3.1 Introduction

No other single food ingredient compares with starch in terms of sheer versatility ofapplication in the food industry. Second only to cellulose in natural abundance, thispolymeric carbohydrate was designed by nature as a plant energy reserve. Man, however,has extended the use of starch far beyond this original design. Modifications – physical,chemical or biochemical – mean that numerous highly functional derivatives haveenabled the evolution of new processing technologies and market trends. To this end,speciality starches have been tailored to: create competitive advantage in a new product;enhance product aesthetics; simplify a label declaration; reduce recipe/production costs;increase product throughput; eradicate batch rejects; ensure product consistency; andextend shelf life. Clearly, understanding the capabilities of starch and how to exploit itspotential is of relevance to all stages of a food product’s lifecycle from development,through production and marketing, to retail.

3.2 Manufacture

3.2.1 SourcesTo ‘plug’ into the sun is to trap the cleanest and ultimate source of energy for the planet.Photosynthesis does precisely this. Leaves – nature’s ‘solar panels’ – trap light energyand, through a cascade of physico-chemical processes, involving carbon dioxide andwater, translate this into sugar molecules, such as glucose. In itself, glucose is too mobileto act as a long-term energy storage system. Nature’s solution to this is to immobilise theglucose by forming a polymer; more precisely, a condensation polymer in which glucosechains are linked together by the elimination (condensation) of water. This is starch, ananhydroglucose polymer. Starch is a member of the ‘polysaccharide’ group of polymers.

3

StarchP. Murphy, National Starch and Chemical, Manchester

The author would like to thank M. Croghan, J. Scott and K. Hughes, National Starch and Chemical and Dr B.Murphy, MMU for the information and support provided.

It is laid down as insoluble, compact and microscopic semi-crystalline granules of size 1–100�m. In the human dimension, this means that 1g of starch typically containssomething of the order of 1,000,000,000 granules with each granule, in turn, containingabout 10,000,000,000,000 starch molecules. Seeds, tubers, roots and stem piths are therepository sites for the starch. When digested, the trapped energy is released as the starchis broken down by hydrolysis back to its constituent glucose molecules and thence furtherback to the original carbon dioxide and water.

Despite the ubiquity of starch in nature, the number of commercially viable sources issmall. The most important origins of starch are maize, potato, wheat, tapioca and rice.‘Corn’ and ‘maize’ starches are American and European terms that actually refer to thesame range of starches. The cereal crops maize, waxy maize and wheat are grown inAmerica and Europe whilst potato is largely derived from the cooler climes of northernEurope. Most cassava (the base crop for tapioca starch) is sourced from Brazil, Thailandand Indonesia, while rice originates mainly from Asia.

3.2.2 ExtractionThe manufacture of starch employs a variety of processes which isolate purified starchfrom the other constituents of the raw material. For example, the extraction of maizestarch from the maize kernel is a process known as wet milling in which the starch isseparated from the fibre, oil and tightly bound protein. Regardless of the extractionprocesses involved, the objective is to recover the insoluble starch as undamaged or intactgranules. In this form it is known as native starch. It can be washed and dried or left as aslurry for future processing into modified starches.

3.2.3 ProcessingUnprocessed native starches are structurally too weak and functionally too restricted forapplication in today’s advanced food technologies. Processing is necessary to engender arange of functionality. Processing can be chemical, biochemical or physical.

Table 3.1 serves as a ready reference to the type of modification, its effect on thenative starch (the objective) and the functionality produced (the benefit). The effects ofmodification processes on starch structure are explored in Section 3.4. Chemicallymodified starches are, in most cases, labelled as ‘modified starch’. Physically treatedstarches (without any additional chemical modification) carry a simple ‘starch’ label.This is elaborated in Section 3.7.

3.3 Structure

3.3.1 Starch moleculeThe glucose polymers that make up starch come in two molecular forms, linear andbranched. The former is referred to as amylose and the latter as amylopectin. The basicglucose building block is a ring-shaped molecule with six atoms in the ring, see Fig. 3.1.Although for simplicity often drawn as a flat structure, the ring is, in fact, mobile and cantake many shapes through ‘puckering’. Of these many shapes or conformations one –known as the chair form – is favoured. Moreover, the chair form comes in two varieties(stereoisomers): �-D-glucose and �-D-glucose. The two are interconvertible using heatand one is transformed into the other by a series of twists around the six-membered ring

42 Handbook of hydrocolloids

Table 3.1 Modifications of starch

Modification* Objective Benefit to user Typical uses

Cross-linking Strengthen starch granule Improved process tolerance toheat, acid and shear

Ambient stableproducts

Delay viscositydevelopment by retardinggranule swelling

Production efficiency: increasedheat penetration allowing shorterprocess time

Bottled sauces

Sterilised soups andsauces

Stabilisation Prevent shrinkage of starchgranule and providestability at low temperatures

Excellent chill and freeze/thawstability to extend shelf life

Chilled and frozenprocessed foods

Lower gelatinisationtemperature

Easy to cook in high solidssystems

High brix fillings andtoppings

Dextrinisation Break down and rearrangestarch molecule providinglower viscosity, increasedsolubility and a range ofviscosity stability fromliquid to gel

Easily handled or applied athigher dosage than parent nativestarch for desired effect

Create film-forming properties

Fat replacers

Bakery glazes

Protective coatingsin confectionery

Blistered, crispycoatings in friedsnacks

Enzymeconversion(Biochemicalmodification)

Produce varied viscosity,gel strength, withthermoreversibility andsweetness

Contributes texture and rheology

Economic dispersant

Fat mimetics

Flavour carriersDry mix fillers

Acid thinning Lower viscosity andincrease gel strength

Enhances textural properties athigher usage concentrations ofstarch

Gums, pastilles,jellies

Oxidation Introduce carbonyl andcarboxyl groups whichincreases clarity andreduces retrogradation ofcooked starch pastes

Improves adhesion of coatings

Creates soft stable gels at higherdosage than parent native starch

Battered meat,poultry and fish

Confectionery

Provide lower viscosity andlow temperature stability

Lipophilicsubstitution

Introduce lipophilic groups Emulsion stabiliser whichimproves quality of any fat/oil-containing product

Reduces rancidity by preventingoxidation

Beverage, saladdressings

Flavour-encapsulating agents

Pregelatinisation Pre-cook starch to givecold water thickeningproperties

Cold water thickening eliminatesneed to cook, offers convenienceand energy savings

Instant soups, sauces,dressings, desserts,bakery mixes

Thermaltreatment

Strengthen starch granule

Delay viscositydevelopment by retardinggranule swelling

Unique functional native starchwith ‘Starch’ on label declaration

Improved process tolerance toheat, acid and shear

Production efficiency: increasedheat penetration allowing shorterprocess time

Ambient stableproducts

Bottled sauces

Sterilised soups andsauces

* Combination treatments are also commonly used to achieve the desired objective. The most popular are cross-linking/stabilisation or cross-linking/stabilisation/pregelatinisation.

Starch 43

Fig. 3.1 Linear and branched starch polymers.

which cause the groups to ‘wag’ sequentially in a molecular-scale ‘Mexican wave’around the ‘arena’ of the glucose ring. It is the �-D-glucose that is used in nature to formthe starch polymers. Once polymerised into starch, �-D-glucose is locked into this chairform. When the ring atoms are numbered as in Fig. 3.1, it is clear that the links(glucosidic links) between carbons 1 and 4 of neighbouring units give rise to amylose,while occasional branches from this linear chain between carbons 1 and 6 give rise to thelarger, more highly branched amylopectin.

