Dough temperature must be regulated during mixing, lest the dough become too hot or cold and thus unusable. Luckily, a wide array of cooling options exist for mixing and other downstream processes.
Imagine arriving at the bakery one morning to discover boxes of dough balls, neatly stacked the night before, upended with over-fermented dough oozing out. The ruined dough forces the bakery to close for the day. This is one worst-case scenario of what can happen after dough emerges from the mixer only 5°F to 10°F warmer than desired. Warm dough does not cool at the necessary rate and ferments longer in the cooler, often resulting in the “blown dough” example above. This is because yeast ferments faster at higher temperatures and slower at lower temperatures. A warm dough temperature also can result in poor crumb structure and crust quality, low volume, unbalanced shapes and sizes within the batch, sticky dough and off flavors.
The temperature of ingredients when they enter the mixer and regional climate also affect final dough temperature. While doughs that are too warm tend to be sticky, doughs that are too cold may not mould well. By controlling temperature, bakers can regulate the batter’s consistency or doughfeel, as well as the rate of fermentation in yeast-leavened products and rate of reaction in baking powder in chemically leavened products.
The energy required during mixing raises the overall dough temperature. Today’s mixing systems strive to use minimal energy and offer solutions to keep dough at the optimum temperature. Most mixers accommodate a myriad of methods for controlling dough temperature, including bowl jacket cooling, bowl end cooling, adding ice or water, bar breaker cooling and agitator bar cooling. Other possibilities include using chilled ingredients. Flour chilling systems, as an example, inject liquid carbon dioxide (CO2) into the flour lines, so the flour is cooled before entering the mixer. Dry ice or CO2 in solid form, or liquid nitrogen also can be used to cool dough. “The greater number of components that have cooling capabilities in the mixer, the more effective the cooling process will be. This may prove valuable in a freezer dough where the final desired dough temperature is colder than a bake-off type dough,” one expert says.
Some manufacturers say relying on only cold water to cool dough can result in products with poor volume or crust quality. Still, water stands as the most inexpensive and common way to chill dough. In pizza dough formulations, chilled water between 60°F and 65°F will maintain a dough temperature of 80°F to 85°F, assuming the room temperature is 70°F to 75°F, says Tom Lehmann, director, bakery assistance, AIB International, Manhattan, Kan.
Some manufacturers prefer adding ice directly to the dough. Chip ice has a five to seven times greater cooling capacity than water, manufacturers say. Chip or shaved ice is preferred over cube ice, because it melts faster, preventing ice pockets that can form when ice is not completely melted into the dough. “It’s imperative that the ice be shaved or chipped ice,” says Lehmann, who is known as “the Dough Doctor.” If less than four minutes of mixing time remains and ice particles are still present, water pockets are a distinct possibility. Water pockets can lead to wet, soggy spots in dough and can create large holes, tunnels or blisters in the product, he adds.
Cubed ice melts slowly and can freeze together, resulting in longer kneading time and higher energy costs. This can cause longer dough times or batch to batch inconsistencies. To combat this challenge, Ziegra offers Micro Ice, which is smaller than traditional chip ice or fine chip ice. At 0.2 in. by 0.2 in. by 0.1 in., Micro Ice is delivered at a constant 31°F, says Mike Hartnett, president, Kaak Group North America, Lithia Springs, Ga., the distibutor for Ziegra systems in the United States. “The combination of its size and constant temperature means Micro Ice does not form lumps because it never freezes together,” he adds. Ziegra also offers Stream Ice for applications using between 20 percent to 40 percent ice to water, although percentages can be adjusted depending on climate or time of day.
“Adding ice to the dough doesn’t have to be messy or inconsistent. The Ziegra systems offer an automated dispensing system directly into the mixer with extremely accurate dosing. This eliminates the need for manually adding ice, which can be inconsistent and messy,” Harnett says. Ziegra currently is working on a system that will use almost 80 percent ice to water and should be available in early 2008.
Harnett says adding ice is a more cost effective way of cooling dough, because to cool dough with a jacketed horizontal mixer, refrigerated arms are also needed, and spiral mixers cannot be jacketed.
