Lit Review Take 3

8/8/2019 Lit Review Take 3

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CHEE4020: Polymer Rheology & Processing ii

1. Contents1. Contents ................................ ................................ ................................ ................................ ii

2. Table of figures ................................ ................................ ................................ .................. iii

3. History of fermentation ................................ ................................ ................................ .... 1

4. Importance of Yoghurt Rheology ................................ ................................ ................... 1

5. Yoghurt Manufacture ................................ ................................ ................................ ........ 2

6. Yoghurt Rheology ................................ ................................ ................................ ............... 3

7. Rheological measurements ................................ ................................ ............................. 5

7.1. Set Yoghurt................................ ................................ ................................ ................................ ............... 6

7.2. Stirred Yoghurt................................ ................................ ................................ ................................ ........ 7

8. References ................................ ................................ ................................ ............................ 9


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CHEE4020: Polymer Rheology & Processing iii

2. Table of figuresFigure 1- The typical structure of yogurt; the bonding between the groups of casein micelles is evident, as are

the whey-filled spaces (Robinson .R .K, 1995) 3

Figure 2- Graphs of shear rate against time for constant shear stress, at three stages in yoghurt production. (a )

Post fermentation, (b) post mixing, (c) post filling. (Mullineux, Simmons, 2007) 5

Figure 3- Stress time curves of set yoghurts deformed at a constant rate (Raphaelides , Gioldasi, 2005). 6

Figure 4 - A mechanical spectra (a) and a time dependency curve (b) with two different started mediums. The

full line shows Yoghurt A while the dotted line shows yoghurt B. (Jaros, Rohm, McKenna (ed.), 2000) 8


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CHEE4020: Polymer Rheology & Processing 1

3. History of fermentationFermentation is one of the oldest methods practiced by human beings for the transformation of milk

into products. The exact date that this started to become common practice is hard to determine but

it is around 1000 to 1500 years ago. (Tamime, Robinson, 2007) According to Persian tradition

Abraham owed his long life to yoghurt and Emperor Francis 1 ofFrance owed his life to yoghurt as it

helped cure his debilitating illness. It is likely that the origin of yoghurt is in the Middle East and the

evolution of the product is due to the nomadic people living there. The production of milk has

always been seasonal being restricted to only a few months of the year. The main reason is that

intensive animal production never occurred as in early history nomadic peopledid most of the

farming and they moved on following the pastures far away from the populated cities where they

could sell their wares. In addition with the extreme heat over summer the milk turns sour and

coagulates in a very short amount of time. This meant that yoghurt could only be made through a

small time frame when they returned to the cities. (Robinson et al, 2002)Even though yoghurt has

many desirable products it is still prone to deterioration at high temperatures and in ancient Persia

they tried many techniques to extend the products life.

Nomads traditionally held the yogurt in animal skins which meant that as the why seeped through

the skin and evaporated, causing the solid content in the yoghurt to rise increasing the acidity and

therefore making concentrated yoghurt less prone to denaturing. However it still became

unpalatable after a week. As a result salted yoghurt became more popular neutralizing the taste of

the acid while preserving the food, and had a storage span of up to 18 years. Over the following

years yoghurt became more popular throughout the world as better fermentation methods where

created and the rheology of yoghurt became better understood.

4. Importance of Yoghurt RheologyFood rheology has become an increasingly important factor in food manufacture because if the

rheological properties are known it helps manufactures predict various quality attributes. Yoghurts

are a growing segment across the world and in particular in the USA with an annual growth rate of

between 3 and 10%. (Montella, Sodini, Tong, 2005)This has been mainly due to their high nutritional

image, health benefits and pleasing taste. By applying rheological measurements to yoghurt it can

help producers predict various quality attributes.(Peng et al, 2009) Yoghurt has a very complex

internal structure which is susceptible to irreversible damage through time-temperature shear

history experienced through their manufacture. This has an obvious impact on the consumer

perception of the product as texture is of the four main quality attributes of any food material, with

the others being flavour, appearance and nutrition. (Mullineux, Simmons, 2008) Rheological

behaviour is directly associated with textural qualities such as mouth feel, taste and shelf lifestability. Other factors such as spreadability and creaminess in a semisolid food is also very

important to consumer satisfaction.(Herh et al, 2000 (Fenelon, Guinee, Wilkson, 2000)

