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6252 YOGURT/The Product and its Manufacture
Bhaduri S, Turner-Jones C, Taylor MM and Lachica VR
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DA (1999) Isolation and detection of Listeria spp,
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Methods 39: 33–43.
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Probes 6: 291–297.
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Journal of Applied Microbiology 90: 358–364.
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Michaelsen TE (1993) Detection of pathogenic Yersinia
enterocolitica in foods and water by immunomagnetic
separation, nested PCR and colorimetric detection of
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in some fish, meat and meat products in India. Journal of
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vitek, API 20E and Gene-trak systems for the identification of Yersinia enterocolitica. Letters in Applied Microbiology 18: 90–92.
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bovine brucellosis. Journal of Veterinary Research 5:
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enterocolitica from raw milk. Applied and Environmental Microbiology 35: 54–58.
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and the definition of a database. Journal of Antimicrobial and Chemotherapy 43: 37–45.
Vishnubhatla A, Fung DYC, Oberst RD et al. (2000) Rapid
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YOGURT
Contents
The Product and its Manufacture
Yogurt-based Products
Dietary Importance
The Product and its
Manufacture
N Shah, Victoria University of Technology, Victoria,
Australia
Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction
0001
At the beginning of this century, Nobel Laureate, Elie
Metchnikoff, at the Pasteur Institute, linked health
and longevity to the ingestion of bacteria present in
fermented foods such as yogurt. This observation
provided a major boost to the manufacture and
consumption of yogurt and other fermented milk
products.
Although no records are available to trace the
origin of yogurt, it is believed that fermentation
was the first technique employed by humans to
preserve foods. The word ‘yogurt’ was derived from
the Turkish word ‘Jugurt.’ Yogurt is defined as ‘a
product resulting from milk by fermentation with a
mixed starter culture consisting of Streptococcus
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YOGURT/The Product and its Manufacture
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thermophilus and Lactobacillus delbrueckii ssp. bulgaricus.’ However, in some countries, including Australia, other suitable lactic acid bacteria are permitted
for use as starter cultures. As a result, some yogurt
manufacturers use Lactobacillus helveticus and Lactobacillus jugurti for yogurt manufacture. The first US
Federal Standards of Identity for yogurt were published in 1981. The standard does not permit the use
of culture organisms other than Lb. delbrueckii ssp.
bulgaricus and Sc. thermophilus for making yogurt,
and the titratable acidity must be at least 0.9%, expressed as lactic acid.
Yogurts differ according to their chemical composition, method of production, flavour used and the
nature of postincubation processing. Based on the
fat content, there are three main types of yogurt:
full-fat yogurt, reduced-fat yogurt, and low-fat
yogurt (Table 1).
On the basis of the method of production and the
physical structure of the coagulum, yogurts are classified as either set or stirred. Set yogurt is the product
formed when the fermentation of milk is carried out
in a retail container, and the yogurt produced is in a
continuous semisolid mass. In contrast, stirred yogurt
results when the coagulum is produced from milk,
and the gel structure is broken before cooling and
packaging. Fluid yogurt can be considered as stirred
yogurt of low viscosity.
On the basis of flavorings, yogurts are divided into
three categories. Plain or natural yogurt is the traditional product, which has a typical sharp ‘nutty’
flavor. Fruit yogurts are made by addition of fruits,
usually in the form of fruit preserves, puree or jam.
Flavored yogurts are prepared from plain or natural
yogurt by adding sugar and/or other sweetening
agents, flavorings and colorings.
The method of yogurt production has changed very
little over the years. However, some newer varieties
have been added including frozen-, concentrated-,
dried-, and pasteurized yogurt. Postincubation processing of yogurt may lead to pasteurized/UHT
yogurt, concentrated yogurt, frozen yogurt, and
dried yogurt. Pasteurized/UHT yogurt is prepared by
heat- treating yogurt after incubation. Heat treatment
destroys yogurt starter bacteria and reduces the levels
Table 1 Compositional standards for yogurt
Fat (%)
Milk solids not fat (%)
Titratable acidity (%)
pH
Full-fat
yogurt
Reduced-fat
yogurt
Low-fat
yogurt
3.0%
8.25
0.9
4.5
0.5–2
8.25
>0.9
4.5
0.5
8.25
>0.9
4.5
6253
of volatile compounds associated with the flavor of
yogurt. In some countries, such as Australia, heat
treatment after production to inactivate the starter
culture is not permitted. In other countries, such
as the USA and several countries in Europe, heat
treatment is permitted; however, auxiliary labeling
‘heat-treated after culturing’ is required if yogurts
are heat-treated after culturing. Frozen yogurt resembles ice cream, and the chemical composition and
manufacturing details up to the freezing stage are
similar to those of yogurt. Frozen yogurt can be either
soft or hard. Dried yogurt can be produced by
sun-drying, spray-drying or freeze-drying of yogurt.
