method for the production of precooked and dehulled corn flour for arepa and tortilla

Precooked and dehulled corn flour for arepa and tortilla is produced in a continuous process. First, the whole cleaned kernel is wetted and crushed to obtain two fractions. The fine fraction only is air-dried and sieved, while segregating both fractions through aspirations to isolate a floury-grit and hull fraction, incorporating the isolated fractions for animal feed by-product, and further producing an endosperm-germ fraction. The endosperm-germ fraction is conditioned and precooked fraction, cooled, air-dried and stabilized as to moisture content in the flaked material. It is then ground to particulate form, separating and collecting fine particles from coarse particles which are further processed, subjecting only the fine particles to centrifugation to yield an integral arepa flour, and, if desired, admixing the fine flour with lime to make a masa flour.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a thermal-mechanical process for the
 manufacture of precooked and dehulled corn flour and, more particularly,
 relates to a continuous column-roller process applied to the production of
 instant corn flour for use in the preparation of arepa and tortilla and
 derivatives thereof.
 2. Description of Related Art
 Corn represents one of the most important cereal grains for the nutrition
 of the Latin American population. The amounts consumed, as well as the
 form of consumption, are quite variable among different countries in the
 region. In Mexico and some Central American countries, the grain is
 transformed into food products by a unique process known as
 "nixtamalization", which basically consists of the alkaline cooking of
 whole corn kernel and breaking of the hull (or pericarp) down toward a
 partially gelatinized state (U.S. Pat. No. 4,513,018). Afterwards, the
 cooked corn is washed, ground and dried to give corn masa flour for
 different product applications. The main edible product of the traditional
 process is the "tortilla" which is a flat, round, unfermented and baked
 thin pancake produced from lime-cooked maize.
 In the northern part of South America, particularly in Colombia and
 Venezuela, hard endosperm corn is processed with dry milling technology
 and it is further converted into a precooked flour for traditional maize
 foods. Its consumption is mainly in the form of "arepa" which is a flat,
 unleavened and baked thick pancake made from dehulled and degermed
 cooked-maize. In other South American countries, corn meal and corn flour
 are used for different bakery and pancake mixes as well as snack products.
 At the present time, there is a large expansion in the consumption of corn
 products in the U.S.A. and tortilla is one raw material for the
 preparation of snack and traditional "Tex-Mex" dishes. Thus, there is an
 increasing demand and use of industrially prepared corn flours so they can
 provided benefits to the consumer, such as convenience, availability,
 price and a more stable quality. The potential market for masa flours in
 the U.S.A., Mexico and Central America is estimated at 17.8 million tons
 per year based on an average annual per capita consumption of 122 kilogram
 (tortilla and snacks: Sustain, 1997).
 In the production of corn products, such as tortillas, chips and the like,
 from food grade corn, it is known that the hard endosperm corn (U.S. Pat.
 No.2: USFGC, 1996) must be partially cooked before it is formed into the
 end product, so as to cause it to be a partially gelatinized corn flour.
 In the past, this has been done by processes wherein the corn is cooked
 and/or steeped in a lime-water solution (traditional nixtamalization) such
 as those disclosed in U.S. Pat. Nos. 2,584,893 (Lloyd et al.), 3,194,664
 (Eytinge) and 4,326,455 and 4,513,018 (Rubio), and subsequently ground and
 dried to produce nixtamalized corn flour (masa flour) from which such corn
 products may be made. As it may be seen from the above, the prior art
 methods for the industrial production of corn dough or masa flour involve
 accelerated cooking times with large amounts of processing water and
 increased costs for water removal. The lime-cooking changes produced in
 the hull or pericarp fraction are connected with the hydrolysis and
 solubilization which releases a highly hydrated dietary fiber having a
 marked effect on the rheological and mechanical characteristics of the
 traditional dough and its tortilla. Furthermore, the lime-water residue
 ("nejayote") is composed of 2.2 to 2.8% total solids from a continuous
 cooker which included 64-76% dietary fiber, 12-20% starch and 1.4%
 protein. The raw corn quality, cooking-time profile and process-type
 (Alvarez and Ramirez, 1995) utilized have been proposed as the main
 variables which determine corn solid losses in nejayote.
