Food freezing has been a subject of research since Clarence Birdseye’s time (and undoubtedly much earlier but not as scientifically). Here’s a brief description of some current areas of research and development:
• Air-Impingement Freezing. In the past few years, companies have been paying more attention to increasing the effectiveness of the heat-transfer medium, said David S. Reid (phone 530-752-8448), Professor of Food Chemistry at the University of California–Davis. One approach has been to develop redesigned air blast freezers to get the same cooling rates as cryogenic freezers. A blast of air doesn’t really contact very well, he said, so some equipment manufacturers are designing systems in which thin, high-velocity jets of air are directed at the food, resulting in much better turbulent heat transfer, approaching the heat transfer rate of cryogenic freezing. Some systems have become commercially available over the past few years, he said, but more are coming on the market, since they give more effective freezing and better throughput.
• Pressure-Shift Freezing. Researchers have been studying pressure-shift freezing as a means of improving frozen food texture. Research conducted by Dietrich Knorr and his coworkers at the Berlin University of Technology has shown that pressure-shift freezing produces smaller and more-uniform ice crystals than conventional freezing. By increasing the pressure to about 200 MPa, the freezing point of water is depressed, allowing the product to be cooled to about -20°C without causing the water present to freeze. When the pressure is released, the water rapidly freezes—ice nucleation occurs uniformly throughout the product, and small, uniform ice crystals are formed. This results in less damage to the product from the crystal growth and finer texture. The method is still under study by Knorr and coworkers and others elsewhere, and a Japanese company may be using it commercially.
Flow International, Inc., Kent, Wash., manufacturer of high-pressure equipment, has done some experiments with pressure-shift freezing of ice cream. Flow’s Vice President Edmund Ting (phone 253-813-3346) said that since the pressures utilized are lower than those currently used for microbiological reduction (200 MPa for freezing, compared to 600 MPa for pasteurization/sterilization), the equipment can be simpler and less expensive. He added that Flow is willing to work with any interested parties in pursuing this approach.
• Extrusion Freezing. In Switzerland especially, Reid said, there is a big push to develop extruder freezers for ice cream. Traditionally, ice cream comes out of the freezer at -7°C then is placed in the hardening room. Using a feeder extruder and higher pressure allows the ice cream to come out at -20°C straight from the freezer, replacing the usual scraped-surface heat exchanger. The crystal size is more uniform because freezing is done under much better control. Nestlé and Unilever have been working in this area, he said.
• Modulated Freezing. This approach uses a conventional freezer but a different thermal profile and nucleation control. It produces a similar pattern of small ice crystals to that produced by pressure-shift freezing, Reid said. But is the quality enhancement of pressure-shift freezing due to use of high pressure or to small ice crystals? If it’s due to small crystals, he said, existing equipment can be reprogrammed, eliminating the need for expensive high-pressure equipment.
• Modeling. Freezing prediction models are being improved and made more user friendly. The warehouse industry routinely makes use of freezing time predictions. For example, the International Association of Refrigerated Warehouses (IARW) made available to its members a model developed by Paul Singh at the University of California-Davis to predict freezing time. We know that models in use are valid for most regular freezing processes because they have been validated, Reid said. The biggest challenge is in putting in the heat transfer coefficient. If we’re freezing something in air, the heat transfer coefficient could differ depending on how powerful the fans are, whether air impingement is being used, and other factors. Someone knowledgeable still has to choose the heat transfer coefficient to use in the model, but Singh and coworkers are developing a bigger database, a new generation of programs that make it easier to choose the coefficient.
• Nondestructive Testing. Reid said that the Educational Foundation of IARW (the World Food Logistics Organization) is seeking nondestructive methods to monitor whether a product has been properly frozen before it is put into storage. Researchers at UC-Davis are looking at ultrasonic and MRI techniques. We can use a calorimeter to measure how much heat is removed in freezing, Reid said, but that’s a destructive method, since we have to thaw the product to do so.
One nondestructive technique being investigated is magnetic resonance imaging. Using calorimetry and computer models, we can determine how heat content changes with time in the freezer, he said. Is the interface actually in same position as we see in the MRI image? If so, we can use MRI to find out where the interface is. Another method to find the interface is ultrasonics, which can be used to determine how much liquid and solid is present.
Reid said that nondestructive testing is feasible, but we need sensors that are robust enough and cheap enough. MRI is feasible, but is it economically practical? Ultrasonics is cheaper, but can it be correlated with heat content?
Three papers presented at IFT’s 2001 Annual Meeting last June addressed various aspects of freezing.
Ferruh Erdogdu of the University of California-Davis discussed development of software to predict freezing times of foods. The objective of the work by Erdoglu and coworkers R. Paul Singh and Jatal D. Mannapperuma was to provide a Windows-compatible, Internet-accessible package for the prediction of freezing times of foods of different geometrical shapes. They developed a numerical method for change in heat conduction with change in enthalpy for 1-, 2-, and 3-dimensional geometrically shaped foods subjected to variable boundary changes (surface temperature and heat transfer coefficient), and a model to predict the thermophysical properties required by the enthalpy method was used to simulate the freezing process. The predictions of the software compared well with experimental data. The software provided a fast calculation of freezing times for different-shaped foods and of temperature distribution and change of thermal properties in the product during freezing.
Suyong Lee of Purdue University discussed use of ultrasonic and magnetic resonance methods to quantify and locate solid and liquid phases in frozen foods during processing and storage. Since the quality of frozen foods depends on the presence of the ice crystals and their distribution during freezing and storage, Lee and coworkers Yong-Ro Kim and Paul Cornillon used nuclear magnetic resonance and ultrasonics as a nondestructive method for measuring the different phases present in frozen foods.
Rutger M.T. Van Sleeuwen of Rutgers University discussed the effect of bacterial extracellular ice nucleators (ECINs) on the temperature history profile in foods during freezing and the effect on ice-crystal size and crystal size distribution in a model food.
According to Van Sleeuwen and coworker Tung-Ching Lee, ice-crystal formation during freezing can lead to deterioration of food, especially large crystals that form during slow freezing. Individual-quick-freezing (IQF) processes produce small ice-crystals but generally can’t be used for large food products because the freezing rates are mostly governed by the heat transfer within.
ECINs are cell-free membrane vesicles, derived from ice-nucleation-active bacteria such as Erwinia herbicola, that have been shown to inhibit supercooling, shorten freezing times, and lead to altered patterns of ice formation. The researchers concluded that ECINs may be used in food freezing to control ice-crystal size and crystal size distribution throughout the product. They are developing a computer model to predict the ice-crystal size and distribution throughout a frozen product.
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by NEIL H. MERMELSTEIN