The drying process transforms the junket into
powder. Drying also causes loss of some flavor compounds and destruction of the starter culture. Another type of yogurt, which may find favor among
diet-conscious consumers is low-calorie yogurt, in
which intense sweeteners are used, and the viscosity
of the product is improved by the addition of stabilizers and thickening agents such as carrageenan and/
or gelatin.
In the Middle East, concentrated yogurt is prepared
by adding wheat flour or parboiled wheat, and the
yogurt–wheat mixture is shaped into rolls and sundried. The product is popularly known as ‘kishk.’ In
some countries, such as India, Nepal, and Bangladesh, yogurt is made in earthenware pots, and
because of evaporation of moisture through the
pores of the earthenware pots, the product becomes
concentrated.
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Processing and Manufacture of Yogurt
Preparation of Mix
A flow diagram for making yogurt is shown in
Figure 1. Traditionally, yogurt is manufactured from
milk, which has been concentrated by boiling to increase the viscosity of the product. Nowadays, milk is
fortified with nonfat dry milk for the same purpose.
The level of fortification varies from as little as 1% to
as much as 6%. However, the generally recommended level of fortification is around 3–4%. This
increases the total solids level in yogurt mix to
15–17%.
Ultrafiltration and reverse osmosis may also be
used to achieve a higher solids content in order to
improve the viscosity of the product. Milk concentrated by ultrafiltration to 18–20% total solids has
been reported to produce good-quality yogurt. The
viscosity of yogurt is almost wholly dependent on the
protein content of milk. Hence, a high protein concentration is essential for the production of viscous
yogurt. Casein is the major contributor of viscosity
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Preparation of mix (standardization, fortification with skim milk powder)
Homogenization (65 ⬚C,1.8 × 104 kPa)
Heat treatment (85 ⬚C/30 min)
Cool to incubation temperature (42 ⬚C)
Inoculation with starter (1% each of Sc. thermophilus and Lb. delbrueckii ssp. bulgaricus)
Pack in retail container (or inoculate in bulk for fruit yogurt)
Incubate (42 ⬚C for ~4 h)
Cool (4 ⬚C) (for fruit yogurt, mix with fruit when the product is cooled to 4 ⬚C)
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Figure 1 Flow diagram for yogurt manufacture.
followed by fat and whey proteins. Stabilizers can
also influence the consistency of yogurt. In practice,
gelatin, starch, vegetable gums, carrageenan, and
pectin are used most widely as stabilizers for yogurt.
The best yogurt texture is achieved by using gelatin at
0.3–0.8%. Good yogurt can be made without the use
of added stabilizers, but yogurt without a stabilizer is
more vulnerable to a number of stress factors than
one that has not been stabilized. When properly
chosen and used, stabilizers play an important role
in improving the body, texture, mouth feel, and appearance of yogurt. The fat content may vary from
0.5 to 3.5%. Milk fat also tends to ‘mask’ the acid
flavor of yogurt. Obviously, when milk fat is incorporated into the mix formulation, homogenization
becomes important for the overall texture quality of
yogurt.
Sweeteners are added to plain yogurts, generally
with fruits. Addition of sugar is a method of cutting
the sharpness of yogurt flavor. An appropriate level of
sugar should be used to mask the full degree of acidity, but enough sugar should remain in the product
after fermentation for a desirable acid–sugar blend.
Up to 10% sucrose is usually used when fruit is added
to yogurt. According to Ravula and Shah, a mix
containing 4% or more sucrose, may reduce acid
production and lower cell counts of both microbial
species when incorporated in the mix prior to fermentation. Nonnutritive or intense sweeteners such as
aspartame (NutraSweet) may find favor among diet
conscious consumers. Since aspartame is approximately 200 times sweeter than sucrose, only a small
amount is required for desirable sweetness. Because
of this, yogurts containing intense sweeteners require
a bulking agent such as polydextrose.
As yogurt mix is prepared, particular attention
should be given to blending, homogenization, and
heat treatment of the mix. The blending and
homogenization steps are important to the uniformity
of ingredient distribution. Homogenization is usually
carried out before heat treatment, but in some cases,
it may take place after the heat treatment.
