A microbiological method for assessing the nutritional value of proteins

The value of feather meal as a protein supplement for growing pigs

The results of a nitrogen balance experiment with growing pigs have shown that 5 % feather meal was inferior to 7 % white-fish meal, when both supplements supplied the same amount of total crude protein in a barleyweatings- minerals-vitamins ration.

A comparative feeding trial in which white-fish meal was partially replaced by feather meal in the diets given to growing pigs gave inconclusive results.

Dipeptide utilization by starter streptococci

Of 8 strains ofStreptococcus cremoristested, 5 grew almost as well in defined media in which various essential amino acids were supplied in dipeptides as they did in media containing the equivalent free amino acids. The remainder grew poorly or not at all in the peptide-containing media. Growth of peptide-utilizing strains was inhibited by also including structurally-related dipeptides in the medium, presumably due to competition for uptake by transport system carriers. Both types of starters produced cell-free dipeptidases recoverable from the medium of exponential phase cultures. Addition of the partly-purified extracellular dipeptidases to dipeptidecontaining test media initiated growth in strains unable to use peptides.Str. lactisgrew in defined peptide media, but the further addition of structurally-related dipeptides did not inhibit growth, either bcause each dipeptide was transported by a specific carrier or because peptides were hydrolysed extracellularly. The presence of cell-bound extracellular dipeptidase was indicated by the hydrolysis of dipeptides with washed whole cells in buffer. This was not observed withStr. cremorisstrains.

Amino acid nutrition of some commercial cheese starters in relation to their growth in peptone-supplemented whey media

Some strains of commercial cheese starters were obtained in higher yield after batch culture in whey-based media supplemented with soy peptones than in those supplemented with papain digested skim-milk. The soy peptones contained a greater proportion of small peptides than did the skim-milk digest. Also the average size of peptides was lower (3·4–3·9 residues) in soy preparations than in the milk digest (6·8). Small peptides isolated from the soy peptones were utilized by growing starters more completely than the equivalent peptides isolated from the papain digested milk. These observations may account for the difference in cell yields with soy and milk-digest peptones.

Mechanisms of heat damage in proteins

1. The preparation of a propionylated protein is described, and the effects of this treatment on amino acid composition and availability are compared with the effects of severe heat treatment (autoclaving) of a protein.

2. Using chemical analyses, changes exceeding 5% for total tyrosine, histidine, methionine and cystine contents were found after propionylation of the protein. Autoclaving of the protein resulted in changes in total serine, lysine, methionine, cystine and tryptophan contents.

3. Microbiological estimates of total amino acid contents were not in close agreement with the chemical estimates for the autoclaved protein.

4. Fluorodinitrobenzene-reactive lysine content was reduced to almost zero by propionylation, and by almost 40% by autoclaving.

5. Both propionylating and autoclaving protein reduced the amount of lysine available to the chick by about half. In contrast, the availabilities of methionine and tryptophan to the chick were unchanged by propionylation, but were reduced to 0.66 and 0.44 respectively, relative to the untreated protein, by autoclaving.

6. Because of the difficulties of obtaining reliable absolute estimates of amino acid availability using chick growth assays, our interpretation of results is mainly based on relative values. The merits of microbiological microbiological estimates of amino acid availability are assessed.

Protein quality of feeding-stuffs

1. Twenty-two samples of high-protein feeding-stuffs, sixteen of them fish meals, were used in a collaborative study of the precision and the limits of discrimination of the Streptococcus zymogenes assay procedure, as applied to the estimation of available methionine, tryptophan and isoleucine contents.

2. All the participating laboratories ranked the test samples in much the same sequence with respect to content for all three amino acids. There were apparently systematic differences between laboratories which impaired the precision of some of the estimates, and these were greatly reduced by including a common reference sample in the tests as an auxiliary standard.

3. Values for available methionine content for eleven test samples were highly correlated (r 0.86) and quantitatively similar to those obtained for chick growth assays, but those for available tryptophan content were markedly lower and were probably in error.

