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CASINGS

Casings are soft cylindrical containers used to contain sausage mixes (Fig. 314). Casings can be of natural origin or artificial. Natural casings are obtained from animal intestines derived from slaughtering. Manufactured artificial casings are made of cellulose, collagen or synthetic materials. Sausage fillings are mostly minced or comminuted meat mixes held together by the casings during further processing steps such as smoking, boiling, frying or roasting. In addition, casings also protect products during storage.


Fig. 314: Casing of different colour

Natural casings

Natural casings are mainly derived from small and large intestines from sheep, goats and pigs, but also from cattle and horses. They

Small intestines of sheep, goats and pigs are popular small calibre natural casings. They are processed in a way that makes them tender (edible) (see page 252) and are mostly eaten with the sausage (Fig. 323). Many other parts of the intestinal tract of slaughter animals can also be used for natural casings. Those casings are processed differently and have stronger and tougher casing walls. Due to their toughness they are generally not considerededible” (although not unfit for human consumption) (Fig. 205, 320) and are usually peeled off before consuming the sausages.


1) Reduction in moisture content -“drying”- is only needed and desirable for raw-fermented sausages (see page 115).

In many parts of the world the proper manufacture of sausage casings from animal intestines is unknown. Intestines, if not used for human food, are often wasted. Many people in the livestock and meat sector are unaware that processing of intestines into natural casings for sausage production is relatively simple and can be a profitable1 business. If natural casings can be produced locally, this may help to reduce overall production costs. Even in remote or rural settings with no access to commercial casing suppliers, natural casings can easily be processed from intestines derived from local slaughter. The availability of locally produced natural casings will considerably facilitate rural meat processing but proper advice and training on casing preparation is essential (for technical instructions see page 253).

Anatomically the walls of the intestinal tract of slaughter animals consist of four layers of intestinal tissue. These layers from inside to outside are: Mucose membrane (I), submucose membrane (II), muscular layer (circular and longitudinal) (III) and serose membrane (IV). For natural casing manufacture, one or more of these layers are removed during casing processing depending on the type of casing (thin/thick, edible/non-edible) to be fabricated.

Fig. 315: Animal small intestine, cross section

From inside:

I Mucose membrane with finger-like outgrowths for enlarged surface area (“slime”)
II Submucose membrane, firm-elastic layer mainly of connective tissue
III Muscular layer, the circular internal one, the outside one longitudinal
IV Serose membrane (blue) thin coating covering the abdominal cavity from inside and surrounding all organs.


1) Annual imports of natural casings into the EU are valued at US$0.5 billion.


Sheep and goat casings

Fig. 317: Gastro-intestinal tract of sheep

Natural casings from sheep:
Sheep casing (small intestines) (1)
Sheep fore-stomach and tripes (cleaned and scalded stomach) (2)

From the gastro-intestinal tract of sheep and goats (Fig. 317) normally only the small intestines are processed to be used as casings for products such as fresh frying sausages, frankfurters, BBQ sausages, hot dogs and thin dried fermented sausages. These casings are processed in a way that they become tender enough to be easily chewed (see Fig. 316 and page 252). Usually they are not peeled off before consumption but eaten together with the sausage filling. Therefore they are called “edible” in this context. All other natural casings are also edible in principle, but most of them are peeled off as they are too tough to be chewed.

Processing of small intestines at medium- to the small-scale level

The processing of natural casings must be started as soon as possible after slaughter, as bacterial spoilage of the intestines tissues sets in rapidly. For ease of processing it is recommended to start the operation while the intestines are still warm.

The small intestines are detached from adhering mesenteric (connective and fatty) tissue (Fig. 318, step 1). The intestinal content is removed manually (Fig 318, step 2). The empty casings are flushed with water and subsequently de-slimed by using either manual or electrically operated casing-cleaning machines. For this purpose, the small intestines are passed through a set of rollers to loosen the tissue layers (Fig. 318, step 3) and to remove the “slime”. “Slime” is the internal layer of the intestine, basically the internal (“mucose”) membrane (Fig. 315, I and Fig. 316). In the slaughtered animal this membrane disintegrates rapidly and can easily be removed. Because of its structure it is commonly known as “slime”.

