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Abstract. The fermentation of milk to yogurt is dependent upon growth and acid production by
Streptococcus salivarius var. thermophiles (commonly named S. thermophiles) and Lactobacillus
delbrueckii var. bulgaricus (commonly named L. bulgaricus). Changes in the carbohydrates and
proteins during fermentation not only alter the flavor and texture of the milk but may also alter
digestibility of these nutrients. The mineral content of the milk is quantitatively unchanged even
though mineral bioavailability may be altered. The presence of a a large number of live and active
bacterial cells and/or metabolites formed during fermentation may have a beneficial effect on
Key words: fermented milks, nutritional value.
Fermentation has been used for thousands of years as a safe way to store perishable food (1).
Fermentation products are obtained from grains (beer and breads), grapes and other fruits
(wines), milk (yogurt, cheese, butter, etc), meat (sausages), fish (nuoc-mâm) and vegetables
(sauerkraut and pickles). In addition to changing the flavor of the original food, fermentation may
improve digestibility of carbohydrates and proteins, increase the levels of some vitamins, make
minerals more available and have other probiotic effects. This issue will review the nutritional
attributes of fermented milks.
DEFINITION OF YOGURT AND FERMENTED MILK
The definition of “yogurt” according ti the Codex Alimentarius (Std n° A-11/a/1975) is “a
coagulated milk product obtained by specific acid fermentation, through the action of
Lactobacillus delbrueckii var. bulgaricus and Streptococcus salivarius var. thermophiles, from cows
milk with or without addition of powdered milk, powdered skim milk etc. The microorganisms in
the final product must be viable and abundant”.
The use of lactic acid bacteria other than the 2 species cited in the definition is not permitted in
most European countries. In this case, a yogurt containing Bifidobacterium spp. or Lactobacillus
acidophilus would be classified as a “fermented milk”. In other countries, such as the UK, Canada
and the USA, the addition of other lactic acid bacteria to the two species used to make yogurt is
To ferment milk, concentrated cultures of bacteria are inoculated into pasteurized milk which has
been enriched in milk protein and is then incubated at 40-44 °C for 4 to 5 hours. During the
fermentation, lactic acid is produced from lactose by the yogurt bacteria, whose population
increases 100- to 1000- fold to a final concentration of approximately 10
ml. The reduction in pH,
due to the production of lactic acid, causes a destabilization of the micellar casein at a pH of 5.1-
5.2 and with a complete coagulation occurring around pH 4.6. At the desired final pH, the
coagulated milk is cooled quickly to 4-10°C to slow down the fermentation.
During milk fermentation, about 20-30% of the lactose in milk is fermented by lactic acid bacteria
through different pathways (figure 1). Yogurt bacteria are homofermentative, producing one
major end product, in this case lactic acid, which accounts for greater than 95% of the
fermentation products. In other types of bacteria, other fermentation products are formed. In
Bifidobacterium spp, for example, the ratio of acetic acid : lactic acid is 3:2.
The final concentration of lactic acid is 0.7 to 1.2% in yogurt, which accounts for the mildly sour,
refreshing taste. This lactic acid is a mixture of both the L(+) and D(-) isomers (see later section).
Although the quantity of each isomer present depends on the specific culture, the L(+) isomer
generally represents between 50-70% of the total lactic acid.
The reduction in the lactose concentration coupled with the presence of a high number of viable
bacteria containing B-galactosidase explains why yogurt can be consumed by lactose-maldigesting
The bacterial cells protect the B-galactosidase from denaturation by the acid in the stomach and
deliver in to the intestine. The action of bile in the small intestine increases the permeability of the
bacterial cells, facilitating the entry and subsequent hydrolysis of lactose.(2)
One study on mice investigated the effects of acid milk and organic acids on the motility of
different parts of the digestive tract (3). Lactic acid resulted in increased peristaltic movement in
the duodenum, jejunum, ileum, cecum and colon, but not the stomach or rectum. Acetic acid only
stimulated movement in the duodenum and colon. The exact role of this acid-stimulated motility
on digestion remains to be clarified.
The proteolytic activity of yogurt bacteria is slight, resulting in a breakdown of only 1-2% of milk
protein (4). This proteolytic activity is necessary to release small peptides and amino acids for the
growth of these bacteria. L bulgaricus is more proteolytic , but both yogurt bacteria contain
peptidases which are necessary to hydrolyze large peptides into smaller peptides for transport
into the cell. The principal substrate for proteolysis is casein, but limited degradation of whey
proteins may also occur (5,6). The net effect of this proteolytic activity is that fermented milks
have a higher content of peptides and free amino acids, especially valine, histidine, serine and
proline than milk.(7,8).
