Module 2. Cereal processing

Lesson 5

CRITERIA OF WHEAT FLOUR QUALITY, IMPROVERS FOR WHEAT FLOUR, TYPES OF WHEAT FLOUR

5.1 Introduction

Flour produced at every grinding machine is different in terms of proportion of endosperm, germ and bran contained in it. Thus, each ‘machine’ flour is distinct in terms of baking quality, colour, granularity and the ash content. This difference in the composition of flour can lead to non-uniform quality in baked products. Thus to improve the quality of bakery products numerous chemical and biological compounds are added to the wheat flour during processing. They are usually added in very minute quantities to get the desired effects.

These additives must be GRAS (Generally Recognised As Safe) and should be permitted under food laws of the country of use. Wheat flour is major ingredient in manufacture of baked goods. But each type of bakery product requires a specific type of flour or combination of flour with certain improvers to get desired quality of end product. These additives are known as flour improvers, as they improve the technical quality of flour. The examples of flour improver are: bleaching agents, maturing agents, surfactants, enzymes, reducing agents, vitamins and minerals, antimicrobial agents, etc.

5.2 Flour Improvers

5.2.1 Bleaching agents

Flour contains a yellowish pigment, of which about 95% consists of xanthophyll or its esters. Bleaching of the natural pigment occurs very slowly when the flour is stored in bulk. Hence, to accelerate the bleaching process chemical bleaching agents are used. Nitrogen peroxide, chlorine, chlorine dioxide, nitrogen trichloride, benzoyl peroxide and acetone peroxide are the principal agents used for bleaching of wheat flour.

5.2.2 Maturing or improving agents

The bread making quality of wheat flour improves during storage for a period of 1 – 2 months. This change in baking quality is known as maturation or ageing. Chemical improver accelerates the maturing process by modifying the physical properties of gluten during fermentation. The stickiness in dough gets reduced and hence improved handling properties, increased tolerance in the dough to varied conditions of fermentation, and in producing loaves of larger volume and more finely textured crumb.

The action of improvers is believed to be an oxidation of the sulphydral or thiol (– SH) groups present in wheat gluten. Hence, the thiols are not available for participation in exchange reactions with disulphide (–S:S–) bonds. This reaction is considered to release the stress in dough and consequently the dough tightened. Potassium bromated, ascorbic acid, azodicarbonamide, stearoyl lactylates, ammonium/potassium per sulfate, acid calcium phosphate, acid sodium pyrophosphate are widely used maturing agents in wheat flour.

Chlorine, chlorine dioxide and acetone peroxide acts as both bleaching as well as maturing agents.

5.2.3 Biological additives

Enzymes such as amylases, proteases and oxidases are added to improve mixing, fermentation, flavour development, texture and storage properties of the baked products. Amylases break down starch molecules into glucose which is used by yeast to liberate carbon dioxide gas. Glucose oxidase enzyme is used to replace the chemical maturing agents such as potassium bromate which is banned in certain countries. Proteases are usually used in biscuit dough to reduce the mixing time. Lipoxigenase is used in bread dough as bleaching and maturing agent.

5.2.4 Emulsifiers

Emulsifier reduces surface tension and thus facilitates mixing of fat and aqueous phases. Emulsifiers are widely used in biscuit dough, icings, cakes, etc. where it helps in improvement of the texture. Mono/diglycerides, lecithin, polygycerol esters, calcium stearoyl lactylate, polysorbates, sodium stearoyl lactylate are some of the widely used surfactants in manufacture of baked food products. Emulsifiers with low HLB (Hydrophilic Lipophilic Balance) value such as mono/diglyceride, lecithin are suitable for high fat products, while emulsifiers with higher HLB values (sodium stearoyl lactylate) are best suited for low fat high moisture foods.

5.2.5 Antimicrobial agents

Bakery products are susceptible to microbial contamination especially with molds and rope formers owing to its high moisture and nutrient rich composition. Products get contaminated generally during processing steps such as cooling, slicing and wrapping. So wheat flour is usually added with certain mold inhibitors such as calcium/sodium propionate, sorbic acid or its calcium/potassium salts, acetic acid, lactic acid, acid calcium phosphate, sodium diacetate.

5.2.6 Vitamins and minerals

Wheat milling involves removal of bran, germ and aleurone layers. These layers are rich in vitamins and minerals. Hence, to meet the deficit of these nutrients, wheat flour is usually enriched with vitamins and minerals. Even lysine which is an essential amino acid is substantially lost during processes such as milling and baking. So, the flour is enriched with lysine too.

5.3 Types of Wheat Flour

Wheat flour is used in manufacture of numerous baked products. Other than baked products wheat flour is used in making other food products such as meat pie, sausages, chapattis, soups, etc. Biscuit dough should be stiff enough to permit rolling and flattening, while bread dough must be plastic mass that can be moulded and shaped. Wafer batter is a liquid suspension, which is able to flow through a pipe. So for each application flour with specific properties is necessary.

Flour from different stages in the mill are not identical in physical appearance, chemical analysis or baking properties. If all the streams of flour are mixed to one composite, then the resultant flour is known as ‘straight-grade flour’. It is also possible to blend the streams in definite proportions/ratios to produce the flour which is called ‘split milling’ or ‘divide milling’. Flour streams head end middlings, primary sizing and in some cases that of second and third breaks originate from the centre of wheat kernel. The blend of these flour streams is called ‘patent flour’.

In Table 5.1 various types of wheat flour along with their specific characteristics are listed.

5.4 Criteria of Wheat Flour Quality

Flour quality is a subjective concept that relates to the final product usage. For different baked goods wheat flour with specific characteristics is required as discussed earlier in types of wheat flour. Quality parameters such as colour, protein, granulation distribution, gluten quantity and quality, and starch damage play important role in deciding the suitability of flour for the baker.

