Minimizing defects

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.

Among the many options bakers should consider when selecting a cooling system is the amount of floor space available in the plant. Different modular designs are available to meet a variety of needs.

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.

Design criteria

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.