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III. MILK
DERIVATIVES
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7
Fermented Milks Past, Present,
and Future
M. Kroger, J. A. Kurmann, and I. 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.
6
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62
FERMENTED FOODS
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 Man (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
(4,51. 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 prod-
ucts 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.
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63
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 mix-
tures, and fermented milk-fishmeal hydrolyzate mixtures. Conse-
quently, 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
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FERMENTED FOODS
indicating optimum bacterial growth temperatures, roughly 70° 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
Current Name
Number of Former
Designations and Synonyms
Genus Lactobacillus
L. delbrueckii
L. delbrueckii subsp. Iactis
L. delbrueckii subsp. bulgaricus
L. acidophilus
L. helveticus
L. cased
L. brevis
L. fermentum
L. kefir
Genus Leuconostoc
L. mesenteroides
L. mesenteroides subsp. dextranicum
L. mesenteroides subsp. cremoris
L. Iactis
Genus Pediococcus
P. pentosaceous
P. acidilactici
Genus Propionibacterium
P. freudenreichii subsp. shermanii
P. freudenreichii subsp. freudenreichii
Genus Streptococcus
S. Iactis
S. Iactis subsp. diacetylactis
S. Iactis subsp. cremoris
S. thermophilus
Genus Bifidobacterium
B. bifidum
B. Iongum'
B. infantis
B. breve
Genus Acetobacter
A. aceti
Yeasts
Torulaspora delbrueckii
Kluyveromyces marxianus subsp. marxianus
Kluyveromyces marxianus subsp. bulgaricus
Candida kefyr
Saccharomyces cerevisiae
8
10
8
2
7
6
29
10
4
2
6
3
1
3
1
31
13
1
3
2
11
3
1
3
o
~In an earlier edition of Bergey's Manual, B. Iongum was listed as having two subspecies: B. Iongum
subsp. Iongum and B. Iongum subsp. animalist The latter was translocated in the new Bergey's into
two species: B. animalis and B. pseudolongum.
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65
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 Lactobacillus Acidophilus and Its
Therapeutic Application (91.
On the practical front at that time, A. D. Burke, head of the Dairy
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FERMENTED FOODS
Department of Alabama Polytechnic Institute, published Practical
Manufacture of Cultured Milks and Kindred Products (101. 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, research-
ers, 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 Man. 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. Docu-
ment 179, International Dairy Federation, Brussels, Belgium.
5. International Dairy Federation. 1989. Monograph on Fermented
Milks: Science and Technology. International Dairy Federation, Brus-
sels, 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.
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67
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.
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8
Lactobacillus GG Fermented Whey
and Human Health
Seppo SaIminen and Karl Salminen
Traditionally, whey has been a troublesome waste product at cheese
factories. New uses have now been developed for cheese whey to
utilize the whey nutrients, including protein and carbohydrates.
Fermented milk products have been reported to have an important
role in the treatment of infant diarrhea in malnourished children (1,21.
More recently, Isolauri and co-workers (3) have shown in a double-
blind controlled trial that Lactobacillus GG bacteria promote recovery
from acute diarrhea in children. These results suggest that whey-based
products may be used in this application.
A process for manufacturing a fermented flavored whey drink has
been developed that combines the nutritional properties of whey and
the health benefits of Lactobacillus strain GG. The objective has been
to improve the utilization of whey through use of a scientifically
selected Lactobacillus strain with proven health benefits. For this
purpose, demineralized lactose-hydrolyzed whey concentrate has been
fermented with Lactobacillus GG. Whey and lactic acid bacteria have
thus been combined to provide a wholesome and nutritious beverage.
WHEY HYDROLYSIS PROCESS
Important steps in whey processing are the hydrolysis of lactose and
demineralization to remove excess salt. A continuous whey hydrolysis
process has been developed using immobilized 13-galactosidase enzyme.
This process is more economical than batch hydrolysis. Lactose
hydrolysis is important for lactose-intolerant populations and for
malnourished children. Malnourished children may experience worsen-
ing of acute diarrhea when lactose is given during treatment (1J. Salt
removal can be completed using an ion exchange process. After
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69
concentration to 60 percent dry matter, a hydrolyzed demineralized
whey syrup is obtained that has a good shelf life and a pleasant rich
taste.
