7 Fermented Milks Past, Present, and Future

M. Kroger, J. A. Kurmann, and J. L. Rasic

Milk is the most important foodstuff for a mammal and has always been the first food of the newborn. One could argue that the deliberate souring or fermentation of milk was one of the key achievements that nurtured mankind to grow and develop into a productive and preeminent species. Had fermented milk been considered spoiled and inedible and thus not have entered the human diet in the thousands of years to come, human development would have taken an entirely different course. Although there is no perfect food, milk is the most nearly perfect food known.

At some stage in the course of human evolution it was recognized that the milk of other mammals was equally satisfying in meeting physiological demands for moisture, energy, and nutrients. Milk from eight species of domesticated mammals (cow, buffalo, sheep, goat, horse, camel, yak, and zebu) has been used to make traditional fermented milk products throughout the world.

From a biological standpoint, fermented milks are characterized by the accumulation of microbial metabolic products. It was realized very early that such microbial metabolites as lactic acid, ethyl alcohol, and dozens of other chemicals collectively called flavor substances, were not altogether unpleasant and even contributed to overall preservative action.

CLASSIFICATIONS

Despite the long historical record and worldwide distribution of fermented milks, few people know more than five or 10 of the several hundred specific products that could be described. Even current food science and dairy technology textbooks fail to do the subject justice.

For example, the latest (fourth) edition of Food Microbiology (1) covers fermented dairy products in only two pages. The textbook used in the Pennsylvania State University dairy technology course is The Science of Providing Milk for Mar' (2). Cultured and acidified milk products occupy 10 pages, and cultured buttermilk, sour cream, yogurt, acidophilus milk, and ymer and lactofil are given only subchapter status. Koumiss and kefir are merely mentioned as being popular in Eastern Europe. Cheese and Fermented Milk Foods (3) is somewhat more comprehensive, but it deals mainly with practical concerns and primarily with cheese.

By far the best compilations on fermented milks have been and are being published as documents of the International Dairy Federation (4,5). One chapter of the latter lists some 80 fermented milks, including both traditional and nontraditional products. A soon-to-be-published encyclopedia of fermented fresh milk products (6) describes some 200 traditional fermented milks and several hundred nontraditional ones.

Traditional and Nontraditional

The most fundamental division of fermented milk products is into traditional and nontraditional types. Traditional fermented milk products have a long history and are known and made all over the world whenever milk animals were kept. Their production was a crude art. It was not until the days of Pasteur - about 100 years ago - that the microbiology underlying fermentations was revealed. In contrast, nontraditional fermented milk products are recently developed. They are based on known scientific principles; their microbial cultures are known; and their quality can be optimized. This is not the case with traditional products made with ill-defined, empirical cultures where you have to take what you get out of the fermentation. Yogurt is both a traditional and a nontraditional product - the latter being represented by ever-changing varieties.

Medium and Procedure

Classification by technology differentiates between fermented milks and fermented products not based directly on milk. It is obvious that products other than fresh milk can serve as the fermentation medium or substrate, such as cream, whey, buttermilk, and dry milk solids It is also possible to further manipulate or change the curd recovered after coagulation.

Further Processing

Neither law nor taboo forbids experimentation with fermented milks. Numerous products are known that are mixtures of milk and other foodstuffs and that have been subjected to fermentation. These include fermented milk-vegetable products, fermented milk-meat extract mixtures, and fermented milk-fishmeal hydrolyzate mixtures. Consequently, we find societies that have utilized specific plants, meat extracts, or fishmeal hydrolyzates to enhance their nutritional status and the flavor and variety of their cuisine.

