Art. LII.—The Chemistry of Flesh Foods.—(5) The Nitrogenous Constituents of Meat-extracts.*
[Read before the Philosophical Institute of Canterbury, 1st December, 1920; received by Editor, 31st December, 1920; issued separately, 12th August, 1921.]
This paper is a continuation of the investigations of flesh foods which are being carried out in the laboratory of the New Zealand Refrigerating Company (limited) (1, 2), and covers a number of investigations dealing with the composition of meat-extracts. These have been carried out since the publication of a former contribution on the subject (3).
In general, commercial meat-extracts are manufactured from finely chopped lean meat (the muscular tissues of flesh), which is placed in tanks containing cold water; steam is admitted, and the material is heated for about half an hour. The liquor obtained from meat which is parboiled in the process of preparing certain canned meats is also utilized. The liquors while hot are pumped into a large tank and there settled in order to separate out in part the particles of meat-fibre which are present; the supernatant liquor is then filtered to remove any solids in suspension, the fat present is skimmed off, and the clear liquid is concentrated in steam-heated pans, either under vacuum or at ordinary atmospheric pressure, the partially concentrated liquor being finally transferred to a finishing-pan and heated until the water content approximates 20 per cent. and the material is of a syrupy consistency.
It is obvious, therefore, that meat-extract can contain only a small part of the nutriment of meat, for there is practically no albumen or fat present, and very little gelatine; the extract consists of salts and extractives of the meat. It is the nitrogenous extractives which give meat-extracts their chief value, and these have been classed under the somewhat loose term of “meat-bases” (3). The meat-bases are products of the breaking-down of proteins in the vital processes of the body, and are excreted for the most part unchanged, and have little or no value as builders of tissue; they cannot be strictly regarded as foods, but possess certain stimulating properties, and apparently furnish relief to fatigued muscle and are powerful excitants of gastric secretion.
The results of an examination and identification of the various nitrogenous constituents of a number of meat-extracts have already been published by one of us (A. M. W.) (3), so that it is unnecessary to record the data covering the work then published.
[Footnote] * The late p. S. Nelson was killed in action during June, 1917.
The commercial valuation of meat-extracts, however, is not based upon the results of an extensive and detailed identification of the various nitrogenous constituents which give the value to an extract, but upon a consideration of the colour, flavour, and the proportion of the extract soluble in 80 per cent. alcohol; it is the amount of the latter which to a large extent determines the value to the manufacturer of the meat-extract.
The method has been criticized, adversely from time to time as being unsound in principle from a scientific point of view; in commercial practice, however, this determination showed results which were in general accord with the demand of the purchaser, although the underlying reason was not apparent.
It is only recently, however, that methods of analyses have been developed which enable the study of nitrogenous constituents to be carried out with a reasonable degree of accuracy and detail.
In the results to be described the following methods were used. For the determination of the moisture, mineral salts, chlorine, nitrogen, meat-bases, the methods outlined by one of us (A. M. W.) (3) were used. The “meat-base” nitrogen is that of the tannin-salt filtrate after deducting the ammoniacal nitrogen determined by the magnesium-oxide distillation method. This probably gives results lower than the actual for the “meat-base” nitrogen, for the reason that the magnesium-oxide distillation method for the determination of the ammoniacal nitrogen probably gives results which are too high. The results of a comparative study of the magnesium-oxide distillation method, and the Folin aeration method applied to the determination of ammoniacal nitrogen in meat-extracts, will be discussed later. As, however, in most of the recent work upon flesh products the magnesium-oxide method has been used, the results will be comparable with those of other workers, For the determination of the 80 per cent. alcoholic precipitate and the soluble extract the method described by Thorpe (4) was used.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Calculated to Moisture-free Basis.|
|Soluble in 80 per cent. alcohol—|
|Insoluble in 80 per cent. alcohol—|
|Mineral salts||9.42||8.01||9.59||9.28||8.70||8 93|
It is thus found that 62.4 per cent. of the organic extractives are soluble in 80 per cent. alcohol, 61.4 per cent. of the total solids, 57.7 per cent. of the total mineral salts, and 63.1 per cent. of the total nitrogen,
while 94.3 per cent. of the meat-bases and 99 per cent. of the chlorides are thus soluble.