Jacket bowl cooling
Another method of chilling dough is jacket-bowl cooling. Picture a double-layered mixing bowl with a food grade glycol, also known as antifreeze, a common cooling medium, running between the two bowl layers. Using this method, about 80 percent of the bowl is jacketed and covered by the cooling agent. Tap water or salt water also can be pumped between the layers in place of glycol. “Water freezes at 32°F, but [with salt water] you can get your temperature down into the mid 20s,” Lehmann says. He adds glycol can get so cold it does not freeze at 0°F.
Regardless of the chosen coolant, jacket bowl cooling is “the primary method of cooling,” says Tim Cook, vice president, sales and marketing, AMF Bakery Systems, Richmond, Virginia. “That’s what’s going to give you the most added coolant to the dough. Then, if you’re in a situation where every one or two degrees matters, that’s when you start adding other measures like bowl end cooling, which I would argue is the next most important area of cooling,” Cook says. Adding breaker bar or agitator bar chilling is the next step. AMF has incorporated the jacket bowl cooling feature into its standard unit as most manufacturers require it. AMF offers three families of mixers, including the stationary bowl mixer, regular and offset tilt bowl mixers of various sizes, all of which offer jacket cooling and can include additional cooling options.
Bowl end cooling
Manufacturers who need an even colder option, might consider bowl end cooling, which chills the ends of the mixing bowl. While bowl end cooling alone does not offer much coverage, when used with jacket bowl cooling, it proves effective.
On a horizontal mixer, “the bowl itself is like a big cylinder or barrel. If you take the barrel or cylinder and lay it down horizontally, the jacket is the horizontal portion, the longest section. The bowl ends are the top and bottom of that barrel. The ingredients splash around that bowl and get to those bowl ends that are cooled and get colder,” says Alain Lemieux, product manager, dough systems, AMF Bakery Systems. “It’s a way of increasing the total refrigerated surface of the bowl,” Cook adds.
With direct expansion, which Lehmann says is most efficient, refrigerated coils inside the mixing bowl become extremely cold. “You can actually freeze the dough to the side of the bowl on the inside if you’re not careful and stop the mixer and leave the dough laying there.”
Direct expansion uses a gaseous refrigerant similar to the one used in household refrigerators, Lemieux notes. “Using a gas instead of a liquid implies different technologies on the mixer bowl jacket and the refrigeration system. In addition, the EPA agency has required the replacement of freon R22 to other non ozone-depleting substances in older installations. This is why most bakeries now use glycol cooling,” he says.
Breaker bars and agitators
Including breaker bars and agitator bars in the cooling process is a relatively new concept. The breaker bar is the solid fixed bar that runs across the top of the bowl.
“The dough keeps striking that bar, so there’s a great opportunity, even though you don’t have much contact surface, for refrigerating that dough or cooling it down,” Lehmann says. By contrast, an agitator sits directly inside the dough mixer.
“Most of our customers opt for bowl end cooling and breaker bar cooling when they are looking for additional cooling options for their mixer,” says Larry Gore, director of product marketing, AMF.
Determining what cooling system to use depends on a customer’s unique dough cooling challenges and the amount they wish to spend on mixing equipment, Cook says. The more cooling components added to a mixer, the greater the expense, he adds.
Today, most horizontal mixers are standardly equipped with a glycol chilled bowl. Bowl end cooling is the most cost-effective method of increasing cooling capabilities, followed by breaker bar cooling.
Agitator bar cooling is the most expensive and adds a lower incremental benefit than bowl end and breaker bar cooling, Gore says. He adds agitators can be shaped to allow for better mixing ability. One example is the YT agitator or a slanted bar-type agitator, which is gentler on the dough and also exposes more of the dough to the bowl cooling surface.
During flour handling, the flour moves through a tube, as it is either pushed by air pressure or pulled by a vacuum. In both cases, this causes friction that heats the flour. Adding warm flour to a mixer puts a greater burden on that mixer to cool the ingredients. Lehmann says this is why many bakeries use cryogenic flour cooling, an effective process that uses cryogenic gas, such as CO2 or liquid nitrogen, to cool the flour before mixing.
Reimelt is one company that examines flour temperature and uses air cooling systems to cool the flour, says Anthony Jeskey, vice president, sales, Reimelt Corp., Odessa, Fla. He adds flour cooling with CO2 can be costly as the bakery must buy the CO2 and maintain a contract with a CO2 supplier for an extended time. Lehmann adds bowl and jacket cooling or some additional cooling process is almost always needed in addition to flour cooling.