The removal of the “slime” can also be done manually by using a tablespoon or a specially shaped piece of wood. With the spoon firmly pressed onto the intestine, and pulling the intestine through in its full length between spoon and finger, the internal (“mucous”) membrane can be loosened and removed. The loosened tissues inside the casing are pressed out manually (see Fig. 318, step 5.) and the remainder rinsed off with water. Parts of the outside layers (“serous” membrane) are automatically removed when detaching the small intestine from the mesenteric tissue (Fig. 318, step 1 and step 4). The rest of the outside layer and the intermediate (“muscular”) layer will be removed during the casing de-sliming and cleaning operation.

The remaining strong-elastic tissue is a layer composed mainly of connective tissues (“submucous membrane”) (Fig. 315 II, Fig. 316, Fig. 318, step 6, b2). This connective tissue membrane forms the edible sheep casing. Sheep casings are not reversed (turned inside out) during their processing. For completion of the processing, the casings are inflated for grading, flushed with salted water, stripped for water removal, dry salted (Fig. 318, step 7) and stored in a cool place, preferably in the chiller. In this form they can be stored for three months, preferably under storage temperatures not exceeding +15°C. By no means should natural casings be frozen, as they would lose their elasticity and firmness.

The average length of the small intestine from sheep is 17 to 24 meters depending on the size of the animal. Sheep and goat casings for the international casing trade are produced in largely mechanized operations, usually packed in hanks (91.4m or 100 yards) and graded according to their diameter and colour coded as follows:

28/ + mm green/white 26/28 mm green
24/26 mm red/white 22/24 mm red
20/22 mm blue/white 18/20 mm blue
16/18 mm yellow/white 14/16 mm yellow

Sheep casings, as well as other natural casings are soaked in water before filling the sausage mix. This treatment removes part of the salt and the casing walls become more elastic, as their collagen fibers absorb water. Addition of organic acids, in particular lactic acid (2% to the water), also assists in this process.

Fig. 318: Processing steps for small intestines


Step 1: Separation of the small intestines from mesenteric tissue


Step 2: Stripping out intestinal content


Step 3: Loosening of tissue layers using a small-scale manual casing cleaning machine

Fig. 318: Processing steps for small intestines (continued)


Step 4: Removal of remaining parts of the serose membrane


Step 5: Removal of "slime" by using spoon (white arrow)


Step 6: Flushing of clean casings The photo shows the remaining submucose membrane b1/b2 ("edible" casing) a = unprocessed casing
b1 = processed casing (slimed and cleaned)
b2 = processed casing (slimed and cleaned and being flushed with water)
c = "slime" removed from inside of casing
d = tissue layers removed from outside


Step 7: Salting of clean "edible" casings for storage


Fig. 319: Small-calibre sausages in "edible" sheep casings


Fig. 320: Large-calibre sausage in large pig intestine (pig middles) (not "edible")

Pig casings

Fig. 321: Gastro-intestinal tract of pigs

Natural casings from pigs:
Oesophagus and tongue (1), stomach (2),
small intestine (3), cap (4), pig middles (chitterlings) (5), pig middles (after ends) (6), pig bung (7), pig bladder (8)

Several parts of pig intestines are processed to casings (Fig. 321). The most important are the small intestines. The processing technique used is similar to the procedure for sheep casings. Hence they are also considered “edible” (Fig. 322, 323).

Small pig intestines, also called rounds, with an average length of 15 to 20m, are mainly used as casings for fresh sausages (e.g. fried sausages, Fig. 322), raw/cooked sausages and dried fermented sausages (e.g. chorizos) (see also pages 103, 115, 127).

