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Bioactive Oligosaccharides Containing Sialic Acid
Breast Milk as Model
Breast milk is ideally suited for the nutrition and development of new born infants because of its nutritional constituents like proteins, fat and carbohydrates. Not only does breast milk differ significantly in its constitution of nutrients from cow milk, but also because of compounds with a certain functionality and the structure of components. Major components are carbohydrates, lactose serves as an energy source.

Infant formula is based on cow milk components, among them fat, milk powder, whey powder and lactose, however, the composition of the formula varies according to age and official regulation for nutrients.

A major difference of human milk compared to cow milk is the up to 30-times higher content of free oligosaccharides (OS). The colostrum which is the first milk or pre-milk for new born infants contains the highest content of free OS, approx. 20 – 23 g/L which decreases in the following weeks to 9 g/L.

Milk oligosaccharides are built with 5 basic blocks: glucose (glc), galactose (gal), N-acetylglucosamine (glcNac), fucose (fuc) and sialic acid (neu5ac). Human milk oligosaccharides consist of a lactose basic unit with (gal-glcNac]-chain elongations with final fucose and sialic acid units. The structural diversity of oligosaccharides is partly due to a variation of basic blocks, but mainly due to the different combination of two monosaccharides. Sialic acid or N-acetylneuraminic acid are predominantly α2-6-bound in human OS, whereas in cow milk they are mainly α2-3-bound.

Physiological Significance
Neutral and acidic OS containing sialic acid are important for the growth of the gut flora bacteria (lactobacillus and bifido bacteria). Furthermore sialo-oligosaccharides play an important role in the development of the central nervous system as studies with mice have shown. No oligosaccharides similar to those in breast milk can be added so far to infant formula, since manufacturing is extremely difficult. In a joint research project in cooperation with the Technical University of Hamburg-Harburg, scientists try to develop a manufacturing process based on biocatalysis (GlykoBioKat).

Sialic Acid Analysis
Predominantly, sialic acid is present in breast milk bound to free oligosaccharides. Colostrum contains approx. 1 g/100 g sialic acid (mainly as sialyllactose) and decreases to approx. 0.2 g/100 g after 120 days. Cow milk contains only a fifth of this amount. Breast milk differs greatly from cow milk in which sialic acid is bound mainly to glycoproteins, for example as a glycosylated subunit of casein.

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