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Coating is a critical operation widely found in food processing. Examples are batters and breading on chicken, fish, potatoes, and snacks. Icings and frostings on baked goods are coatings. Chocolate is used to coat various centers, such as nuts, raisins, cherries, mint patties, cookies, crackers, and cakes. Hard and soft sugar shells are coated onto nuts, gums, gels, chocolate lentils, and tablets.
The critical issues in coating include adhesion, uniformity, texture of the coating, and surface appearance, in many cases. Replenishment of the coating material, pot life, and proper formulation are also important.
Battering and Breading
Many fried foods, or foods that are finished by frying, are coated first with a relatively thin liquid, such as egg, milk, or water with some flour and seasoning, called batter, and then with a dry powder or thick liquid paste. The dry powder may be bread crumbs, seasoning, flour, starch, or a mix of such ingredients.
Equipment to apply batter usually involves a flexible conveyor chain, often of parallel rods, running through a chamber or bath containing the batter. Pieces to be coated are laid on the chain and immersed in the bath. Usually, there is a portion of conveyor above the bath to allow draining of excess coating back into the bath.
A second, thicker coat can be applied in the same way in a second bath, or dry powders can be dusted onto the wet pieces. Some pieces may need to be turned over to get full coverage.
After coating, pieces may be partially or fully fried, and then most are frozen and packaged. Flavored French fries have a thin batter with seasoning applied. Chicken nuggets, fish sticks, poppers (fried peppers), and mushrooms are made in a similar way.
To achieve consistent organoleptic properties of the coated food, it is important to control the viscosity of the batter, according to Ramesh Gunawardena, Manager of Technology and Process Development, FMC FoodTech, Sandusky, Ohio (phone 419-626-0304x14315). The viscosity of the batter is dependent on the temperature, and the temperature has a tendency to rise as a run proceeds. FMC’s batter machine chills the batter and maintains a close tolerance on temperature.
A critical issue in this type of coating is the pot life of the batter. Because of its high moisture content and the fact that other foods are constantly being immersed, the batter can become contaminated with bacteria and begin to spoil. It also can contribute to contamination of the food to which it is applied. Frying helps to reduce the bacteria count in many cases, but battered and breaded frozen foods are rarely sterile. If they are allowed to thaw and warm up, then are frozen again, they can be a source of food poisoning.
Because batter is continuously removed on the coated food, a given batch of batter is eventually consumed. It is good practice to keep the bath relatively small and to dump and replace the batter after a controlled period of time, usually just a few hours. Leftover batter should not be reused.
Coating with solids is challenging because it involves handling powders which can bridge, flood, and form clumps. Usually, powders for coating are fed from a hopper by a specially designed screw or vibratory feeder. The powders are spread over the target pieces by a slot or vibrating table to form a curtain through which the pieces pass. Excess powder falls through the open-chain conveyor and is recycled to the hopper, often manually.
Fried snacks, such as potato and corn chips, are coated with dry seasoning in a rotating drum with several baffles. The process relies on the hot surface frying fat to help the seasoning adhere. Control of particle size of salt and seasoning is important to obtain consistent coating.
Coating with Liquids
Breakfast cereals, pet foods, and some snacks are often coated with liquids by spraying in rotating drums. Breakfast cereals often have vitamin mixtures and sweeteners applied to dry flakes or extruded pieces. Pet foods have fat, liquid flavors, and dry powders such as yeast and egg applied. Raisins may be lightly coated with oil to prevent clumping and inhibit drying.
Spray Dynamics, Ltd., St. Clair, Mo., designs and builds liquid and dry coating systems, according to Doug Hanify, Director of Technology (phone 800-260-7366). One of the more interesting applications is a dough-coated peanut, said to be popular in Japan. That operation uses a centrifugal batch coater in which the bottom spins to expose centers, in this case peanuts, to the liquid. The coating layer is built up by drying and then repeated spraying.
A continuous coater uses a rotating drum with perforated sides through which air is pulled after contacting the pieces. Such equipment can help control allergens in a plant by removing dust and powders.
Continental Products Corp., Milwaukee, Wis., promotes use of its proprietary Rollo-Mixer for coating, agglomerating, and impregnating solids with liquids, according to Bill Callaghan, Manager of Engineering and R&D (phone 414-964-0640). The Rollo-Mixer is offered in sizes ranging from 3.5 cu ft to 1,500 cu ft of effective mixing volume. The principle is to expose all the surface area of all the solids to a spray of liquid by rotating a drum at a low speed (3 rpm). On each rotation, there are 25 divisions of the solid mass, achieved by a special design of the internals.
As with other solids mixers and contactors, the working volume of the machine is less (about 50%) than its total volume, to allow the solids to be moved and exposed.
Chocolate is a special case of coating which relies on solidification rather than absorption or drying to convert the liquid to a solid shell. Chocolate and other fat-based coatings, such as compound coatings and yogurt coatings, melt at about 100ºF. Chocolate, in particular, has a relatively sharp melting point near body temperature, which accounts for its popular sensory impact.
Fat-based coatings are applied warm to a piece that is cooler. It is important that the piece be neither too cool nor too warm. The molten coating is applied with a water fall, in an enrober, or by spraying successive layers in a batch or continuous pan coater, according to Bob Boutin, Executive Vice President, Knechtel Labs, Skokie, Ill. (phone 847-673-4477). After the coating is applied, it is cooled to convert it into a solid. In an enrober, the cooling occurs in a refrigerated tunnel. Chocolate-coated candy bars and cookies are made this way. Cakes are usually iced, meaning they are not completely covered, and the icing is not usually all chocolate. Some snack cakes are covered with compound coating using an enrober.
In pan coating, pieces or centers are loaded into a spherical vessel mounted on a rotating shaft. The speed of rotation and the angle of the shaft are varied. Vessel diameters range from 12–18 inches holding a few pounds to 36–48 inches holding 150–300 lb per batch. The motion of pieces against each other distributes the coating and polishes the product. As Boutin points out, depending on the center, it may be necessary to apply a gum or starch layer before and after the chocolate, to improve adherence and to help protect the chocolate. A glossy appearance is achieved by applying an alcohol solution of shellac and then carnauba wax.
A substitute for shellac for glazing chocolate can be made from whey proteins, according to John Krochta, Professor of Food Science at the University of California, Davis (phone 530-752-2164). The whey is applied in an aqueous solution and therefore takes longer to dry than the alcohol solutions. It may also be necessary to apply a gum or starch solution to promote adherence.
Some of our most-popular confections, such as jelly beans, candy-coated chocolate, and chewing-gum tablets are sugar coated using pans similar to those described earlier, but relying on drying of a sugar solution to form the shell. Panning is an art as well as a science, because ambient conditions of temperature and humidity, as well as the nature of the centers, can cause variations to which a skilled operator adjusts. Some panning operations can take several days, while most take about 5 hr. It is common to remove pieces for overnight drying before final waxing and polishing.
Hard sugar shells use relatively simple syrups of sucrose and water, while soft shells use sucrose, corn syrup, and water. Colors and flavors are often added in the last few stages.
From fried chicken to jelly beans, coating operations have certain principles in common but use dramatically different types of equipment. The results are some of our favorite foods.
by J. PETER CLARK
Consultant to the Process Industries
Oak Park, Ill.