When meat animals are processed, considerable quantities of trim are produced which is sorted according to the lean meat content and sold to sausage makers or ground for hamburger meat. Surplus fatty trimmings are rendered to recover the fat from beef and lard from pork as tallow (Clark, 2005). Rendering involves cooking, usually by direct steam injection, followed by centrifugation and drying of the fat and protein fraction. The protein from edible rendering can be used in animal feeds, while that from inedible rendering can be used as a fertilizer. Tallow and lard are used in baking and frying, although recent nutritional concerns with respect to saturated fats are affecting their use. The aqueous phase from the above centrifugation step is usually called 'stick water' and may be concentrated as a source of beef flavour. If the 'stick water' is not concentrated, it can represent a significant waste disposal challenge (Clark, 2005).
Body condition score (BCS) of beef cows has been shown to have a significant effect on by-product yield and value. By-product weights - including blood, bone meal and a variety of organs - have been obtained during the slaughter process and by-product yields have been calculated as a percentage of the animal's live weight taken 24 h before slaughter. Total byproduct value was quadratically related to BCS. The results from this study indicated that the BCS of cows at the time of slaughter had a profound influence on by-product yields and values of by-products that are credited back against the cost of production to the beef cattle producer (Apple et al., 1999).
Refined meat protein isolates are produced from non-blood sources such as pig rind, which is a source of collagen. Ultrasonic fragmentation of the myofibrils and subsequent solubilization in buffer have been used to isolate intramuscular collagen in high yield and purity, and maintains intact fibres (Avery and Bailey, 1995). Many meat protein isolates contain over 90% protein and are used for their emulsifying properties in hotdogs, replacing carrageenan. The majority of such products are derived from pork, with smaller amounts coming from chickens.
Collagen is a natural constituent of bovine skin and can be converted by suitable procedures to a very thin collagen film which has food-grade properties and is edible. In the production of net-wrapped raw sausage, such a collagen film may be used both for enclosing the sausage meat and as a separating or release layer between the sausage meat and the net (Bisson and Weisenfels, 1992).
Computer-assisted simulation has been used to study the effect of collagen content on the biological value of meat proteins. It has been shown that an increase in the collagen content from 2.5% to 15-20% of the total amount of proteins contained in minced meat tangibly enhanced protein utilization for tissue synthesis. The above collagen content in meat products heightens their nutritional and biological value and renders them more suited to human metabolism (Rogov et al., 1992).
It has also been reported that collagen, in conjunction with soy protein and carrageenan, is able to increase the water holding capacity and improve the texture of deli rolls (Daigle et al., 2005). Collagen proteins have also been used as aids for the improvement of technological and sensory properties of meat products and ready meals (Marggrander, 1995).
The effects of pork collagen in emulsified and whole muscle products has been evaluated. Eight frankfurter treatments (0-3.5% pork collagen) and four ham treatments (0-3% pork collagen) were formulated and frankfurters and hams were evaluated for cooked yields, purge, colour, texture and sensory characteristics. Incorporation of pork collagen at 1% and above, significantly increased cooked and chilled yields in frankfurters but did not have any effect in hams. Purge was significantly reduced in both frankfurters and hams after 4 weeks of storage, while sensory difference testing showed no significant difference up to a 2% usage level of pork collagen in both frankfurters and hams (Prabhu et al., 2004).
The quantity of collagen has also been used as an index of quality of raw materials employed in the preparation of meat products. It has been reported that a high content of collagen and, as a consequence, of connective tissue found in meat samples, was a fairly good index of poor quality of the raw materials employed in the preparation of meat products (Bonafacci et al., 1992), with collagen also being a good predictor of protein quality (El, 1995).
Freeze drying of collagenous material from chicken feet skins and tendons, has been shown to affect physical functionality by increasing solubility, gel strength, emulsion stability and water holding capacity at 60 °C. Such modifications to this co-product of chicken processing would suggest the potential use of these products as functional ingredients in meat products (Alves and Prudencio-Ferreira, 2002).
