Extruded Snacks

Extruded snack products fall into two broad categories: directly expanded or puffed, and pellets (dense half-products). Extrusion literally means forcing out. Many food formers such as hydraulic presses and double rollers are called extruders. However, the extruders used in snack food processing resemble those of the plastic industry. Rotating screws enclosed in a barrel convey a cereal dough and force it through a die. Products are cut to size with rotating knives. Both single and twin-screw extruders are used. Twin-screw extruders are more complex, more flexible, more easily controlled and operate over a wider range of moisture conditions. However, they are more costly. Single-screw extruders still are used extensively, particularly in the manufacture of expanded corn snacks and pellets.

Direct Expansion

Directly expanded snacks are made by extruding cereal grains and flours at rather low moisture levels (12-16%) and forcing them through a die. Corn grits are the most common ingredient. Rice flour, wheat, potato starch flour, and tapioca all expand well. Due to its high native fat content, oat flour does not provide the expanded pufly texture desired in a snack base. In puffing extrusion friction and shear combine to heat and plasticize the grains or flours into a pressurized mass of molten gelatinized dough. The pressure at the die end of the extruder is much greater than the saturated pressure of steam at the temperature where the dough is extruded, about 350°F. Thus the water becomes superheated. Consequently when the dough is forced through the die into the atmosphere, water and steam are released explosively and cause the gelatinized starch to expand into a foamed structure that sets up on cooling. The extruders used for puffed snacks range from extremely short length single-screw collet extruders that have a length-to-diameter ratio (L/D) of only 2-3 to 1 to long single- and twin-screw models of 20:30 L ID (12). Expanded puffs contain 5-9% water and must be dried or baked to below 2% moisture to achieve the desired crisp-ness. Once dried, puffs are coated with oil and flavors. Cheese puffs are an example of a baked, coated expanded snack.

Fried Expanded

In a variation of direct expansion, corn grits of 15-18% water are extruded in a collet type extruder that contains a die or auxiliary rolling device modified to restrict the expansion of corn grit extrudate. The extrudate, in the form of irregular rods, is deep fried to remove residual water, impart a fried flavor, and slightly expand the extrudate. After frying the rods are coated, usually with cheese flavoring.

Pellets

The use of extruders to manufacture pellets is actually a new version of an old process. A number of Asian cultures make a starch paste of about 50% water containing flavoring such as shrimp. The paste is molded, gelatinized with steam, sliced, dried, and fried to a puff. Extruded pellets are made in much the same way. A blend of flours and starches that have good puffing characteristics, such as corn and potato, is combined with water and flavor (if desired) and extrusion cooked to gelatinize the starch. Moisture content ranges from 25 to 35%. The cooked mass must be cooled before it is formed to avoid expansion and produce a dense, glassy pellet. If a twin-screw extruder is used the dough can be cooled by venting steam and by external heat transfer before it is forced through a pasta-type die and knife assembly. In another version, two single-screw extruders are used (13). The first cooks and gelatinizes. As the dough exits the first extruder, water evaporates, and cools the dough mass that has foamed. The second extruder recompresses the mass and forces it through a shaping die. Pellet shapes are virtually unlimited. Shapes include grills, wagon wheels, spirals, shells, tubes, and playing card suits among others. Freshly extruded pellets are tacky, therefore they must be predried with air to prevent sticking in the final dryer. Final drying takes place in a rotating drum dryer equipped with temperature and humidity control. Pellets of about 10% moisture emerge after 6-8 h of drying (14). The shelf-stable pellets are extremely versatile and can be fried in only 10-15 s, puffed in hot air without any fat and can even be puffed in a microwave. Fried pellets contain 20-25% fat,

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