Processed chicken feathers as feedstuff for poultry and swine. A review

Processed chicken feathers as feedstuff for poultry and swine. A review

Agricultural Wastes 14 (1985) 275-290 Processed Chicken Feathers as Feedstuff for Poultry and Swine. A Review Manthos C. Papadopoulos Department of ...

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Agricultural Wastes 14 (1985) 275-290

Processed Chicken Feathers as Feedstuff for Poultry and Swine. A Review

Manthos C. Papadopoulos Department of Animal Science, Agricultural University, P.O. Box 338, 6700 AH Wageningen, The Netherlands

ABSTRACT Feather waste at poultry processing plants has been of htterest hi nutritional studies because of its high protein content. This material must be hydrolyzed to be digested by the animal, because, in its natural state, it is of no nutritive vahte. Methods of feather meal processhzg (autoclave hydrolysis, chemical and enz)'matie treatments) and its use hzpoultry and swine feeding are described. Because feather meal is deficient hi lyshle, methionhle and histidine and also varies in quality, discrepancies have been found h~ the amount of meal which can be effectively used under practical conditions. The total amhto acid profile of feather meal, as affected by different processhtg conditions, is discussed, together with the possibility of ushlg in vitro protehz digestibility as a criterion to detect feather meal proteht quality. Emphasis is given to the major problem currently affecthtg the quality of feather meal--the re&tction ofmttritive vahte ht terms ofamhto acid content and digestibility. Causes of amhto acid changes &re to processhlg are discussed, together with the possibility of ushlg the unnatural amhto acid, lanthionhze, present hi feather meals, bat not ht feathers, as a reasonable htdicator of treatment damage. It was conchtded that feather meal must be evahtated by quantitative in vivo digestibility measurements of the hulividual amhzo acids attd should be used in poultry and swhze rations on the basis of the digestible amhlo acid~'it supplies. 275 Agricuhural Wastes 0141-4607/85/S03.30 9 Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

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INTRODUCTION At present the principal sources of animal protein for animal feeding are fish and meat meals. These are expensive and it would be advantageous if other, cheaper, animal b37-products could find a use in animal feeds. Poultry industries now exist in many countries and generate large amounts of residues that require disposal. Of these, a considerable proportion consists of poultry feathers. Although this keratinous material has a high protein content (85-99 ~), it is virtually indigestible in its natural state. Consequently, a great deal of interest has been aroused over the possibility of processing to make it more digestible. If this could be achieved economically it would provide an additional and cheap source of animal protein for livestock feeding to help meet the growing demand for animal products by an ever increasing world population. Keratin proteins have generally been considered to be of little, or no, nutritive value to most animals. Mangold & Dubiski (1930) failed to show any digestion of white goose feathers by cats, owls, dogs and rats. Routh (1942) reported that powdered chicken feathers as the sole source of protein for rats were capable of supporting a moderate growth rate, but only when supplemented by tryptophan, histidine and lysine, while Moran et al. (1966) found that ground'raw feathers failed to support growth i~ chicks even after supplementation with amino acids. Furthermore, McCasland & Richardson (1966) showed that rats fed on ground raw feathers as the sole source of protein lost weight and had a mortality rate of 100~o. This was reduced to 2 5 ~ by amino acid supplementation. Feather keratin is very rich in the sulfur-containing amino acid cystine. Because the cystine disulfide bonds within the keratin contribute to the insolubility and indigestibility of this protein, they must be destroyed before feather protein can be digested by animals.

METHODS OF PROCESSING FEATHER MEALS Since feather protein in its natural state is very poorly digested, various methods have been developed for processing feathers to convert their keratinous proteins to a more digestible form.