LACTOBACILLUS GG
La ctoba cillus cased strain GG (La ctoba cillus GG) is a new La ctoba-
cillus strain that is of human origin and has been shown to colonize
the intestinal tract (41. This strain was originally isolated from a healthy
human volunteer based on its ability to tolerate acid and bile, to
produce an antimicrobial substance, and to adhere to human intestinal
cells (5,6~. It is among the first strains with clinically proven health
benefits in various intestinal disorders in adults, children, and infants.
The most important evidence of its health benefits comes from studies
of infant diarrhea. Isolauri and co-workers (3) published the first study
on infant rotavirus diarrhea in which the duration of diarrhea was
reduced by 50 percent through the use of either freeze-dried Lactobacil-
lus GG or Lactobacillus GG fermented milk products.
LACTOBACILLUS GG FERMENTED WHEY DRINK
A new fermented flavored whey drink has been manufactured from
demineralized lactose-hydrolyzed whey concentrate using Lactobacil-
lus GG. It is a low-lactose product that contains no fat and is lightly
sweetened with fructose. It has special sensory characteristics-
smooth texture, mild acidity, and the rich taste from whey. Fruit juices
or fruit flavoring have been used to modify the Havor to appeal to
different people.
Fermentation of whey may also influence lactose content when
suitable bacteria are used. Additionally, whey proteins may undergo
slight changes to ease their digestibility. The end product may offer
alternatives for people not currently attracted to fermented milks.
CONCLUSIONS
This development in whey processing offers new alternatives for
utilizing cheese by-products and applies new technologies to nutri-
tionally important products. Combining whey processing with lactoba-
cilli that have been obtained using new selection methods may prove
to be beneficial to human health in many intestinal imbalances. It may
also offer possibilities in utilizing new technologies in food production
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TABLE 1 Average Physical-Chemical Composition of Moroccan Traditional Dairy Products
Composition
Lben(4) Jben(3) Raib(3) Zabda(1 )
pH 4.25 4.1 4.2 4.5
% lactic acid 0.81 1.04 0.62 0.77
% total solids 6.5 37.5 10.7 76.7
% fat 0.9 16.47 2.22 73.7
% protein 2.5 15.8 3.1 1.8
% lactose 2.7 4.1 4.2 1.2
% chlorides 0.17 0.5 0.17 ND
% ash ND 1.26 0.54 ND
ND, not determined.
preparation of these products and/or poor bacteriological quality of the
raw milk used for their manufacture. In addition to the indicator
microorganisms, pathogens such as Salmonella sp., Yersinia enterocoli-
tica, Listeria monocytogenes, and enterotoxigenic Staphylococcus
aureus have been recovered mainly from samples of Iben and jben.
Although there are no epidemiological reports of outbreaks linking
Moroccan traditional dairy products with diseases caused by these
pathogens, their presence in these products indicates potential health
hazards for consumers. Therefore, there is need to implement correc-
tive procedures to eliminate or reduce this risk. This can be achieved
by the use of heat-treated milk instead of raw milk and through the
use of selected starter cultures for preparation of these products.
OBJECTIVES
The application of modern technology to Moroccan traditional dairy
products aims to assure the following:
~ Large-scale production of these products year-round by replacing
raw milk with dry milk and butter oil. This will solve the problem of
seasonality in Moroccan milk production.
· Production of dairy products with standardized chemical and
microbiological composition so that their quality can be more easily
controlled and standards for each product can be established.
Streptococci
Lactobaciiii
Leuconostocs
T. conforms
F. coliforms
Enterococci
Fungi
Total flora
TABLE 2 Average Microbiological Counts of Moroccan Traditional Dairy Products (cfu/g or ml)
_ . . .
Microorganism Lben(4) Jben(3) Raib(3) Zabda(2)
5.0x 106
2.4x 105
1.8x 104
6.5 x 1 04
2.1 x 104
8.6x 104
ND
4.6x 107
7.6x 108
1.0x 103
1.7x 105
5.0x 104
1.0x 103
1.0x 105
8.5x 102
2.9x 109
5.1 x 108
3.2x 108
2.6x 108
4.3 x 105
2.7x 104
2.4x 105
2.3x 106
8.2x 108
1.4x 108
2.6x 106
2.8x 106
1.7x 105
4.2x 103
2.2x 104
2.3x 104
3.5x 108
ND: not determined.
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MOROCCAN DAIRY PRODUCTS
77
· Elimination of massive contamination of these products and
reduction of health hazards associated with these contaminations by
using heat-treated milk and improving the sanitation and fermentation
conditions.