In fact, the ‘linear’ amylose has a small degree of branching but it is predominantlyregarded as a single chain. The chain length can vary with the botanical origin of thestarch but will be of the order of 500 to 6000 glucose units. Because of its more simplepolymeric structure, amylose has a greater propensity to deposit in a regular mannerforming crystals. In nature, three crystalline forms of amylose, A, B and C exist,depending upon the source: cereals (A), tuber (B) and certain pea and bean varieties (C).Precipitated starch complexes (with iodine, long-chain alcohols and fatty acids) are foundin the V form. The so-called linearity of the amylose is further complicated by a twistingof the polymer into a helix. It is different degrees of hydration of the helix that gives riseto the A, B and C forms.

In contrast to amylose, each branched chain of amylopectin contains only up to 30glucose units. However, the multitude of branching in amylopectin gives it a molecularweight that is 1000 times that of amylose. Indeed, amylopectin is a titan of a moleculefrom nature: one of the largest with a molecular weight of 400 million. The ratio ofamylose and amylopectin in any native starch is dependent not only on its source, Table3.2, but also on selective crop breeding, a process known as hybridisation. Waxy and highamylose starches are widely used examples of this.

Table 3.2 Starch granule characteristics

Starch Type Diameter Morphology Gelatinisation Pasting Amylose Cookedmicrons temp. ºC temp. ºC content properties(�m) (a)

Maize (b) Cereal 5–30 Round 62–72 80 25 Opaque gelPolygonal

Waxy Cereal 5–30 Round 63–72 74 � 1 ClearMaize Polygonal cohesiveTapioca Root 4–35 Oval 62–73 63 17 Clear cohesive,

Truncated tendency to gel‘kettle drum’

Potato Tuber 5–100 Oval 59–68 64 20 Clear cohesive,Spherical tendency to gel

Wheat Cereal 1–45 Round 58–64 77 25 Opaque gelLenticular

Rice Cereal 3–8 Polygonal 68–78 81 19 Opaque gelSphericalCompoundgranules

Sago Pith 15–65 Oval 69–74 74 26 Opaque gelTruncated

High Cereal 5–30 Polygonal 63–92 (c) � 90 50–90 Very opaque,Amylose Irregular very strong gelMaize Elongated

(a) Measured for 5% starch suspension.(b) Maize is also often referred to as ‘corn’, ‘dent corn’ or ‘regular maize’.(c) High amylose maize starches are not completely gelatinised in boiling water.

Starch 45

3.3.2 Starch granuleFigure 3.1 also shows that the starch backbone has numerous ‘OH’ (hydroxyl) groupsprojecting into the surrounding space. Hydroxyl groups have a particular affinity forother hydroxyl groups and can serve as a driving force in bringing starch chainstogether in an ordered manner through hydrogen bonding (Section 3.3.3). Where suchordering occurs, crystalline regions are deposited in the granule. The remaining regionsof unordered starch are referred to as amorphous. It is the crystalline regions that givethe granule its structure and facilitate identification of a raw (uncooked) starch. Undera microscope, starch granules illuminated with polarised light, show a characteristic‘Maltese Cross’ pattern. This phenomenon is also known as birefringence. Themicroscopic appearance of each granule is diagnostic of its source: Table 3.2 –morphology. Microscopic identification can be enhanced using iodine to stain theamylose to a characteristic blue-black colour. This is the result of an associationbetween the two to form a complex in which the amylose forms a helical coil aroundiodine molecules. Waxy maize starch, which contains negligible amounts of amylose,does not form a complex and as a result takes on the red/brown solution colour ofiodine.

3.3.3 Hydrogen bondingHydroxyl groups are the very same groups that make up water – the usual medium forstarch in the food industry. There is consequently a strong interaction (hydration) andaffinity, through hydrogen bonding, between the colossal starch and the diminutive watermolecules. Hydration, when brought about by cooking, produces an irreversible changein the structure of the starch granule whereby the starch–starch interactions are‘unzipped’ and replaced by starch–water interactions. This forces the chains apart and thegranule swells. Eventually the granule ruptures and starch polymers are dispersed insolution producing a viscous colloidal state. This is a form of water management thatcontrols structure and texture in food products. ‘Gelatinisation’ and ‘pasting’ are,respectively, the technical descriptions of the hydration within the granule and theirreversible granule swelling that builds viscosity.

Dispersal of the simpler and more linear form of starch, amylose, allows for greatermobility which can result in the molecules self-assembling into a more ordered structure.Aligning themselves parallel to each other, the hydrogen bonding that previously mayhave involved water, can be reduced and replaced by hydrogen bonding between thealigned chains. Ordering in this manner produces a three-dimensional network thatconstitutes an opaque gel. Otherwise referred to as ‘retrogradation’ or ‘set-back’, onlycooking and cooling can bring about this phenomenon.

3.4 Modifications

Starch modifications are a means of altering the structure and affecting the hydrogenbonding in a controllable manner to enhance and extend their application. The alterationstake place at the molecular level, with little or no change taking place in the superficialappearance of the granule. Therefore, the botanical origin of the starch may still beidentified microscopically. Each chemical and biochemical modification described belowis represented schematically in Fig. 3.2.

46 Handbook of hydrocolloids

3.4.1 Cross-linkingCross-linking is the most important chemical modification in the starch industry. Itinvolves replacement of the hydrogen bonding between starch chains by stronger, morepermanent, covalent bonds. In this manner, the swelling of the starch granule is inhibited,pre-empting disintegration either by chemical attack, mechanical attrition (shear) orcooking. Simply put, the starch granule is, in the molecular dimension, ‘spot welded’ atrandom locations to reinforce it. Distarch phosphates and distarch adipates are the mostcommonly encountered cross-linked starches wherein a phosphate or adipate bridge ispresent; the latter containing a longer bridge than the former. Because of the morepermanent nature of the covalently bonded bridge or cross-link, only a small degree isnecessary to produce beneficial effects: typically one cross-link per 100–3000anhydroglucose units of the starch. As the number of cross-links increases, the starchbecomes more resistant to gelatinisation. Consequently, cross-linked starches offer acid,heat and shear stability over their parent native starches.

3.4.2 StabilisationStabilisation, the second most important modification, is usually used in conjunction withcross-linking. The primary objective of stabilisation is to prevent retrogradation andthereby enhance shelf-life through tolerance to temperature fluctuations such as freeze-thaw cycles. In this modification, bulky groups are substituted onto the starch in order totake up space and hinder (steric hindrance) any tendency for dispersed (cooked), linearfragments to re-align and retrograde.

Fig. 3.2 Chemical and biochemical modifications of starch.

Starch 47

H H

H

H

H

H

H

H X

X

The effectiveness of stabilisation depends upon the number and nature of thesubstituted group, of which there are primarily two food-approved types: acetylated andhydroxypropylated. The Degree of Substitution (DS) is a measure of the number ofsubstituents per 100 anhydroglucose units. Low DS starches, those with a DS below 0.2(i.e. 2 substituents per 10 anhydroglucose units), are the most important commercially.As the DS level is raised, the starch–starch interactions in the granule are weakened and,consequently, hydration and gelatinisation by cooking is achievable at lowertemperatures. Such starches benefit from easy cooking and are particularly useful inlow-moisture environments and where the moisture level is restricted by competitionfrom co-ingredients.

3.4.3 ConversionsConversion is a collective term for a range of chain–cleavage reactions of starch.

Acid hydrolysisAcid-thinned, thin-boiling and fluidity starches are all terms which refer to starcheswhich have been subjected to acid hydrolysis. This form of hydrolysis differs fromdextrinisation in that it is conducted in aqueous conditions. The acid predominantlyattacks and depolymerises the amorphous regions of the granule such that when the starchis heated beyond its gelatinisation temperature the granules rupture quickly. The effect oncooking is a lower hot viscosity and, due to the increase in the ratio of smaller, linearmolecules, a stronger gel develops on cooling (set-back) compared to the native parentstarch.

