Developments in Food Freezing

A number of interesting developments are occurring in the area of freezing research and applications.

• Ice Cream Processing and Formulation. One of the more interesting papers I was actually able to hear at the recent IFT Annual Meeting concerned a new process for ice cream. The developer is E.J. Windhab of the Institute of Food Science, Swiss Federal Institute of Technology, Zurich.

His ultra-low-temperature ice cream concept began with the observation that in conventional ice cream, about 40% of the water is frozen when the mixture exits a scraped-surface freezer at about 21ºF. The rest of the water is frozen in a hardening tunnel at about –40ºF, which leads to relatively large ice crystals and a rough texture. Windhab observed that even fully frozen ice cream is still plastic and reasoned that it could be transported and frozen at much lower temperatures, giving smaller ice crystals and better texture.

He uses a twin-screw extruder cooled with liquid nitrogen. The mix is fed by pump and kept under pressure so that entrained air is retained. Exit temperatures are close to 0ºF, and no further hardening is needed. Traditional hardening tunnels have significant capital costs and consume energy. Not only does ULTICE, as the process is called, give an improved product, but it also uses less energy. It will be interesting to see if the process is adopted commercially.

The symposium in which Windhab’s paper was presented, “The Changing Landscape of Ice Cream,” also included some other interesting papers. C.G.J. Bisperink of DMV International, Veghel, Netherlands, spoke about specialized milk proteins. He pointed out that using whey proteins to replace milk solids non fat (MSNF) for cost reduction could affect quality unless the whey proteins are specially processed and the entire system of other ingredients and processing conditions is optimized.

H.D. Goff of the Dept. of Food Science at the University of Guelph, Canada, discussed the interest in use of natural ice-modifying proteins as food ingredients. These are the compounds that enable fish to survive in arctic waters and plants to tolerate cold. They can be nucleating or anti-nucleating in their effects. At low concentrations in ice cream they can prevent recrystallization of ice, but at higher concentrations they may negatively affect texture by promoting nonspherical crystals.

C.M. Bruhn of the Center for Consumer Research at the University of California at Davis discussed the nutritional impact of ice cream, frozen yogurt, and sherbets. She pointed out that vanilla-flavored ice creams surveyed in supermarkets ranged from 70 to 270 kcal and from 0 to 18 g of fat per half-cup serving. Some are good sources of other nutrients, such as vitamin A, vitamin C, calcium, and even iron (from chocolate and nuts). There is an opportunity to use ice cream to deliver other nutrients such as probiotic cultures.

• Versatile Freezing Equipment. An equipment company with its roots in ice cream hardening has found important new applications for its technology. Intec USA LLC, now based in Morrisville, N.C., was established in New Zealand to market an automated tray freezer system, according to Joe Vozella, Vice President of Sales & Marketing (phone 919-467-4155). Vozella is a veteran of FrigoScandia, another freezer company now owned by FMC. He and partners have successfully marketed Intec to the poultry and bakery industries.

The Intec system has several decks or paths through a large freezer box. An automated loader can read bar codes and deliver packages to any of the decks. Each deck can have a different residence time, hence the name VRT, for variable residence time. Most applications are for cases or other relatively large packages which might otherwise be frozen in static blast freezers.

Static freezers require considerable labor to load and unload. To get high density, they are closely packed, which can inhibit air flow and may require that all packages receive the longest residence time required by any size. Residence time is related to size by about the square of the characteristic dimension.

Thus the advantage of the Intec system is that each package size gets the residence time it requires. The same system can handle various size packages, which are sorted as they are discharged. The system also reduces freezing time by providing high air velocities and good exposure to the cooling air. Compared to a static blast freezer, the Intec machine is more expensive to build, but it offers savings in labor and energy costs and helps with order tracking.

• Research in Freezing. R. Paul Singh, Professor in the Dept. of Biological Systems Engineering at UC-Davis (phone 530-752-0811) is a well-known food engineer who is investigating heat transfer in freezing. He has previously studied frying and air impingement in baking. Now he is looking at air impingement in freezing. Air impingement uses nozzles to direct cold air at 30–40 m/sec at the surface of pieces to be frozen. The concept is to reduce the insulating boundary layer at the surface which controls the external removal of heat. It does not affect the internal heat transfer, which is by conduction and which depends on the thermal conductivity of the material. By reducing the external heat transfer resistance, the system assures that the rate is governed solely by the internal properties, which should then give the fastest freezing rate. Fast freezing normally gives the highest quality by reducing moisture loss and forming small ice crystals, which do less damage to texture. Tests on hamburger patties have shown positive effects on quality.

Optimizing air-impingement freezing can be complex, as it is sensitive to the distance from nozzle to food, to velocity, and to the pattern of nozzles. Singh has a walk-in freezer at UC-Davis in which he can conduct precise research on these variables.

On a recent trip to Japan, he discovered a company that is using air-impingement freezing to freeze cooked rice patties intended for sushi. The Japanese are famous for their focus on high quality in food, so the fact that this product is evidently well accepted is strong support for its potential.

Singh is also interested in cryogenics, especially the issue of stress cracks in some food frozen at very low temperatures. Gas companies such as Air Products, Allentown, Pa., Praxair, Tarrytown, N.Y., Carbonic Systems, Elmira, N.Y., and Matheson Tri-Gas, Irving, Tex., provide both equipment and cryogenic gases (liquid nitrogen and liquid carbon dioxide). Cryogenic freezing equipment is normally less expensive than mechanical refrigeration equipment because it does not require the compressors, evaporators, and condensers of a mechanical system. However, the operating cost of cryogenics is usually higher because one must continuously buy the gas. The cost is sensitive to the cost of electricity, since operating the liquefaction compressors is the major cost.

Which gas to use depends on the application and, usually, local costs. Sometimes nitrogen is plentiful and inexpensive because it is produced as a by-product of purifying oxygen for steel making or other industrial processes. Sometimes carbon dioxide is available from fermentation or ammonia plants.

The equipment is tailored to the gas. Most of the cooling with nitrogen comes from sensible heat, so the liquid is injected countercurrently to the product flow and warmed vapor is exhausted near the entrance of the typical tunnel. With carbon dioxide, the heat removal is by sublimation of the dry ice snow, so the liquid is injected cocurrently with the food and vapor exhausted near the exit. Both gases can displace oxygen, causing an asphyxiation hazard if they are not properly exhausted.

Cryogenic freezing is especially appealing when products are frozen before packaging and when weight loss is important. The very low temperatures reduce moisture loss and so increase yield. They also form small ice crystals, which usually helps quality. However, as Singh points out, freezing too fast can have a negative impact on quality.

An interesting variation on cryogenics is an experimental system in which air is liquefied on site and used as a cryogen without separation. This makes some sense when local electricity costs are low. The benefits of low temperature are achieved without a third party or any delivery costs. On the other hand, a small liquefaction plant is probably less efficient than a larger one because of economies of scale and greater integration of heat recovery. In addition, this technology might be a little sophisticated for some food plants. I know of no current commercial applications.

by J. PETER CLARK
Contributing Editor
Consultant to the Process Industries
Oak Park, Ill.