· Adoption of simple and standardized processes for the preparation
of these products that could be easily applied in the dairy industry.
PRELIMINARY STUDY
Preparation of traditional dairy products using improved technologi-
cal processes requires, for each type of product, determination of the
characteristics that constitute an excellent-quality product. For this
purpose, samples of each product were evaluated by a gustatory panel.
The best products were then analyzed to determine their physical
characteristics, chemical composition, and microbiological profiles.
The objective of the study was to assess the censorial and compositional
parameters (e.g., acidity, total solids) that the improved product should
have to be acceptable to consumers.
Selection of Starters
Microbiological analysis of the different traditional fermented dairy
products showed that an important proportion of their microflora
was represented by lactic acid bacteria. Lactic streptococci were
predominant in Iben, raid, and zabda, while streptococci, lactobacilli,
and leuconostocs were found in jben at almost the same average levels
(108 cfu/g or ml) (colony forming units). From each product isolates
from the predominant lactic flora were identified using biochemical
tests. The principal species found in Iben, raib, and zabda were
Streptococcus lactis, and S. diacetylactis, while 5. Iactis, Lactobacillus
cased casei, and Leuconostoc lactis were the main species recovered
~ r
In Joen.
Owing to the nature of traditional Moroccan dairy products (fresh
fermented products), the major criterion considered for selection of
lactic starters was their acid production ability at different incubation
temperatures. Production by lactic strains of certain substances contrib-
uting to the overall aroma of these products also was taken into
account. Thus, several lactic strains were retained to be used for
preparating improved products.
Manufacture of Traditional Dairy Products from Heat-Treated
Milk
To prepare each type of product, a simple and economically feasible
technology, which industrial dairy plants could easily adopt, was used.
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The improved processes proposed for use with raib (fermented milk)
and jben (fresh cheese) are as follows:
· Manufacture of raib:
Reconstitution of dry milk to 90 percent water and 10 percent
solids.
Pasteurization at 63°C for 30 minutes.
Addition of calcium chloride and storage at 7°C for 10 hours.
Addition of fresh pasteurized milk (60 percent of the total volume).
Inoculation (S. Iactis, S. diacetylactis @ 3.0 percent).
Distribution into plastic containers and incubation at 30°C for 3
to 4 hours.
Refrigeration at 4° to 6°C.
· Manufacture of jben:
Reconstitution and pasteurization of powdered milk.
Addition of calcium chloride and storage at 7°C for 10 hours.
Addition of fresh pasteurized milk (60 percent of the total volume).
Inoculation (S. Iactis, S. diacetylactis, L. cased cased (it? 3.0
percent).
Storage of inoculated milk at 20° to 25°C until 0.25 percent lactic
acid is formed.
Addition of rennet (5 to 10 milliliters/100 liters).
Fermentation at 20° to 25°C until 0.60 percent lactic acid is formed.
Curd cutting and whey draining.
Unmolding when titratable acidity reaches 0.9 percent lactic acid
and total solids content reaches 28 to 30 percent.
Cutting of cheese into suitable pieces (150 grams/piece).
Surface dry salting, if desired (1 percent salt) and wrapping.
RESULTS
This study is still in progress. The final results regarding censorial
quality, chemical composition, and microbiological quality of tradi-
tional dairy products made with the improved technology are not yet
available. Nonetheless, preliminary data obtained for raib and jben are
very encouraging:
~ Sensorial quality: Laboratory samples of improved raib and jben
gave similar or even higher sensory scores than market samples. The
characteristics considered in this evaluation are mainly acidity, texture,
and aroma.
o Chemical composition; Because standard procedures were used
for making raib and jben, the samples obtained had uniform composi-
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MOROCCAN DAIRY PRODUCTS
79
lions. This information will be useful in establishing standards for these
products.
· Microbiological quality: The use of heat-treated milk in the
manufacture of raib and jben had a profound effect on the microbiologi-
cal quality of the products. The improved products were free from
pathogens such as S. aureus, Salmonella, L. monocytogenis, and Y.
enterocolitica. They were either free of or contained very few coliforms
(<10 cfu/g). Their microbiological quality was substantially improved
compared with currently marketed traditional products.
CONCLUSIONS
Although data on all traditional dairy products are not yet available,
information on the quality of laboratory-made raib and jben indicates
that the use of modern technology in their manufacture has enhanced
their bacteriological quality and reduced the risks of dairy-borne
infections. This new technology has also begun to establish standards
for these products.
