Feather meal : evaluation of the effect of processing conditions by chemical and chick assays

Abstract

Feather waste at poultry processing plants, has been of interest in nutritional studies because of its high protein content. This material must be hydrolyzed in order to be digested by the animal, because in its natural state it is of no nutritive value. However, this product will be of variable quality because hydrolysis has not only beneficial effects but can also reduce the nutritive value in terms of amino acid content and digestibility. Although feather meal is used in poultry feeding, our present knowledge of its protein-amino acid quality is inadequate. The value of feather meal as a component of animal feeds could be better assessed if more were known about the effect of different processing conditions on the content and digestibility of its amino acids. Therefore, the investigation described in this thesis was conducted in order to study chemical, nutritional and physiological aspects of feather meal treated under different conditions.The first major part of this study was an evaluation of feather meal proteinamino acid quality by chemical methods (Chapter 2). Crude protein analysis and digestibility determinations in vitro are often used in practice as a rapid quality-control method. The amino acid composition in relation to the amino acid requirements of the animal is also taken into account in formulating rations.The effects of five processing times (30, 40, 50, 60 and 70 min), five moisture contents (50, 55, 60, 65 and 70%), five added levels of sodium hydroxide (0.2, 0.3, 0.4, 0.5 and 0.6%) and proteolytic enzyme (0.2, 0.3, 0.4, 0.5 and 0.6%) upon amino acid concentration and nitrogen solubility of feather meal were studied by multiple regression techniques in three experiments (section 2.3.2). The first experiment examined the combinations of time and moisture. The second and third experiments contained the combinations of time, moisture, sodium hydroxide and of time, moisture, enzyme respectively. All test products were autoclaved at a constant temperature of 146°C.The results of these experiments showed that the individual amino acids have distinct variations in their response to various processing variables (time, moisture, sodium hydroxide, enzyme). There are losses, in qeneral, of amino acids during the processing of feather meal. These losses were more pronounced in the chemically treated feather meals, followed by the enzymatically treated ones compared with feather meals treated without additions. In the last treatments some amino acids, valine, leucine, tyrosine and phenylalanine, proved to be rather stable during processing. The protein solubility of the testfeather meals in pepsin-HCl solution and alkali or acid solvents was increased as a result of processing. However, the amino acid contents and especially that of cystine were decreased. This suggests that the amount of cystine may be a reliable index of the degree of processing since feather meals with lower cystine content showed higher protein solubility/digestibility values.The reduction of amino acids can be explained by changes in the protein structure as a result of protein cross-linking reactions and the formation of new amino acids (section 2.5.3). Evidence of the nature of these chemical changes is given in section 2.4.5 in which the formation of the unnatural amino acid lanthionine concurrent with the drastic destruction of cystine is described.Summarizing the above, it can be concluded that:a) there is, in general, a negative processing effect on amino acid contents and a positive one on nitrogen solubility;b) amino acids and nitrogen solubility/digestibility, estimated by chemical methods, have very clear variations in their response to different process variables, leading to the conclusion that it is difficult to find an adequate criterion of optimal processing conditions;c) there is an inverse relationship between amino acid contents and nitrogen solubility/digestibility, suggesting that, in this stage, it is difficult to evaluate the effect of processing on feather meal protein as a whole from the corresponding effects on the amino acid contents and nitrogen solubility.The losses of amino acids as demonstrated in Chapter 2 may not be of much nutritional significance unless other structural modifications to the protein affect in vivo digestibility of crude protein and amino acids. There is a need for a bioassay of the protein because physico- chemical analyses have a limited value in that they give little or no indication of individual amino acid availability for digestion, absorption and metabolism by the animal. Therefore, a second major objective of this study was the use of biological methods for estimating the value of the differently processed feather meals (Chapter 3). In this chapter the effect of three processing times (30, 50 and 70 min) and three methods of treatments (without additions, added sodium hydroxide 0.4% and added enzyme 0.4%) were investigated. The digestibility of the test feather meal protein/amino acids was determined by quantitative excreta collection over a period of 36 hrs. All the test chicks were force- fed with 12 9 of feather meal (dry matter) given in 2 doses with a 3 hr interval between feeds. Correction was made for metabolic plus endogenous excretions by using a nitrogenfree diet to distinguish apparent from true digestibility.The apparent and true digestibility of all individual amino acids were influenced by the processing conditions. The main significant effect was the processing time. There was a negative time-linear effect on the digestibility of all amino acids of the feather meals, with the exception of leucine. Significant differences in digestibility coefficients due to the method of treatment were found for some amino acids, cystine, tyrosine, phenylalanine, serine and alanine. Their values were lower in the chemically treated feather meals. Comparison between the three methods of feather meal treatment revealed higher values for the enzymatically treated samples and lower values for the chemically treated ones, compared with samples treated without additions.An interesting feature of this experiment was the considerable variation between individual amino acids in their digestibility values. They ranged in apparent digestibility values from 22.5% for lysine to 82.4% for isoleucine and, in true digestibility values, from 36.3% for aspartic acid to 86.5% for isoleucine. The three limiting essential amino acids lysine, histidine and methionine, in increasing