Low Pressure Extrusion for Great Tasting Baked Snacks

Pretzel & Snack History

The history of pretzels dates to the 6th or 7th century A.D. Europe. During that time, European monks used scraps of leftover bread dough to form a shape resembling a person with arms folded across their chest in prayer. These dough scraps were placed in the oven and baked. The monks gave these baked “pretzels” to children as rewards for correctly learning their prayers. In fact, the term of pretzel is believed to be derived from the Latin word “pretiola,” which means little reward.

In contrast to these crude beginnings, today’s pretzels are made throughout the world on highly automated production lines capable of producing several thousand pounds of pretzels per hour.  

These flexible and configurable production systems are also used to make all types of popular low pressure extruded snacks including filled products, flat pretzels, sushka, pretzels, expanded nuggets, broken pretzels, bread snacks, biscuit products, potato‐based snacks, and braided rods.

This paper explains the process parameters and demonstrates how different varieties of snacks can be made using the same types of equipment and process.

The Baked Snack Process

Production parameters are proprietary to each individual factory but can generally be broken down into the following steps:

• Formulation & Mixing
• Low Pressure Extrusion
• Proofing
• Cooking
• Salting & Seasoning
• Baking
• Drying

Formulation / Mixing: 
Most snack doughs consist primarily of low protein flour, a sweetener such as corn syrup or malt syrup, oil or vegetable shortening, and leavening agents such as yeast and/or sodium bicarbonate. A typical formula may consist of the following:

• Flour 100.0 lb
• Oil 2.0 lb
• Corn Syrup 2.0 lb
• Instant Yeast 25.0 g
• Water (90°F) 42.0 lb

Dry ingredients can be blended in a mixer or pre‐ blender prior to adding any liquid ingredients. This helps ensure the dough ends up in a homogeneous mass, which is critical for consistent product quality.

Traditionally, double‐arm Sigma horizontal batch mixers have been used to mix snack dough. Average mix time is 1 to 2 minutes on slow speed followed by 3 to 5 minutes on fast speed. Finished dough temperatures typically range between 80 and 100°F.  

Handling the dough properly from batch creation to forming can be challenging.  Automated dough handling systems should be leveraged whenever possible to ensure consistent loading of the production line.  

Automated RBS Doughbot System

Batch weights depend on the size and type of the production line and the type of ingredients used. Most processors specify three batches per hour so that all dough from any given batch is processed within 30 minutes. Some dough types should not be allowed to rest longer than this since as the dough viscosity changes, so does the rate of extrusion, causing product piece weight variability.  Wide variability from one piece to the next can make consistent production and packaging difficult and create poor quality product.  This is a common issue in the industry that has been largely resolved by converting from batch mixing to a continuous dough mixing system.

In the mid 1990’s, many manufacturers switched to a continuous method of dough production introduced by Exact Mixing. This mixing method virtually eliminated the problem of dough sitting too long before forming, since the dough is made continuously rather than in large batches. All ingredient streams are brought together, relying on mass‐flow technology for liquid ingredients along with metering and gravimetric dry feeders resulting in highly consistent dough. 

Today, nearly all new snack production lines in the US have been equipped with continuous mixing systems. Additionally, many manufacturers have converted their existing batch mixing to new, continuous mixing systems.

Exact Continuous Mixer

Low Pressure Extrusion: 
Of course, almost all snacks were originally made by hand.  Early mass production systems incorporated mechanized systems that mimicked the human process but did so much faster and cheaper.

Extrusion: Ropes (above) Traditional Shape (below)

In the mid‐1960s, Reading Bakery Systems developed the low‐pressure extruder as an alternative to the pretzel twisting machine. This machine has been re‐ designed almost 10 times as technology improved, but the general function of the machine is still the same. The Low Pressure Extruder uses a twin‐screw design to push dough into a compression head and through an attached die plate which has the desired shape cut into it.

Die plates can be made for almost any shape or size. The dough is forced through the die then cut off with a bandcutter. Dough pressure is monitored throughout the process using a pressure gauge. Most pretzel doughs are extruded at 100 psi or less. This ‘low pressure’ process minimizes “crystallization” – a breakdown of the gluten structure of the dough due to too much pressure.  This results in a very dense and fragile product.

Using the Low Pressure Extruder, rows of pretzels or snacks up to two meters wide can be cut at a rate of up to 265 cycles per minute for very high‐capacity snack manufacturing.

The Low Pressure Extruder provides a flexible, cost‐effective forming solution for a wide variety of snacks.   The compression head and dies are quickly and easily changed to make different shapes and sizes, and changing the base recipe or using a slightly different process can produce different product options.    Some of the most popular pretzels and snacks made on the extruder include braids, flat and filled products and broken pretzel pieces.

Extrusion: Braided Ropes cut by Guillotine Cutter

Proofing:  

The proofing conveyor receives the product directly from the extruder.  It is a long belt typically made of cotton or a cotton/poly blend to allow the dough pieces to breathe on the underside and rest after the forming process. Proof times range from a few minutes to an hour, depending on the type of snack. This proofing time allows the yeast or other leavening agents to expand and create air pockets in the dough, which aid in producing a pleasant texture in the finished product.

Guillotine Cutter & Proofing Conveyor

For products that are extruded out of the die as a ‘rope’ or continuous stream of dough, a Guillotine Cutter is often built into or over top of the proofing conveyor.  The Guillotine Cutter cleanly and consistently cuts the dough into discrete pieces of a designated length.  Length is determined by the number of cuts per unit of time and can be modified as needed.   