 With the aim of solving the above described problems by the traditional
 and/or industrial processes, some workers have developed alkaline-cooking
 methods with extruders. In this connection reference is made to the
 following U.S. Pat. Nos: Rubio 4,250,802, Martinez-Bustos et al. 5,532,013
 and Bazua et al. (1979). Although the above-described prior art provides
 means for rendering grain, such as corn or dehulled corn, into corn dough
 and/or masa flour under process conditions involving reduced amounts of
 water with shortened processing times, an industrial extruder capable of
 producing such a corn dough or masa flour was still unavailable in the
 market.
 In view of the limitations of the prior art accelerated methods for
 alkaline-cooking (thermal process) and alkaline-extrusion
 (thermal-mechanical process) for producing instant masa flour, they are
 not yet suitable for obtaining a corn flour with "different degrees of
 starch gelatinization" during precooking of the corn endosperm. In
 accelerated alkaline-cooking with simultaneous "impact grinding and flash
 drying processes", the peripheral zone of the endosperm might present
 starch gelatinization and, while this permits an increase in water
 adsorption (dough yield) during preparation of the dough, it may also
 facilitate water evaporation (dehydration) during tortilla cooking,
 whereby the mechanical properties are negatively affected by hardening or
 staling of the tortilla without texture additive.
 In view of the above, Wimmer et al. in U.S. Pat. No. 3,404,986 proposed a
 mechanical method for separating germ, endosperm and hull (pericarp) of
 the corn kernel and then prepared mixtures of moisturized standard corn
 meal and degerminated meal or flour (containing 18 to 35% moisture) and
 subjected it to the action of heated rotating rolls to cause controlled
 starch gelatinization (precooking) with subsequent drying and grinding to
 produce a corn flour. This instant corn flour can be used for a reduced
 fat snack and may be used in tortilla preparation. Hart, in U.S. Pat. No.
 4,329,371, describes a hybrid method without the use of alkali-cooking, by
 providing a step of dehulling the corn grains with a dehusking apparatus,
 separating the hull from the endosperm and germ fractions, using the hull
 for other low-value applications, and cooking the endosperm and germ
 fractions with steam. This cooked-dehulled fraction is then dried and
 ground to produce a corn flour for tortilla.
 Vaqueiro et al., in U.S. Pat. No. 4,594,260, discloses another hybrid
 method of obtaining corn flour which essentially comprises removing the
 hull from the germ and endosperm of the corn kernel until a
 hull-containing fraction and an endosperm-germ-containing fraction are
 produced and then nixtamalizing by the traditional method only the hull
 fraction and remixing the same with the uncooked endosperm-germ fraction
 (containing 30 to 35% moisture). This moisturized mixture is ground and
 dried to obtain an instant corn masa flour used in producing tortilla.
 Herbster, in U.S. Pat. No. 5,176,931, defines a short-time, low-pollution
 hybrid method of preparing masa flour for making tortilla chips. This
 procedure preferably comprises subjecting alkali-treated debranned and
 moisturized corn (15 to 35%) to near infrared radiation in order to
 partially cook the dehulled grain. The precooked-dehulled fraction can be
 additionally cooked, cooled and milled to desired particle size similar to
 industrial masa flour (Masa mixta brand) having 55% of non-gelantinized
 starch granules, 25% partially gelatinized and 20% fully gelatinized
 (according to microscopic examination).
 According to Watson (1987), the corn hull or pericarp (bran) makes up 5-6%
 of the kernel dry weight. In reference to the value-enhanced corn report
 (USFGC, 1996), a white-hard-endosperm kernel may contain: 11.5-11%
 moisture, 72.2-73.2% starch, 10.5-9.8% protein, 4-3.7% fat, and 2.1-2.3%
 of ash and crude fiber. Furthermore a dry-milled sample might yield, on a
 dry weight basis, 76.2-74.8% of total endosperm, 18.9-20.5% of germ and
 4.9-4.8% of pericarp. A typical materials balance for a Venezuelan arepa
 plant can yield a minimum of 65% flour on a wet basis (Cuevas, 1985). By
 1993, a whole or integral arepa flour was produced with more fiber and
 less protein and fat as compared to the traditional degermed flour which
 is enriched by Venezuelan law with vitamins (B and A) and iron.