From a consideration, therefore, of the results it is seen that the meat-bases, to which is due the principal physiological value of a meat-extract, are nearly completely soluble in 80 per cent. alcohol; consequently the results obtained by the commercial method of valuation are in agreement with the physiological.
The nitrogenous constituents insoluble in 80 per cent. alcohol are principally compounds similar to gelatine; and, while gelatine has a physiological value as a sparer of protein in metabolism, it has but little value as a food.
Non-putrescence of Solid Meat-extracts.
It has been a matter of common knowledge that solid meat-extracts do not undergo bacterial decomposition. While this fact has been noted in connection with the report of the Commission appointed to investigate the methods of manufacture of meat-extracts (8), the question has arisen as to whether in the absence of special precautions solid meat-extract remains free from bacterial growth and decomposition. Commercially it is known that even after a period of several years solid meat-extract has been found to be undeteriorated. In order, however, to ascertain whether there is any evidence of bacterial or other decomposition we have made a number of determinations covering extracts which have been held in jars with loosely fitted tops after exposure to the atmosphere; these extracts have in some cases been held for as long as six months.
As is well known, ammonia is one of the decomposition products of nitrogenous foods, and the determination of the loosely bound nitrogen as ammonia which occurs in the nitrogenous constituents of meat-products has proved to be one of the most reliable methods for indicating the decomposition or otherwise of such substances; it has been shown that a marked rise in the amount of ammoniacal nitrogen occurs in meat products before the senses can detect any decomposition (11, 12, 13, 14, 15).
The methods used in the determination of the ammoniacal nitrogen were (a) the magnesium-oxide distillation method (9), and (b) Folin's aeration method (10, 16).
(a.) The magnesium-oxide method used was as follows: 1 gramme of the extract (or an aliquot portion of a solution of the extract equal to 1 gramme of extract) was placed in a distillation-flask with 300 c.c. of water and 5 grammes of magnesium oxide free from carbon dioxide; after connecting the flask with a condenser, 100 c.c. of the liquid was distilled into N/50 acid, and titrated as usual, using congo-red as an indicator.
(b.) The Folin aeration method used was as follows: An aliquot portion of a solution of the extract equal to 1 gramme of extract was placed in a large tube; 0.5 c.c. saturated solution of potassium carbonate and 1 C.c. saturated solution of potassium oxalate with. 2 c.c. kerosene (to minimize frothing) were added. The mixture was aerated from a water-blower with water-injector pump, the air being passed through 30 per cent. sulphuric acid in order to remove any traces of ammonia before passing through the aeration-tube. The period of aeration was four hours, at the rate of 80 litres of air per hour; the ammonia from the extract was collected in N/50 acid through which the, air from the aeration-tube-was passed; congo-red was used as an indicator.
The results are shown as follows:—
|Magnesium-oxide Distillation Method.||Folin's Aeration Method.|
|Per Cent.||Per Cent.|
In a former paper (3) the ammoniacal nitrogen in freshly prepared extracts was shown to average 7.06 per cent. of the total nitrogen, using the magnesium-oxide method.
It will thus be seen that even after six months' storage and after ordinary atmospheric exposure no decomposition is found by either method.
Ammonia Test for Spoilage of Solutions of Meat-extracts.
In a former paper it was shown that the ammoniacal nitrogen determined by the magnesium-oxide method increases markedly in known cases of decomposition of meats, and it was established that this method was capable of demonstrating incipient decomposition (1). As, however, the magnesium-oxide distillation method is somewhat empirical—because if a further quantity of water is added to the solution in the distillation-flask, and another 100 c.c. are distilled, it will be found that the distillate contains a further amount of ammonia—it has been customary to determine the ammonia in the first 100 c.c. of the distillate only. It is apparent that, in addition to liberating the loosely bound ammoniacal nitrogen, the magnesium oxide is capable of producing hydrolysis, with the result that ammonia is being continuously split off from the nitrogenous compounds. Our experiments confirm those of others, that after four hours' continuous aeration at a rate of 80 litres of air per hour little, if any, ammoniacal nitrogen is liberated by continuing the aeration.