OxidationThe production of oxidised starches employs alkaline hypochlorite as a reagent. Twoimportant modifications occur: the relatively bulky carboxyl (COOH) and carbonyl(C�O) groups are introduced together with partial depolymerisation of the starch chains.Oxidised starches, like acid-thinned starches, exhibit a significantly reduced hot viscositydue to breakdown of the starch beyond its gelatinisation temperature. Unlike acid-thinnedstarch however, the steric hindrance of the bulky groups disrupts any tendency towardsre-association (set back) of the shorter chains thereby significantly reducing the gelstrength. This is an advantage of oxidised over acid-thinned starches. ‘Bleaching’ is, infact, a milder form of oxidation with less than 0.1% of carboxyl groups.

DextrinisationDextrinisation, also known as pyroconversion, refers to two aspects of the structuralmodification of starch. The first is a partial depolymerisation achieved throughhydrolysis. Hydrolysis is the reverse of condensation. It is the addition of water acrossa bond resulting in cleavage of that bond. It is usually brought about by dry roasting thestarch either alone, making use of its natural 10–20% moisture content, or in the presenceof catalytic quantities of acid. This gives rise to a range of polymer fractions of varyingchain length (low conversion). The second aspect involves a recombination of thesefragments (repolymerisation) but this time in a branched manner (high conversion). Thestarches so produced are called dextrins or pyrodextrins. They are typically classified aswhite dextrins, yellow dextrins or British gums depending on their range of viscosity,cold-water solubility, colour, reducing sugar content and stability.

48 Handbook of hydrocolloids

Enzyme hydrolysisSelective enzyme hydrolysis is a form of biochemical modification. This reaction canresult in a wide range of functionalities. Depending on the extent of enzyme hydrolysis, arange of chain lengths corresponding to glucose (dextrose), maltose, oligosaccharides andpolysaccharides can be produced. The exact composition can be determined bymeasuring the dextrose equivalent (DE), where a DE of 100 corresponds to puredextrose and a DE of 0 to the native starch. Hydrolysates with a DE below 20 are referredto as maltodextrins whilst those with a DE between 20 and 100 are referred to as glucosesyrups. A range of enzymes is employed to produce such a spectrum of hydrolysates.These commonly include amylases for breaking up the straight chains as well as otherenzymes to break off the branched segments of starch. �-Amylase selectively andrandomly attacks the 1,4-linkages of the starch to produce maltodextrins and low DEsyrups. �-Amylase is more thorough and attacks every other 1, 4-linkage to give lowermolecular fragments and higher DE syrups, e.g., maltose which has a DE of 50. Othercommonly used enzymes include iso-amylase and pullulanase which also give high DEsyrups through hydrolytic attack at other specific sites on the starch, such as de-branchingreactions at the 1, 6-linkages.

3.4.4 Lipophilic substitutionThe hydrophilicity of starch, its propensity to interact with water, can be transformed intoa schizophrenic hydrophilic-hydrophobic duality. This is particularly useful forstabilising interactions between materials such as oil and water. To achieve this thealready hydrophilic starch must be given a characteristic that mirrors that of an oil,namely, a long hydrophobic, i.e. lipophilic, hydrocarbon chain. Octenylsuccinate groupscontaining an eight-carbon chain provide the lipid-mimicking characteristic. Starchoctenylsuccinates are attracted to, and stabilise, the oil–water interface of an emulsion.The glucose part of starch binds the water while the lipophilic, octenyl part binds the oil.In this way complete separation of the oil and water phases is prevented.

3.4.5 PregelatinisationPregelatinisation is a physical rather than a chemical modification. Certain starchesrequire cooking to develop their function. These are referred to as ‘cook-up’ starches andthe process of pregelatinisation is designed to remove the necessity for cooking.Pregelatinisation may be applied to native or modified cook-up starches to achieve aversatile range of cold thickening starches. The starch is pre-cooked or ‘instantised’ bysimultaneously cooking and drying using one of the following processes:

• drum drying (starch suspensions or starch pastes) – very widely used• extrusion (semi-dry starch) – rarely used• spray-drying (starch suspensions) – increasing in use

Drum-dried starches give a slightly lower viscosity than their base starch because ofthe damage sustained by the granules when the drum-dried flakes are ground to thedesired particle size, of which there are two, fine and coarse. Further, the finer the particlesize required, the greater the damage incurred. Consequently drum drying, albeit atraditional and economic method of processing, is not without its drawbacks. Moreover,the fine and coarse grades perform differently in respect of dispersion, smoothness and

Starch 49

rate of viscosity development. As a result, spray-dried starches are finding popularity.They offer properties that are closer to their base cook-up starch.

3.4.6 Thermal treatmentAs recently as 1997 it was claimed that: ‘the search for physical modification techniquesthat are ‘‘greener’’ and ‘‘more natural’’ alternatives to chemical modification continues. . . but today there is no economically viable alternative to chemical modification to meetthe many requirements of food processing in the late 1990s and beyond’ (Imeson, 1997).This market deficit was remedied while the above article was in press in 1996. This is atrue revolution in starch technology and offers manufacturers ‘thermally-treated’ or‘functional native starches’ – new terms in the field – with all the process tolerance ofchemically modified starches whilst, crucially, remaining classified as ‘native’.Processing involves physical rather than chemical modification under proprietarytechnology. These functional native starches are simply labelled as ‘starch’, thus avoidingthe need for the somewhat confusing ‘modified starch’ label.

3.5 Technical data

3.5.1 Structure–function relationshipNative starchesThe technical attributes of a native or modified starch are brought to the fore throughcooking. Depending upon the origin or modification, phases such as gelatinisation (1),pasting (2) and retrogradation (3) can be achieved within temperatures and times that aresuited to the food product in which the starch is an ingredient. These three phases,characterised by the viscosity profiles of Fig. 3.3, are captured using a viscometer such asthe Brabender Viscoamylograph, and the light microscope. The viscoamylograph is theindustry-standard instrument used to control mixing, heating and cooling during viscositymeasurement. The unit of viscosity measured in this way is the arbitrary Brabender Unitand measurements are always made relative to an internal standard. It must be stressedthat Brabender measurements are not comparable with viscosity measured by rheometerssuch as, the Brookfield or Bostwick instruments. The viscosity changes of the nativestarches are related to the structural transformations of swelling, rupture, dispersion andset-back (all of which have been detailed previously in Section 3.3.3). Thesetransformations can be observed using the light microscope with iodine-staining forfurther enhancement.