[Activity of and requirement for selected amino acids in growing female pigs. 1. Lysine]

In N balance experiments with a total 59 growing female pigs in the live weight range of 33 to 55 kg the lysine efficiency (bc-1 value) and the lysine efficiency coefficient (kLys) of various cereal proteins were ascertained under the consideration of various charges and varieties. The range of kLys = 0.65 ... 0.96 shows a wide spectrum of lysine efficiency, which results in an analogous differentiation of the derived lysine requirement values. Assuming kLys = 1.0 (lysine efficiency 100%, i.e. the maximum value for bc-1 found as yet for native proteins) a daily lysine requirement (efficient amino acid) of 8.9, 10.8 and 12.6 g resp. was ascertained for 100 g daily protein retention at 30, 40 and 50 kg live weight resp.

Proteolytic systems in lactic acid bacteria

The proteolytic systems of lactic acid bacteria are important as a means of making protein and peptide N available for growth and as part of the curing or maturation processes which give foods their characteristic rheological and organoleptic properties. The proteolytic systems of lactic acid bacteria are described in relation to their growth and their functions in protein-rich foods. Their role in the manufacture of milk products is discussed.

Assessment of the nutritional quality of proteins: the use of "ileal" digestibilities of amino acids as measures of their availabilities

1. A comparative study was made of two biological techniques for assessing protein quality in wheat and barley, and in a soya-bean-protein isolate both as received, and after being heat damaged by autoclaving. Amino acid digestibility was determined by the "ileal" analysis technique while amino acid availability was measured in growth assays. Some chemical and microbiological tests were also done. 2. Heat treatment of the soya-bean-protein isolate caused little change in amino acid composition but the digestibility of all amino acids and the availability of lysine and methionine were severely reduced, lysine being most affected. The reduced availability of lsyine was not entirely attributable to impaired digestibility. 3. The amino acids in wheat and barley were highly digestible. The availability of methionine and lysine in barley and of methionine in wheat was high, whereas the availability of lysine in wheat was apparently much lower. 4. The results showed that for methionine, digestibility values are a fair measure of the availability. In heat-damaged soya-bean-protein isolate however, digestibility of lysine over-estimated the availability, indicating that amino acid digestibility may sometimes provide a misleading indication of nutritional value.

Protein quality in cereals and pulses. 1. Application of microbiological and other in vitro methods in the evaluation of rice (Oryza sativa L.), sorghum (Sorghum vulgare Pers.), barley and field beans (Vicia faba L.)

1. The Streptococcus zymogenes assay procedure was modified and used in the estimation of available methionine and relative nutritional value in rice (Oryza sativa L.), sorghum (Sorghum vulgare Pers.), barley and field beans (Vicia faba L.). The results were assessed in relation to the content of tannins and 'dye-binding lysine', and to published information on the nutritional quality of some of the test samples. 2. In grain of ten varieties of sorghum, for which other workers had reported a correlation (r -0.82; P less than 0.01) between tannin content and amino acid digestibility in chicks, the available methionine content ranged from 6.3 to 17.7 g/kg protein (nitrogen x 6.25) and was highly correlated with tannin content (r -0.97; P less than 0.001). The content of total methionine and dye-binding lysine varied little between varieties and was not related to tannin content. 3. In nine samples of rice the availability of methionine was uniformly high, in accordance with literature values for true digestibility of the N. 4. In field beans the presence of tammins in the seed coat was associated with a significant (P less than 0.001) reduction in the availability of methionine. In eleven tannin-free varieties the coefficient of availability averaged 0.791, compared with 0.685 in eleven tannin-containing varieties. The standard deviation associated with both values was 0.04. There was no such difference between the corresponding dye-binding lysine values. 5. Thirty-three samples of barley were examined, representing fourteen varieties, four of which had been grown at several widely different latitudes in England and Sweden. There was little difference in methionine content between samples. The average availability coefficient for eighteen samples grown in England was 0.995+/-0.044, and for fifteen varieties grown in Sweden it was 0.851+/-0.042. The difference was not associated with any difference in tannin content, which was uniformly low. The findings contradict published evidence for marked increase in tannin content with increase in geographical latitude of cultivation, associated with decrease in digestibility of N.

