Generating added value through extrusion in the flour mill
Extrusion has provided the means to produce new and creative snack foods. Extrusion processing equipment has become the touchstone operating equipment in most snack food companies throughout the world. Food extrusion is a process in which a food material is forced to flow, under one or more varieties of conditions of mixing, heating and shear, through a die, which is designed to form and/or puff-dry the ingredients (Rossen and Miller, 1973).
Food extruders can be visualized as a high temperature short time (HTST) device that can transform a variety of raw ingredients into intermediate and finished products. During extrusion, the cooking temperature could be as high as 180-190° C, but residence time is usually 20-40 seconds (depends on types of extruders and speed of the shaft). For this reason, the extrusion process can be called an HTST process. It is very important to understand extrusion terminology, since all manufacturers like to use their own terminology based on their equipment.
Extrusion technology provides several different advantages over traditional methods of food and feed processing. Extrusion is simply the operation of shaping a plastic or dough-like material by forcing it through a restriction or die. A food extruder is a device that expedites the shaping and restructuring process for food ingredients. Extrusion is a highly versatile operation that can be applied to a variety of food processes. Extruders can be used to cook, form, mix, texturize and shape food products under conditions that favor quality retention, high productivity and low cost.
Food extruders can be visualized as a high temperature short time (HTST) device that can transform a variety of raw ingredients into intermediate and finished products. During extrusion, the cooking temperature could be as high as 180-190° C, but residence time is usually 20-40 seconds (depends on types of extruders and speed of the shaft). For this reason, the extrusion process can be called an HTST process. It is very important to understand extrusion terminology, since all manufacturers like to use their own terminology based on their equipment.
Extrusion technology provides several different advantages over traditional methods of food and feed processing. Extrusion is simply the operation of shaping a plastic or dough-like material by forcing it through a restriction or die. A food extruder is a device that expedites the shaping and restructuring process for food ingredients. Extrusion is a highly versatile operation that can be applied to a variety of food processes. Extruders can be used to cook, form, mix, texturize and shape food products under conditions that favor quality retention, high productivity and low cost.
Extruders
There are several different styles of extruders available in the market. This may cause difficulty for snack food manufacturers to select a proper extruder for snack food production. However, it increases the use of extrusion technology for snack foods. During food extrusion, a number of functions occurr in short time, i.e., conveying, mixing, homogenization, deaeration, heating/cooling, cooking, sterilization, forming/shaping, expansion, texturization, flash drying, and center filling. In general, snack food extruders are divided into two major categories: single-screw and twin-screw.
Single-Screw Extruders
Single-screw cooking extruders have compressive screws with decreasing channel depth turning at high speeds to increase shear and mechanical energy input for heating. The resulting friction induces heating of a product. In some cases, the barrel is jacketed for steam to allow additional contact heating in the metering section. To increase capacity and efficiency, it is common to preheat ingredients in a pre-conditioner by adding steam before they enter the extruder. Categories of single-screw extruders include (Harper, 1981):
Cold forming (Pasta-type) Extruders: deep flight, smooth barrel, low shear speed. Little or no cooking. Used for pasta, pastry dough, cookies, egg-rolls, ravioli, processed meat and certain candy.
High-Pressure Forming Extruders: grooved barrels to prevent slippage at the wall and greater compression in the screw design. Used for pre-gelatinize cereal and fried snack foods.
Low-Shear Cooking Extruders: moderate shear machines with high compression and grooved barrels to enhance mixing. Used for soft-moist foods and meat-like snacks such as simulated jerky.
Collet Extruders: high shear machines with grooved-barrels and screw with multiple shallow flights. Used for puffed snacks and expanded curls or collets.
High Shear Cooking Extruders: high shear machines, with screws of changing flight depth, HTST devices. Make pet food, Ready-to-Eat Cereal (RTE), candy, crisp breads, precooked food ingredients, pre-gelatinized corn flour, dried food mixes, instant beverage powder, croutons and breading, crackers and wafers, enzyme deactivations of full fat soy flour, imitation nuts, famine relief feeding, texturized vegetable protein (TVP), and deactivation of enzymes in cereal and oilseeds.
Twin-Screw Extruders
Twin screw extruders consist of two parallel screws in a barrel with a figure-eight cross section. The use of twin-screw extruders for food processing started in the 1970s, with an expanding number of applications in the 1980s. Twin-screw extruders are generally one and one-half times or more expensive than single a screw machine of the same capacity (Lusas and Riaz, 1994). Yet the degree of quality control and processing flexibility they offer can make them attractive to food industries. Twin screws produce a more uniform flow of the product through the barrel due to the positive pumping action of the screw flights. Some other advantages of twin screw are:
Four types of twin-screw extruders are possible:
There are several different styles of extruders available in the market. This may cause difficulty for snack food manufacturers to select a proper extruder for snack food production. However, it increases the use of extrusion technology for snack foods. During food extrusion, a number of functions occurr in short time, i.e., conveying, mixing, homogenization, deaeration, heating/cooling, cooking, sterilization, forming/shaping, expansion, texturization, flash drying, and center filling. In general, snack food extruders are divided into two major categories: single-screw and twin-screw.
