Cracker tech 101
Nov. 1, 2014
by Mihaelos N. Mihalos and Supapong Siris, Mondelez Global LLC
Nothing is more basic to successful cracker production than sheeting, gauging and cutting. These activities consist of unit operations such as sheeters, laminators, gauge rolls and cutters that process a sheeted dough prior to baking.
Typically, sheeting can be considered a series of mechanical operations that provide the dough a level of cohesion by compression to form a continuous band with a targeted thickness permitting the desired geometric cracker shapes to be formed prior to baking. Sheeting, gauging and cutting are the most versatile means for forming cracker pieces.
Start with the dough
Sheeting is a critical unit operation in the biscuit manufacturing process. The typical production line consists of various unit operations and associated equipment.
Doughs — the fermented types that yield soda and saltine crackers or chemically leavened styles for making butter and graham crackers — are typically mixed in an upright mixer. Once mixed, and depending on the formulation, they are allowed a certain amount of residence time in conditioning rooms (usually held at 80°F and 80% relative humidity) for either proofing or water hydration before being sent to the next stage in the process: forming.
Depending on the line setup, doughs move to the forming steps in two ways. The first involves dumping the doughs into troughs, moving them into a proofing room and, eventually, transporting them to the forming step. The second option is to turn doughs out onto an environmentally controlled lay-time conveyor that slowly moves them to the forming step.
Then the dough needs to be formed into a continuous mass. It is typically transferred into a dough dump and down a feed chute into a set or sets of feed rolls to be formed into a continuous sheet. These systems usually consist of two-, three- or four-roll sheeters. Currently, most cracker lines use either a three-roll or four-roll sheeter. The old two-roll sheeters have been replaced to improve the quality of the dough sheet.
Three rolls or four
Biscuit and cracker lines use two types of three-roll sheeters: front and rear discharge. Both collect dough from the dough chute or a pre-sheeter. The dough is forced into the compression rolls, which are corrugated with various patterns and groove depths depending on the formulation. Then the dough follows the path to a main roll or a finishing smooth roll, which can be adjusted to alter the gap between rolls and, therefore, the dough band thickness. The smooth roll surface ensures that there is no impression left on the final product.
The option of front or rear discharge is solely determined by the layout of the line and how its operator looks at the dough sheet. Depending on the viewing angle and the surrounding infrastructure, certain line layouts make it easier to see the dough sheet emerging from the rear of the sheeter; others offer a better view from the front.
Because adding a third roll to the old two-roll sheeter improved dough quality, OEMs and equipment vendors reasoned that adding a fourth roll to a three-roll system would be of further benefit. Therefore, they developed the four-roll sheeter.
The alignment of four rolls offsetting the top two grooved rolls with the bottom two finishing rolls creates two compression chambers. If the alignment of these rolls is not offset, however, then one large compression chamber is formed. There does not appear to be any process advantage for one configuration over the other.
The dough path is similar to the three-roll system. Both collect dough from the dough chute or a pre-sheeter. The dough is forced into compression rolls corrugated with various patterns and groove depths depending on the formulation.
The four-roll sheeter more evenly “pulls” dough through the hopper compared with the three-roll system. The fourth roll enables the pressure chambers to improve sheet quality by producing a smooth-edged, even sheet width. The configuration prevents the buildup of old dough in the sheeter and provides consistent dough density of the sheet. The smooth rolls guide a more even, relaxed dough sheet from the sheeter.
It should be noted that the cost of the four-roll unit vs. the three-roll design is almost double. Therefore, the food manufacturer needs to make a business decision. Consider the desired final product attributes from a technical perspective, and ask, “Will the additional capital dollars be worth spending to further improve the quality of the dough sheet?”
Layer upon layer
Now that the sheeters have provided a uniform dough sheet, the next unit operation, lamination, is to achieve the layering responsible for developing a light, tender texture in the finished product. However, if the final product attributes do not require this, then the lamination step is not implemented, and the final product will have more of a chip-type texture.
The lamination step provides strong, firm doughs that are extensible and can be sheeted without tearing or crumbling. Another function of the laminator is to give the dough sheet an even thickness for the gauging and cutting unit operations.
There are two types of laminators: sweep or swing-arm and cut-sheet. The older style laminator employs a sweeping or swinging arm design that produces a continuous sheet. Because this method folds the continuous sheet at the edges of the conveyor belt, it causes higher dough weights at the conveyor edges.
With the newer cut-sheet laminator, individual dough sheets are cut with a knife and layered one on top of another. This produces a uniform dough weight across the width of the conveyor. These laminators, however, are more expensive, have many more moving mechanical parts requiring constant maintenance and significant space on the forming line.