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Autoclave hydrolysis The commercial use of feathers up to the present has been confined mainly to hydrolyzed (auto~laved) feather meal and several reports concerning different methods of hydrolyzing feathers have been published. In early studies, Draper (1944) observed that autoclaving the feathers for various periods from 2 to 8 h, at different pressures between 200 and 240 kPa, appeared to have little positive effect on their nutritive value as measured by chick growth assays. A method developed by Binkley & Vasak (1950) for processing feathers into a friable, high-density meal, stimulated new investigations into the nutritive value of feather keratin. This method is essentially a wet cooking process in which the feathers are treated with saturated steam at pressures of 275-415 kPa for 30 to 60 min with constant agitation. The feathers are dried and ground to produce a free-flowing meal of relatively high density. Binkley and Vasak also noted that, with a steam pressure above 415 kPa and constant agitation, the feathers tended to 'gum', leading to a non-free-flowing meal. Sullivan & Stephenson (1957) found that variations in processing methods--with 200 to 340 kPa for 20 to 60 min--influenced the nutritive value of hydr,olyzed feather meal as measured by chick growth. Moran et al. (1966) showed that commerical feather meal, hydrolyzed at 142 ~ for 30 min, with appropriate amino acid supplementation, supported chick growth equivalent to that with soybean protein. Raw feathers autoclaved at 121 ~ for 18h, however, did not show the same ability to support growth. Morris & Balloun (1973b) demonstrated, in chick growth trials, that feather meal cooked for 60 min at 445 kPa with intermittent agitation resulted in the highest net protein values of all the feather meals tested. These processing conditions, however, were not confirmed as favourable by the results of Papadopoulos (1984) in which, from digestion experiments with broiler chicks, feather meal processed for 30 min had higher amino acid digestibility values than that processed for 60 min. Chemical treatments

Increasing energy costs have generated interest in developing alternative methods that will reduce autoclaving requirements, since alkali

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treatments resulted in disulfide bond cleavage (Goddard & Michaelis, 1934). Few reports have been published on the use of chemically treated feather meal as a protein source for animal feeding. Draper (1944), in his studies, used feathers treated with sodium sulfide by adding 700g of feathers to 454 g of Na2S plus 16 g NaOH dissolved in 6 litres of water. The mixture was allowed to stand for 24 h, with occasional stirring. The author reported that 50 70 supplementary protein from sodium sulfide treated feathers added to a basal cereal diet resulted in a significantly higher growth rate than that produced by the basal diet alone when fed t o chicks. Moran et al. (1966) treated feather meal samples with reducing agents such as sodium thioglycolate and sodium sulfide. They concluded that the meal prepared with-the lowest concentrations of the sodium thioglycolate (a quarter of the molar quantity of cystine), when supplemented with methionine, histidine, tryptophan, lysine and glycine and fed to chicks at a 15 70 level in a diet as the sole source of protein, gave a similar growth response to that of commercial feather meal similarly supplemented. When higher levels of sodium thioglycolate and a single level of sodium sulfide (1 mole per mole of cystine) w,ere used in the preparation of the feather meal, the chick growth response was depressed, suggesting that toxic f~ctors may have been present. Treating feathers with sodium hydroxide during autoclaving has been reported by Gruhn & Zander (1977). They used low pressures of 200 and 300 kPa for 2 h with different concentrations of sodium hydroxide from 0.25 70 to 1.0 70. Their results from feeding trials with laying hens led them to suggest that treated feather meals could be used more widely. Digestion experiments, however, with young chickens showed that sodium hydroxide (0.2 70 to 0.6 70) added during autoclaving of feathers may have a negative effect on feather meal amino acid digestibility values (Papadopoulos, 1984). Enzymatic treatments The use of proteolytic enzymes as digestive aids has been practised in the processing of various foods (Beddows & Ardeshir, 1979; Rexen, 1981). The literature contains some reports on microorganisms with keratinase activity, although not necessarily in the native state. Enzymes from Streptono,cesfi'adiae, isolated from soil (Noval & Nickerson, 1959),

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S. microflavus (Kuchaeva et al., 1963) and from Trichophyton gramdosium, a fungus of human and mammalian dermatophytes (Day et al., 1968; Yu et al., 1968) have been shown to be effective proteases with keratinolytic activity. It was concluded that the unusual ability of the organism to decompose keratin (wool and chicken feathers) rapidly and completely, must be due t Oits ability to reduce disulfide bonds in keratin. Other microorganisms have been introduced to utilize keratin (Koh et al., 1958; Frey, 1975) but their use is rather complicated. Although these reports indicated that some microorganisms hydrolyzed keratins, intentional enzymic modifications of keratin proteins are still largely found only in the patent literature and are practised to only a very limited extent. Elmayergi & Smith (1971) compared commercial feather meal, fermented by Streptooo, cesfradiae, with unfermented meal, in feeding trials with chicks. They found no significant difference in nutritional value between the two products, although the fermented meal was 9 0 ~ digestible by pepsin-HCl solution as compared with 65-70 ~ for unfermented meal. Recently Papadopoulos (1984) observed, in preliminary work, that feather meals enzymatically treated by a commercial proteolytic enzyme, 'maxatase', showed increased amino acid digestibility values determined by chick assays, in comparison with feather meals treated without enzymes. The results seem to be promising, but further work is needed to determine the full potential of the method employed, because it actually measured the combined effect of the different enzymatic constituents used, therefore, the keratinolytic activity of the enzyme used could not be estimated adequately.

