371 24.2 Mechanical, Thermal and Hydrothermal Modification of Flour

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24.2 Mechanical, Thermal and Hydrothermal Modificationof Flour

U. Müller and V. Schneeweiß

24.2.1 Introduction

Numerous processes are available to themilling industry for achieving specific effectson the functional properties of flours. Theyinclude conventional grinding of defined lotsof grain, "selective" grinding (multiple-passagemilling), fine grinding followed by fractiona-tion and thermal or hydrothermal methods.

Special flours produced by the mills solelyby physical means do not require modifiedingredient statements, and this is one of thereasons why customers often ask for them.

Moreover, special flours of this kind permitnew combinations with additives for optimizingthe process or products; this further widensthe scope for making up customized rawmaterials based on cereals.

24.2.2 Milling Methods

Conventional GrindingFlours are usually made from analyticallydefined lots of bread cereal that conform tothe regulations of the country concerned. It isprimarily the formulation for grinding the grainthat results in the analytical values and func-tional properties of the flours; in the case ofwheat, for example, these are the protein andwet gluten content, gluten quality, sedimenta-tion value, enzymatic activity and waterabsorption.

By selecting individual multiple-passageflours and combining them according to a"flour recipe" it is also possible to exert a certaininfluence on further properties in addition tothe above parameters, for example mineralcontent, colour, brightness and particle-sizedistribution. However, when specific customerrequirements have to be met, these possibilitiesare limited by the diagram of the milling system

and/or the availability of a particular grainquality. For that reason, both conventionaland multiple-passage flours are more andmore often subjected to processes that have amuch greater effect on the constituents andfunctional properties.

Fine Grinding and Air ClassificationOf course milling is not simply a question ofgrinding on cylinder mills. Millers have longused combinations of different grindingmethods to produce specific flours. But it hasonly recently become possible to achievefurther really significant changes in respect ofconstituents, for example with special impactmills and subsequent fractionation.

If conventional grinding (possibly of selectedwheat varieties) is followed by fine grindingusing pin or impact mills and air classification,it is possible to influence criteria such asparticle size distribution, water content,colour, brightness, water absorption and therheological properties of doughs over a verywide range.

Fig. 225 shows the principle of an impact millwith an integrated vane ring classifier toensure a defined upper limit to particle size.Flour re-ground in this manner has a differentparticle size distribution, as Fig. 226 shows.

24.2 Mechanical, Thermal and Hydrothermal Modification of Flour

Fig. 225: Alpine Zirkoplex classifier mill, ZPS (Courtesy of Hosokawa Alpine AG & Co. OHG, Augsburg, Germany)

24-2-Mechanica 20.10.2006 14:05 Uhr Seite 371

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In a second unit, the fine classifier, the groundproduct is divided into two fractions withdifferent particle size distribution and specificgravity by a further vane ring classifier (Fig.227). Separation can also be performed bymeans of cyclones instead of the vane ringclassifier.

Fig. 228 shows the particle size distribution ofthe two fractions. However, the real achieve-ment of this grinding and separation processis not the particle size distribution but the"shift" of constituents in the fractions resultingfrom the morphological condition of the wheatendosperm.

Fig. 229 shows the mix of particles consistingof small-grain starch and protein in the finefraction of the flour. Fig. 230 shows cleanlyseparated large-grain starch practically withoutadherent protein. Intensive fine grinding removes most of theproteins adhering to the starch grains of thewheat endosperm. Some of the "wedgeprotein" is released, and some of the "adherentprotein" is detached from the starch grains.The fine classifier separates this protein offtogether with small wheat starch grains andmakes it accessible for further special applica-tions. The "coarse" fraction of the originalflour is removed via a lock at the bottom end


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conventionally groundimpacted

Fig. 226: Particle size distribution of a wheat flour Type 550 before fine grinding and after (impacted)

