Extrusion: The Revolution Of Mixing

Continual evolution of mixing technology within the food industry has led to new applications for existing machines, and the development of processes have made some existing methods redundant. Examples include high-speed mixers finding new applications across the whole of the bakery industry, and extrusion replacing complete production systems to make snack crackers.

High-speed mixers have been developed over the years to meet the demands of high-output bakeries for product consistency, high efficiency and low levels of operator input. Up till now, it has been mainly bakers of tin or pan bread that have benefitted; bakers of other products have been reluctant to embrace high-speed mixing because they believe that either their output does not warrant it, or that the end product would be different.

However, several integral features combine to make high-speed mixing process ideal for many other products, with a range of machines covering the lower and medium outputs required, as well as high-output bread plants.

Products include pizza, burger buns, baguette, focaccia, ciabatta, batter, rye, soda, pita and artisan breads, brioche, breadcrumbs and Panko, bagels, cinnamon rolls, and burger buns. The scope is still being extended.

Short mixing cycles are a decisive advantage. Automated ingredient feeds minimise dead-time, while the design of the bowl and beater—plus the speed of rotation—combine to produce a 3D mixing action that stretches and shears more of the dough for more of the time.

The result is rapid structural development, giving up to 12 batches every hour. The process is the same as in conventional mixers—just much quicker. Spiral and roller bar mixers can, in contrast, produce three mixed batches every hour.

Pressure or vacuum mixing to control the dough texture is another fundamental reason to look at the high-speed mixer. Applying pressure at the start of mixing helps development by trapping more air in the dough, providing oxygen to enhance the action of ascorbic acid. This improves the development and gas retention in the dough.

Applying vacuum towards the end of the cycle controls the size of the bubbles and refines the crumb structure.

The proportion of pressure and vacuum applied determines whether the dough texture is fine (e.g. for bagels) or open (such as for baguettes). And because these proportions are simply altered, it is easy to switch to a different product—say between an open textured deep pan pizza and a denser crispy pizza base.

Rapid product changeover is a benefit to every baker, particularly where a variety of products are needed each day. The high-speed mixer combines short mixing cycles with rapid and effective cleaning to minimise changeover times.

A time-saving automatic washout system developed for the mixing bowl can give an effective cleaning action, eliminating operator involvement. This rapid, thorough and fully automatic cleaning cycle therefore minimises downtime.

This is particularly effective when making products where cross-contamination could cause quality or food safety problems. For example, coloured breadcrumb coatings for fish and meat, or products containing allergens such as nuts, eggs or dairy.

Extrusion is a technology still expanding within the food industry, and new applications continue to be developed. Some of these bring an end to the involvement of traditional mixing in the production of specific foods. The most recent development enhances the capability of twin-screw extrusion and adds new products; a new die produces a thin, wide sheet of dough that is cut into regular, geometric shapes by an in-line rotary cutter.

The conventional production method uses mixing, sheeting, gauging and rotary cutting equipment familiar in the cracker and biscuit sectors. This equipment is expensive and dedicated to sheeted products. Extrusion sheeting instead (versus conventional mixing, sheeting and gauging) reduces both the capital investment needed for a snack cracker line and the floor space required.

The extruded products are the same as those produced on a conventional dough sheeting line, however the equipment required is considerably simpler and more flexible. The lines are easy to operate and, having fewer units, give significant reductions in cleaning time and maintenance costs.

The shapes produced may be fried as a conventional snack, or oven baked for a lower fat content, and then flavoured. The extrusion system is suitable for any type of baked or fried snack that is cut from a sheet of dough. Wheat and maize are the most common ingredients, but many types of flour can be processed— either on their own or as part of a blend.

Cutting cracker-type snacks from this sheet of extruded dough before baking and flavouring creates a low-fat alternative to conventional frying. Profiled snacks from a dough stream can be rippled, curved or have V-shaped channel effects. Using extruders to make sheeted snacks therefore taps into a trend for thinner snacks—plenty of crunch but less product and hence fewer calories.

It is not possible to adapt a conventional line to make other snack products, and in a market driven by innovation, manufacturers need the ability to switch production to a completely new product, or add to their range without investing in a complete new line—easily achieved with an extrusion system.

Breadcrumb is another example where conventional mixing has become unnecessary. The traditional process involves a conventional mixer, plus forming and baking bread before discarding the crusts and grinding. The same product can be made with no waste using only a twin-screw extruder and dryer.

Similarly for croutons; a bread-like texture can be developed in the extruder and the pieces cut to size at the die or a post-extrusion cutter. Croutons for the ingredients market (such as soups and salads) or a crunchier variant for the snacks market can both be produced by this method with rapid changes between them.

Extrusion is also an excellent way of addressing the market for gluten-free bread products as it can handle with ease the various alternative flours and starches used to make the dough. Screw profiles can be adjusted to achieve the desired characteristics much more readily than conventional processes, and it also overcomes the problems caused by stickiness of the dough.

While mixing can be handled within the extruder barrel for the majority of products in the industries in which extrusion dominates (particularly the snacks sector), mixing is often handled in a pre-conditioner. This simple unit mixes and heats wet and dry ingredients ready for cooking in the extruder, and automatically feeds the pre-treated product into the extruder.

The effect is to significantly increase output: the extruder is entirely focused on processing rather than preparation. A wider range of materials can be processed, but because of the long hydration time, it can be uneconomical to extrude some ingredients.

It is easy to see why extrusion is making deep inroads into food production. This is perhaps the most versatile process available to the food industry. It is capable of producing a wide range of end products from a variety of different ingredients and rapidly switching between them. It also can replicate products made using other, less flexible, methods and the breadth of process options offers plenty of scope for innovation.

If it is possible to make a product in an extruder, it is almost certainly the most cost-effective and compact way to make it. Conventional processing inevitably involves a great deal more time, equipment and cost.

Extrusion is hygienic and energy efficient—all the action is in the short barrel: ingredients in, and product out seconds later. Energy use is concentrated in a very compact area so most of it goes into the product, not the environment. And if a market changes, the extruder can be adapted for a wide choice of other applications.

By changing screw configurations and barrel lengths, the amount of mixing and shear can be controlled; exact management of temperature ensures a precise amount of cooking for each individual product. Post-extrusion, a wide choice of different dies, cutters and forming equipment creates the finished product. Output from one machine can vary from a light, crispy piece to dense inclusions and crumbs or powders.