Flour Treatment

A. Flour Treatment and the Improvement of Flour

There is a huge demand for flour improvers these days, in the baking trade and in the food industry. 

In the past, fifty or sixty years ago, millers were not faced with this topic – except, perhaps, in the context of some very special sideline tasks such as the first attempts to vitaminize light-coloured flours (now generally known as "flour fortification"). 

After the hardship and privation of the war and the years that followed, the world's populations were satisfied with what grew on the soil, what farmers and cooperatives delivered to the mills and what could be shipped across the oceans in the context of world trade. 

And if the worst came to the worst one delved back into the past. Holger Jörgensen discovered it in 1935; P.R.A. Maltha confirmed it fifteen years later: ascorbic acid offers a reliable way of closing many a gap in quality (Jörgensen, 1935; Maltha, 1950). 

This, along with the intelligent use of malt flour, was the only possibility that existed for some long time.

But the mid 1960 s saw the start of a new development. The useful properties of certain mono-diglycerides, and especially lecithin, as an aid to baking had been discovered, and it was now possible to use compounds for virtually all the stages of the improvement process. 

At the same time an extremely practical new theory had become known. Flour improvement is not a question of achieving particularly "good" rheological or analytical values. The secret is to adjust certain empirical data precisely. 

The rheology must "be just right". And of course the results of baking must correspond to the parameters set: it is this that determines the specific nature and intensity of flour treatment.

Ultimately it does not matter what technical means are used to "train" a commercial flour for the tasks it is to perform; but there can be no doubt that in practice the specific use of certain additives and ingredients plays the leading role throughout the world. 

However, the demands made have changed in a characteristic manner in recent years. Ten years ago it was an attractive objective to construct an "all-round" flour from which biscuits and also bread rolls and toast slices could be made in excellent quality. 

These days – especially in the food industry – priority tends to be given to top qualities for limited, special purposes. There has always been, and there remains, a great need for research.

B. Flour Treatment is an Essential Part of the Industrial Production of Flour

Flour Treatment is an Essential Part of the Industrial Production of Flour. Wheat is of increasing importance for feeding the world's population, whose growing number and changing food consumption habits require more and more wheat flour. 

At the same time the processing of wheat flour is changing; although small craft bakeries still predominate in many parts of the world, industrial bakeries are taking an increasing share of the market and thus of the raw material, flour. 

In these bread factories it is not uncommon to find engineers at the production line rather than bakers. 

The direct interaction of the raw material or dough with the person responsible has been reduced to a minimum, and this necessitates a minimum of fluctuation in the properties of the raw materials since the machines do not (yet) have a "feeling" for the dough. 

Consequently, the flour producer, the mill, is considered responsible for uniform flour quality. This goal of standardization is achieved by the art of milling and with flour improvers. At bakeries, specialization of the flour is carried out by means of bread improvers. 

The ingredients used on both levels are often the same. The main difference lies in the dosage of the improver preparation: typically 10 – 100 ppm at the mill, 1,000 - 100,000 ppm at the bakery (0.1 - 10%). 

The continuous production of large amounts of flour at the mill requires additives with good flow properties as well as low dosages. 

This means that additives such as emulsifiers or fat powders that tend to form lumps are not suitable, and salt and sugar are not added at the mill either because of their high dosage. 

Flour mills generally apply single ingredients separately – only occasionally in premixes – whereas bakeries apply bread improvers composed of several additives. This chapter mentions the most common flour improvers. They may also be part of bread improvers.

C. Oxidation and Flour Maturation

The present necessity for oxidative treatment might be regarded as a disadvantage of the fast and gentle processing of grain into flour. Natural ageing of the flour by exposure to the atmosphere alone is no longer possible, so maturation has to be speeded up with oxidative preparations. 

Oxidation primarily affects sulphur containing amino acids that are constituents of the gluten. The oxidation of two adjacent hydrogen sulphide (thiol) groups results in the formation of a disulphide bridge between different sections of the long gluten molecule or between different gluten molecules. 

This causes a hardening of the protein.

1. Ascorbic Acid

By far the most important substance for this purpose is ascorbic acid. 

Using a complex biochemical method starting with starch as the raw material it is produced in a very pure form and sold as a fine or crystalline powder in various concentrations to facilitate dosing. Less often, ascorbic acid of purely biological origin is used. 

The most common product is Acerola fruit powder, the dried juice of the Acerola cherry, with 17 - 19% pure ascorbic acid. However, this more natural variant is up to 50 times more expensive than the synthetic product. 

Other substances on the market are ascorbic acid obtained from rose hips and mixed preparations, some of them containing ascorbic acid of biochemical origin. At the mill, flour is treated with typically 0.5 – 3 g of pure ascorbic acid per 100 kg. 