In addition, the manufacture of traditional dairy products at an
industrial scale will increase the production of these products and
assure better distribution and marketing.
On the other hand, the use of dry milk, which is more economical
than raw milk for preparing products such as raib and jben, has the
advantage of being available any time of the year. This is very important
in Morocco, where seasonal variabilities in milk production are a major
problem for the dairy industry. Furthermore, the availability of dairy
products that are rich in nutrients (e.g., proteins, fat) at a modest price
and throughout the year will contribute to reduced malnutrition
especially among children in rural areas.
REFERENCES
1. El Marrakchi, A.M., M. Berrada, M. Chahboun, and M Benbou-
hou. 1986. Etude chimique du smen marocain. Le Lait 66:117-133.
2. H-amama, A. 1989. Studies on the hygienic quality of certain
Moroccan dairy products. Ph.D. thesis, University of Minnesota.
3. Hamama, A., and M. Bayi. 1991. Composition and microbiological
profile of two Moroccan traditional dairy products: raib and jben.
Journal of the Society of Dairy Technology.
4. Tantaoui Elaraki, A., M. Berrada, A. El Marrakchi, and A.
Berramou. 1983. Etude du Iben marocain. Le Lait 63:230-245.
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11
Fermented Milk Products in
Zimbabwe
Sara Feresu
Fermentation is the oldest means of preserving milk (11. Originally,
unpasteurized milk was left to ferment naturally, and fermentation
involved microorganisms present in the raw milk and surrounding air.
With the development of modern technologies, specific lactic-acid-
producing microorganisms are now introduced to carry out fermenta-
tions under controlled conditions. In this way fermented products of
superior nutritional, physical, chemical, and sanitary qualities are
produced.
In Zimbabwe one finds the modern fermented products such as
yogurt and different types of cheese. The rural population, however,
still ferment their milk traditionally. Fresh unpasteurized cow's milk
is allowed to stand, at ambient temperature, in an earthenware pot
loosely covered by a plate. This allows microorganisms inherent in the
milk, from the pot, and from the surrounding air to ferment the milk.
Fermentation takes 1 to 2 days depending on the ambient temperature
(20 to 25°C). The fermented milk is not refrigerated and has an estimated
shelf life of 3 days at ambient temperature.
In response to the urban population's desire for fermented milk, the
Zimbabwe Dairy Marketing Board produces a fermented milk called
Lacto on an industrial scale. Milk is standardized, pasteurized at 92°C
for 20 minutes, cooled to 22°C, and inoculated with 1.2 percent of an
imported mesophilic starter culture, similar to that used to produce
"filmjolk," a Scandinavian fermented milk. The milk is immediately
packaged into sachets, left to ferment at ambient temperature for 18
hours, and stored at 5°C ready for the market. The shelf life of
refrigerated Lacto is 7 days.
Our studies have compared traditionally fermented milk with Lacto.
We included traditionally fermented pasteurized milk, since substitu-
tion of unpasteurized with pasteurized milk might be an alternative for
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81
upgrading hygienic standards. The initial study was concerned with
the effects of pasteurization and of the container used during fermenta-
tion on the total microbial cell counts, the counts of lactic acid bacteria,
the amount of lactic acid produced, and the acceptability of the
fermented milk by a panel (2~.
We have characterized 10 predominant lactic acid bacterial isolates
from traditionally fermented milk and four isolates from Lacto (3~. We
have also carried out studies to determine the fate of pathogenic and
nonpathogenic Escherichia cold during fermentation of Lacto and
traditionally fermented pasteurized and unpasteurized milk. The sur-
vival of E. cold was also tracked during storage of the fermented
products at ambient (20°C) and refrigeration temperatures (5°C) for 4
days (4), since it is possible that pathogenic bacteria may gain access
to these products before, during, and after fermentation. In the case
of traditionally fermented milk, coliform contamination from cattle
dung or from the milker's hands is possible. Contamination with
coliforms during Lacto production can occur through bulk starter
cultures and from inadequately sanitized equipment.
