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TABLE 2. Milling yields and polyphenol content in the flour fractions using single-step milling process Milled fraction yield (%) Polyphenol content(a)
| Variety and tempering degree (%) | Bran | Shorts II | Flour I | Flour II | Total flour | Flour I | Flour II | Total flour |
| BR | ||||||||
| 12 | 3.7 | 23.5 | 65.5 | 8.5 | 72.0 | 1.40 | 1.90 | 1.55 |
| 14 | 5.2 | 22.5 | 60.2 | 10.3 | 70.5 | 1.21 | 1.7 | 1.38 |
| 16 | 7.4 | 24.5 | 52.1 | 14.6 | 66.7 | 0.93 | 1.1 | 1.10 |
| 18 | 7.5 | 25.0 | 51.5 | 15.0 | 66.5 | 0.92 | 1.12 | 1.10 |
| Giza 15 | ||||||||
| 12 | 4.6 | 15.7 | 66.9 | 12.3 | 79.2 | 0.00 | 0.00 | - |
| 14 | 5.8 | 18.5 | 59.9 | 15.1 | 75.0 | 0.00 | 0.00 | - |
| 16 | 6.8 | 18.3 | 55.5 | 18.9 | 74.4 | 0.00 | 0.00 | - |
| 18 | 7.7 | 18.5 | 55.2 | 19.0 | 73.2 | 0.00 | 0.00 | - |
| NES 1007 | ||||||||
| 12 | 5.2 | 21.7 | 63.5 | 9.2 | 72.5 | 0.008 | 0.01 | 0.009 |
| 14 | 5.9 | 23.5 | 56.4 | 12.6 | 70.2 | 0.007 | 0.009 | 0.008 |
| 16 | 6.3 | 24.2 | 52.9 | 16.1 | 69.0 | 0.000 | 0.005 | 0.001 |
| 18 | 6.5 | 24.7 | 50.2 | 18.0 | 68.2 | 0.000 | 0.004 | 0.001 |
a. As catechin (g/100 g) by vanillin-hydrochloric acid method.
Among sorghum varieties, the thin-pericarp Giza 15 variety gave the highest yield of polyphenol-free total flour, while the thick-pericarp BR variety gave a low flour yield with a high polyphenol content. Because of the presence of testa in the BR variety, its flour fractions, particularly flour II, contained large amounts of polyphenols compared with the two Egyptian varieties.
According to the theoretical calculation, the flour yield of sorghum should be more than 80%, as in wheat, but in fact it was lower than that. This might be attributed to the differences in both the morphological and histological characteristics of the sorghum grains as compared with wheat grains. After tempering, the endosperm became soft and friable, and when sheared by the first roll or break, it split into some endosperm particles (endosperm middlings) and flour.
These particles found their way into shorts, and consequently, the milling yield was lower than expected. Therefore, tempering of sorghum grain at 16% moisture content and at 20°C-22°C was found to be sufficient to allow the endosperm to be scraped away carefully from the bran.
The specifications of the milling fractions obtained at 16% tempering are shown in table 3. Generally, two types of products were obtained from sorghum milling, flour and millfeed. The proportion of the first type varied from 66% to 74%, which was composed of 51%-55% flour I (break flour plus reduction, or "patent," flour) and 14%-19% flour II (from remilling of shorts I in a small Bühler mill). The millfeed (bran plus shorts II) represented 25%-32%, composed of 6.8%-7.4% bran (seed coat plus part of the germ) and 18.3%-24.5% shorts (coarse middlings and germ).