Gelatin is a protein product commercially made from pork skin, cattle bones, calf skin or fish skin. It is prepared by the thermal denaturation of collagen (the protein in connective tissue and bones) isolated from animal skin and bones with very dilute acid. Gelatin contains a large number of amino acids such as glycine, proline and 4-hydroxyproline residues (Frost & Sullivan, 2005b).
Gelatin is a heterogeneous mixture of water-soluble proteins obtained from collagen material and is a highly digestible food protein that can be added to other foods to increase protein content. However, gelatin is used primarily for its numerous functional properties. Gelatin is probably the most versatile of the animal proteins in terms of functionality. Gelatin has a variety of applications as an ingredient, particularly in the confectionery industry - where it provides mouth-feel sensory attributes - but also in baked goods, dairy products, the meat industry, pet foods and the pharmaceutical industry. In confectionery products, the addition of gelatin results in improved foaming, gelling and final solidification of the candy in such a way that it dissolves slowly once ingested. In certain products such as marsh-mallows, it lends characteristics such as texture and prevents the sugar from crystallizing. The dairy industry has used gelatin to stabilize and improve the texture of yoghurt and sour cream products and to provide a smooth texture and mouth feel to cheese products (Frost & Sullivan, 2005b).
Gelatin is also used in the photographic film, cosmetics, adhesives and printing industries; it can also be used in microencapsulation processes and as an additive in finishing products for the leather industry (Cabeza et al., 1998). Specifically, gelatin is used in food applications as a gelling agent, a thickener, a film former, a protective colloid, an adhesive agent, a stabilizer, an emulsifier, a foaming and whipping agent, and a beverage-fining agent. The pharmaceutical industry uses gelatin to manufacture hard- and soft-gel capsules, to coat vitamins and tablets, and to manufacture blood plasma substitute. Soft-gel capsules are being used more extensively in over-the-counter pharmaceuticals, vitamins, nutraceuticals and cosmetics. Soft-gel capsules are odour-free and are easier to swallow than hard capsules and also tend to be more aesthetically appealing to consumers and have longer shelf lives. Gelatin is the primary material used in soft-gel capsules (Frost & Sullivan, 2000). In cosmetics, gelatin is used to encapsulate bath oils, while the photography industry uses high technical specification gelatin for silver halide photography and other imaging applications (Frost & Sullivan, 2005a).
Gummy products have been in high demand among American consumers for over a decade and are continuing to drive market growth for gelatins. Gummy candies are popular because of the wide variety of shapes, colours and flavours available for these products. Gelatin gives gummy candies their chewy structure and is the primary gelling agent of American gummy candies (Frost & Sullivan, 2000).
Hydrolyzed gelatin has a protein content greater than 90% and is used as a dietary supplement in sports drinks and protein bars. It is rich in the amino acids glycine and proline, which are suited to support of collagen synthesis. Medical studies indicate that hydrolyzed gelatin may improve joint pain and enhance mobility in people with joint disease. Hydrolyzed gelatin also has direct applications in the treatment of joint pain in strength-training athletes. Studies have also indicated that hydrolyzed gelatin may have a positive impact on post-menopausal bone maintenance by extending the effect of calcitonin. Hydrolyzed gelatin also has applications in the treatment of periodontal disease and is used in the form of a biodegradable matrix to deliver subgingival sustained-release chlorhexi-dine for supplemental treatment of periodontal pockets (Frost & Sullivan, 2000).
Prion diseases are a group of mammalian diseases that produce large vacuoles in the cortex and cerebellum of infected subjects. Prion diseases are caused by proteinaceous infectious particles called prions. Examples of prion diseases include BSE or 'mad cow disease', which affects cattle, and CJD, which affects humans. Outbreaks of BSE in the UK and deaths resulting from CJD have created worldwide scepticism about the safety of bovine-derived products originating from BSE-infected countries. Gelatin has been placed under scrutiny since it is produced in some instances from the bones and hides of cattle. There is presently no evidence that BSE can be transmitted to humans through gelatin products. However, uncertainty about the BSE infectivity of gelatin has raised questions about the safety of gelatin and has been a large contributing factor to the rise in popularity of gelatin replacers (Frost & Sullivan, 2000).