Understanding the rheological behaviour of yoghurt helps improve the quality control of the

products, the design of process equipment, storage and the characterization of food products for

consumer acceptability. (Vanovas, Velez-Ruiz,1997)These rheological, physical properties and the

microstructures of yoghurts can be very easily affected by milk composition, such as dry matter


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fortification and by varying process variables in yoghurt manufacture like incubation temperature,

fat content and incubation time. (Lee, Lucey, 2004)In low fat yoghurts and unfortified yoghurts there

are quality concerns such as, weak body and poor texture die to the low total solid content. To

overcome this problem many yoghurt companies add hydrocolloids such as milk proteins therefore

improving the texture of the yoghurt making it more pleasing to the consumer. (Peng et al, 2009)

5. Yoghurt ManufactureThe basic methodology to yoghurt manufacture is the fermentation of milk with a lactic acid bacteria

which converts lactose into lactic acid reducing the pH. Polysaccharides and stabilizers can be added

to improve the texture of the end product. Manufacturing methods vary considerably dependent on

the raw materials, type of product manufactured etc. However there are a number of common

principles which are outlined below: (Jaros D, Rohm H, McKenna (ed.), 2000)

y The total solids content of the base milk is enhanced to increase the water holding capacityy A heat treatment of the base milk at 80oC for some time to cause the proper denaturation of

the why proteins also increasing the water binding capacity.

y The inoculation with a specific starter culture and incubation with a time dependent profiledepending on the properties of the starter and technical requirements

y Cooking and addition of ingredient i.e. flavours, fruity Packaging and chilled storage.

The normal bacterium used is Streptococcus thermophilus and Lactobacillus delbrueckii (Lee, Lucey ,

2004) causing the properties of milk to change in an irreversible way achieving a final protein count

of4050 g kg1.( Montella, Sodini, Tong, 2005) Another property of Lactic acid bacteria apart from

lowering the pH is the formation of extracellular polysaccharides (EPS). The presence of this is

perceived as giving the yoghurt a better mouth feel and a more cohesive texture. Therefore the

bacteria can be classified as ropy or non ropy depending on whether they produce EPS. The milk is

then subjected to a high level of heat causing the denaturing of whey proteins. Why proteins

constitute around 20% of the total protein content in milk and can remain soluble at pH values low

enough to cause agglomeration of casein. The principle whey protein is -lacto globulin. (Haquea,

Richardson, Morris, 2001) These denatured why proteins interact with casein on the surface of

casein muscelles and form disulphide bridges with -casein, forming a 3D network structure. Around

eighty percent of the total protein content in milk is Casein which comprises of four main

components s1, s2, and . Approximately 95% ofCasein exists as micelles which are sphericallyshaped and range in size from 50 to 300 nm. The integrity of casein micelles is controlled by a

localized balance between the hydrophobic interactions and electrostatic repulsionwhich is highly

dependent on the pH temperature and time of fermentation. (Peng, Horne, Lucey, 2009)Finally the

lactic acid production in the fermentation stage leads to the pH lowering. As the isoelectric point is

reached, (the point at which the molecule carries no net electrical charge) around pH4.6 for casein


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y bonds in partic lar hydrophobic are established between the proteins causing the

mixture to become moreviscoelastic (Remeuf, et al, 2003)

The gelsconsist of a coarse network ofcasein particles linked together in clusters or strands The

network has pores or void spaces where an aqueous phasecan beseen. Thiscan beseen in figure1

where thecasein has bonded together in a series ofstrands leaving an aqueous phase which will be

filled with soluble why proteins. (Peng, Horne, Lucey, 2009) This microstructure has been studied by

a scanning electron microscopy, fluorescence microscopy and confocal scanning laser microscopy.

