulose ,467 ,Thickener, stabiliser, emulsifier
Cross-linked sodium carboxymethyl cellulose, 468, Stabiliser, binder
Sodium carboxymethyl cellulose, enzymatically hydrolysed ,469 ,Thickener, stabiliser
Thickening characteristics
Hydrocolloids are widely used to thicken food systems and a much clearer understanding of their rheological behaviour has been gained over the last thirty years or so, particularly through the development of controlled stress and controlled strain rheometers capable of measuring to very low shear rates.
Main hydrocolloid thickeners:
Xanthan gum
Very high low-shear viscosity (yield stress), highly shear thinning, maintains viscosity in the presence of electrolyte, over a broad pH range and at high temperatures.
Carboxymethyl cellulose
High viscosity but reduced by the addition of electrolyte and at low pH.
Methyl cellulose and hydroxypropyl methyl cellulose
Viscosity increases with temperature (gelation may occur) not influenced by the addition of electrolytes or pH.
Galactomannans (guar and locust bean gum)
Very high low-shear viscosity and strongly shear thinning. Not influenced by the presence of electrolyte but can degrade and lose viscosity at high and low pH and when subjected to high temperatures.
Very high low-shear viscosity (yield stress), highly shear thinning, maintains viscosity in the presence of electrolyte, over a broad pH range and at high temperatures.
Carboxymethyl cellulose
High viscosity but reduced by the addition of electrolyte and at low pH.
Methyl cellulose and hydroxypropyl methyl cellulose
Viscosity increases with temperature (gelation may occur) not influenced by the addition of electrolytes or pH.
Galactomannans (guar and locust bean gum)
Very high low-shear viscosity and strongly shear thinning. Not influenced by the presence of electrolyte but can degrade and lose viscosity at high and low pH and when subjected to high temperatures.
Viscoelasticity and gelation
Hydrocolloid gels are referred to as ‘physical gels’ because the junction zones are formed through physical interaction, for example, by hydrogen bonding, hydrophobic association, cation-mediated crosslinking, etc., and differ from synthetic polymer gels which normally consist of covalently crosslinked polymer chains.
Some hydrocolloids form thermoreversible gels and examples exist where gelation occurs on cooling or heating. Some form non-thermoreversible gels. In such cases gelation may be induced by crosslinking polymer chains with divalent cations.
Gels may be optically clear or turbid and a range of textures can be obtained. Gel formation occurs above a critical minimum concentration which is specific for each hydrocolloid. Agarose, for example, will form gels at concentrations as low as 0.2%, while for acid-thinned starch, a concentration of ~15% is required. Gel strength increases with increasing concentration.
Molecular mass is also important. It has been shown that gel strength increases significantly as molecular mass increases up to ~100,000 but then becomes independent of molecular mass at higher values,The principal hydrocolloid gelling agents and a comparison of their relative gel textures is illustrated. An increase in brittleness is usually accompanied by an increase in the tendency to undergo syneresis and is attributed to an increase in the degree of aggregation of molecular chains.
Main hydrocolloid gelling agents:
Thermoreversible gelling agents:
Gelatin
Gel formed on cooling. Molecules undergo a coil-helix transition followed by aggregation of helices.
Agar
Gel formed on cooling. Molecules undergo a coil-helix transition followed by aggregation of helices.
Kappa Carrageenan
Gel formed on cooling in the presence of salts notably potassium salts. Molecules undergo a coil-helix transition followed by aggregation of helices. Potassium ions bind specifically to the helices. Salts present reduce electrostatic repulsion between chains promoting aggregation.
Iota Carrageenan
Gel formed on cooling in the presence of salts. Molecules undergo a coil-helix transition followed by aggregation of helices. Salts present reduce electrostatic repulsion between chains promoting aggregation.
Low methoxyl (LM) pectin
Gels formed in the presence of divalent cations, notably calcium at low pH (3–4.5).
Molecules crosslinked by the cations. The low pH reduces intermolecular electrostatic repulsions.
Gellan gum
Gels formed on cooling in the presence of salts. Molecules undergo a coil-helix transition followed by aggregation of helices. Salts reduce electrostatic repulsions between chains and promote aggregation. Multivalent ions can act by crosslinking chains. Low acyl gellan gels are thermoreversible at low salt concentrations but non-thermoreversible at higher salt contents (> 100mM) particularly in the presence of divalent cations.