Fig. 227: Alpine Turboplex Classifier, ATP(Courtesy of Hosokawa Alpine AG & Co. OHG, Augsburg, Germany)


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method undergo a number of changes inrespect of their constituents, which are relatedto the significant differences in protein content.


of the classifier. It consists mainly of the largeand medium-sized wheat starch grains. Tab. 137 shows that flours produced by this

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2.5 7.5 12.5 20 27.5 36.5 52

Particle size (µm)

finecoarse

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Fig. 228: Particle size distribution in the fine and coarse fractions

Fig. 229: Fine fraction (1000x) Fig. 230: Coarse fraction (1000x)

Tab. 137: Changes in respect of constituents resulting from fine grinding and air classification

Property Fine fraction Original flour Coarse fraction

Moisture % 10.8 14.6 9.5

Protein (d.b.) % 7.0 11.8 24.2

Wet gluten % 20.3 27.7 not extractable

Sedimentation mL diffuse 39 diffuse

Falling Number s 342 309 190

Water Absorption Index g/g 2.1 2.2 2.8


Applications of Finely Ground Wheat FloursThe changes in the constituents of finelyground and separated flours result in signifi-cant differences in respect of the functionalproperties of the flours, and these in turnopen up different applications as Tab. 138shows.

24.2.3 Thermal and Hydrothermal Modification of Flours

Millers have long been familiar with thermaland hydrothermal processes for modifyingcereals and flours. In particular the drying ofgrain in years with a wet harvest is still one ofthe important and hygienically relevantquality assurance measures carried out atmills. For decades, flours with a water contentreduced by pneumatic conveyance have beenpart of the standard product range of cerealmills. For some years mills have also been making aclose study of various different thermal andhydrothermal methods of modifying flours. In fact there are a number of very differentcooking and/or drying methods available tothe milling industry. They include fluidized-bed drying, drum drying, high-temperatureshort-time extrusion cooking, the turbothin-layer technique, puffing, micronizing,combined steaming and drying methods andvarious roasting techniques.

Drum-DryingIn this method a flour-and-water suspension isdried on chromium-plated rollers heated bysteam. The result is a thin, dry film of productwith a high degree of gelatinization of the

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starch component. The film of product isground and classed in order to achieve a particlesize distribution resembling that of flour.Further characteristics of drum-dried productsbesides a high degree of gelatinized starchand the resulting high viscosity when cold area very good microbiological status and dena-tured protein.

HTST ExtrusionAlthough it works with a comparatively lowwater content in the raw materials, high-temperature short-time (HTST) extrusioncooking may be regarded as a hydrothermalmethod of modifying flours. Changes to theconstituents of the extrudates as compared tothe initial raw materials result from the simul-taneous effects of pressure, temperature,shear forces and water content. This basicprinciple applies both to single and doublescrew extruders, although processing with thelatter permits a wider range of modificationsin respect of constituents and functionalities.As a rule, products made by HTST extrusionare subsequently dried, then ground andclassed. Like drum-dried products, groundextrudates have a high degree of starch gela-tinization and thus high viscosity in the coldstate. On the other hand, a large proportion ofsoluble components is also typical of groundextrudates. A further feature of such productsis a very good microbiological status resultingfrom the process.

Flour Heat Treatment (FHT)In this method, a combination of steaming anddrying, flours are first treated with a definedamount of steam or water. The flour thus mois-tened/heated is agitated by means of heatedscrews and held at a defined temperature.At the end of the holding time the flour iscooled and classed. Agglomerated particlesare ground and added to the main product inspecific amounts. From the point of view ofconstituents/functionalities the method hasthe advantage of permitting different levels ofstarch gelatinization and thus a range of coldand hot viscosities. The procedure is also capableof improving the microbiological status offlours. Roasting is not possible, however.