Homogenization
Homogenization is the typical industrial process used
to effect stabilization of the lipid phase against separation by gravity. The specific gravity of milk fat is 0.9,
whereas that of skim milk is 1.036. Fat, being lighter,
tends to separate from the serum or skim milk if left
undisturbed. The diameter of the fat globules in milk
ranges from 1 to 15 mm, with an average of 3–4 mm.
Homogenization reduces the average diameter of fat
globules to 1 or 2 mm. As a result, the fat globules do
not cream during the incubation of yogurt. Because of
the size reduction, there is usually a four-to sixfold
increase in the surface area.
The fat-globule membrane protects the fat globules
from lipase. Upon homogenization, the fat-globule
membrane is destroyed, and the fat globule is vulnerable to attack by lipase, which is naturally present in
milk. Because of this, the milk is pasteurized to inactivate lipase before homogenization. If the milk is to be
homogenized before pasteurization, the milk must
be pasteurized immediately to prevent lipolysis.
Since the efficiency of homogenization is much
better at higher temperatures, the mix is warmed to
around 65 C to liquefy fat and then forced at high
pressure (1.8 104 kPa) through a small orifice to
reduce the size of the fat globules.
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Heat Treatment
Yogurt mixes are heated to a high temperature, typically 85 C for 30 min. Such a high heat treatment has
many objectives: (1) to destroy all pathogenic bacteria, (2) to inactivate all the enzymes that may be
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YOGURT/The Product and its Manufacture
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present in milk, including lipase, (3) to destroy most
of the spoilage-causing bacteria, including thermodurics, and, most importantly, (4) to denature whey
proteins, b-lactoglobulin and a-lactalbumin. This
heat treatment denatures more than 90% of the
b-lactoglobulin compared with 60% of the a-lactoglobulin. A complex is formed between k-casein and
denatured b-lactoglobulin, which increases the
hydrophilic properties of the casein, reduces the propensity of the gel to syneresis, and facilitates the
formation of a stable coagulum. The hydration effect
of the protein is maximal when milk is heated at
85 C but decreases as the temperature is raised
above 85 C. A higher heat treatment decreases the
hydrophilic properties of the b-lactoglobulin–kcasein complex. As a result, syneresis occurs, and the
structure of the yogurt gel becomes weak and fragile.
After heat treatment and complex formation between b-lactoglobulin and k-casein, the heated milk
forms a smooth gel-like structure when the pH drops
during fermentation to 4.6, which is the isoelectric
point of casein (the pH at which casein proteins carry
no electrical charge).
Inoculation and Incubation
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After pasteurization, the mix is cooled to 45 C and
inoculated at a level that varies from 0.5 to 5%.
The former level (0.5–1%) may be susceptible to
growth conditions and a longer incubation time.
The maximum amount recommended is 5%. This
level will cause very rapid acid production, but leads
to defects in aroma, and a large amount of culture
must be prepared. The optimum level is 2%, 1%
each of Lb. delbrueckii ssp. bulgaricus and Sc.
thermophilus.
Lb. delbrueckii ssp. bulgaricus hydrolyzes milk
proteins, the caseins, thus releasing essential amino
acids, including valine, which stimulate the growth of
Sc. thermophilus. Initially, Sc. thermophilus grows
rapidly, reducing the pH to around 5.4, which stimulates the growth of Lb. delbrueckii ssp. bulgaricus,
which is acid-tolerant and produces large amounts of
lactic acid, which reduces the pH. Sc. thermophilus
uses oxygen during its growth, which makes
oxidation–reduction potential more favorable for
Lb. delbrueckii ssp. bulgaricus; it also produces
formic acid, which stimulates the growth of the lactobacillus.
The starter bacterial cells are distributed in the
inoculated mix by stirring for 10–15 minutes after
the addition of the cultures. This is followed by dispensing the mixes into consumer-sized containers and
incubating at 42 C for approximately 4 h until the
pH decreases to 4.5. For the manufacture of stirred
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yogurt, the inoculated mix is incubated in bulk, and
when the pH reaches the desired value, the yogurt
is stirred, cooled, mixed with various flavorings, if
required, and filled into retail containers.
The incubation temperature of 42 C is optimal for
the associated growth; the optimum temperature of
the growth of Sc. thermophilus alone is 37 C, and
that for Lb. delbrueckii ssp. bulgaricus is 45 C. Incubation temperatures above 42 C will promote the
growth of the Lactobacillus, whereas an incubation
temperature of less than 42 C will favor the growth
of Sc. thermophilus, and lack of flavor results due to
the poor growth of Lb. delbrueckii ssp. bulgaricus.