Fiber Components of Corn Kernel Parts
 % Dry Fiber Fiber
 Fiber % Kernel
 Part Matter Insol. Hemicellulose Cellulose Lignin Soluble
 Total Fiber
 Whole Kernel 100 9.5 6.7 3 0.2 0.1
 9.5 100
 Starch endosperm 80.9 1.0 -- -- -- 0.5
 1.5 12
 Aleurone endosperm 2.0 50.0 -- -- -- 25.0
 75.0 15
 Germ 11.0 11.0 18 7 1.0 3.0
 14.0 16
 Pericarp (bran) 5.3 90.0 67 23 0.1 0.6
 90.7 51
 Tip cap 0.8 95.0 70 -- 2.0 --
 95.0 6
 SUMMARY AND OBJECTS OF THE INVENTION
 Accordingly, it is an object of this invention to provide a complete
 departure from the prior art precooking methods of thermal and mechanical
 processing of dehulled corn in order to control starch gelatinization
 during production of instant corn flour for arepa and tortilla and
 derivatives thereof.
 Another object is to produce this instant corn flour utilizing a continuous
 precooking process which is not only efficient but also less expensive
 than prior art accelerated methods for producing masa flour.
 Still another object is to produce instant corn flour for arepa and
 tortilla wherein such flour is relatively uniform and homogeneous in its
 physical and chemical properties.
 The above and other objects and advantages of the invention are achieved by
 a new continuous process applied to the production of precooked and
 dehulled corn flour or instant corn flour for arepa and tortilla,
 embodiments of which comprise the steps of: wetting the whole cleaned
 kernel to condition the same; crushing the wetted kernel to produce fine
 and coarse fractions; air-drying and classifying the fine fraction;
 separating from both fractions a floury grit and hull fraction as a corn
 by-product; conditioning the dehulled fraction to partially hydrate the
 starchy endosperm; precooking the conditioned material in a column with
 steam injection and further roller flaking to obtain a desired degree of
 gelatinization; cooling, air-drying and stabilizing the flaked material
 moisture to a desired optimum level for grinding; grinding the dried flake
 in a series of primary and secondary mills and producing air-suspendable
 particles; separating and recovering the fine particles so produced from
 the coarse particles while the latter are further processed to render from
 them a desired particle size; subjecting only the fine grind to a high
 speed centrifugation to further control of filth material for arepa flour
 and to admixing with lime to produce masa flour for tortilla and the like.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Referring first to FIG. 1, there is depicted, in flow diagram form, an
 embodiment of the present invention. It includes a conditioner 1; a
 crusher 2; a drier 3; a sieve 4; a primary aspirator 5; a secondary
 aspirator 6; a conditioner 7; a cooker 8; a roller 9; a drier-cooler 10; a
 drier 11; a preconditioner 12; a primary mill 13; a series of secondary
 mills 14; a cyclone with an associated sifter 15; and an entoleter 16.
 Whole corn kernel, which has been freed of foreign material, is fed to a
 conditioner 1, where the corn is sprayed with water to wet the hull and
 germ fractions. The wet kernel is passed through a crusher 2, which breaks
 the hull loose from the kernel, tears out the germ, and coarsely grinds
 the endosperm into two fractions.
 The large portion of corn endosperm is known as the coarse grit fraction
 (or "flaking grits"), while the small portion is described as the fine
 grit fraction composed of endosperm, germ and hull which is also known as
 "through stock or standard meal".
 The fine grit thus obtained is next directed to an air-drier 3, which
 reduces the moisture content of the ground fine mixture to about 9-11%,
 similar to the incoming whole corn, depending on the kernel hardness and
 its granulation being produced.
 The dried fine grit is further conveyed to a sieve 4, wherein two fractions
 are obtained, the floury grit with a softened hull fraction (containing
 from 10%-12% moisture) which is removed and sold as corn by-product, and
 the fine grit which is thereafter fed to the primary aspirator 5, together
 with the coarse fraction directly obtained from the crusher 2.
 The density separation steps which are performed in both primary 5 and
 secondary aspirator 6, each produce two fractions in a series of
 operations. The floury grit and hull fraction segregated by aspirators 5
 and 6 is isolated as corn by-product, and the dehulled fraction is
 conveyed to a conditioner 7 (representing from about 80% to 85% of the
 total weight incoming corn).
