Developments and Trends
in Extruded Snacks
By Gordon R. Huber
Developments in the snack-food industry are numerous and ever-changing. So much so, that it would be a difficult task to cover each of the developments in these pages. However, four areas of discussion will hopefully give a general idea of the recent extruded-snack trends in the industry:
- Multidimensional snack food production
- Use of super/subcritical fluids in snack food production
- New applicator/dryers for low- or no-fat applications
- New health benefits for extruded snack foods
Multidimensional snack basics
Third-generation snack products or pellets, sometimes referred to as semi- or half-products, are not new to the snack-food industry. In fact, they have been very popular in many regions of the world. Extrusion systems for the production of multidimensional third-generation snacks are efficient, economical to run and result in a product with built-in marketing flexibility due to long shelf-life and high bulk density prior to frying or puffing.
For those not familiar with this type of product, ingredients are blended together and then passed through a cooking extruder where they are cooked and formed into a dense pellet. Following extrusion-cooking and forming, the pellets are dried to a stable moisture content to assure shelf stability. Once dried, the pellets may be distributed to a snack processor where they are expanded or puffed by immersion in hot oil, heating with hot air, hot salt, infrared heating or microwaving. The expanded products then are seasoned with salt and/or various spices, packaged and sold to the consumer as a ready-to-eat (RTE) snack. The pellets also may be sold directly to the consumer for in-home preparation. This type of snack adds new dimensions to its marketing potential because of its high bulk density and stability for transporting to various smaller distribution sights.
Producing a successful product is a fine balance between the consumers’ needs, likes, tastes and interests vs. a manufacturer’s production capabilities, economics and quality control. For example, raw-material cost has a large impact on the finished product’s selling price. Therefore, it is an advantage to use the lowest-cost ingredients. However, if a desirable or sellable final product cannot be made with the lower-cost ingredients, the customer won’t buy the product — essentially eliminating the market for that product.
With the multidimensional snack system, a wide range of raw ingredients may be selected and blended together to make excellent recipes for many types of third-generation snacks. Generally, the combination of ingredients contains relatively high starch levels to maximize expansion of the final product during exposure to hot oil, air, salt, infra-red or microwaving. Levels of 60% or less of total starch in a recipe will result in less final-product expansion, and will yield a final product with an increased crunchiness and firmer texture. Examples of this type of product would be those containing mainly whole cereal grains, such as whole ground corn or masa (as in traditional corn-chip snacks).
As the total starch level in a third-generation product is increased above 60%, the final product will yield more expansion, resulting in lighter and softer textures. Shortenings, vegetable oils, salts and occasionally emulsifiers are included in recipes as processing aids, to reduce stickiness, control expansion and to impart a more uniform cellular structure in the final product. Monoglycerides may reduce expansion significantly during secondary puffing if they are used at levels of 0.5% or above.
When designing recipes, careful consideration must be given to the selection of cereal grains, starches, proteins and other minor ingredients. For example, starch makes many contributions to the final product, including expansion, binding, caloric value, functionality, flavor, viscosity development and resilience.
It also is important to consider ingredients available locally because ingredient cost is the major production expense. For example, in the United States, corn or wheat are major starch sources, while in Europe, potato is a major source, and Asia uses tapioca or rice as a major starch source.
Each individual starch source has its strong points. The five main cereal-grain starch options include rice (long, medium or short grain), wheat (soft, hard or durum), corn (white or yellow), barley and oats. Rice offers the smallest granule size and is the most-digestible when cooked; its bland flavor is easily flavored; it has good expansion, a white color and the highest energy requirements for cooking. Wheat has fairly large granules and provides good expansion and mild flavor; it has a white- to off-white color and medium to low energy requirements for cooking. With medium-size granules, corn offers good expansion, definite flavor, yellow color and medium to high energy requirements. Barley has medium- to large-size granules, which provide fair expansion; it has light-brown to gold color and low energy requirements for cooking. Oat’s small-size granules offer poor expansion, strong flavor, light-brown color and an energy requirement that varies depending on the fat content.
Tuber starches, which are lower in protein, include potato and tapioca. Potato offers very large starch granules that break down easily with definite flavor, excellent swelling and binding power, very high viscosity when cooked, a gold to light-brown color and low energy requirements for cooking. Tapioca, whose medium-size starch granules provide high viscosity, has a bland flavor, white color, excellent binding properties and low to moderate energy requirements.