Even though the limited proteolytic action of yogurt bacteria does not significantly alter the
nutritional value of milk proteins (9), yogurt is more digestible than the milk mixture from which it
was made (10). A study with rats found that feeding yogurt compared to the milk from which it
was prepared resulted In increased feed efficiency (11). The increased digestibility of proteins in
fermented milks may be related to the fine flocculation of caseins resulting from the joint action of
proteolysis and acidification. This increased digestibility is especially important for people with
gastric atrophy, gastrointestinal disturbances and protein malnutrition.
Yogurt and fermented milks contain small amounts of dairy lipids (from 0 to 4 g. 100ml
fermentation and shelflife the lipids are hardly metabolized. All nutrition studies show that yogurt
can be included in a healthy diet: daily consumption of yogurt seems unlikely to alter blood lipid
MINERALS AND VITAMINS
While the lactic acid bacteria require some minerals and vitamins for growth, the change of these
constituents, compared to the milk before fermentation, is negligible, with the exception of some
B vitamins. The pasteurization of milk before fermentation may destroy some vitamins such B6,
B12 and folic acid while the level of thermostable vitamins (niacin and pantothenic acid) remains
unchanged. Some lactic acid bacterial strains produce a net increase in B vitamins, natably folates,
during fermentation while others result in a net loss (table 1). In general, L. bulgaricus uses folic
acid whereas S. thermophilus produces it (14). During cold storage after fermentation, the levels of
some vitamins, especially B12 and folic acid decrease.
Yogurt, like milk, is an excellent source of calcium and phosphor which are essential for bones.
Moreover yogurt also contains relatively high amounts of potassium and can be consider a good
source of these minerals. Since yogurt is produced from milk which has been enriched either
through concentration or addition of milk powder, it is on a unit-to-unit basis a richer source if
these minerals than milk (table2). For lactose-maldigesting individuals, yogurt provides a rich,
easily digestible source of these minerals.
HEAT TREATED FERMENTED MILKS
Post-fermentation heat treatment significantly alters some properties of fermented milks. Heat
treatment above 65°C appreciably reduces the level of some thermosensitive vitamins.
In addition enzymic activity, notably B-galactosidase, is markedly reduced. This reduced enzymic
activity dramatically lowers the ability of lactose maldigestors to tolerate the same amount of
lactose that otherwise could be tolerated with live yogurt.
This modification of properties may not be limited to lowering vitamin content and lactose
absorption but may also affect some other probiotic properties.
LACTIC ACID ISOMERS
Lactic acid is produced from lactose by the bacteria fermenting milk. The L(+) isomer is produced
by S. thermophilus and the D(-) from in produced by L. bulgaricus. Both D(-) and L(+) isomer are
metabolized by humans but the D(-) isomer metabolized at a much slower level.
In 1965, the World Health Organization prescribed maximum levels for intake of D(-) lactic acid,
but since 1974 it has modified its recommendation and eliminated all restrictions on intake for
adults and children older than 3 months. The limitations are still valid for babies under the age of 3
EFFECTS OF REGULAR CONSUMTION OF FERMENTED MILK
S. thermophilus and L. bulgaricus are not inhabitants of the intestinal tracts of humans and animals
but are normally isolated from green plant material and milk, respectively. These bacteria are not
highly acid- and bile-resistant.
Only about 15% survive passage through the stomach and about 1% reach the large intestine (17)
but cannot colonize it (18). However, they may still exert an effect in vivo due to intracellular
enzymes, cell surface antigenic receptors or metabolites produced during fermentation.
Although yogurt bacteria and other lactic acid bacteria have been shown to inhibit pathogenic
bacteria in vitro by the production of organic acids and antibiotic-like substances, this interaction
has not been clearly demonstrated in vivo. It has been shown in studies with mice (19,20) that
feeding yogurt resulted in an alteration in the intestinal flora, stimulating the growth of lactobacilli
and bifidobacteria. Such changes in intestinal microflora are known to affect the intestinal transit
time which may have a significant impact on nutrient absorption.
Beyond their good taste, fermented milks and especially yogurts have unique nutritional
attributes. They supply high quality proteins, are an excellent source of calcium, phosphor and
potassium, and contain significant quantities of several vitamins. The carbohydrate content is
easily absorbed even by lactose maldigestors. Fermented milks and yogurt are a valuable adjunct
to any healthy diet.
A NEW TENHNIQUE FOR EARLY DETECTION OF GRAM-NEGATIVE BACTERIA IN MILK.