Availability of sulphur amino acids in protein foods

In the first paper of this series (Miller & Carpenter, 1964) it was reported that analysis of commercial animal protein concentrates for ' cystine + methionine' had given values which showed little correlation with the value of the materials in the rat net protein utilization (NPU) test even though the sulphur amino acid content was the limiting factor.Similarly Morrison & Sabry (1963) have noted some samples of fish flours in which the nutritive values, measured under conditions of methionine deficiency, were not related to the methionine content.Again, Ford (1962) has shown that in a series of whale-meat meals of widely differing protein quality there was little difference in the levels of several essential amino acids including methionine.Using the proteolytic organism Streptococcus zymogenes, Ford (1962) obtained 'available ' methionine values for thirty-nine samples of animal protein concentrates which in all but six instances were less than two-thirds of the corresponding total values, and these lower 'available' values showed a closer correlation with rat NPU values.In our second paper (Miller, Carpenter, Morgan & Boyne, 1965) we confirmed, by direct determination of available methionine by a growth assay procedure with chicks, that a large proportion of methionine of animal protein concentrates can be present in an unavailable form.By comparison with these chick assay values the original procedure of Ford (1962) underestimated the available methionine content, but by increasing the concentration of papain used in the enzymic predigestion of the samples we obtained results in close agreement with the chick values.Subsequently Ford (1964) has confirmed the effect of higher papain concentration and also that other enzymes either singly or in combination do not bring about any further increase in available methionine value.One explanation for the incomplete availability of methionine in some animal protein concentrates could be the indigestible nature of some of the raw materials used in their manufacture.On the other hand, it has been shown that excessive heating results in a decrease in fluorodinitrobenzene (FDNB)-available lysine (Carpenter, Ellinger, Munro & Rolfe, 1957; Lea, Parr & Carpenter, 1960; Carpenter, Morgan, Lea & Parr, 1962), and it has been considered that the low FDNB-available lysine values found for some commercial meals must reflect heat damage.There is a considerable amount of indirect evidence that excessive heat also decreases the level of available sulphur amino acids.Thus Miller (1956) showed a decrease in NPU for rats