(Lee, Lucey, 2004)

Figure 1- The typical structure of yogurt; the bonding between the groups of casein micelles is evident, as are the

whey-filled spaces (Robinson .R .K, 1995)

Yoghurt typescan also be distinguished to their physical state in the retail container which can bedescribed asset or stirred yoghurt. Set style is where the milk is fermented in the package until the

required pH is reached therefore giving a continuously gelled structure in the final product. The

continuous viscoelastic gel network consists of aggregated spherical casein particles forming a

continuousstructure with enclosed fat globules. Stirred yoghurt is where the fermentation iscarried

out in large fermentation tanks and the acid gel is disrupted by stirring and sieving to give it a

greater fluidity which is often a base for the inclusion of fruit before packaging.(Haquea, Richardson,

Morris, 2001)By breaking up the gel a highlyviscous, non Newtonian liquid is formed, which shows a

strong shear rate and time dependent flow behaviour. During the pH decrease casein and why

proteins coagulate and form a structure that is characterized by aggregated proteins and pores.

(Lorenzi, Pricl, Torriano, 1995)

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Yoghurts are non Newtonian fluids and are highlystructured materials making their characterization

through normal rheological methods increasingly hard. Somestudies have preferred to useempirical

methods to help characterize the textural properties of yoghurt. (Harte, Clark, Barbosa-Cnovas,

2007)Nearly all yoghurts with the exception of drinkable yogurts act as a viscoelastic material if


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undisturbed with strong time dependence of both the thixotropic and viscoelastic types. (Delorenzi,

Procl, Torriano, 1995)

The consistencies of gels among other things have been dependent on the integrity of the casein gel

structure. Therefore any interruption to this will result in altering the yogurts texture. (Suwonsichon,

Peleg, 1999) There viscoelastic properties however are very hard to determine as they behave as

very weak gels with a narrow linear viscoelastic region. This means that that using standard

rheological methods to determine the stress strain relationship quickly causes a structural

breakdown. (Harte, Clark, Barbosa-Cnovas, 2007)

Many different methods have been proposed with Ozer, Bell and Robinson (1997) stirring the

mixture before measurement and then describing the properties of the recovered structure. The

fundamental problem with this method is that it is no longer the same material being described.

Yoghurts also have a yield stress, the amount of stress needed to initiate flow. This is another

property which is hard to measure as it depends on the makeup off each sample and the

characteristic time of the process to which the sample is being subjected.

Rheological properties depend on the number and strength of the bonds between the casein

particles, and between those of the why proteins. The dynamic module can indicate the strength

and the number of bonds in the network and the yield stress points towards the likelihood of the

strands breaking. The loss tangent which is the ratio of viscous to elastic properties indicate the

relaxation of the bonds. (Peng, Horne, Lucey, 2009)

The main factors that determine the physical properties of yoghurt include: milk heat treatment,

incubation temperature, acid development, total solids (protein and fat) contents and type and

concentration of stabilizers. High milk treatment causes why protein denaturation which interact

with the casein micelles increasing the voluminosity and water binding capacity of why protein and

decreases their solubility. Dynamic rheological measurements showed a much higher storage

modulus when the heat was increased over 80oC indicating a higher gel firmness. This results in a

higher gelation pH and a stiffer network. The loss of colloidal calcium phosphate from the casein

particles, that are a part of the gel network increases the susceptibility of the network to rearrange

which continues after gelation. This leads to a firmer texture described by the consumers. (Herh et al

