This method was previously described in AACC (2000).
A. Samples, Ingredients, and Reagents
• Different samples of soft and hard wheats
• Distilled water
• Quadrumat Sr. or Jr. Brabender mills
• Buhler mill
• Tempering device or bottles with lids
• Scale
• Graduated cylinder (100 mL)
• Air-forced convection oven
• Rotap (U.S. nos. 35, 60, and 100 sieves)
• Rotap collector pan
• Rotating sifter with a U.S. 100-mesh sieve
• Sieve cleaners C. Procedure
1. Obtain a representative grain sample, making sure to determine grade and class (refer to Chapter 1). Clean the grain sample, preferably by using a clipper or Carter dockage test meter. Determine the percentage of foreign material and moisture content (refer to procedure in Section 2.2.1.1).
2. Weigh the predetermined wheat sample with an accuracy of 0.1 g.
3. Temper the grain according to its class:
a. Soft wheats: 15% to 15.5% moisture for 12 to 15 hours.
b. Hard wheats: 16% to 16.5% moisture for 16 to 24 hours.
c. Durum wheats: 16.5% to 17% moisture for 16 to 24 hours.
Use the following equation:
{[(100 − % original moisture)/(100 − % desired conditioning moisture)] − 1]} × amount of wheat to temper.
4. Place the wheat sample in the tempering device or bottle and add the predetermined amount of conditioning water. Place the lid of the con-tainer and manually shake contents to promote water distribution. If a tempering device is used, place the container and turn on the apparatus that mechanically shakes the sample. If the tempering is manually done, make sure to shake contents
128 Cereal Grains: Laboratory Reference and Procedures Manual for at least 10 minutes or until no free water is
observed. Allow the tempered wheat to temper at room temperature for the predetermined time.
Milling Procedure Using the Experimental Chopin Mill
See Figure 5.2.
1. Weigh the predetermined amount of tempered wheat (i.e., 300 g) and gradually grind it through the break rolls. Recuperate the resulting three milling fractions: break flour that passed the U.S.
100-mesh sieve, middlings and shorts that passed the U.S. 35-mesh sieve, and bran that was retained by the U.S. 35-mesh sieve. Alternatively, the bran can be reground and sifted again to recuperate the attached endosperm. Weigh each of the three frac-tions with an accuracy of 0.1 g.
2. Grind the middlings/shorts fraction at least six times through the reduction rolls. After each
pass, make sure to separate the two resulting fractions: reduction flour that passed the U.S.
100-mesh sieve and the reground middlings that did not pass the U.S. 100-mesh sieve. Weigh the two fractions and regrind only the milling frac-tion that has not achieved the proper granula-tion. After the sixth pass, weigh and determine yield of shorts/red dog that did not meet granu-lation. Blend all break and reduction flours and estimate yield or extraction rate with the follow-ing equation:
Extraction rate = (refined flour weight/original grain weight) × 100.
3. Build a table indicating yields of flour as the wheat is progressively milled. Break, first reduc-tion, second reducreduc-tion, third reducreduc-tion, etc.
4. Determine the moisture content, chemical com-position (protein, starch, ash, fat, and fiber),
1
2
3 4
(a) (b)
(d) (e)
(c)
FIgure 5.2 Laboratory and experimental equipments for the production of refined wheat flour. (a) Tempering device; (b) Quadrumat Jr.
Mill; (c) Quadrumat Sr. Mill; (d) Chopin Experimental Mill; (e) wheat dry-milling fractions (clockwise: 1, whole wheat; 2, bran; 3, refined flour; 4, shorts).
129 Dry-Milling Processes and Quality of Dry-Milled Products
color, starch damage, and functional ties (falling number, dough rheological proper-ties, and viscoamylograph) of samples of break, reduction, and straight grade flours.
Milling Procedure Using the Quadrumat Jr. Mill See Figure 5.2.
1. Weigh the predetermined amount of tempered wheat (i.e., 100 g) and grind it through the Quadrumat Jr. Mill. Recuperate the ground wheat and place it on top of a set of sieves (35, 60, 100, and bottom collection pan) for 5 min-utes of rotaping. Separate the milled fractions into bran (+35), middlings (U.S. +60 and 100 mesh), and break flour (U.S. −100 mesh sieve or material collected at the bottom of the pan).
Weigh each of the four fractions with an accu-racy of 0.1 g. Separate the bran and break flour.
2. Grind the middlings/shorts fraction at least two times through the Quadrumat Jr. reduction rolls and then separate the reduction flour form the shorts/red dog in a rotating U.S. 100-mesh sieve furnished with at least one sieve cleaner. Weigh the two fractions. Blend the break and reduction flours and estimate yield or extraction rate with the following equation:
Extraction rate = (refined flour weight/original grain weight) × 100.