Metabolism of heat-damaged proteins in the rat. Inhibition of amino acid uptake by "unavailable peptides" isolated from enzymic digests of heat-damaged cod fillet

1. An extract containing "unavailable" small peptides was isolated from an enzymic digest of heat-damaged cod fillet and examined for its influence on uptake of leucine in the rat small intestine, using the everted-sac technique. 2. The extract strongly inhibited the uptake of leucine. It had no effect on uptake of glucose or its metabolism to lactate. 3. The findings are discussed in relation to the concept (Buraczewski, Buraczewska & Ford, 1967) that the accumulation of "unavailable peptide" material in the intestine, found in rats given heat-damaged protein, might hinder the absorption of amino acids by blocking a mechanism involved in their transport across the mucosal barrier.

The tryptophan requirements of young laying pullets

Two experiments were conducted with laying pullets between 32 and 47 weeks of age. In each trial 1728 White Leghorn and 1728 cross-bred pullets were used. A series of diets of increasing protein content was offered. Protein quality was identical in all diets and tryptophan was demonstrated to be the most limiting amino acid in the protein mixture used. The daily tryptophan requirement of the individual pullet was estimated, by indirect methods, to be 2.25 mg/g egg output plus 10.25 mg/kg body weight. Response curves for flocks of pullets are illustrated. Calculated optimum intakes of tryptophan for various ratios of costs of input to value of output are tabulated. It is estimated that for a flock of mean body weight 1.5 kg, producing 55 g egg mass/hen d and consuming 110 g food/hen d, the optimum dietary tryptophan concentration is 1.7 g/kg when the marginal cost of supplying 1 kg tryptophan is 20 times the marginal value of 1 kg egg output.

[14C]leucine incorporation into proteins of pancreatic juice in rats fed soya-bean flour

1. The effect of a diet containing a trypsin inhibitor on the incorporation of radioactively labelled leucine into the pancreatic proteins secreted during stimulation with cholecystokinin-pancreozymin (CCK) was studied in rats. 2. The total output of protein was significantly greater in the rats given raw soya-bean flour (RSF) compared with those given heat-inactivated soya-bean flour (HSF) (controls) in response to the sub- and supramaximal stimulation with CCK, but similar responses were obtained to maximal stimulation with CCK. Total protein output decreased continuously with time after reaching peak values at 90--120 min after the start of stimulation with CCK. 3. The total output of radioactively labelled protein in RSF-fed rats was not different from that of the controls with sub- and supramaximal dose rats of CCK, but was significantly lower than that of the controls in response to the dose rate of CCK which produced maximal rates of pancreatic secretion. 4. The specific activity of radioactively labelled protein increased continuously, while the output attained a constant rate during stimulation with all doses of CCK. 5. We concluded that feeding the trypsin inhibitor-containing diet led to increased secretion of stored pancreatic protein, while secretion of newly synthesized protein was not altered. During the course of prolonged stimulation with CCK, irrespective of diet, there was increasing secretion of the newly synthesized protein compared with the pre-existing stored proteins of the pancreas, it was unable to compensate for the decreased secretion of pre-formed protein.

Comparison of chemical and microbiological methods in the estimation of methionine in cowpea (Vigna unguiculata) seeds

1. Meals were prepared from the seeds of fifteen varieties of cowpea (Vigna unguiculata), one of lima bean (Phaseolus lunatus) and one of yam bean (Sphenostylis stenocarpa), and their methionine content was determined by six different methods.

2. Total methionine content was determined by two chemical methods (ion-exchange chromatography and a colorimetric procedure) and by two microbiological methods. The ‘available’ methionine content was determined by microbiological assay with Streptococcus zymogenes.

3. All the different methods for total methionine determination gave similar results, with much the same high extent of precision.

4. The values for ‘available’ methionine content were similar to or marginally higher than the corresponding microbiological assay values for total methionine content. There was no indication that the methionine in any of the test samples was not completely available.