The Brabender profiles of Fig. 3.3 illustrate the limitations of native starches. In thefigure, the viscosity changes are related schematically to changes in granularmorphology, or shape. During the initial stages of heating, no viscosity change isnoted as hydration occurs at the molecular level within the granule. This is thegelatinisation stage which, in the absence of a detectable viscosity change, is bettergauged using the microscope. The first indication of gelatinisation is the loss of thediagnostic ‘Maltese Cross’ or birefringence as viewed under plane polarised light. Atthis stage the hydration and swelling are reversible. If cooled and dried at this juncture,sufficient molecular ‘memory’ remains in the starch interactions for the granule torevert to its original state. However, once the hydration has reached the critical pastingstage, a rapid onset in the development of viscosity is seen. At this point the structuralchanges in the granule are irreversible. Pasting temperatures differ with starch origin

50 Handbook of hydrocolloids

Fig. 3.3 Changes in traditional native starch during processing.

due to the varying accessibility of the granule to hydration. The accessibility isdetermined by: the ratio of amylose to amylopectin; the concentration of lipid material;and other factors such as the detrimental effect of phosphate groups. The cerealstarches, maize and wheat, in contrast to waxy maize, tapioca and potato, reach theirpasting temperatures at a slower rate due to an amylose-lipid complex that reinforcesthe granular structure. With continued heating the paste viscosity climbs to a peak andthen decreases as the granules rupture. This is over-cooking. On cooling, retrogradationand gel formation in cereal starches sets in quickly and strongly due to the reasso-ciation of a high proportion of small and mobile (dispersed) amylose polymers.Significantly, waxy maize contains next to no amylose thereby allowing the granules toswell more freely to a peak viscosity and ultimately to produce, not a gel, but a thick,colloidal amylopectin dispersion. Potato starch exhibits the greatest swelling due itslarger granule size. In this case, the very rapid viscosity development at lowtemperatures is due to naturally occurring phosphate groups which are responsible forstarch-starch repulsions that weaken the granule and accelerate its rupture. Thephosphate groups also impart a strong salt/electrolyte sensitivity.

For native starches, the process tolerance is extremely fragile and, in general, nativestarches suffer from over-cooking.

Process-tolerant starchesIn modified starches that are cross-linked, the peak viscosity region is drawn out into anextended viscosity plateau. In this region, the modified starch is tolerant of over-cookingand retains its high viscosity-building potential (Fig. 3.4). Thermally treated starches cannow achieve the same profiles.

Fig. 3.4 Effect of cross-linking and stabilisation.

52 Handbook of hydrocolloids

Stabilisation is used in conjunction with cross-linking to produce process tolerantstarches with freeze-thaw stability. In addition to enhanced process tolerance,stabilisation also lowers the gelatinisation temperature.

3.5.2 NutritionFat mimeticsMarketing initiatives in the ‘diet’ and ‘health’ sector of the food industry presents newchallenges for the food industry. The thrust of starch technology in this direction has beento support the development of new wholesome and balanced foods without sacrificing thedesirable characteristics associated with the traditional high-calorie fat-containing foodproducts. In fat-reduction programmes, the key functions of starches are in achieving asignificant calorie reduction coupled to the rebuilding of features which hitherto wouldhave been provided by the fats in the product. These are viscosity, body andmouthcoating.

The traditional approach is the partial replacement of fat using starches which, whendissolved in water, create stable thermoreversible gels. Soft, fat-like gels can be createdby conversion modifications to the degree necessary to produce thermoreversible,spreadable gels. Typically, 25–30% solids, i.e. starch in water, form an optimal stablestructure for fat replacement. New generation fat replacers are tailored to mimic moreclosely the many and complex properties of fats or oils in a particular application. Theseare referred to as fat mimetics. Whilst normal, viscosifying starches require intact,swollen starch granules for maximum viscosity and stability, fat mimetics generallyrequire granule rupture and solubilisation for their functionality. Therefore, the lubricitythey are designed to contribute is not generally affected by extended heating, shear oracid. Maximising the synergies of functional ingredients such as hydrocolloids generallyin combination with specific starch fat mimetics can mean that 100% fat reduction isachievable.

Resistant starchIn modern societies, great emphasis is frequently placed on the relationship betweenhealth, lifestyle and diet. With major infectious disease under control, focus has shiftedfrom reactive cure to proactive prevention. Given the nutritional connotation that ‘you arewhat you eat’, there is much debate surrounding food fortification. Resistant starch (RS),be it natural or commercial, has a role to play with regard to the nutritional benefits offibre fortification. RS goes under many definitions but, in essence, it is starch that isresistant to digestion in the stomach and small intestine. RS offers advantages overcellulosic sources of fibre such as bran. It provides low water-holding capacity therebyaiding processing; it enhances the organoleptic qualities of food as a replacement for, orcomplement to, natural fibre and it can be labelled as ‘dietary fibre’. Although research inthis area is still in its infancy, there are potential physiological benefits in relation to thebiochemistry of the colon and glucose/insulin metabolism. In the former, it is said that‘desirable’ microflora are encouraged in the gut from the prebiotic action of RS leadingto improved colonic health. In the latter, it offers a controlled rate of digestion to glucosewithout the peaks and troughs produced by less complex carbohydrates.

There are four types of resistant starch. RS II, III and IV are commercially available.RS I is the physically inaccessible starch found in grain, seeds and legumes. In general, itis not suited as a food ingredient since processing can destroy it. RS II is a granular starchwhich, in an uncooked state, is naturally resistant to enzyme attack. It is found in green

Starch 53

bananas, potatoes and very high amylose starch. RS III and IV are formed through:thermal modification, e.g., as in bread crusts, cornflakes or retrograded high amylosestarch; or chemical modification, e.g., as in repolymerisation to alter the glycosidiclinkages such that they are no longer recognised by �-amylose. Commercial sources ofRS assay at 30–60% dietary fibre (as measured by the Prosky AOAC method). Assuming50% of the RS product is indigestible (i.e. reaches the colon) then for labelling purposes itcan be considered as non-calorific and has a theoretical calorific value of only 1.9 kCal/g.This makes resistant starch products ideal ingredients for the fibre fortification of low-calorie products. Note: Given that material reaching the colon does provide energy to thebody the actual physiological calorific value will be approximately 2.8 kCal/g.

Processing of high-fibre products has traditionally been fraught with problems relatedto the high water-binding capacity of cellulosic fibres. RS offers processing advantages,not only due to its low water-binding capacity, but also due to a negligible impact ondough viscosity and rheology, since it does not compete for water. There is even atextural benefit observed with RS in low moisture systems where, for example, cerealsand snacks containing RS are more expanded and retain a light, crispy texture. This is incontrast to the texture attributed to oat bran-substituted snacks and cereals which aredense and hard, and accordingly have a reputation of limited palatability. However, asynergistic action of RS complimenting traditional cellulosic fibres can dispel thisreputation and open up opportunities for dietary enhancement with texturally appetisingand nutritionally balanced foods.

3.6 Uses and applications

3.6.1 Starch selectionAs a multifunctional and user-friendly ingredient, starches are found widely appliedacross the food and beverage industry. Matching the right starch within any of theseapplications requires a multitude of criteria to be considered. The process of starchselection can be rationalised through a knowledge of the range of features in a foodproduct or process that starch can control or facilitate. These include sensory properties,method of manufacture, co-ingredients and shelf-life expectations. All are dealt with ingreater detail in the following sections.

Sensory propertiesFigure 3.5 displays some key sensory descriptions for appearance, structure, taste andmouthfeel together with examples of how such attributes can be achieved.

Ingredient factorsIt is not uncommon, in complex food-product formulations, to encounter effects arisingfrom ingredient interactions. Indeed, on occasions when synergistic interactions areobserved, manufacturers will seek to optimise them and, where the opportunity arises,protect their invention through patents. In cooking a starch-based formulation, the effectsof three critical co-ingredients should be considered.

AcidMany foods contain acids either as a means of preservation or for flavour. In such cases,the acidity or pH of the food product is very important to the selection of a suitable starch.Viscosity can vary significantly as a function of pH, not only for native starch but also for

54 Handbook of hydrocolloids

modified starch. Acids disrupt naturally occurring hydrogen bonding causing the starchgranule to swell more easily and, in extreme pH conditions (e.g. pH 2.5), cooking canlead to premature rupture of the granules with an accompanying breakdown in viscosity.To overcome the effects of acid and provide maximum viscosity at minimum starchusage levels, the starch should be inhibited, i.e., strengthened by chemical cross-linkingor thermal treatment, to the degree necessary. This will usually include consideration ofthe manufacturing process involved for the food product.