3. Determine the moisture content, chemical com-position (protein, starch, ash, fat, and fiber), color, starch damage, and functional ties (falling number, dough rheological proper-ties, and viscoamylograph) of samples of break, reduction, and straight grade flours.
Milling Procedure Using the Quadrumat Sr. Mill See Figure 5.2.
1. Weigh the predetermined amount of tempered wheat (i.e., 1000 g) and grind it through the break and reduction rolls of the Quadrumat Sr.
Mill. Make sure to properly adjust wheat feed rate to optimize milling and sifting. The whole milling operation is automated. Recuperate the resulting milling fractions: flour, shorts, and bran. Weigh each of the three fractions with an accuracy of 0.1 g.
2. Calculate extraction rate using the following equation:
Extraction rate = (refined flour weight/original grain weight) × 100.
3. Determine the moisture content, chemical com-position (protein, starch, ash, fat, and fiber),
color, starch damage, and functional properties (falling number, dough rheological properties, and viscoamylograph) of flours.
Milling Procedure Using the Buhler Roller Mill 1. Weigh the predetermined amount of tempered
wheat (i.e., 1–3 kg) and grind it through the break and reduction rolls of the Buhler Roller Mill. Make sure to properly adjust wheat feed rate to optimize milling and sifting. The whole milling operation is automated. Recuperate the resulting milling fractions: flour, shorts, and bran. Weigh each of the three fractions with an accuracy of 0.1 g.
2. Calculate extraction rate using the following equation:
Extraction rate = (refined flour weight/original grain weight) × 100.
3. Determine the moisture content, chemical com-position (protein, starch, ash, fat, and fiber), color, starch damage, and functional ties (falling number, dough rheological proper-ties, and viscoamylograph) of the refined wheat flour.
5.2.3 Dry-Millingof rice—ProDuction of regularanD ParboileD White rice
Rice is the most important staple food for Asians. Most of the crop is dry-milled into white kernels that are free from glumes, pericarp, germ, and aleurone. The milling pro-cess that converts paddy into white rice basically consists of six sequential operations: drying, grain cleaning, dehul-ling, decortication, polishing, and sizing. First, the moist paddy rice is dried in preparation for dehulling. Paddy rice is dehulled to selectively remove the glumes or husks, yield-ing brown rice. Duryield-ing this operation, approximately 20%
of the paddy rice weight is lost. Then, the brown rice is abrasively decorticated to remove pericarp, germ, and the outer endosperm that contains the multilayered aleurone.
Finally, the white rice is sized into head, second head, and broken pieces. The yield of head white rice is affected by the grain’s physical properties and drying. Vitreous or hard kernels yield higher amounts of head rice. Drying should be aimed at minimizing the formation of stress cracks or fis-sures. Commercially, the maximum expected yield of head rice is 65% (Juliano 1985).
Approximately 15% of the world rice is parboiled before milling. Parboiling is defined as a hydrothermal process applied to rough rice to improve milling yield especially when low-quality rices are parboiled. There are many types of parboiling methods; however, the key sequential opera-tions are conditioning, heating, and drying. The rough rice is
130 Cereal Grains: Laboratory Reference and Procedures Manual usually tempered to 30% to 35% moisture and then subjected
to a thermal treatment for short periods of time (2–5 min-utes). During these operations, the starch gelatinizes and acts as glue, sealing microfissures or stress cracks. Finally, the parboiled rough rice is dehydrated to decrease the mois-ture to approximately 13% to 14%. Parboiling modifies the appearance and culinary properties of the milled rice and its physical, chemical, and nutritional characteristics. Upon parboiling, kernels become somewhat glassy, translucent, and slightly discolored (light yellow or amber). The harder kernels are also less susceptible to insects and more stable during storage. However, parboiled rough rice loses its via-bility or germination capacity. Parboiled milled rice has dif-ferent cooking quality and textural properties because of its lower cooking water uptake. Cooked parboiled rice retains its shape, is firmer, fluffier, and less sticky compared with regular rice (Juliano 1985).
Laboratory milling equipment for rice consists of a dehul-ler and an abrasive decorticator that removes the husks and abrades brown rice to obtain white rice. The white rice, sec-ond heads, and broken kernels are removed by sifting. The most broadly used laboratory mills are the shellers and whit-eners from McGill and Satake, which only require small samples of rough rice (125–200 g). These laboratory tests highly correlate with commercial milling yields.