Durum wheat milling is different from other forms of wheat milling in that there are differences in the wheat, the actual milling process, final product characteristics and the utilization of those final products.
According to the "2001 Grain and Milling Annual," compiled by World Grain, durum milling comprises approximately 9.5% of the total milling capacity in United States. Durum milling capacity has fluctuated in the past decade from 41.7 million cwts (1.9 million tonnes) in 1991, to 50.2 million cwts in 1993, down to 43.8 million cwts in 1996 and back to 49.8 million cwts in 2001.
Durum wheat is different than bread wheats. Genetically, it has 14 pair of chromosomes compared with the 21 pair of chromosomes in the common wheats. The endosperm is characterized by a yellow color.
In the United States, the classification system runs from Number 1 to Number 5 with three subclasses. The subclasses are determined by the percentage of hard and vitreous kernels present, with vitreousness being described as a "flinty" appearance to the endosperm when an individual kernel is broken open. This is a visual determination.
In the United States, the north central states of North Dakota and Montana produce approximately 80% to 85% of the durum crop. The remainder is grown in the desert southwest. Besides differences in growing times, there are physical differences between northern-grown durums and desert durums.
The functions of a durum mill are to produce a maximum amount of high-value semolina that meets customer specifications with a minimum amount of byproducts. Semolina is a granular product which, according to the code of federal regulations FD 137.320, is "that food prepared by grinding and bolting cleaned durum wheat to such fineness that when tested … it passes through a No. 20 sieve, but not more than 3% passes through a No. 100 sieve."
However, customers often specify a finer granulation. One important specification is the degree of speckiness.
Other products produced are various grades of flour that can be used for noodles or industrial purposes, and various feed streams. The bran can be used as a fiber source in the health food industry.
Most of the same issues of wheat selection for wheat milling apply to durum semolina milling, including test weight, kernel size, the aforementioned degree of vitreousness, protein content, and ash content. Lastly, quality cannot be milled into the wheat.
CLEANING. Once the wheat is selected, it must be prepared for the milling process. In durum milling, the cleaning process is exceedingly important because of the need to prevent the introduction of any discolored foreign material into the mill.
There is the potential for such discolored material to be ground up and manifest itself as specks in the final semolina product. These specks in turn will show up in the final pasta product.
Wheat from the field contains extraneous material that must be removed before milling. The foreign material has physical characteristics that can be taken advantage of to aid in its removal. Magnets, found both at the beginning and the end of the cleaning process, are used to remove ferrous metals.
Size is another physical principle that can be used to remove foreign material. Screening machines are used to remove material larger and smaller than the desired grain. Light material such as chaff and shrunken wheat and heavy material such as stones and non-ferrous metal can be used by machines that use specific gravity as a separating principle. Gravity tables, stoners and stone concentrators, and paddy machines are examples of such devices.
Indent machines are used to remove materials that are the same size but a different shape than the desired grain. Cylinder separators and disc machines utilize this principle.
Scouring machines remove adhering dirt and help reduce microbial counts in the final product. Impact machines when combined with scouring are used to remove any unsound wheat, knock loose any adhering dirt, and minimize any potential insect infestation. This process also serves to remove beeswing and germ.
Aspirators also are beneficial in this area. It is desirable to have both a "dry" impact/scouring and a "wet" impact/scouring after the tempering process.
Ancillary machines such as spiral seed separators, color sorters, cracked and broken wheat systems can be incorporated into the cleaning flow to assure the removal of virtually all foreign material.
All of the cleaning machinery is kept under negative aspiration to control dust that is dislodged during the cleaning process. The unwanted material removed during cleaning is ground and can be added to bran byproducts not destined for human consumption.
Adhere to all capacity specifications outlined by the cleaning equipment manufacturer. There are some who believe the cleaning equipment should be operated at approximately 80% rated capacity to make sure cleaning is done to the optimum. As with any wheat milling operation, the cleaning system should be oversized to allow for blend changes, maintenance and bin sequencing.
TEMPERING. Once the wheat is cleaned, it goes to the tempering process. Tempering is the process of adding the optimum amount of water to wheat for the optimum amount of time to enhance its milling properties.
Ideally, water should be added in three stages, with about 40% temper added at the first stage, 59% added at the second stage and 1% added about one-half hour before milling. Total temper time varies with the natural moisture and type of wheat in the mill mix.
Too short a temper time will cause the bran to shatter, with increased specks in the final product. Too long a temper time will produce excess fines, which could lead to the production of lower value flour.
The temper bins, like those in common wheat mills, should be of first-in, first-out design. In colder climates, wheat should be heated to the point of condensation prevention in equipment and optimal water uptake by the wheat. Chlorine can be added to the temper water to reduce microbial counts.
MILLING. Once the wheat is properly tempered it enters the mill. The flow of a dedicated durum mill is longer compared with common wheat flows in order to handle the in-process material in the gentlest way possible. A shorter milling process could lead to the production of lower value flour.
The break system may contain from five to seven passages, with fine break passages incorporated. Grinding action, spiral, differential and corrugation choice should be such that the initial passages open up the wheat berry with subsequent passages scraping the endosperm out in large particles.
Sizing systems also can contain five to seven passages. Likewise, grinding action, spiral, differential and corrugation is chosen to produce a minimum amount of flour.
The sizing system can be composed of two stages. The first stage is used to gently grind equally sized endosperm to loosen adhering bran and prepare it for purification. The second sizing stage adjusts the granulation of clean purifier stock to meet the semolina granulation specification.
Stocks not suited for semolina production can be reduced on the midds and tail system. Additional equipment for bran finishing, dusting and filter stock processing is included.
The sifter sections are comparable in function and purpose to common wheat flows. The sifters make their separation based on size. The characteristics of the material exiting the sifter determine its destination and subsequent processing. Material can go to another break roll, one of the sizing systems, the purification system, the flour reduction system, or to by-products.
The utilization of grader sifter sections can be advantageous in a durum flow. The purpose of the grader section is to take ground endosperm and produce streams of narrow particle ranges for more efficient purification.
PURIFYING. The purification system is where the final semolina product is made. Material going to the purifiers undergoes separation using the principles of size and specific gravity. Ideally, this material is composed of equally sized particles of bran, endosperm with adhering bran, and pure endosperm.
The combination of air, purifier action, and cloth aperture size makes the separation. The material is introduced at the head end of the purifier. The combination of air and purifier motion stratifies the stock, with the heavy, dense endosperm settling close to the separation media and the lighter, branny material rising to the top.
The heavy, pure endosperm drops out first and can be considered final product or go to a sizing roll.
The endosperm with adhering bran will drop out next and be directed to a different sizing roll to knock the adhering bran loose. From there it will go to a sifter and be sifted and repurified. The final material at the end of the purifier is composed of light bran and goes to a roll to be ground with the ultimate purpose of flour removal.
Purifiers should be clothed in accordance to the sifter section preceding it and loaded so that when the air is cut off, all the stock should pass through the sieves at the three-quarter length.
Purifier action should be such that the material being purified stratifies and the air adjusted so that the stock does not bubble. Aperture cleaning devices should have a gentle action that does not upset the stratified stock.
The valves under the purifiers send the purified material to either the final product or a roll for further processing. A durum semolina mill may have up to three times the purifier capacity as a wheat mill. This is why a well-filtered air supply is needed.
The final semolina product can go directly to the pasta manufacturer or to a regrind system where the granulation can be adjusted further and then to the pasta manufacturer. Prior to shipping, the product should be sifted and passed through an impact machine to eliminate potential infestation problems.