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Populations of large and small milk fat globules were isolated and analyzed to determine differences in fatty acid composition. Globule samples were obtained by centrifugation from milks of a herd and of individual animals produced under both pasture and barn feeding. Triacylglycerols of total globule lipids were prepared by thin layer chromatography and analyzed for fatty acid composition by gas chromatography. Using content of the acids in large globules as 100%, small globules contained fewer short-chain acids, -5.9%, less stearic acid, -22.7%, and more oleic acids, +4.6%, mean values for five trials. These differences are consistent with alternative use of short-chain acids or oleic acid converted from stearic acid to maintain liquidity at body temperature of milk fat globules and their precursors, intracellular lipid droplets. Stearyl-CoA desaturase (EC 1.14.99.5), which maintains fluidity of cellular endoplasmic reticulum membrane, is suggested to play a key role in regulating globule fat liquidity. Possible origins of differences between individual globules in fatty acid composition of their triacylglycerols are discussed.

Nonimmunological components in human milk can protect breast-fed infants against infection by microorganisms. The structural and functional characteristics of four such components are discussed. The mucin inhibits binding of S-fimbriated Escherichia coli to bucal epithelial cells; lactadherin prevents symptomatic rotavirus-induced infection; glycoaminoglycans inhibit binding of human immunodeficiency virus gp120 to its host cell CD4 receptor, and oligosaccharides provide protection against several pathogens and their toxins.

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Purified plasma membrane fractions from lactating bovine mammary glands and membranes of milk fat globules from the same source were similar in distribution and fatty acid composition of phospholipids. The sphingomyelin content of the phospholipid fraction of both membranes was higher than in these fractions from other cell components, ß-carotene, a constituent characteristic of milk fat, was present in the lipid fraction of the plasma membrane. Cholesterol esters of plasma membrane were similar in fatty acid composition to those of milk fat globule membranes. Disc electrophoresis of either membrane preparation on polyacrylamide gels revealed a single major protein component characteristic of plasma membrane from other sources. Distinct morphological differences between plasma membrane and milk fat globule membranes were observed in both thin sections and in negatively stained material. Plasma membrane was vesicular in appearance while milk fat globule membranes had a platelike aspect. These observations are consistent with derivation of fat globule membrane from plasma membrane accompanied by structural rearrangement of membrane constituents.

One aspect of flavor research appears to be evolving rapidly as a science. This concerns identification of (volatile) flavor compounds and results from recently developed chromatographic and spectroanalytical tools. Such tools have enabled identification on a microscale heretofore impossible. It seems probable that comprehensive analyses of volatiles from many foods will soon find their way into the literature. When identification data are in hand, the question arises : which of the identified compounds are of flavor significance! Because of the relative nature of flavor and odor, answers to this question ordinarily have been presented as personal opinions. This state of affairs indicates the need for a more objective approach. EXPERIMENTALIn determining whether a particular compound is of significance in a flavor, 2 principal problems must be solved: (a) the concentration of the compound in the food product must be determined and (b) the flavor threshold-value for the compound must be revealed. The premise is that compounds which exceed threshold-level in a food are significant in its flavor, whereas those occurring below threshold are not. Although this premise may not hold in all instances, i t does appear to have merit as a working principle. Concerning the problem of determining concentration of the compound in the food product, the answer frequently is inherent in the identification work. If not, it usually can be secured by one of the many sensitive analytical methods. The second problem, that of determining threshold-values for flavor compounds, need not be overly difficult. Threshold-studies may be very precise, elaborate and time-consuming. However, a fairly simple version, which appears adequate for the purpose, has been employed a t this laboratory. This procedure is as follows:A series of samples are prepared containing the compound in a range from zero concentration to well above the anticipated threshold area. Samples can be made by using water or any other convenient and appropriate medium. The samples are adjusted to approximately room temperature and, using a dentist's chair type of environment, are presented by spray technique to the taste obselvcrs. I n this technique, a 3-ml. sample is drawn into a 3-ml. graduated pipette by means of an attached rubber bulb. The sample is then sprayed into the mouth of the taste observer who, after evaluation of the sample, merely indicates his judgment as positive or negative concerning presence of the compound. One o r two pieliminary presentations of the zero and highest concentration samples are made to each observer so that he will be properly oriented to make judgments. Samples are then presented in randoin order. Rinsing the mouth and rest periods between observations are left to the diseretion of the individual observer. Five observers, recruited from available personnel, are used. A t times individuals showing extremes of sensitivity or insensitivity t o the compound under study will be encountered. These should be replaced by individuals who conform...