Keeping the Crunch in Breakfast Cereals

June 2001

It’s morning again and you’ve grabbed that favorite box of cereal out of the cupboard, poured it into a bowl and covered it with fresh, cold milk. You can’t wait for the sensation of the cold milk and crunchy cereal in your mouth. You eat it as quickly as possible to keep that crispy, crunchy sensation going. If you dislike soggy cereals, like most of us, you’re looking for the texture of that first bite for many more spoonfuls. Luckily, food technologists have designed many specialized processes and ingredients to keep the crunchy texture of ready-to-eat (RTE) cereals in milk through more and more bites in the bowl.

Perfect processing
The birth of the cereal industry began with Corn Flakes, developed by William K. Kellogg in the 19th century, considered the most traditional RTE cereal product known. Whether you’re eating a flaked cereal, a puffed whole grain or an extruded cereal, all RTE breakfast cereals have common ingredients and utilize similar processes.

Eight categories of breakfast cereals show up on the shelf: flaked cereals (such as corn, wheat, rice flakes), including extruded flakes; gun-puffed whole grains; extruded gun-puffed cereals; shredded whole grains; extruded and other shredded cereals; oven-puffed cereals; extruded expanded cereals; and granola cereals. Regardless of the cereal category, each one goes through a step that cooks the grain — usually corn, wheat, oats or rice — in combination with flavor materials, sweeteners and sometimes heat-stable, nutritional fortifying agents. They undergo one of two cooking methods: a direct steam injection into the grain mass using a rotating batch vessel or a continuous extrusion cook. The cooking stage creates the physical properties necessary for the development of the final texture.

Gelatinization of the starch grain fractions is the primary goal of the cooking stage. This textural development is required to make some form of expansion or development of cellularity and is an important factor in the cereal’s organoleptic acceptability. The creation of water vapor from rapid heating (as in toasting or oven-puffing) or by the sudden release of pressure from the product in a superheated state (seen in gun- or extrusion-puffing) causes structural development. The cereal mix must be tough, elastic and fairly homogenous so it can respond to the vapor pressure. The processes and the grains used for making the different categories of cereals are the defining factors of their textural characteristics.

Flaky cereals
Flaked cereals are made directly from whole-grain kernels or parts of corn, wheat or rice kernels. The objective of a flaked cereal process is to process the grain to yield particles that form individual flakes. A flaked cereal’s finished quality depends on grain selection. Sizing or screening operations may be necessary to achieve flakable-sized particles, called flaking grits. Corn or wheat flakes usually are made from whole-grain kernels or parts of kernels that are the correct size for flaking grits. The basic processing steps of a flaked cereal include mixing, cooking, dumping, delumping, drying, cooking and tempering, flaking and toasting. “Unmodified corn starch is added to cereal flakes to improve the ability of the flake to withstand processing,” says Mike Augustine, director of food ingredient applications, A.E. Staley Manufacturing Company, part of the Tate & Lyle Group, Decatur, IL.

A typical corn-, wheat- or rice-flake formula will contain 100 lbs. of the native grain, 6 to 12 lbs. granulated sugar, 2 lbs. malt syrup, 2 lbs. salt and enough water to achieve 28% to 32% moisture after cooking. The cooking time varies depending on the grain used. Corn grits may require about two hours, while wheat kernels need 30 to 35 minutes, and rice kernels cook in about one hour. “Starches may be used to standardize the hydration properties of cereal grains to improve their processability with seasonal changes in the grain,” says Augustine.

Control of texture and moisture in the finished cereal flake begins after cooking, drying and tempering, or after extrusion in an extruded product. Moisture control in the finished product is critical to maintaining product integrity. Moisture content greater than 3% will reduce the crunchiness; less than 1% will promote breakage. Both conditions will ultimately decrease the cereal’s shelf life. A too-high moisture before the flaking rolls may cause the flakes to stick to the roll knives and become wrinkled rather than flat. These flakes will not blister correctly, or at all, and will become hard and flinty rather than crisp and tender. If the tempered grits are too dry prior to flaking, they won’t be gripped and drawn through the “nip,” or gap, between the rolls and no flaking will take place.

Shred it
Shredded cereals usually are made from whole-kernel wheat. After cooking and cooling the grain, the critical point in controlling the strength of the final shreds is tempering the kernels for up to 24 hours before shredding. Tempering allows moisture of the kernels to equilibrate and firm up from the starch retrogradation. Insufficient tempering will cause crooked, gummy, sticky and difficult-to-cut shreds.