FEEDING ANIMALS ON FEATHER MEAL

Poultry Hydrolyzed feather meal in poultry diets has been used for several years. However, because it is characterized by amino acid imbalances--which are critical in most primary feed ingredients--and also varies in quality, discrepancies have been found in the amounts of meal which can be effectively used under practical conditions. A number of investigators have demonstrated that broiler chicks and laying hefts performed normally when up to 4 ~ of their diet was supplied

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by hydrolyzed feather meal, while 5 ~ to 8 ~o dietary feather meal induced methionine, lysine, histidine and tryptophan deficiencies and worsened chicken performance (Naber et al., 1961 ; Moran et al., 1966; Morris & Balloun, 1973a; Vogt et al., 1975; Luong & Payne, 1977; MacAlpine and Payne, 1977). These authors also showed that supplementation of the diet containing the high levels of feather meal with the synthetic form of the deficient amino acids resulted in improvement in performance. Further studies of Baker et al. (1981) demonstrated that, with methionine plus lysine supplementation, up to 40 ~ of the dietary protein (24 ~) could be supplied by feather meal. Recently, Bielerai et al. (1982) reported that even 15 ~o feather meal may be included in the diet without affecting normal chick growth, by comparison with chicks receiving the control diet. This level was calculated on the basis of nitrogen absorption values, experimentally found to be 55 ~ for feather meal and 85 ~ for the control diet (without feather meal). Swine

Although feather meal has been used extensively in practical swine feeding, the available data in the literature are quite limited. Combs et al. (1958) reported no sighificant differences in the growth rates of l?igs fed the basal diet and 7-5 ~ feather meal plus lysine rations. The lififitations observed in poultry, as far as the level of feather meal in diet is concerned, are similar for swine. This is understandable because dietary amino acid balance and protein digestion by proteolytic enzymes have the same importance for simple-stomached animals as for birds.

I N V I T R O PROTEIN DIGESTIBILITY

Hydrolyzed feather meal is characterized by a variable nutritive quality, dependent upon processing methods. Consequently, the animal-feed industry needs a rapid quality control method in order to produce meals of good quality. For this purpose, crude protein analysis and digestibility determinations /n vitro are often used in practice. However, a crude protein analysis does not distinguish between raw and hydrolyzed feathers and gives relatively little or no information about the quality of the product for animal feeding, as this depends largely upon the efficiency of p~tein hydrolysis in the digestive tract.

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Several attempts have been made to simulate hi vitro conditions of digestion h7 vivo so as to predict the relative digestibility of proteins, since digestibility of protein is related to its usefulness as a source of individual amino acids. Pepsin digestibility is an accepted means of evaluating the digestibility of proteins for monogastric species. Standardization of the pepsin test has proved to be difficult due to the impurity and low activity of the commercial pepsin preparations and due to the use of varying levels of pepsin-HCl solution for pepsin digestion. Feather meal showed a wide range of digestibility values when the pepsin-HCl test was applied (Naber et al., 1961; Morris & Balloun, 1973b; Johnston & Coon, 1979; Aderibigbe & Church, 1983; Papadopoulos, 1984). The data suggested that there was a definite trend for feather meals to have higher pepsin digestibility values and increased degradation of cystine,..as processing time and pressure increased. However, no standard definition of a desirable pepsin digestibility of the feather meal protein has been adopted. The'pepsin-HCl test measures only solubilization of protein and not the extent of its digestion and absorption by an animal. Such an ht vitro test should be compared with ht vivo digestibility measurements of protein and amino acids in order to prove its validity. Recent studies, however, on the relationships between h~ vitro and h~ vivo tests were not encouraging (Papadopoul.os, 1984). It was found that laboratory (pepsin-test included) evaluations of feather meals are not a reliable index to detect inferior amino acid digestibility in hydrolyzed feather meals treated under different processing conditions. AMINO ACID COMPOSITION Effect of autoclaving hydrolysis