Industrial veal hydrolysate has been produced enzymatically for possible use as a gelatin-replacing ingredient for human consumption. Protein digestibility was determined by a pH-stat method and cell dialysis. Amino acid composition including 4-hydroxyproline, allowed determination of connective tissue, amino acid score and protein digestibility-corrected amino acid score. A high correlation was found between true digestibility and the pH-stat method. The meaty flavour and gelling properties of veal hydrolysate could make it useful for high-quality soups, sauces and gravies (Linder et al., 1997).
Dehydrated meats are primarily used as flavour enhancers in gravies, soups, stews and other food applications, but are also found in ready-to-eat military rations, survival rations, and outdoor sports and recreation rations. Beef, chicken and pork are the primary sources for dehydrated meats. Dehydrated meats are also referred to as bouillon in many food applications (e.g. beef bouillon, chicken bouillon). Dehydrated meats come in powder, granule, diced or jerky form and have protein contents ranging from 20% to 52% (Frost & Sullivan, 2000).
Meat proteins, in general, have good functionality - such as water binding and emulsification capabilities - and form strong elastic gels, they have good flavouring properties and a high nutritional value (related to the high essential amino acid content and minerals such as calcium, potassium, phosphorus and iron).
The effects of time and temperature on the water binding ability of chicken skin connective tissue (CCT) and its ability to form model gels have been studied, and it has been shown that CCT gels can be used as water binders in reduced-fat bologna. Processing qualities, and textural and sensory attributes were also analyzed to assess the feasibility of manufacturing a reduced-fat processed poultry product containing a modified poultry by-product. Heating (60 °C) CCT for 0.5 h allowed the formation of model CCT gels with added water decreasing CCT gel fat, protein and collagen content, and also decreasing hardness due to a protein (collagen) dilution. All bologna treatments exhibited acceptable sensory attributes and the data indicated the feasibility of using lower-water CCT gels as texture-modifying agents in reduced-fat comminuted meat products (Osburn and Mandigo, 1998). Another study by the same group has also demonstrated similar properties for pork connective tissue gels (Osburn et al., 1997).
MBM is a high-protein agricultural commodity that currently has few applications other than as an animal feed. Unmodified MBM has poor functional properties, due to its low solubility. A recent study has demonstrated that MBM can be extrusion-processed along with sodium caseinate to produce a useful plastic material which has been developed for use as a dog chew toy. For this application, elastic modulus (stiffness) was a key characteristic. The influence of moisture content on the glass transition temperature and elastic modulus reflected the plasticization of this material by water. On the basis of a comparison with a commercially available dog chew, the range of stiffness achieved, 0.25-2.50 GPa, encompassed the values appropriate for a dog chew. The results showed that a particular desired stiffness can be maintained by applying an edible moisture barrier to the surface of the material (Garcia et al., 2004).
MBM contains appreciable amounts of nitrogen (N), phosphorus and calcium, making it an interesting fertilizer for various crops. The effect of MBM as an N fertilizer has been evaluated in pot and field experiments. The soils used in the pot experiment were peat and a sand-peat mixture, both low in content of plant nutrients. The field experiment was carried out on a silt loam. In the pot experiment, increasing amounts of MBM gave significantly increased yields although there was, in part, N immobilization shortly after seeding the soil based on peat organic matter. In the field experiment there was no period of N immobilization and a good N effect was found for small amounts of MBM (total N, 50 kg/ha). At a total N content of 100 kg/ha there were no significant differences in grain yield of spring wheat between the treatments with MBM, mineral N fertilizer, and a combination of MBM and mineral N fertilizer (N 50 kg/ha from each). The results indicated that the relative N efficiency of MBM compared with mineral fertilizer was 80% or higher, if MBM was applied to cereals in spring (Jeng et al., 2004).
De-fatted meat and bone meal protein concentrate (MBMPC) has also been reported to have significant adhesive properties. Adhesiveness increased linearly as the MBMPC concentration increased up to 20%. The highest adhesiveness was observed in the range of 70-90 °C with improved adhesiveness and water resistance being observed with 0.05% glutaralde-hyde treatment (Park et al., 2000).
