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Corrieu G., and Béal C. (2016) Yogurt: The Product and its Manufacture. In: Caballero, B., Finglas, P., and
Toldrá, F. (eds.) The Encyclopedia of Food and Health vol. 5, pp. 617-624. Oxford: Academic Press.
© 2016 Elsevier Ltd. All rights reserved.
Yogurt: The Product and its Manufacture
G Corrieu, Bioval Process Co., Canet en Roussillon, France
CBe
´al, AgroParisTech INRA, Thiverval-Grignon, France
ã2016 Elsevier Ltd. All rights reserved.
Introduction
An ancestral version of yogurt probably appeared 9000 or 8000
years BC in Mesopotamia and Egypt and subsequently spread
in the northeast of Africa, in the Middle East, in Central Asia,
and later in Balkan countries, offering a large variety of
‘fermented milks.’ Originally, yogurt resulted from a spontane-
ous, accidental lactic acid fermentation leading to the acidifi-
cation and coagulation of milk, thus resulting in an efficient
way to preserve this raw material otherwise vulnerable to spoil-
age. Along the following centuries, the knowledge for making
homemade yogurts and fermented milks spread all over the
world. The first attempts to produce yogurt industrially were
performed in Barcelona in 1919 by Isaac Carasso. Since the
1950s, the production of yogurt has grown significantly thanks
to the intervention of several powerful agrofood groups setting
up international activities. If ‘set-type’ and ‘stirred’ yogurts are
the basic products, a large number of new recipes and processes
have been created to diversify the products and address market
demands. These alternations include the modification of the
texture, resulting in drinking, frozen, and concentrated yogurts;
modification of aroma and taste by addition of fruit, jam, and
aroma compounds; modification of nutritional properties by
changing the fat and sugar contents; and development of
health-related elements with the use of probiotic strains or
the inclusion of vitamins or phytostanols. The coverage of
these issues has been contributing to a significant expansion
of the overall consumption of yogurt and fermented milks
worldwide.
As a result, the industrial production of yogurt and fermen-
ted milks in the world reached about 32–35 million tons in
2012, for an expected global market in 2015 of about $67
billion. In Europe (27 countries), the annual production was
around 9.3 million tons, with Germany, France, and Spain
being the three main producers covering 20%, 18%, and 9%
of the European production, respectively. For the last 5 years in
Europe, the total production increased by 10%, whereas for the
last 30 years in France, the increase was of 38%. Besides
Europe, the main producers of yogurt are China, Russia, Iran,
and the United States, with 4.0, 2.5, 2.2, and 2.0 million tons
per year, respectively. Production in South America and Cen-
tral America and in Africa remains low with 1.5 and 0.4 million
tons per year, respectively.
Definition and Main Types of Yogurts
Definition of Yogurt and Probiotic Fermented Milks
In essence, yogurts are fermented dairy products obtained from
lactic acid fermentation by two species of lactic acid bacteria,
that is, Streptococcus thermophilus and Lactobacillus delbrueckii
subsp. bulgaricus. This fermentation leads to acidification and
milk coagulation, without addition of rennet (as in cheese),
and allows an increase of the shelf life as a result of the low pH.
The sensory properties of yogurts rely on three main characters:
(1) the composition of milk as raw material, which differs
according to the milk source (e.g., cow, goat, or sheep; con-
ventional or organic) and the fat content that can be adjusted
to obtain full-fat, low-fat, or nonfat products; (2) the addition
of ingredients that allow modifying the sensory properties
(flavor, color, and texture) of the products, such as sweetening
agents (sugar or other sweeteners for low-calorie products),
flavoring agents (fruit aromas or vanilla) or fruits (small pieces
enriched with sugar or jam), stabilizers (pectin, starch, or
gelatin), or emulsifiers; and (3) the technology employed for
the manufacture, which may vary depending on the operations
during milk pretreatment (fat and nonfat solid standardiza-
tion, homogenization, or heat treatment) or yogurt post-
treatment (stirring, concentration, mixing, cooling, drying, or
freezing).
Even if yogurt is the most consumed within the wider
family of fermented milks, many probiotic fermented milks
are traded around the world. They involve probiotic bacteria,
which are defined according to the FAO/WHO in 2011 as ‘live
microorganisms that, when administered in adequate
amounts, confer a health benefit on the host.’ As European
regulation has stated that these health benefits have to be
demonstrated before using the probiotics in foods and supple-
ments, the health claims are not anymore maintained. In other
countries, and depending on the country regulation, the fol-
lowing health benefits are recognized: reduction of lactose
intolerance, prevention of microbial infections, stimulation
of the immune system, and regulation of immune responses.
