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Preterm and term transitional milks of human subjects and mature milks of human subjects, nonhuman primates and non-primates were analysed for free amino acids (AA) using precolumn phenylisothiocyanate derivatization and liquid chromatography. Differences in free AA between three types of human milk were small. Milks of pinnipeds (seals and sea lions) contained the highest levels of total free AA (8634-20 862 prnom), while the milks of cows and sheep had the lowest levels of total free AA (1061-1357 pmol/l). The milks of human subjects, chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), elephants (Elephas mnrirnus), horses and pigs had intermediate levels of total free AA (3069-7381 pmol/l). Glutamic acid was the most abundant free AA in milks of human subjects (1339-2157 pmol/l), non-human primates (423-2528 pmoV l), elephants (1332 pmol/l), horses (1 119 pmol/l), and cows (349 pmol/l). Taurine was the most abundant free AA in milks of pinnipeds (5776-13643pmol/l), pigs (1238pmol/l), goats (1 150 pmol/l) and sheep (341 pmol/l). Taurine was the second most abundant free AA in milks of human subjects and non-human primates, while histidine was the second most abundant free AA in milks of pinnipeds. Milks of each species had a distinctive free AA pattern which may reflect the relative importance of the free AA during early postnatal development. Amino acids: MilkThe total (including protein-bound) amino acid (AA) concentrations in the milks of human subjects, non-human primates and pinnipeds have been determined (Davis et al. 1994a(Davis et al. ,b, 1995. Despite the wide variation in total AA concentration among the species, there was a general similarity in the AA patterns of the milks. Human milk is known to contain a pool of free AA, which may have a beneficial role during early postnatal development (Pamblanco et al. 1989). Information on free AA in transitional human milks and milks of other species is limited. Only one comparative study on free AA in mature milks of man and some other mammals has been reported (Rassin et al. 1978). The transitional milks of human subjects and of pinnipeds were not studied by these workers. Thus, in the present investigation free AA were studied, using accurate and standardized methods, in preterm and term transitional milks of human subjects and mature milks of human subjects and other species including pinnipeds. MethodsThe preterm and term transitional human milk samples (5-10d postpartum) originated from mothers of preterm (gestations of 25-32 weeks) or term (gestations of >36 weeks) infants admitted to the Toronto Hospital for Sick Children (Sarwar et al. 1996). The details of the collections of the mature milks of human subjects, non-human primates (baboon (Papio cynocephulus anubis), chimpanzee (Pan troglodytes), gorilla (Gorilla gorilla) and rhesus monkey (Macaca mulatta)), non-primates (cow, goat, sheep, pig, horse and elephant (Elephas maximus)) and pinnipeds (Northern elephant seal (Mirounga angustirosAbbreviations: AA, amino acids.

Values (%) for true digestibility of crude protein and individual amino acids in 20 selected foods were determined by the rat balance (fecal) method. The products were fed as the sole source of protein in diets containing 8% crude protein (N x 6.25). Lowest true protein digestibility values (79-84) were obtained for pinto beans, kidney beans and lentils; intermediate values (89-92) were obtained for chick peas, beef stew, skim milk (over heated), rolled oats, whole wheat cereal, and pea protein concentrate; and highest values (94-100) were obtained for sausage, macaroni-cheese, rice-wheat gluten cereal, skim milk, tuna, soy isolate, peanut butter, chicken frankfurters, beef salami, casein and casein + methionine. In animal foods, peanut butter and soy isolate, the differences between true digestibility of crude protein and most individual amino acids were less than 5%. However, the values for true digestibility of methionine and cystine were up to 44% lower than those of crude protein in pinto beans, kidney beans, lentils, chick peas and pea concentrate. In these legumes, digestibility of crude protein was not a good predictor of digestibility of the limiting amino acids.

Protein and amino acid digestibility and protein quality of liquid concentrate and/or powder forms of infant formulas were studied by rat balance and growth methods. Casein plus methionine (control) and eight formulas were fed to weanling rats as the sole source of protein in diets containing 8% protein (nitrogen X 6.25). Values for true digestibility of protein, lysine, methionine, or cystine (85-92%) in liquid concentrates were up to 13% lower than those in powders. Similarly, the 2-wk relative protein-efficiency ratio values (64-85%) or the relative net protein ratio values (78-94%) of liquid concentrates were up to 25% lower than those for powders. Lower levels of bioavailable lysine and methionine plus cystine in liquid concentrates compared with powders (prepared by the same manufacturer) would suggest that inferior protein quality of liquid concentrates may be due to more heat treatment involved in their preparation.

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