Impingement freezing can improve product quality and safety, as well as processing efficiency, by providing a fast, controlled method for surface freezing meat products. Since the development of the first freezer 45 years ago, the food industry has produced increasingly sophisticated equipment designed to keep foods fresher longer. Recent research has shown that a process previously thought to be negative to food products, known as surface freezing or crustfreezing, is actually a viable way to rapidly cool meat cuts, provided the freezing is extremely fast and even. In fact, using the impingement technique to crustfreeze meat cuts can result in better production yields, improved product quality and a longer shelf life than was possible using old-fashioned bulk storage rooms and traditional fast-cooling systems.

The Time/Temperature Equation

Microbiological quality is a crucial factor in the handling of chilled products and is especially important when products of animal origin and prepared foods are involved. A higher degree of processing results in a more sensitive product. Recipe changes such as a reduction in salt, sugar or other ingredients can alter the conditions for microbiological growth dramatically. Assuming that high-quality raw materials are optimally processed and packaged, time and temperature will be the main parameters that determine the shelf life of a product.

Time and temperature can be influenced, but both factors have their limits. The time factor is difficult to alter radically because the time between production and consumption has already been minimized to achieve the lowest possible costs. Temperature can be changed more easily and at a cost that is often negligible considering the benefits of increased shelf life and safety.

All food products are contaminated with microorganisms during handling; the number of microorganisms depends on hygienic conditions. During slaughter and subsequent handling, meat is contaminated from the animal itself and from the environment. In general, spoilage bacteria thrive and multiply to a greater extent on the product surface. When the product is processed and the meat is portioned, the new surface becomes infected. As the surface-to-volume ratio increases with the degree of cutting, microbiological problems increase. In these cases, fast cooling is essential to reduce the bacterial growth rate.

Another important food quality factor influenced by cooling is drip loss, which occurs during storage and distribution. Equally important is the dehydration loss during the cooling process. These factors are economically significant and also can negatively affect the sensory properties of the food. Not only does the product lose weight, but the appearance is also negatively affected since the fluid is contained in the package. The loss of fluid -- drip loss or dehydration -- depends on a number of factors, but time and temperature are the most important during the cooling process.

The Demand for Faster Cooling

For many years, a chill room kept at 36 to 46oF (2 to 8oC) or a simple cooling tunnel has been the industry's most common cooling equipment. Most products are packed when cooled, often in master cartons weighing between 22 and 66 lb (10 and 30 kg). In large operations, products are cooled in single packs in automated tunnels. In both cases, the products are sometimes subjected to very low temperatures (-4 to -22oF [-20 to -30oC]) before the product temperature is equalized at air temperatures above the freezing point.

Figure 1 illustrates the difference in product temperature during cooling in a chill room and a tunnel with a low air temperature. The products to be cooled were wrapped in poly sheets and packed in cartons weighing 55 lb (25 kg) each. The cooling rate in the chill room was obviously very low, but even if the rate was increased when low temperatures and forced air circulation were used, the cooling time was in the range of 24 hr, including equalization.

A simple way to reduce the cost of large-scale equipment -- and to some extent, the cooling time -- is to blow carbon dioxide snow into the cartons. In this process, cooling is carried out during handling and distribution, but still takes considerable time -- 6 to 24 hr, depending on the type of product. The initial investment costs for this method are low, but the operating costs are high.

Systems using air temperatures below the freezing point or carbon dioxide snow systems normally create an uneven partial freezing of the product surface. This partial uncontrolled slow freezing often leads to an increased loss of fluid from the product.

For these reasons, an increasing number of companies have begun searching for systems that provide faster cooling to low temperatures. Fast cooling provides improved sensory and nutritional quality as well as better safety for the consumer. It also provides improved economics throughout the distribution chain by increasing product shelf life, reducing waste due to returns from retail and catering and minimizing weight loss and dehydration during processing. Efficiency also is increased because less space is required in the production line where a continuous inline cooling is achieved. Most importantly, a lower capital cost from production to retailing and catering is achieved because of the reduced lead time for handling and distribution.

A Chilling Advance

Controlled, extremely fast surface freezing of meat cuts and meat products makes it possible to cool those products to an equilibrated temperature of 32 to 25oF (0 to -4oC) within 1 to 8 min of holding time. The product temperature equilibration time is in the range of 90 min. Some examples are given in table 1.

