Food Engineering

Tech Update: Mixers Migrate to Continuous Processing

February 1, 2008
For many products, batch processing begins at a mixer. Thanks to technological advances, continuous mixing systems are making batch an option, not a necessity.

A mixer operator hand-loads ingredients into a mixing bowl, while micro-ingredients wait on the pallet to the right. More powerful mixers create emulsions faster than older units, but some manufacturers would like to leapfrog to continuous mixing.

Batch processing long has defined food and beverage manufacture, with continuous production mostly a vague dream. Thanks to advances in mixing and blending technology, the dream is becoming a reality for companies willing to rethink their processes and break with tradition.

The transition is not easy. The brewing segment has been especially aggressive in trying to make the transition, with mixed results. On the other hand, new products are helping win acceptance for continuous production, particularly in the noncarbonated beverages segment. Sports drinks and enhanced water are some of the fastest growing product categories, and the formulations and processes used to produce them are not bound by tradition. Makers of these still drinks are installing new lines to meet demand, giving them an opportunity to challenge convention and incorporate continuous options.

Ingredients like aspartame, caffeine, modified starch and tea powder are particularly susceptible to lumping, clogging, foaming and other issues in conventional powder mixing, notes Alger Barragan, an industrial engineer and product manager for Tetra Pak Inc.’s Almix product line. If those issues are not adequately addressed early in a process, “they ripple throughout production,” Barragan says.

Entrained air from the mixing process creates multiple problems in food processing. Foaming, fouling in heat exchangers and increased risk of oxidation are just some of the issues. Almix’s use of vacuum doesn’t completely resolve them, but it can significantly reduce the amount of time needed for deaeration, Barragan says. As an example, he cites a trial comparison of 700kg (1,540-lb.) syrup batches containing aspartame. A conventional mixer took 15 minutes to blend the ingredients, followed by a two-hour hold in a syrup tank for deaeration. Using a high-shear vacuum tank, mixing was completed in three minutes, followed by a 30-minute hold.

The 2006 acquisition of a French firm specializing in powder handling has led to a number of innovations in the Almix, most notably the addition of direct or indirect steam injection for simultaneous mixing and cooking. That advance, coupled with mixing heads for varying degrees of shear, is opening up applications beyond dairy in sauces and other prepared foods, Barragan says.

The need to create an emulsion involving oil and water gave birth to the homogenizer for milk processing, but most food processes didn’t require the same intense energy inputs. When the need did arise, manufacturers often used a secondary mixing step. An example is salad dressings. Consumers don’t want dressings that require shaking, and manufacturers have responded by moving to high-shear units. An example is T. Marzetti Co.’s Horse Cave, KY facility, Food Engineering’s 2007 Plant of the Year (see “Opportunity knocks,” Food Engineering, April 2007).  Marzetti’s other facilities used “dixie mixers” backed by a colloid mill to emulsify oil and water, according to Jeff Harris, vice president-engineering. For the Horse Cave project, the units installed from Scott Turbine Mixer Inc. created enough shear to make the colloid mill unnecessary.

Bill Scott, founder of the Adelanto, CA company that bears his name, credits engineer Dave Muskoff of Kent Foods for leading the shift to “a continuous-batch process” for dressings, mayonnaise and sauces. Scott fabricates units with mixing shafts up to 25-ft. long for 6,000-gallon bowls. The design puts the paddles at the bottom of the bowl, pulling product into the impeller and propelling it upward for optimum circulation. Each shaft serves two bowls, resulting in a near-continuous feed (after shaft cleaning) to a tank farm where product is staged before packaging.

It took Scott decades to convince food processors he had a better mixing mousetrap. A UK-based company with offices in Norwalk, CT began a similar journey in recent years. Pursuit Dynamics plc was formed in 2000 to adapt marine propulsion technology to food processes (see “Supersonic processing,” Food Engineering, August 2005). The technology relies on the energy differential (thermal and momentum) between steam and a fluid stream to suck fluid and powders into a mixing chamber. Instead of moving parts, the steam does the mixing by condensing and creating an energy implosion that atomizes the product and transfers a controlled amount of heat.

Like Tetra Pak’s Almix, Pursuit’s system can simultaneously heat and mix a fluid stream, though the company’s head of product and process implementation argues the efficiency of the PDX Sonic reactor is unparalleled. “You’re using 99% of the steam on the product,” says Stuart Rigby. “With steam jacketing, you’re constantly adding steam.”

Pursuit’s early successes have come with barbecue sauces, ketchup, salad dressings and other products, particularly those dependent on heat-sensitive starches to bind with water. Products with a lot of oil can be a problem because of rapid temperature increases, but if water is the main ingredient, “we can fix it so thermally sensitive products don’t go above 69

Stuart Rigby conducts a mixing trial at Pursuit Dynamics’ pilot plant in Huntingdon, UK. Ketchup, sauces and salad dressings were an early focus of the firm’s sonic mixer, and applications are expanding into beer brewing and pasteurization. Source: Pursuit Dynamics plc.