Main Types of Yogurts and Fermented Milks
Yogurt diversification is reflected by various textures (set-type
or firm, stirred, drinking, frozen, concentrated, or powder
yogurts), numerous flavors (natural, sweetened, flavored, or
with added pieces of fruits or honey), and diverse shelf life and
nutritional (fat content and residual lactose content) proper-
ties. Figure 1 proposes a synthetic scheme of the several char-
acters that lead to yogurt diversity.
Classification of fermented milks is also based on their
potential health properties. They are mainly related to the
decrease in lactose content in the product, as a result of lactic
acid production. Some strains of S. thermophilus and
L. delbrueckii subsp. bulgaricus are able to survive in the gastro-
intestinal tract, thus being able to play a role in the gut health.
Finally, the development of US versions of Greek yogurt is
associated to high protein but low carbohydrate contents,
thus contributing to the weight management of consumers.
Encyclopedia of Food and Health http://dx.doi.org/10.1016/B978-0-12-384947-2.00766-2 617
The Encyclopedia of Food and Health, (2016), vol. 5, pp. 617-624
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Microbiological and Biochemical Mechanisms
Involved in Yogurt Manufacture
Microbiological Characteristics of Yogurts
The two thermophilic lactic acid bacteria, S. thermophilus and
L. delbrueckii subsp. bulgaricus, which trigger yogurt fermenta-
tion, are considered as ‘Generally Recognized as Safe’ in the
United States and possess the ‘Qualified Presumption of Safety’
status in Europe, as a consequence of a long history of safe use
in food and an absence of pathogenicity. They are Gram-
positive, anaerobic, aerotolerant, and catalase-negative, do
not form spores, and have less than 55% GþC content in
their DNA. They are able to grow between 42 and 50 C, but
not at 10 C. S. thermophilus forms linear chains of rods,
whereas L. delbrueckii subsp. bulgaricus grows as ovoid cells.
They convert lactose into galactose that is not metabolized and
glucose that is fermented predominantly to lactic acid, thus
corresponding to homofermentative metabolism.
In milk, these two species demonstrate a positive interac-
tion called protocooperation, which is mutually favorable.
This phenomenon induces a more rapid growth and acidifica-
tion, higher production of aroma compounds and exopolysac-
charides, and more pronounced proteolysis. An upregulation
of biosynthesis pathways for nucleotides and sulfur-containing
amino acids is also observed. Growth of S. thermophilus is
promoted by free amino acids and small peptides that arise
from milk proteins by the action of the cell wall protease PrtB
of L. delbrueckii subsp. bulgaricus. In return, L. delbrueckii subsp.
bulgaricus is stimulated by formic acid, folic acid, and CO
2
that
are synthesized by S. thermophilus in milk. As a consequence of
this interaction, growth of S. thermophilus starts first by using
the nitrogen compounds and stops early as this species is very
sensitive to lactic acid inhibition. Growth of L. delbrueckii
subsp. bulgaricus begins later but is prolonged even at low
pH, due to the better resistance of this species to acidity.
Probiotic bacteria involved in fermented milk production
other than yogurt include different lactobacilli and bifidobac-
teria. The main bacterial species found in commercial products
are Lactobacillus acidophilus,L. casei,L. paracasei,L. rhamnosus,
Bifidobacterium animalis subsp. lactis, and B. breve. These bacte-
ria demonstrate numerous interactions with the classical yog-
urt cultures, which depend on specific strain associations.
Biochemical and Physicochemical Changes During Lactic
Acid Fermentation
Growth of lactic acid bacteria in milk induces many changes
that are desirable in yogurt. These changes include the synthe-
sis of different metabolites (lactic acid, exopolysaccharides,
and aroma compounds) and the modification of the texture
and the nutritional value of the product.
Lactic acid production
As a result of glycolysis, one mole of lactose is theoretically
transformed into one mole of galactose and two moles of lactic
acid, together with the production of intracellular energy in the
form of two moles of ATP. S. thermophilus,L. delbrueckii subsp.
bulgaricus, and L. acidophilus operate these reactions according
to a homofermentative metabolic pathway that is summarized
in Figure 2.
