As more fresh bakeries venture into the frozen arena, caution must be taken to minimize changes to product quality.
Cooling is a standard part of any baking process. Proper cooling is important to the integrity of finished baked foods and can truly impact a product's shelf life. Residual heat can cause a buildup of condensation after packaging, which negatively affects product texture and may become a catalyst for mold growth. “Bread needs to be uniformly cooled to a crumb temperature of 86°F to 95°F (30°C to 35°C) to avoid condensation in packaging and improve the slicing operation,” says Chris Hanssen, freezer sales support and business development manager, FMC FoodTech Inc., Northfield, Minn.
In some cases, bakers who primarily market fresh products have expanded their portfolios with frozen items. Freezing offers many advantages, chiefly among which is cost savings. Freezing also allows the bakery to become more dedicated to longer runs, eliminates peak demands, extends storage stability and gives the bakery far more leverage over its distribution area, notes Tim Sieloff, baking instructor, AIB International, Manhattan, Kan., who presented a seminar on freezing at the American Pie Council's Pie Industry Seminar in Orlando last month.
As with cooling, freezing requires a good measure of control; perhaps even more so, since frozen products are subject to far more environmental changes than those that are cooled. “Anytime the environment temperature changes, product is a recipient of that temperature change,” Sieloff says. As a result, using the proper equipment and freezing method is critical to ensuring finished product quality.
While moving from fresh to frozen processing might seem straightforward, freezing technology is far more complex than anyone might think. Any baker considering such a move should run small-scale shelf life testing through various freeze/thaw cycles before taking on full- scale production.
When a product is frozen, its water phase effectively changes from a liquid to a solid state. A product's freezing point is affected by its formulation and chemical composition. For instance, the amount of solutes in the formula, such as sugar, will affect the freezing point. The higher the sugar amount, the lower the freezing point. Thus, there can be as much as 10 percent water left unfrozen if the product is not frozen properly. Sieloff recommends freezing leaner products at 20°F to 25°F (-4°C to -7°C) and richer products, or those containing higher levels of solutes and fat, at 13°F to 18°F (-8°C to -11°C).
“In freezing, two types of heat must be removed: sensible heat and latent heat,” Sieloff explains. “Sensible heat is the heat that must be removed to lower the core temperature of the product closer to the freezing point, and once cooled, latent heat is the heat that must be removed to change the physical state of the product from liquid to solid form.” If product isn't through its latent heat phase before it is placed in a holding freezer, where the temperature may not be sufficiently cold enough to drop its temperature further, it may increase in temperature. When the temperature rises back above the freezing point, water within the product remains in a liquid state. This free water can cause a variety of quality issues.
Any amount of water left unfrozen can damage the product, particularly during freeze/thaw cycles that occur throughout storage and distribution. Residual unfrozen water left in the product can create problems with moisture migration, dehydration and crystallization during storage. Moisture loss of 0.8 percent to 1 percent is typical, but it can be as high as 2 percent, Sieloff notes.
The effects of freezing differ from product to product based on a product's formulation, its chemical composition, size, shape, packaging, rate of freezing and method of freezing. Improper freezing methods or excessive freeze/thaw cycling can result in changes to a product's eating quality — changes that continue throughout shelf life. These changes include crust separation and icing defects, as well as microscopic damage, dehydration and staling, all of which affect product eating quality and texture, Sieloff notes.
The rate of cooling, or how quickly heat is removed from a product is a critical quality parameter. For example, chocolate enrobed donuts need to be cooled at a controlled rate, notes Rickey Woods, president, Woods Fabrication Inc., Taylorsville, Ga. If these donuts are cooled too quickly, the chocolate coating skins over, preventing heat from being released from the product's inner core. Chocolate “blooming” problems can also be avoided by proper cooling rates.
Under certain conditions, freezing too quickly dries out a product's surface, which can cause the crust to crack, flake or blister. Pre-cooling baked products to 104°F (40°C) using gentle airflow at ambient temperatures or slightly below can help prevent these defects, notes Hanssen. “In products, such as buns, the crust may separate from the internal part of the bread. This happens when the surface contains less water than the rest of the product and when the temperature gradient through the product is too high. One way to solve this problem is have a shorter holding time in the freezer. An outfeed temperature of 14°F (-10°C) is recommended,” Hanssen adds.
Freezing rate also is critical as product passes through the cooling temperature range of 50°F (10°C) to 23°F (-5°C), which is the Critical Staling Zone, where staling is the highest. “Slow freezing and thawing of baked products results in significant staling and reduced shelf life,” Hanssen explains. “Test results have shown that bringing product quickly through the Critical Staling Zone increases shelf life. Freezing a single, unpackaged bun can reduce shelf life by two hours, whereas slowly freezing a master carton of packaged buns can result in shelf life loss of one day. The same is true in reverse for thawing. Quick thawing reduces the risk of staling.”
Cooling systems configurations
Photo Courtesy of Sarah Simonis.
Photo Courtesy of Joanne Ho-Young lee.
The type of cooling method used depends on the product, manufacturing footprint and throughput, among other criteria. Ambient spiral coolers are the most common method used for cooling baked products produced for fresh distribution, particularly when moving large capacities of product using long belt lengths, Hanssen notes. Spiral coolers, which have taller vertical clearance, are better suited for larger products, such as buns and bread loaves.
Cooling tunnels are widely used for smaller baked products, including snack cakes, donuts, pastries and cookies, Woods notes. A cryogenic cooling tunnel that uses liquid CO2 or liquid nitrogen can reportedly minimize dehydration better than a mechanical spiral cooling system with high velocity air movement, due in part to the difference in the amount of open and exposed surface area inherent in each unit. Still, an impingement, blast or cryogenic freezer that combines cold temperatures with high velocity air movement removes heat faster than a holding freezer that has no air circulation. The baker must select the cooling system that removes heat from the product effectively without causing excessive dehydration.
Thin, flat products, such as pitas, tortillas and pizza crusts, can be cooled in impingement tunnels. Pre-cooled thin, flat products are more easily handled during automated packaging processes. FMC FoodTech's patented impingement technology directs high-velocity air toward the top and bottom surfaces of the product. The air jets remove the boundary layer of air that holds heat in the product, which effectively cools the product rapidly. “For instance, tortillas can be cooled from 150°F (66°C) to below 30°F (-1°C) in the ADVANTEC™ impingement tunnel using -32°F (-36°C) air temperature in 17.5 sec.,” Hanssen says.
Shape-sensitive, delicate or sticky products may require contact plate freezing before entering a spiral freezer. As product is conveyed over the contact freezer plate, its bottom surface freezes, ensuring that it maintains its shape as it moves into and through the spiral freezer.
Aside from the importance of food safety and hygienic standards, ease of maintenance and sanitation, bakers value certain design aspects in a cooling system. Among these criteria are safe and user-friendly operator handling, fast and efficient freezing, a modular design with minimal footprint requirements, product handling flexibility, belt flexibility options, reliability and larger capacities that are capable of longer run times.
“Air circulation and zone controlling of temperature on certain products is very important,” Woods says. “Another important design aspect is controlling the humidity within the system.”
Matching the appropriate cooling system to the type of bakery operation is an important factor in maximizing product quality and shelf life. In the end, the cooling method must be selected on the basis of product type, throughput, capacity, run time, available floor space and capital investment. Suppliers of cooling systems can help bakers acquire the right type of cooling technology for their needs. Ultimately, finished product integrity may depend on it.