Single-Screw Extruders
Single-screw cooking extruders have compressive screws with decreasing channel depth turning at high speeds to increase shear and mechanical energy input for heating. The resulting friction induces heating of a product. In some cases, the barrel is jacketed for steam to allow additional contact heating in the metering section. To increase capacity and efficiency, it is common to preheat ingredients in a pre-conditioner by adding steam before they enter the extruder. Categories of single-screw extruders include (Harper, 1981):
Cold forming (Pasta-type) Extruders: deep flight, smooth barrel, low shear speed. Little or no cooking. Used for pasta, pastry dough, cookies, egg-rolls, ravioli, processed meat and certain candy.
High-Pressure Forming Extruders: grooved barrels to prevent slippage at the wall and greater compression in the screw design. Used for pre-gelatinize cereal and fried snack foods.
Low-Shear Cooking Extruders: moderate shear machines with high compression and grooved barrels to enhance mixing. Used for soft-moist foods and meat-like snacks such as simulated jerky.
Collet Extruders: high shear machines with grooved-barrels and screw with multiple shallow flights. Used for puffed snacks and expanded curls or collets.
High Shear Cooking Extruders: high shear machines, with screws of changing flight depth, HTST devices. Make pet food, Ready-to-Eat Cereal (RTE), candy, crisp breads, precooked food ingredients, pre-gelatinized corn flour, dried food mixes, instant beverage powder, croutons and breading, crackers and wafers, enzyme deactivations of full fat soy flour, imitation nuts, famine relief feeding, texturized vegetable protein (TVP), and deactivation of enzymes in cereal and oilseeds.
Twin-Screw Extruders
Twin screw extruders consist of two parallel screws in a barrel with a figure-eight cross section. The use of twin-screw extruders for food processing started in the 1970s, with an expanding number of applications in the 1980s. Twin-screw extruders are generally one and one-half times or more expensive than single a screw machine of the same capacity (Lusas and Riaz, 1994). Yet the degree of quality control and processing flexibility they offer can make them attractive to food industries. Twin screws produce a more uniform flow of the product through the barrel due to the positive pumping action of the screw flights. Some other advantages of twin screw are:
- Handle viscous, oily, sticky or very wet material and some other products, which will slip in single screw extruder (for example it is possible to add up to 25% fat in a twin-screw extruder)
- Less wear in a smaller part of the machine than in single screw extruder
- Wide range of particle size (from fine powder to grains) may be used, whereas single screw is limited to a specific range of particle size.
- Because of the self-wiping characteristics clean up is very easy.
Four types of twin-screw extruders are possible:
- Non-intermeshed, co-rotating
- Non-intermeshed, counter rotating
- Intermeshed, co-rotating
- Intermeshed, counter rotating
Advantages of Extrusion
Extrusion technology provides several different advantages over traditional methods of food and snack processing. Some of these advantages as described by Smith (1971) and Riaz (2000) include the following.
Adaptability: Ample varieties of products are feasible by changing the minor ingredients and the operation conditions of the extruder. The extrusion process is remarkably adaptable and can accommodate consumer demands for new products.
Product characteristics: A variety of shapes, texture, colour and appearances can be produced, which is not easily done using other production methods.
Energy efficient: Extruders operate at relatively low moisture while cooking food products, so less re-drying is required.
Low cost: Extrusion has a lower processing cost than other cooking and forming processes. We can save 19 percent raw material, 14 percent labour and 44 percent capital investment (Darrington, 1987).
Less Space: Extrusion processing need less space per unit of operation than other cooking systems.
New foods: Extrusion can modify protein (vegetable and animal), starches (almost all sources), and other food material to produce a variety of new and unique snack food products.
High productivity and automated control: An extruder provides a continuous high-throughput processing and we can have a fully automated control for extruders.
High product quality: Since extrusion is a HTST heating process, it minimizes degradation of food nutrients, while improving the digestibility of proteins (by denaturing) and starches (by gelatinizing). Extrusion cooking at high temperature also destroys anti-nutritional compounds, such as trypsin inhibitors, and undesirable enzymes, such as lipases, lipoxidases and microorganisms.
No effluent: Very few process effluents are produced.