Many types of proteins and protein enrichments may be added to third-generation snack recipes, such as meats (whole fresh shrimp, chicken or beef), dairy products (cheese, yogurt or milk solids) and legume proteins (soy, pea or bean). Levels up to 30% to 35% may be added and still maintain high-quality final products.
Several minor ingredients have very useful effects on the texture, quality and flavor of the final products. After drying, salt assists with uniform moisture migration throughout the third-generation pellet 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.
Extrusion processing steps
After uniformly blending the dry ingredients, liquid ingredients, such as shortenings, flavors or water, may be applied as a spray in the batch mixer or injected into the preconditioner of the extruder in a cooking-extrusion system. During extrusion-cooking, it is vital that raw materials be completely cooked (unless the recipe contains pregelled starches) if the objective is to maximize the final-product expansion.
A good cook is defined as the combination of temperature, residence time and moisture content during extrusion to fully gelatinize the starchy components. The temperature profile in the extruder — which depends on ingredient characteristics, extruder configuration and processing conditions — will be higher than the gelatinization temperature of the starches in the formula, unless pregelled starches are used in the formula.
Typical extrusion-cooking processing conditions may vary depending on the type and the amount of starches used. Generally, temperatures in the extruder’s cooking zone will range from 80° to 150°C (175° to 302°F) and barrel temperatures in the forming zone will range from 65° to 90°C (150° to 194°F). Extrusion moisture contents will range from 25% to 30% with a residence time of 30 to 90 seconds.
The total energy requirements for producing third-generation snacks may be quite low when using tuber starches or pregelled starches in a formula. However, whole cereal grains or high-protein wheat flours make increased mechanical energy desirable to fully gelatinize the recipe and lower the molecular weight of the starch granules. This allows improved expansion characteristics during frying, hot-air puffing or microwaving.
Thermal energy sources include the use of steam, hot water or other thermal fluids circulated through the jacketed barrel. This provides heat transfer into the extrudate (external heating) and/or steam and hot water may be injected directly into the product in the preconditioner or extruder barrel. The cooking extruder has segmented barrels (heads) and screws for product versatility. Incorporating various head, screw and steamlock designs into the configuration will produce the desired cooking conditions.
For example, recipes consisting mostly of tuber starches require low-shear screw configurations to make pellets that yield a light-density, soft-textured final product. However, large amounts of wheat flour require screw configurations designed to impart higher levels of mechanical energy into the pellets to make the same light-density, soft-textured final product.
Following this cooking step, the material is passed into a venting zone and then into a forming extrusion zone, which cools and densifies the cooked, plasticized mass. This forming step may be accomplished in a separate extruder or in a secondary zone of the cooking extruder.
This forming portion of the process contains a low-shear screw configuration containing a minimum of restrictions, except for the final die. The forming zone usually is characterized by a positive, forward-transport screw configuration complemented by maximum cooling to reduce product temperature to between 70° and 95°C (158° to 203°F).
The cooled, viscoelastic product then is shaped by the final forming die which contains sufficient open area to prevent excessive pressure build-up and thus expansion of the cooked dough. Many different shapes and sizes of third-generation snacks may be produced. The cooked, densified, shaped product extruded through the former die contains between 20% and 28% moisture after a slight flashoff during the small decompression step.
For multidimensional snack products, the viscoelastic dough is formed into sheets that are sized to the proper thickness, and then embossed with a pattern to give a specified design to the product’s surface. The sheets then are brought together and passed through a rotary die cutter, which cuts the product into the desired shape. In many cases, multicolored products are made with two extruders feeding one common die. This makes it possible to laminate two sheets having different formulas; and therefore, two completely different textures may be incorporated into the same product piece. Stenciling or imprinting logos, designs or even wording on the sheets provides further uniqueness.
Proper drying reduces the moisture content of the pellet to approximately 9% to 11% moisture, which is most suitable for frying. The drying step is critical to the production of good-quality third-generation snacks. Drying temperatures of 65° to 85°C (150° to185°F), humidity control and retention times of one to three hours accomplish this important processing step. Immediately after exiting the dryer, some moisture in the extruded pellet is distributed close to the product’s periphery, but most of it still is located in the pellet’s central points. Various studies and experience show that properly dried third-generation snack pellets fry up or expand more uniformly following an equilibration period of one or two days, during which time the moisture migrates to reach an equilibrium within the final product.