Lactose is first internalized into the cell with the help of a
lactose permease energized by a proton gradient. Intracellular
lactose is then hydrolyzed into glucose and galactose by the
enzyme a-galactosidase. Glucose is catabolized to pyruvate via
the glycolytic pathway (Embden–Meyerhof–Parnas), whereas
galactose is excreted out of the cell. Pyruvate is then reduced
into lactic acid through a lactate dehydrogenase, together with
the reoxidation of NADH formed earlier. Two different isomers
are synthesized: L(þ) lactic acid by S. thermophilus and D()
lactic acid by L. delbrueckii subsp. bulgaricus. Finally, the intra-
cellular lactate is excreted out of the cell via a symport with
protons, thus inducing acidification of extracellular medium
and progressive inhibition of bacterial growth.
Other lactobacilli and bifidobacteria may use hetero-
fermentative pathways to produce their intracellular energy
together with lactic acid. L. rhamnosus, L. casei, and
L. paracasei produce one mole of each lactic acid, ethanol,
CO
2
, and ATP from one mole of glucose. Bifidobacteria syn-
thesize three moles of acetic acid and two moles of (Lþ) lactic
acid and ATP, without generation of CO
2
from two moles of
glucose. These different pathways induce different acidification
Yoghurt
diversity
Flavor:
Plain
Sweetened
Flavored
Added with fruits
Texture:
Firm
Stirred
Concentrated
Drinking
Frozen
Powder
Storage :
Chilled (4 °C)
Dehydrated (Ambient)
Frozen (–20 °C)
Fat content:
Not-fat
Low-fat
Full-fat
Type of milk:
Cow
Goat
Sheep
Conventional
Organic
Figure 1 Classification scheme for yogurts and fermented milks.
Lactose
Lactose
Glucose + Galactose
Pyruvate
Lactic acid
2 ADP
2 ATP
1 NADH
1 NAD+
Lactic acid
Galactose
Extracellular mediumIntracellular medium
Figure 2 Simplified scheme of metabolic reactions involved in
homofermentative metabolism in yogurt bacteria.
618 Yogurt: The Product and its Manufacture
The Encyclopedia of Food and Health, (2016), vol. 5, pp. 617-624
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rates and, consequently, different sensory properties of the
final product.
Proteolysis
During milk fermentation, the proteolytic system of lactic acid
bacteria degrades caseins into peptides and free amino acids
that are essential to bacterial growth and participate in the
generation of flavor. L. delbrueckii subsp. bulgaricus possess
the cell surface proteinase PrtB that is highly active and pro-
motes fast growth and rapid acidification of milk. In contrast,
even if a few strains of S. thermophilus possess the cell wall
proteinase PrtS, most strains are protease-negative. Growth of
S. thermophilus is nevertheless effective in mixed cultures, since
available nitrogen compounds are supplied from the protein-
ase of L. delbrueckii subsp. bulgaricus.
Flavor compound production
Yogurt taste is mainly characterized by an acid character due to
the presence of lactic acid in the product. Yogurt aroma is
characterized by about hundred volatile compounds that con-
sist of carbonyl compounds (mainly acids and esters), alco-
hols, and heterocyclic and sulfur-containing compounds.
Among them, acetaldehyde is the major flavor compound of
yogurt, where it confers a pleasant fresh and fruity aroma. It is
produced by the lactic acid bacteria at a final concentration
comprised between 5 and 40 mg kg
1
. Most of the acetalde-
hyde is directly synthesized from pyruvate with the aid of
pyruvate decarboxylase or indirectly from acetyl coenzyme A,
through the action of pyruvate dehydrogenase and aldehyde
dehydrogenase. L. delbrueckii subsp. bulgaricus is also able to
convert threonine into acetaldehyde and glycine, through the
action of serine hydroxyl-methyl transferase. In addition,
S. thermophilus produces a-acetolactate that is partially metab-
olized into diacetyl or acetoin through the action of a-
acetolactate decarboxylase that allows regulating leucine and
valine biosynthesis.
Coagulation
Acidification of milk leads to coagulation as a result of desta-
bilization of the casein micelles. The mechanism relies on two
concomitant phenomena. During acidification, the net nega-
tive charge on casein micelles decreases, thus reducing electro-
static repulsion between charged groups. In the same time, the
colloidal calcium–phosphate complex is solubilized, which
results in the depletion of calcium in the micelles. Then, elec-
trostatic and casein–casein attractions increase due to
enhanced hydrophobic interactions. When the isoelectric
point of caseins (pH 4.6) is achieved, coagulation occurs as a
result of the formation of a three-dimensional network con-
sisting of clusters and chains of caseins, which leads to the
formation of the yogurt gel.