The major difference in amino acid composition between raw and processed feather meal is the drastic reduction in cystine concentration after treatment (Gregory et al, 1956; Davis et al., 1961; Moran et al., 1966; Eggum, 1970; Papadopoulos, 1984). This is an indication that disulfide linkages have been broken, thus making the feather protein more soluble and susceptible to proteolytic enzymes. As far as the amino acid pattern of processed feathers was concerned, Gregory et al. (1956) reported that the amino acids in commercially hydrolyzed feathers (determined by the method of Binkley & Vasak,

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1950) were relatively stable during processing with steam and pressure, with the exception of arginine, phenylalanine, isoleucine and cystine. Of these, the only considerable loss was in cystine content. Davis et al. (1961) studied the time-pressure combinations necessary for hydrolyzing feathers a~d evaluated several methods to determine their effect on the amino acid composition of the processed feathers. The methods evaluated were 185kPa for 16h, 310kPa for 30min to 4h and 515 and 720 kPa for 20 and 6 min, respectively. They reported that the amino acids, with the exception of cystine, did not appear to be affected substantially by the processing treatments employed. In their tests cystine appeared to be lowered in extreme processing conditions. They also observed the appearance of the unnatural amino acid, lanthionine, in feather meal but not in feathers. The fact that the amount found approximated to the loss ofcystine during processing indicates that most of the cystine lost is converted to lanthionine (Papadopoulos, 1984). Morris & Balloun (1973b) presented results indicating that the level of the limiting amino acids (lysine, methionine and histidine) in the processed feather meal was correlated with the conditions of time, temperature and pressure at which the treatment was carried out. The maximum level was attained by hydrolysis for 1 h at 445 kPa pressure and with intermittent stirring. Similar result~ for the sulfur amino acids have been reported by Wheeler & Latshaw (1980). Recently, Papadopoulos (1984) studied the effect of varying lengths of processing time (30 to 70 min) and moisture content (50 % to 70 %) on feather meal amino acid contents. The heterogeneous behaviour of the individual amino acids to the different processing variables led to the conclusion that there is no single set of processing conditions that is optimal in all circumstances. Effect of chemical treatments

The amino acid composition of chemically treated feathers has been reported by Eggum (1970). In his trials, the addition of 1% HCl-solution to hydrolyzed feathers reduced the fall in cystine compared with feather meals processed under the same conditions of heat and pressure without HC1. It was also shown that the addition of HCI reduced the content of other amino acids and that the decreases were most pronounced in the case of lysine, tyrosine, arginine and tryptophan, when compared with feather meals treated without HCI.

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Wolski et al. (1980) observed that in feathers treated with dimethylsulphoxide (DMSO) the content of all the amino acids increased in comparison with the non-modified feathers, except for cystine, methionine, lysine and histidine. Further studies by Papadopoulos (1984) showed that, in chemically treS.ted feather meals, there was a significant variation in the amino acid responses to the effect of added sodium hydroxide. All the essential amino acids, with the exception of valine, were reduced, while the non-essential ones, with the exception of serine, were increased, as sodium hydroxide concentration increased. Effect of enzymatic treatments

Concerning the effect of microorganisms on the amino acid contents of feather meal, Elmayergi & Smith (1971) published results showing that levels of methionine, tyrosine, lysine and histidine, usually present in small quantities in feather meal, were increased considerably during fermentation with Streptomyces fradiae. They concluded that the concentration of cystine decreased because the acid was used for methionine synthesis. The changes in amino acid contents of enzymatically-treated feather meals were studied by Papadopoulos (1984), using a commercial proteolytic enzyme, 'maxatase'. It was found that most of the amino acid concentratior~, with the exceptions of leucine, tyrosine and phenylalanine, were lower in the enzymatically-treated feather meals than in the samples with no addition. A M I N O ACID DIGESTIBILITY AND AVAILABILITY Amino acid contents of individual feedstuffs have, in general, been determined by physico-chemical methods of analysis. Because these determine the total amount of amino acid present, they are of limited value only, since not all of each amino acid in a protein is made available to the animal in the course of digestion, absorption and metabolism. There is need, therefore, for a bioassay to estimate the value ofa feedstuff. For several years, research workers have tried to determine the amino acid digestibility/availability of hydrolyzed feather meals for several monogastric species, but the reports have been relatively few. In digestion experiments with pigs (Dammers, 1964), feather meal had a rather high true digestibility of the individual amino acids, of between 80 % and 90 %. Similar results were obtained by Eggum (1970) using rats.