Pharmaceutical preparations are unique in that they emphasize microorganisms only instead of milk nutrients or product flavor. The subject of probiotics (a word coined in 1974) will undoubtedly emerge as a major field of study. We see it in animal science now where some work is being done to get specific bacteria implanted or colonized in the gastrointestinal tract of animals, obviously in the interest of animal health and improvement of farm animal food production. So-called health food stores make available preparations that provide people with specific doses of bacteria, such as Lactobacillus acidophilus, commonly found in some fermented milk products. The subjects of health and probiotics, as well as myth and faddism, are beyond the scope of this paper.

End Uses

Traditionally, fermented milk products have been consumed as beverages, as meal components, or as ingredients in cookery. As social patterns have changed, however, meal eaters have become snackers and grazers. Furthermore, food technologists and food innovators have created a multitude of new products for the shelves of modern supermarkets. Most of the developments have been in the dessert and confectionery category.

Microbial Actions

Homemade fermented milk products, especially in nomadic or village environments, are still occasionally made by spontaneous fermentation, but most likely they are made by the use of an empirical culture. In other words, the inoculum is obtained from a previous production and its microbial identity is unknown.

The bacteria utilized are either mesophiles or thermophiles, terms indicating optimum bacterial growth temperatures, roughly 70°C and 100°F (22° and 38°C), respectively. More specific and important is the bacterial species present. A fermented milk is mainly characterized by its sensory properties, and the sensory properties, such as taste, odor, and viscosity, are the direct results of specific bacterial action. The current names of microorganisms recognized in fermented milks are listed in Table 1.

TABLE 1 Current Names of Microorganisms In Fermented Milks

	Number of Former
Current Name	Designations and Synonyms
Genus Lactobacillus
L. delbrueckii	8
L. delbrueckii subsp. lactis	10
L. delbrueckii subsp. bulgaricus	8
L. acidophilus	2
L. helveticus	7
L. casei	6
L. brevis	29
L. fermentum	10
L. kefir	4
Genus Leuconostoc
L. mesenteroides	2
L. mesenteroides subsp. dextranicum	6
L. mesenteroides subsp . cremoris	3
L. lactis	1
Genus Pediococcus
P. pentosaceous	3
P. acidilactici	1
Genus Propionibacterium
P. freudenreichii subsp. shermanii
P. freudenreichii subsp. freudenreichii
Genus Streptococcus
S. lactis	1
S. lactis subsp. diacetylactis	1
S. lactis subsp. cremoris	3
S. thermophilus	1
Genus Bifidobacterium
B. bifidum	13
B. longum(1)	1
B. infantis	3
B. breve	2
Genus Acetobacter
A. aceti	11
Yeasts
Torulaspora delbrueckii	1
Kluyveromyces marxianus subsp. marxianus	3
Kluyveromyces marxianus subsp. bulgaricus	1
Candida kefyr	3
Saccharomyces cerevisiae	0

(1)In an earlier edition of Bergey's Manual, B. longum was listed as having two subspecies: B. longum subsp. longum and B. longum subsp. animalist The latter was translocated in the new Bergey's into two species: B. animalis and B. pseudolongum.

With regard to bacterial species, a number of products have evolved that are now characterized by the presence of specific organisms. Modern yogurt is now defined by the regulations of many governments to be made from and to contain only Lactobacillus bulgaricus and Streptococcus thermophilus. But there are no hard-and-fast rules, and, theoretically, any combination of organisms could be utilized to make a fermented milk product. The ultimate test is palatability. Frankly, there is still much confusion over the microbial identity of most of the known traditional fermented milk products in the world. Some have never been studied in depth. Some are very variable from batch to batch. Only yogurt has been given a proper definition by regulatory authorities in some countries. All other products are only loosely defined.

RESEARCH

Milk has always turned sour, but at some point in human history artisans deliberately caused milk to coagulate. However, the scientific principles behind the phenomenon of milk fermentation have remained unrevealed until recent decades.