SugarsIn ‘high-solids’ or ‘high-Brix’ systems, the presence of water-soluble solids, notablysugars, can have a detrimental effect on starch hydration. By competing for, andpreferentially tying up, the water necessary for hydration, sugars can cause an increase inthe starch gelatinisation temperature which makes it more difficult to cook-out the starchand achieve the desired functionality. A formulation containing 60% sugar raises thestarch gelatinisation temperature to above 100ºC. To achieve such temperatures in water,pressurised cooking would be needed to cook-out the starch. As an alternative in simpleoperations, the solution may be to add the sugar after the starch has been cooked; or, tolimit the sugar addition to 20% whilst cooking the starch, the remainder of the sugarbeing added at the end. Complex manufacturing, however, may not be amenable to suchalteration. In these circumstances, it is advisable to use a light to moderately inhibitedstarch which is also highly stabilised or, indeed, a pregelatinised starch. Hydroxypro-pylated, stabilised starches offer processing advantages in this respect. Their lowergelatinisation temperature allows easy hydration and cooking, even in high-solidsformulations. Their low hot viscosity allows excellent heat penetration and affords a

Fig. 3.5 Starch sensory attributes.

Starch 55

shorter process time. This, in combination with their better freeze-thaw stability, providesfor a premium quality product with good shelf-life.

Fats and oilsThe rate of viscosity development is often lowered in high-fat or oil products. This occursdue to the fat or oil coating the starch and delaying hydration. This feature can be takenadvantage of to achieve easier dispersion and rehydration of pregelatinised starches. Infood emulsions, lipophilic starches are used as emulsion stabilisers to ensure productquality and shelf-life stability.

Process factorsThere are essentially three parameters to consider here, namely, time, temperature andshear during manufacturing and reconstitution. These parameters should not be viewed inisolation since, in most processed foods, at least two of the parameters are involved.Table 3.3 lists examples of processing equipment together with the conditions associatedwith them. In general, higher temperature, longer hold time and greater shear forces willpromote granule swelling and, consequently, the starch will be more susceptible torupture and breakdown. Tolerance to such processing factors is achieved bystrengthening the granule through cross-linking or thermal treatment.

Shelf stabilityThe conditions that a finished food product has to withstand between production andconsumption will also influence the selection of an appropriate starch. Ensuring a processedfood reaches the consumer in the same ideal state as it left the factory, is the considerationof shelf stability. The most critical factor is temperature during storage, especially if thetemperature fluctuates. Assuming the relevant modified starch in the product is optimallycooked, and the level of cross-linking is matched to the required degree of processtolerance, then a combination modification is required for temperature-storage tolerance.The key combination with cross-linking is stabilisation which prevents freeze-thawintolerance, i.e., retrogradation, water loss or any textural changes. In this context, the morebulky blocking groups of hydroxypropylated starches, as opposed to those of acetylatedstarches, offer better freeze-thaw stability and longer term storage stability.

Table 3.3 Effects of food processing*

Equipment Time Temperature Shear Pressure

Steam jacketed kettle Long Low–medium LowPlate heat exchanger Short Medium–high HighScraped-surface heat exchanger Short Medium–high HighJet cookers Short Medium–high Medium–highDirect steam injection cookers Short High Very highRetorts Very short High High Low–mediumExtruder cookers Long High Low HighFlash cooling Short High High HighPiston pump HighMono pump Low–moderateCentrifugal pump ModerateColloid mill, homogenisers Moderate–high Low–HighMicrowave Very high

* High temperature, shear and pressure are often referred to as ‘high stress’ processes. For such environments,specific modified starches will be required.

56 Handbook of hydrocolloids

End useThe end use of a product defines how it will be reconstituted or prepared for consumption.The meticulous consideration of factors affecting starch selection during manufactureshould be extended to include those factors which will influence the starch cook duringreconstitution, and which will consequently affect product quality. If a product is to bereheated, then the method used is important since, not only time and temperature, butshear may also need consideration. Flexibility in reconstitution is a selling point. Forexample, in a three-way cook using either hob, casserole or microwave, temperature andtime are involved in two of them whilst the microwave additionally includes shear.Microwaves have a different mechanism for heating than the conventional oven cooker.Microwave radiation produces heat by ‘activating’ friction between dipoles such as thehydroxyl groups of molecules (water, starch and sugar have these in abundance).Consequently, microwave processing is a high shear environment and the assessment ofthe degree of cross-linking needed should therefore include consideration of the means ofreconstitution. Fats, compared to water, exhibit little dipolar charge but their much lowerspecific heat means that they warm considerably faster than water in microwave cooking.

3.6.2 ApplicationsBaked goodsWheat flour is the basis of many baked goods. It is an economic commodity but is over-stretched in respect of modern products such as frozen, chilled, low-fat and gluten-freefoods, where aesthetics and processing can be limiting without recourse to starch co-ingredients. Whilst formulations will vary significantly depending on the desired finalproduct, typical ingredients, apart from starch, could include wheat flour (8–16% protein,71–79% carbohydrate), fats, sugars, eggs, emulsifiers, milk and/or water. Processingconditions will also vary with the formulations. The effect of these ingredients andprocesses on the use of starch or modified starch can be exacerbated since baked goodshave a limited amount of moisture. Gelatinisation of the starch (in the wheat flour as wellas in the added starch) is critical to building structure and texture in bakery products.Since wheat starch will only thicken during baking, pregelatinised starches can be used tobind the limited water available early on. This creates a host of benefits: suspension ofparticulates (as in muffin mixes); reduction in the stickiness of doughs; improvedhandling and machinability; increased cake volume; improved water binding forincreased moistness; and softer textures. A common problem in bakery products is stalingcaused by retrogradation. Stabilised starches, in particular hydroxypropylated, pregela-tinised starches bind water more effectively thereby providing baked goods withenhanced shelf-life through extension of the perception of freshness. Native maize, waxymaize or tapioca starches are alternatives to wheat flour for gluten-free products forcoeliac sufferers who are intolerant to wheat gluten.

Glazes and icings are used to enhance aesthetics and add value to baked goods. Aglaze is typically a thin wash containing sugar, water and/or milk, whilst an icing may beapplied as a thickened layer and usually contains fats. Pregelatinised starches and,especially dextrins, find application here where the main functions are viscosity control,colouring, softening and texturing.

Batters and breadingsThe concept of a breaded or battered coating is to create additional value for meat,poultry, seafood and vegetables. Flours are a major component in batters and breadings

Starch 57

but a common problem for food manufacturers is fluctuation of the quality of commoditynative flours. Speciality starches are used to solve this and extend the features of thenative base flour. The wide array of substrates for coatings means that the accompanyingwide range of storage and reconstitution methods become key considerations, in additionto textural attributes, in the selection of a starch.

Starches in batters provide viscosity control which, in turn, controls the quantity andthickness of the batter layer; the adhesion efficiency; visual effects (smooth to blistered);texture; storage; and reconstitution stability. Pregelatinised starches are used to controlthe cold viscosity. These can be cross-linked and stabilised to provide shear and freeze-thaw stability for batter slurries which are to be recycled or used in chilled or frozenproducts. The textural attributes can be enhanced by high amylose starches for fried oroven-baked poultry and meat products. High amylose starches need higher temperatures,as in frying, to functionalise them. Stabilisation lowers the gelatinisation temperature andtherefore makes the starches suitable for applications with lower cooking temperatures.Dextrins are used for vegetable coatings where they will enhance colour and, at highdosage rates (~30%), will create blistered effects to transform the surface of a coating.Very high levels of dextrins or incompletely gelatinised, high amylose starches, maycause stickiness. This problem is overcome by reducing the dextrin level or increasing theamylose content to reach the desired crispiness. To achieve a smooth, uniform coating atlower cooking temperatures, a modified high amylose starch is recommended. Sinceamylose has excellent film-forming properties, the incorporation of high amylosestarches in batters can reduce oil pick-up in fried, battered products.