Types of Snack Foods
Each snack processor may use a specific unit operation and somewhat different technologies to produce unique snacks. There are many ways to classify snacks. Manufacturers use three main terms to identify snacks:
Types of Extruded Snacks
Simply Extruded Snacks
This category has the greatest potential for growth among snack foods. These snacks can be made to produce innovation, which captures consumer imagination. Some examples are three-dimensional snacks, a variety of animals, cartoon, and alphabets shapes etc. Producing a successful snack is a fine balance between the consumer’s needs, like tastes and interests, and a manufacturer’s production abilities, economics and quality control. Raw material costs play an important role in the finished product’s selling price. Therefore it is advantageous to use the lowest cost raw material to produce a successful snack.
Expanded Snacks
The majority of extruded snacks are in this category. This group is also referred to as “collet” or “second generation snacks”. In general, expanded snacks are made on high-shear extruders. These are high-fiber, high-protein, and low calorie snacks. Some examples are corn curls, onion rings, three dimensional snacks and potato sticks. These types of snacks can be seasoned with a variety of different flavors, oils, salt, sugars, etc. The quality of an expansion-cooked product depends upon the conditions of operation of the extruder and the main raw material used in the formulation. Several other factors can influence the degree of puffing of snacks during extrusion, for example the amount of moisture in the feed material, dough residence time in the extruder barrel and cereal particle size.
Fried Collets
These are the most familiar extruded snacks in the market. A special die arrangement gives the product a twisted, puffed shape. These collets are made on collet extruders. The product is then fried in vegetable oil, and coated with cheese and some other flavor. During frying, the moisture level in the product reduces from 8 percent to 1-2 percent. The most common material used for fried collet is corn meal. Some other cereal grains can also be used for this type of product.
Baked Collets
Baked collets are another example of expanded, extruded snacks. This includes products such as baked corn curls, onion rings and potato sticks. Baked collets can be made with different cereal grains and tuber flours. Protein, fibers, cellulose, and bran can be blended with cereal grain up to 20 percent to make healthy snacks (Huber and Rockey, 1996). Potato sticks are usually made by mixing potato flour with corn or rice flour.
Third-Generation Snacks
Third generation (3G) snacks, also referred to as “half products” or pellets, provide an alternative to fully prepared puffed snack foods. Third generation snacks or half products are extrusion cooked, and formed at low pressure to prevent expansion, and then dried to a final moisture content of about 10 percent to form a glassy pellet. In developing third generation snacks, "half" of the process is completed to prepare "pellets" which are shelf-stable for periods of up to a year without refrigeration, provided they are properly packed to retain their moisture.
Many types of proteins and protein enrichments may be added to third generation snack recipes, such as meats (whole fresh shrimp, fresh chicken, beef, etc), dairy products (cheese, yoghurt, milk solids) and legume proteins (soy, pea, bean). Levels of up to 30-35 percent may be added and still maintain high quality final products (Sunderland, 1996).
Several minor ingredients have very useful effects on the texture, quality and flavor of the final products. Salt is very useful in assisting with uniform moisture migration throughout the third generation pellet after drying during the moisture equilibration period. Baking soda will give special flavor and textural attributes to the finished products after frying, puffing or microwaving. Oils or emulsifiers reduce stickiness during cutting and other processing steps (Huber, 2002).
Drying is very critical in the production of good quality third generation snacks. Proper drying will reduce the moisture content of the pellet to approximately 12 percent. Temperatures of 70-95° C and retention times of one to three hours are required. These products are economical to run and have built-in marketability due to their high-bulk density. Third generation snacks can be prepared in homes or restaurants. Unlike typical snack foods, half-products do not yet contain oil that can oxidize to give off-flavor to the products. These pellets can be shipped from a central manufacturing distribution point, held until needed for the market, and then puffed, flavored and packed fresh and locally.
New variations of third generation snacks are arriving, using infrared heating, hot air, or microwaving. The use of hot air systems reduces the oil uptake that occurs in frying and allows a controlled addition of oil to be made as required for flavoring. With consumer concerns about fats and oils, a half product snack that expands using hot air, offers snack food manufacturers an oil-free snack with perceived health benefits. Elimination of frying oils reduces calories and allows a marketing opening for snacks with a "lite" image.
With the multidimensional snack system, a wide range of raw ingredients can be used to blend together to make an excellent formulation for many types of third generation snacks. The extruder feed must contain a high level of starch to maximize expansion of the collet during exposure to hot oil or air. Levels of 60 percent or less total starch in the formula give only slight expansion in the puffing step and yield a final product with a crunchy, hard texture. Wheat, corn and tubers are widely grown crops in both developing and industrialized countries, and they are cheaper and more easily available in the market than the other cereal crops.