The determination of methionine in proteins by gas-liquid chromatography

Intact methionine residues in proteins were rapidly and precisely determined by measuring methyl thiocyanate released during the reaction with CNBr and separated by g.l.c. Conditions for the reaction and for chromatography on columns of Porapak P-S are described. The recovery of methyl thiocyanate from several methionine derivatives and analogues were examined. Carbamoylmethionine was adopted as a stable primary standard and ethyl thiocyanate as internal standard. The measured methionine content of several isolated proteins was close to the theoretical value indicated by previous work and the results for these and a range of food proteins agreed well with results obtained by ion-exchange chromatography after performic acid oxidation. Since CNBr does not react with methionine sulphoxide and a preliminary hydrolysis is not required, the method discriminates between methionine and any methionine sulphoxide that may be present. It could be useful in studies on the nutritional availability of methionine in processed foods.

An improved procedure for the assay of available lysine and methionine in feedstuffs using Tetrahymena pyriformis W

1. A study was made of the assay of available lysine and available methionine using Tetrahymena pyriformis W, as applied to a variety of protein-rich feedstuffs. 2. Results were strongly influenced by the conditions under which the analyses were done. With most of the test samples predigestion with papain caused a large increase in the values obtained. 3. An improved assay procedure was developed which included enzymic predigestion of the test samples, "all-in" sterilization of the medium constituents, and measurement of growth from extinction values. It gave results for available lysine that were broadly similar to those obtained in growth tests with rats, and in chemical tests by the method of Carpenter (1960). Values for available methionine agreed closely with those obtained in microbiological tests with Streptococcus zymogenes.

The effect of methods of sterilization on the nutritive value of protein in a commercial rat diet

1. The effect on protein quality of treating a commercial rat diet by autoclaving at various temperatures for different periods of time, or by irradiation with 2-5 or 10 Mrd, was studied. True digestibility (TD) and biological value (BV) were measured and the available and total amino acids in the diets were estimated using microbiological and chemical methods. 2. Autoclaving at 121 degrees for 60 min reduced BV, TD and net protein utilization (NPU) more than autoclaving at 134 degrees for 3 min. Availability of amino acids was reduced by both treatments but to a greater extent by autoclaving at 121 degrees for 60 min. Total amino acids were essentially unaffected. Irradiation had no effect on BV, TD, NPU or total amino acids, and the availability of amino acids was also unaffected, with the exception of lysine which was slightly reduced. 3. When the diet was autoclaved at 115 or 121 degrees for 15, 30 or 60 min, or at 134 degrees for 3 min the availability of the amino acids was reduced with increasing time and temperature of treatment. Treatment at 134 degrees for 3 min had an effect on available amino acids similar to treatment at 121 degrees for 15 or 30 min. 4. Ethylene oxide fumigation of the diet caused reduced availability of histidine, methionine and tryptophan but had negligble effect on arginine, leucine and lysine. 5. It is concluded that from a practical point of view irradiation causes least damage to proteins in rodent diets. If such diets are to be autoclaved they should be supplemented with complete protein to counteract amino acid destruction.

Protein quality of feeding-stuffs. 6. Comparisons of the results of collaborative biological assays for amino acids with those of other methods

1. Twelve commercial samples of high-protein feeding-stuffs, six of them fish meals, were used for a study of the reproducibility of chemical, microbiological and biological assay values for individual amino acids. Comparisons were also made of results obtained by different methods for the same amino acid.

2. For lysine there was quite good agreement between the results from the different laboratory tests for ‘available’ lysine. The values for ‘total’ lysine were, on average, 10 % higher than those for ‘available’ lysine.

3. For methionine the correlations between the chick assay and each of the laboratory tests were similar.

4. The results from the ‘total protein efficiency’ test of over-all protein quality, using chicks, showed a close correlation with the results of lysine or methionine assays according to which of the two amino acids was most deficient.

5. The results with tryptophan were difficult to interpret and no recommendations could be made as to procedures suitable for general use.

6. Although most of the present samples proved to be of good quality, only screening of a much larger number of samples will reveal to what extent commercial protein concentrates of low availability are still liable to come onto the market.