Tab. 138: Applications of finely ground and separated wheat flours

Low-protein flour High-protein flour(coarse fraction) (fine fraction)

Wafers Heavy yeast doughs

Wafer-thin biscuits Rusks

Madeira-cake mixtures Sandwich / toast slices

Sponge mixtures Processing methods such as:- Retarded fermentation- Freezing- Frosting


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Turbo Thin-Layer TechniqueModification of flour by the turbo thin-layertechnique (TTLT) can be carried out alterna-tively by one or two units of the same type(cooker/dryer). Both consist of double-walledcylinders arranged horizontally, with fast-running paddle shafts. The inner surfaces ofthe cylinders are heated by thermal oil orsteam. The position of the paddles and thespeed of the shafts can be varied over a widerange. The rotating paddles take up the flourand convey it to the heated inner surfaces ofthe cylinders. Immediately after this the productis taken up by paddles again; a moving film ofproduct is formed and constantly divided andre-heated. In the broadest sense this principlemay be termed a high-temperature short-termprocess with the corresponding benefits inrespect of the functionality of the con-stituents, for example gluten. Steam can alsobe introduced into both units. Moreover, thedryer can work with hot air and hot water canbe fed into the cooker. The hollow shaft of thecooker also enables various liquids to beadded.

Modification of the Functional Properties of FloursWith regard to the objectives to be achieved inthe product there are five main fields in whichthermal or hydrothermal modification isappropriate:• Drying of flour• Reduction / inactivation of the enzymes

in the flour• Positive effect on the microbiological

status of the flour • Influence on the rheological properties

of the dough • Effects on hot or cold viscosity• Effects on colour or taste.The most important results of the changesthat take place in the starches and proteins ofthe cereals are the rheological changes tothermally modified flours and the possibilityof achieving very different hot and cold viscosi-ties. In the case of wheat flour, for example,thermal modification can stabilize the proteinsof the gluten that are necessary for makingbaked goods. But if the function of the gluten

is not needed, or if it is even undesirable, itcan also be inactivated by the use of heat.(Tab. 139).Using thermal and hydrothermal methods, thestarches naturally present in the grain can bemade to differ widely in their degree of gela-tinization and thus in their functional properties.But thermal modification in conjunction withmechanical stress on the biopolymers may alsoincrease the proportion of soluble substancesin the end products, as Tab. 140 shows.But for special purposes the possibilities ofspecifically influencing the enzymatic activityof flours and their microbiological status arealso very interesting. Since enzymes are in the nature of proteins, itis fairly easy to inhibit their activity by thermaltreatment, although this depends on theirspecific actions. Amylases, for example, canbe inactivated quite easily, but to inactivateperoxidases completely is much more difficult.Tab. 141 is a comparison of various differentthermal and hydrothermal processes from thepoint of view of modification of enzymaticactivity, expressed indirectly by the parametersfalling number, degree of gelatinization andgelatinization temperature. In particular the more recent developments infoods with a long shelf life and high-conven-ience products require the use of flours with a"non-critical" microbiological status. Allthermal and hydrothermal processes bringabout positive changes in respect of themicrobiological quality of flours; a few caneven produce flours in "dairy quality" by a

Tab. 139: Thermal processes and theirinfluence on the functionality of the gluten in wheat flour

Modification Gluten vitality

Native flour +

Flour merely dried +

TTLT - less intensive +

TTLT - intensive -

FHT - combination of steaming and drying -

Drum-drying -

HTST extrusion -


Under certain conditions thermal processescan also modify the taste and colour of flours.This applies especially to methods with alonger dwelling time and the direct effect ofcontact heat. Under these conditions it is quitepossible to produce roasted flours for use asrecipe ingredients to add flavour or colour to

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kind of pasteurizing effect, or even sterilizethe flour (Tab. 142). In both cases use of theflour as a raw material for baking is restricted,but the baking capability that remains never-theless makes it possible to produce a smallselection of baked goods.