Cooking:  

Dough pieces are deposited directly into the cooker after proofing. The cooker is an important part of the pretzel process, but is useful in other snack products as well. The cooker normally consists of a sodium hydroxide and water solution heated to between 190 and 210°F.  

Pretzel Cooker

Bakers use other solutions made with sodium bicarbonate or potassium carbonate to produce specific types of product surface textures. Pretzel Cooker solutions contain between 0.7 and 2.0 causticity for most pretzel types, but some snacks may only use hot water or a different type of solution to help form a shell on the outside of the product.  The pieces are usually dipped into the solution for a specific amount of time (generally less than 15 seconds). They may also be bathed in a waterfall of solution or subjected to a steam environment.  Alternatively, the cooker may be used to prepare the product so that seasoning sticks to the surface.   

The cooking solution gelatinizes the dough and forms a shell around the outside of each dough piece. This shell may have a pH as high as 13.0. It is this combination of outer alkaline shell to slightly acidic inner core that can provide a unique flavor.  

Salting & Seasoning: 

After the above steps are taken in the process, the gelatinized dough piece travels under a curtain of salt, seeds or other seasoning before entering the oven. 

Salt Dispensed on to Product

Because the hot cooker bath made the dough shell warm and sticky, the toppings stick to this shell as it falls on top of the dough piece. The dispensing unit includes a storage hopper located over a grooved or pocketed roller. The speed of the roller is variable and is set to the desired seasoning application rate.

Seasoning that does not stick to the product can be “recycled” back into the storage hopper via a vacuum system.  A slightly clear crystal called rock pretzel salt is used on most hard pretzels in the market. Other variations include both pretzel “I” and “M” salt, which have a larger and a much more white appearance. Application coverage on a typical snack ranges up to 15% of the surface area.

Baking: 

 After salting or seasoning, the dough pieces are ready for initial baking. Most manufacturers use tunnel ovens for baking, and these can be as long as 45 meters. Some ovens are direct‐fired, which means combustion happens inside the baking chamber, so the product is exposed to the actual fire. While this is more traditional, it also risks uneven baking (hot spots) and an overall inconsistent bake caused by product breaks or dead zones.

Many modern snack ovens use convection technology for a more efficient and consistent bake.  In these ovens, hot air is precisely circulated across the product resulting in a very even bake with no flame scorching. These tunnel ovens may contain several baking zones and each zone can be customized to a specific baking temperature, air velocity and exhaust. These ovens may also contain radiant burners, in addition to the forced air, to give the operator more baking flexibility. Combining these various baking settings allow the operator to customize the oven to achieve specific product characteristics. Oven temperatures generally range between 400 and 600°F, and baking times from 1.5 to 5 minutes. Moisture levels are typically between 8 and 15% as product exits the oven.

SPECTRUM OVEN® Over Dryer

Most products are finished in a dryer, but some thinner, softer products – may be removed and packaged right after baking.  Product that is not dried further will have a softer, bread like texture, but will have a shorter shelf life.   

Oven Discharging Baked Product

Drying:  After baking, the product passes into the dryer. The dryer is a low temperature oven (220 to 300°F) designed to reduce the product moisture gently and evenly to between 2 and 5%, depending on the thickness. This very low moisture content gives the product a shelf life of up to 6 months. The product travels in the dryer for between 5 and 50 minutes, depending on the type and desired moisture content. This slow process enables moisture to escape before transferring the product to packaging. 

As opposed to the single layer, or bed, of pretzels passing through the oven, the dryer may have several centimeters of pretzel bed passing through it.

Other Pretzel / Baked Snack Product Types:

Co‐Extruded, Filled Snacks
The Filled Snack Extrusion System is used to produce snacks that have a hard outer shell but include a soft filling inside. The most common fillings are peanut butter, cheese, and chocolate.

Peanut Butter Filled Pretzel Nuggets

Dough is deposited into a low pressure extruder equipped with a special compression head and die. The dough forms a continuous tube as it exits the die onto the proofer belt.

Filled Stick Extruder & Filling System

Simultaneously, filling is pumped into an inner nozzle thereby continuously filling the inside of the tube. The filled “rope” is cut/crimped on the proofer belt to the desired length and then follows the same process as the hard pretzel.  

Potato / Corn Based Products
The extruder can also be built to handle potato or corn‐based doughs.  Adding components like powered pre‐feed rollers that force the more granular doughs into the extruding augers ensures consistent product density.  A free float section can be installed under the die discharge to allow product to flow directly into a frying system after extrusion. 

Fried Potato Rings

“Braided” Products
A specialized compression head that includes powered, rotating dies can be fitted to the extruder and produce dough streams of “braided” ropes.  The ropes can be cut to consistent lengths by a guillotine cutter and cooked and / or baked to a desired finish. 

Broken Pieces
A breaker can be fitted at the end of the oven or kiln / dryer to intentionally break the product into randomly sized pieces that are perfect for seasoning or using in other snack processing applications.  

Seasoned Broken Pretzel Pieces

Conclusion
As you can see, the Low Pressure Extrusion System from Reading Bakery Systems is capable of producing a wide variety of snack products, with just a few of them mentioned here. If you would like to discuss a possible new product idea or need more information on any of our snack systems, we invite you to contact us directly or visit our website at www.readingbakery.com
 

Photo source: Reading Bakery