2000) With fermentation temperatures the values of log G (storage modulus) show an essential

linear increase with increasing fermentation. Storage modulus expresses the energy stored in the

material from rearrangements in the structure that take place during the stress. Solidstend to

return to the original state after stress is released so G confirms the elastic characteristics of the

product. The loss modulus G records the energy lost during the cycle of deformation indicating the

viscous component of the material. Using these two modulus helps give an accurate picture of thegel structure. (Tamime, Robinson, 2007) The total amount of solid in the base milk has a drastic

effect on the overall physical properties with a higher solid content leading to increased firmness

and viscosity. This is due to the increasing amount of bound water as according to Snoeren eta#

(1982), milk casein is able to immobilise 2.82g of H2O per g of protein, therefore leading to an

increased firmness of the gel. (Jaros D, Rohm H, McKenna (ed.), 2000)


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As stated before the properties of yoghurt can be affected by the inoculum level, a higher level

causes an increased amount of time taken to rich the maximum acidification rate which in turn

causes a decrease in the maximum storage level G. This is due to a decreased permeability, pore

size and why separation, suggesting that that the aggregation level is modified by a fats acidification

process caused by the high inoculums level. The integrity of the casein micelle and the changes inthe internal muceller structure due to the changing pH are important factors in the determination of

the viscoelastic properties of yogurt. (Lee, Lucey, 2004) Therefore yogurts with a low inoculation

rate has finely dispersed protein clusters while those with higher inoculation rates exhibit

interconnected clusters of aggregated protein particles with larger pores.

7. Rheological measurementsA common way of determining the properties of yoghurt is to subject a sample to a constant shear

stress and measure the shear rate. By repeating this diagram a rheogram can be created however as

yoghurt is thixotropic the shear rate for a constant shear stress is not constant but instead varies

with time. This means that a single rheogram only gives information about a specific time howeverthe rest of the time dependent information is not represented. (Mullineux, Simmons, 2007)

For thixotropic materials is easier to deal with shear rate against time curves which are shown in

figure 2. The graphs show three stages through the production process, after fermentation, mixed

with fruit and post filling.

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The further down the process the more shear thinning occurs increasing the gradient of the curves.

The graphs for b and c show a typical form where there is an initial steep gradient followed by alinear line. The differences between the graphs is dependent on the work done on the yoghurt

through its processing. (Mullineux, Simmons, 2007)


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7.1.Set P oQ hu R tThe rheological properties ofset yoghurt gels that arestill intact can only be measured when the

fermentation process is performed within the rheometer as long assome fundamental requirements

are fulfilled. Most importantly is that the there is sufficient protection against theevaporation ofmilk during fermentation which be achieved by using a low viscosity oil at the milk air interface.

(Suwonsichon, Peleg, 1999) Thestrain applied to the gelling system has to beverysmall to stop any

interference to the gelation process. This is achieved by using strain controlled rheometers however

it isvery hard in theearlystages of fermentation where anystress applied towards the almost liquid

milk will cause deformation. Slippage occursveryeasily when using horizontal geometrics likecone

and plate whereas cup and bob systems are a lot more robust towards to the process. (Jaros D,

Rohm H, McS

enna (ed.), 2000) One of the requirements in a rheological experiment is that there is

good contact between the tested fluid and thesensor surface. This requires that the fluidsedge in

contact with the moving part of the sensor travels at the same velocity. If this doesnot occur

normally due to a change of interface then the fluid pattern and the velocity profiles changes

dramatically undermining the result of theexperiment. Insertion ofyoghurt into the narrow gap of

viscometers can produce enough shear to cause such adisruption and since the extent of this is

unknown the relationship between the measured properties and those in the yoghurt remains

largely unknown. This is a very serious problems in set yoghurts where gel disruption cases an

irreversible process. (Suwonsichon, Peleg, 1999)Vane geometry also offers minimal destruction of

the weak gel structure and offers three main advantages over other rheological methods.

y it minimizes damage during sample preparationy it avoids wall-slipy

thesample preparation is reproducible

The disadvantageous is the fact, that the vane geometry cannot produce any absolute values,

because it is a relative measuring system. (Montella, Sodini, Tong, 2005). When measured theset

yoghurts have a muchstronger consistency then that at thestirred yoghurts due to the fermentation

occurring inside thecontainer so that the gel structure is not disrupted.Figure3 below shows the

stressversus timecurves for set yoghurt.The curveshown in figure2shows a shape

that is characteristic for a structured

material such as gel which issubjected to a

large deformation. On applying a constant

deformation rate the stress increases

proportional to the deformation.