Pig rounds are packed in hanks of 100 yards (91.4 m), consisting of 15 to 20 single casing strings of 18 ft each (5.5m), sorted according to their diameter and colour coded as follows:

- /26 mm yellow 26/28 mm yellow/white
28/30 mm blue 30/32 mm blue/white
32/34 mm red 34/36 mm red/white
36/40 mm green 40/ + mm green/white

 


Fig. 322: Fresh sausages in pig casings (for frying)


Fig. 323: Raw-cooked sausages in pig casing, left cut open

Pig middles (large intestines with an average length of 3 m) and the cap (Fig. 324) are used as casings for coarse liver sausage (see Fig. 320) and sometimes also for salamis. The bung (last part of the gastro-intestinal tract with an average length of 0.8 m) is due to its strength and shape used as casing for products such as cervelat (finely chopped dry fermented salami) and fine emulsified liver paste. Also the bladder can be used for products such as black pudding or gelatinous meat mixes (see page 164, 166).


Fig. 324: Salted pig stomach and cap (above), pig middles and bung (below)

These parts of the pig intestines are stripped of their intestinal content and must be reversed (turned inside out), washed and slimed (removal of internal slimy cover, now situated outside due to reversing the intestine).

In contracts to the processing of “edible” sheep and pig casings from small intestines, only the mucose membrane is removed through “sliming” from the large intestines and most of the serosa will automatically be detached during separating from the mesenteric tissue. The casing wall is therefore composed of a submucose membrane and muscular layer. These casings are relatively strong and tough and are usually not eaten with the sausage (Fig. 320).

Pig stomachs can be processed in two ways. If the stomachs are to be incorporated into meat mixes for sausage, they are scalded before further processing. If they are used as casings, only a small opening is made, through which they are cleaned by flushing with plenty of clean water. Thereafter they are turned inside out and kept in salt. They are used as casing for precooked-cooked sausages such as gelatinous meat mixes (Fig. 205) and blood sausages such as black pudding (see page 164). Before being used they need to be soaked in warm water to regain elasticity and to wash out the adhering salt.

Beef casings1

Several parts of the gastro-intestinal tract of cattle (Fig. 325) are used as casings in sausage production. Small intestines -“rounds”- have a typical circular shape and are used for stuffing sausages such as lyoner, liver and blood sausages and dried fermented beef products. Rounds are 40 m long and are normally readily available where cattle are slaughtered. They are used for all types of sausages in Muslim countries. The middles are around 7 m long and used for dried fermented and precooked-cooked sausages such as hunter’s sausage and coarse liver sausage. The blind gut is also used for precooked-cooked sausages and raw-cooked products such as large bologna etc. Their diameter varies from 76 to 102mm. Beef bladders are used for mortadellas and other specialities.

In preparation for processing, beef rounds are turned inside out and slimed. The mucose and serose membranes are removed from the intestines, leaving the submucose and muscular layer. The processing of beef small intestines does not remove the muscular layer (see Fig. 316) as it is the case when processing small sheep and pig intestines.


1) If should be noted that from 2001 in the EU no beef casings can be processed but the cattle tract from duodenum to rectum must be condemned due to BSE concern. Certain non-EU countries are authorized to produce beef casings for the EU.

Although these natural casings are edible they are usually not eaten due to their tough casing walls. After submerging the casings in water and thorough washing, the beef rounds are calibrated, tied and salted. Salted rounds are marketed in sets of around 100 yards (91.4m), each set containing a maximum of five pieces.

Fig. 325: Gastro-intestinal tract of cattle

Natural casings from cattle:
Beef gullet (1)
Beef tripes (2)
Beef rounds (3)
Beef caps (4)
Beef middles (5)
Beef rectum (6)
Beef bladder (7)

The beef middles are separated from the mesenteric fat (ruffle), flushed out with water, trimmed free of fat, turned inside out, slimed and salted. Beef middles include the “straight” casing (long, not curved part) and are packed in sets each measuring about 17m after salting and composed of 5 pieces. Beef middles (narrow end, wide end and fat end) are used as containers for different salamis and other large-diameter sausage products.

Beef bladders are washed, turned inside out and either salted or inflated with air and dried, before they are used for different sausage specialties. Beef bladders are usually graded in large, medium and small sizes.

Recommended treatment of natural casings

Natural casings are usually available and best stored dry-salted. Prior to the filling of sausage mix into such kind of casings all the adhering salt must be washed off with cold water. Dry-salted casings need to be then soaked in water for several hours (3-5 hours in lukewarm water or over night in cold water). Soaking in water does not only remove remaining salt but also serves to make the connective tissue fibres of the casing wall more elastic in order to optimally enclose and hold the sausage mix to be filled. Addition of lactic acid (2%) to the water can support this process further.