The shredding process itself is a simple concept that requires a lot of skill. The wheat is squeezed between one smooth and one grooved roll. A comb runs along the grooved roll to separate the shreds from the roll to form one layer of the finished biscuit. It takes 10 to 20 pairs of rolls to produce a deep enough web for a large shredded-wheat biscuit. Bite-size products require fewer layers than the large biscuits. The cutting of individual biscuits also crimps the ends to hold them in biscuit form. Shredded biscuits are baked from a moisture of 45% to a final moisture of about 4%.

Puff it up
Typically, puffed rice and wheat cereals fall into the category of gun-puffed whole grains. Gun-puffing is a unique process that requires two conditions for the grain to puff. The grain must be cooked, followed by a fast pressure drop in the atmosphere surrounding the grain. This releases steam from the grain as the pressure within the grain tries to equilibrate with the lower pressure surrounding it.

Rice and wheat are the only whole grains used for gun-puffing. Corn and wheat also undergo this process, but not in whole-grain form. Puffing can be done with a batch single-shot gun, automatic single-shot gun or automatic multiple-shot guns. Generally, the safety and speed of production increases from the batch to the automatic, multiple-shot guns. In all cases, the processes involve temperatures between 400º to 500ºF and pressures of about 200 lbs. psi. The resulting moisture of the puffed grains ranges from 5% to 7%, which is too high for a crisp, crunchy cereal. A drying step follows to reach the desired 1% to 3% moisture. The extremely porous puffed grain picks up moisture easily. These cereals rely on coatings to help maintain crispness in milk and good moisture-barrier properties in their packaging materials.

Oven-puffed cereals primarily use rice, corn or mixtures of the two. Wheat and oats will not puff with the presence of moisture and high heat. Oven-puffing requires oven temperatures from 550º to 650ºF in later stages of baking. As in other cereal processes, the cooking of the grain and the expansion of the product during baking dictate the final texture.

Extrusion etiquette
Extruded gun-puffed cereals use flours instead of whole grains for their starting material. Processors often combine cooking and extrusion into one step. Cooking usually occurs in the extruder and then the dough is formed into the desired shape as it is extruded through a die. One of the first cereals of this type was General Mills’ Kix® cereal.

Some of the basic dry ingredients are flours, starches, heat-stable vitamins and minerals, sugar, salt, malt, flavors, color and water. The dry ingredients are fed into the extruder and a solution of the sugar, salt, malt, flavor, color and water is added into the first section of the extruder. “There are three textural and sensory benefits of using modified wheat starch in extruded, gun-puffed cereal. First, it helps in obtaining a high expansion volume. Secondly, it provides film-forming ability to slow down the absorption of milk into the cereal. Finally, it has a low fat content, which contributes to improved flavor characteristics of the cereal,” says Clodualdo Maningat, Ph.D., corporate director of research and development and quality control, Midwest Grain Products Inc., Atchison, KS.

After the cooked dough leaves the cooking extruder it is fed into a forming extruder to achieve the desired shape. Moisture of the cooked pieces has reached 20% to 24% at this point. A drying step follows and then the shapes are gun-puffed and dried again to a moisture content of 1% to 3%. “Native tapioca and potato starches have very good expansion properties which enhance the expansion of extruded gun-puffed cereals and extruded and direct-expansion cereals,” says Dale Bertrand, manager of research and commercialization, Avebe America Inc., Princeton, NJ.

Extruded flakes form the grit for flaking by extruding the mixed ingredients through a die hole and cutting pieces of dough to a specific size. Extruded corn flakes may utilize flours and other finer materials since the flakable grit size is achieved mechanically. “The film-forming properties of native starches, in either an uncooked or pregelatinized form, also contribute to the crunchiness of the cereal by preventing the milk from penetrating the cereal,” adds Bertrand.

One difference in the extrusion of flakes is more mechanical working of the mix. The flakes may look dull or slightly gray — especially so if the formula is low in sugars or without malt syrup, reducing the amount of Maillard browning. Adding natural or artificial color will alleviate the problem. “In flaked products, such as wheat-bran flakes, inulin can be added to the malt syrup and added into a preconditioning cylinder of the extruder. If added to the premix, inulin can be easily added at a level of 30% or about 9 grams per 30-gram cereal serving. If added to the malt syrup, the level is limited somewhat to inulin’s solubility in the syrup, or to about 8%,” says Brian Tungland, vice president, scientific and regulatory affairs, Imperial Sensus, Sugar Land, TX.