Non-stop dough

Few segments of food manufacture are as closely identified with batch as bakery. Fewer still have worked as diligently to exploit the throughput advantages of continuous processes. Brew systems for no-time dough, such as the one built at Pepperidge Farms’ Bloomfield, CT bakery (“Built for speed,” Food Engineering, April 2004) are one piece of the puzzle. Scientists at Cornell University have applied supercritical fluid extrusion as a dough-mixing alternative, though short-circuiting the development step altogether impairs flavor development and creates other issues.

Raw-material handling specialist Reimelt Corp. recently overhauled a continuous system that had difficulties with soft dough. Separate zones were created for mixing and kneading, with a rest stage in between for gluten development (see related story on page 64). Likewise, Memphis, TN-based Exact Mixing Systems has made continuous refinements over the last decade to produce consistent chunks of dough, and Sancassiano also is improving a system that comes closer in delivering on continuous mixing’s promise. Significant improvements in liquid dosing are a notable advance, delivering more consistent, well-hydrated dough to the makeup line.

Even the promise of throughput gains usually isn’t enough to get a manufacturer to scrap the tried and true for the new and improved, allows Marcelo Ferrer, Alger Barragan’s counterpart for the Alblend system at Vernon Hills, IL-based Tetra Pak. “A lot of people are skeptical about continuous blending,” he says, “but we will write performance guarantees into the contract. The input costs for new beverages are rising, and consumers are demanding products that are more functional and expensive.”

Carbonated soft drinks were still the most attractive segment of beverage manufacturing when Tetra Pak introduced Alblend to the US market. It was positioned as an alternative to a syrup room but drew little interest. The boom in still beverages, on the other hand, has created an opportunity for inline blending. In recent years, engineers have refined the system by incorporating a buffer tank to capture out-of-spec concentrates at the end of a production run.

Processors are rethinking multiple-stream blending for complex beverages because of the uncertainty the correct stream is being adjusted when finished product is out of spec, says Ferrer. Some are concluding blending water with higher concentrations of dissolved solids and liquid ingredients in fewer, smaller batching tanks can also free up floor space and reduce cleaning and chemical costs. If water used to be blended with a 10 Brix solution, for example, they may shift to 50 Brix.

Waste from such a system costs five times as much, of course. Three years ago, Tetra Pak engineers incorporated a buffer tank with a Brix analyzer. If the concentration drops, the Alblend system recirculates the stream from the pasteurizer back to the buffer, where it is held until it is brought back into spec, Ferrer explains.

“Pasteurizers always have an issue with the initial mix phase,” he says. “A lot of product ends up going to drain. You’re still going to get some losses when the system shuts down, but the buffer tank saves ingredients and reduces the load on the sewage system.”

The 2000 animated film Chicken Run presented a vision of continuous chicken-pie production that Rube Goldberg would love. Real-world continuous processing looks somewhat different, and mixing systems that eliminate steps, as well as labor and energy inputs, qualify as advances in continuous processing. Bit by bit, technology is leading food manufacturers in that direction.  

For more information:

Lou Beaudette, Admix Inc.,

603-627-2340, lou@admix.com

Stuart Rigby, Pursuit Dynamics plc, 44-1480-422050,

stuartrigby@pursuitdynamics.com

Stephen Marquardt, Reimelt,

813-920-7434,

smarquardt@reimelt.com

Bill Scott, Scott Turbine Mixer Inc. 760-963-5720, bill@scottmixer.com

Alger Barragan, Tetra Pak Inc.,

905-780-4947,

alger.barragan@tetrapak.com

Marcelo Ferrer, Tetra Pak Inc.,

847-955-6320,

marcelo.ferrer@tetrapak.com

Small batches of dough exit continuously from a Codos mixer and are conveyed up an incline. After a 2-10 minute rest, the dough moves into a continuous low-shear kneader before going to a divider or other downstream machinery. Source: Reimelt Corp.

Continuous system mixing and kneading

While the double-spiral mixers engineered by Odessa, FL-based Reimelt Corp. trace back decades, recent advances bolster the company’s claim to the first continuous mixing and kneading process for all types of dough.

The US division of Germany’s Reimelt GmbH is commissioning the first North American version of the Codos system that is working in 20 European bakeries. Unlike conventional mixers, separate chambers perform mixing and kneading functions, with a 2-10 minute rest period in between. The core technology was acquired by Reimelt in 1994, but extensive development work has followed. “We did a lot of research and development on the mixing tools themselves, the motors and other elements,” according to Stephen Marquardt, product manager for Codos. The result is a system that minimizes stress on the dough and optimizes the biochemical process of dough development.

In the system’s first stage, flour and micro-ingredients are blended in a loss and weight feeder with water to achieve immediate hydration. Because fat disrupts biochemical reactions, it is introduced at the end of the feeder, before the dough is allowed to rest. In the second stage, interlocking kneading arms in a double-jacketed chamber gently pull dough into each flight, simulating hand-kneading and minimizing mechanical shear. Dough temperature increases about 4