Animal Sciences Research and Reviews

Special Circular 156

Natural Casings

H.W. Ockerman
Department of Animal Sciences

Introduction

Animal casings can be used for a variety of different purposes, such as surgical sutures, collagen sheets (used for burn dressing), strings for musical instruments, casings, human food, pet food, meat meal, tallow, or fertilizer. Certainly the area having the highest economic value combined with volume of utilization would be sausage containers. This paper originally was prepared in 1996 as a chapter in the UN book titled, "Manual on Animal By-Products Utilization in Developing Countries".

Ground and comminuted meat often require some type of a casing to hold the product intact while it is being processed, pre-cooked, distributed, and final cooked. The meat processor has the option of using natural casings, reconstituted collagen casings, or artificial casings made primarily from cellulose. The various advantages and disadvantages of the different types of casings are shown in Table 1.

Animal casings have the advantage of contributing to old world appearance, often not having to be peeled prior to consumption, and having many of the physical and chemical properties that make a very desirable container for sausage items; but they are usually the most expensive type of casing. Therefore, animal casings are usually used for the more expensive sausage items. Areas of the animal used to manufacture casings are the small and large intestine, weasand, urinary bladder, stomach, and rectum from most meat producing animals. For the sections of the intestinal tract utilized for beef, see Figure 1; for sheep, see Figure 2; and for hogs, see Figure 3. The utilization of these products varies tremendously from country to country and for different sausage types in which they are going to be utilized. A diagrammatic cross section of the intestinal tract of animals is shown in Figure 4 (described in Table 2). This also indicates the areas that are removed from the intestinal tract in order to transfer the product into useable casings.

As received from the slaughter floor, the intestinal tract always is highly microbiologically contaminated. The cleaning and removal of various internal, and sometimes external layers, are necessary to convert this product into a useful casing. Many factors influence the quality of the casing, such as health of the animal, specie, age of the animal, breed, fodder consumed, conditions under which the animal was raised, portion of the intestinal tract utilized, and how the product is handled and processed after the animal is slaughtered. Some determinants for evaluating casings include:

1. Cleanliness -- casings should be clean, free from stains, odor, fat particles, parasites, nodules, and ulcers, and should be sound and free from pin holes.

2. Strength -- casings should be strong enough to withstand the pressures that will be put on them in the areas of filling, stuffing, cooking, processing, storage, and consumer cooking. Only the submucosal part of the intestine has the required strength to make this possible.

3. Length -- The number of pieces per hank or bundle often varies according to the country in which the casings were collected. The country in which the product is going to be utilized also influences the desirable length. On the average, sheep and hog casings are 299.8 feet (91.4 meters), beef rounds are packaged in bundles of 54 feet (18 meters), and tennis rackets and surgical strings in lengths of 9.24 feet (6 meters).

4. Caliper -- Diameter of the casing desired is determined by the country of use and also according to type of sausage that is going to be placed in the casing. Modern sausage processing equipment requires, however, that the caliper of the casing be uniform in order to have adequate machinability. The greatest demand for sheep casing is in the 0.78 to 0.95 inch (20 to 24 millimeters) diameter. In some cases, small calibre hog casings can be substituted for the larger sheep casings. The greatest demand for hog casings is in the 1.38 inches (35 millimeters) and over range. Beef rounds are normally 3.09 inches (33 millimeters) and over, and middles are 2.17 inches (55 millimeters) and over. Over sizes are in greatest demand.

5. Curing -- Casings are normally cleaned, then salted, and only in a few cases are they dried. Also, in some cases, some of them may be frozen, but this primarily is utilized for surgical cat gut. Some countries, even in this area, use salted products. The curing should be accomplished with high quality, fresh, and small practical-size salt grades.

6. Packaging -- The type of packaging is either wooden or plastic containers or tins. Often these containers are lined with plastic bags. Sometimes plastic bags are utilized and protected by sacks. However, the trend seems to be in favor of plastic containers because of cost and ease of sanitation.