TRADITIONALLY FERMENTED MILK AND LACTO
In an earlier study (2) unpasteurized milk and pasteurized milk were
fermented in clean nonsterile earthenware pots and sterile glass
containers. At the same time, Lacto was fermented in plastic sachets
and sterile glass containers. Bacterial counts and lactic acid levels
were determined. The acceptability of the fermented milks was ranked
by 11 panelists. Comparisons of all parameters were made after 24 and
48 hours of fermentation, when Lacto and traditionally fermented milk
are likely to be consumed.
The numbers of lactic acid bacteria, lactic acid production, and
acceptability were always higher for unpasteurized than pasteurized
traditionally fermented milk irrespective of the container used.
Earthenware pots are better containers for traditional fermentation
of milk. This is because earthenware pots have micropores in their
walls, which, if not sterilized, may harbor lactic acid bacteria from
the previous fermentation, which then act as inocula for the next
fermentation. Our results suggest that earthenware pots are good
containers to ferment milk in and may still have a place in milk
fermentation in the home.
Unpasteurized milk fermented traditionally in either container was
significantly more acceptable to the panel than Lacto, although the
products were similar in all the other parameters assessed. It was
therefore impossible to explain the differences in the acceptability of
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traditionally fermented milk and Lacto on the basis of this work. We
suggested that the differences were probably due to the types of
microorganisms involved in the fermentation of the two milk products
rather than pasteurization or the container used for fermentation. Thus,
we set out to isolate and characterize the lactic acid bacteria in
traditionally fermented milk and Lacto.
ISOLATION AND IDENTIFICATION OF LACTIC ACID BACTERIA
From the previous study (3), 10 predominant morphologically
different lactic acid bacteria colony types from plates inoculated with
traditionally fermented milk and four morphologically different types
of colonies from Lacto plates were selected and isolated into pure
culture. The isolates were identified using numerical taxonomic tech-
niques and reference strains. The isolates and reference strains were
examined for 32 characteristics. Data were analyzed using the simple
matching coefficient, and clustering was by unweighted pair group
average linkage (51.
All the isolates from traditionally fermented milk belonged to the
genus Lactobacillus. Seven of the isolates could be identified as
belonging to L. helveticus, L. plantarum, L. delbrueckii subspecies
lactis (two isolates), L. cased subsp. cased (two isolates) and L. cased
subsp. pseudoplantarum. Three of the isolates could only be identified
as either betabacteria or streptobacteria. The four isolates from Lacto
were identified as Lactococcus lactis. They could not, however, be
identified to subspecies level.
From this study we concluded that the differences in acceptability
of traditionally fermented milk and Lacto are probably due to differ-
ences in the biochemical pathways and resulting types and levels
of end products produced by the different bacteria responsible for
fermentation of the two products. We suggested that more work should
be done to determine the particular flavors and aroma present in
traditionally fermented milk that are absent in Lacto and to determine
whether any of our isolates are responsible for producing these desired
properties.
E. COLI STRAINS IN LACTO AND TRADITIONALLY FERMENTED
MILK
In another study (4) the growth and survival of pathogenic and
nonpathogenic strains of E. cold were determined in traditionally
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83
fermented pasteurized and unpasteurized milk and Lacto. Unpasteur-
ized and pasteurized milk and freshly inoculated Lacto, together with
sterile control milk, were each inoculated with two strains of pathogenic
and one strain of nonpathogenic E. cold to give approximately 103 cells/
milliliter. All the milk treatments were left to ferment at ambient
temperature (20°C) for 24 hours. One set of the fermented products
was stored at ambient temperature, and the other set was refrigerated
(5°C) for another 96 hours. Samples were taken at 24-hour intervals
and tested for numbers of E. coli, pH, and percentage of lactic acid.
Lacto inhibited all three E. cold strains. Two strains (one pathogenic
and one nonpathogenic) could not be recovered, and the third (patho-
genic) survived only in very low numbers after 24 hours of storage of
Lacto at both 20° and 5°C.
All three E. cold strains survived and multiplied to maximum cell
numbers in the range 107 to 109/milliliter during traditional fermentation
of unpasteurized milk. Cell numbers decreased to 103 to 106 and 102 to
105 during storage of the fermented product at 20° and 5°C, respectively.