TABLE 3. Yields and proximate composition of products from single-step milling process of sorghum and wheat (whole grain at 16% tempering)-percentages on dry-weight basis
| Yield | Crude Proteina | Ether extract | Ash | Crude fibre | Poly Phenolsb | Carbohydrate |
|||
| Totalc | Starch | Otherc | |||||||
| BR | |||||||||
| bran | 7.4 | 11.7 | 9.2 | 4.5 | 15.6 | 12.1 | 46.9 | 36.8 | 10.1 |
| shorts II | 24.5 | 16.3 | 3.8 | 1.9 | 2.1 | 4.0 | 71.9 | 63.8 | 8.1 |
| flour I | 52.1 | 8.3 | 1.9 | 0.8 | 0.4 | 0.93 | 87.7 | 81.4 | 6.3 |
| flour II | 14.6 | 14.6 | 5.6 | 2.5 | 1.6 | 1.10 | 74.6 | 67.2 | 7.4 |
| total flour | 66.7 | 10.66 | 2.7 | 1.4 | 0.8 | 1.00 | 84.0 | 77.4 | 84.0 |
| Giza 15 | |||||||||
| Bran | 6.8 | 13.9 | 10.2 | 4.0 | 16.6 | 0.033 | 55.2 | 34.2 | 21.0 |
| shorts II | 18.3 | 19.1 | 5.2 | 1.9 | 2.9 | 0.025 | 70.9 | 58.6 | 12.3 |
| flour I | 55.5 | 10.2 | 1.9 | 0.9 | 0.30 | 0.00 | 87.7 | 80.9 | 6.8 |
| Flour II | 18.9 | 17.8 | 6.0 | 2.9 | 1.7 | 0.00 | 71.6 | 63.5 | 8.1 |
| Total flour | 74.4 | 12.0 | 3.1 | 1.6 | 0.8 | - | 82.5 | 75.0 | 7.5 |
| NES 1007 | |||||||||
| Bran | 6.3 | 14.5 | 11.4 | 4.0 | 17.5 | 0.06 | 52.5 | 30.0 | 22.5 |
| Shorts II | 24.2 | 19.3 | 4.4 | 1.1 | 2.5 | 0.03 | 72.7 | ' 60.0 | 12.70 |
| Flour I | 52.9 | 13.3 | 2.1 | 1.4 | 0.5 | 0.00 | 82.7 | 77.8 | 4.9 |
| Flour II | 16.1 | 17.4 | 6.2 | 3.2 | 1.0 | 0.005 | 72.2 | 65.1 | 7.1 |
| Total flour | 69.0 | 15.1 | 3.2 | 2.1 | 0.9 | 0.001 | 78.5 | 72.7 | 5.8 |
Flour I = break flour + reduction flour. Flour
II and shorts II are produced by remilling of shorts I. Total
flour = flour I + flour II.
a. N x 6.25 for sorghum protein and N x 5.7 for wheat protein.
b. As catechin by vanillin-hydrochloric acid method.
c. By difference.
Because the Giza 15 variety is a thin-pericarp sorghum and has a hard endosperm, it gave the highest flour yield and the lowest millfeed, followed by the NES 1007 and then the BR variety.
Concerning the bran from the three varieties, the protein and crude fibre contents were lowest and that of ash was highest in the BR variety, while the protein was highest in NES 1007. In the case of shorts, protein, fat, and crude fibre were lowest in BR and starch was lowest in Giza 15.
The flour from the BR variety was found to contain less protein, fat, and ash and more starch than that from the other two varieties. Ash content was greatest in the flour from NES 1007.
The polyphenols were mostly concentrated in the bran, followed by shorts, flour II, total flour, and flour I. The distribution was clear in the case of the BR variety, which was distinct in its testa layer. Polyphenols were absent completely from the flour obtained from Giza 15, and were present only in traces in that from NES 1007.
Carbohydrates other than starch were highest in the bran fraction, followed by shorts, flour II, and flour I. As shown by these data, large amounts of endosperm found their way into the shorts fraction and this led to a reduction in the milling yield. Moreover, flour from the BR variety had a high polyphenol content (1%), which affects the flour colour and perhaps its products. Sorghum bran and part of the germ are easily pulverized to fine particles that are difficult to separate from the flour by sieving [8]. Thus the problem is how to remove them from the flour. To improve the separation, a two-step milling process was suggested in which the grains would be decorticated to remove the bran layer in the first step, while in the second step the dehulled grains would be ground into flour in a roller mill.
Two-step milling process
Dehulling
The results of subjecting the sorghum grains to different times of dehulling (one, three, and five minutes) and at different tempering percentages (12%-18%) are shown in table 4.