Tab. 140: Thermal processes and their effect on the properties of the starch (wheat)

Modification WAI WSI Degree of gelatinization Damaged starchg/g % % %

Native flour 2.3 6.0 Approx. 15 4.7

Flour merely dried 2.2 6.6 Approx. 20 6.2

TTLT - less intensive 2.3 6.2 Approx. 20 6.6

TTLT - intensive 2.7 4.7 Approx. 50 13.1

FHT - combination of steaming and drying 2.8 5.0 Approx. 50 19.0

Drum-drying 9.6 8.8 100 43.4

HTST extrusion 9.0 16.8 100 54.5

Tab. 141: Thermal processes and their effect on the enzymatic activity of wheat flour

Modification Falling Number Peak viscositys AU

Native flour 265 280

Flour merely dried 275 290

TTLT - less intensive 290 330

TTLT - intensive 430 590

FHT - combination of steaming and drying 410 480

HTST extrusion 70 300

Drum-drying 85 430

Tab. 142: Thermal processes and their effecton the microbiological status of wheat flour

Modification Total bacterial count

CFU/g

Native flour > 10,000

Flour merely dried 3,000 - 8,000

TTLT - less intensive < 100

TTLT - intensive < 10

FHT - combination of steaming and drying < 100

Drum-drying < 100

HTST extrusion < 100

Tab. 143: Thermal processes and their effecton the colour and taste of flours

Modification Colour Taste

Native flour 0 0

Flour merely dried 0 0+

TTLT - less intensive + +

TTLT - intensive ++ +++

FHT - combination of steaming and drying + +

Drum-drying + +

HTST extrusion 0+ ++


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the baked products. Tab. 143 gives anoverview of the different processes in respectof colour and taste.In practical terms the most relevant possibilityis that of modifying the rheological propertiesof flours by thermal or hydrothermal methods.


The following comparison of two Farinograms,one of a native wheat flour and the other ofthe same flour after treatment by the turbothin-layer technique, serves to illustratethe considerable possibilities (Fig. 231 andFig. 232).

Fig. 231: Farinogram of a native wheat flour

Fig. 232: Farinogram of the same flour after treatment with the turbo thin-layer technique (less intensive)


If flour-and-water suspensions are used it isnot so much the rheological properties ofthe dough that are of interest as the

A glance at the Extensograms of these floursreveals the following information (Fig. 233 andFig. 234):

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Fig. 233: Extensogram of the original flour (native wheat flour)

Fig. 234: Extensogram with the turbo thin-layer technique (less intensive)


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dynamics and intensity of the viscositiesthat develop in both the cold and the hotstate. The various thermal and hydrothermalprocesses differ considerably in their ability


Fig. 235: Viscogram of a native wheat flour (measuring range 250 g.cm)

Fig. 236: Viscogram of wheat flour with the turbo thin-layer technique (intensive; measuring range 250 g.cm)

to achieve very high cold or hot viscosities.In Fig. 235 - Fig. 237 below the hot extrusionmethod and the turbo thin-layer techniqueare compared as an illustration, taking theviscograms of a wheat flour as an example.


Bakery ProductsThermally and hydrothermally modified flourscan make a considerable contribution toprolonging the shelf life of bread and smallbaked products. They enable refrigerated orliquid doughs to be kept longer and optimizethe texture of the baked goods. The use ofthermally modified flours also increases thedough yield and improves the processingcharacteristics of doughs. Moreover, "thermalflours" usually make it unnecessary to chlorinatethe flour. Taking sandwich slices as an example,the two diagrams below show the effects of awheat flour treated by the (intensive) turbothin-layer technique in respect of shelf life(Fig. 238 and Fig. 239).Tab. 144 is a comparison of the relevant watercontents and aw values.