Co-Extruded Snacks
This is a relatively new technology introduced in 1984 to the snack food industry. In this process, two different materials are extruded from one die. The two materials can come from two extruders or from one extruder and one pump. This process can produce a snack with two different flavors, or two textures or two colors. The most common snack produced by co-extrusion is a cereal-based outer tube with a cheese filling inside. There are three basic types of co-extruded snacks in the market; cereal-based tubes with cereal-based fillings, cereal-based tubes with fat-based fillings, and cereal-based tubes with water-based fillings. The shelf life of these snacks is limited, because of migration of moisture and/or oil from the filling to the outer shell.
Common Ingredients
Presently, snack products are being made from a variety of ingredients. However, the selection of ingredients was limited by equipment availability. The introduction of extrusion processing, and better knowledge of extrusion technology, have led to more diverse and complex formulations for snack foods.
The most common source of ingredients is corn, wheat, rice, potato, tapioca, and oats. This is not an inclusive list and someone should not limit his/her snack food formulation based on these ingredients. There are several other sources of ingredients for snack food all over the world. A major ingredient in snack formulation is starch. In its natural form, the starch is insoluble, tasteless, and unsuited for human use. To make it digestible and acceptable, it must be cooked.
Cereal Sources
Almost any cereal can be extruded, but if expansion is a major objective, the numbers of functional cereals are limited to de-germed corn/grits and rice. Cereals that have high amounts of lipids are more difficult to expand due to dough slippage within the extruder barrel. This type of cereal usually requires high moisture and high temperature before significant puffing will occur. In general, starches with 5-20 percent amylose content will significantly improve expansion as well as the texture of snack foods. The most common cereals used in snack food formulations are described below.
Corn
Extruded snacks are a growing segment of the corn-based market. Corn (Maize) is a primary ingredient for corn collets and many pellet products. For most corn-based extruded snacks, dry-milled corn meal is used. Large quantities of corn meal are used in puffed extruded snack production and some is used in corn chips. Cornmeal, corn grits, corn flour and corn cones are all different forms of dry-milled dent corn, and in general vary only in particle size distribution. Selection of the granulation depends upon the type of snack and type of extruder. For example, for fine texture and cell structure, or softer bite, a fine granulation of corn meal should be used. For a crunchy texture snack with a slightly large cell structure, coarse granulation of corn meal is desired. Similarly, the twin screw extruder can handle fine as well as coarse granulation corn flour, while collect extruders require coarse granulation. Mostly, de-germed corn is used in extruded snacks because it expands better than whole corn
Wheat
In snack food industry, wheat flour is used for baked/fried snacks, flavoured crackers, snack cakes, pretzels, bread, and the like. Semolina (coarse particles), usually produced from milling hard wheat, is also used in snack foods. The semolina product had an expansion ratio and bulk density about the same as corn meal. Snack foods with all-semolina will produce a very crispy texture. Wheat starch granules are fairly large (20-40 µm) as compared to other cereal grain starches. In extruded snacks, wheat gluten provides nutritional value, crispness and desired texture. Milling by-products of wheat (bran) can be used with soy protein and some other ingredients to produced expanded snack foods of high nutritional and fibre value. The use of wheat in snack food formulation is limited because of cost.
Rice
Rice is the one of the largest crops in the world. In the US, rice ingredients are not commonly used in snack food formulation. In Japan, however, most snacks are made with rice or rice flour. One major reason is the cost of rice compared to other snack food ingredients. Broken rice can be used as an ingredient in expanded or puffed snack products, since rice has a good expansion quality. Rice starch granules are the smallest (2-8µm) of all grain starches and it digests very easily. The functional properties of rice are very different from corn or wheat starches. Selection of the rice starch in the snack food formulation will depend on the amylose content of the common rice varieties. Chips made with 100 percent rice flour absorb 20-30 percent less oil during frying. In a formulation where a rice and potato blend is used, the potato flavor and texture remain distinctive even though it is mixed with the less costly rice blend.
Oats
In general, oats are marketed as rolled oats or as an ingredient for breakfast cereal. Oats have not been used in grain-based snacks as have wheat and corn. Recent discoveries that oat bran can reduce serum cholesterol levels in humans have boosted the use of oats in the snack food industry. The major problem with oats is high oil content (7-9 percent) and lipase enzymes.
Before using oats in snack food formulation, it is desirable to inactivate the lipase. Otherwise, lipase will catalyze the hydrolysis of oil, which would lead to the production of bitter tasting free fatty acid. Oat starch granules are comparatively small (2-12 µm) in size as compared to other starches. Amylose content of oats varies from 16-27 percent. Oat starch has very strong flavour and it gives a light brown color to the snack product. It requires relatively low gelatinization temperatures, but higher amount of energy input for cooking because of higher oil content. Oats contain high level of fibre.
Snacks extruded with oat starch expand poorly. For this reason it has only found its way into products at a low level. By using longer barrel extruders with pre-conditioner, a higher level of oats can be used in snack foods. Among the snacks that have traditionally included oats in their formulation are cookies and granola. With new technologies and more interest in oats due to health claims, oat-based snack products may be popular in the future.