The products are crustfrozen in an impingement freezer at an air temperature of -22 to -40oF (-30 to -40oC) at high air velocities. An even ice zone is formed to a depth of 0.07874 to 0.2362" (2 to 6 mm), as shown in figure 3, which results in a product equalization temperature in the range of 32 to 28oF (0 to -2oC).

The fast freezing of the product's surface minimizes dehydration compared to other air-cooling methods. Comparative cooling tests between an inline carton chiller and impingement freezer resulted in 0.6 and 0.2 percent dehydration, respectively. In both cases, the infeed product temperature was 46oF (8oC). The equilibrated product temperature was 36.5oF (2.5oC) when chilled in the carton chiller for 3 hr, compared to 32oF (0oC) when chilled for 4 min in the impingement freezer. The air temperature in the carton chiller was kept no lower than 28oF (-2oC) to avoid any uncontrolled freezing of the surface. This is how most single packs and unpacked products are handled when cooled individually.

Benefits of Fast, Controlled Freezing

Compared to a non-frozen products, surface freezing during the cooling process can lead to a larger loss of fluid if the freezing occurs at a slow or moderate rate (10 to 60 W/m2-oC), which is illustrated in table 2. This loss of fluid will not appear until the product has been equalized to the desired temperature.

The first ice crystals are formed outside the cells where the freezing rate is higher because the fluid is more diluted than inside the cells. Once started, the rate of ice crystallization is a function of the speed of heat removal, as well as the diffusion of water from within the cells to the intercellular space. If the freezing rate is low, few ice nuclei are formed in this space. During the freezing process, water diffuses out from the cells and crystallizes on the existing few crystals or nuclei, which thereby grow in size. This results in cell damage, and can provide an increase in drip loss when the product is thawed.

Ice nuclei can also form within the cells during fast freezing. However, the diffusion is less pronounced and the original small ice crystals are kept intact, which results in less cell damage and less or no drip loss. The difference in impact on the cell structure is illustrated in figure 3, which shows the cell structure of hamburgers frozen at three different rates.

A standard inline spiral freezing system would freeze this hamburger in about 20 min with a heat transfer coefficient of 25 to 80 W/m2-oC. In an impingement freezer, the same hamburger would require just 2 min, 40 sec freezing time at a heat transfer coefficient of 250 to 300 W/m2-oC. This difference explains the lower and sometimes non-measurable drip loss from impingement-cooled products compared to alternative crustfreezing processes.

A pilot-scale test comparing impingement to carbon dioxide cooling of pork chops showed significantly lower drip losses immediately after temperature equalization. However, after eight days the difference was less apparent (table 3). No significant difference in dehydration losses was evident in this investigation.

Improved Microbiological Quality. The microbiological status of the products used in the pilot test comparing impingement and carbon dioxide cooling indicates a slightly lower number of microorganisms, 1 log10, in the material cooled by surface freezing. These results were confirmed in a model study with Pseudomonas fluorescens.

It is known that under certain conditions, freezing can be lethal to certain microorganisms. Ongoing research is focusing on identifying the variables that might be used to optimize the freezing, thawing and equalization processes as a future food safety technology. Further research is needed to specifically study the effect of different freezing rates on the survival and viability of microorganisms in general, and the most important pathogens specifically.

A Viable Process

Research shows that crustfreezing is a viable, efficient process for rapidly cooling meat and other products, provided the heat transfer coefficient is in the range of 250 to 300 W/m2-oC or higher. This fast chilling method achieves positive results related to dehydration, drip loss, sensory properties such as texture and appearance, and processing efficiency.

Equipment capable of fast surface freezing, such as an impingement freezer, provides short holding times that can be tailored to the product, resulting in a more efficient cooling operation. Improved temperature control at a low temperature results in improved product safety and longer shelf life. Crustfreezing with impingement also increases flexibility, reduces processing times and energy consumption and provides for a more efficient use of factory space. The end result is a higher-quality product to the consumer at a lower capital cost throughout the distribution chain.

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Stefan Goransson and Goran Londahl are engineers with Frigoscandia Equipment AB, Helsingborg, Sweden, a manufacturer of coating, cooking, freezing, frying equipment for food processing and production. For more information, call FMC FoodTech at (312) 861-6000; e-mail