Beverage emulsions and flavour encapsulationLipophilic starches have replaced gum arabic, the traditional emulsion stabiliser, inconcentrated flavour emulsions for soft drinks and in encapsulated ingredients, e.g.,spray-dried flavours and creamers. Indeed, lipophilic starches can be extended beyondgum arabic applications to high-load encapsulation of flavour oils. Generally thesestarches provide improved oxidation resistance and low temperature emulsionstability.

ConfectioneryStarches are found in a wide range of confectionery products, contributing from softthrough to hard gels, and from brittle through to chewy textures. Starch is also active asthe structure builder in coatings and even as the moulding medium to support the shapingof confections. Starches are selected primarily on their ease of cooking in high-sugarenvironments and their ease of handling during production.

The majority of confections are high in sugar, sugar syrups or polyols, with solids inthe region of 68–72%. In these products there will be considerable competition for water,with the starch suffering in that gelatinisation temperature is increased and the productmore difficult to functionalise under 100ºC. There are several methods used inconfectionery processing to functionalise ingredients: the traditional steam-jacketedkettle (low-to-medium temperature, low shear); heat exchangers (medium-to-hightemperature, high shear); or direct steam injection, as in jet cooking (high temperatureand very high shear).

Converting amylose and high amylose starches to different degrees of hydrolysiscreates a suitable range of acid-thinned or oxidised starches for confections. Thesestarches have a range of low hot viscosities and consequently they allow rapid andefficient cooking of starch solutions in the presence of concentrated sugar syrups. A

58 Handbook of hydrocolloids

further useful modification is stabilisation where the reduced starch gelatinisationtemperature allows easier cooking, especially of high amylose starches, so that strongergels can be achieved with increased clarity and extended shelf life for the confections.Converted starches are also used in confections on the exterior of products, as in pan-coating. Dextrins, with their good film-forming properties are used with high-sugarsolutions to create stable, flexible coatings as in jelly beans or shell-coated chocolates.Texture can be further modified by using starches in combination with otherhydrocolloids.

Starches also serve as a process aid rather than an ingredient in confections. Theshaping of confectionery pastes normally occurs in starch moulds where the desired shapeor design has been imprinted into trays of moulding starch. The moulding starch is alsotreated with a small percentage of mineral oil which enables it to hold the mould imprintand to minimise dusting during the manufacture process. The moulding starch has twokey functions, its shape and the absorption of moisture. The moisture content of themoulding starch is critical in obtaining a high-quality confection. Above 9% moisture, thedrying time is extended – this reduces production rates – whilst below 6% moisture, theconfection has a crust of hardened exterior as the rate of moisture loss is too fast.

Dairy productsDairy products have flourished as they are convenient at most daily eating occasions. Thelatest developments are primarily focused on chilled and frozen products, but thereremains a substantial market for ambient and dry mix products. Chilled, frozen andambient dairy products are processed in order to deliver the required shelf-life.Processing factors which influence the selection of starch are heat and shear. Heating isachieved through direct or indirect processing, and shear is predominantly achieved bythe design of the heating equipment or by homogenisation of the product (in this instancebefore or after heating). Direct processing involves heating the product by steam injection(where steam is forced into product through a nozzle) or by steam infusion (where thesteam and product are mixed in a chamber). These systems involve very hightemperatures and high shear but short process times. Indirect processing relies on heatexchangers such as the ‘scraped surface’ or ‘tubular’ varieties. More viscous products arebest handled through scraped surface heat exchangers where, despite their higher shearintensity, the shear profiles are still lower than those experienced in direct processing.Heat penetration is critical to the production of a high-quality, safe and stable dairyproduct. Low initial viscosity during processing contributes to a better heat penetrationwhich, in turn, means increased product throughput with less fouling and downtime,together with improved plant efficiency through shorter process times. Hydroxypropy-lated and cross-linked, waxy maize or tapioca starches offer lower hot viscosities thantheir acetylated counterparts and yet produce up to 50% greater cold viscosity after 24hours storage. Hydroxypropylated starches also provide better freeze-thaw stability andproduce richer, creamier textures. They are more effective thickening and stabilisingagents in low-fat or low-calorie dairy products due to their greater compatibility withmilk protein. Retorted (canned) dairy-based products can bring out the advantages ofusing hydroxypropylated starches. By comparison, acetylated starches may de-acetylate,creating dramatic localised changes in pH which cause the product to curdle orprecipitate.

Achieving the desired texture of desserts is dependent upon an optimal cooking of thestarch. Understanding the impact of shear on the product is necessary to select a cross-linked or thermally treated starch which provides the level of process tolerance necessary

Starch 59

for production. Upstream homogenisation of products at temperatures below thegelatinisation temperature of starch, 60ºC, will not adversely affect starch functionality.Downstream homogenisation is where the starch is subjected to high shear well above itsgelatinisation temperature. In this instance, very highly cross-linked or thermally treatedstarches are recommended in order to deliver an optimum viscosity.

Starch selection will be influenced not only by the process requirements but also bythe product characteristics. Blandly flavoured starches, such as modified tapioca starchesor certain thermally treated starch, are particularly important in dairy products due to thehigh dosage levels required. Starch produces a higher viscosity when cooked in wholemilk (3.5% fat) than in either skimmed milk (0.5% fat) or water. Typically, 1–3% starchis used for pouring consistencies, 3–4% for medium viscosity and 4–6% for thick,spoonable textures. Gelled textures, as in cheese analogues, are achievable with someconverted starches or high amylose starches. Gelled dairy desserts will also use otherhydrocolloids, notably carrageenans, in combination with starch to create the desiredtextures.

Fruit preparationsRegardless of season, there is now a wide range of fruit bases available throughout theyear. For the food manufacturer, this presents a range of conditions, such as pH, to becontended with. This is compounded further by the fact that, even within season, therewill be a further natural variation in pH (3–4.5) and pectin within and between fruit types.To achieve consistent fruit preparations, a thickener or stabiliser is required to even outthese variations. But the thickener and stabiliser must also be able to tolerate theprocessing involved to provide the desired texture, freeze-thaw or bake stability. Fruitpreparations find application in: yoghurts, layered dairy desserts and ice-creams; asfillings for baked goods, e.g. doughnut; and, as toppings for pastries such as cheesecakes.Formulations will include 20–90% fruit, and sugar solids from 7% (no added sugar) to70%. Sugar solids (measured as percentage Brix) above 40% increase the gelatinisationtemperature beyond 100ºC. In these circumstances a jet cooker is preferred over the kettlevariety to functionalise the starch. Alternatively, adding the sugar at the end of theprocess, allows the starch to fully gelatinise. In systems where a ‘one-shot’ addition ofdry ingredients is only possible, then cross-linked and stabilised, pregelatinised starchesor highly stabilised and light-to-moderately cross-linked cook-up starches are suitable asthickeners. The starches must be capable of developing sufficient viscosity to suspend thefruit, yet be pumpable and mixable with, for example, a yoghurt base. Fruit pieces areparticularly delicate during cooking. Therefore, reducing the process time through anincreased heat penetration will protect fruit integrity. This is achieved by selecting across-linked and stabilised starch which will thicken enough to suspend the fruit pieces;provide a low viscosity for effective heat penetration; provide acid stability; and excellentchill and freeze-thaw stability. Flavour release in fruit preparations is particularlyenhanced with process tolerant, thermally treated starches.