Exopolysaccharide production
Some strains of lactic acid bacteria contribute to the physical
properties of stirred fermented milks through biosynthesis of
extracellular polysaccharides (EPS), which are either homo- or
heteropolysaccharides. These polymers are composed of
several hundreds to thousands of repeating units of monosac-
charides such as D-glucopyranose, D-fructofuranose, D-glucose,
D-galactose, L-rhamnose, N-acetyl-D-galactosamine, and
N-acetyl-D-glucosamine, withmolecular masses ranging between
410
4
and 6 10
6
Da. S. thermophilus and L. delbrueckii subsp.
bulgaricus produce EPS during growth, to final concentrations
comprised between 30 and 600 mg l
1
in milk.
Biosynthesis of exopolysaccharides is governed by well-
characterized gene clusters. It involves the production of
precursors that are formed in the cytoplasm, related to the
sequential addition of activated carbohydrates (UDP-glucose,
UDP-galactose, and dTDP-rhamnose) by specific glycosyl-
transferases. These repeating units are coupled to lipid carriers
and translocated across the membrane before polymerization.
Changes in nutritional value
Nutritional characteristics of yogurts differ from those of milk
by the three main following aspects: (1) as a consequence of
lactic acid production, the lactose content is lower in yogurt
(30gl
1
) than in milk (50 g l
1
), which is important for
lactose-intolerant people, by limiting the formation of organic
acids, hydrogen, methane, and carbon dioxide in the human
gut; (2) the levels of calcium and potassium are higher in
yogurt than in milk (200 and 255 mg 100 g
1
, respectively),
as a consequence of nonfat solid adjustment; and (3) the
concentration of folic acid is higher in yogurt than in milk.
The latter essential vitamin is biosynthesized by S. thermophilus
in the range of 20–150 mgl
1
, during which it is consumed by
L. delbrueckii subsp. bulgaricus, thus leading to an average net
level in yogurt of 80 mgl
1
, which exceeds the one in milk
(40 mgl
1
).
Yogurt Manufacture
General Diagrams of Yogurt Manufacture
The industrial manufacture of yogurts is organized along three
main steps: (1) the preparation of the mix and all correspond-
ing physical treatments such as homogenization, heat treat-
ment, cooling, and deaeration; (2) the fermentation process
starting after inoculation of the mix; and (3) the yogurt harvest-
ing, post-treatment, and packaging. Depending on the steps
performed, at least four types of yogurt can be considered,
whose manufacture is presented in Figure 3. One has to notice
that each step of the manufacture affects the final quality of the
yogurts and that, except for set-type yogurts, the product fla-
voring and the cup filling are performed after fermentation.
Preparation of the Mix
Milk standardization
In order to obtain the mix to be fermented, milk preparation
involves mainly fat and protein content standardization and
optional addition of sweeteners and stabilizers. Fat standardi-
zation consists of fat removal by centrifugation (at about
55 C), followed by cream reincorporation to reach the tar-
geted fat content, ranging from nonfat (0.01%), to low- or
light-fat (1–2%), to whole-fat yogurts (>3.2%). Protein stan-
dardization aims at increasing the protein content of the mix
(from 3% to 5–15%) in order to improve the yogurt firmness
(texture) and reduce its syneresis. It is mostly done by addition
of milk powder, which is the easier and traditional way. The
use of milk proteins or milk replacers as caseinates or whey
Yogurt: The Product and its Manufacture 619
The Encyclopedia of Food and Health, (2016), vol. 5, pp. 617-624
Author's personal copy
powders is also common. A complete mixing of the dry ingre-
dients without air incorporation is recommended. Concentra-
tion of milk by membrane processes (ultrafiltration and
reverse osmosis) is an alternative method to increase the pro-
tein content of the mix.
For some yogurt recipes, sugars or other sweetening agents
are added to the mix, generally after the physical treatments
described in the succeeding text. In some countries, the use of
thickeners and stabilizers (gelatin, pectin, xanthan gum, carra-
geenan, starch, etc.) at concentrations varying from 5% to 10%
is allowed by FAO/WHO to improve the yogurt texture.
Physical treatments of the mix
Heat treatment is an essential step of the mix preparation. It
allows removing spoilage microorganisms, inactivating lacto-
peroxidases and producing stimulatory compounds in milk. In
parallel, heat treatment contributes to improved yogurt texture
by allowing whey protein denaturation and interaction with
casein, resulting in a decrease of gel syneresis and an increase of
gel firmness. During industrial yogurt manufacture, the mixes
are generally heated at 90 or 95 C for 3–7 min before cooling
down to fermentation temperature. Plate heat exchangers, with
a tubular holding zone, are generally used and are designed in
order to cool the mix accurately at the fermentation tempera-
ture (between 37 and 43 C).
