Galactosemia

Digestion of lactose in the intestine, and fermentation of milk gives increased concentrations of galactose. Galactose is catabolized by the Leloir pathway by phosphorylation at position 1, and then converted to UDP-galactose and glucose-1-phosphate [162]. Defects in enzymes in this pathway may result in galactosemia in humans, and early onset cataract. In young women ovarian failure at a very early age has been observed following galactose accumulation. Cramer et al. [163] studied the relation between age-specific fertility rates, the prevalence of adult hypolactasia and per capita milk consumption. They found that fertility at high ages is lower with high per capita consumption of milk and greater ability to digest its lactose component. These demographic data thus add to existing evidence that dietary galactose may deleteriously affect ovarian function.

The level of galactose in fermented milk products depends on growth conditions of the different organisms and fermentation time, and for example after 24 h fermentation, the concentration of galactose has been reported to about 20 g/litre [164]. A study in rats showed that administration of galactose in the form of lactose seemed to be less toxic than when galactose was fed [165]. High levels of galactose as well as glucose may cause glycation of proteins, form advanced glycation end products, and the activation of polyol metabolism. This may accelerate generation of reactive oxygen species (ROS) and increases in oxidative chemical modification of lipids, DNA, and proteins in various tissues.

Possible concerns of milk in current use

Within modern societies the milk has to be treated in different ways to keep for several days. This processing includes steps that may be of concern. In fresh milk each lipid globule is surrounded by apical plasma membrane from the mammary epithelial cell. It is not known whether the milk homogenisation, when the fat globules with their globule membrane are broken up into many new small lipid droplets with just a small fragment of the originating membrane, might have health implications.

Proteins and peptides are heat sensitive, and their bioactivity may be reduced by pasteurisation of milk. Heating of milk may also result in the formation of potentially harmful new products i.e. when carbohydrates in milk react with proteins [166]. Also the amount of some vitamins and antioxidants may be reduced by heating. Glutathione may easily be destroyed during storage [167]. The glutathione concentration in human breast milk was reduced by 81, 79 and 73 % by storage at either -20 degrees C, 4 degrees C or at room temperature for 2 h, respectively [167]. To treat milk in a way that preserves the vitamins, proteins and peptides is therefore an important task and a challenge for the dairy industry. Some dairies now membrane filtrate the milk in stead of pasteurisation, and application of non-thermal processing technologies may give health benefits.

Further improvements of the nutritional quality of bovine milk

Several components in bovine milk which are of great importance in human nutrition may be significantly altered by the feeding regime [168]. The principal effects of feeding on milk content of these components are summarized and briefly discussed below.

Fat content and composition

The fatty acids of bovine milk are derived from two sources. The first source is fatty acids supplied to the udder by the blood, composed of fatty acids absorbed from the intestine and mobilized from the adipose fat tissue, mainly palmitic acid (16:0), stearic acid (18:0) and longer chained fatty acids. The second source is derived from circulating blood acetate and butyrate produced during fermentation in the rumen (de novo synthesis), and fatty acids up till 14 carbon atoms are synthesised in the udder. Palmitic acid in milk originates from both de novo synthesis and from circulating blood. Due to the extensively biohydrogenation of dietary unsaturated fatty acids in the rumen, the supply of these fatty acids to the udder is low. However, in the udder desaturation of fatty acids like 12:0, 14:0, 16:0 and 18:0 take place, and the products being 12:1, 14:1, 16:1 and 18:1, respectively. The preferred substrate for the desaturating enzyme; delta-9-desaturase, is stearic acid. Therefore is bovine milk a relatively good source of oleic acid (18:1, cis 9). The udder enzymes can not make double bonds in omega-3 and omega-6 positions. Consequently, milk content of linoleic acid and alpha-linolenic acid depends on the supply of these to the udder.

Conjugated linoleic acid (9c,11t-CLA) in milk originates from two sources. A small part originates from incomplete biohydrogenation of linoleic acid in the rumen which are absorbed from the small intestine transported to the udder and included in the fat synthesis. Most of the 9c,11t-CLA originates, however, from vaccenic acid which is an intermediate from biohydrogenation of unsaturated fatty acids in the rumen. After absorption and transportation by the blood to the udder, a portion of the vaccenic acid is desaturated by delta-9-desaturase to CLA. There is a close positive correlation between milk content of vaccenic acid and 9c,11t-CLA [86,169].

The effect of feed on milk fat content and fatty acid composition is comprehensively discussed [68,170-172]. There are large variations in fat synthesis in the udder, and the fat content and fatty acid composition are the most modifiable of the main components in milk. Some feeding strategies to obtain milk with altered fatty acid composition are summarized in Table 2. There are seasonal variations for the major fatty acids [67,82]. Milk from grazing dairy cows contain significantly higher proportion of oleic acid than milk produced on traditional indoor feeding composed of concentrates and conserved roughages [67]. Typically, CLA content in milk produced on pasture is at least twice of that obtained by indoor feeding [67,82]. Moreover, the proportion of alpha-linolenic acid increases more than linoleic acid, resulting in a lower ratio between omega-6 and omega-3 fatty acids. Milk fat from cows fed an in-door diet consisting of conserved grass and concentrate have a ratio between omega-6 and omega-3 fatty acids of about 4:1 [67,82], but in summer when the cows are out on pasture and have a high intake of grass the ratio may be reduced to about 2:1 [8,67,82,172]. These positive effects of pasture on the fatty acid composition of milk are mainly attributed to the high content of polyunsaturated fatty acids, especially alpha-linolenic acid, in grasses at early stage of maturity [173].