Very soft glutens or flours for certain applications (mainly frozen dough) require a larger dose of 6 - 10 g. 

Ascorbic acid does not act on the protein directly; it may be seen rather as an agent protecting against the loss of protein stability by counteracting glutathione, a reducing (softening) agent, that occurs naturally in the flour. 

This is only possible if ascorbic acid is oxidized to dehydro ascorbic acid (DHAA) at the beginning of the kneading process with the aid of the flour's own enzymes (i.e. ascorbate oxidase and glutathione dehydrogenase). 

In this process, glutathione is oxidized to glutathione disulphide, thus eliminating the gluten-softening effect of glutathione (Grosch and Wieser, 1999; Fig. 107).

Fig. 107 : Reaction of ascorbic acid in wheat dough (modified from Grosch and Wieser, 1999)

Proof of an adequate quantity and homogeneous distribution of the product in the flour can easily be obtained with Tauber's reagent in conjunction with a Pekar test (Fig. 108). A convenient and storable set with the two solutions required is available on the market. 

Titration with iodine, which is more accurate but less convenient, is still common practice as well. In biochemical laboratories, test kits with ascorbate oxidase and also HPLC are used to determine ascorbic acid very accurately.

Fig. 108 : Wet Pekar sample with Tauber's reagent for determination of ascorbic acid

2. Enzyme-Active Soy Flour

One enzyme from soy flour, lipoxygenase, also has an oxidative effect on the protein of the gluten. During the oxidation of the lipids by lipoxygenase, peroxides are formed that have a cross-linking effect on thiol groups. 

However, the gluten-strengthening effect of soy flour is comparatively slight; its bleaching effect is more important. There are several types of lipoxygenases with different action patterns. 

While type I lipoxygenase only acts on free unsaturated fatty acids, types II and III also oxidize unsaturated fatty acid bound to the glycerol backbone. Bean flour contains mostly types II and III, which makes it an efficient agent for oxidizing all unsaturated lipids in flour. 

The use of lipoxygenase is limited because the enzyme creates a "green" flavour that is not desirable in this application.

3. Glucose Oxidase

The enzyme glucose oxidase (GOD) is usually derived from the mould Aspergillus (in a similar manner to amylase) and sometimes Penicillium. Honey is also a rich source of GOD, where the enzyme enters the honey from the pharyngeal glands of the bees (Molan, 1992). 

However, its suitability is greatly restricted by its taste.

Fig. 109 : Hypothetical reactions of glucose oxidase in wheat dough

One effect of GOD in the dough is to oxidize glucose into gluconic acid with the aid of atmospheric oxygen (the slight souring that occurs in the process is negligible); its other effect is to transform water into hydrogen peroxide (Fig. 109). 

This oxidizing agent is not very specific and acts, for example, on the thiol groups of the gluten, on glutathione, or on ascorbic acid, all reactions resulting in tightening – or protection – of the gluten. The limiting factor in this process is the availability of oxygen.

Besides other chemical reactions that consume oxygen, yeast also takes up oxygen before starting its actual fermenting activity as it initially breathes instead of fermenting. 

This means that the conditions for GOD are really only good on the surface of the dough, as plenty of oxygen is always available there. The problem can be solved by technical measures during dough preparation, for example overpressure or the supply of extra oxygen (Lösche, 1996). 

The addition of an oxygen source, e.g. calcium peroxide, does not have sufficient effect within reasonable dosage limits. Saturation of the water with air by agitation at low temperatures improves the oxygen supply to a limited extent.

A typical GOD preparation is dosed in similar quantities to other enzymes, for example 100 - 500 ppm on flour (about 1,500 to 7,500 units of GOD per 100 kg of flour), but this depends to a very great extent on the product and process. 

In long fermentation and sheeted dough applications, GOD is more effective due to prolonged exposure to atmospheric oxygen.

4. Cystine

Cystine is the dimer of the amino acid cysteine in which two molecules of cysteine are linked by a disulphide bridge (Fig. 110). This sulphur bridge gives the molecule a certain oxidative effect. 

But at low doses it is possible that the gluten may soften, as reducing cysteine is released when cystine reacts with thiol groups of the protein. 

Although this has yet to be thoroughly investigated, cystine is used in spite of its high price compared to ascorbic acid because it is occasionally found to have a positive effect on the properties of the dough.

Fig. 110 : Chemical structure of cysteine (left) and cystine (right)

5. Dehydroascorbic Acid

DHAA is the oxidized form of ascorbic acid (Fig. 111). This means that if DHAA were used instead of AA it would be possible to dispense with the initial step of oxidation. Tests have shown that this is quite possible. 