Gravies, soups and saucesThis sector includes an enormous range of food products that span the spectrum of shelfstability from immediate consumption to very long ambient shelf life. Starch selection inthis instance depends upon the production process which, in turn, is usually influenced bythe pH of the product. Acidic products (especially pH� 4.5) will require a higher degreeof cross-linking than neutral products, provided the heat process is the same. Normallyacidic products will have a shorter process time or temperature to reach their target

60 Handbook of hydrocolloids

sterilisation factor or F0 value. Cross-linking or, more recently, thermal treatmentintroduces process tolerance by strengthening the starch granule. In ‘wet’ sauces,processing factors to consider are preheat temperature and hold time; pasteurisation/sterilisation temperature and time; method and rate of cooling; mechanism of pumping;and, indeed any other forms of shear used during cooking or in the introduction ofdifficult-to-disperse powders. Reference has already been made to the contribution ofthese factors during food processing and their effect on starch gelatinisation in Section3.1.3. The shelf-life requirements of this group of products are achieved with stabilisedstarches which imply freeze-thaw stability. This feature is particularly important in thedevelopment of chilled, frozen and ambient-stable products which are recommended forrefrigerated storage after opening. Hydroxypropylated starches are the best to use here,especially since their excellent compatibility with milk or milk proteins leads to anenhanced texture and mouthfeel of cream-based sauces.

Besides processing and shelf-life considerations, there are two other important criteria,fill-viscosity and heat penetration. The function of starch as a fill-viscosity aid is toprovide sufficient high viscosity during the initial stages of cooking, typically duringpreheat, that particulates are homogeneously suspended and then evenly distributed intothe container. The high initial viscosity is also designed to prevent splashing and reduceunhygienic conditions around the production filling line. Fill-viscosity starches arecommonly native starches, primarily waxy maize, which thicken easily and quickly toachieve their peak viscosity. They then breakdown during the sterilisation stage with littleor no residual viscosity. The final desired viscosity is achieved by cross-linked and,usually, stabilised starches. This combined, modified starch facilitates heat penetrationwhich supports a higher quality of product with shorter, optimal heat processing.Increasing the level of cross-linking delays viscosity development which is desirable forbetter heat penetration. However, there will be an optimum economic level for processtolerance. In general, viscosity-building starches are used at dosages of 2–6%, whilst fill-viscosity starches may be used at 1–3%.

Dry mixes are often required by consumers, food caterers and manufacturers forreasons of convenience, ease of handling and storage stability. Starch selection willdepend on the specific requirements of the product and its use. In the case of foodmanufacturers, time, temperature, shear, pH and storage conditions apply as for wetsauces. Dry mixes for the consumer will demand dispersibility in hot or cold liquid as thecritical factor. Whisk-and-serve applications require easy dispersion with fast hydrationof the starch to achieve the target viscosity within a convenient time. Pregelatinised oreasy-to-cook, ‘cook-up’ starches are recommended. However, pregelatinised starchesmay ‘lump’ as they try to wet-out too quickly, particularly in hot liquids. Lumpingrestricts viscosity development thereby compounding the problem. Agglomerated,pregelatinised starches can disperse more easily than the traditional drum-dried starchesas they are more granulated. These easily dispersed starches are particularly desirable inlow-sugar/salt formulations as there is less granulated material to blend. Alternatively, ahighly stabilised, hydroxypropylated starch with a lower gelatinisation temperature willovercome the lumping to hydrate quickly and produce a rich, thick and creamy product.In some applications, e.g., microwave reconstitution or in cold pies requiring additionalbake stability, a combination of pregelatinised and cook-up starch is used. Thepregelatinised starch builds viscosity immediately to suspend the other ingredients,including the cook-up starch. This is essential in microwaveable dry mixes since stirringduring cooking is impossible. The cook-up starch will contribute to the final viscosity andstability.

Starch 61

Potato starch has the highest swelling capacity combined with a low gelatinisationtemperature. Native or lightly modified potato starches find application in economy-styledry mixes. When products require greater flexibility, e.g., three-way cooking on the hob,casserole or microwave, followed by chilled or frozen storage, as in catering, then cross-linked or stabilised waxy maize or tapioca starches are preferred. Tapioca starches aremore suited to delicately flavoured sauces as in fish dishes.

Often dry mixes will require low-moisture starches (2–8%) depending upon the otheringredients and the required water activity for long-term shelf stability. Native starchessuch as maize or wheat, may be used to increase opacity and change mouthfeel or asbulking agents to aid dispersion. For all varieties of gravy, soup and sauce, whentraditional native starches are used, the recommendation for superior product quality isone part native to two parts modified starch. This ratio ensures the beneficial effects ofthe modified starch compensates for the limitations of the traditional native starches.Thermally treated starch in total replacement of the modified starch, can retain theattributes of the modified whilst dispensing with the ‘modified’ label declaration (Section3.7).

Mayonnaise and salad dressingsThe main function of starches in this category is to thicken and stabilise. Cross-linked andstabilised starches are commonly found here. More recently, thermally treated starchesare finding increased popularity. Two further starch types find specialised applicationhere: lipophilic starches to stabilise emulsions, particularly in egg-free dressings and fatmimetics in low-calorie or fat-free formulations. The processing conditions are veryrigorous for mayonnaise and salad dressings. Starches need to be able to tolerate theacidity, where pH can be as low as pH 2.8; a heat process, which generally involves ‘hot-fill’ as a key sterilisation factor; shear for the emulsification, which will depend on the fatcontent; and the type of emulsification unit used. Products which are hot-filled anddirectly stacked onto pallets have a tendency to suffer from ‘stack-burn’. This occurs asthe core temperature of the stack of product on the pallet remains relatively high forprolonged periods and product viscosity begins to break down as the starch is degraded.A highly cross-linked starch will provide additional heat and acid tolerance to overcomestack-burn. Starches are also used to ensure product quality over a long shelf life, whichcould be up to 12 months. While moderate-to-high cross-linked starches are essential forthe heat, acid and shear tolerance stabilised, and in particular, hydroxypropylatedstarches, bring added benefits by introducing freeze-thaw stability. This is a benefit forcatering and consumer products which require refrigeration after opening. Low-calorie orlow-fat dressings also benefit from hydroxypropylated starches as the main thickenerssince they develop rich, creamy textures. Mouthfeel can be further enhanced using starch-based fat mimetics. These are available as replacements for vegetable oils. They bring aunique rheology suitable for pourable dressings, or as thermoreversible gels they aresuited to spoonable dressings. A gelled structure can be further enhanced using amylose-containing starches but typically, they are used in combination with cross-linked andstabilised waxy maize starches since these contribute to the process and storage stability.

Salad dressings and mayonnaise will vary in oil content. Here the processingequipment is designed to create the emulsion by dispersing the oil as fine droplets with adiameter of less than 1 �m throughout the aqueous phase. Lecithin, the natural emulsifierin egg yolk and/or any added emulsifier, will stabilise the emulsion by reducing thesurface tension of the oil droplets. This makes large droplets unstable and prevents‘creaming’ (where oil migrates to the surface). Strictly, starches act as emulsion

62 Handbook of hydrocolloids

stabilisers rather than as true emulsifiers. Viscosity-building starches achieve this bystructuring the aqueous phase and restricting the movement of the oil droplets. Lipophilicstarches are introduced under agitation for optimal dispersion so that they interact withboth water and oil to produce a stabilising effect at the oil–water interface. Anotheradvantage of using lipophilic starches is that, as modified starches, they may already becovered on the label declaration if a modified starch has been used as the viscositybuilder – this simplifies the list of ingredients on the label.