In order to determine whether the aeration method is applicable to the determination of ammoniacal nitrogen in known cases of decomposition of aqueous solutions of meat-extract, solutions of various dilutions were prepared and contaminated by exposure to the ordinary bacteria of decomposition present in the air. The results of the determinations of the ammoniacal nitrogen by the aeration method are as follows:—
|After 9 days||8.52||5.13|
|After 10 days||10.40||6.28|
|After 11 days||12.56||8.74||7.99|
It is thus seen that the method is capable of detecting decomposition of dilute solutions of meat-extract, and it is therefore applicable in the
examination of the solid meat-extracts. It can therefore be assumed that had the solid meat-extracts been even incipiently decomposed the method would have revealed the fact. Further work on the point is being carried out in order to determine the factors which inhibit the decomposition.
Copper in Liver-extract.
Our work in connection with meat-extracts from various sources has led to the examination of extracts manufactured from edible portions of the carcase other than true muscle-tissue, and it has been found that the mineral Baits of an extract manufactured from liver invariably contain copper. Of course, if copper utensils were used in the preparation of these extracts the presence of copper might be expected, but we have been able to detect this metal in liver-extracts manufactured under conditions which exclude the possibility of casual contamination from copper utensils; moreover, in extracts manufactured from true muscle-tissue no copper has been found in the mineral salts. The presence of copper in liver has, however, been recorded, notably by Aston (5) in his investigations upon bush sickness, and its presence as a normal liver-constituent is also noted by Hammerstein (6) and Emery and Henley (7); it is thus not surprising to find it a normal constituent of extracts manufactured from liver.
The presence of up to 10 per cent. of glucose is also recorded by us as a normal constituent of liver-extract.
The commercial valuation of a meat-extract based upon the percentage of the material soluble in 80 per cent. alcohol is in accord with the physiological value, which depends upon the “meat-bases” present.
The incipient decomposition of nitrogenous foods can be detected by the determination of the percentage of ammoniacal nitrogen before such decomposition is evident to the senses.
A comparative study of two methods of determining ammoniacal nitrogen is given.
Solid extract of meat is a non-putrescible substance.
In extracts manufactured from liver both copper and glucose are found to be present.
1. A. M. Wright, Trans. N.Z. Inst., vol. 45, pp. 1–17, 1913.
2. —– ibid., vol. 47, pp. 569–72, 1915.
3. —– ibid., vol. 43, pp. 1–6, 1915.
4. T. E. Thorpe, Dict. App. Chem., vol. 3, p. 428, 1912.
5. B. C. Aston, Trans. N.Z. Inst., vol. 44, pp. 288–98, 1912.
6. O. Hammerstein, Text-book Physiol. Chem., p. 367, 1911.
7. J. A. Emery and R. R. Henley, Jour. Agric. Res., vol. 17, p. 16, 1919.
8. Report Extract-of-Meat Commission, Lancet, Oct. 24, 1908, p. 1241.
9. Method of Analysis, Bull. 107 (rev.) U.S. Dep. Agric., pp. 9–10, 1912.
10. Method of Analysis, J.A.O.A.C., 2, pp. 274–75, 1916.
11. N. Henrikson and G. C. Swan, Jour. I. Eng. Chem., vol. 10, p. 614, 1918.
12. M. Pennington and H. S. Greenise, Jour. Am. Chem. Soc., vol. 33, p. 561, 1911.
13. E. D. Claek and L. H. Almy, Jour. Biol. Chem., vol. 33, pp. 483–98, 1918.
14. K. G. Falk, E. I. Bauman, and G. McGuire, Jour. Biol. Chem., vol. 37, p. 525, 1919.
15. K. G. Falk and G. McGuire, Jour. Biol. Chem., vol. 37, p. 547, 1919.
16. O. Folin. Jour. Biol. Chem., vol. 8, p. 497, 1910.