One reason why it is so rarely used, however, is its instability, but this could be improved by coating. A further problem is that it is more difficult, and thus more expensive, to synthesize.

Fig. 111 : Chemical structure of ascorbic acid (left) and dehydroascorbic (right)

6. Bromate

The powerful oxidizing agent bromate (more precisely: potassium bromate, KBrO3) is still being used as a flour improver in many countries. 

Although it has a very long-lasting effect, this effect starts later than that of ascorbic acid and allows better processing of the doughs, for bromate clearly oxidizes glutathione only very slowly without the need for an enzyme (cf. ascorbic acid). 

It results in very good fermentation tolerance and a high volume yield. In the main, bromate acts directly on the gluten. Because of doubts about its effects on health it has gradually been replaced by ascorbic acid since the 1950s. 

A further problem is that it accelerates fire and explosion (bromate is a constituent of fireworks, especially rockets; Fig. 112).

Fig. 112 : Laboratory waste bin that caught fire because of mixed residues of potassium bromate and ascorbic acid

In countries that are now replacing bromate, combinations of ascorbic acid and enzymes offer good alternative ways of achieving satisfactory dough and baking properties. 

Because of the low doses required (similar to ascorbic acid or less) and its lower price, bromate can hardly be replaced without intervention by public authorities. Bromate is easily detected and determined semi-quantitatively with a kit in a similar manner to ascorbic acid.

7. Azodicarbonamide (ADA)

Azodicarbonamide (ADA) is foaming agent (Fig. 113) used in the manufacture of expanded plastics (not only does it have an oxidative effect; it also decomposes into large-volume gases upwards of 120 °C) has been used as a temporary replacement for bromate and in some cases still is. 

A great disadvantage is its low dosing tolerance; a slight overdose causes the bread to split badly. The dosage is roughly the same as that of ascorbic acid or bromate. 

The product most often used – in a correspondingly larger dose – is azodicarbonamide mixed with calcium sulphate to reduce its inflammability, usually with 23% of the pure substance. 

Fig. 113 : Chemical structure of azodicarbonamide

The ADA concentration of a premix can be determined by titration if no other oxidizing agent is present (AACC Method 48.71A), or by the Kjeldahl method if the carrier does not contain nitrogen. 

Another restriction on the use of the Kjeldahl method is the azo group (N=N) which is not fully accessible. In flour, only determination by HPLC seems to be a reliable procedure (Ahrenholz and Neumeister, 1987).

8. Chlorine and Chlorine Dioxide

These oxidizing agents have been banned in many countries because of their possible harmful effects on health and the technical risks they involve. There is no doubt that with certain baked goods (for example cake with a high proportion of fat and sugar) chlorination of the flour – that can only be carried out at the mill – produces the best results. Products: Cl₂, ClO₂ (typical dosage 20 - 250 ppm), hypochlorite (NaOCl, Ca(OCl)₂)

Chlorine reduces the pH because it is converted into hypochloric acid through reaction with water according to the following equation:

Cl₂ + H₂O → HCl + HClO.

The pH usually drops to 4.5 - 4.7, and in flour for certain cookie applications it even falls as low as 3.5. But the acidity is not responsible for the improving effect, since the latter is retained even after neutralization (Kulp et al., 1985).

The resulting hypochloride is a strong oxidant, reacting with flour pigments and other components. ClO₂ is a green gas that dissolves in water. It does not react with water, but with unsaturated chemical bonds and other reducing groups. 

Chlorine and its derivatives affect pigments (bleaching), starch (partial breakdown of amylose and amylopectin that alters the pasting properties), proteins (improved solubility), fats (saturation) and pentosans (degradation, and hence reduced water absorption) (Kulp et al., 1985).

Heat-treated flours have a certain similarity to chlorinated flour when combined with wheat starch, but chlorinated flour still achieves much better results (Seibel et al., 1984).

9. Calcium Peroxide

Calcium peroxide is yet another commonly used oxidizing agent. Upon heating, CaO2 releases oxygen that can be used in various oxidation reactions, for instance oxidation of ascorbic acid or water to hydrogen peroxide with the help of glucose oxidase. 

The effect of calcium peroxide is not very pronounced, but it is appreciated for its surface-drying property. For this reason it is always used in conjunction with more effective oxidizing agents. Calcium peroxide increases the pH of the dough. 

In certain limits this can be beneficial, particularly if the flour has high amylase activity. Larger amounts reduce the volume yield and cause excessive browning.

10. Other Oxidizing Agents

Tab. 86 summarizes the oxidizing agents that have been suggested for use in flour improvement. The "action in the dough" is based on the author's experience. For some of the substances, different information can be found in the literature. 

