Only a few results are available on the size of human milk fat globules (MFG), despite its significance regarding fat digestion in the infant, and no data are available at <24 h postpartum (PP). We measured the MFG size distribution in colostrum and transitional human milk in comparison with fat globules of mature milk and infant formula. Colostrum and transitional milk samples from 18 mothers were collected regularly during 4 d PP and compared with mature milk samples of 17 different mothers and 4 infant formulas. The size distribution was measured by laser light scattering. For further characterization, the zeta-potential of some mature MFG was measured by laser Doppler electrophoresis. The MFG diameter decreased sigmoidally in the first days. At <12 h PP, the mode diameter was 8.9 +/- 1.0 microm vs 2.8 +/-0.3 microm at 96 h PP. Thus, the surface area of MFG increased from 1.1 +/-0.3 to 5.4 +/-0.7 m2/g between colostrum and transitional milk. In mature milk, the MFG diameter was 4 microm on average and increased with advancing lactation, whereas the droplets in infant formula measured 0.4 microm. The zeta potential of mature MFG was -7.8 +/- 0.1 mV. The fat globules are larger in early colostrum than in transitional and mature human milk and in contrast with the small-sized fat droplets in infant formula. Human MFG also have a low negative surface charge compared with bovine globules. These structural differences can be of nutritional significance for the infant.

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This study presents milk fat globule refractive index (n i) and absorption coefficient (k a) values that can be used reliably for particle size distribution measurements by Laser Light Scattering at two different wavelengths. A k a Ϸ10-5 was obtained for liquid milk fat. The n i of milk fat globules at 20 o C was measured at 589 nm and was calculated to be 1.470 at 466 nm and 1.460 at 633 nm. Because the software of the LLS equipment does not allow for differences in n i,water at the two different wavelengths, n i,fat was corrected to reflect the difference between the refractive indices of the fat globule and its surrounding medium at each wavelength. For example, if n i,water is taken to be 1.33, then for milk fat globules the corrected value is 1.458 at 633 nm and 1.460 at 466 nm. These optical parameters led to reliable milk fat globule size distributions. In any case, k a must never be overestimated, since this leads to artefactual peaks.

-Camembert cheeses were produced using either small (~3 mm, SFG) or large (~6 mm, LFG) native milk fat globules obtained by a patented microfiltration process. The composition of the cheese milk did not depend on the fat globule size. Less whey was extracted from the SFG cheeses, that subsequently contained more moisture than LFG cheeses throughout the ripening period. The SFG curds were less rigid and less firm than the LFG ones and underwent greater proteolysis during ripening. Camembert cheeses with small fat globules had a higher melting and elastic texture, a higher flowing aspect and were less yellow. The results were explained (i) by the greater surface area of native milk fat globule membrane for SFG vs. LFG, at a given fat content, and (ii) by the thinner casein strands in SFG cheeses due to the smaller interglobular distance. The use of native milk fat globules with different sizes can thus lead to new products with different technological and sensory properties.Milk fat / fat globule / Camembert cheese / cheese yield / rheology / particle size / microfiltration / sensory analysis Résumé -Des propriétés physico-chimiques et sensorielles du Camembert sont affectées par la taille des globules gras natifs du lait. Des camemberts ont été fabriqués en utilisant des globules gras natifs du lait de petit diamètre (~3 mm) ou de grand diamètre (~6 mm), obtenus par un procédé de microfiltration breveté. La composition des laits de fromagerie ne dépend pas de la taille des globules gras. Il s'égoutte moins de sérum des camemberts à petits globules, qui sont ensuite plus humides que les fromages à gros globules au cours de l'affinage. Les caillés à petits globules sont moins fermes et moins rigides que ceux à gros globules et sont plus protéolysés durant l'affinage. Les camemberts à petits globules ont une texture plus fondante et plus élastique, sont plus coulants et moins jaunes. Ces résultats peuvent être expliqués (i) par la plus grande surface de membrane native pour les petits globules par rapport aux gros globules à une teneur donnée en matière grasse et (ii) par la moindre épaisseur des brins de caséine dans les camemberts à petits globules, à cause de la plus faible distance moyenne entre globules. L'utilisation de globules gras natifs du lait de différentes tailles peut donc aboutir à de nouveaux produits présentant des propriétés technologiques et sensorielles différentes.Matière grasse du lait / globule gras / Camembert / rendement fromager / granulométrie / rhéologie / analyse sensorielle / microfiltration

The fatty acid composition of small and large naturally occurring milk fat globules Native milk fat globules of various mean diameters, ranging from d 43 = 1.5 to 7.3 µm, were obtained using microfiltration of raw whole milk acquired in winter and spring. After total lipid extraction, fatty acid composition was characterized by methyl and butyl ester analysis using gas chromatography. The oleic and linoleic acid content of milk obtained in winter increased with fat globule size, whereas myristic and palmitic acid decreased. There was significantly more lauric, myristic and palmitoleic acid, and less stearic acid in small fat globules compared to large fat globules in milk obtained in both winter and spring. The relative content of oleic and linoleic acids were found to depend on fat globule size and season. Results are interpreted on the basis of the relative content of milk fat globule membrane depending on fat globule size, and on consequences of compositional variations on milk fat globule melting behavior.

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-Emmental cheeses were produced using either small (~3 µm, SFG) or large (~6 µm, LFG) native milk fat globules obtained by a patented microfiltration process. Ripened SFG cheeses were less flexible and less firm than LFG cheeses. Indeed, the SFG cheeses contained more moisture and underwent greater proteolysis than the LFG cheeses 52 d after manufacture. However, lipolysis was three-fold lower in SFG cheeses. This was linked to the milk fat structure in Emmental cheese: larger inclusions of denuded destabilized fat were observed in LFG cheeses, whereas more fat globules were observed in SFG cheeses. One Emmental cheese was produced using homogenized milk fat globules, in which lipolysis was dramatically increased and unacceptable. For a given size range, the structure of milk fat is thus of tremendous importance regarding lipolysis and small natural milk fat globules cannot be replaced by homogenized ones. The melting temperature and solid fat content of fat in Emmental cheese increased with native fat globule size. This is related to the improved functional properties of SFG cheeses, i.e., increased stretching, elastic increase and melting, and lower extrusion force compared with LFG cheeses. This study confirms, as previously pioneered using Camembert cheese manufacturing technology, that the use of native milk fat globules with different sizes can lead to a range of new dairy products with different physico-chemical and functional properties.Milk fat / fat globule / Emmental cheese / cheese yield / rheology / particle size / microfiltration / functional property Résumé -Des propriétés physico-chimiques et fonctionnelles de l'emmental sont affectées par la taille des globules gras natifs du lait. Des emmentals ont été fabriqués en utilisant des globules gras natifs du lait de petit diamètre (~3 µm) ou de grand diamètre (~6 µm), obtenus par un procédé de microfiltration breveté. Les fromages affinés à petits globules sont moins souples et moins fermes que ceux à gros globules. En effet, les fromages à petits globules sont plus humides et plus protéolysés à 52 j d'affinage. Cependant, la lipolyse est trois fois moindre dans les fromages à petits globules, ce qui est lié à la structure de la matière grasse dans les emmentals : les emmentals à gros globules présentent de plus grandes inclusions de matière grasse déstabilisée, tandis que ceux à petits globules semblent contenir plus de globules gras. Un emmental a été fabriqué avec des globules * Corresponding author: mcmichal@labtechno.roazhon.inra.fr 344 M.-C. Michalski et al.

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