Nutritive Value

Studies on the nutritive value of fermented milks predominantly deal with yoghurt and are mainly carried out on animals. Determining the significance of the observed effects for human health is not easy. Following are the most important aspects when a fermented milk product is compared with plain milk.

i. Composition

a) Lactose content: Fermentation decreases the lactose content but not to such a low pH that any further sugar breakdown is impossible because the resulting product would become too acidic. At a lactic acid content of, say, 0.9 per cent the fermentation is often slowed down by cooling. About 20 per cent of the lactose in the milk has then been split, if both glucose and galactose are fermented.In yoghurt twice as much lactose is split since most of the yoghurt bacteria do not decompose galactose.

b) Vitamin content: Lactic acid bacteria often require certain B vitamins for growth, and can produce other vitamins. Accordingly, the properties of the culture involved largely determine the extent to which the concentrations of vitamins in the fermented milk differ from those in the original milk. In yogurt the level of most of the vitamins is somewhat reduced; the folic acid content may be increased but the utilizability by humans of the folic acid thus formed is not certain. The vitamin content in fermented products is also affected by the storage conditions and especially by the pretreatment of the milk. For instance, heat treatment of milk results in a decrease of vitamins B1, B12, C, and folic acid.

c) Other changes due to bacterial action are nutritionally insignificant.

d) Composition can be changed by such process steps as standardization and ultrafiltration, and by addition of skim milk powder, caseinate, stabilizers, flavourings, or fruit pulp.

ii. Nutritional Aspects

e) Edible energy: The fermentation process per se does not cause a substantial change of the energy content of milk. The conversion of lactose to lactic acid reduces the energy value by only a small percentage.

f) Digestibility

Protein and fat: The digestibility may be improved by a slight pre-digestion of the compounds by enzymes of the lactic acid bacteria. People with a weakened intestinal function may take advantage of the pre-digestion, but healthy people digest these compounds efficiently. In the stomach, the protein in fermented milk coagulates into finer particles than plain milk, which may increase digestibility. The gastric juice of babies contains little acid and, accordingly,sometimes (dextrorotatory) lactic acid is added to baby formulas.

Lactose: Lactose-intolerant users digest a sour milk product like yoghurt much better than plain milk due to decrease in the concentration of lactose in fermented milk. In addition, factors must exist in fermented product that lead to easier digestion of lactose. The lactase activity of the yoghurt bacteria as well as the stimulation of the lactase activity of the intestinal mucosal by yoghurt have been held responsible. Alternatively, the depletion of the stomach contents into the duodenum may be retarded when fermented milk is consumed; thereby, the contact time of lactose hydrolyzing enzymes with the substrate in the stomach would be extended, resulting in a better digestion of lactose.

g) pH adjustment: The consumption of fermented milks causes a smaller decrease of the pH of the stomach contents and thereby diminishes the risk of passage of pathogens. This is of particular importance for people suffering from a weakened secretion of gastric juice, e.g., many elderly people and babies.

h) Antimicrobial action: Lactic acid bacteria can form antibiotic compounds that injure pathogens in-vitro. The in-vivo significance of these compounds in suppressing gastroenteritis is not quite clear.

i) Absorption of minerals: Due to the low pH of fermented milks, some minerals are better soluble than in plain milk; it is sometimes assumed that a better absorption of minerals is thus to be expected. However, the absorption of various elements, especially that of magnesium and zinc, is enhanced by lactose.The lactose content decreases during fermentation, causing the net absorption from sour milk to be lower. Animal tests with yoghurt have confirmed the effect;the absorption of phosphorus, which is less affected by lactose, proved to be somewhat increased. Clearly, as far as the uptake of minerals is concerned, the fermentation of milk offers no distinct nutritional advantages.

j) Some additional positive and negative effects: 

•  Intestinal flora: The consumption of living lactic acid bacteria through fermented milk is supposed to result in the implantation of a favourable flora of lactic acid bacteria in the large intestine; the flora might repress pathogens. The most probable effects result from bacteria that do not only survive the action of gastric juice in the gastrointestinal tract but can also colonize in the intestine, such as strains of the intestinal bacteria Lactobacillus acidophilus, L. Salivarius, and Bifidobacterium bifidum.When yoghurt is eaten frequently, the common yoghurt bacteria survive the transport through the gastrointestinal tract but do not colonize. Thus far investigations have not permitted the conclusion that effects for humans are favourable.
• Cholesterol level: Some animal tests suggest that consuming fermented milk might contribute to decreased cholesterol content of the blood and,accordingly, could reduce the risk of heart and vascular diseases. However,even if this is true, the effect would be small. Consumption of fermented milk could further contribute to an increased resistance to pathogens by activating the immune system and to a decreased risk of colon cancer.
• Dental caries: Fermented milks have not been shown to cause caries due to damage to the enamel at low pH. The lactic acid bacteria of the mouth flora form non sticky dextrans from lactose and they consequently cause no dental plaque. Obviously, the saliva has adequate counteracting activity to prevent dental caries.
• Contract: Supposedly, the consumption of yoghurt can cause this eye disorder. Rats exclusively fed with yoghurt (made of concentrated milk)went blind because of the accumulation of galactitol in the eye lens.However, unlike the rat, humans can readily convert galactose to glucose;therefore, the galactose content of the blood does not increase and no galactitol is formed.
•  Lactic acid type: The type of lactic acid formed has physiological significance. Two stereoisomers of lactic acid exist: detrorotatory L (+) lactic acid and levorotatory D(-) lactic acid. L (+) lactic acid can readily be metabolized in the body, but D(-) lactic acid at a slower rate. The latter acid is partly removed from the body through the urine. In traditional yoghurt some 40 per cent to 60 per cent of the lactic acid is levorotatory and is formed by Lactobacillus delbrueckii ssp. bulgaricus. Ingesting excessive quantities of (D(-) lactic acid may cause acidosis, resulting in some tissue injury.
• The major constituents of yoghurt and health-promoting attributes of fermented milk are presented in below Tables respectively

Typical values of the major constituents of milk and yoghurt

                    Some of health-promoting attributes of yoghurt/ fermented

milks