Extruded Snack foodsVandana Mishra,Neelam Yadav and Vinita PuanikCentre of Food TechnologyUniversity of Allahabad
Extrusion is a process which combinesseveral unit operations including mixing,cooking, kneading, shearing, shaping andforming.
Extrusion cooking (HTST) is a popular meansof preparing snacks and ready to eat foods.Its advantages include energy efficient, lack of process effluents and versatility with respect to ingredient selection,and the shapes and textures of products thatcan be produced.Extrusion processing is used for processing ofstarch as well as pertinacious material.It is an important food processing technique forpreparation of nutritious food
Snack foods: The snack food processes, which use extrusion cooking, includethe production of direct expanded extrudate and extruded pelletsor half products of the third generation snacks. The extruded half products require a secondary puffing step. Priorto consumption, the third generation snacks are puffed in a fryer,or expanded in hot air or a microwave oven. Snack food technology of direct expanded, and shaped snacks issimilar to RTE cereals processes, but normally performed at lowermoistures, so that a higher energy input from mechanical sourcesoccurs. In general, direct expanded products are made using high-shearextruders. Other categories of the direct expanded foods are crisps or flatbreads, which are produced using the twin-screw extrudertechnology. Also, twin-screw extruders can be used for crackerproduction.
Types of Snack FoodsEach snack processor may use a specific unit operation andsomewhat different technologies to produce unique snacks.There are many ways to classify snacks. Manufacturers use threemain terms to identify snacks: First generation snacks: Simply Extruded SnacksIn this category all the natural products used for snacking,nuts, potato chips and popped popcorn are included. Second generation snacks: ExpandedSnacksThe majority of the snacks fall in this category.All the single ingredient snacks, simple shaped products likecorn tortilla chips and puffed corn curls and all directlyexpanded snacks are included in this category. Third generation snacks, half products” or pelletsIn this category, multi ingredient formed snacks andpellets, made by extrusion cooking are included.
Most snack manufacturers use some form of existingtechnology as the basis for creating snack products, butincorporating variation is very necessary, especially toincrease the snack’s health image appeal by lowering fatand calories or adding nutrients. This variation can beaccomplished by using extrusion technology. Simply Extruded SnacksThis category has the greatest potential for growth amongsnack foods. These snacks can be made to produceinnovation, which captures consumer imagination.Some examples are three-dimensional snacks, a variety ofanimals, cartoon, and alphabets shapes etc.Raw material costs play an important role in the finishedproduct’s selling price. Therefore it is advantageous touse the lowest cost raw material to produce a successfulsnack.
ExpandedSnacks The majority of extruded snacksare in this category. This group is also referred toas “collet” or “second generation snacks”. Expanded snacks are made on high-shear extruders. These are high-fiber, high-protein, and low caloriesnacks. Some examples are corn curls, onion rings, threedimensional snacks and potato sticks. These types of snacks can be seasoned with a varietyof different flavors, oils, salt, sugars,etc. The quality of an expansion-cooked product dependsupon the conditions of operation of the extruder andthe main raw material used in the formulation.
Fried ColletsThese are the most familiar extruded snacks inthe market. A special die arrangement gives the product atwisted, puffed shape. These collets are made on collet extruders. The product is then fried in vegetable oil, andcoated with cheese and some other flavor. During frying, the moisture level in the productreduces from 8 percent to 1-2 percent. The most common material used for fried colletis corn meal. Some other cereal grains can also be used forthis type of product.
Baked ColletsBaked collets are another example ofextruded snacks. This includes products such as baked corncurls, onion rings and potato sticks. Baked collets can be made with differentcereal grains and tuber flours. Protein, fibers, cellulose, and bran can beblended with cereal grain up to 20 percent tomake healthy snacks. Potato sticks are usually made by mixingpotato flour with corn or rice flour.
Third-Generation Snacks These are extrusion cooked, and formed at lowpressure to prevent expansion, and then dried toa final moisture content of about 10 percent toform a glassy pellet. To create formed or pelletized products, a highmoisture dough is used at a lower temperature. When the extrudate is forced through theforming die, the product takes the shape of thedie, instead of expanding. The result is a dense intricately-shaped pellet
Flow diagram for theproduction of secondand third generationsnacks
Second generation snacks or expanded snacks,where most extruded snacks are classified, areusually low in bulk density and are oftenmarketed as high-fibre, low-calorie, high proteinand nutritional products. Different raw materials used to produce thesekinds of snacks (i.e. flours and/or cereals andtubers starches and proteins) are processed in aextruder resulting in a continuous mass, that iscut into pieces of uniform size, being afterwardsdried, flavoured and stored.
Third generation snacks or pellets are produced almostthe same way as second generation snacks, however,when the product exits the extruder, it has the form of thedie, that is, it is not expanded, being dried in this form. The expansion of the product occurs afterwards throughfrying, heating by hot air or in a microwave oven. This kind of product presents a low moisture content(between 7 and 10%), high density and stability to bestored for a long time without microbial damage.
Co-Extruded SnacksThis is a relatively new technology introduced in 1984 to thesnack food industry. Coextrusion is a process that combines two different extrudedstreams to obtain two-component products characterized by adual texture and/or color. The two materials can come from two extruders or from oneextruder 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 acereal-based outer tube with a cheese filling inside. There are three basic types of co-extruded snacks in themarket; cereal-based tubes with cereal-based fillings, cereal-based tubes with fat-based fillings, and cereal-based tubes with water-based fillings.
