After almonds are collected from the field, they undergo two processing phases, post-harvest processing and finish processing. These phases are typically conducted at two different facilities. There are two basic types of almond post-harvest processing facilities: those that produce hulled, inshell almonds as a final product (known as hullers), and those that produce hulled, shelled, almond meats as a final product (known as huller/shellers). Almond precleaning, hulling, and separating operations are common to both types of facilities. The huller/sheller includes additional steps to remove the almond meats from their shells.
Almond harvesting and processing are a seasonal industry, typically beginning in August and running from two to four months. However, the beginning and duration of the season vary with the weather and with the size of the crop. The almonds are harvested either manually, by knocking the nuts from the tree limbs with a long pole, or mechanically, by shaking them from the tree. Typically the almonds remain on the ground for 7 to 10 days to dry. The fallen almonds are then swept into rows. Mechanical pickers gather the rows for transport to the almond huller or huller/sheller. Some portion of the material in the gathered rows includes orchard debris, such as leaves, grass, twigs, pebbles, and soil. The fraction of debris is a function of farming practices (tilled versus untilled), field soil characteristics, and age of the orchard, and it can range from less than 5 to 60 percent of the material collected. On average, field weight yields 13 percent debris, 50 percent hulls, 14 percent shells, and 23 percent clean almond meats and pieces, but these ratios can vary substantially from farm to farm.
The almonds are delivered to the processing facility and are dumped into a receiving pit. The almonds are transported by screw conveyors and bucket elevators to a series of vibrating screens. The screens selectively remove orchard debris, including leaves, soil, and pebbles. A destoner removes stones, dirt clods, and other larger debris. A detwigger removes twigs and small sticks. The air streams from the various screens, destoners, and detwiggers are ducted to cyclones or fabric filters for particulate matter removal. The recovered soil and fine debris, such as leaves and grass, are disposed of by spreading on surrounding farmland. The recovered twigs may be chipped and used as fuel for co-generation plants. The precleaned almonds are transferred from the precleaner area by another series of conveyors and elevators to storage bins to await further processing. (In some instances, the precleaned almonds may be conveyed to a dryer before storage. However, field drying is used in most operations.)
Almonds are conveyed on belt and bucket conveyors to a series of hulling cylinders or shear rolls, which crack the almond hulls. Hulling cylinders are typically used in almond huller facilities. Series of shear rolls are generally used in huller/shellers. The hulling cylinders have no integral provision for aspiration of shell pieces. Shear rolls, on the other hand, do have integral aspiration to remove shell fragments from loose hulls and almond meats. The cracked almonds are then discharged to a series of vibrating screens or a gravity table, which separates hulls and unhulled almonds from the in-shell almonds, almond meats, and fine trash. The remaining unhulled almonds pass through additional hulling cylinders or shear rolls and screen separators. The number of passes and the combinations of equipment vary among facilities. The hulls are conveyed to storage and sold as an ingredient in the manufacture of cattle feed. The fine trash is ducted to a cyclone or fabric filter for
collection and disposal.
In a hulling facility, the hulled, in-shell almonds are separated from any remaining hull pieces in a series of air legs (counter-flow forced air gravity separators) and are then graded, collected, and sold as finished product, along with an inevitable small percentage of almond meats. In huller/shellers, the in-shell almonds continue through more shear rolls and screen separators.
As the in-shell almonds make additional passes through sets of shear rolls, the almond shells are cracked or sheared away from the meat. More sets of vibrating screens separate the shells from the meats and small shell pieces. The separated shells are aspirated and collected in a fabric filter or cyclone, and then conveyed to storage for sale as fuel for co-generation plants. The almond meats and small shell pieces are conveyed on vibrating conveyor belts and bucket elevators to air classifiers or
air legs that separate the small shell pieces from the meats. The number of these air separators varies among facilities. The shell pieces removed by these air classifiers are also collected and stored for sale as fuel for co-generation plants. The revenues generated from the sale of hulls and shells are generally sufficient to offset the costs of operating the almond processing facility.
The almond meats are then conveyed to a series of gravity tables or separators (classifier screen decks), which sort the meats by lights, middlings, goods, and heavies. Lights, middlings, and heavies, which still contain hulls and shells, are returned to various points in the process. Goods are conveyed to the finished meats box for storage. Any remaining shell pieces are aspirated and sent to shell storage.