Deviation in either direction will lead to a disturbance
in the ratio of lactobacilli to streptococci. The ratio of
lactobacilli and streptococci in natural yogurt for
optimum flavor should be 1:1. By using monocultures, this ratio of 1:1 can be maintained easily by
adjusting the inoculum level of both Lb. delbrueckii
ssp. bulgaricus and Sc. thermophilus.
When a 2% inoculum is used at an incubation
temperature of 42 C, the milk will coagulate, and a
firm gel will form in 3.5–4 h, and the pH will decrease
to 4.4–4.5. When frozen concentrated cultures are
used, an incubation period of 5–8 h may be required.
Similarly, freeze-dried cultures may require longer
times than fresh and active cultures.
A new trend in yogurt making is to incubate the
yogurt mix at a significantly lower temperature than
normal for a longer period. One of the advantages is
that products require less starter cultures and shorter
cooling times. An incubation temperature of 30 C
with 0.25% starter culture may take 12–14 h. If the
incubation temperature is too low, the growth of
yogurt starter may be affected adversely.
The final acidity of yogurt is 0.9–1%. The US
standard requires a titratable acidity of 0.9% or
higher, whereas the Australian standard requires a
pH of 4.5 or lower. According to IDF, the minimum
acidity of yogurt should be 0.7%.
The yogurt cultures produce acetaldehyde, which
gives natural yogurt its typical flavor that resembles
that of green apples. Other volatile compounds
include acetic acid and diacetyl.
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Cooling
Once the desired acidity has been reached, the product is cooled to <10 C as quickly as possible. In the
case of stirred yogurt, in one-phase cooling, the product is cooled from the incubation temperature to
<10 C prior to the addition of flavoring material
and filling. In two-phase cooling, the temperature of
the product is reduced to 15–20 C during the first
phase cooling before addition of flavoring materials
and filling of containers followed by the second stage
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6256 YOGURT/The Product and its Manufacture
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cooling to <10 C in a cold store. Yogurt quality may
be improved by packaging at 24 C followed by final
cooling of the product in the container. To maximize
the effect, the second stage of cooling should be
carried out as slowly as possible over a 12-h period.
The viscosity of yogurt improves during storage for
1–2 days. During the first 24–48 h of cold storage, an
improvement in the physical characteristics of the
coagulum is observed, mainly hydration and/or stabilization of casein micelles. Proper hydration is required to avoid syneresis. It is therefore important to
delay the sale or distribution of yogurt for 24–48 h.
During cold storage, it is important to minimize
rough mechanical handling of the packaged yogurt,
and to maintain the temperature of <5 C. During
transport, especially in summer, shaking of yogurt
can lead to a reduction in viscosity and syneresis.
Flavored yogurts
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Flavored yogurts are prepared by adding flavorings to
plain yogurt. Sundae-style yogurt is prepared by
layering 15–18% of total weight of yogurt with fruit
purée or syrup on the bottom of the containers and
then filling the containers with warm inoculated mix,
followed by sealing the containers and incubation.
The fruit in the product may be mixed with the yogurt
gel by consumers before eating.
Swiss or stirred-style yogurts are prepared by
blending fruit purée, sucrose, or glucose into bulk
prepared fresh plain yogurt. Since the coagulum is
broken during blending, plain yogurt is usually prepared with a higher level of stabilizer (0.7%) than
normal (0.3%). The product after mixing with fruit is
chilled to 4 C.
Frozen yogurt
0030
Frozen yogurt is a fermented milk or yogurt mix
containing a typical yogurt flavor and has been subjected to fermentation by the two yogurt bacteria to a
pH of 4.5 or lower or a titratable acidity of 0.9% or
higher, expressed as lactic acid. Frozen yogurts,
frozen flavored yogurt desserts, yogurt frozen on a
stick, or frozen fermented dairy desserts are relatively
new products that were developed in the 1960s.
Frozen yogurts are prepared in a similar fashion as
icecream, and therefore are made most conveniently
in icecream factories.
Low-lactose Yogurt
0031
Lactose malabsorbers do not produce sufficient lactase (b-d-galactosidase) and thus cannot hydrolyze
the ingested lactose completely. In addition to the
gastrointestinal discomfort, such as stomach upset
and diarrhea brought about by ingestion of milk by
these individuals, a general impairment in the normal
digestion process has been observed.
