YOGURT/YOGHURT

YOGURT/YOGHURT
Garima Pande
FDST 8120
Summer 2010
History
Food historians generally agree the genesis of yogurt
and other fermented milk products was discovered
accidentally by Neolithic peoples living in Central
Asia. These foods occurred naturally due to local
climate and primitive storage methods (1). Yogurt
first gained international prominence in the early
1900s when Ilya Metchnikov, a Russian bacteriologist,
observed that the long life span of Bulgarians
was mainly due to their diet which included the
consumption of large quantities of soured milk (2).
The first industrialized production of yogurt is attributed
to Isaac Carasso in 1919 in Barcelona – his
company “Danone” was named for his son, “Little
Daniel”. Turkish immigrants brought yogurt to
North America in the 1700s but it really didn’t catch
on until the 1940s when Daniel Carasso, the son of
Danone founder Isaac, and Juan Metzger took over
a small yogurt factory in the Bronx, New York – the
company is now called Dannon in the United States
(3). The word is derived from Turkish: yoğurt, and is
related to yoğurmak ‘to knead’ and yoğun “dense” or
“thick”.
Traditional name 9
Jugurt/eyran/ayran
Busa
Kissel mleka/naja/yaourt
Leban/laban/laban rayeb
Zabady/zabade
Mast/dough/doogh
Yiaourti
Viili
Iogurte
Dahi/dadhi
Skyr
Tarho/taho
Country
Turkey
Turkestan
Balkans
Lebanon and some Arab
countries
Egypt and Sudan
Iran and Afghanistan
Greece
Finland
Brazil and Portugal
India
Iceland
Hungary

What is yogurt?
According to the Code of Federal Regulations of the FDA (4), yogurt is defined as the “food produced by culturing
one or more of the optional dairy ingredients (cream, milk, partially skimmed milk, and skim milk) with a
characterizing bacteria culture that contains the lactic acid-producing bacteria, Lactobacillus delbrueckii subsp.
bulgaricus and Streptococcus thermophilus”. “Yogurt, before the addition of bulky flavors, contains not less than
3.25% milkfat and not less than 8.25% milk solids not fat, and has a titratable acidity of not less than 0.9%, expressed
as lactic acid. Yogurt may be heat treated after fermenting to destroy viable microorganisms for a longer
shelf life of the food”. Lowfat yogurt and nonfat yogurt are similar in description to yogurt but contain 0.5 to 2%
and less than 0.5% milkfat, respectively (5, 6).
National Yogurt Association proposed a yogurt standard that (i) requires a minimum level of active cultures of
107 colony-forming units (CFU) per gram (g); (ii) requires an acidity of pH 4.6 or lower; (iii) requires a minimum
level of total dairy ingredients of 51 percent; (iv) provides for pre-culture homogenization and pasteurization;
(v) permits the use of reconstituted milk and whey protein concentrate as ``standard dairy ingredients;’’
(vi) provides for the use of any milk-derived ingredients as optional dairy ingredients; (vii) permits the use of
safe and suitable sweeteners, emulsifiers, and preservatives; (viii) permits the optional use of any safe and suitable
ingredients added for nutritional or functional purpose; and (ix) makes provisions for lowfat and nonfat
yogurts based on total fat content of the food per reference amount customarily consumed (7).
smoked
yogurt
Yogurt Related Products 9
Yogurt Production 9
frozen
yogurt
drinking
yogurt
dried
yogurt
yogurt
yogurt
cheese
yogurt
butter
or ghee
strained
yogurt
Classification of Yogurt 9
Yogurt
Physical
Set
Stirred
Drinking
Chemical
Full fat
Medium fat
Low fat
Flavor
Plain/natural
Fruit
single/mixed
Miscellaneous
Enzyme
hydrolysis
Vitamin
fortification
Vegetable oils
Heat treatment
INGREDIENTS
• Milk: Whole milk, partially skimmed milk, skim milk or
whole milk
• Other Dairy Products: concentrated skim milk, nonfat dry
milk, whey, lactose
• Sweeteners: glucose or sucrose, high-intensity sweeteners
(e.g. aspartame)
• Stabilizers: gelatin, carboxymethyl cellulose, locust bean
gum, guar, alginates, carrageenans, whey protein concentrate
• Flavors
• Fruit Preparations: including natural and artificial
flavouring, colour