Two other physical treatments of the mix, deaeration and
homogenization, are closely associated with the heat
treatment, and the design of the heat exchangers takes into
account the temperature favoring their effect. Homogenization
is compulsory for yogurt quality, as it increases the gel texture
and reduces syneresis. It provokes a reduction of the size of the
fat globules (near 2 mm) and a better link between fat and
hydrophilic proteins. Homogenization of the mix is done at
high pressure (20 or 25 MPa) and at a temperature close to
70 C. Associated with the heat treatment of the mix, it takes
place just after the holding section of the heat exchanger.
Double-stage high-pressure homogenizers are recommended
for high-fat yogurts. Vacuum deaeration of the mix is per-
formed at large industrial scale to reduce its oxygen content
and consequently shorten the fermentation time, as to improve
the yogurt texture and to remove off-flavors. This step is gen-
erally performed at 70 C, before homogenization.
The Fermentation Process
Inoculation of the mix
At industrial scale, yogurts are prepared through inoculation of
the mix with concentrated starter cultures of the two yogurt
bacteria (S. thermophilus and L. delbrueckii subsp. bulgaricus).
The commercial starter cultures are composed of specific
Set-T
y
pe Stirred
Fermentation in cup (42 ⬚C)
Fermentation in tank (42 ⬚C)
Filling (10 ⬚C, aseptic conditions)
Cooling (18–25 ⬚C)
Mixing of the coagulum
Filling (10 ⬚C)
Reception and storage of milk (4 ⬚C)
Fat standardization (not-fat, low-fat, full-fat)
Protein standardization (5–15%)
Homogenization (20–25 Mpa at 70 ⬚C)
Deaeration (70 ⬚C)
Inoculation (106–107 CFU ml–1)
Cooling to incubation temperature (42 ⬚C)
Flavoring
Flavoring
Drinkin
g
Homogenization
Flavoring
Concentrated
Concentration
Cooling at 10–12 ⬚C
Heat treatment (90–95 ⬚C, 3–7 min)
Cooling (5 ⬚C), cold storage, transportation, delivery
Figure 3 Schematic diagram of the production processes of set-type, stirred, drinking, and concentrated yogurts.
620 Yogurt: The Product and its Manufacture
The Encyclopedia of Food and Health, (2016), vol. 5, pp. 617-624
Author's personal copy
blends of selected and well-defined strains, at a concentration
higher than 10
10
colony-forming units (CFU)g
1
, and are
preserved as frozen or freeze-dried formulations. The inocu-
lated mix contains generally 10
6
–10
7
CFU ml
1
of bacteria.
After mixing, it is transferred to the fermentation tanks (for
stirred, drinking, or concentrated yogurt manufacture) or
directly to the packaging machine for fermentation in cups
(for set-type yogurt manufacture).
Fermentation step
During the lactic acid fermentation of milk, numerous param-
eters vary as a function of time, as shown in Figure 4. The
growth of S. thermophilus occurs first, followed by that of
L. delbrueckii subsp. bulgaricus, reaching final concentrations
close to 10
9
CFU g
1
(Figure 4(a)). The consumption of lac-
tose and nitrogenous compounds permits the growth of both
strains and leads to the accumulation of many relevant metab-
olites. Lactic acid, galactose, acetaldehyde, and exopolysacchar-
ides are the most important ones, contributing to flavor and
texture of the yogurt (Figure 4(b)). The synthesis of extracel-
lular lactic acid provokes an acidification of the mix character-
ized by a decrease of the pH (Figure 4(a)), the coagulation of
proteins, and the subsequent gel formation. Acetaldehyde con-
fers to yogurt its particular aroma, and exopolysaccharides
contribute to its texture.
The acidification process is controlled by the final pH of
the yogurt and the acidification rate, which are key factors to
master quality. The fermentation is stopped (by a fast cooling
of the product) when the final pH of the yogurt is reached.