The almond meats are now ready either for sales as raw product or for further processing, typically at a separate facility. The meats may be blanched, sliced, diced, roasted, salted, or smoked. Small meat pieces may be ground into meal or pastes for bakery products. Almonds are roasted by gradual heating in a rotating drum. They are heated slowly to prevent the skins and outer layers from burning. Roasting time develops the flavor and affects the color of the meats. To obtain almonds with a light brown color and a medium roast requires a 500-pound roaster fueled with natural gas about 1.25 hours at 118°C (245°F).
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. 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 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 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. There are 2 primary methods for roasting peanuts, dry roasting and oil roasting.
Dry Roasting –
Dry roasting is either a batch or continuous process. Batch roasters offer the advantage of adjusting for different moisture contents of peanut lots from storage. Batch roasters are typically natural gas-fired revolving ovens (drum-shaped). The rotation of the oven continuously stirs the peanuts to produce an even roast. Oven temperatures are approximately 430°C (800°F), and peanut temperature is raised to approximately 160°C (320°F) for 40 to 60 min. Actual roasting temperatures and times vary with the condition of the peanut batch and the desired end characteristics. Continuous dry roasters vary considerably in type. Continuous roasting reduces labor, ensures a steady flow of peanuts for other processes (packaging, candy production, peanut butter production, etc.), and decreases spillage. Continuous roasters may move peanuts through an oven on a conveyor or by gravity feed. In one type of roaster, peanuts are fed by a conveyor into a stream of countercurrent hot air that roasts the peanuts. In this system, the peanuts are agitated to ensure that air passes around the individual kernels to promote an even roast. Dry roasted peanuts are cooled and blanched. Cooling occurs in cooling boxes or on conveyors where large quantities of air are blown over the peanuts immediately following roasting. Cooling is necessary to stop the roasting process and maintain a uniform quality. Blanching removes the skin of the peanut as well as dust, molds, and other foreign material. There are several blanching methods including dry, water, spin, and air impact.
Dry blanching is used primarily in peanut butter production, because it removes the kernel hearts which affect peanut butter flavor. Dry blanching heats the peanuts to approximately138°C (280°F) for 25 minutes to crack and loosen the skins. The heated peanuts are then cooled and passed through either brushes or ribbed rubber belting to rub off the skins. Screening is used to separate the hearts from the cotyledons (peanut halves).
Water blanching passes the peanuts on conveyors through stationary blades that slit the peanut skins. The skins are then loosened with hot water sprayers and removed by passing the peanuts under oscillating canvas-covered pads on knobbed conveyor belts. Water blanching requires drying the peanuts back to a moisture content of 6 to 12 percent.
Spin blanching uses steam to loosen the skins of the peanuts. Steaming is followed by spinning the peanuts on revolving spindles as the peanuts move, single file, down a grooved conveyor. The spinning unwraps the peanut skins. Air impact blanching uses a horizontal drum (cylinder) in which the peanuts are placed and rotated. The inner surface of the drum has an abrasive surface that aids in the removal of the skins as the drum rotates. Inside the drum are air jets that blow the peanuts counter to the rotation of the drum creating air impact which loosens the skin. The combination of air impacts and the abrasive surface of the drum results in skin removal. Either batch or continuous air impact blanching can be conducted.
Oil Roasting –
Oil roasting is also done on a batch or continuous basis. Before roasting, the peanuts are blanched to remove the skins. Continuous roasters move the peanuts on a conveyor through a long tank of heated oil. In both batch and continuous roasters, oil is heated to temperatures of 138 to 143°C (280 to 290°F), and roasting times vary from 3 to 10 minutes depending on desired characteristics and peanut quality. Oil roaster tanks have heating elements on the sides to prevent charring the peanuts on the bottom. Oil is constantly monitored for quality, and frequent filtration, neutralization, and replacement are necessary to maintain quality. Coconut oil is preferred, but oils such as peanut and cottonseed are frequently used. Cooling also follows oil roasting, so that a uniform roast can be achieved. Cooling is achieved by blowing large quantities of air over the peanuts either on conveyors or in cooling boxes.