Low-lactose or lactose-free milk for lactose sensitive individuals can be prepared either by the physical
removal of lactose by ultrafiltration or by hydrolysis
of lactose into the corresponding monosaccharides,
glucose, and galactose.
Lactose can be hydrolyzed using strong mineral
acid or enzymes. The enzyme b-galactosidase (bd-galactoside galactohydrolase, EC 3.2.1.23), commonly called lactase, catalyzes the hydrolysis of the
b-1 ! 4-galactosidic linkage present in lactose. Lactase has been isolated commercially from the fungi,
Aspergillus niger, A. oryzae, and A. flavus, or the
yeast Saccharomyces lactis or from the bacterium
Escherichia coli. Yogurt bacteria (Lb. delbrueckii
ssp. bulgaricus and Sc. thermophilus) possess the
highest amount of b-galactosidase activity among
the lactic acid bacteria.
b-Galactosidase from Saccharomyces (known as
neutral lactase) has an optimum activity at pH 6.8–
7.0, is stable in the pH range of 6.0–8.5, and works
best at 35 C. It is suitable for treating milk (pH 6.6)
or sweet whey (pH 6.2), but the lack of stability
below pH 6.0 precludes its use for treating acid
whey (pH 4.5). A. niger lactase (also known as
acid lactase), with a pH optimum of 4.0–4.5, good
stability over a wide range of pH (3.0–7.0), and an
optimum temperature of 55 C, is suitable for the
modification of acid whey.
Low-lactose yogurt is then produced by the use of
low-lactose milk during processing.
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Heated or Pasteurized Yogurt
Even though natural yogurt is held at 4 C, the titratable acidity increases due to residual activity of the
starter bacteria. This process is referred in the industry as ‘post-acidification,’ as a result of which the
product becomes too tart. Without heat treatment,
the shelf-life of yogurt is 4–5 weeks at 4 C. Heating
destroys most of the starter bacteria and yeast and
molds, and if post-processing contamination is
avoided, the shelf-life of the product can be extended
to 8 weeks. No postacidification occurs in heated
yogurt. Heated or pasteurized yogurt can be prepared
by heat-treating yogurt in the package at about 55 C
for 30 min, followed by cooling. The main problems
associated with pasteurized yogurt are loss of flavor
and syneresis. Stabilizers can be used to overcome the
latter problem.
Controversies still exist regarding the heat treatment of yogurt after fermentation as heat-treated
yogurt contains very few or no viable yogurt bacteria.
It also raises questions about the definition of yogurt,
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YOGURT/The Product and its Manufacture
which must contain an abundant viable count of Sc.
thermophilus and Lb. delbrueckii ssp. bulgaricus.
Yogurt is considered to be an excellent nutritional
food, and consumers ingest several million live cells
through a typical serving of yogurt.
Prebiotics, Probiotics, and Synbiotics
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The term ‘probiotic’ is derived from two Greek words
meaning ‘for life.’ Probiotic organisms and substances produced by these organisms contribute to
the microbial balance in the intestines. The generally
accepted definition of probiotics is that ‘they are live
microbial food or feed supplements that provide a
beneficial effect on hosts (human or animal) by
improving the microbial balance in the intestine.’
The intake of these bacteria is reported to help restore
the balance in the intestinal microflora, which may
have been lost due to stress, antibiotic use, or illness.
The major strains of bacteria used in probiotics, Lb.
acidophilus and Bifidobacterium spp., are dominant
organisms of human large intestines. These microorganisms are claimed to inhibit the growth of pathogenic organisms through the production of organic
acids and bacteriocins. Other benefits include reduction in lactose malabsorption, suppression of potentially harmful enzymes, increased immune response
due to increased production of secretory immunoglobulins A, reduction in serum cholesterol, and
antimutagenic effects.
The beneficial effects of the presence of bifidobacteria in the gastrointestinal tract are dependent on
their viability and metabolic activity. The growth
of bifidobacteria is dependent on the presence of
complex carbohydrates such as oligosaccharides and
other substrates such as N-acetyl glucosamine
and lactulose. These carbohydrates that stimulate
the growth of bifidobacteria are known as ‘bifidogenic factors.’ Some oligosaccharides, due to their
chemical structure, are resistant to digestive enzymes
and therefore pass into the large intestine, where they
become available for fermentation by bifidobacteria.