Binding SystemsAs binding systems or as components ofcomplex binding systems, thermally andhydrothermally modified flours offer variousdifferent benefits, the most important criterionbeing specifically adjustable hot or coldviscosities. Because of their good microbio-logical status and low enzymatic activity the

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Thermally and hydrothermally modified flourshave an extremely wide range of applications.To permit an overview we have classified theseapplications in five groups: a) use as a constituent of the recipe in bakery

products; b) use in binding systems; c) benefits in the production of snacksd) use as coatings; e) use as a carrier.


Fig. 237: Viscogram of wheat flour, extruded (measuring range 250 g.cm)

Tab. 144: Water contents and aw values of sandwich slices with and without TTLT flour

Length of storage TTLT flour Moisture aw valued % %

1 0 35.38 0.947

15 37.08 0.950

2 0 34.64 0.942

15 36.71 0.943

5 0 33.96 0.941

15 35.65 0.942


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Hardness Elasticity Adhesion ofthe crumb

Chewiness

Day 1 Day 2 Day 5

Fig. 238: Texture profile of sandwich slices made from a standard recipe

0

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Hardness Elasticity Adhesion ofthe crumb

Chewiness

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Fig. 239: Texture profile of sandwich slices made with 15 % flour treated by the turbo thin-layer technique (intensive)


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24.2.5 Outlook for the Future

Changes in the market, especially in connectionwith the development of new products by thefood industry, will increase the demand forflour products with specific functional properties.Changing eating habits and expectations onthe part of consumers, stricter requirementsin respect of food safety, a further increase inthe convenience of foods and a demand forspecific raw materials for products that aredifficult to replace are factors that willcontribute to this trend. In the milling industrythis development will result in a widening ofthe product range.

In cooperation with other manufacturers ofraw materials for the food industry, too, thethermally and hydrothermally modified floursoffer an interesting field of activity for productdevelopers in the milling industry.

In the next few years the product developersof the milling industry will have to deal withthe following topics in order to create furtherspecial applications for flour:

• Improve the visual transparency of flour-and-water suspensions and minimize signsof syneresis;

• Improve the freezing and thawing stabilityof the products; increase the stability offlour-and-water suspensions towards acidsand shear forces;

• Optimize emulsifying properties and makeincreased use of the nutritional benefits ofspecial flours.

In cooperation with other manufacturersof raw materials for the food industry, too,thermally and hydrothermally modified floursare an interesting field of work for the productdevelopment departments of the mills.


products help to prolong the shelf life of thefoods concerned. They can also help to optimizesuch quality criteria as colour, sheen andprevention of skin formation and enhance thehaptic properties. As a substitute for conven-tional binding systems they may also reducethe cost of the formulation. No speciallabelling is necessary.

SnacksIn the production of snacks, the benefits ofthermally and hydrothermally modified floursfor the process and the products lie chiefly intheir specifically adjustable degree of gela-tinization. They help to optimize the shapingof snack pellets and give a more even surfaceto heat-extruded 3-D products. Pre-gelatinizedraw materials also increase the throughput ofthe extruders. Moreover, thermally modifiedflours have advantages over other vegetableproducts (e.g. potato granulate) in respect ofcost.

CoatingsIn this sector we have to distinguish betweendry and liquid applications. The liquid applica-tions include all uses in the field of coatingbatters; the advantages lie in the goodadherence of the products and their excellentmicrobiological and enzymatic status. Dryapplications mainly include uses in theproduction of sweets, in particular productsfor panning and powdering.

CarriersOne field in which the functional properties offlours are of little or no importance is that ofcarriers. In the production of flavourings,spices or enzyme preparations, for example,there is a need for flours as carriers or fillers thatcontribute to the safety of foods and protectionof the consumer but do not otherwise affectthe end products through a "functionality" oftheir own. Enzyme-inactivated and microbio-logically excellent products have interestingmarket opportunities in this field. Products ofthis kind do not require special labelling andoften reduce the cost of the formulation ascompared to carriers and fillers based onstarch.


Documents

371 24.2 Mechanical, Thermal and Hydrothermal Modification of Flour