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In chick-growth assays, Smith (1968) found that the availability of most of the amino acids in commercial feather meal was poor compared with fish and soybean meals. This applied especially to lysine and histidine. In contrast, MacAlpine & Payne (1977) reported that lysine had an availability of 72.5~o, estimated by a chemical (FDNB) method. Similar results on 'available' lysine present in pepsin-digested (hi vitro) feather meal were found by Morris & Balloun (1973b). They also showed that the more severely processed feather meals (445kPa for 60min) contained more 'available' lysine, methionine and histidine than did feather meals processed under milder conditions. In digestion experiments with laying hens Vogt & Stute (1975) showed that the apparent digestibility of amino acids in feather meal was 79 ~. The true digestibility of amino acids from feather meal, determined by faecal analysis irkchicks, has been found by Burgos et al. (1974) and Kirby et al. (1978) to be as high as 97~o and 94~o, respectively. Kim et al. (1980) studied the available sulfur amino acid content of five feather.meals, different in terms of plant source and processing method, using the growth of chicks as the criterion. They found that the range in availability of the sulfur amino acids was from 4 1 ~ to 82~. Furthermore, Parsons et al. (1982), using a faecal analysis method, reported the average true digestibilit~ of amino acids in commercial feather meal to be 82 ~ compared with 76 ~ for cystine. Recently, Bielorai et al. (1983), Papadopoulos et al. (1983b) and Papadopoulos (1984) concluded that there are certain consistencies in the digestibility of amino acids from feather meals. The most important variations in amino acid digestibility values were the rather high digestibility of isoleucine, phenylalanine, arginine and valine, in contrast to cystine, histidine, lysine and aspartic acid which were generally among the less digestible amino acids. The highest and lowest true digestibility values were reported for isoleucine (87 ~) and aspartic acid (36 ~o) from feather meals enzymatically treated or autoclaved without addition, respectively (Papadopoulos, 1984). The low digestibility values of some amino acids may be related to the sensitivity of these amino acids to different treatments. PROCESSING TREATMENTS AND AMINO ACID DIGESTIBILITY Featrrer meals, commerically prepared, may vary widely in digestibility and nutritional quality, probably due to variations in pretreatment of the

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source materials in the dressing plants and due to different processing conditions. Systematic studies in our laboratory showed that processing conditions, in particular, increased processing time, have a substantial negative effect on aminb acid digestibility of feather meals (Papadopoulos et al., 1983a,b; Papadopoulos, 1984). In these studies the formation of the unnatural amino acid, lanthionine, in parallel with the drastic destruction ofcystine and other amino acids, led to the conclusion that autoclaving feather meal may alter the protein structure in such a way that the enzymic attack necessarily-associated with the digestion process is hindered. It was also concluded that the amount of lanthionine present in the test samples may be a reasonable index of treatment damage, since feather meals with higher lanthionine contents showed lower digestibility values. The nutritional value o~ the cysteine residue, lanthionine, is not well known. Robbins et al. (1980) and Baker et al. (1981), in feeding experiments with chicks, reported that U-DL-lanthionine could be partly used as a source of cysteine. Recently, Papadopoulos (1984) found, in digestion experiments, that the total lanthionine recovery in poultry excreta was always less than 100 70, which means that lanthionine is either transformed by the intestinal flora, metabolized or retained in the organism. Because the existing data are rather limited, more extensive work is neede~ before definite conclusions can be drawn about the utilization of lanthionine. Possible mechanisms for the reduction of protein quality caused by heat have beeen discussed by several authors (Hurrell et al., 1976; Bender, 1977; Whitaker, 1980). It is now generally accepted that the effect of heat on proteins in the absence of carbohydrates and fats is to impair their nutritive value. It has been suggested that heat causes the formation of new cross-linkages within the protein molecules which lead to the formation of new amino.acids such as lysinalanine, ornithoalanine and lanthionine. The hypothesis is that cross-link formation reduces the rate of protein digestion, possibly by preventing enzyme penetration or by blocking the sites of enzyme attack. In addition to cross-link considerations discussed above, the exposure of protein to alkali (Provansal et al., 1975; Masters & Friedman, 1980) and also to heat (Sternberg et al., 1975; Liardon & Hurrel, 1983) leads to racemization of amino acid residues. Even if small amounts of racemization occur in a protein during alkali processing, an extended range of the peptide chain around the racemized amino acid residues