An alternative way of storing natural casings is in saturated salt solutions. This is the ready-to-fill natural casing type, as it requires only brief soaking periods ranging from minutes to up to one hour, and proper rinsing. This type must always be stored chilled.

Recommended periods for soaking in water

Dry salted natural casings:

  General : 10-12 hours
 

For pig large intestines

: up to 24 hours
 

For cattle intestines

: 5-10 hours

Ready-to-fill natural casings (stored in saturated salt solution)

  General : maximum 10-60 min.
  For large pig intestines: 2-3 hours

Transport and storage of natural casings

The storage periods for natural casings depend on the storage temperature. Dry-salted casings in closed containers, which also protect them against light impact causing fat-rancidity, can be stored at 6-8°C from six months to 3 years. Storage periods are reduced with higher storage temperatures. Adhering fat reduces the shelf-life.

The casing industry has established the following minimum requirements regarding storage and transport:

Dry-salted maximum +15°C

at least 3 months

Ready-to fill maximum +10°C

at least 4 weeks

(saturated salt solution)

Sensory and hygienic quality

The principle for optimal natural casing production is to start processing the casing as soon as possible after slaughter. Intestines should ideally be processed when still warm as they are easier to manipulate (cleaning, sliming, washing) and bacterial growth can still be contained. The subsequent salt treatment, usually dry salting, will create high salt concentrations in the casing tissue, which easily reach the concentration of 15%, at which bacterial growth is halted (see page 33).

Some countries have established requirements for imported natural casings most of which derive from developing countries. A summary of such requirements is given hereunder:

Sensory quality:

Odour:

Free of signs of putrefaction

No rancidity

No sour (acidic) smell

Appearance:

Colour may vary from white to pink to grey

Microbiological norms (per gram)

Fully acceptable

Critical numbers (not to exceed)

Total aerobic colony count

<105

5 x 106

Enterobacteriaceae

<102

1 x 104

Staphylococcus aureus

<102

1 x 103

Clostridium (sulphite reducing)

<102

1 x 103


Artificial (manufactured) casings

Artificial casings were developed at the beginning of the 20th century when, in some countries, the supply of natural casings could no longer cope with the demand for such natural casings from the growing meat industries. Following the development of highly automated sausage filling equipment, artificial casings proved to be better suited to those systems, mainly due to their uniformity.

Also from the hygienic point of view, there were certain advantages to artificial casings as the microbial contamination is negligible, refrigeration is not needed and there are no spoilage problems during transport and storage. Nowadays, for wide sausage calibres, artificial casings are the material of choice, while for smaller calibre products, artificial and natural casings remain equally important.

According to their structure and composition of material1, artificial casings can be subdivided into

1) casings made of natural materials, with two groups:

     a) casings made of organic plant material, namely cellulose
     b) casings made of animal by-products, namely collagen

2) casings made of synthetic substances deriving from thermoplastic materials (“synthetic casings” which can be subdivided in “polymer casings” and “plastic casings”).


1) There are also casings made from textiles or co-extruded coatings based on alginate used for special products. They are not discussed here.

Cellulose casings

Cellulose as a natural material derived from wood or cotton has proven to be suitable for sausage casings as it is:

Simple thin cellulose casings are used as so called peeling casings for frankfurter type sausages. The batter is filled into such casings (calibre range 12-42 mm) and portioned. Thereafter the products undergo smoking and cooking (at 74˚C), which causes the build-up of a firm layer of coagulated protein under the casing. After this heat treatment, the cellulose casings are removed and the sausages maintain their shape due to the firm external layer of coagulated protein. As ready-to-eat sausages do not have a casing, they are also known as “skinless sausages” (Fig. 326, 327, 328).

Cellulose casings are not suitable for larger sausage calibres as frequent breakages may occur due to rupture of the cellulose wall. In order to solve this problem, fibrous casings were developed. Fibrous casings are cellulose casings reinforced with strong cellulose fibres. These fibrous casings are resistant enough for large sausage calibres and still suitable for smoking (Fig. 329).