Table 1. Advantages and disadvantages of casing types.1
	Natural	Collagen	Cellulose
Most expensive, cost/ pound product	Most expensive	Less expensive	Least expensive
Refrigeration storage	Yes	Yes	No
Degree of tenderness	Most tender	Less tender	Peeled
Break during processing	Most likely	Less likely	Least likely
Casing preparation cost	Most expensive	None	None
Soaking & flushing before use	Yes	No	Sometimes soaking
Ease of smoke penetration	Most penetration	Less penetration	Least penetration
Best machinability	Least	Less	Best
Best product yield, per foot of casing	Least	Less	Best
Finished product yield, per foot of casing	Least	Less	Best
Finished product uniformity	Least	Less	Best
Cost of casing removal	None	None	Most
Printability	None	Limited	Best
Old World appearance	Best	Less	None
Ease of plant storage	Least storage	Less storage	Best storage
1 Source: Ockerman, 1996. Chemistry of Meat Tissue. The Ohio State University, Columbus, Ohio, USA.

Figure 1. Digestive track of cattle. Modified from Mann, 1962. Animal By-products, FAO.

Figure 2. Digestive track of sheep. Modified from Mann, 1962. Animal By-Products. FAO.

Figure 3. Digestive track of hogs. Modified from Mann, 1962. Animal By-Products, FAO.

Figure 4. Cross section of intestine and casing.

Table 2. Composition of natural casings.1
Natural casings are acquired from the alimentary tract of meat animals. The construction of the intestine is composed of five layers, which from inside to outside, are shown, along with the layers that are remaining after it is processed into a casing.
Hog and sheep casing Beef casing		1. Mucosa - glands aid in secretion, digestion, and absorption.
		2. Submucosa - collagen, and often fat. 3. Circular muscle. 4. Longitudinal muscle. 5. Serosa - collagen, elastin.
1Source: Ockerman, 1996. Chemistry of Meat Tissue. The Ohio State University, Columbus, OH, USA.

Table 3. Steps for the conversion of the digestive tract into sausage casing.1

Removing the intestine from the animal. Running - removal of loose mesentery fat. First stripping - squeezing to force out intestinal contents. Washed and cooled (50oF water). Mechanical stripper or hand stripping. Brushes used to remove fat. Slimming - removal of tissue layers - revolving drums and warm (115F) water or hand slimming. Strippers or hand removal of appropriate tissue layers. Stored overnight in ice and 15 to 20% saturated salt solution. Graded - species, size, quality. Cured - rubbing with salt; allow to set for 1 week. Removal from cure, shaken free of excess salt, rubbed with fine salt, and packed (40% salt). Flushed prior to use.

1 Source: Ockerman, 1996. Chemistry of Meat Tissue. The Ohio State University, Columbus, OH, USA.

Removal of the Viscera

The first step in casing preparation (Table 3) is removing the viscera and separating it from the internal organs. This is primarily a hand and/or knife operation. The viscera is placed on a table and separated from the mesentery fat. Again, this is a hand operation which can be facilitated by using an air operated knife. The puller usually will start at the stomach and pull the casing away from the ruffle fat.

The next step in casing manufacturing is to run the casing through a manure stripper to squeeze out the liquid and manure using large rollers similar to a laundry ringer. These rollers are usually rubber and sometimes are wrapped with burlap. This operation also can be done by hand by pulling the casing through the fingers. In either case, a great quantity of potable water is needed to wash the casings and to keep the operation clean. The casing should be soaked for approximately 30 minutes in 100 to 108F (38 to 42C) water. In some areas of the world, casings now go through a fermentation cycle, but in many areas, processing casings by fermentation is no longer legal (e.g., USA). If fermentation is to be conducted, the casings usually are soaked overnight in 72^oF (22^oC) water or until the mucosa and muscular coatings are loosened. Excessive fermentation will soften the casings. If fermentation is used, the casings usually are stripped after fermentation, soaked, restripped, and often restripped again. Next, they are run through a cleaning machine (which is often a drum with a revolving scraper blade).

If the casing is not fermented, the next step is to run it through a crushing machine and soaking tank. The purpose of this is to break the intermucosal membrane and separate it from the casing. This machine also has two adjustable rollers with eccentric bearings. Again, a great quantity of 108^oF (42^oC) water is used in this operation. Crushing also can be accomplished by a hand operation wherein the casing is scrapped with a dull-bladed knife or an oyster shell.