Extruded expanded cereals use flour or meal rather than whole or broken kernels of grain along with the other traditional cereal ingredients. “A pregelatinized, unmodified wheat starch acts as a structure-forming agent in extruded breakfast cereals,” says Maningat. Cooking is done in the cooking section of the extruder or in its own cooking extrusion unit. The cereal expands when the moisture in the mix is released from the elevated temperature and pressure into ambient conditions. “If you formulate with an amylopectin base or waxy base starch you get greater expansion and a softer bite,” says Augustine.

“A 100% amylopectin wheat starch is in development now for introduction in two to three years, which may have some application in breakfast cereal. Some major universities, like Kansas State, Nebraska and Washington State, along with some major seed and biotech companies, are actively pursuing this technology,” says Maningat.

“Adding a starch with higher amylose content, like wheat starch or corn starch, will increase the hardness, or crunchiness of a cereal,” says Augustine. Holes in the end of the extruder control the shape of the cereal. A rotating knife on the outside of the die cuts the pieces. These pieces often are sugar-coated, colored and flavored to create even more variety.

Like extruded flaked or puffed cereals, extruded and other shredded cereals use ingredients such as starches, sugar, corn sweeteners, malt, salt, color, flavors and vitamin/mineral mixes. “Dextrins can be incorporated into a non-traditional, shredded-wheat-type product to add crispness and to provide a moisture barrier,” says Bertrand. All four basic grains can be used in the form of whole or parts of kernels or flours, depending on the product being made and the type of cooking used prior to shredding. Either pressure cooking or extrusion cooking may be used. Extrusion cooking provides better control of the piece size used for shredding. Shreds from cooked dough typically are made into bite-sized products. Making these products with cross grooves on the shredding rolls provides extra strength to the web and a more uniform appearance.

Shredded corn and rice cereals are toasted or baked at lower temperatures in the initial stages of baking and extremely high temperatures in the last half of the oven to puff and open up the shred structure. Without these conditions, the texture will become hard and flinty. Shredded wheat and oat cereals do not require these specialized baking conditions.

Great granola
Granola isn’t just for the “natural-food” eaters anymore. Although many granola cereals are made with all-natural ingredients, they have gained broad appeal as a cereal and a topping for products such as yogurt or fruit. Granola cereal is basically a combination of whole rolled or quick oats mixed with nuts, coconut, brown sugar, honey, malt extract, dried milk, dried fruits, water, vegetable oil and spices. The wet and dry ingredients are combined separately, mixed together, and spread out over a continuous oven band or dryer. The mixture is baked at temperatures from 300º to 425ºF to achieve a light-brown color and a moisture of about 3%.

After baking or drying, the mixture is broken up into chunks. The size of the chunks, their moisture content and the presence of crystallizable sugars, makes granola inherently crunchy. Adding other ingredients to granola formulations can enhance crunchiness. “In granola cereals, inulin is generally used at about 5% with other carbohydrates to effectively bind the granola together,” says Tungland “This results in a higher-fiber, more crunchy cereal.”

Coat it
The traditional sugar coating for cereal combines a sugar formula with an application method that produces a coating of sugar crystals having the desired size, structure, color and flavor when dry. The main ingredient remains cane or beet sugar, between a 4X and 10X particle size. Sugar-based flavoring materials, such as brown sugar or honey, can partially replace white sugar and adding small amounts of oil can help prevent clumping. In sweetened cereals, the coating visibility often adds appeal, so up to 50% or more of the coating by weight is applied to the cereal.

Besides adding sweetness, sugar coatings provide another layer between the milk and the cereal to prolong its crispness. Other ingredients can be added to the sugar coating to enhance its functionality. “The addition of starches, dextrins or maltodextrins to the surface of cereals will improve the storage stability of the sweetener system,” says Augustine.

When an extended bowl life is the desired goal, there are more ingredient options. “In coating systems, inulin can be used with other non-digestible carbohydrate systems to help minimize moisture migration by acting to form a film, to help extend bowl life by coating the pores,” says Tungland. The coating further reduces sweet taste and provides a golden-brown color.

Spraying dextrins or maltodextrins onto the surface of a cereal also can provide a crisp barrier without adding sweetness. “White dextrin is used a lot in a clear-coat application that is further heat set, as in the toasting process,” says Bertrand.