Methyl cellulose and hydroxypropylmethyl cellulose
Gels formed on heating. Molecules associate on heating due to hydrophobic interaction of methyl groups.
Xanthan gum and locust bean gum or konjac mannan
Gels formed on cooling mixtures. Xanthan and polymannan chains associate following the xanthan coil-helix transition. For locust bean gum the galactose deficient regions are involved in the association.
Gel formed on cooling. Molecules undergo a coil-helix transition followed by aggregation of helices.
Agar
Gel formed on cooling. Molecules undergo a coil-helix transition followed by aggregation of helices.
Kappa Carrageenan
Gel formed on cooling in the presence of salts notably potassium salts. Molecules undergo a coil-helix transition followed by aggregation of helices. Potassium ions bind specifically to the helices. Salts present reduce electrostatic repulsion between chains promoting aggregation.
Iota Carrageenan
Gel formed on cooling in the presence of salts. Molecules undergo a coil-helix transition followed by aggregation of helices. Salts present reduce electrostatic repulsion between chains promoting aggregation.
Low methoxyl (LM) pectin
Gels formed in the presence of divalent cations, notably calcium at low pH (3–4.5).
Molecules crosslinked by the cations. The low pH reduces intermolecular electrostatic repulsions.
Gellan gum
Gels formed on cooling in the presence of salts. Molecules undergo a coil-helix transition followed by aggregation of helices. Salts reduce electrostatic repulsions between chains and promote aggregation. Multivalent ions can act by crosslinking chains. Low acyl gellan gels are thermoreversible at low salt concentrations but non-thermoreversible at higher salt contents (> 100mM) particularly in the presence of divalent cations.
Methyl cellulose and hydroxypropylmethyl cellulose
Gels formed on heating. Molecules associate on heating due to hydrophobic interaction of methyl groups.
Xanthan gum and locust bean gum or konjac mannan
Gels formed on cooling mixtures. Xanthan and polymannan chains associate following the xanthan coil-helix transition. For locust bean gum the galactose deficient regions are involved in the association.
Thermally irreversible gelling agents:
Alginate
Gels formed on the addition of polyvalent cations notably calcium or at low pH (< 4). Molecules crosslinked by the polyvalent ions. Guluronic acid residues give a buckled conformation providing an effective binding site for the cations (egg box model).
High methoxyl (HM) pectin
Gels formed at high soluble solids (e.g. 50% sugar) content at low pH < 3.5. The high sugar content and low pH reduce electrostatic repulsions between chains. Chain association also encouraged by reduced water activity.
Konjac mannan
Gels formed on addition of alkali. Alkali removes acetyl groups along the polymer chain and chain association occurs.
Locust bean gum
Gels formed after freezing. Galactose deficient regions associate.
Alginate
Gels formed on the addition of polyvalent cations notably calcium or at low pH (< 4). Molecules crosslinked by the polyvalent ions. Guluronic acid residues give a buckled conformation providing an effective binding site for the cations (egg box model).
High methoxyl (HM) pectin
Gels formed at high soluble solids (e.g. 50% sugar) content at low pH < 3.5. The high sugar content and low pH reduce electrostatic repulsions between chains. Chain association also encouraged by reduced water activity.
Konjac mannan
Gels formed on addition of alkali. Alkali removes acetyl groups along the polymer chain and chain association occurs.
Locust bean gum
Gels formed after freezing. Galactose deficient regions associate.
Synergistic combinations
Mixtures of hydrocolloids are commonly used to impart novel and improved rheological characteristics to food products and an added incentive is a reduction in costs. Classic examples include the addition of locust bean gum to kappa carrageenan to yield softer more transparent gels and also the addition of locust bean gum to xanthan gum to induce gel formation. The nature of the synergy can be due to association of the different hydrocolloid molecules or to phase separation.
If the two hydrocolloids associate then precipitation or gelation can occur. Oppositely charged hydrocolloids (e.g. a protein below its isoelectric point and an anionic polysaccharide) will associate and may form soluble or insoluble complexes depending on the pH, mixing ratio and ionic strength of the solution.