These results indicated that traditional methods of fermenting milk in
Zimbabwe pose a potential health hazard because, if milk is contami-
nated during milking or fermentation, E. coli, and possibly other enteric
pathogens, are able to multiply to infective doses and retain relatively
high numbers during storage of the product at both refrigeration and
ambient temperatures. The results also indicated that more than acid
production alone is involved in the fate of E. cold during fermentation
and storage of Lacto and traditionally fermented unpasteurized milk
since more E. cold survived in unpasteurized fermented milk despite
similar final lactic acid and pH levels of both milk products. We
suggested that, since in our earlier studies we found that different lactic
acid bacteria were responsible for fermentation of the two milk
products, it is likely that these organisms produce different types and
quantities of other inhibitory products (antibiotics, volatile acids,
hydrogen peroxide) during fermentation.
Higher maximum numbers, 109 to 10'° of the three strains of E. coli,
were attained during traditional fermentation of pasteurized milk. The
numbers decreased to 105 to 108 and 104 to 107 during storage of the
fermented product at 20° and 5°C, respectively. Under our experimental
conditions there appeared to be more danger in traditionally fermenting
pasteurized milk than unpasteurized milk; since less acid was produced,
more E. cold multiplied and survived during fermentation and during
storage of the pasteurized fermented milk. The practical relevance of
this result should be interpreted with caution, since pasteurization also
removes milk-borne organisms such as E. cold and Salmonella spp.
and since it is unlikely that airborne recontamination of the milk by E.
cold would result in initial numbers as high as 103 cells/milliliter. Thus,
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use of pasteurized milk in practice may not be as inappropriate as it
might appear in theory.
Generally, fewer E. cold survived when the fermented milk products
were stored at refrigeration than at ambient temperature. However,
most people in rural areas of Zimbabwe do not have access to
refrigerators.
CONCLUSIONS
We are currently determining the amounts of some B vitamins and
of aroma and flavor compounds in traditionally fermented unpasteur-
ized milk and Lacto. Preliminary results indicate that traditionally
fermented milk contains more thiamine, riboflavin, pyridoxine, and
folic acid than Lacto. Again, traditionally fermented unpasteurized
milk is performing better than Lacto.
From the work we have done so far there are two options to follow
in our future studies. We know that traditionally fermented milk has
similar amounts of lactic acid and a pH level similar to that of Lacto
and that it might also have higher amounts of some B vitamins;
however, it is not hygienically acceptable. We know some of the lactic
acid strains involved in the fermentation, but we also know that in a
situation where raw milk is used and fermentation is carried out under
conditions where asepsis is not observed, other microorganisms, in
addition to lactic acid bacteria, contribute to the production of aroma
and flavor compounds. Supposing we were to develop a starter culture
based mainly on members of the genus Lactobacillus, it is debatable
whether we would have the same organoleptic properties in a tradition-
ally fermented pasteurized milk as found in traditionally fermented
unpasteurized milk. If we developed and sold this starter culture for
home use in fermentation of boiled milk, it is also unlikely that poor
rural people would adopt such a fermentation since it has an added
cost when compared with traditional fermentation.
Alternatively, we could incorporate some isolates from traditionally
fermented milk into the Lacto starter culture and see whether the
organoleptic properties of Lacto can be improved. Such a product
would have to taste much better than traditionally fermented unpasteur-
ized milk so as to entice rural populations to abandon traditional
fermentation and adopt Lacto. Educational programs would have to
be instituted for the public to appreciate the wisdom of spending money
on buying Lacto, a hygienically safer product. At present, it is unlikely
that Lacto will replace traditionally fermented milk in the foreseeable
future.
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REFERENCES
85
1. Robinson, R. K., and A. Y. Tamime. 1981. Microbiology of
fermented milks. Pp. 245-278 in: Dairy Microbiology, Vol. 2. R. K.
Robinson, (Ed.~. London: Applied Science Publishers.
2. Feresu, S., and M. I. Muzondo. 1989. Factors affecting the
development of two fermented milk products in Zimbabwe. MIRCEN
Journal of Applied Microbiology and Biotechnology 5:349-355.
3. Feresu, S., and M. I. Muzondo. 1990. Identification of some lactic
acid bacteria from two Zimbabwean fermented milk products. World
Journal of Microbiology and Biotechnology 6: 178-186.
4. Feresu, S., and PI. Nyati. 1990. Fate of pathogenic and non-
pathogenic Escherichia cold strains in two fermented milk products.
Journal of Applied Bacteriology 69:814-821.
5. Sneath, P. H. A., and R. R. Sokal. 1973. Numerical Taxonomy: The
Principles and Practice of Numerical Classifications. San Francisco: W.
H. Freeman, pp. 228-234.
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Representative terms from entire chapter:
lactic acid