Barley
Barley is used in small quantity in some snack food formulations. It has a mild flavor and nutritionally it is almost the same as wheat, except it contains considerably more fibre. Barley starch granules are medium to large in size as compared to other cereals. A reasonable amount of expansion can be obtained during extrusion of snack food using barley starch. It gives a light brown to gold colour to the product. Snack food formulation containing barley starch needs low cooking temperatures during extrusion. Barley fibre can be used in healthy snack foods as a fiber supplement. Some manufacturers use barley in multi-grain snack foods in order to add one extra cereal on the label.
Other Cereal Sources
Cereals such as rye, sorghum, millet, amaranth, and triticale has been used in snack foods. However, these cereals are not major ingredients in the snack food formulation.
Tuber Sources
Roots and tubers belong to the class of foods that provide energy in the human diet in the form of carbohydrates. According to a recent FAO estimate, virtually every country in the world grows some species of root crops. Potato and tapioca (cassava) are two main tuber crops used for extruded snack foods.
Potato
Different forms of potatoes (granules, flake, flours and starches) are used in snack food formulations. The potato granules are made from diced potatoes, which are tempered to allow enzymes to soften cell walls before cooking and drying. The dried granules contain the natural cell structures of the potato.
Potato flour and flakes are made by roller drying a cooked potato slurry on hot rolls with small amounts of monoglycerides to inhibit excessive starch adhesion to metal surfaces. Potato starch is often used in snacks to provide extra expansion. Potato starch has a wide range of sizes with some larger granules (60-100µm) than the other cereals. This starch contains 20-25 percent amylose and has very low oil content. Potato starch develops high viscosity during extrusion cooking. It has an excellent swelling and binding power. In snack food it has a definite flavour and it gives a gold to light brown colour to the product. It requires low cooking temperatures, since the granules break down easily. Potato flour is the major ingredient for two common snack products, direct expanded snacks (the product looks like French fries) and fabricated chips.
Tapioca
Tapioca (cassava) is a basic source of low calories, or as a supplement to cereal. In general, tapioca starch is used in third generation snack food formulations. Tapioca starch grains vary in shape and size from 5-35 µm. The amylose content is about 17 percent. Good quality starch should have a pH of 4.7-5.3, a moisture content of 10-13.5 percent and should be uniformly white in color. Tapioca starches develop very high viscosity and it is an excellent binder with a bland flavour. Tapioca requires moderate cooking temperatures during extrusion cooking.
REFERENCES
Dr Mian Riaz is Head of the Extrusion Technology Program at Texas A&M University, College Station, Texas, USA. He can be contacted at [email protected] or Tel: +1 979 845 2774 and Fax: +1 979 845 2744. The Extrusion Technology Program website is www.tamu.edu/extrusion.
Extrusion technology provides several different advantages over traditional methods of food and snack processing. Some of these advantages as described by Smith (1971) and Riaz (2000) include the following.
Adaptability: Ample varieties of products are feasible by changing the minor ingredients and the operation conditions of the extruder. The extrusion process is remarkably adaptable and can accommodate consumer demands for new products.
Product characteristics: A variety of shapes, texture, colour and appearances can be produced, which is not easily done using other production methods.
Energy efficient: Extruders operate at relatively low moisture while cooking food products, so less re-drying is required.
Low cost: Extrusion has a lower processing cost than other cooking and forming processes. We can save 19 percent raw material, 14 percent labour and 44 percent capital investment (Darrington, 1987).
Less Space: Extrusion processing need less space per unit of operation than other cooking systems.
New foods: Extrusion can modify protein (vegetable and animal), starches (almost all sources), and other food material to produce a variety of new and unique snack food products.
High productivity and automated control: An extruder provides a continuous high-throughput processing and we can have a fully automated control for extruders.
High product quality: Since extrusion is a HTST heating process, it minimizes degradation of food nutrients, while improving the digestibility of proteins (by denaturing) and starches (by gelatinizing). Extrusion cooking at high temperature also destroys anti-nutritional compounds, such as trypsin inhibitors, and undesirable enzymes, such as lipases, lipoxidases and microorganisms.
No effluent: Very few process effluents are produced.
Types of Snack Foods
Each snack processor may use a specific unit operation and somewhat different technologies to produce unique snacks. There are many ways to classify snacks. Manufacturers use three main terms to identify snacks:
- First generation snacks: In this category all the natural products used for snacking, nuts, potato chips and popped popcorn are included.
- Second generation snacks: The majority of the snacks fall in this category. All the single ingredient snacks, simple shaped products like corn tortilla chips and puffed corn curls and all directly expanded snacks are included in this category.
