21-11-2012, 12:23 PM
Peanut Processing
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General
Peanuts (Arachis hypogaea), also known as groundnuts or goobers, are an annual leguminous
herb native to South America. The peanut peduncle, or peg (the stalk that holds the flower), elongates
after flower fertilization and bends down into the ground, where the peanut seed matures. Peanuts
have a growing period of approximately 5 months. Seeding typically occurs mid-April to mid-May,
and harvesting during August in the United States.
Light, sandy loam soils are preferred for peanut production. Moderate rainfall of between
51 and 102 centimeters (cm) (20 and 40 inches [in.]) annually is also necessary. The leading peanut
producing states are Georgia, Alabama, North Carolina, Texas, Virginia, Florida, and Oklahoma.
Process Description
The initial step in processing is harvesting, which typically begins with the mowing of mature
peanut plants. Then the peanut plants are inverted by specialized machines, peanut inverters, that dig,
shake, and place the peanut plants, with the peanut pods on top, into windrows for field curing. After
open-air drying, mature peanuts are picked up from the windrow with combines that separate the
peanut pods from the plant using various thrashing operations. The peanut plants are deposited back
onto the fields and the pods are accumulated in hoppers. Some combines dig and separate the vines
and stems from the peanut pods in 1 step, and peanuts harvested by this method are cured in storage.
Some small producers still use traditional harvesting methods, plowing the plants from the ground and
manually stacking them for field curing.
Harvesting is normally followed by mechanical drying. Moisture in peanuts is usually kept
below 12 percent, to prevent aflatoxin molds from growing. This low moisture content is difficult to
achieve under field conditions without overdrying vines and stems, which reduces combine efficiency
(less foreign material is separated from the pods). On-farm dryers usually consist of either storage
trailers with air channels along the floor or storage bins with air vents. Fans blow heated air
(approximately 35°C [95°F]) through the air channels and up through the peanuts. Peanuts are dried
to moistures of roughly 7 to 10 percent.
Local peanut mills take peanuts from the farm to be further cured (if necessary), cleaned,
stored, and processed for various uses (oil production, roasting, peanut butter production, etc.). Major
process steps include processing peanuts for in-shell consumption and shelling peanuts for other uses.
In-shell Processing -
Some peanuts are processed for in-shell roasting. Figure 9.10.2.2-1 presents a typical flow
diagram for in-shell peanut processing. Processing begins with separating foreign material (primarily
soil, vines, stems, and leaves) from the peanut pods using a series of screens and blowers. The pods
are then washed in wet, coarse sand that removes stains and discoloration. The sand is then screened
from the peanuts for reuse. The nuts are then dried and powdered with talc or kaolin to whiten the
shells. Excess talc/kaolin is shaken from the peanut shells.
Shelling -
A typical shelled peanut processing flow diagram is shown in Figure
Shelling
begins with separating the foreign material with a series of screens, blowers, and magnets. The
cleaned peanuts are then sized with screens (size graders). Sizing is required so that peanut pods can
be crushed without also crushing the peanut kernels.
Next, shells of the sized peanuts are crushed, typically by passing the peanuts between rollers
that have been adjusted for peanut size. The gap between rollers must be narrow enough to crack the
peanut hulls, but wide enough to prevent damage to the kernels. A horizontal drum, with a perforated
and ridged bottom and a rotating beater, is also used to hull peanuts. The rotating beater crushes the
peanuts against the bottom ridges, pushing both the shells and peanuts through the perforations. The
beater can be adjusted for different sizes of peanuts, to avoid damaging the peanut kernels. Shells are
aspirated from the peanut kernels as they fall from the drum. The crushed shells and peanut kernels
are then separated with oscillating shaker screens and air separators. The separation process also
removes undersized kernels and split kernels.
Following crushing and hull/kernel separation, peanut kernels are sized and graded. Sizing and
grading can be done by hand, but most mills use screens to size kernels and electric eye sorters for
grading. Electric eye sorters can detect discoloration and can separate peanuts by color grades. The
sized and graded peanuts are bagged in 45.4-kg (100-lb) bags for shipment to end users, such as
peanut butter plants and nut roasters. Some peanuts are shipped in bulk in rail hopper cars.
Roasting -
Roasting imparts the typical flavor many people associate with peanuts. During roasting,
amino acids and carbohydrates react to produce tetrahydrofuran derivatives. Roasting also dries the
peanuts further and causes them to turn brown as peanut oil stains the peanut cell walls. Following
roasting, peanuts are prepared for packaging or for further processing into candies or peanut butter.
Typical peanut roasting processes are shown in Figure 9.10-2.2-3. There are 2 primary methods for
roasting peanuts, dry roasting and oil roasting.