Some substances are rather risky, for instance acetone peroxide that tends to explode when exposed to even slight shock or friction. The substances not mentioned in the text above do not offer any considerable benefit as compared to the standard oxidizing agents.

Tab. 86 : Oxidizing agents suggested for flour treatment, and their typical reaction pattern

D. Reduction and Dough Softening

Gluten that is too short is difficult to process and results in a low volume yield, since the gas formed by the yeast is not able to expand the dough as it should. 

The problem can be solved by using substances with reducing properties that break down surplus disulphide bridges and thus give the protein molecules more room to move. 

Short gluten properties may result from the varieties used, but they are sometimes caused by the storage and processing of the grain (overheating) or the use to which the flour is put (for instance, freezing shortens the gluten). 

Some applications, in particular biscuits and crackers, require extensive softening of the dough for optimum processing and product properties.

1. Cysteine

A suspected "opponent" of ascorbic acid is cysteine, a simple amino acid that is a constituent of all proteins and produced either by hydrolysis of extremely cysteine-rich proteins such as those from feathers or hair and complex purification procedures, or by synthetic means.

As cysteine splits disulphide bridges like other reducing agents, one would expect it to counteract the effect of ascorbic acid if used at the same time. Initially it was only discovered empirically that this is not the case (Kieffer et al., 1990). 

In fact ascorbic acid and cysteine complement each other. One makes the gluten firmer, while the other ensures adequate elasticity. This is possible – as was proved later – because the two substances act on different constituents of the gluten and attack it at different sites.

The use of these flour improvers, in frozen doughs especially, makes it necessary to add large doses of both substances, for on the one hand good fermentation stability it required (ascorbic acid) and on the other hand the deep-freezing process shortens the gluten, a problem that can be solved at least in part by cysteine. 

The amount of cysteine added is often two-thirds of the quantity of ascorbic acid.

Cysteine is usually sold as L-cysteine hydrochloride, anhydrous or monohydrate, as it is more easily synthesized and has better water solubility in the latter form. 

Sodium nitrocyanoferrate/ammonium hydroxide can be used for detection, for instance on the wet pekar sample, but this is an unreliable method as the blue spots are sometimes difficult to see and fade quickly.

2. Reducing Yeast Preparations

Yeast also produces reducing substances, but these are only released when the cells die. There are now preparations made of inactivated, killed yeast on the market that have a softening effect similar to that of cysteine (Fig. 114). 

But as the dose required is about 100 times higher (100 - 1,000 g to 100 kg of flour), even the lower price (about 1/10 or even less) cannot make up for it. 

This is true even of so-called glutathione yeast, a variant with a very high reductive potential. So one might say that the main advantages of inactivated yeast are in the field of labelling.

Fig. 114 : The effect of reducing agents (L-cysteine or inactivated, glutathione-rich yeast) on the Extensogram (45 min)

3. Sodium Metabisulphite and Sulphur Dioxide

These powerful reducing agents are especially good at breaking down the gluten fast and reliably (Fig. 115), which greatly simplifies the production of biscuits, crackers and wafers. 

But as these substances are known to destroy vitamin B1 (thiamine) and to cause health problems in sensitive persons, their use should be avoided. They also impart a sulphur taste which can only be tolerated as long as no comparison to products without sodium metabisulphite (SMB) can be drawn. 

The dosage of SMB varies from 10 ppm to more than 1,000 ppm. Whereas 10 ppm are hardly effective, concentrations above 100 ppm are detectable sensorily, and concentrations larger than 500 ppm even cause a clear off flavour. 

Alternatives based on enzymes are now available; they achieve the same results but react rather more slowly and often require some process adjustment and hence knowledge on the part of the user. 

On the other hand, certain enzyme preparations based on proteases and amylases permit the reduction or omission of expensive ingredients, e.g. sugar, milk or whey powder, and therefore offer economic advantages.

Fig. 115 : Sodium Metabisulphite

Furthermore, sulphites are often not permitted, or they are restricted to low dosages or have to be declared in the labelling. Enzymes, on the other hand – natural proteins classified as processing aids – are not subject to restrictions and do not have to be declared in most countries.

Tab. 87 : Reducing substances used in baking applications

4. Other Substances

with Reductive Potential Tab. 87 lists substances commonly used in bread production which have a certain potential for reducing disulphide bonds to thiol groups. Even malt flour has been found to have a softening effect through its reductive potential (Tab. 88). 

Ascorbic acid acts as a reducing agent at elevated levels which cannot be completely oxidized by ascorbate oxidase because of limited oxygen in the dough. The threshold depends largely on the processing conditions.

Tab. 88 : Reducing potential of malt flour ^a

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