Prior to consumption, third-generation snacks are fried in hot oil, or expanded in hot air, hot salt, a microwave or infrared oven. The frying procedure usually involves complete immersion of the pellet in 165° to 200°C (325° to 390°F) oil for 10 to 40 seconds (depending on the product recipe). The expansion of the pellets with hot air, infrared heating, sand or salt-puffing has gained popularity due to the final product’s low caloric content.
The frying process can be divided into three phases. During the first phase, the periphery of the snack pellet loses moisture into the oil. The snack pellet becomes warmer and more plastic in texture. Heat penetrates to the center of the pellet and the moisture distributed throughout the pellet turns to steam. A thoroughly cooked and properly formed product will have gas-retaining properties that are critical to the expanded product’s textural development. The moisture in the product will evaporate due to the oil’s high temperature and expand inside the product where it is trapped. This process results in an expanded product having fine cell structure with a desirable mouthfeel and texture.
Super- and subcritical fluids
Supercritical fluid injection, coupled with the continuous twin-screw extrusion cooking process, opens many opportunities for new engineered processing techniques for developing new products and product concepts. This supercritical fluid extrusion technology is a patented process that already has resulted in new developments in cereals, confectioneries, pastas, flavorings, pharmaceuticals, snacks and other products left only to the imagination.
“Supercritical” seems to be an eye-catching phrase for many researchers when they read or hear about using CO2 to expand extruded products. A definition and some background of the term supercritical, related to fluids and gases, is important to understanding the terminology and concepts related to this fascinating technology.
Fluids become supercritical when their temperature and pressure are above the critical temperature (TC) and critical pressure (PC). The critical temperature is unique for every fluid and is defined as the temperature above which a gas cannot be liquefied by simply increasing its pressure. The critical pressure, also unique for every fluid, is the gas/liquid equilibrium pressure that corresponds to the critical temperature. The various phases of CO2 include the triple point; the point at which the gas, liquid and solid phases all exist in equilibrium; and the critical point.
Carbon dioxide has been selected as the solvent of choice for the extrusion process and may be used as the solvent for both supercritical extraction and supercritical deposition techniques. In many cases, supercritical CO2 may be used as an alternative to water as an expansion agent simply because of its physical properties. The gas/liquid equilibrium pressure at room temperature (6500 kPa at 25°C) allows us to handle liquid CO2 at a reasonable temperature and pressure, while CO2 becomes supercritical at a temperature and pressure particularly well-suited to the extrusion process. Comparing the critical temperature and pressure of other candidates for alternative expansion agents in the extrusion process reveals that their critical temperature and pressure are not compatible with the pressure and temperature normally encountered in the extrusion process.
Product expansion by CO2 offers several advantages as compared to steam expansion in the extrusion process: a closed cell structure; CO2 does not condense, resulting in cell collapse; and the product’s interior is very nearly oxygen free.
The cell structure of steam-expanded products depends on several extrusion-processing variables, including formula, moisture, energy input, etc. The cell structure of CO2-expanded products also relies on these same variables, but in general will have much smaller, more uniform and closed cells. With steam-expanded products, as the product exits the die and cools, the steam condenses. If the cells have not ruptured and the product has not yet passed from the flowable plastic to the rubbery or glassy state, the condensing steam causes cell collapse just as the vaporizing water caused cell expansion. With CO2 expansion, the gas does not condense and therefore a closed cell structure can be maintained without complete cell collapse.
Extrusion cooking has been recognized as enhancing oxidative rancidity, which results in shortened shelf life. However, in the case of a CO2-expanded product where cell rupture and collapse has not occurred, the cells are filled with CO2 and are nearly oxygen free. If promptly packaged in a modified atmosphere before the CO2 has an opportunity to diffuse through the cell walls letting air (containing oxygen) replace it, a packaged product that is very nearly free of oxygen on both the interior and exterior of the product results.
The question may arise, “Is supercritical CO2, as opposed to liquid CO2, necessary to obtain the benefits of CO2 expansion in the extrusion process?” Ongoing research is searching for answers to this question. Nevertheless, because of the critical temperature and pressure of CO2, you normally will end up operating under temperature and pressure conditions that result in supercritical CO2 within the extruder barrel.