Adding CO2 directly to the mixer is an effective method for maintaining but not lowering dough temperatures, Lehmann says. “If we have a target temperature of 80 degrees and the dough has gotten up to 85 degrees and we want to cool it down to 80, injecting a cryogen into the mixing bowl is not very efficient at all, but if you have a PLC monitoring the temperature of the dough, and then you are continually injecting lesser amounts of cryogen into the mixer to maintain the dough at 80 degrees, it’s very efficient,” Lehmann adds.
There is no danger to the dough because liquid CO2 is one of the natural gases produced by fermentation. The liquid CO2 itself is not sent into the mixer, but blown in as a cryogen snow, which is cold but not cold enough to damage yeast, Lehmann says.
Cooling methods post mixing
Post mixing, dough sometimes can use additional assistance in maintaining its proper temperature. Turkington APV USA, Goldsboro, N.C., offers the SofTouch™ Divider, which uses liquid cooled rollers to maintain consistent optimal dough temperature and stable dough pressure during dividing.
Using a cooling system in the rounding bed is another option. When the dough exits the mixer it is processed into dough balls that travel across a rounder bed where they are shaped. This bed can be chilled to further control dough temperature, AMF’s Lemieux notes. In a hot, humid climate, when the cool dough hits a warm rounder bed, condensation can form on the bed, and unless an oil drip is installed, the dough sticks. If the bed is cooled, however, condensation does not form and sticking is not a problem, Lehmann says.
Regardless of the cooling system used, problems arise when cooling dough. It can prove difficult to determine what the temperature of ingredients, such as water, must be, to achieve correct dough temperature at the end of mixing. Other issues involve deciding how much ice to use and managing the timing of the cooling system.
One challenge when using ice during the mixing process is counteracting the additional mixing time required to melt the ice, as longer mix times increase temperature, Hartnett says. “Micro Ice and Stream Ice dissolve quicker in the dough, so you don’t have to heat the dough to melt the ice. Micro Ice and Stream Ice also are the same temperature every dosing,” Hartnett notes. The warmer the incoming ingredients, the more difficult it is to obtain the desired temperature. A humid climate also can pose problems.
“A plant down in the middle of Florida in the summertime is going to have more challenges cooling dough than a plant located in the North,” Cook says. Controlling the inputs during mixing also can be tricky. A company using manual operation might rely on someone to set the length of the mix time and the amount of coolant, leaving more opportunity for human error and thus inconsistencies in the dough, Cook notes.
In the past five years, advancements in dough cooling have escalated. Among the most notable is “adding control tied to a probe in the mixer that can monitor the temperature of the dough as it’s being mixed,” Cook says. “That allows us, through a series of controls, to know whether the dough is reaching the correct temperature and whether it needs more refrigeration time or lower temperature to achieve the correct finished dough temperature,” he adds.
“Automation and the control process of the cooling circuit remove a lot of the human error factor and also the seasonal change factor in order to obtain [regularity in] dough temperature even across the season changes,” Lemieux adds.
In addition, the baking industry in the United States is seeing a resurgence in growth, with more small-to intermediate-size bakeries flourishing, whereas 25 years ago, small bakeries were flailing alongside wholesale conglomerates, Lehmann notes. Today’s small bakeries do not require large horizontal mixers that hold 800 to 2,800 pounds of dough, Lehmann says. Instead, smaller operations can benefit from open bowl spiral mixers, which hold 200 to 300 pounds. “It’s been difficult to use those mixers mostly because temperature control is lacking, but now we’re seeing cryogen gas injection into those mixing bowls. CO2 is heavier than air, so it tends to settle in that mixing bowl and stay there, and we’re finding that by injecting the cryogen gases into these bowls, these mixers are becoming more realistic for small and intermediate size bakers to use, and they’re relatively cost efficient too,” Lehmann says.
Some manufacturers say the future will hold more advancements in flour cooling, which has come into focus in the past five years. “The optimal system could include a combination of both flour cooling with water or glycol and ice,” Hartnett says. Most agree new and improved ways of adding coolant to mixing will arise, as mixing begins to use less energy than previously required. With all the advancements and options available, “cooling becomes a pretty straight, helpful issue,” Lehmann says.