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In any discussion of sausage casings, it is necessary to begin with the oldest of our packaging materials for sausage: the animal casing. For centuries, animal casings have been the traditional containers for sausage materials and are still associated with high-quality sausage products. They offer a unique combination of properties that provide valuable processing characteristics and fit the requirements for natural products, which are increasingly more popular. The material, made from the animals’ intestinal tract that is used for the casings, is actually the submucosa, a collagen layer that provides the strength to the particular organ. For the most part, with a few exceptions, the fatty outer layers of muscle are removed along with the inner layer, the mucosa. Because the material used is collagen, the treatment of the casing at the slaughtering plant and during subsequent steps in casing processing has a very definite effect on the utility of the final product.
As in the case of all collagen materials, the collagen in casings is hardened and rendered less soluble by the application of salt. This is essentially the same process used in the curing of hides. Thus, the salting of the casing at the slaughtering plant has a great deal to do with appropriate curing and the final quality of the finished casing.
The collagen material that is the basic structure of animal casings has unique characteristics that are applicable during processing. Initially, as the collagen is exposed to heat and drying, it becomes less permeable to moisture. This drying also affects smoke penetration. For this reason, the initial step in the processing of a sausage in animal casings requires drying to develop the appropriate smoke permeability. Once the smoke is applied, and the desired smoke color and concentration are attained, further drying will render the casing virtually impermeable to moisture. It can then be cooked in a variety of ways, including steam cooking, without injuring the final product. In fact, at one time it was customary to apply the smoke and dry the casing in the smokehouse and then finish, particularly small-diameter products, by water cooking. This variable permeability becomes a very useful tool in further processing and enables the animal casing to accommodate a wide variety of conditions.
Small-diameter animal casings are derived from the small intestines of hogs and sheep (Figures 1 and 2). The sheep casings are the smallest in diameter and are also the most tender. These are the most adaptable to fresh sausages, where there is no further processing to tenderize the casing, as well as to small-diameter cooked and smoked sausages. There are different grades of sheep casing. The B grade has small holes and is applicable primarily to coarse-ground fresh sausage. The A grade is most applicable to an emulsion-type sausage. Hog casings are used for some types of fresh sausage as well as a variety of smoked, dry, and semidry sausages. They are somewhat less tender than sheep casings.
Sheep and hog casings are sold in bundles or hanks comprising of 91 m of casing. Diameters of sheep casings range from 16 mm to approximately 28 mm. Hog casings range from 30 mm to approximately 44 mm. They are normally grouped in 2 mm increments.
Casings of a small diameter are available in a variety of forms. They can be dry salted, and then require flushing before use. During the flushing process, the casing can be examined for strength, punctures, and conformance to size specifications. Preflushed casings are packed in a brine solution. Pretubed casings are shirred onto a plastic sleeve; these can then be transferred directly to the stuffing horn. To improve production efficiency, some processors will flush the casings in a separate location and then shirr them onto a stainless steel mandrel, so that they can be quickly applied to the stuffing horn.
During smoking and cooking, the initial critical steps of drying and smoke application must be watched very carefully. Before smoke is applied, the casing should be dried to the point at which it is tacky. If it is not, the smoke will penetrate through the casing and be deposited on the meat surface, thereby allowing for casing separation and causing a pale, dull appearance. By the same token, if the sausage is overdried, the smoke will essentially be deposited only on the outside surface with very little flavor effect. Animal casings are ideally suited to liquid smoke application, either by drenching or by atomization.
There is a variety of larger-diameter natural casings available (Figure 3). Beef rounds, from the small intestine, are generally used for ring-type sausages. In terms of handling characteristics, the same type of handling as applied to sheep and hog casings can be used. Beef rounds are probably the largest-diameter casings that one would classify as still being edible from a palatability standpoint. Diameters range from 35 to 46 mm. They are sold in bundles or sets of 9, 18, or 30 m. Other beef casings are beef bung caps, ranging from 75 to 125 mm. These are used for some cooked as well as dry and semidry sausages and are sold by the piece. Beef middles, used for various dry and semidry sausages, range in diameter from 45 to 65 mm and are also sold by the piece. Beef bladders are used for some specialties such as mortadella. Some of these larger-diameter sausages require support when hanging; the support can be either a series of string loops or netting. Most beef casings would typically be removed from the sausage before consumption.
One type of large-diameter casing that is used quite often is the sewn casing, particularly the sewn bung. Sewn hog bungs are used not only for some salamis and cervelats but also for various types of liver products. Sometimes these sewn casings are lined with paper or with another type of animal casing such as hog bungs lined with beef middles. In the case of sewn bungs, the fat layer of the bung is left on the inside of the casing. Particular care should be applied to these casings to check the rancidity level of this fat layer. Salt triggers the development of rancidity and, unless these casings have been carefully and properly handled, rancidity of this fat layer could develop. Subsequently, this rancidity could be transferred to the finished product in the form of an off-flavor. Animal casings should be stored under refrigeration at a temperature of less than 4 °C.
Even when carefully graded, animal casings lack a degree of uniformity and are therefore somewhat limited in their machinability. For this reason, handling animal casings requires somewhat higher labor input than one would experience with the machine-made casings. This is where the processor must make some management decisions depending on the type of the product that is manufactured and the clientele to which it is to be sold.