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cannot be used by the substrate-binding site of proteases. This causes a major decrease in proteolysis (Hayashi & Kameda, 1980, Masters & Friedman, 1980). This was further confirmed by the results of Papadopoulos (1984) who found that feather meals processed with increasing sodium hydr6xide concentrations had reduced amino acid digestibility values in comparison with samples with no alkali treatments. CONCLUSIONS The published information suggests that: feather meal has to be treated in order to increase its digestibility for use as a feedstuff in animal nutrition, and autoclave hydrolysis seems to be the most used method, but prolonged treatment (longer than 30 min at 146~ is not desirable. For the chemical and enzymatic modifications to feather meal protein, the former seem to'be unfavourable while the latter are promising, but further work is needed on the development of the enzymic treatment. The use of feather meal in poultry and swine feeding is limited because it is deficient in some of the essential amino acids (lysine, methionine and histidine) and also varies in quality. Recent studies suggest that the modifications of amino acid digestibility values by processing treatments are of considerable importance (Papad,opoulos, 1984). In particular, the poor digestibility of the most frequently limiting amino acids, lysine, histidine,,and methionine, seems to be of interest from the nutritional point of view. The ht vitro pepsin test is not adequate to evaluate feather meal protein quality because it revealed differences between the differently processed products which were not detectable by the ht vivo amino acid digestibility determinations. The lanthionine present in feather meals may be a reasonable indicator of treatment damage since the amino acid digestibility values of processed feather meals are inversely proportional to the lanthionine contents of the test samples (Papadopoulos, 1984). The variations between individual amino acids in their digestibility values are sufficiently extensive to suggest that, in the formulation of diets for simple-stomached animals, it is essential that the dietary feather-meal protein be balanced on the basis of digested amino acids. REFERENCES Aderibigbe, A. O. & Church, D. C. (1983). Feather and hair meals for ruminants. I~ Effect of degree of processing on utilization of feather meal. Journal of Animal Science, 56, 1198-207.

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Baker, D. H., Blitenthal, R. C., Boebel, K. P., Czarnecki, G. L., Southern, L. L. & Willis. G. M. (1981). Protein-amino acid evaluation of steam-processed feather meal. Poultry Science, 60, 1865-72. Beddows, C. G. & Ardeshir, A. G. (1979). The production of soluble fish protein solution for use in fish sauce manufacture. I. The use of added enzymes. Journal of Food Technology, 14, 603-12. Bender, A. E. (1977). The effect of heat on protein rich foods. In: Food quality and nutrition--Research priorities for thermal processhlg. (Downey, W. K. (Ed.)) Applied Science Publishers Ltd., London, 411-26. Bielorai, R., losif, B., Neumark, H. & Alumot, E. (1982). Low nutritional value of feather-meal protein for chicks. Journal of Nutrition, 112, 249-54. Bielorai, R., Harduf, Z., Iosif, B. & Alumot, E. (1983). Apparent amino acid absorption from feather meal by chicks. British Journal of Nutrition, 49, 395-9. Binkley, C. H. & Vasak, O. R. (1950). Pro&wtion of afriable mealfrom feathers. USDA, ARS Bulletin No. AIC-274. Burgos, A., Floyd, J. I. & Stephenson, E. L. (1974). The amino acid content and availability of different samples of poultry by-product meal, and feather meal. Pottltr), Science, 53, 198-203. Combs, G. E., Alsmeyer, W. L.. & Wallace, H. D. (1958). Feather meal as a source of protein for growing-finishing swine. Journal of Animal Science, 17, 468-72. Dammers, J. (i 964). Verteringsstudies bij het varken. Factoren van hlvloed op de rertering tier voeder-componenten op tie ,rerteerbaarheid der aminozuren. Thesis. University of Leuven. Davis, J. G.,~Mecchi, E. P. & L.ineweaver, H. (1961). Prgcesshlg of poultry byproducts and their utilization hi feeds. I. Processing of poultr), by-products. USDA, ARS, Utilization Research Report 3. Day, W. C., Toncic, P., Stratman, S. L.., L.eeman, U. and Harmon, S. R. (1968). Isolation and properties of an extracellular protease of Trichophyton gramdosum. Biochemica et Biophysica Acta, 167, 597-606. Draper, C. I. (1944). The nutritive value of corn oil meal and feather protein. Iowa Agr. E.vpt. Sta. Res. Bull. 326. Eggum, B. O. (1970). Evaluation of protein quality of feather meal under different treatments. Acta Agriculturae ScandinaL,ica, 20, 230-4. Elmayergi, H. H. & Smith, R. E. (1971). Influence of growth of Streptomyces fradiae on pepsin-HCl digestibility and methionine content of feather meal. Canadian Journal of Microbiolog)', 17, i 067~-72. Frey, D. (1965). Isolation ofkeratinophilic fungi from soils collected in Australia and New Guinea. Mycologia, 57, 202-16. Goddard, D. R. & Michaelis, L. (1934). A study of keratin. Journal of Biological Chemistry, 106, 604-14. Gregory, B. R., Wilder, O. H. M. & Ostby, P. C. (1956). Studies on the amino acid and vitamin composition of feather meal. Poultry Sc&nce, 35, 234-5. Gruhn, ~K. & Zander, R. (1977). Technisch und chemische Bearbeitung yon Schweineborsten und Hiihnerfedern zur Gewinnung proteinreicher Futtermittel. Tierzucht, 31,374-6.