- Third generation snacks, also called half-products or pellets: In this category, multi ingredient formed snacks and pellets, made by extrusion cooking are included. Most snack manufacturers use some form of existing technology as the basis for creating snack products, but incorporating variation is very necessary, especially to increase the snack’s health image appeal by lowering fat and calories or adding nutrients (Kuntz, 1996). This variation can be accomplished by using extrusion technology.
Types of Extruded Snacks
Simply Extruded Snacks
This category has the greatest potential for growth among snack foods. These snacks can be made to produce innovation, which captures consumer imagination. Some examples are three-dimensional snacks, a variety of animals, cartoon, and alphabets shapes etc. Producing a successful snack is a fine balance between the consumer’s needs, like tastes and interests, and a manufacturer’s production abilities, economics and quality control. Raw material costs play an important role in the finished product’s selling price. Therefore it is advantageous to use the lowest cost raw material to produce a successful snack.
Expanded Snacks
The majority of extruded snacks are in this category. This group is also referred to as “collet” or “second generation snacks”. In general, expanded snacks are made on high-shear extruders. These are high-fiber, high-protein, and low calorie snacks. Some examples are corn curls, onion rings, three dimensional snacks and potato sticks. These types of snacks can be seasoned with a variety of different flavors, oils, salt, sugars, etc. The quality of an expansion-cooked product depends upon the conditions of operation of the extruder and the main raw material used in the formulation. Several other factors can influence the degree of puffing of snacks during extrusion, for example the amount of moisture in the feed material, dough residence time in the extruder barrel and cereal particle size.
Fried Collets
These are the most familiar extruded snacks in the market. A special die arrangement gives the product a twisted, puffed shape. These collets are made on collet extruders. The product is then fried in vegetable oil, and coated with cheese and some other flavor. During frying, the moisture level in the product reduces from 8 percent to 1-2 percent. The most common material used for fried collet is corn meal. Some other cereal grains can also be used for this type of product.
Baked Collets
Baked collets are another example of expanded, extruded snacks. This includes products such as baked corn curls, onion rings and potato sticks. Baked collets can be made with different cereal grains and tuber flours. Protein, fibers, cellulose, and bran can be blended with cereal grain up to 20 percent to make healthy snacks (Huber and Rockey, 1996). Potato sticks are usually made by mixing potato flour with corn or rice flour.
Third-Generation Snacks
Third generation (3G) snacks, also referred to as “half products” or pellets, provide an alternative to fully prepared puffed snack foods. Third generation snacks or half products are extrusion cooked, and formed at low pressure to prevent expansion, and then dried to a final moisture content of about 10 percent to form a glassy pellet. In developing third generation snacks, "half" of the process is completed to prepare "pellets" which are shelf-stable for periods of up to a year without refrigeration, provided they are properly packed to retain their moisture.
Many types of proteins and protein enrichments may be added to third generation snack recipes, such as meats (whole fresh shrimp, fresh chicken, beef, etc), dairy products (cheese, yoghurt, milk solids) and legume proteins (soy, pea, bean). Levels of up to 30-35 percent may be added and still maintain high quality final products (Sunderland, 1996).
Several minor ingredients have very useful effects on the texture, quality and flavor of the final products. Salt is very useful in assisting with uniform moisture migration throughout the third generation pellet after drying during the moisture equilibration period. Baking soda will give special flavor and textural attributes to the finished products after frying, puffing or microwaving. Oils or emulsifiers reduce stickiness during cutting and other processing steps (Huber, 2002).
Drying is very critical in the production of good quality third generation snacks. Proper drying will reduce the moisture content of the pellet to approximately 12 percent. Temperatures of 70-95° C and retention times of one to three hours are required. These products are economical to run and have built-in marketability due to their high-bulk density. Third generation snacks can be prepared in homes or restaurants. Unlike typical snack foods, half-products do not yet contain oil that can oxidize to give off-flavor to the products. These pellets can be shipped from a central manufacturing distribution point, held until needed for the market, and then puffed, flavored and packed fresh and locally.
New variations of third generation snacks are arriving, using infrared heating, hot air, or microwaving. The use of hot air systems reduces the oil uptake that occurs in frying and allows a controlled addition of oil to be made as required for flavoring. With consumer concerns about fats and oils, a half product snack that expands using hot air, offers snack food manufacturers an oil-free snack with perceived health benefits. Elimination of frying oils reduces calories and allows a marketing opening for snacks with a "lite" image.
With the multidimensional snack system, a wide range of raw ingredients can be used to blend together to make an excellent formulation for many types of third generation snacks. The extruder feed must contain a high level of starch to maximize expansion of the collet during exposure to hot oil or air. Levels of 60 percent or less total starch in the formula give only slight expansion in the puffing step and yield a final product with a crunchy, hard texture. Wheat, corn and tubers are widely grown crops in both developing and industrialized countries, and they are cheaper and more easily available in the market than the other cereal crops.