A matter of reduction
Once the dough is laminated, the sheet’s depth needs to be reduced to the proper thickness. A series of reduction rolls compress the laminated layers to achieve a single dough sheet of uniform thickness. Gradual reduction in thickness imparts less stress to the dough and precludes deformation of the dough pieces during cutting and baking. A 2:1 reduction ratio at each roll station is suggested.
The number of auxiliary roll stations on each line can vary from one to four, depending on the line layout and the available floor space. After the dough sheet has been reduced to the desired thickness, it then travels through the final gauge roll. This roll regulates the finished weight of the product, which is extremely important.
If the dough piece is too heavy or too light, it will affect the final product moisture. The piece weight will also impact the finished package weights, especially for saltines or butter crackers placed in slug wrappers. The heavier the final baked piece, the fewer the crackers in the slug. Vice versa, the lighter the final baked piece, more crackers are in the final slug, resulting in product giveaway.
Cut into shape
At this point, the dough sheet has achieved its desired thickness, and it is time to cut the individual pieces. This is accomplished by the use of a rotary cutter in most of the industry. However, there may be biscuit manufacturers that use the older reciprocating cutters. For the purposes of this paper, the discussion will center on rotary cutter operations.
The rotary cutter has three main parts: the cutter roll, backup roll, and cutter apron. The cutter roll is specifically designed for an individual product. The piece shapes and dockers are embossed and, in certain circumstances depending on the formulation, coated to prevent dough from sticking.
Typically, the backup roll is composed of a hard rubber anvil roll. It provides pressure to maintain uniform cutting. The cutter apron is usually seamless or woven to prevent uneven wear and provide uniform cutting. The types of aprons selected depend on the dough characteristics and the requirement that it assist the clean extraction of dough piece from the cutter’s cups.
One important point is the way the dough travels from the final gauge roll to the cutter. If the dough sheet stretches from the final gauge roll, it is likely to produce a “short” strip when cut. To correct this problem, check the conveyors after the final gauge roll to ensure they are not running too fast.
Conversely, if the dough sheet is too relaxed, it is likely to bunch and change the product size, or strip length. To correct this condition, make sure the final gauge roll out-feed conveyor and cutter apron speeds are not too slow.
If space is available in the line, some manufacturers install relaxation conveyors between the final gauge roll and the cutter. These conveyors typically run slower than the final gauge roll causing the dough sheet to ripple slightly and, thus, allowing the dough sheet to relax properly prior to the cutting unit operations.
Scrap or no scrap
One more comment on cutters. There are two types: scrap and tied. Depending on the cracker shape and cutter design, the appropriate cutter is selected.
Scrap cutters are used for round or irregular shaped crackers where the individual pieces are cut out. The interval or spacing between the shaped pieces in the cutter pattern is called the scrap. Greater quantities of scrap are generated using this cutter and are required to be recombined back into the process at the sheeter.
Typically, 30 to 50% trim may need to be reincorporated. However, the amount of scrap impacts dough texture and rheology. The resulting sheeted dough is less extensible because part has already gone through the sheeting process once, compared with fresh dough. It also needs to be recombined evenly in the sheeter to minimize rheological impact with the fresh dough. Usually, scrap dough passes underneath a heater to warm it prior to incorporating with fresh dough. Additionally, a swing-arm conveyor can be used to place scrap and trim dough uniformly into the sheeter hopper.
The other type of cutter is the tied cutter. It is the more efficient of the two styles because it permits full loading of the oven band. The cut pieces remain interlocked in a continuous sheet. Edge trim is again recombined into the sheeter process but at minimal rates. Typically, fresh dough consists of 98 to 99%, and the scrap/trim is only 1 to 2%. Crackers shaped by tied cutters bake as a peel. Individual crackers are separated from the sheet after baking.
Role of dockers
Another important component is the docker, mounted in tandem with cutters. It produces an integral part of the distinctive product patterns. The docker’s pins perform several important functions. They help control the thickness of the baked product by pinning the laminated layers together. They provide a uniform bake through the product’s interior where residual moisture levels tend to be higher, and they help reduce blisters in the baked product.
After the dough pieces have been individually cut, they are transferred to a conveyor where either salt or other toppings maybe applied before being placed onto the pan-on conveyor that deposits the individual pieces onto the oven band for baking.
Like other bakery processes, successful biscuit and cracker manufacture depends on giving the dough a good start and managing it throughout makeup. Each unit operation — sheeter, laminator and cutter along with the transfers between these machines — has a material effect on the end product and bakery bottom-line results.