CHEMISTRY OF YOGURT
Yogurt is a cultured dairy product made by fermentation of heated milk with lactic
acid bacteria, which convert lactose into lactic acid resulting in a reduction in pH.
Casein micelles are the building blocks of acid milk gels, so the nature and type
of casein interactions are very important for the properties of acid milk gels. The
integrity of casein micelles is controlled by a localized balance between hydrophobic
interactions and electrostatic repulsion (10). Yogurt mix is normally heated at
a higher temperature and longer time than normal pasteurization, ranging from
90 to 95 °C for 5 to 10 min, to help improve product consistency through whey
protein denaturation (9). Whey proteins which participate in casein aggregation
in yogurt are α- lactalbumin and β-lactoglobulin. The former has a denaturation
temperature of 62 °C and the latter, 78 °C. During heat treatment there are two
possible events depending on heat intensity; very high heat treatment such as 90
°C for 10 min induces both α-LA and β-LG to participate in protein aggregation
resulting in less susceptibility to syneresis in yogurt, but a lower heat treatment (vat
pasteurization) causes β-LG to participate rather than α-LA because 80 to 90% of
denatured α-LA is reversible after heating (10). Incorporation of β-LG at the micellar
surface gives a long chain casein micelle linked by a finely flocculated protein
resulting in a loose structure and an increase in syneresis (11). Finally, the lactic
acid production during the fermentation step leads to the destabilization of the micellar
system which results in the gelation of the proteins. As the isoelectric points
of denatured whey proteins (pH 5.2), and caseins (pH 4.6) are reached, low-energy
bonds, mainly hydrophobic, are progressively established between the proteins
(10). Hydrophilic colloids bind water and consequently increase the viscosity of
yoghurt; they also help prevent the separation of whey from the yoghurt, a problem
known as syneresis. Common stabilisers are, gelatin, pectin, agar, starch.
MICROBIOLOGY OF YOGURT
Early investigators of yogurt microorganisms found rod and coccal-shaped bacteria, yeasts and molds in yogurt
(9). Yogurt bacteria are now characterized as lactic acid bacteria belong to the Lactobacillaceae and
Streptococcaceae genera. Generally, yogurt cultures are L. delbrueckii subsp. bulgaricus and S. thermophilus.
Some other strains such as L. helveticus, L. jugurti, L. acidophilus and Bifidobacterium spp. are also sometimes
used as adjuncts. S. thermophilus is a Gram-positive, facultative anaerobic homofermentative bacterium
with a spherical or ovoid shape, a diameter

CHEMICAL AND PHYSICAL ANALYSIS OF YOGURT
Analysis of the qualitative parameters of yogurt can be divided into two parts as chemical and structural analysis.
The former includes yogurt flavor analysis (taste/aroma compounds), milk constituents analysis (e.g., carbohydrates,
fat, total nitrogen, free amino acids, proteins, and antibiotics), and assays for chemical indices (e.g., Zeta
potential of casein micelles and enzymes activity). Structural analysis includes textural and rheological as well as
microstructural analysis. Textural and rheological analysis is associated with the methods evaluating gel (set-type
yogurt) and liquid (stirred-type yogurt) properties of yogurts. Microstructural analysis consists of two parts:
microstructural images (studying detail structure) and microstructural assays (such as those obtained by EPS,
exopolysaccarides secreted by bacteria, and those for the determination of the mean diameter of fat globules/
particles) (15). Yogurt is a typical non-Newtonian fluid, exhibiting a shear-thinning, yield stress, viscoelasticity,
and thixotropic (time-dependency) flow behavior. The rheological properties of yogurt are indicative of gel formation
and are important in the design of flow processes, quality control, processing and storage, and predicting
the texture of yogurt. Textural properties for yogurt include viscosity, firmness and syneresis. Viscosity of yogurt
is affected by composition, type of starter cultures, heat treatment and stabilizer (15).
MICROSTRUCTURE OF YOGURT
Microstructure of yogurt is usually studied by transmission electron microscopy (TEM) of thin section and
scanning electron microscopy (SEM) (16). Sample preparation for TEM is time consuming but it allows for the
visualization of casein aggregation, whereas with SEM sample preparation is easier and aids in the study of gel
structure (16). Heat treatment of a yogurt mix causes changes in the microstructure of finished product. As
previously mentioned, denaturation of whey proteins is responsible for the changes in gel formation. Protein
micelles in yogurt mix when heated at 90 °C for 10 min have appendages which are complexes of whey proteins
and κ- casein, but without casein fusion (9). During the fermentation process, these appendages inhibit casein
fusion resulting in a loose structure (9). In a recent development, confocal scanning laser microscopy was used
to study the microstructure of yogurt during fermentation in real time without sample preparation (16). Microstructure
studies on yogurt gels have demonstrated that these gels consist of a coarse particulate network of
casein particles linked together in clusters, chains, and strands (16). The network has pores or void spaces where
the aqueous phase is confined.
4X 10X
YOGURT MICROSTRUCTURE BY
LIGHT MICROSCOPY
20X 40X