TABLE 4. Yield and polyphenol content of sorghum at different dehulling times and tempering percentages
| Tempering % | Dehulling time (min.) | BR | Giza 15 | NES1007 | |||
| Dehulling yield (%) | Polyphenolsa (%) | Dehulling Yield (%) | Polyphenolsa (%) | Dehulling yield (%) | Polyphenolsa (%) | ||
| 12 | 0 | 0.0 | 3.40 | 0.0 | 0.020 | 0.0 | 0.02 |
| 1 | 93.2 | 2.42 | 94.0 | 0.017 | 94.8 | 0.021 | |
| 3 | 85.2 | 1.21 | 87.6 | 0.009 | 88.4 | 0.012 | |
| 5 | 76.2 | 0.17 | 85.8 | 0.005 | 86.5 | 0.008 | |
| 14 | 0 | 0.0 | 3.19 | 0.0 | 0.019 | 0.0 | 0.027 |
| 1 | 92.4 | 2.10 | 94.3 | 0.012 | 94.5 | 0.017 | |
| 3 | 84.0 | 0.97 | 87.4 | 0.005 | 88.0 | 0.008 | |
| 5 | 76.0 | 0.15 | 85.1 | 0.004 | 85.7 | 0.006 | |
| 16 | 0 | 0.0 | 3.00 | 0.0 | 0.015 | 0.0 | 0.023 |
| 1 | 90.7 | 1.95 | 93.1 | 0.010 | 94.1 | 0.014 | |
| 3 | 83.1 | 1.07 | 86.6 | 0.002 | 85.0 | 0.004 | |
| 5 | 75.0 | 0.10 | 84.4 | 0.002 | 83.6 | 0.004 | |
| 18 | 0 | 0.0 | 3.02 | 0.0 | 0.017 | 0.0 | 0.025 |
| 1 | 90.1 | 1.99 | 92.9 | 0.013 | 93.8 | 0.018 | |
| 3 | 82.8 | 1.09 | 85.1 | 0.002 | 84.1 | 0.004 | |
| 5 | 74.1 | 0. l0 | 83.1 | 0.002 | 82.5 | 0.004 | |
a. Vanillin hydrochloric acid (V HCI) expressed as g catechin/g sample (%).
Dehulling yields decreased when both time and tempering percentage increased. Both of the Egyptian varieties gave higher yields than the BR variety. This might be attributed to the smaller grain size and the presence of testa in BR. The dehulling yield (abrasive dehuller) varied from 69% to 98% for 31 non-testa-containing and 41% for testa-containing sorghum varieties [9].
Three minutes of dehulling were suitable to remove the bran from the Egyptian varieties, while five minutes were needed for the BR variety (with a testa layer). Increasing the tempering over 16% moisture content reduced the dehulling yield and did not reduce the polyphenol content. These conditions also caused a reduction in polyphenol content from 1.03% to 0.1% in the BR variety but not in the Egyptian varieties.
The results of the proximate composition of whole and dehulled grains at 16% tempering showed marked reductions in polyphenol tannins (84%-96%), physic acid (70%-80%). crude fibre (70%-75%), ash (32%-53%), and oil (27%-39%). These were accompanied by a slight decrease in protein (4.5%-8.3%) and an apparent rise in starch content after three to five minutes of dehulling. Generally, these reductions were more pronounced in the BR than in the Egyptian varieties.
TABLE 5. Proximate composition of the three sorghum grain varieties after different dehulling times at 16% tempering (percentages)
| Variety and dehulling time (min.) | Crude Proteina | Crude ether extract | Ash | Crude fibre | Phytic Acidb | Poly- Phenolsc | Starch | Other carbo hydrated |
| BR | ||||||||
| 0 | 11.9 | 3.6 | 1.7 | 1.9 | 0.20 | 2.9 | 69.9 | |
| 1 | 11.6 | 3.1 | 1.5 | 1.5 | 0.07 | 1.95 | 70.3 | 11.48 |
| 3 | 11.2 | 2.4 | 1.1 | 0.6 | 0.06 | 1.03 | 73.8 | 9.81 |
| 5 | 11.0 | 2.2 | 0.8 | 0.3 | 0.04 | 0.10 | 75.9 | 9.66 |
| 0 | 11.3 | 3.8 | 1.8 | 1.9 | 0.19 | 0.02 | 69.5 | |
| 1 | 12.2 | 3.0 | 1.6 | 1.6 | 0.06 | 0.01 | 70.9 | 12.23 |
| 3 | 12.2 | 2.7 | 1.0 | 0.4 | 0.05 | 0.002 | 74.0 | 10.00 |
| 5 | 12.1 | 2.3 | 1.0 | 0.3 | 0.04 | 0.002 | 74.3 | 9.6 |
| NES 1007 | ||||||||
| 0 | 15.7 | 4.0 | 1.9 | 2.0 | 0.25 | 0.03 | 68.5 | |
| 1 | 15.6 | 3.2 | 1.2 | 1.2 | 0.11 | 0.014 | 69.9 | 10.0 |
| 3 | 15.0 | 2.9 | 0.6 | 0.6 | 0.07 | 0.004 | 71.9 | 8.9 |
| 5 | 14.8 | 2.6 | 0.5 | 0.5 | 0.06 | 0.004 | 73.1 | 8.44 |
a.Nx6.25.
b. As phosphorous phytate (g/100) g).
c. Vanillin-hydrochloric acid as catechin equivalent (g/100) g).
d. By difference.