Next, the casing goes through a mucosa stripper, which looks essentially like the manure stripper. Again, 108^oF (42^oC) water is used to keep the operation sanitary. If labor is very economical, this operation also can be accomplished by hand scraping.

Next, the casing goes through a finishing machine to remove any string-like material and remaining mucosa. Rollers again are used in this operation. Finishing also can be accomplished by hand if a great deal of attention is paid to detail. In either system, large quantities of 108^oF (42^oC) potable water are essential to keep the operation clean. After finishing, the casings are soaked again in 50 to 60^oF (10 to 16^oC) water and/or a salt brine tank to remove excess blood. This tank normally has continuous flowing water to remove the stained liquid. If continuous running water is used, salt normally is not used at this stage. The soaking time usually ranges from 30 minutes to overnight.

After the soaking operation, casings usually are salted either by hand or again by machine. The salting and shaking of the casing usually are continued until the casings absorb 40% salt, at which point they are packed into a container. If the casings are packed in a slush container, the container will hold 10 to 15% salt water brine. The advantage of using a dry pack is that the casings become less tangled. Also, the casings will be darker in color.

Items such as hog bung, hog stomach, blind end or cecum, bladder, beef bung, and weasand usually are handled by hand, trimmed of excess material, and salted again.

If attention is paid to detail, large quantities of cool water are used to keep the casings clean, labor is inexpensive, and strict sanitation is maintained, then suitable casings can be produced by hand operation and will make very desirable casings for the sausage industry. However, a breakdown in any one of these processes will result in an unsanitary product that has little use in modern sausage processing.

Casings names and locations, as well as sausage products for which they are utilized and yield per animal can be found in Table 4. How to use sausage casings in the meat packing industry can be located in Table 5.

If all steps of the process are performed satisfactorily, a very gourmet sausage product can be produced by utilizing value added and upgraded animal casings which, in their native state, have almost no value but can be transformed to a very desirable container for high quality sausage products.

Table 4. Classifications of natural casings used for meat purposes.1
Natural casing	Location	Appearance & comments	Sausage	Average yield / animal
BEEF
Round	Small intestine	Ring-like, tougher, easily handled, less breakage	Ring bologna, Polish sausage	90 to 135 feet long
Bung	Cecum		Capocolla, salami	4 to 5 feet long
Bladder	Bladder	Oval or molded	Minced specialty, Mortadella	7 to 14 inches wide
Middles	Large intestine	Sewed, most expensive, adds uniformity	Bologna, salami	20 to 25 feet long
Weasand	Windpipe			18 to 26 inches long
HOG
Round	Small intestine	May be eaten or peeled	Large frankfurter, hog sausage	42 to 52 feet long
Bung	Cecum	May be sewed	Braunschweiger	30 to 72 inches long
Middles	Large intestine	Curly	Chitterlings	12 to 16 feet long
Stomach	Stomach		Headcheese, souse
Bladder	Bladder			5 to 9 inches wide
SHEEP
	Small intestine	Most tender, most breakage	Small frankfurter, pork sausage	90 feet long
1 Source: Ockerman, 1996. Chemistry of Meat Tissue. The Ohio State University, Columbus, OH, USA.

Table 5. How to use casings.1
In using natural casing, there should be a drying period in the smoking cycle to obtain uniform smoke color, and great care should be exercised in cooking and smoking these products. Since these products are primarily collagen they behave in the following manner:
	Dry : They become brittle and shrink - also smoke will not penetrate. Some drying is necessary to reduce smoke penetration to keep from forming a skin under the casing.
	Wet: Smoke goes through the casing to the sausage.
	Acid (smoke): Firm, harder and tougher.
	Heat in presence of moisture: They become soft and hydrolyze slowly (sheep is the worst).
If these casings are handled properly, the meat product can stand more abuse (less fatting out) than with cellulose casings.
1 Source: Ockerman, 1996. Chemistry of Meat Tissue. The Ohio State University, Columbus, OH, USA.