There is evidence to show that some stiff polysaccharide molecules (xanthan gum, carrageenan, etc.) will associate with galactomannans and glucomannans leading to gel formation. If the two hydrocolloids do not associate, as is commonly the case, then at ‘low’ concentrations they will appear to exist as a single homogeneous phase while at higher concentrations they will separate in time into two liquid phases each enriched in one of the hydrocolloids. The phase separation process involves the formation of ‘water-in-water’ emulsions which consist of droplets enriched in one hydrocolloid dispersed in a continuous phase enriched in the other. Whether the hydrocolloid is present in the dispersed or continuous phase depends on the relative concentrations. If either or both of the hydrocolloids can form gels independently, then phase separation and gelation will occur simultaneously. The characteristics of the resultant gel will depend on the relative rates of these two processes. Careful selection of hydrocolloid type and concentration can, therefore, lead to the formation of a broad range of gel textures and this is currently an area receiving considerable attention.
Hydrocolloid fibres
Because there is a growing belief throughout the world that natural fibre foods are an integral part of a healthy lifestyle, food producers source an increasing proportion of their raw materials from nature itself. There is a growing demand from an increasingly health-conscious consumer for reduced fat and enhanced fibre foods of all types. If this can be achieved using materials which have low calorific value, further health benefits will result.
Foods containing such ingredients will need to match the quality of the original product and without
adverse dietary effects. This target cannot be achieved without the scientific use of thickeners, stabilisers and emulsifiers, particularly of the ‘natural type’. This calls for fibres, which can interact with water to form new textures and perform specific functions, which itself requires the use of hydrocolloids.
adverse dietary effects. This target cannot be achieved without the scientific use of thickeners, stabilisers and emulsifiers, particularly of the ‘natural type’. This calls for fibres, which can interact with water to form new textures and perform specific functions, which itself requires the use of hydrocolloids.
In 1998 the world market for such hydrocolloids of the fibre type was US$2.83 million and
is set to grow significantly to meet the health aspirations of the consumer. It is the task of the food scientist to provide the hydrocolloids in the most appropriate form for inclusion in the food product. This requires an understanding of their structure and the way in which they act to produce the desired function in the food.
is set to grow significantly to meet the health aspirations of the consumer. It is the task of the food scientist to provide the hydrocolloids in the most appropriate form for inclusion in the food product. This requires an understanding of their structure and the way in which they act to produce the desired function in the food.
Dietary fibre was first described as the skeletal remains of plant cell walls, which are resistant to hydrolysis, by the digestive enzymes of man. Since this excluded polysaccharide fibres in the diet, the definition was subsequently expanded to include all polysaccharides and lignin, which are not digested by the endogenous secretions of the human digestive tract. Dietary fibre thus mainly comprises non-starch polysaccharides, and indeed has been defined by Englyst and others as the ‘polysaccharides which are resistant to the endogenous enzymes of man’. Industrialised countries now generally recognise the healthgiving properties of increased consumption of fibre and reduced intakes of total and saturated fat. In this respect ‘fibre’ is used in a non-specific way, but is generally taken to mean structural components of cereals and vegetables. More recently the concept of ‘soluble fibre’ has emerged which assists plasma cholesterol reduction and large-bowel fermentation.
The properties of such soluble and insoluble fibre allow them to perform both in a physical role and also to ferment through colonic microflora to give shortchain fatty acids (SCFA), mainly acetate, propionate and butyrate. These have a very beneficial effect on colon health through stimulating blood flow, enhancing electrolyte and fluid absorption, enhancing muscular activity and reducing
cholesterol levels. The various hydrocolloids described in this handbook fall into this category.
The physical effect
To be effective dietary fibre must be resistant to the enzymes of the human and animal gastrointestinal tract. If physically suitable it can work effectively as a result of its bulking action. In the stomach and small intestine the fibre can increase digested mass, leading to faecal bulking, which readily explains the relief of constipation, which is one of fibre’s best documented effects. It can increase stool mass and ease laxation very efficiently. This behaviour has considerable human and agricultural importance.
The growth of the ruminant animal depends on the fermentable fibre content of the stockfeed.
Soluble as well as non-soluble fibres exert their actions in the upper gut through their physical properties. Those which form gels or viscous solutions can slow down the transit in the upper gut and delay glucose absorption, best explained in terms of ‘viscous drag’. Thus the reduction in glycemic response by soluble fibres can be explained.
Soluble as well as non-soluble fibres exert their actions in the upper gut through their physical properties. Those which form gels or viscous solutions can slow down the transit in the upper gut and delay glucose absorption, best explained in terms of ‘viscous drag’. Thus the reduction in glycemic response by soluble fibres can be explained.