Data from animal feeding trials have indicated that the protein value of MBM and poultry by-product meal is limited by the amount of metaboliz-able methionine they contain (Klemesrud et al., 1997). The potential for improving the efficiency and rate of dietary nitrogen utilization in Holstein steers, by feeding an amino acid-balanced mixture of animal by-product protein sources, in combination with urea, has also been evaluated. A combination of porcine meat and bone meal, fish meal, hydrolyzed feather meal and blood meal was also formulated as an undegradable intake protein blend to complement amino acids derived from microbial protein synthesis. The results demonstrated that use of an amino acid-balanced blend of animal by-product protein sources did not improve the efficiency of dietary nitrogen usage when added to corn-based diets formulated to meet the nutrient requirements of rapidly growing steers (Knaus et al., 2001). MBM has also been evaluated as a potential substitute for fish meal in the diet of various fish species, although the results have not generally been positive with some studies reporting that the final body weight of fish, the weight gain, feed efficiency and protein efficiency ratio of fish fed MBM diets tended to decrease as the proportion of MBM in the diet increased (Kikuchi et al., 1997).
Brine-incorporated meats have become major novelty food items in the present fresh meat retail market in the United States. Brines are used to 'enhance' products' moistness and juiciness; however, the physicochemical processes involved in water binding and entrapment in the meat protein matrix have not been completely elucidated. The results of a recent study have shown that the dynamics of brine penetration into muscle fibrils were specific to phosphate types, with pyrophosphate and tripolyphosphate having the highest efficacy. Both phosphate treatments produced a transverse expansion of the myofibrils with a simultaneous extraction of myosin from the ends of the A-band in the sarcomere. These structural/biochemical changes resulted in substantial swelling of muscle fibres, i.e. an enhanced water uptake and immobilization. Furthermore, depending on the type of proteases employed, soy protein hydrolysates or peptides interacted differently with myofibrillar proteins, producing an array of morphologies and rheologies of protein gels that played a major role in water immobilization in salted pork products (Xiong, 2005).
A macromolecular meaty-flavour enhancer has been fractionated from a commercial beef extract. The macromolecular fraction was obtained by dialysis and separated by anion-exchange chromatography, copper-chelate chromatography and gel-filtration chromatography. Two fractions were isolated as active meaty-flavour enhancers. Determinations of the amino acid compositions and amino acid sequences of the isolated fragments showed that the two active fractions consisted of collagen and tropomyosin. The macromolecular material obtained from heated collagen and tropomyo-sin in the low-molecular-weight fraction of beef soup stock enhanced the meaty flavour. These results suggested that collagen and tropomyosin were precursors of the macromolecular meaty-flavour enhancer (Kuroda and Harada, 2004).
Functional protein isolates have been separated from coarsely and finely ground mechanically deboned (MDB) turkey using a combined alkaline solubilization and isoelectric precipitation method. Although the two types of MDB turkey were quite different in original composition, the compositions of the resultant protein isolates were very similar. Almost all of the fat, collagen and calcium originally present in the MDB turkey were removed during the processing. Gels made from the resultant protein isolates showed good textural properties and water retention ability with a lighter colour than the original MDB turkey (Liang and Hultin, 2003).
The enzymatic activities of a bovine spleen lysosomal-enriched extract have been characterized for their proteolytic potential. The extract showed a high activity of cathepsins, i.e. a considerable ability to degrade both myofibrillar proteins and collagen, as well as of exoprotease. Proteolytic activity was accompanied by acid lipase and esterase activities, and a minor peroxidase activity. The bovine spleen lysosomal-enriched extract may therefore be a useful tool for the tenderization and ripening of muscle foods such as meat, fish and their products (Melendo et al., 1998).
Market growth for meat co-products has been curtailed somewhat due to the farming crisis (BSE, foot and mouth) and due to high production costs (in comparison to plant-derived proteins such as soy proteins). While some of the restrictions (related to the BSE crisis) on the use of animal proteins in the food industry have eased, restrictions in the European Union on the use of meat proteins in animal feed applications are expected to remain until 2007 and therefore, the recovery of such markets is expected to be slow (Frost & Sullivan, 2005a).
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