Table 5.1 Types of wheat flour and their characteristics

Sr. No.	Type of wheat flour	Specific characteristics
1	Bread flour	· Flour with high level of good quality protein · Protein content: 10.8 – 11.3% (Mechanical process), 11.8 – 12.3% (Fermentation process)
2	Biscuit flour	· Milled from weak wheat of low protein content · Protein content: 8 – 9.5%
3	Household flour	· Weak wheat of low protein content with admixture of up to 20% of strong wheat
4	Self-raising flour	· Flour with added raising agents · Moisture should not exceed 13.5% to avoid premature reaction of aerating chemical
5	Flour for confectionery	Cakes · Undamaged starch granules, free from adhered protein and unattached by amylases · Protein content: 8.5 – 9.5% · Particle size: 90 μ Buns · Bread making flour Pastry · Strong baker’s flour
6	Flour for soups	· Steamed flour in which enzymes have been inactivated
7	Flour for sausage rusk	· Low protein flour milled from weak wheat
8	Batter flour	· Low protein flour milled from grist comprising 90% weak wheat and 10% strong wheat

5.4.1 Composition of wheat flour

The quality requirements of flour for different products are different. In Table 5.2 proximate composition of flour suited for different products are given.

Table 5.2 Proximate composition of flour suited for different products

Sr. No.	Component (%)	Biscuit	Bread	Cake
1	Protein	9.0	11.5	8.0
2	Fat	1.3	1.4	1.3
3	Fiber	0.15	0.1	0.05
4	Ash	0.6	0.5	0.4
5	Carbohydrate	72.6	70.1	73.5

5.4.2 Physical characteristics of wheat flour

5.4.2.1 Colour

Flour is tested for colour for evaluating either its whiteness, which primarily determines the extent of the oxidation of carotenoid pigments by bleaching agents, or the presence of bran particles indicating milling performance. Hunter device with L*a*b system, ‘Slick’ (Pekar) test, reflectance meter, NIR reflectance meter are some of the analytical techniques used for determination of wheat flour colour.

5.4.2.2 Thousand grain weight

Thousand grain weight or thousand kernel weight is usually expressed as weight of a thousand grains of wheat in grams. It is used to predict how much flour will be extracted from a given weight of wheat.

5.4.2.3 Granularity

Particle size of flour directly affects the amount of water required and rate of hydration for dough. The simple method for estimation of particle size is use of standard sieves with different mesh size.

Other physical characteristics such as test weight, hardness, experimental milling are also performed to evaluate suitability of flour for manufacture of various baked goods.

5.4.3 Alpha-amylase activity and Hagberg Falling Number

Alpha-amylase is an inherent enzyme of the wheat which breaks down the wheat starch into simple sugar during germination. It is used as one of the key indicators of wheat baking quality. The flour having high level of alpha-amylase activity requires less amount of water for mixing, softens the dough, weakens the bread structure and produces a soft, sticky crumb. Alpha-amylase activity is measured using the Hagberg falling number test. A sample of ground wheat is suspended in water using a standard glass tube. The tube is placed in the Falling Number apparatus, where the suspension is heated in water bath at 100°C and stirred for 60 sec. The  alpha-amylase breaks down the starch in the suspension, causing a reduction in the viscosity which is measured using the time taken for a plunger to fall through the suspension. The time in seconds is taken as the Hagberg falling number. The greater the number, the higher the viscosity and the lower the alpha-amylase activity. Falling number values of greater than 250 sec are generally acceptable for breadmaking.

5.4.4 Flour quality testing

5.4.4.1 Ash

It is indicator of the amount of mineral matter present in the flour and is commonly considered a quality index for flour. The flour having high bran content will contain more of ash content as bran has a higher mineral content the endosperm. High ash content of flour is not good for baking. The ash content is measured by the combustion of flour at 550°C.

5.4.4.2 Starch damage

Some of the starch granules get damaged during the milling process. Damaged starch absorb more water than undamaged starch and also susceptible to attack by alpha-amylase. Damaged starch can cause problems such as sticky crumb and weak bread structure.

5.4.4.3 Sedimentation value

Sedimentation test is used to assess the gluten quality and bread making potential of the flour by observing the way in which a ground wheat or flour suspension coheres and settles in the presence of sodium dodecyl sulphate. It takes upto 30 minutes for ground wheat and 50 minutes for white flour. Hard wheat flour having high content of glutenin proteins showed high sedimentation value as compared to soft wheat flour.

5.4.5 Rheological characteristics of wheat flour

Dough forming properties or rheological properties of wheat flour dough is important as they determine the suitability of dough for making of various baked food products. Even these properties determine performance of dough during various stages of manufacture. Following are the methodology usually adopted to determine the rheological characteristics of dough:

Dough mixing characteristics can be measured by use of ‘farinographs’, which give idea about water absorption, mixing time, mixing tolerance, strength of dough against machine abuse. To correct any deficits found, baker can change the wheat mix. Bread flour should have higher water absorption capacity and good mixing characteristics than biscuit dough. Bread flour should have water absorption capacity of 60 – 65%, dough development time of 5 – 7 min and stability of 7 – 9 min. Biscuit flour have 55 – 60% water absorption capacity.

‘Extensograph’ is used to determine structural transformations of dough during fermentation and maturing in terms of strain/stress relation indicating degree of maturation. This test give idea about ageing of flour, requirement of oxidizers and heat conditioning.

To analyze the gelatinization characteristics of starch in oven, ‘amylographs’ are used. This test indicates any need of enzymes or diastatic malt.