THE TORRY KILN ITS DESIGN AND APPLICATION WITH PARTICULAR REFERENCE TO THE COLD SMOKING OF SALMON, HERRING, AND COD Alex M cK . Bannerman, M BE A. I. F . S. T. Independent Consultant Swanland, North Ferriby Yorkshire HU14 3QT En gland 44! 0482-633124 In 1933, I was privileged to join the Torry Research Station in Aberdeen, Scotland. I worked for the director, the late Dr. G. A. Reay, investigating fish proteins, freezing, cold storage, salting, and dehydration of herring and cod, and the analytical techniques to test these processes ~ In 1936 37, I became involved in smoke curing fish, working with Dr. Charles Cutting. We started a research program to investigate the process of curing fish in traditional kilns. Some of these kiln s were 800 ft square and 30 ft high. We measured air flow, temperatures, humidities, and weight loss of fish during smoking. From the data, we built a picture of the irregularities and disadvantages of smoking in these old kilns. However, this also led to improving the process. After several attempts, we developed a simple tunnel design in which fish could be smoked under controlled conditions. The process was more economical, faster, and the product more uniform. This is a bit of history, but it was from this work that we built our expertise and knowledge of fish. A previous speaker suggested that with modern, programmable smoking equipment it is only necessary to push buttons, and "witch doctors" are no longer needed. I would agr ee with this for products such as salami, sausages, and other products manufactured from uniform ingredients in skins of uniform length and thickness ~ However, fish come in all shapes and sizes, with differing fat, protein and water content. In any smoking process, these differences have to be considered by the processor. Based on my knowledge of the old kilns, I became one of the "witch doctors" of modern smoking equipment. Only very fresh or good quality thawed fish should be used for smoking since modern methods will not preserve flesh or mask off-flavors. All fish have to be prepared for smoking as in Figure 1. Whole fish are first washed in fresh or chlorinated water to remove any blood, loose scales, and other debris. With few exceptions Table 1!, fish are cut. Herring with the head on the gut in are used to produce bloaters and buckling. A buckling is a German herring cure which is hot-smoked. There is a little buckling production in the United Kingdom, but the herring are prepared by nobbing: removing the head and long gut, but retaining any roe or milt. This is done by cutting the fish behind the head, stopping before going through the gut cavity, then pulling off the head with the long gut attached. The Russians use "nobbed" herring and mackerel for freezing, canning, smoking, and fried products. 135 Figure 1. Basic steps in the processing procedure. 136 Table 1. Preparation of fish for smoking Treatment S Finished Cured washed loose scale removed! all washed loose scale removed! herring bloaters and buckling head on, gut and gills trout and mackerel for hot smoking removed "gibbed" herring red herring herring buckling head on "split" herring kippers head off "split" haddock "Finnan" "block, " "butter fly" ! any small white golden cutlets or boned fillet! fish, herring and boned kippers mackerel filleted lug bone on! salmon! for cold smoking large sea trout! filleted and trimmed all white fish, for cold smoking mackerel for hot smoking washed, cooked, and shucked mussels and oysters for hot smoking All cut fish to be washedfree of any loosescales, blood and pieces of gut. 137 "Gibbing" is a Scottish term for herring which have gills and gut removed by cutting into the throat with a small knife. "Gibbed" herring are used for salting, and subsequently for red herring, probably the oldest commercial form of smoked cure. The herring are salted for weeks or months, desalted in running water, and hung in the kiln where they are smoked for 24 hours, cooled, then smoked again. This process repeats for several weeks. The cooling forces internal moisture to the surface where it is dried off with the warm smoke. Red herring are heavily smoked, very salty, dried, and usually exported to tropical countries. For more popular kippers, herring are split along the backbone and the gut and roe removed, usually by machine. Boneless kipper s and block haddock fillets are prepared by cutting behind the head, continuing the cut down either side of the backbone, removing the head and backbone so the two side fillets are left, still joined down the back. The "boneless" kipper was introduced after World War II, when iced herring were shipped overland in boxes. This resulted in some deterioration in the belly walls. Gut enzymes caused the walls to burst, producing a high proportion of second-class kipper. Most of the white fish cures are prepared from fillets, apart from "finnan" haddocks and Arbroath smokies. Salmon are gutted, the gut cavity scraped, and all blood removed. The fish are then beheaded, but the lug bones are left intact, and the fish filleted. String should be threaded through the shoulder of each fiHet under the lug bone to form a loop. This helps when handling the fish during further processing. Filets from thawed fish can be very soft and must be handled carefully. All fish have to be quickly washed in running water after cutting. After that, most fish are ready for salting. Prior to smoking, all fish are either dry salted or brined. Most fish which have been cut open are brined. Salmon is the exception in the United Kingdom, where it is always dry salted. Salmon processing has, over the years, been looked upon as a mysterious ritual and a closely guarded secret. From time to time, various ingredients have been used in addition to common salt. These include brown sugar, molasses, spices, and rum. I personally would use only salt, put the brown sugar in my coffee and drink the rum. In the United Kingdom, only fine, vacuum-dried salt is used. The salmon fillets are laid skin side down on a bed of salt about one-half to 1 inch thick. The cut surface of the fillet is then covered with salt about one-half inch deep at the thick end tapering down to the tail. At the thinnest part of the tail the salt is only lightly sprinkled over, in order to prevent the tail from becoming too salty and dry. Salting time depends on the size and fat content of the fish. A rough guide would be six to eight hours for a 2 lb side and up to 24 hours for a 6 lb side. It is only from experience that a processor will get to know how much salt and salting time will be required. Salmon fillets can be brined, but the fillets will not lose any weight in brine, and may even gain some. This extra water has to be removed in further processing so the fillet will have the texture for slicing. Dry salting can remove from 6 to 9 percent of water. 138 After salting, any excess salt is washed off and the fish are allowed to drain before they are smoked. The salt content in a finished side of smoked salmon averages 5 percent. In general, the tail will have 7 percent, the middle section about 5 percent, and the thickest part of the fillet about 2. 5 to 3 percent. It is not possible to equalize the concentration of salt throughout the fillet by either method of salting and this applies to all fish, either whole or filleted. An 18 to 25 percent brine solution is used for salting smaller fillets from cod, haddock, herring, and mackerel. The fillets were usually soaked in tanks of brine with added coloring in batches of 56 lbs of white fish and up to 112 lbs of herring split for kippers. This bulk brining resulted in variations in salt uptake between batches, and also within a batch. There is now an automatic brining machine manufactured to handle these smaller fillets Figure 2! . The fish are fed into one end of the brine tank in a single layer and moved along by paddies to an off-take mesh belt which allows the surplus brine to drain back into the tank. This allows a uniform salt content in the fish. Mechanical brining can. reduce the brining time from one-half to one-third of the bulk brining time. For example, herring fillets with a 12 percent fat content could be bulk brined for up to 5 minutes, whereas only 1. 5 minutes is required in the mechanical briner. This particular briner has a solid salt make up tank, variable paddle speed, complete recirculation system, and a filter for removing most of the debris. This briner is constructed of stainless steel and at the end of a day's production can be emptied out, washed down, and sterilized. Most of the cold-smoked fish produced in the United Kingdom is colored with a dye added to the brine. This color, in some people's opinion, gives the finished product a more attractive appearance, however, some of these dyes do contain carcinogens and are being investigated by food additives committees. The fish are all either brined or salted, hung on various pieces of equipment, or laid on wire mesh trays on trolleys, and allowed to drain for at least two hours. If the trolleys of fish can be stored at 3 C, so much the better.