As a further step in the development of strong fibrous casings for large calibres, a layer of synthetic material, (e.g. PVDC) was added to the inside or outside of the casings (coated fibrous casings). The coating made the casing mechanically very resistant and created a complete barrier for gases, i.e. no evaporation losses can occur (Fig. 330).

However, fibrous casings with an inside or outside synthetic coating cannot be used for products to be smoked, as no smoke penetration is possible, and for products to be dried and fermented, as no water vapour evaporation is possible. They are mainly used for cooked sausages of the raw-cooked and the precooked-cooked type. The main advantage of coated fibrous casings for cooked sausages is the casing wall tightly enclosing the sausage mix and the easy peeling. As smoke does not penetrate through coated fibrous casings, smoke flavour can be added during manufacture of the sausage mix if desired.



Fig. 326: Products in transparent cellulose casings (cal. 22) (after filling and before smoking/cooking)


Fig. 327: Product in red coloured cellulose casings (cal. 22) used to transfer colour to sausage surface


Fig. 328: Sausage after removal of peeling casing (middle); removed casing (left); peeling casing still on (right)


Fig. 329: Fibrous casings (medium calibres)


Fig. 330: Coated fibrous casings


Fig. 331: Frankfurters in collagen casing


Fig. 332: Larger calibre (raw-fermented) sausage in collagen casing

Collagen casings

This type of casings is fabricated from collagen, which is obtained from the corium layer of selected split cattle hides1. The collagen-rich tissue is homogenized under high pressure, ring-extruded (hose-shaped) and hardened and results in a mechanically strong casing. Collagen casings are permeable for smoke and water vapour. While wide calibres must have a relatively thick casing wall as increased stability is required, small calibres can be made with relatively thin casing walls. As collagen is an animal tissue fit for human consumption, the thin collagen casings are easy to chew and “edible”. They are an alternative to replace natural sheep, goat or thin pig casings (page 251, 255). The advantages of collagen casings are their standard diameter and strength and that they can be “shirred” i.e. folded together, in long lengths and used for manual or automatic filling stations without pre-soaking in water (Fig. 331).

Traditionally many consumers still prefer frankfurter type sausages in the natural casing, although with recent advances in edible collagen casings there is not much difference between both types terms of in tenderness and mouth-feel.

The edible collagen casings are also used for fried sausages (including the typical breakfast sausages) and small calibre dry sausages such as beef sticks, etc. Collagen casings of 32 mm and above are not intended to be eaten as part of the sausage, they have to be peeled off. They can be used for most fresh sausages, raw-cooked and smoked sausages or raw-fermented sausages (Fig. 332).


1) For leather fabrication the middle portion of the cattle hide, also called corium, is used. The corium can be separated by using splitting machines in up to three layers for leather fabrication. Tissues from the corium middle layer are used for production of collagen casings.

Synthetic casings

These casings are made of synthetic thermoplastic materials (Fig. 203, 333, 334). Suitable materials are Polyamide (PA), Polyethylene (PE), Polypropylene (PP), Polyvinylidenchloride (PVDC) and Polyester (PET).

While previously only synthetic casings from individual synthetic substances (mono-materials) could be fabricated, recently developed co-extrusion1 techniques can be used to produce casings composed of combinations of several synthetic materials. Synthetic casings can therefore be manufactured with tailor-made properties.

The resulting casings are mechanically strong, relatively heat resistant, impermeable for smoke, gases and water vapour. Synthetic casings are particularly well suited for:

The latest development in synthetic casings are casing walls consisting of two to five layers of synthetic material with extreme barrier properties for gases and temperature resistance from -18° to 105/121°C They are suitable for production of sausages with long shelf life as they can be mildly sterilized and stored frozen if necessary.

Synthetic casings cannot be used for products which have to undergo drying, ripening and fermentation, such as dry sausages, as the casings are impermeable for gases and water vapour.


Fig. 333: Liver sausages in synthetic casing


Fig. 334: Meat in jelly in synthetic casing, casing end closed by clip


1) Co-extrusion is the combination of thin layers of different synthetic materials, which are fused during the extrusion process.

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