Riboflavin

Milk is a good source of riboflavin, 1.83 mg riboflavin/l milk (Table 1). Daily recommended intake is 1.1 and 1.3 mg for women and men, respectively [4]. Riboflavin is part of two important coenzymes participating in a numerous metabolic pathways in the cell. It has a role in the antioxidant performance of glutathione peroxidase and DNA repair via the ribonucleotid reductase pathway.

Vitamin B12

Milk is also a good source of vitamin B₁₂, being 4.4 ug/l [9]. The daily recommendation is 2.4 μg [4]. Vitamin B₁₂ is found only in animal foods, and plays a central role in folate and homocysteine metabolism, by transferring methyl groups. Vitamin B₁₂ deficiency may cause megaloblastic anaemia and breakdown of the myelin sheath.

Bacterial flora of milk

Milk samples from normal healthy mammary glands contain many strains of bacteria [141]. To prevent diseases caused by pathogenic bacteria in milk and to lengthen the shelf life of milk, treatment such as cooling and pasteurization or membrane filtration is needed. To preserve milk, addition of selective, well-documented strains of starter cultures for fermentation is a method that has been used for centuries.

Fermented milk

Historically, the seasonal variation in milk production made it necessary to preserve milk. The Nordic countries including Iceland have a long tradition for using fermented milk, and the consumption of fermented milk is about 20 kg per person [2].

During fermentation bacteria and yeasts convert lactose in the milk to various degradation products depending on the species present. Lactobacilli and streptococci give rice to lactic acid and monosaccarides (especially galactose). Bifidobacteria give rice to lactic acid, acetic acid and monosaccarides, while yeasts, present only in some few fermented milk products, produce CO₂ and ethanol [2]. Different bacterias may be used for fermentation, giving products of special flavour and aroma, and with several potential health beneficial metabolites [142]. The bacteria contain cell wall components that bind Toll-like receptors on dendritic cells (and also other leucocytes) found in the mucosa of the small intestine and colon, thus stimulating the Th1 immune response [143]. It has been shown that fermented milk stimulates the Th1 immune response, and down-regulates the Th2 immune response [144]. The immune system may thus be strengthened against cancer, virus infections and allergy [145]. Bacterial DNA has also a similar effect, binding to Toll-like receptor-9 [146]. Some bacteria can also improve the intestinal microbial balance, and the fermented milk may have positive health effects both in the digestive channel and in metabolism. During the fermentation of milk, lactic acid and other organic acids are produced and these increase the absorption of iron. If fermented milk is consumed at mealtimes, these acids are likely to have a positive effect on the absorption of iron from other foods [147]. Lactic acid is also a poorer substrate for growth of pathogenic bacteria than glucose and lactose [148].

The low pH in fermented milk may also delay the gastric emptying from the stomach into the small intestine and thereby increase the gastrointestinal transit time [149]. Also, full-fat milk has been shown to increase the mean gastric emptying half-time compared to half-skimmed milk [150], and accordingly it might be favourable to gastric emptying and thus may have an effect on appetite regulation [150,151].