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Hayashi, R. & Kameda, I. (1980). Racemization of amino acid residues during alkali-treatment of protein and its adverse effect on pepsin digestibility. Agricultural and Biological Chemistry, 44, 891-5. Hurrell, R. F., Carpenter, K. J., Sinclair, W. J., Otterburn, M. S. & Asquith, R. S. (1976). Mechanisms.of heat damage in proteins. 7. The significance of lysine-containing isopeptides and of lanthionine in heated proteins. British Journal of Nutrition, 35, 383~95. Johnson, J. & Coon, C. N. (1979). A comparison of six protein quality assays using commerically available protein meals. Poultry Science, 50, 408-10. Kim, S. M., Winegardner, M. & Dudley, W. A. (1980). Available sulfur amino acid content of commercial feather meals. Poultry Science, 59, 1627-8. Kirby, L. K., Nelson, T. S., Johnson, Z. & Waldroup, P. W. (1978). Content and digestibility by chicks of the amino acids in wheat, fish meal and animal byproducts. Nutrition Reports htternational, 18, 591-8. Koh, W., Santro, A. & Messing, R. (1958). Keratinolytic enzymes from Aspergilhts flartts and Aspergilhts niger. Bacteriological Proceedings, 38, 18. Kuchaeva, A. G., Taptykova, S. D., Gesheva, R. L. & Kpasilnikov, V. A. (1963). Keratinase activity of acthzomycetes of the "Fradiae" group. Doklady Akademii Nauk SSSR, 148, 1400-2. Liardon, R. & Hurrell, R. F. (1983). Amino acid racemization in heated and alkali-t~eated proteins. Journal of Agricultural and Food Chemistry, 31, 432-7. Luong, V. B. & Payne, C. G. (1977). Hydr~iysed feather protein as a source of amino acids for laying hens. British Poultry Science, 18, 523-6. MacAIpin~, R. & Payne, C. G. (1977). Hydrolysed feather protein as a source of amino acids for broilers. British Poultry Science, 18, 265-73. Mangold, E. & Dubiski, J. (1930). Die Verdauung des Keratins, besonders der Hornsubstanz von Vogelfedern, durch S/iugertiere und V6gel. Wiss. Arch. Landw., Abt. B., Tierern/ihr. Tierzucht, 4, 200-11. Masters, P. M. & Friedman, M. (1980). Amino acid racemization in alkalitreated food proteins--Chemistry, toxicology, and nutritional consequences. In: Chemical deterioration of protehzs (Whitaker, J. R. & Fujimaki, M. (Eds.)), ACS Symposium series 123, 165-94. McCasland, W. E. & Richardson, L. R. (1966). Methods for determining the nutritive value of feather meals. Poultry Science, 45, 1231-6. Moran, E. T., Jr., Summers, J. D. & Slinger, S. J. (1966). Keratin as a source of protein for the growing chick. 1. Amino acid imbalance as the cause for inferior performance of feather meal and the implication of disulfide bonding in raw feathers as the reason for poor digestibility. Poultry Science, 45, 1257-66. Morris, W. C. & Balloun, S. L. (1973a). Effect of processing methods on utilization of feather meal by broiler chicks. Poultry Science, 52, 858-66. Morris, W. C. & Balloun, S. L. (1973b). Evaluation offive differently processed feather meals by nitrogen retention, net protein values, xanthine d~laydrogenase activity and chemical analysis. Poultry Science, 52, 1075-84.

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