The targeted final pH varies from 4.8 to 4.5, as a function of
the type of yogurt. A significant postacidification during the
yogurt’s cooling, harvesting, and storage has to be considered
in defining this target. Generally, online measurement of pH
of the mix is avoided, because the glass pH probes may break
inside the mix and need to be submitted to cumbersome
protocols of cleaning and calibration. Consequently, only
manual sampling is done during the acidification process to
allow offline pH measurements, and the decision to stop the
fermentation by cooling requires a good expertise of the
process. The acidification rate acts directly on the fermenta-
tion time, so that its knowledge and control are very impor-
tant to properly schedule industrial production. It is
influenced by various factors, such as starter composition
and activity, mix composition and physical treatments, and
fermentation temperature. However, accurate temperature
control is quite impossible during yogurt fermentation
because of the coagulation phenomenon that occurs at
about pH 5.2. As a consequence, the fermentation time can
vary in important ranges. For probiotic yogurt, fermentation
time can reach 6–8 h, whereas for stirred yogurt, a 3–4 h
0
2
4
6
8
10
0
10
20
30
40
50
01234
(
g
l–1)
Galactose and lactic acid concentrations
Lactose and acetaldehyde concentrations
(g l–1)
Fermentation time (h)
Lactose
Acetaldehyde
Lactic acid
Galactose
3
4
5
6
7
1,E+06
(a)
(b)
1,E+07
1,E+08
1,E+09
1,E+10
01234
pH
Bacterial concentrations (CFU ml–1)
Fermentation time (h)
S. thermophilus
L. bulgaricus
pH
Figure 4 Growth of S. thermophilus and L. delbrueckii subsp. bulgaricus and milk acidification (a) and evolution of lactose, galactose, lactic acid, and
acetaldehyde concentrations (b) as a function of fermentation time.
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Author's personal copy
process is the common target. However, industrial manufac-
turers have difficulties to master perfectly all the parameters of
the process, and longer fermentation times (5 or 8 h) are
frequently attained.
The fermentation of set-type yogurts is generally performed
in cabinets, incubation rooms, or large tunnels in which the
pallets move forward gradually with forced ventilation of
warm air. The fermentation of stirred yogurts is performed in
large tanks (15–20 m
3
for the largest ones) equipped with
mixing devices for mix homogenization, starter mixing, and
gel breaking after fermentation.
Yogurt Harvesting and Packaging
Cooling and harvesting of yogurt
The first step in yogurt harvesting corresponds to a fast cooling
of the product in order to stop its acidification. It takes place
when the required final pH of yogurt is obtained. Set yogurts
are cooled within 1 or 2 h to 4 or 5 C using cold air in
ventilated cabinets, cooling rooms, or tunnels, as a function
of the size of the manufacturing unit. For stirred yogurt, the
cooling is performed in an external heat exchanger reaching an
intermediate temperature (between 18 and 25 C) in less than
1 h (20–60 min for industrial tanks). At this temperature,
some additives as aroma compounds, sweeteners, and fruits
(jam, pulp, and pieces) can be added to stirred yogurts. In
modern large plants, these additions are generally performed
online at the level of the packaging machine, using metering
pumps and mixers.
The final texture of yogurts, especially stirred ones, is a
critical factor for consumer acceptance. As the texture is influ-
enced by many factors (mix composition, strains used, and
processing conditions), it is a real challenge to obtain the
targeted texture. The mechanical constraints exerted on stirred
yogurt by all the harvesting devices (pumps, heat exchangers,
pipes, mixers, filling machine, etc.) tend to reduce its texture
but can give them some smoothness.
Packaging of yogurt
Yogurt packaging ensures its hygiene and protection during
distribution. If plastic and glass cups are always used for set-
type yogurts, large up-to-date packaging units use the
‘form–fill–seal’ technology. The same packaging machine real-
izes the three main following operations: (1) the thermo-
formation of the containers at 150–200 C, using multilayer
thermoplastic materials, (2) the filling of the preformed con-
tainers under a closed environment and sterile air overpressure,
and (3) the thermosealing of the filled containers with an
aluminum lid labeled to deliver product information. These
high-tech packaging machines allow reaching high security
and high capacity (up to 70 000 cups per hour) standards.
Consequently, they correspond to the most expensive invest-
ment in an industrial manufacturing unit of yogurt.
Overpackaging is then carried out, in the form of multipacks
of 2, 4, 8, or 16 cups, with the help of an automatic tray packer.
After packaging and overpackaging, the yogurts are stored at
low temperature (4 or 5 C), which is maintained during
transportation and commercialization. This low temperature
maintenance permits limiting the postacidification in the prod-
ucts and preserving their safety.