Fats and oils play an important role in frying, coating andfilling of the extruded snack foods. Coating with oil and seasonings is the final processing step inthe production of snack foods. The seasonings allow the manufacturer to create a variety offlavors. The oil is the carrier of seasonings and gives the extrudate abetter mouth feel. The oil and seasoning coating usuallymakes up to 35% of the finished product. The shelf life of these snacks is limited, because of migrationof moisture and/or oil from the filling to the outer shell.
Common IngredientsExtrusion technology, have led to more diverse andcomplex formulations for snack foods.The most common source of ingredients is corn, wheat, rice, potato, tapioca, and oats.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 becooked.
CerealSourcesAlmost any cereal can be extruded, but ifexpansion is a major objective, the numbers offunctional cereals are limited to de-germedcorn/grits and rice. Cereals that have high amounts of lipids are moredifficult to expand due to dough slippage within theextruder barrel. This type of cereal usually requires high moistureand high temperature before significant puffing willoccur. In general, starches with 5-20 percent amylosecontent will significantly improve expansion as wellas the texture of snack foods.
Rice as a Starch Source1) Small, tightly packed starch granules thathydrate slowly2) Becomes sticky when it gelatinizes3) Choose long grain varieties over mediumand short grain varieties as they aremuch less sticky when cooked4) Rice is very digestible even when cookvalues are low5) Rice bran may contain up to 40% starch
Corn as a Starch Source1) Good expansion2) Excellent binding3) Sticky at high levels (>40%)
Wheat as a Starch Source1) Good binding2) Good expansion3) Can be sticky if overcooked4) Contains gluten (good binder)5) Most widely available starch source6) Often utilized as wheat flour which hasmost of the bran removed
Tubers as a Starch Source(Potato & Cassava)1) Excellent binding (at 5% levels)2) Requires less total starch in diet3) Good expansion4) Often precooked5) Smooth pellet surface6) Increased cost
Factors influencing the degree of puffing ofsnacks during extrusion The amount of moisture in the feed material,dough residence time in the extruder barreland cereal particle size. To manufacture expanded products, thepressure and temperature are increased, whilethe moisture level is accurately controlled. When the product exerts the extruder throughthe forming die, the change in atmosphericpressure causes the internal moisture to turnto steam. This puffs the fully-cooked doughinto an expanded product.
The factors are-a) Raw materialsThe most used raw materials in the extrusion process are- starch and- protein based materials.The structure of the extruded products may be formed fromstarch or protein polymers.In general, the chemical or physicochemical changes inbiopolymers that can occur during extrusion cooking include:binding, cleavage, loss of native conformation, fragmentrecombination and thermal degradation.Thermally labile compounds such as flavors and vitamins maybe injected immediately before the die to minimize exposureto heat and shear.
Moisture If high expansion is required in a low moisture product, finelymilled forms of harder endosperm types will give excellentresults. If the product requires low to medium expansion, some of thehard material may be replaced by soft flour; and for lowexpansion in a dense product such as breading crumb, soft flourmay be used. Maximum expansion degree is closely related to starch content.Maximum expansion is obtained with pure starches (an increaseof 500% in product diameter), followed by whole grains (400%)and with lower expansions for seeds or germ (150-200%); thestarch content of these products is 100, 65-78, 40-50 and 0-10,respectively. The minimum starch content for expansion is 60-70%. In the extrusion process of expanded products with low moisture,the expansion of the final product is inversely related to themoisture of the raw material and directly related to the increasein extrusion temperature; however, the effect of moisture is moresignificant
SuperheateddoughProduct puffs as moisture flashes off
Fibre The physical presence of fibres in air cell walls reduces theexpansion potential of the starchy film, larger particles, suchas bran, tend to rupture air cell walls of the extrudedproduct, causing a reduction in expansion index. Non-starch polysaccharides, such as fibres, may bind watermore strongly than proteins and starch during extrusion. Thiswater binding capacity inhibits water loss at the die, that is,at the exit of the extruder, reducing expansion. The starch present cannot be totally gelatinized in thepresence of fibre and is thus not capable of supportingexpansion. Lipids The presence of lipids in quantities lower than 3% does notaffect expansion properties, however, in amounts above 5%,reduction in expansion rate is considerable. The increase in lipid content can be corrected through thereduction in conditioning moisture content, so as not to affectthe expansion index of second generation products (directlyexpanded snacks).
Temperature Another important parameter for extrudate expansion is processtemperature. Products do not expand if the temperature does not reach100°C. Expansion increases with the increase in temperature when moisturecontent of the material is close to 20%, due to lower viscosity, permittinga more rapid expansion of the molten mass, or due to an increase inwater vapour pressure. The reduction of expansion at very high temperatures is attributed to anincrease in dextrinization, weakening starch structure.. Expansion occurs in both radial and axial directions, at different degrees,depending on the viscoelastic properties of the melt. Vaporization of moisture and cooling of the extrudate serve to bring theproduct from a molten to a rubbery state; and further drying is usuallyused to produce the brittle, fracturable texture typical of these products.
ConclusionIn the future science and technology of theextrusion field, scientists and engineersshould focus on the relationship betweencomposition changes and product quality,evaluating and enhancing nutritional, sensoryand functional properties of extruded foods.