Flour extraction rate

Martmez-Villaluenga, C., Michalska, A., Frias, J., Piskula, M.K., Vidal-Valverde, C., and Zielinski, H., 2009. Effect of flour extraction rate and baking on thiamine and riboflavin content and antioxidant capacity of traditional rye bread. Journal of Food Science. 74 49-55. 

Rich sources are some vegetables, such as peas and cauliflower (about 0.1 mg/kg), cereals, cereal products and legumes. The biotin content in flour is strongly dependent (as is the content of all B group vitamins) on the flour extraction rate. In white flour a httle over 10% of the original vitamin present remains in the grain. However, only a small proportion of biotin in wheat is available, better availabflity comes from maize and soybeans. 

Many current triticales have flour extraction rates of more than 70% comparable with those of wheat. Triticale flour can be used to partially replace wheat flour in most food applications. In unleavened or flat breads such as chapatis, wheat flour tortillas, and Injera, triticale behaves like soft wheat flours and the bread-making process needs no modification. This makes the crop especially promising in many countries of Asia, Africa, and Latin America where these prepared foods are staples. In leavened breads, triticale has not been able to match the performance of hard wheats. However, it has been successfully utilized to produce noodles, breakfast cereals, and porridges (National Research Council 1989). 

One property of the flour that is controlled by the miller is the extraction rate. Wholemeal flour has a 100% extraction rate, with brown, white and patent white having progressively reduced extraction rates. One obvious difference is the colour. Another is that the quality of the protein increase towards the middle of the wheat berry from which patent flour is produced. Thus, patent flour is sometimes used not to produce whiter bread but in products like hlo pastry or West Indian patties where the strength that patent flour gives is important and the colour is irrelevant. 

All of the above flours are white flours of ordinary whiteness. If the extraction rate is reduced still further, whiter flour known as patent flour is obtained. A patent flour can be produced from the grist of baker s grade or higher flours. The resulting flour will not only be whiter it will have a lower protein content. The quality of the protein will be higher. 

Although the 100% extraction rate ensures a bigger yield, wholemeal is not particularly popular with millers as a stronger grist is needed. In addition, its shelf life is only three months (cf. a year for white flour). The reduction in shelf life is supposed to be caused by oxidation of the lipid fraction that is absent from white flour. Furthermore, wholemeal flour must be kept apart from white flour less that is contaminated. 

Brown flour is a term that covers the extraction rates above 70% and below 100%, i.e. between white and wholemeal flour. Unlike wholemeal flour the full range of flour improvers are legal. The sale of brown bread is lower than that of wholemeal. Some nutritional thinking points to... 

Bread has been made in Norway from wheat flour 78% extraction (50 parts) barley flour 60% extraction rate (20 parts) wholemeal flour (30 parts) with extra fat added. 

Typically wheat flour with a 75-82% extraction rate is used. The flour used in the UK is a strong bread flour. 

Depending on the colour and use of the finished wafer some wafers are made from brown flours. While the increased fibre content has nutritional advantages the real reason for this is that the higher extraction rate makes the flour cheaper to produce. 

Figure 9-14 Relation Between Extraction Rate and Proportion of Total Vitamins of the Grains Retained in Flour. Source Reprinted with permission from W.R. Aykroyd and J. Doughty, Wheat in Human Nutrition, 1970, Food and Agriculture Organization of the United Nations.

In the present study, bread in the form of rolls was prepared from whole grain flour (100% extraction rate) with a zinc content of 22 mg/kg and from white wheat flour of about 72% extraction rate, where the zinc content was 7 mg/kg. The water added to the dough, contained an amount of almost carrier free isotope solution, corresponding to 0.25y/ Ci of Zn for each roll. The isotope solution was made by diluting a stock solution of 65 Zn CI2 in 0.1 M HCl (0.1 to 0.5 Ci/g Zn, Radiochemical centre, Amersham, England) with physiological saline to a final radioactivity of Ci/ml. 

As can be seen, there is a significantly higher per cent absorption in meals consisting of white bread (low extraction rate) than in meals consisting of whole flour bread, 38.2% versus 16.6%. The amount of zinc was however considerably higher in whole flour bread (1.3 mg versus 0.4 mg), which causes a higher absolute absorption of zinc from whole flour bread. 

In most countries, cereals are the largest single source of dietary iron. The iron content of white flour is considerably lower than that of whole grain flour and flours of higher extraction rate. In Norway we have no fortification of the white flour, and the contribution of iron from cereals to the diet accounts for only 30%. Our neighbor country Sweden does enrich its white flour, with the result that over 60% of the iron in the diet comes from cereals. The frequency of iron anemia is, nevertheless, the same in the two countries. 

The extraction rate in Norway of 78-80% must be considered relatively high compared with that of other countries, but even such a flour has significantly less iron than whole wheat flour (Table I). 

The dietary fiber content of various Norwegian wheat flours (9) with different extraction rates were analyzed. To analyze the dietary fiber content we used a gravimetric method, based on digestion of the samples with pepsin and pancreatin. The method is a modification of the procedure described by Hellendorn et al. and modified by Asp et al (10). The pepsin digestion was carried out at pH 1.5 for 1 hr, and the pancreatin digestion at pH 6.8 for 1 hr. 

An ascorbic acid oxidase (AO) occurs in wheat flour (Table 15.25), which oxidizes L-threo- and D-erythro-2iSCOYhic acid at comparable rates. In addition, a substance has been found in flour extracts which oxidizes L- /ir 6>-ascorbic acid at pH 10 at a maximal rate. In comparison with AO, this activity does not decrease on incubation with proteases nor is it inhibited by the addition of the AO inhibitors KCN and NaF. It obviously catalyzes a nonenzymatic oxidation of ascorbic acid. 

Wheat flour contains 1.5-2.5% fipids, depending on milling extraction rate. Part of this lipid is nonstarch lipid. This portion is extracted with a polar solvent, water-saturated butanol, at room temperature. Nonstarch lipid comprises about 75% of the total lipid of flour (Fig. 15.19). The residual lipids (25%) are bound to starch (cf. 15.2.4.1). Nonstarch- and starch-bound lipids in wheat differ in their composition (cf. Table 15.28 and Table 15.32). In nonstarch-bound lipids the major constituents are the triacylglycerides and digalactosyl diacylglycerides, while in starch-bound lipids, the major constituents are lysophosphatides in which the acyl residue is located primarily in position 1. A decrease in amylose content is accompanied by a decrease in the lipid content (Table 15.28). The ratios of nonstarch-bound lipid classes are dependent... 

The chemical composition of the flour depends on the milling extraction rate, e. g., flour weight obtained from 100 parts by weight of grain. Examples are given in Table 15.36. Increasing the rate of flour extraction decreases the pro-... 

Fig. 15.23. Content of B vitamins and minerals in flour as affected by milling extraction rate (according to...

The commercial product semolina Cgriess ) is made from endosperm cells of hard durum wheats. Semolina keeps its integrity during cooking and is used mostly for pasta production. Since semolina is a milled flour of low extraction rate, it contains few minerals and vitamins. 

Flour acidity (ml of O.lmol/1 NaOH/lOg, titrated in the presence of phenolphthalein) depends upon the extraction rate of the flour and ranges between 2.0ml/g (flour type 450) and 5.5 ml/g (flour t)fpe 1800). Too low acidity often reflects poorly aged flour. Acidity above 7.0 suggests microbial spoilage. 

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