Industrial Design of Yogurt Manufacturing Units
All equipments used for milk storage, mix preparation, fermen-
tation and yogurt cooling, and harvesting and packaging are
especially designed to allow for the cleaning in place (CIP)
procedures commonly used in dairy industry. These proce-
dures assume the existence of a CIP kitchen in the factory to
automatically provide the cleaning mixtures at the right tem-
perature and for the right duration.
Yogurt fermentation is a batch process, but some opera-
tions such as mix preparation and treatment and yogurt cool-
ing and packaging are designed and managed as continuous or
semicontinuous processes. In industrial manufacturing units,
automation and process control systems are more and more
popular. They encompass (1) sensors that essentially measure
physical parameters such as temperature, pressure, level, and
weight; (2) programmable logic controllers controlling valves,
pumps, and motors that permit the regulation of the main
process parameters; and (3) computer supervision that allows
traceability. Nevertheless, as an accurate control of the yogurt
acidification rate remains limited, optimization of yogurt man-
ufacture is not possible.
Finally, even if the acidity of yogurt contributes to its safety,
industrial manufacturers observe good manufacturing prac-
tices to control the microbial risk. They act mainly on food
contact surfaces that have to be cleaned and sanitized before
use and impose clothing and hair covering for the staff.
Control of Yogurt Quality
Yogurt quality requires controls of the raw materials, during
the course of the manufacturing process and on the final
products.
Quality Controls of Raw Materials
Microbiological controls are carried out on raw materials, in
particular fresh milk, powder milk, fruits, sweeteners, and
starters. Somatic cell counts are also verified on fresh milk. In
addition, many physicochemical properties are checked: (1)
temperature, titratable acidity, and fat and protein contents of
the fresh milk; (2) the absence of antibiotics, solubility,
moisture, and fat content of the milk powder; and (3) pH,
viscosity, and Brix of the added fruits. Acidification activity of
starter cultures is also assessed, mostly by using the Cinac
system that allows determining various quantitative kinetic
descriptors such as the absolute value of the maximum acidi-
fication rate (Vm, in min
1
) and the time (tm, in min) neces-
sary to reach Vm. The lower the value of tm and the higher
the value of Vm, the higher the acidification activity will be.
Figure 5 shows typical acidification curves and corresponding
first derivatives (kinetics) of two yogurt starter cultures growing
in milk and displaying low (Vm ¼1.33 pH unit h
1
;
tm¼2.55 h) or high acidification activity (Vm ¼1.44 pH
unit h
1
;tm¼2.05 h).
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Controls During the Process
During yogurt manufacture, controls are accomplished to
ensure repeatability of the productions and maximal levels of
quality and food safety of the products. They refer mainly to
the control of temperature (in fermentation tanks, heat
exchangers, incubation rooms, and cooling systems), pH (by
sampling either in the fermentation tanks or directly in cups),
and duration of the different steps of manufacture.
In addition to these controls, the use of food safety man-
agement systems such as ISO 22000 or International Food
Standards is requested to control foodborne safety hazards
and guarantee the products’ safety. By implementing Hazard
Analysis and Critical Control Point systems according to the
Codex, three steps are identified as critical control points for
microbiological hazards during yogurt manufacture: pasteuri-
zation of milk, refrigerated packaging, and cold storage. In
addition, packaging is also related to physical hazards.
Quality Controls of the Final Product
Various controls are performed on the final products at the end
of their manufacture and during their shelf life. The frequency
of sampling is defined by each dairy factory, as stated by its
own good hygiene practices.
Counts of S. thermophilus and L. delbrueckii subsp. bulgaricus
are controlled to verify that the targeted value of 10
7
CFU g
1
at shelf life is achieved. The presence of spoilage and patho-
genic microorganisms, including Listeria monocytogenes,Salmo-
nella spp., coliforms, yeasts, or molds, is also checked.
Physicochemical analyses consist of the verification of the
fat and total solid contents, the titratable acidity or the pH of
the products, and the assessment of some texture parameters
including firmness, consistency, or viscosity, depending on the
type of yogurt (set-type, stirred, or drinking yogurt). Sensory
evaluations are completed with experienced panelists. The fol-
lowing main sensory attributes are generally employed:
appearance (syneresis and color), texture (palatability, firm-
ness, and consistency), aroma and odor, and taste and after-
taste (freshness, acidity, and persistency).