order, were particularly low in digestibility.The reduction in amino acid digestibility can be explained by the fact that autoclaving feather meal may have altered the protein structure in such a way that the enzymic attack, necessarily associated with the digestion process, is hindered. It has been suggested that heat/alkali causes the formation of new cross-linkages within the protein molecules and this leads to the formation of new amino acids (section 2.5.3), as in the case of lanthionine in our experiments. Cross-linkage formation reduces the rate of protein digestion possibly by preventing enzyme penetration or by blocking the sites of enzyme attack (section 3.5.3). Our results showed that feather meals with higher lanthionine content had lower amino acid digestibility values.Comparing the digestibility values of amino acids with those of crude protein, it appeared that the latter revealed differences between the differently processed feather meals which were not shown by the former. Furthermore, we found negative correlations between in vivo and in vitro tests.The following general conclusions may be drawn from this experiment:a) processing can affect amino acid digestibility of feather meal indicating that prolonged time (longer than 30 min) and use of NaOH are not desirable; a study on the effect of shorter than 30 min processing periods would be of interest;b) crude protein digestibility estimations cannot be used as predictors of amino acid digestibility;c) laboratory (in vitro) tests are not reliable in detecting inferior protein/amino acid quality in differently processed feather meals;d) the variations between individual amino acids in their digestibility values are sufficiently extensive to suggest that, in the formulation of diets for poultry, it is essential that the dietary feather meal protein must be balanced on the basis of digested amino acids;e) the lanthionine present in test 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.The digestibility of the individual amino acids in dietary feather meal has also been studied in broiler chicks by their changes in the intestinal contents (Chapter 4) and blood plasma (Chapter 5). In these nutritional-physiological studies the feather meals prepared in Chapter 3 were also used. The test feeds were given to the birds by force-feeding in 3 equal doses at 3 hr intervals, to insure an adequate and controlled intake and to get a uniform distribution of the passage of feed. Nitrogen-free diet, standard amino acid mixture and casein were used as reference diets. Intestinal amino acid concentrations in jejunal and ileal contents of chicks Were measured at 21 hrs after the last force-feeding. These concentrations reflected the relative digestion of dietary feather meal protein as affected by different processing conditions. Digesta taken from the jejunum showed higher levels of amino acids derived from the chemically treated feather meals and lower levels from the enzymatically treated ones. The variations between intestinal amino acid levels were more pronounced in ileal digesta, where samples derived from feather meals processed for 30 min showed the lower amino acid levels. This was true for feather meals treated with enzymes and for products without additions, while the chemically treated feather meals showed an inverse trend. It should be noticed that feather meals treated for 30 min and the enzymatically treated products had the higher amino acid digestibility values while the chemically treated products were less well digested (Chapter 3).Comparing the ileal digesta derived from chicks fed on nitrogen-free diet and standard amino acid mixture, there were differences in the proportions of meth ionine, leucine, tyrosine, histidine and proline. Comparing the ileal digesta with the excreta, there were very close similarities in the proportions of all individual amino acids, with the exception of cystine, indicating an insignificant microbial influence on amino acids in the large intestine.It can be concluded that:a) the composition of a nitrogen-free diet may influence the amount of endogenous amino acids. These findings in relation to the small differences between apparent and true amino acid digestibility values found in this study (Chapter 3), suggest that apparent digestibility determinations are reliable measures for practical purposes;b) the similarity of the proportions of amino acids in the ileal digesta and excreta derived from the test feather meals, as well as from the nitrogen free diet, indicates that digestibility values for feather meal amino acids based upon fecal analysis are not substantially different from those obtained by using ileal assays. Comparative studies of ileal and fecal amino acid digestibility may be needed in order to prove the validity of the very close similarity of ileal and excreta amino acid composition. It issuggested, how ever, that fecal analysis is a more practical and reliable criterion for routine amino acid digestibility determinations, from the point of view of applied nutrition.Plasma amino acid concentrations in broiler chicks were estimated at 1 hr after the last force-feeding. There was a relationship between plasma amino acid levels and the relative digestibility of the feather meal protein subjected to various processing treatments. Comparisons between dietary/digested amino acids and their levels in plasma revealed significant correlations for the essential but not for the non-essential ones. Possible reasons for the lack of a precise relationship between dietary/digested and plasma amino acids are discussed in section 5.5.It can be concluded that:- although blood plasma studies may provide useful information on the relative digestion of dietary feather meal, it seems difficult to interpret plasma amino acid changes as a practical quantitative measurement of amino acid digestibility.The final conclusion, referring to the points of investigation which are mentioned in the introductory chapter is that:a) Feather meal protein-amino acids are affected in both their contents and digestibility, by different processing conditions, time being the most significant one.b) Chemical methods (total amino acids, soluble nitrogen) and qualitative assays in blood plasma and intestinal amino acids are not adequate to evaluate feather meal protein quality as affected by different processing conditions.c) Feather meal must be evaluated by quantitative in vivo digestibility measurements of the individual amino acids and should be used in poultry rations on the basis of the digested amino acids it supplies.<p/