Commercial meat meals have shown low net protein utilisation values with rats of about 32, which is only 40% of the corresponding value for freeze-dried meat or gut. Biological assays for methionine and tryptophan, based on the weight gains of chicks, have shown a similar inferiority of meat meals despite 81-87% digestibility of their N. It is concluded that the lysine, methionine and tryptophan of the meat meals are approximately 80% available and that dilution of muscle protein with tendon and ossein in the raw material is at least as important as processing damage in reducing the quality of the products. It is argued that previous estimates of only 30% availability of methionine in meat meals are erroneous. Further problems in the use of values for fluorodinitrobenzene-available lysine and Streptococcus zymogenes-available trytophan are discussed.
IntroductionNo single test can provide all the information required about the protein quality of a test material since this is a function of the quantities of each of the essential amino acids that it provides. It has long been realised that meat meals are low in some essential amino acids and this has been related to raw materials used in their manufacture.l Previous reports have shown only small differences among the digestibility values of meat meal samples, which were generally similar to those of fishmeals. All values are high, both by in vitro and in vivoThe presence of amino acids in an unavailable form in meat meals has been reported.5.7-lo On the basis of microbiological determination,*, 9 meat meals, on average, appeared to have 1.8 g/16 g N total and 0*6g/16 g N 'available' methionine indicating a high degree of damage to methionine during meat-meal processing. However, lower total and higher available methionine figures by chemical and chickassay methods respectively, have been found.7.11 The biological performance of meat meals has been correlated with their available amino acid content. 5J2-15 In the present study both commercial meals and laboratory preparations have been used to investigate the protein quality of meat meals.
Experimental
MaterialsMost of these have already been described in Part I of this series.16 Ossein samples were prepared by removing the connective tissues, etc., from bone which was then degreased at a low temperature. The mineral matter was removed by treatment with dilute hydrochloric acid at a temperature below 10" and the material was then washed until acid-free and dried at a temperature not exceeding 40". This material was given the code X824. Half of it was heated in tallow at 110" for 14 h and then refluxed with light petroleum ether for an hour and the solvent was removed; the product was given the code X825.
Evaluation procedures
Amino acid analysesChemical procedures.-Acid hydrolysates, prepared as for the determination of total lysine,17 were analysed for amino acids on an accelerated-column chromatographic system after the basic method.18For further materials, samples were oxidised with performic acid and then hyd...
