Department of Animal Science, Gansu Agriculture University, Lanzhou 730070, Gansu, P.R. China
Due to economic limitations, the processing and use of by-products, including blood, internal organs, bone and waste hair from slaughtered yak, have not been fully attempted. However, previous reports have shown that medicines and other biochemical products could be extracted from these by-products. Slaughter by-products can also be made into feeds and feed additives for livestock and poultry. These can improve the overall economic value of the yak.
Keywords: By-product, development, slaughtered yak, use
Numbering some 13 million head, the yak is a special livestock on the Qinghai-Tibetan Plateau and its surroundings covering total grassland of 3 million km2 (Lu 1994). Yak is also the main production resource and life support for the people living in this area.
A development focus on the yak therefore provides an important avenue for addressing the needs of the people. However, because of the obstacles imposed by the natural environment and economic factors over a long period of time, the use of yak products, especially the intensive processing of the slaughter by-products, is still very limited. A small proportion of these by-products has been processed and sold as primitive materials at very low prices. It is estimated that, if all such by-products could be used effectively, one slaughtered yak could contribute an extra of 100 RMB Yuan (US$ 1 = 8.2 Yuan). This would significantly increase the overall economic value of the highland livestock production system, with important implications for the living conditions of local herders. This paper reviews the possible uses of the by-products of slaughtered yak.
It has been shown that some valuable biochemical substances such as haemoglobin and super oxide dismutase (SOD) can be extracted from yak blood leaving the rest of the product to be made into blood and protein meal.
The serum albumin can be separated from the yak serum (Yang et al. 1988; Chen 1990). The albumin of cattle has been used in medicine as a biochemical reagent. Compared with the method of ammonium sulphate and salt extraction, the albumin from yak serum produced by the cold alcohol procedure had a larger quantity and a higher quality (after ã-globin had separated). Furthermore, because the procedure is simple, low-cost protocols could easily be developed and applied in pastoral areas.
Haemoglobin and SOD are important substances in erythrocyte. Hematochrome affects the cell's respiratory, biological oxidation and metabolism of the intro-organism. It is also widely used in the domestic and international markets as the main medical composition of anticancer drugs. SOD is a kind of metal enzyme, which could eliminate a super-oxygen anion (O2-). It has functions of antiphlogistics, hemostasis and prevents cells from aging.
A simple protocol for extracting hematochroma by the 'acetone method' from yak blood has been developed with a product yield of 3.2–4.0 g per litre (Yu and Zhang 1991). SOD had also been extracted from yak blood with a yield of 1g per 15 to 20 L (Yu and Zhang 1990; Hua et al. 1991; Xu 1991). It has been reported that the SOD obtained from yak blood not only has higher vitality, but also has higher purity than that from the blood of other animals, probably due to the low oxygen tension in the yak environment.
Yak blood cell plasma has also been manufactured into blood meal and fermented blood meal. The blood meal contained high quality animal protein and essential amino acids. The protein was up to 80% of the whole blood meal with a higher nutritive value than meat meal and fish meal (Yu and Zhang 1990). Thus, the blood meal of yak could be a very important raw material in the compound feed. The palatability of fermented blood meal is also quite high. Because of the simple processing techniques, the blood meal and fermented blood meal can be manufactured even in a small slaughterhouse. Livestock in pastoral areas can, thus, make use of the blood meal obtained from the waste blood of yak as a supplementary source of protein.
Brewing soy sauce from the yak blood is a new way to produce food additive using yak slaughter by-products (Yang and Zhang 1988). Through proper sanitary and physiochemical screening, the soy sauce from yak blood can be produced that meets the required food standards. Moreover, it contains larger quantity of free amino acids. Because there is more glutamic acid in the yak blood soy sauce than in the soybean soy itself; the yak blood soy is sweeter.
Applying the protocol used in pig, the cytochrome C has been successfully extracted from fresh or frozen yak heart (Yang et al. 1989). The yield was up to 135.0417 ± 5.3751 mg per kg.
Semi-finished product of heparin can be extracted from the lungs and small intestines of yak by the salt lyses and ion exchange method (Yang et al. 1989; Zhang et al. 1991). The yield was up to 43,416.87 U per kg milled lung. The average anticoagulant effect was 81.94 U/mg, which met the national standard (>80 U/mg).
Bilirubin is the major part of natural bezoar and it can be extracted from bile collected from either a living or slaughtered animal (Zhang et al. 1989; Wang et al. 1992).
Besides the use in the making of elegant crafts, the yak bone is used to extract bone gelatin and bone fat or to process bone meal and jam. These substances can be further made into health foods or the feed supplements both of which are in great demand in the domestic and international markets because they are rich in phosphorus, calcium, and proteins (Zhang et al. 1989).
There are 18 amino acids in the yak hair, of which cystine makes up 7–10% of the total. It is possible to extract the cystine from the wasted yak hair in textile industry, which can be used as additive in chemistry and feed supplement (Kuang and Zhao 1997).
It has been said that all parts of yak have high value. However, the problem is how to develop and use them efficiently. The use of the meat, milk, hide, wool and undercoat of yak has been fairly popular and successful, but large-scale processing is still not developed. Not only does non-use of these by-products represent an economic loss, it also presents a problem to environmental health when these products are discarded following slaughter. These products should be further processed and marketed. Both research and development are needed to make this possible.
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