EFFECT OF MILK SOLID % (LEFT TO RIGHT,10, 15, 20% ) ON YOGURT MICROSTRUCTURE
http://www.magma.ca/~pavel/science/Yogurt.htm
Microstructure of yogurt obtained from milk bases enriched with SMP, WPC, NaCn, and CaCn (in clockwise
order), and heated at 90 °C for 5 min. Bar—20 mm (17).
SEM image of yogurt. Left upper corner and right lower corner show the protein matrix
of yogurt made by the action of bacteria: streptococci (yellow) and lactobacilli (blue).
http://www.magma.ca/~pavel/science/Yogurt.htm
Transmission electron microscopy of sheep's milk labneh:
Fat particles (yellow - small arrows) are encapsulated in
aggregated protein particles (dark structures - large
arrows). Bar: 1 µm.

Ref:18
REFERENCES:
(1) Brothwell, D.; Brothwell, P., Food in Antiquity: A Survey of the Diet of Early Peoples, expanded ed.; Johns Hopkins University
Press, Baltimore, MD, 1997.
(2) Whitman, J., Craig Claiborne’s The New York Times Food Encyclopedia,Times Books,New York, 1985 (p. 489)
(3) Mariani, J. F., Encyclopia of American Food and Drink, Lebhar-Friedman, New York, 1999 (p. 355).
(4) FDA. 1996c. Yogurt. 21 CFR 131.200, Code of Federal Regulations. U.S. Dept. of Health and Human Services, Washington,
DC.
(5) FDA. 1996a. Lowfat yogurt. 21CFR 131.203, Code of Federal Regulations. U.S. Dept. of Health and Human Services,
Washington, DC.
(6) FDA. 1996b. Nonfat yogurt 21 CFR 131.206, Code of Federal Regulations. U.S. Dept. of Health and Human Services,
Washington, DC.
(7) http://aboutyogurt.com/index.asp?sid=1
(8) International Dairy Foods Association, (2008), “Dairy facts2008”, International Dairy Foods Association Publications,
Washington DC, USA.
(9) Tamine A.Y.; Robinson, R.K., Yoghurt science and technology. 2nd ed.; CRC Press LLC, Boca Raton, FL,1999.
(10) Lucey, J. A., Formation and physical properties of milk protein gels. J. Dairy Sci. 2002, 85,281–294.
(11) Wong, N. P.; Jenness, R.; Keeney, M.; Marth, E. H., Fundamentals of Dairy Chemistry. 3rd ed.; Van Nostrand Reinhold
Co. Inc., New York, NY, 1998.
(12) Chandan R. C., Manufacturing of yogurt and fermented milks. 1st ed.; Blackwell Publishing Professional, Ames, Iowa,
2006.
(13) Lee W.J.; Lucey J. A., Rheological properties, whey separation and microstructure in set type yogurt: effects of heating
temperature and incubation temperature, J. Texture Studies. 2004, 34, 515-536.
(14) Ozer B., Destructive effects of classical viscometers on the microstructure of yoghurt gel, Turkish J. Agri. For. 2004, 28,
19-23.
(15) Mortazavian, A.M; Karamatollah, R.; Sohrabvandi, S., Application of Advanced Instrumental Methods for Yogurt
Analysis. Crit. Rev. Food Sci. Nutr. 2009, 49, 153-163.
(16) Kalab, M.; Allen-Wojtas, P.; and Phipps-Todd, B. E., Development of microstructure in setstyle nonfat yoghurt - a
review. Food Microstruct. 1983, 2, 51-66.
(17) Sandoval-Castillaa, O.; Lobato-Callerosa, C.; Aguirre-Mandujanoa, E.; Vernon-Carterb, E. J., Microstructure and texture
of yogurt as influenced by fat replacers. Intl. Dairy J. 2004,14,151–159
(18) Hassan, A. N.; Frank, J. F.; Farmer, M. A.; Schmidt, K. A.; Shalabi, S. I. Formation of yogurt microstructure and threedimensional
visualization as determined by confocal scanning laser microscopy. J. Dairy Sci. 1995, 78,2629-2636