Some ingredients provide more than sweetness. “Crystalline fructose or high-fructose corn syrups can be used in sweetener coatings to provide sweetness and adhering properties for dry flavor-bit applications,” says Augustine.

Crunchy inclusions
A tasty way to add crunch to a cereal is by adding nuts — most commonly almonds, walnuts and pecans — and other crunchy formulated bits. These can be adhered to the cereal or added as individual pieces.

“In a survey conducted in 1999, 55% of food technologists said that texture or crunch was the major benefit of almonds and 60% of cereal technologists listed almonds as their favorite cereal nut,” says Ann Martin, senior account executive, Ketchum Public Relations, San Francisco, CA.

Almonds come in any number of sizes and cuts. “Sliced, split and whole natural almonds are typically used in granola, while chopped or diced cuts are used in a flaked cereal because they will stick to the cereal flake,” says Martin. “If you are looking for the most crunch to add to a cereal, add the larger pieces, like the splits or whole almonds, but all of the other cuts will add crunch as well.” For more information on the health benefits of various nuts, and their many applications, see “Simply Nuts” in the March 2000 issue of Food Product Design.

Formulated bits can add variety without adding nuts to a cereal formulation. “Cereal inclusions add crunch and texture appeal and can also be used as carriers for fortification,” says Freddie Corcoro, senior development scientist, Kerry Food Ingredients, New Century, KS. “We target a bowl life of at least three minutes for our cereal inclusions.”

Many inclusions come in the form of marshmallow bits, available in many shapes and colors, and exclusively are used in kids’ cereals.

However, other varieties provide crunch with less sweetness. “We have developed a new type of cereal inclusion that combines a crunchy shell with a soft chewy center,” says Corcoro. “They can be developed as multi-grained, fruit or nut-based inclusions.” Flaked cereals often incorporate inclusions such as granola clusters. These clusters contain the typical ingredients used in granola cereals.

Measuring up
If cereal texture is so important to consumers, there must be a way of measuring it. When it comes to measuring crunch, there’s more than just eating the cereal.

Crunchiness is a very complex attribute, not merely the sensation felt in the mouth or resistance to chewing. “Crunchiness is an auditory sensation,” says Boine Johnson, president, Texture Technologies, Scarsdale, NY. “Some work that McGill University (Montreal, Quebec, Canada) did in the 1950s said that people who were nerve deaf in their ears could not detect crunchiness,” says Johnson. Similar to the relationship between taste and smell, the sensation in the mouth and the sound in the ears define our sense of crunch. Sensory evaluation is a common method of evaluating crispness of a cereal in milk. Mechanical methods provide a more quantitative measure of texture.

Some instruments, called food-texture analyzers, used by the cereal industry include Stevens, Instron and Texture Analyzer. “Texture analysis on cereal is often done by exposing the cereal to milk for certain times, 30 seconds to two minutes, using the Kramer Shear Press, a multiple Puncture Rig or even an Ottowa Cell attachment on a TA.XT2i Texture Analyzer,” says Johnson.

In one application study measuring bowl life using a Kramer Shear Press, a popular brand of corn flakes, rice flakes, wheat flakes and an extruded oat-based cereal were compared. The corn flakes exhibited similar bowl life to the wheat flakes, while the rice flakes softened at a faster pace than all of the cereals. The extruded oat cereal was firmer than all of the flake products.

When measuring crunch mechanically, more than just the force required to penetrate the cereal is being measured. “When you crunch something, there are two features of the fractures that you must measure to get a repeatable index of crunchiness: one is the frequency of the fractures and the other is the amplitude of those fractures,” says Johnson. “The TTC Multiple Puncture Rig is used for a bulk amount of sample but is designed to allow for the individual product failures normally associated with individually tested products.” This setup allows for a clearer picture of the frequency and amplitude of fractures. The resulting graph of the measurement can yield a somewhat smooth line or a very jagged line. The more jagged a line is, the longer the line’s length will be. “Counting the number of peaks or fractures is not enough. It is a first approximation, but the length of the line analysis provides the measure that combines both the frequency and the amplitude of the various peaks and thus is the best measure of the crunchiness of cereal or other products,” he adds.

During your next bowl of cereal, you might think a little bit more about what you are tasting and hearing. You will probably appreciate more of the science behind the crunch.

Kimberlee J. Burrington is the whey applications program coordinator for the Wisconsin Center for Dairy Research in Madison, WI. She received her B.S. and M.S. degrees in food chemistry from the University of Wisconsin-Madison. Her industry background is in bakery and dairy.

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