Fermentation product effects
Large bowel micro-organisms attack the soluble fibres, in fermentation resembling that in the rumen of obligate herbivores such as sheep and cattle.The products too are similar: short-chain fatty acids (SCFA), gases (hydrogen, carbon dioxide and methane) and an increased bacterial mass. The principal SCFAs are the same in humans as in ruminants, and the concentrations are similar too, particularly for omnivorous animals with a similar digestive physiology (for example, the pig). The increased bacterial cell mass also has a
positive effect on laxation. Faeces are approximately 25% water and 75% dry matter. The major components are undigested residuals plus bacteria and bacterial cell wall debris. These form a sponge-like, water-holding matrix which conditions faecal bulk and cell debris. The ability of different fibres to increase faecal bulk depends on a complex relationship between chemical and physical properties of the fibre and the bacterial population of the colon.
The production of SCFAs and their beneficial effects in humans and ruminant species has been well established for a considerable time, but the effect was not thought to be relevant to the carnivorous dog and cat. Now this too has been demonstrated.
Health benefits
Whether by physical bulking action or through the production of SCFAs, several health advantages are now established. Increasing fibre (20–30 grams per day in humans) can eliminate constipation through increased faecal bulking and waterholding. The fermentation to produce SCFAs can also assist, since propionate stimulates colonic muscular activity and encourages stool expulsion.It was at one time thought that fibre lodged in the colon could lead to inflammation and herniation. This has now been disproved, and fibre can now relieve diverticular disease conditions, probably in the same way as it relieves constipation. Applying a solution of SCFA into the colon of ulcerative colitis patients or into the defunctioned portion of surgical patients has given rise to substantial remission in colitis. It could be that the condition arises due to a defect in the fermentation process in these patients or in the products.
SCFAs stimulate water and electrolyte absorption by the mucosa and enhance their transport through improving colonic blood flow. Fibre fermentation also reduces the population of pathogenic bacteria such as Clostridia and can prevent diarrhoea due to bacterial toxins. Epidemiological studies have shown repeatedly that populations with high levels of fibre in their diet have reduced risk of colon cancer. Protection may be through the SCFA butyrate, which inhibits the growth of tumour cells in vitro. When applied to the companion animal, the increased production of SCFAs increases gut acidity marginally, which reduces the activity of putrefaction and pathogenic bacteria and so lowers toxin and thus reduces bad odours and bad smelling faeces.
The low level of toxin production reduces the load on the liver and results in better coat and skin quality. Therefore, the ageing animal can look better and produce less offensive faeces.
The behaviour of SCFAs in the intestine can influence the immune system. Thus protection is possible against colonisation by opportunistic bacteria, and the improved colonisation of beneficial indigenous bacteria in the gut gives greater resistance to infectious bacteria.
The behaviour of SCFAs in the intestine can influence the immune system. Thus protection is possible against colonisation by opportunistic bacteria, and the improved colonisation of beneficial indigenous bacteria in the gut gives greater resistance to infectious bacteria.
The health effects of food hydrocolloids are dependent on how they are incorporated into foods and in the diet. There are many hydrocolloid carbohydrates naturally present in plant foods as part of the cell wall, such as hemicelluloses and pectin, or with other more specific roles within the plant such as storage polysaccharides like guar gum, exudates like gum acacia, and husk polysaccharides such as ispaghula. There are also alginates and bacterially produced hydrocolloids such as gellan and xanthan. However, the contribution of each individual hydrocolloid in the diet is small and epidemiological studies cannot identify and separate the health benefits of these compounds from those of other non-digestible carbohydrates such as insoluble non-starch polysaccharides,
resistant starch and oligosaccharides.
resistant starch and oligosaccharides.
Hydrocolloids can also be incorporated in small amounts into food products as stabilisers, emulsifiers and fat substitutes. Guar gum levels of <1 are="" nbsp="" span="">typically added to food products; however, health beneficial effects of guar gum are achieved with higher levels (3–5%). Increasing the amount of dietary fibre within food formulation may result in compromising the product’s organoleptic
properties. However, hydrocolloids such as partially hydrolysed guar gum have a higher potential to be successfully incorporated into different foods due to their lower viscosity.
Large doses of hydrocolloids can be prepared as potential therapeutic agents against constipation as with ispaghula or psyllium in medications or for other health benefits.