Grinding
Table 6 gives the milling fractions of the dehulled grain. The two-step method increased the flour I fraction yield and gave a lower yield of both flour II and total flour than the single-step method.
TABLE 6. Yields and proximate composition of products from the two-step milling process of sorghum (dehulled grain)- percentages on dry weight basis
| Yield | Crude proteina | Ether extract | Ash | Crude fibre | Poly- Phenolsb | Carbohydrate | |||
| Totalb | Starch | Otherc | |||||||
| BR | |||||||||
| bran | 0.7 | 12.7 | 4.6 | 2.7 | 8.5 | 17.0 | 53.9 | 40.6 | 13.9 |
| shorts II | 10.3 | 9.0 | 3.1 | 1.0 | 0.7 | 0.7 | 75.5 | 65.4 | 10.1 |
| flour I | 54.2 | 8.3 | 1.3 | 0.6 | 0.2 | 0.050 | 90.9 | 84.3 | 6.6 |
| flour II | 9.5 | 15.0 | 3.7 | 0.8 | 0.3 | 0.045 | 80.1 | 72.2 | 7.9 |
| total flour | 63.7 | 10.5 | 1 7 | 0.7 | 0.3 | 0.048 | 86. 8 | 81.3 | 5.5 |
| Giza 15 | |||||||||
| bran | 0.8 | 14.0 | 5.6 | 2.5 | 8.8 | 0.01 | 69.1 | 50.2 | 18.89 |
| shorts II | 15.1 | 18.0 | 5.0 | 1.2 | 0.7 | 0.000 | 75.1 | 62.0 | 13.1 |
| flour I | 60.5 | 9.9 | 1.7 | 0.8 | 0.2 | 0.00 | 87.2 | 83.1 | 4.3 |
| flour II | 10.0 | 19.7 | 5.2 | 1.6 | 0.4 | 0.00 | 73.1 | 65.5 | 7.6 |
| total flour | 70.5 | 11.7 | 2.0 | I.0 | 0.3 | - | 85.0 | 80.0 | 5.0 |
| NES 1007 | |||||||||
| bran | 0.8 | 15.1 | 5.5 | 2.5 | 8.9 | 0.018 | 68.4 | 46.0 | 21.6 |
| shorts II | 17.1 | 20.8 | 5.8 | 1.4 | 1.0 | 0.00 | 77.0 | 60.00 | 11.0 |
| flour I | 53.8 | 13.0 | 1.9 | 1.2 | 0.4 | 0.00 | 83.5 | 79.4 | 4.1 |
| flour II | 12.8 | 17.1 | 5.6 | 1.7 | 0.5 | 0.00 | 75.1 | 67.0 | 8.1 |
| total flour | 66.6 | 14.5 | 2.2 | 1.3 | 0.5 | - | 81.5 | 76.5 | 5.0 |
a. Nx6.2.
b. Vanillin-hydrochloric acid method as g catechin/100) g sample.
c. By difference.
The shorts fraction yield was lower with the two-step process. The bran fraction yield with that process increased over that removed during dehulling and by Bühler mill grinding. This technique led to a marked reduction in polyphenol content in all milled fractions except bran as compared with the single-step milling process. It also resulted in an increase in starch and a decrease in ether extract, ash, and crude fibre in the milled fractions as compared with the single-step method.
Generally, to obtain a satisfactory flour yield associated with good quality (colour, chemical composition, and nutritional value), the hulls of grain must be removed before dry milling. This is especially true when sorghum grain has a pigmented testa layer and thick pericarp. The resulting fractions by the two-step method were very low polyphenol levels, high starch levels, and nearly the same protein values as testa, with a white or yellow pericarp and large grain those obtained from the single-step method. In the size, dehulling before milling was not necessary. case of low-polyphenol sorghum varieties, free from
References