Compounds that are either partially degraded or
not degraded by the host and are preferentially utilized by bifidobacteria as carbon and energy sources
are defined as ‘prebiotics.’ Some of the bifidogenic
factors that are of commercial significance include
fructo-oligosaccharides, lactose derivatives (such as
lactulose, galacto-oligosaccharides), isomalto-oligosaccharides, xylo-oligosaccharides, gluco-oligosaccharides and soybean oligosaccharides. Resistant
starch and nonstarch polysaccharides are classified
as colonic foods but not as prebiotics because they
are not metabolized by certain beneficial bacteria.
Oligosaccharides have been recognized for their
6257
health benefits in Japan, and many products containing oligosaccharides were developed during the
1990s. Products that contain both prebiotics and
probiotics are referred to as ‘synbiotics.’ An example
of synbiotic includes SymBalance yogurt, which contains inulin as prebiotic and Lb. reuteri, Lb. acidophilus, and Lb. casei as probiotics.
It is not clear how probiotics work. Acidification of
the gut is claimed to be one of the mechanisms.
Breast-fed infants have a much higher percentage of
Bifidobacterium bifidum than formula-fed infants.
The intestinal microflora of the latter group is more
like that of adults; it is a mixed microflora, including
coliform bacteria. Bifidobacteria produce acetic acid,
butyric acid, lactic acid, and pyruvic acid. The lactic
acid and acetic acid account for more than 90% of
organic acids produced. It is widely accepted that
because of acid production by Lb. acidophilus and
bifidobacteria, the enteropathogenic bacteria are
unable to grow. The growth of clostridia and E. coli,
when cocultured with bifidobacteria, has been found
to be inhibited, even at a neutral pH, suggesting that
acid production may not be solely responsible for
inhibition. Metabolites produced by bifidobacteria
may be partly responsible for the inhibition of
pathogens.
0041
Acidophilus and Bifidus Yogurt (AB Yogurt)
Lb. acidophilus and bifidobacteria are normal inhabitants of the intestine of many animals including man.
Lb. acidophilus is Gram positive and rod-shaped,
while bifidobacteria are Gram-positive rods of variable morphology that show branching and pleomorphism. Bifidobacteria were first isolated by
Tissier at the Pasteur Institute, Paris, France, and
predominate the gut flora in breast-fed infants.
Yogurt containing Lb. acidophilus and bifidobacteria has gained popularity in many countries, including Japan, France, Germany, Canada, Australia, and
USA, and more than 70 products containing Lb. acidophilus and bifidobacteria, including sour cream,
buttermilk, yogurt, milk powder, and frozen desserts,
are produced world-wide. It is estimated that about
11% of all yogurt sold in France now contains
Lb. acidophilus and Bifidobacterium spp. In 1978,
the Yakult Co. launched a Bifidus fluid yogurt named
MilmilTM, which contains Bifidobacterium breve,
B. bifidum and Lb. acidophilus. More than 54 different types of milk products containing Lb. acidophilus
and Bifidobacterium bifidum are marketed in Japan.
Lb. acidophilus and Bifidobacterium spp. are
difficult to propagate because of their specific nutritional requirements. Bifidobacteria are not as acidtolerant as Lb. acidophilus, and the growth of
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6258 YOGURT/The Product and its Manufacture
Bifidobacterium species is significantly retarded
below pH 4.0. Lb. acidophilus and Bifidobacterium
spp. are slow acid producers; the slow growth rate of
these organisms can be compensated by adding a
higher level of inoculum, such as 5 or 10%. Yogurt
bacteria are usually added to carry out fermentation.
If pure cultures of Lb. acidophilus and/or Bifidobacterium spp. are used, the time required to reduce the
pH of milk to 4.5 could be as long as 18–24 h at
37 C. When yogurt bacteria (Lb. delbruecckii ssp.
bulgaricus and Sc. thermophilus) and AB (Lb. acidophilus and Bifidobacterium spp.) cultures are used,
the incubation time is about 4 h.
Recent Advances in Probiotic Yogurt
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The most commonly used species in commercial probiotic products are Lb. acidophilus, Lb. casei, Lactobacillus GG (a close relative of Lb. casei subgroup
rhamnosus, ATCC 53103), B. bifidum, B. longum,
B. breve, and B. infantis. Additional blends are also
being investigated, such as Lb. reuteri, Lb. plantarum, and Lb. casei.
In Australia and Europe, yogurt containing
Lb. acidophilus and Bifidobacterium spp. is referred
to as AB yogurt. The recent trend is to incorporate
Lb. casei in addition to Lb. acidophilus and Bifidobacterium spp, and such products are known as ‘ABC
yogurt.’