It is believed that one of the important processing characteristics of expanding with CO2 in the extrusion process is its ability to solubilize in water. Carbon dioxide solubility in water increases by increasing pressure and decreasing temperature. The extrusion process will nearly always operate at temperatures above the critical temperature of CO2; therefore, to prevent CO2 in the extruder barrel from flashing to gas, the pressure must also be above the critical pressure. Nothing dramatic happens to its solubility as the pressure is increased above the critical pressure. Therefore, one might conclude that there is nothing “magical” about using supercritical CO2 as an expansion agent as opposed to liquid CO2, but the thermodynamic constraints of the extrusion process simply dictate that you operate under supercritical conditions for CO2.
Water solubility can be transformed to see what amount of CO2can be effectively solubilized in the extrudate in the CO2-extrusion process. The maximum level that can be solubilized in the extrudate under typical conditions is approximately 1.5%. This has been confirmed in experimental trials where the level of CO2 addition was increased at pressures below the critical pressure until it became evident that the extrudate was expanding before it left the die.
Supercritical CO2 or high-pressure liquid CO2 can be generated for injection into the extrusion process by a simple process. Liquid CO2 is drawn from the bottle at the equilibrium temperature and pressure. It is then cooled further into the liquid phase to prevent vaporization on the suction stroke of the compressor. If it vaporizes in or prior to the compressor, the pumping capacity of the compressor dramatically decreases. A pressure control loop around the compressor controls the pressure of the fluid being injected into the extruder. After the compressor, the fluid flows through a second heat exchanger to adjust the temperature so that liquid CO2 may be maintained, or its temperature can be increased to above the critical temperature to obtain a supercritical fluid. After the heat exchanger, the fluid is introduced into the extruder barrel either as a liquid or as a supercritical fluid, depending on the fluid temperature at this point.
New equipment for reducing fat
An integral component in any snack line is an applicator/dryer. Originally designed for sugar-coating and frosting breakfast cereals, this system has gained significant value in the snack-food industry as a superior method of coating snack products with colors and flavors. Snack producers introducing low-fat snacks to the marketplace commonly include applicator/ dryer processing components. This equipment is used in conjunction with fat-free gums, which serve as the adhesion agent instead of fat, to greatly reduce the total caloric content from fat in the finished product. This technology makes it possible to add sweeteners and savory spices without adding fat.
The applicator/dryer consists of a large perforated drum designed for alternating coating and drying zones. Slots in the drum introduce heated air into the drying zone, reducing the moisture of the flavoring or sweetener being applied. The coating and drying is alternated several times during this process to ensure consistent coating of the product and to achieve a proper moisture suitable for packaging or downstream processing. To give the final product a new and improved appearance, nut bits, cinnamon pieces, granola bits, banana bits, savory spices or any granular additive may be added while an adhesive coating is being applied to the base product.
Precise coating application may be accomplished with simultaneous drying to produce a whole new spectrum of products impossible to achieve with other types of coating and enrobing systems. All systems have optional automatic ratio control.
New health benefits
While extruded snacks are not generally touted for health benefits, that may change in the future. Recently, Carol Klopfenstein, Department of Grain Science and Industry, Kansas State University, Manhattan, has been studying the cholesterol-lowering effect of extrusion cooked cereal grains over other means of cooking cereal grains. Initial studies show that extruded products have improved cholesterol-lowering effects over baked products.
A paper presented by Klopfenstein and associates at the 1998 AACC Annual Meeting, showed that animal studies using diets containing several different extruded ingredients, but particularly soy cotyledon fiber, resulted in lower serum-cholesterol levels. A possible mechanism is the redistribution of insoluble to soluble dietary fiber during extrusion, but other studies are underway in an effort to explain the ingredients’ enhanced hypocholesterolemic properties. Klopfenstein will be continuing her work in this area covering a broader range of food products.
No matter what the future brings, it’s easy to see that, from simple corn curls and balls to complex filled products, extrusion gives the snack-food producer a flexibility and choice of processing technologies.
Gordon R. Huber is the director of new concept development for Wenger, Sabetha, KS. With more than 25 years of extrusion experience, he directs Wenger’s extension Technical Centers, the development of new concepts and conducts technical process seminars. Gordon is a member of the AACC and IFT. Wenger specializes in the manufacture of state of the art commercial extrusion cooking systems and ancillary equipment for the food- and grain-processing industries. He can be reached at GHuber@wenger.com. Other inquiries should be directed to firstname.lastname@example.org.
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