Meat productsIn processed meats, starches are used as water binders to increase yields, reduce cookinglosses, improve texture, sliceability and succulence, and extend shelf life. Reformed orcomminuted meat products require cooking to a minimum internal temperature of 72–75ºC for product safety. In order to function at such low temperatures with little or noshear or acid, potato starches, which swell easily at low gelatinisation temperatures, arerecommended. Waxy maize and tapioca starches, with a high degree of stabilisation toreduce their gelatinisation temperatures, have the additional benefit of excellent waterbinding during chilled or frozen storage. Lipophilic starches, too, find application incomminuted meats and pates as stabilisers of the emulsion and in the strengthening of thefat- and water-binding properties of the meat protein. Coarse grind, pregelatinisedstarches bind water quickly and increase the viscosity which assists the moulding ofreformed or comminuted meats whilst enhancing a more open, coarse texture.

Savoury snacksSavoury snacks are low-moisture products (15–30% water used in the dough, less than2% moisture in the final product) and the choice of starch is critical since manipulatingthe formulation and process equipment will affect gelatinisation and, consequently, willinfluence the final product characteristics.

‘Expansion’ is a very important process for snacks. Several processes are available forexpanding snacks: extrusion (puffed corn snacks); baking (low fat, sheeted bakedsnacks); frying (traditional crisps) and microwave (popcorn). Pregelatinised starches arerecommended in snacks that are manufactured under low-to-medium shear and mediumtemperatures (75–120ºC). Cross-linked or thermally treated ‘cook-up’ starches arerequired to withstand the rigours of higher shear, pressure and temperature (160–180ºC)found in the extrusion process for directly expanded products.

In snacks, texture and appearance are of high priority. The starch polymers of amyloseand amylopectin exhibit a very obvious effect on these. In general, the highly branchedamylopectin polymer increases dough viscosity and expansion, and a waxy maize starchtherefore is the preferred base starch in the production of light, crispy expanded products.Amylose, in contrast, is responsible for strengthening the dough which improves forming,cutting and a harder, more crunchy final texture. In this instance, the linear chains canalign very closely. Amylose also exhibits excellent film-forming properties which can beadvantageous in the reduction of oil pick-up in fried snacks.

3.6.3 Viscosity-troubleshootingWhen a starch containing product fails to meet its viscosity specification, it is a commonand expensive mistake to add further starch to correct the viscosity. This leads, at worst,to a scenario in which the initial product is overcooked and the additional starch isundercooked. The result will be an unstable product which will taste ‘starchy’. Rather

Starch 63

than compensating for viscosity it is better to investigate the root cause. Ensuring that thestarch is optimally cooked is a good place to start. Table 3.4 illustrates the diagnoses ofthe various stages of a starch cook. For example, in the case of undercooking, eitherfurther cooking to achieve a ‘good cook’ or the adoption of a less inhibited starch isrequired. The converse will apply for overcooking.

Sliced and diced vegetables in a product can present viscosity problems due to naturalenzyme activity from the high surface area of the vegetable. �-Amylases are the source ofthe problem. The viscosity loss from enzyme hydrolysis of the starch can be so dramaticas to leave either ‘ghost granules’ or no granules at all under microscopic examination.The most problematic sources of �-amylase are cabbage, onions and kidney beans.Unheat-treated wheatflour is an important and often over-looked cereal source of �-amylase. Heat treatment provides a straightforward means of inactivating the enzyme.

3.7 Regulatory status: European label declarations

Food ingredient suppliers are obliged to label their products with information whichassists the food manufacturer when declaring the ingredient list of the final food product.Table 3.5 outlines the current status of native and modified starches; it highlights thosethat are classified as ingredients and those as additives. The majority of chemicallymodified food starches are approved as permitted food additives within the EuropeanUnion and, as such, have been assigned E-number classifications. Food additives includeartificial sweeteners, colours, preservatives, flavour enhancers, emulsifiers, stabilisers,thickeners and most modified starches. Modified starches which are classed as additivesare defined as ‘substances obtained by one or more chemical treatments of edible starchwhich may have undergone a physical or enzymatic treatment and may be acid- or alkali-thinned, or bleached’. Thermally treated starches, native starches, dextrins, starchmodified by acid or alkali treatment, bleached starch and starch treated by enzymes areclassed as food ingredients and have no E-number classification. Starch food additivesare regulated under the terms of the EC Miscellaneous Additives Directive, 95/2/EC (andamendment 98/72/EC), on food additives other than colours and sweeteners.

As a broad class of additive, modified starches (E1404–E1451) have ‘horizontal’approval throughout the European Union for food use. They may be added to foodstuffsfollowing the quantum satis principle (no maximum level indicated) unless otherwise

Table 3.4 Viscosity troubleshooting – evaluation of starch cook*

Clarity Cloudy Cloudy Clear Clear Clear

Viscosity/ Starch will Thin Heavy-bodied, Reduced, Reduced,texture sediment unless short texture cohesive cohesive

agitated

Taste Starchy Starchy Neutral Neutral Neutral

Stability Poor Poor Good Poor Poor

* Light microscope @ 200 magnitude

64 Handbook of hydrocolloids

specified or their use is restricted in food for which there is existing ‘vertical’, or specific,legislation, e.g., as in chocolate, fruit juices, jams, jellies, etc. For labelling purposes,manufacturers of food products sold to the consumer need only list the generic term‘modified starch’ within the ingredient list. This must be accompanied by an indication ofits specific vegetable origin, when the modified starch may contain gluten.

3.8 BibliographyCROGHAN, M. (1998) ‘The search for a high fibre snack’ Kennedy’s Ready Meals & Snacks, vol. 1, no. 1, 58–60.CROGHAN, M. and MASON, W. (1998) ‘100 years of starch innovation’ Food Science & Technology Today, March,

17–24.EMSLEY, J. (1994) The Consumer’s Good Chemical Guide, London, W. H. Freeman & Co. Ltd.IMESON, A. (ed.) (1997) Thickening and Gelling Agents for Food, London, Chapman & Hall.LIGHT, J. M. (1990) ‘Modified food starches: why, what, where and how?’ Cereal Food World, 35 (11), 1081–92.MACDOUGALL, A. (1999) ‘Native vs Modified starch’, Food Manufacture, February, 16–18.THOMAS, D. J. and ATWELL, W. A. (1997) Starches, Eagan Press, USA.WHISTLER, R. L., BEMILLER, J. N. and PASCHALL, E. F. (eds) (1984) Starch: Chemistry and Technology, 2nd edn,

Academic Press, New York.WURZBURG, O. B. (1986) Modified Starches: Properties and Uses, CRC Press, Florida.Food Starch Technology, National Starch and Chemical Brochure.‘Stability and flavour in fruit preparations’, (1998) Milk Industry International, Technical and Research

Supplement, MII, 11/98, 4–5.

Table 3.5 List of food starches permitted under European Food Law

Starch E Number Classified as Food product labelclassification

Physically modified – Ingredient StarchEnzymatically modified – Ingredient StarchDextrinised – Ingredient Dextrin or modified starchAcid treated – Ingredient Modified starchAlkali treated – Ingredient Modified starchBleached – Ingredient Modified starchOxidised starch E1404 Additive Modified starchMonostarch phosphate E1410 Additive Modified starchDistarch phosphate E1412 Additive Modified starchPhosphated distarch phosphate E1413 Additive Modified starchAcetylated distarch phosphate E1414 Additive Modified starchStarch acetate E1420 Additive Modified starchAcetylated distarch adipate E1422 Additive Modified starchHydroxypropyl starch E1440 Additive Modified starchHydroxypropyl distarch phosphate E1442 Additive Modified starchStarch sodium octenyl succinate E1450 Additive Modified starchAcetylated oxidised starch E1451 Additive Modified starch

Starch 65