A wide variety of probiotic cultured products are
now available world-wide such as Nu-Trish a/B, USA
(acidophilus þ bifidus), AB-yogurt, Denmark (acidophilus þ bifidobacteria þ yogurt culture), Miru-Miru,
Japan (Lb. acidophilus þ Lb. casei þ B. breve), Bifighurt (B. longum þ Sc. thermophilus). Products that
contain both prebiotics and probiotics include SymBalance yogurt (Lb. reuteri þ Lb. acidophilus þ Lb.
casei þ bifidobacteria, inulin) produced in Switzerland and Fysiq (Lb. acidophilus þ Raftiline brand
prebiotic).
Several probiotic products contain both yogurt
bacteria and one or more types of probiotic bacteria.
Because of sensitivity to acid, Lb. acidophilus and
Bifidobacterium spp. in yogurt begin to die within a
few days after manufacture because of acid produced
during manufacture and storage. In order to provide
health benefits, the suggested level for probiotic
bacteria is 106 cfu per gram of a product. However,
studies have shown a low viability of probiotics in
market preparations. Many yogurt manufacturers use
a starter culture devoid of Lb. delbrueckii ssp. bulgaricus but a combination of Lb. acidophilus, bifidobacteria and Sc. thermophilus (known as ‘ABT’) as
starter cultures to overcome the postacidification
problem. However, the use of ABT starter culture
increases incubation time significantly as Sc. thermophilus is the main organism responsible for fermentation in ABT starter cultures, and this organism is less
proteolytic than Lb. delbrueckii ssp. bulgaricus. Bifidobacteria are anaerobic and are fastidious organisms, requiring specific growth factors and prefering
a low oxidation–reduction potential for growth.
Cysteine is usually incorporated in the media used
for selective enumeration of bifidobacteria. Cysteine
is an essential growth factor for bifidobacteria and
also reduces the oxidation–reduction potential for
optimum growth of anaerobic bifidobacteria. Dave
and Shah have shown a better viability of bifidobacteria, in yogurt made from ABT starter cultures, with
an additional source of nitrogen, such as acid casein
hydrolysate, than with lowering the oxidation–
reduction potential with cysteine. These authors
have also shown that incorporation of cysteine (at
> 250 mg l1) to lower the redox potential affected
the growth of Sc. thermophilus, as this organism is
considered aerobic. Since Sc. thermophilus is the sole
fermenting organism in ABT starter cultures, the
incubation time increased significantly.
The viability of bifidobacteria in yogurt made with
starter cultures containing both yogurt bacteria was
not affected as much, as Lb. delbrueckii ssp. bulgaricus is proteolytic and provides peptides and amino
acids for the growth of bifidobacteria.
See also: Bifidobacteria in Foods; Casein and
Caseinates: Uses in the Food Industry; Heat Treatment:
Chemical and Microbiological Changes;
Homogenization; Lactic Acid Bacteria; Pasteurization:
Principles; Prebiotics; Probiotics; Starter Cultures;
Sweeteners: Intensive; Others; Yogurt: Yogurt-based
Products; Dietary Importance
Further Reading
Augustin MA, Cheng LJ and Clarke PT (1999) Effects of
preheat treatment of milk powder on the properties of
reconstituted set skim milk yogurts. International Dairy
Journal 9: 415–416.
Dave RI and Shah NP (1997a) Viability of yoghurt
and probiotic bacteria in yoghurts made from commercial starter cultures. International Dairy Journal 7:
31–41.
Dave RI and Shah NP (1997b) Effect of cysteine on the
viability of yoghurt and probiotic bacteria in yoghurt
made with commerical starter cultures. International
Dairy Journal 7: 537–545.
Dave RI and Shah NP (1998) Ingredient supplementation
effects on viability of probiotic bacteria in yoghurt.
Journal of Dairy Science 81: 2804–2816.
Guzman-Gonzalez M, Morais F and Amigo L (2000) Influence of skimmed milk concentrate replacement by dry
dairy products in a low-fat set yoghurt model system.
0049
0050
YOGURT/Yogurt-based Products
Journal of the Science of Food and Agriculture 80:
433–438.
Hekmat S and McMahon DJ (1997) Manufacture and
quality of iron-fortified yogurt. Journal of Dairy Science
80: 3114–3122.
IDF (1996) Oligosaccharides and Probiotic Bacteria.
Bulletin 313. Brussels: International Dairy Federation.
Kosikowski FV and Mistry VV (1997) Cheese and
Fermented Milk Foods. Westport, CT: FV Kosikowski.