Co-Extruded Snacks
This is a relatively new technology introduced in 1984 to the snack food industry. In this process, two different materials are extruded from one die. The two materials can come from two extruders or from one extruder and one pump. This process can produce a snack with two different flavors, or two textures or two colors. The most common snack produced by co-extrusion is a cereal-based outer tube with a cheese filling inside. There are three basic types of co-extruded snacks in the market; cereal-based tubes with cereal-based fillings, cereal-based tubes with fat-based fillings, and cereal-based tubes with water-based fillings. The shelf life of these snacks is limited, because of migration of moisture and/or oil from the filling to the outer shell.
Common Ingredients
Presently, snack products are being made from a variety of ingredients. However, the selection of ingredients was limited by equipment availability. The introduction of extrusion processing, and better knowledge of extrusion technology, have led to more diverse and complex formulations for snack foods.
The most common source of ingredients is corn, wheat, rice, potato, tapioca, and oats. This is not an inclusive list and someone should not limit his/her snack food formulation based on these ingredients. There are several other sources of ingredients for snack food all over the world. A major ingredient in snack formulation is starch. In its natural form, the starch is insoluble, tasteless, and unsuited for human use. To make it digestible and acceptable, it must be cooked.
Cereal Sources
Almost any cereal can be extruded, but if expansion is a major objective, the numbers of functional cereals are limited to de-germed corn/grits and rice. Cereals that have high amounts of lipids are more difficult to expand due to dough slippage within the extruder barrel. This type of cereal usually requires high moisture and high temperature before significant puffing will occur. In general, starches with 5-20 percent amylose content will significantly improve expansion as well as the texture of snack foods. The most common cereals used in snack food formulations are described below.
Corn
Extruded snacks are a growing segment of the corn-based market. Corn (Maize) is a primary ingredient for corn collets and many pellet products. For most corn-based extruded snacks, dry-milled corn meal is used. Large quantities of corn meal are used in puffed extruded snack production and some is used in corn chips. Cornmeal, corn grits, corn flour and corn cones are all different forms of dry-milled dent corn, and in general vary only in particle size distribution. Selection of the granulation depends upon the type of snack and type of extruder. For example, for fine texture and cell structure, or softer bite, a fine granulation of corn meal should be used. For a crunchy texture snack with a slightly large cell structure, coarse granulation of corn meal is desired. Similarly, the twin screw extruder can handle fine as well as coarse granulation corn flour, while collect extruders require coarse granulation. Mostly, de-germed corn is used in extruded snacks because it expands better than whole corn
Wheat
In snack food industry, wheat flour is used for baked/fried snacks, flavoured crackers, snack cakes, pretzels, bread, and the like. Semolina (coarse particles), usually produced from milling hard wheat, is also used in snack foods. The semolina product had an expansion ratio and bulk density about the same as corn meal. Snack foods with all-semolina will produce a very crispy texture. Wheat starch granules are fairly large (20-40 µm) as compared to other cereal grain starches. In extruded snacks, wheat gluten provides nutritional value, crispness and desired texture. Milling by-products of wheat (bran) can be used with soy protein and some other ingredients to produced expanded snack foods of high nutritional and fibre value. The use of wheat in snack food formulation is limited because of cost.
Rice
Rice is the one of the largest crops in the world. In the US, rice ingredients are not commonly used in snack food formulation. In Japan, however, most snacks are made with rice or rice flour. One major reason is the cost of rice compared to other snack food ingredients. Broken rice can be used as an ingredient in expanded or puffed snack products, since rice has a good expansion quality. Rice starch granules are the smallest (2-8µm) of all grain starches and it digests very easily. The functional properties of rice are very different from corn or wheat starches. Selection of the rice starch in the snack food formulation will depend on the amylose content of the common rice varieties. Chips made with 100 percent rice flour absorb 20-30 percent less oil during frying. In a formulation where a rice and potato blend is used, the potato flavor and texture remain distinctive even though it is mixed with the less costly rice blend.
Oats
In general, oats are marketed as rolled oats or as an ingredient for breakfast cereal. Oats have not been used in grain-based snacks as have wheat and corn. Recent discoveries that oat bran can reduce serum cholesterol levels in humans have boosted the use of oats in the snack food industry. The major problem with oats is high oil content (7-9 percent) and lipase enzymes.
Before using oats in snack food formulation, it is desirable to inactivate the lipase. Otherwise, lipase will catalyze the hydrolysis of oil, which would lead to the production of bitter tasting free fatty acid. Oat starch granules are comparatively small (2-12 µm) in size as compared to other starches. Amylose content of oats varies from 16-27 percent. Oat starch has very strong flavour and it gives a light brown color to the snack product. It requires relatively low gelatinization temperatures, but higher amount of energy input for cooking because of higher oil content. Oats contain high level of fibre.