 The dehulled or endosperm-germ fraction is thereafter subjected to a
 tempering step in conditioner 7, wherein this fraction is sprayed with
 water in order to partially hydrate from 9% to 12% to about 20% to 22% for
 a period of about 20 to 30 minutes. After completing the conditioning
 step, the moisturized material is transferred to a cooker 8, wherein a
 partial gelatinization or precooking is performed through a column with
 the addition of live steam at the bottom end (70 to 98 psi and 154.degree.
 to 162.degree. C.) in order to precook for a period of from about 40 to
 about 100 minutes. The main degree of partial starch gelatinization is
 further controlled by passage of the precooked material between closely
 adjusted heated rotating steel drums or rollers 9. They are heated
 internally with saturated steam (450 to 500 psi and 210.degree. to
 240.degree. C.) so precooking is closely controlled by roll pressure, roll
 temperature, separation and speed variables which govern the material
 residence time. Any granular material is squeezed to form precooked flakes
 with an additional partial gelatinization at a temperature between
 88.degree. and about 93.degree. C. having a moisture content between 17%
 and about 19%.
 The precooked flake is thereafter passed through a drier-cooler 10, whereby
 an adiabatic drying with atmospheric air allows the moisture content of
 the material to be adjusted to about 14% and 17%, depending on the flake
 granulation and its incoming temperature (55.degree. to 60.degree. C.).
 The cooled flake is discharged into a drier 11, wherein the material is
 further reduced to a moisture content of about 10% to 12% by steam-tube
 drying (45 to 60 psi and 135.degree. C. to 145.degree. C.) and incoming
 air for a period of time of from about 2 to about 5 minutes. The dried
 flake is then passed to a preconditioner 12, where the flake moisture is
 further stabilized between 30 and about 90 minutes transit time and
 further enhancing the grinding which is to follow because the moisture is
 more nearly uniform.
 The stabilized flake thus obtained is thereafter ground in a primary mill
 13, wherein a coarse grind is obtained by a roller mill from which the
 premilled flake mixture is passed to a series of secondary mills 14. The
 premilled flake is further fed to a series of four secondary roller mills
 14, where the material is reduced to a coarse and fine grind which are
 further conveyed by a pneumatic pipe system to a cyclone and sifter 15.
 The grind material thus obtained is fed to a cyclone and sifter 15,
 whereby three different fractions are obtained, namely, a light
 floury-hull which is segregated in the cyclone whose design is know per se
 and discarded as corn by-product, a fine grind which is directly fed to a
 single secondary roller mill 14, and a third coarse grind which is
 thereafter reprocessed in a series of three secondary roller mills 14.
 The fine grind which is obtained from a single secondary roller-mill 14, is
 further centrifuged in an entoleter 16, whereby two fractions are
 produced, namely, a light filth material, and a heavy fine flour which is
 known as instant corn flour for arepa.
 In this method, arepa flour is produced in a 80% to 85% yield per kilogram
 of corn kernel. There is an improved yield gain from 15% to 20% of the
 total corn weight as compared to the typical arepa process which yields
 from 65% to 70%.
 Furthermore, and surprisingly, this method produces an arepa flour having
 higher protein content (by 10% to 20%), fat content (by 50% to 100%) and
 fiber content (about 100% higher) than dehulled and degermed corn flour
 for arepa and the like produced by conventional methods.
 Finally, this centrifugated fine thus obtained from an entoleter 16, may be
 admixed with lime (0.3% weight, based on flour) from which an instant masa
 flour is also produced (U.S. Pat. No. 3,730,732).
 From the foregoing, it will be clear that it is possible to manufacture a
 precooked and dehulled corn flour or an instant masa flour for arepa and
 tortilla with a new continuous process which is highly efficient because
 of the minimum use of water, which in turn makes the grinding process more
 uniform, and avoids expensive costs of removing the water and wasted corn
 solids that would have been present but for the features of this
 invention.
 It is to be understood that the embodiments of this invention herein
 illustrated and described, are by way of illustration and not of
 limitation, and that other embodiments may be made by those skilled in the
 art without departing from the spirit or scope of this invention.