However, many of the feeding studies in animals and humans have used high doses of the
hydrocolloids which may not reflect the actions of these polysaccharides in a normal diet or even the effects of palatable supplements. It would be impossible to cover the health aspects of all of the different dietary hydrocolloids and supplements, so in this chapter the role of hydrocolloids will be discussed as part of the dietary fibre story and then more specific health effects of pure hydrocolloids ingested in significant amounts will be reviewed.
hydrocolloids which may not reflect the actions of these polysaccharides in a normal diet or even the effects of palatable supplements. It would be impossible to cover the health aspects of all of the different dietary hydrocolloids and supplements, so in this chapter the role of hydrocolloids will be discussed as part of the dietary fibre story and then more specific health effects of pure hydrocolloids ingested in significant amounts will be reviewed.
Much of our understanding of the impact of food hydrocolloids on gastric emptying, small intestinal digestion and absorption, postprandial glucose and insulin and stool output is now well established with most of the research being carried at the end of the last century (Schweizer and Edwards 1992). However, more recent work has explored new sources of non-digestible polysaccharides and oligosaccharides including those of algal origin , new fruits (e.g., Chinese Jujuba; Huang et al. 2008),
seeds (e.g., fenugreek; Hannan et al. 2007), and nuts (e.g., almonds; Mandalari et al. 2008). The mechanisms of action of hydrocolloids related to their fermentation in the large intestine to short chain fatty acids and their impact on the intestinal microbiota have also been of more recent interest along with the potential actions of the products of their fermentation on plasma lipids and
satiety. This is of increasing importance given the current global epidemic in obesity and increase in associated chronic diseases such as cardiovascular disease (CVD). The randomised control trials of soluble fibre and their effects on weight management, appetite and plasma lipids will be reviewed in this chapter.
Future trends
The introduction of totally new hydrocolloids for food use is restricted by the large financial investment required to obtain the necessary legislative approval.Probably the last hydrocolloid to go through this process was gellan gum. There are certain hydrocolloids, however, that have a long history of use in food in other parts of the world that have potential for use as food additives in the USA and Europe. A typical example is konjac mannan which has been used for hundreds of years in Japan to produce noodles and is eaten as a food in own
right. This material has only recently gained approval for use as an additive in the West. When dissolved in water konjac mannan has similar properties to locust bean gum but produces higher viscosity solutions and also has a stronger synergistic interaction with kappa carrageenan and xanthan gum.
The search for new synergistic combinations continues and this is becoming more fruitful as
our understanding of the interactions and phase behaviour of hydrocolloid mixtures increases at the molecular level. New processing procedures are also being introduced and an area of particular interest at present is the formation of sheared gels to give novel rheological characteristics. This involves applying shear as the hydrocolloid is undergoing gelation and usually results in the
formation of micron-size hydrocolloid gel particles.
our understanding of the interactions and phase behaviour of hydrocolloid mixtures increases at the molecular level. New processing procedures are also being introduced and an area of particular interest at present is the formation of sheared gels to give novel rheological characteristics. This involves applying shear as the hydrocolloid is undergoing gelation and usually results in the
formation of micron-size hydrocolloid gel particles.
At a sufficiently high concentration, the systems formed can have a very high low-shear viscosity and display strong shear thinning characteristics. As discussed above, although hydrocolloids have historically been used in foods to control the rheological properties and texture, consumers are being
made increasing aware of their nutritional benefits. Many hydrocolloids (e.g., locust bean gum, guar gum, konjac mannan, gum arabic, xanthan gum and pectin) for instance, have been shown to reduce blood cholesterol levels. Others (e.g., inulin and gum arabic) have been shown to have prebiotic effects.
made increasing aware of their nutritional benefits. Many hydrocolloids (e.g., locust bean gum, guar gum, konjac mannan, gum arabic, xanthan gum and pectin) for instance, have been shown to reduce blood cholesterol levels. Others (e.g., inulin and gum arabic) have been shown to have prebiotic effects.
They are resistant to our digestive enzymes and pass through the stomach and small intestine without being metabolised. They are fermented in the large intestine to yield short chain fatty acids and stimulate the specific growth of beneficial intestinal bacteria, notably, bifidobacteria, and reduce the growth of harmful micro-organisms such as clostridia.
All in all the hydrocolloid market is currently very buoyant and the prospects for future growth are excellent.