We had to wait for the pioneers in microbiology to lead the way. Louis Pasteur (1822-1895) studied alcohol fermentation; Heinrich Anton DeBary (1831-1888) studied the infection of plants by fungi; and Robert Koch (1843-1910) studied human disease caused by bacteria. It was Elie Metchnikoff (1845-1916) who, while working at the Pasteur Institute in Paris, moved milk fermentations and the unheard-of subject of probiotics into the limelight. In 1908 he shared the Nobel Prize in Physiology and Medicine. Metchnikoff developed a theory that lactic acid bacteria in the digestive tract could, by preventing putrefaction, prolong life. His book, The Prolongation of Life (7), was translated into English in 1907 (reviewed in Harper's Weekly, February 8, 1908) and received much exposure worldwide. In a way it made Metchnikoff the godfather to everyone who, to this day, believes in the therapeutic value of fermented milk.

World War I put a damper on this type of human diet/health preoccupation. In the United States, it was 1921 before an American figure emerged who should be given much more credit, Leo Frederick Rettger. Rettger was a professor of bacteriology at Yale for most of his career. Two of his publications are A Treatise on the Transformation of the Intestinal Flora with Special Reference to the Implantation of Bacillus Acidophilus (8) and Lactolbacillus Acidophilus and Its Therapeutic Application (9).

On the practical front at that time, A. D. Burke, head of the Dairy Department of Alabama Polytechnic Institute, published Practical Manufacture of Cultured Milks and Kindred Products (10). Burke's book is, according to the subtitle, "a complete and practical treatise on the manufacture of commercial cultured buttermilks of all types - lactic, Bulgarian, acidophilus, kefir, kumiss, yogurt." It is also a practical treatise on commercial casein, cottage cheese, cream cheese, and commercial sour cream, with information on dried, condensed, and fruit-flavored buttermilk.

Then came World War II, and until about 1950 very little research and development was seen on fermented milks. Since then increasing attention has been paid to fermented milk products worldwide. The American Cultured Dairy Products Institute was created in the United States in 1965. Several good books have been published, and scientific publications on the subject are proliferating. Manufacturers, researchers, and the public are experimenting with cultured dairy products in North America - and not only with yogurt but with other products as well. Kefir has been available in Los Angeles for more than a decade. In 1985 a New Jersey corporation began producing kefir for the East Coast, and in 1987 several major grocery chains began selling leben.

The future of fermented milk in North America and elsewhere will undoubtedly be exciting and complex.

REFERENCES

1. Frazier, W. C., and D. C. Westhoff. 1987. Food Microbiology. 4th ed. New York: McGraw-Hill Book Co.

2. Campbell, J. R., and R. T. Marshall. 1975. The Science of Providing Milk for Ma''. New York: McGraw-Hill Book Co.

3. Kosikowski, F. V. 1977. Cheese and Fermented Milk Foods 2nd ed. Ann Arbor, Mich.: Edwards Brothers, Inc.

4. International Dairy Federation. 1984. Fermented Milks. Document 179, International Dairy Federation, Brussels, Belgium.

5. International Dairy Federation. 1989. Monograph on Fermented Milks: Science and Technology. International Dairy Federation, Brussels, Belgium.

6. Kurmann, J. A., J. L. Rasic, and M. Kroger. 1990. Encyclopedia of Fermented Fresh Milk Products. New York: Van Nostrand Reinhold.

7. Metchnikoff, E. 1906. The Prolongation of Life. New York: G. P. Putnam and Sons.

8. Rettger, L. F., and H. A. Cheplin. 1921. A Treatise on the Transformation of the Intestinal Flora with Special Reference to the Implantation of Bacillus Acidophilus. New Haven, Conn.: Yale University Press.

9. Rettger, L. F., M. N. Levy, L. Weinstein, and J. E. Weiss. 1935. Lactobacillus Acidophilus and Its Therapeutic Application. New Haven, Conn.: Yale University Press.

10. Burke, A. D. 1938. Practical Manufacture of Cultured Milks and Kindred Products. Milwaukee, Wis.: The Olsen Publishing Co.