Snacks extruded with oat starch expand poorly. For this reason it has only found its way into products at a low level. By using longer barrel extruders with pre-conditioner, a higher level of oats can be used in snack foods. Among the snacks that have traditionally included oats in their formulation are cookies and granola. With new technologies and more interest in oats due to health claims, oat-based snack products may be popular in the future.
Barley
Barley is used in small quantity in some snack food formulations. It has a mild flavor and nutritionally it is almost the same as wheat, except it contains considerably more fibre. Barley starch granules are medium to large in size as compared to other cereals. A reasonable amount of expansion can be obtained during extrusion of snack food using barley starch. It gives a light brown to gold colour to the product. Snack food formulation containing barley starch needs low cooking temperatures during extrusion. Barley fibre can be used in healthy snack foods as a fiber supplement. Some manufacturers use barley in multi-grain snack foods in order to add one extra cereal on the label.
Other Cereal Sources
Cereals such as rye, sorghum, millet, amaranth, and triticale has been used in snack foods. However, these cereals are not major ingredients in the snack food formulation.
Tuber Sources
Roots and tubers belong to the class of foods that provide energy in the human diet in the form of carbohydrates. According to a recent FAO estimate, virtually every country in the world grows some species of root crops. Potato and tapioca (cassava) are two main tuber crops used for extruded snack foods.
Potato
Different forms of potatoes (granules, flake, flours and starches) are used in snack food formulations. The potato granules are made from diced potatoes, which are tempered to allow enzymes to soften cell walls before cooking and drying. The dried granules contain the natural cell structures of the potato.
Potato flour and flakes are made by roller drying a cooked potato slurry on hot rolls with small amounts of monoglycerides to inhibit excessive starch adhesion to metal surfaces. Potato starch is often used in snacks to provide extra expansion. Potato starch has a wide range of sizes with some larger granules (60-100µm) than the other cereals. This starch contains 20-25 percent amylose and has very low oil content. Potato starch develops high viscosity during extrusion cooking. It has an excellent swelling and binding power. In snack food it has a definite flavour and it gives a gold to light brown colour to the product. It requires low cooking temperatures, since the granules break down easily. Potato flour is the major ingredient for two common snack products, direct expanded snacks (the product looks like French fries) and fabricated chips.
Tapioca
Tapioca (cassava) is a basic source of low calories, or as a supplement to cereal. In general, tapioca starch is used in third generation snack food formulations. Tapioca starch grains vary in shape and size from 5-35 µm. The amylose content is about 17 percent. Good quality starch should have a pH of 4.7-5.3, a moisture content of 10-13.5 percent and should be uniformly white in color. Tapioca starches develop very high viscosity and it is an excellent binder with a bland flavour. Tapioca requires moderate cooking temperatures during extrusion cooking.
REFERENCES
- Darrington, H. A long-running cereal. Food Manuf. 1987. 3:47.
- Harper, J. M. Extrusion of Food. Vol. 1. CRC Press Inc. Boca Raton, FL. 1981. p127-128.
- Huber, G and Rokey G.J. Extruded Snacks. In “Snack Food” Ed. Booth, R. G. Published by Van Nostrand Reinhold, NY. 1990. P. 107-138.
- Huber, G. R. Recent developments and trends in the snack food industry. In M. Riaz, L. Rooney and M. Barron Eds., Snack Food Processing Shot Course Manual. Food Protein Research & Development Center, Texas A&M University, College Station, TX. 2002.
- Kuntz, L. Creating healthful salty snacks. Food Product Design. 1996. Vol. 6(9).31-56.
- Lusas, E. W. and Riaz, M. N. An introduction to extruders and extrusion principles. Extrusion Communiqué. 1994. 7(4):9.
- Riaz, M.N. Extruders in Food Application. CRC Press. Boca Raton, FL. Originally published by Technomic Publishing. 2000
- Rossen, J. L. and Miller, R. C. Food extrusion. Food Technol., 1973. 27:46-53.
- Smith, O.B. Why use extrusion. Symposium on Extrusion: Process and Product Development. American Association of Cereal Chemists. St. Paul. MN. 1971.
- Sunderland, R. Production of Third Generation Snacks. Cereal Food Word. 1996. (41)1:12-14.
Dr Mian Riaz is Head of the Extrusion Technology Program at Texas A&M University, College Station, Texas, USA. He can be contacted at [email protected] or Tel: +1 979 845 2774 and Fax: +1 979 845 2744. The Extrusion Technology Program website is www.tamu.edu/extrusion.