Kurmann JA and Rasic JLJ (1991) Health potential of
products containing bifidobacteria. In: Robinson RK
(ed.) Health Potential of Products Containing Bifidobacteria: Therapeutic Properties of Fermented Milks.
London: Elsevier Applied Science.
Mazza G (ed.) (1998) Functional Foods: Biochemical
and Processing Aspects. Lancaster, PA: Technomic
Publication.
Modler WH (1994) Bifidogenic factors – sources, metabolism and applications. International Dairy Journal 4:
383–407.
Ordonez GA, Jeon IJ and Roberts HA (2000) Manufacture
of frozen yogurt with ultrafiltered milk and probiotic
lactic acid bacteria. Journal of Food Processing and
Preservation 24: 163–176.
Ravula RR and Shah NP (2000) Influence of water activity
on fermentation, organic acid production and viability
of yoghurt and probiotic bacteria. Australian Journal of
Dairy Technology 55: 127–131.
Robinson RK (ed.) (1986) Modern Dairy Technology,
vol. II. Advances in Milk Products. London: Elsevier
Applied Science.
Shah NP (2000) Probiotic bacteria: selective enumeration
and survival in dairy foods. Journal of Dairy Science 83:
894–907.
Shah NP, Lankaputhra WEV, Britz M and Kyle WSA (1995)
Survival of L. acidophilus and Bifidobacterium bifidum
in commercial yoghurt during refrigerated storage.
International Dairy Journal 5: 515–521.
Tamime AY and Deeth HC (1980) Yoghurt: technology and
biochemistry. Journal of Food Protection 43: 939–977.
Tamine AY and Robinson RK (1999) Yoghurt: Science and
Technology. Cambridge: Woodhead Publishing Ltd.
Yogurt-based Products
A Y Tamime, Ayr, UK
Copyright 2003, Elsevier Science Ltd. All Rights Reserved.
Introduction
0001
The transformation of milk into yogurt, with a starter
culture composed of Streptococcus thermophilus and
Lactobacillus delbrueckii ssp. bulgaricus, can easily
extend the shelf-life of milk from a few days to about
3 weeks. However, the starter cultures employed in
the manufacture of biofermented milks including
6259
yogurt-related products have been reviewed recently.
(See Yogurt: The Product and its Manufacture.) The
main organisms belong to the following genera: Lactobacillus, Bifidobacterium, Enterococcus, and Pediococcus. The application of the different preservation
techniques known to humans for thousands of years
could extend the shelf-life of yogurt to a few months,
or even indefinitely. Examples of such postfermentation processes include heating, concentration, freezing, and/or drying, but it is evident that such
treatments will alter the characteristics of the end
product. Some of these processes are still carried out
using traditional methods, but limited data are available in the literature; however, some of these processes
have been mechanized and developed by industrial
organizations, and as a consequence, sometechnical
data are somewhat limited. (See Starter Cultures.)
Methods of Manufacture
Pasteurization
Traditionally, natural yogurt in rural areas in the
Middle East is heated gently for a few hours over
low fires using a special wood, and the product is
known as ‘laban mudakhan’ or ‘smoked yoghurt.’
The application of heat inactivates the starter culture
organisms and their enzymes, as well as other
contaminants, e.g., undesirable bacteria, yeasts, and
molds. The use of such methods enables the nomads
in that region to extend the shelf-life of yogurt for a
few weeks, or until they reach a market to sell their
product. Alternatively, the hot yogurt is placed in a
clean jar and covered with a layer of olive oil or
tallow, so that the ‘smoked yogurt’ is preserved over
the winter months.
In mechanized plants, natural/plain, fruit, flavored,
or drinking yogurts are subjected to heat treatment
after the fermentation stage to prolong the shelf-life
of the product. The time–temperature relationships,
which are used to achieve this objective, are dependent on: (1) the level of acidity, (2) the method of
heating and packaging, and (3) the storage conditions.
Set-type yogurt is heated between 60 and 85 C for up
to 50 min, and in some instances, the heating is carried
out under a pressure of about 0.2 MPa. The other
types of yogurt are pasteurized/ultrahigh temperature
(UHT), which is heated at 65 to > 100 C for up to
50 s. In general, the heating and packaging methods
can be divided into the categories of pasteurized/UHT
yogurts given in Table 1. (See Heat Treatment: Ultrahigh Temperature (UHT) Treatments.)
Postfermentation heating of yogurt causes
the separation of the aqueous phase from the suspended casein particles. This is mainly due to the
0002
0003
0004
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