Impingement freezing uses thousands of high velocity air jets to direct air at the top and bottom surfaces of a product. These jets of air blast away the boundary layer of air that holds heat around the product, resulting in rapid freezing speeds. In 1994, impingement technology was developed for flat products as a way of delivering the same speed and quality as cryogenic freezing but at half the cost. By 2001, the technology was being used in the processing of more than half of the frozen hamburger patties processed in the United States.
Impingement freezing results in less dehydration than other methods because dehydration is a function of holding time within the freezer unit. If products are frozen slowly, there is time for moisture to migrate, which causes larger crystals to form. Products with larger crystals are not as moist, and as a result, the quality of the frozen product changes. A faster freezing time results in formation of smaller ice crystals, which means less cellular damage to the product. In food applications, this means the product is juicier, has better texture and exhibits less drip loss when thawed. For instance, in the case of white fish fillets, freezing in cold storage results in dehydration of 5 percent to 6 percent. Air blast freezing in static tunnels creates 4 percent to 5 percent dehydration. In conventional inline spiral freezers, it ranges from 1.5 percent to 2 percent, and in impingement freezers for flat precuts, the dehydration ranges from 0.6 percent to 1 percent.
Impingement Goes Beyond FreezingImpingement technology works just as well for cooling and chilling as it does for freezing applications. One confectionery producer, for example, required quick cooling of a bed of licorice. Using an impingement freezer to cool a three-layer, 0.43" (11 mm) thick sheet of product with an in-feed temperature of 149oF (65oC) and an out-feed temperature of 59oF (15oC) gave a dwell time of just 90 sec, permitting processing of 5,291 lb (2,400 kg)/hr in two modules.
In another application for a processor of fully cooked, 1.1 oz (31 gram) chilled chicken tenders, the in-feed temperature was 176oF (80oC) and the target out-feed temperature 34oF (1oC). The freezer permits a dwell time of 3.5 min on a belt width of 4.1' (1,250 mm) and belt load of 9.9 lb/ft (4.5 kg/m). Dehydration is a relatively modest 2 percent, and the capacity is 992 lb (450 kg)/hr in a single, 18' (5.5 m) long module.
Some impingement freezers offer more flexibility than a flat product freezer and are designed to fit into the same facilities that would typically use liquid nitrogen tunnel freezers. For example, one type of impingement technology is characterized by air jets that impinge directly on both sides of the flat product, removing the thermal boundary layer that surrounds it, which results in better heat transfer than conventional spiral freezers using mechanical refrigeration.
These more flexible versions of flat product freezers can be applied to a range of processors, plants and configurations, accommodating conveyor belt widths up to 6' (1,800 mm) or up to four 1.5' (450 mm) wide lines operating simultaneously. The latter configuration allows several independent product lines to run simultaneously. Additionally, certain units are able to handle product thicknesses up to 1.9" (50 mm) for freezing and up to 7.9" (200 mm) for chilling operations.
For a processor of uncoated mozzarella cheese sticks, where drying is the first step of the process, one benefit of its impingement freezer is that it can take the place of a dryer. Due to the stringent cleanability needs of food processors, ease of cleaning is designed into impingement freezers, incorporating all curved surfaces and corners while offering high visibility.
Freezing on the High SeasImpingement freezing is not confined to land-based plants. Freezers that have been specially designed for installation onboard fishing vessels permit quick freezing of fish products and provide improvements in product quality.
In many cases, fish processors are interested in the thickness of the ice glaze that the impingement process can apply to fresh fish, keeping them chilled during their trip through a supply chain that ends in open-air markets with little or no refrigeration. A rule of thumb used by chefs is that every degree of temperature above 32oF (0oC) that fish are exposed to adds the equivalent of one day in the supply chain. These fishing vessel freezers are designed to virtually eliminate that deterioration.
Since its foray into the food processing industry nearly a decade ago, impingement freezing technology has made great strides. Now recognized as valuable across a variety of applications and industries, it can only be expected that impingement freezers will grow increasingly sophisticated with time. PCE