Conclusion
Yogurt is an ancient traditional fermented food that has
known, since half a century, a tremendous industrialization
of its manufacturing conditions. In more recent years, an
intense innovative diversification of recipes and products
occurred, which partly explain the increase of their consump-
tion. Although artisan production schemes persist, including
homemade yogurt manufacture, the industrial manufacture of
yogurt is now well established. Breakthrough innovations are
limited and the improvements in process productivity relate
mainly to the design and the management of the manufactur-
ing units. Nevertheless, the interesting nutritional properties of
yogurts and more generally of fermented milks, their diversity,
and the opening and increasing of new markets offer impor-
tant perspectives for their development at world scale.
See also: Bifidobacteria in Foods: Health Effects;Fermented Foods:
Use of Starter Cultures;Lactic Acid Bacteria;Packaging: Aseptic Filling;
Probiotics;Rheological Properties of Food Materials;Yogurt: Dietary
Importance;Yogurt: Yogurt Based Products.
Further Reading
Aureli P, Capurso L, Castellazzi AM, et al. (2011) Probiotics and health: an evidence-
based review. Pharmacological Research 63: 366–376.
Be
´al C and Helinck S (2014) Yogurt and other fermented milks. In: Ray RC and
Montet D (eds.) Microorganisms and fermentation of traditional foods,
pp. 139–185. Boca Raton, FL: CRC Press.
Benezech T and Maingonnat JF (1993) Flow properties of stirred yogurt: structural
parameter approach in describing time-dependency. Journal of Texture Studies
24: 455–473.
Cheng H (2010) Volatile flavor compounds in yoghurt: a review. Critical Reviews in
Food Science and Nutrition 50: 938–950.
Corrieu, G., Spinnler, H. E., Jomier, Y. and Picque, D. (1988). Automated system to
follow up and control the acidification activity of lactic acid starters. Fr. Pat.
2 629 612.
Courtin P, Monnet V, and Rul F (2002) Cell-wall proteinases PrtS and PrtB have a
different role in Streptococcus thermophilus/Lactobacillus bulgaricus mixed
cultures in milk. Microbiology 148: 3413–3421.
4
4.5
5
5.5
6
6.5
7
012345
pH
Time (h)
–
1.6
–
1.4
–1.2
–1
–0.8
–0.6
–0.4
–0.2
0
012345
dpH/dt (h
–1
)
Time (h)
Figure 5 Acidification activity of two yogurt starter cultures, displaying high acidification activity (—) and low acidification activity (—) determined
using the Cinac system.
Yogurt: The Product and its Manufacture 623
The Encyclopedia of Food and Health, (2016), vol. 5, pp. 617-624
Author's personal copy
FAO/WHO (2001) Health and nutritional properties of probiotics in food including
powder milk with live lactic acid bacteria: report of a joint FAO/WHO expert
consultation. Cordoba, Argentina: FAO/WHO.
FAO/WHO (2011) Codex Alimentarius: codex standards for fermented milks 243-2003.
In: FAO/WHO (ed.) Milk and milk products, 2nd ed., pp. 6–16. Rome: FAO/WHO.
Lee WJ and Lucey JA (2010) Formation and physical properties of yogurt. Asian-
Australasian Journal of Animal Sciences 23: 1127–1136.
Ruas-Madiedo P and de los Reyes-Gavila
´n CG (2005) Methods for the screening,
isolation, and characterization of exopolysaccharides produced by lactic acid
bacteria. Journal of Dairy Science 88: 843–856.
Saint-Eve A, Levy C, Le Moigne L, Ducruet V, and Souchon I (2008) Quality changes in
yogurt during storage in different packaging materials. Food Chemistry
110: 285–293.
Shiby VK and Mishra HN (2013) Fermented milks and milk products as functional foods
– a review. Critical Reviews in Food Science and Nutrition 53: 482–496.
Sodini I, Remeuf F, Haddad S, and Corrieu G (2004) The relative effect of milk base,
starter, and process on yogurt texture: a review. Critical Reviews in Food Science
and Nutrition 44: 113–137.
Tamime AY and Robinson RK (2007) Yoghurt: science and technology. Boca Raton, FL:
CRC Press.
Vinderola CG, Mocchiutti P, and Reinheimer JA (2002) Interactions among lactic acid
starter and probiotic bacteria used for fermented milk products. Journal of Dairy
Science 85: 721–729.
Relevant Websites
http://infos.cniel.com/actualite/economie-laitiere-en-chiffres.html.
http://www.nutraceuticalsworld.com/issues/2010-03/view_market-research/the-global-
yogurt-market.
http://www.produits-laitiers.com/les-produits-laitiers/produits-laitiers-leurs-circuits-
de-fabrication/les-yaourts-leur-circuit-de-fabrication/.
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