(Raphaelides, Gioldasi, 2005). On further

deformation an increasing structural

breakdown occurs until it reaches the

overshoot value. Before thisvaluecrosslinks

could continuously break and reform

Figure 3-T

tress time curves of set yoghurts deformed at a constant rate

(Raphaelides , Gioldasi, 2005).


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however at the yield value reformation can no longer compete with the structural breakdown, and

stress level decreases as a result.

7.2.Stirred YoghurtAfter fermentation of yoghurt in large vats the gel is broken by stirring thus forming a viscous non

Newtonian fluid which is strongly shear rate thinning. Changing the stirring regime has a crucial

effect on the final rheological properties of the yoghurt. At a given shear rate the viscosity of stirred

yoghurt is dependent on the firmness of the gel before stirring giving higher viscosities with higher

firmness. The higher the gel firmness the more vigorous the stirring can be leading to smoother

products. (Jaros D, Rohm H, McKenna (ed.), 2000)The structure then rebuilds to some extent

through storage into a 3D structure. (Velez-Ruiz, Canovas, 1997) This process however is largely

unknown hampering the understand of the factors that control the final viscosity.

As this is a complex viscoelastic fluid which exhibits shear thinning and time dependent properties,

to be able to characterise the complete flow properties of yoghurt requires a large set of

experiments to consider both the shear rate and time effects. There is a stress yield that can be

observed in yoghurt where under no flow can be observed. Applying constant shear rates for specific

time results in a normal decay curves for viscosity. Graphs showing the time dependency can also be

created which depends heavily on the set up of the test i.e. the acceleration of the shear rate caused

by the time dependent viscosity decay of yoghurt. Other information can be drawn from the graph

including the area between the upwards and downwards curves.(Jaros D, Rohm H, McKenna (ed.),

2000) The curves are created due to the dominant gel like structure resisting at low shear rates

however the viscosity decreases at higher shear rates hence the structure is destroyed. This area can

be treated in terms of the power necessary for the structure degradation. (Delorenzi, Procl,

Torriano, 1995) Rotational viscometer s have proved to be very useful in time dependent values

unlike tube viscometers as the very easily allow materials to be subjected to alternate periods ofshear and rest. (Steffe, 1992) Figure four shows a comparison between two samples of stirred

yoghurt which have been produced at identical conditions however yoghurt A has a highly viscous

starter while yoghurt B used a normal starter.


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The figure shows that the normal mechanical spectra showed no noticeable difference while the

time dependency graph showed a huge difference between the two samples. (Jaros, Rohm,

McKenna (ed.), 2000) The mechanical spectra showed that the stirred samples still retain

predominantly gel like response to some deformations but the module is substantially lower thanthat of set yoghurt which is expected due to the disruption of the structure through stirring. (Velez-

Ruiz, Canovas, 1997) The time dependency graph however shows that yoghurt A had a completely

different histolysis cycle with a much larger area enclosed between the upwards and downwards

slope. This shows that yoghurt A needs an increased amount of power for structure degradation to

occur. (Jaros, Rohm, McKenna (ed.), 2000)


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8. ReferencesDelorenzi L, Procl S, Torriano G, 1995, Rheological behaviour of low-fat and full-fat stirred yoghurt,

Internationau

Dairy Journau

, vol 5, No 7, pp661-671

Harte .F, Clark .S, Barbosa-Cnovas. G.V, 2007, Yield stress for initial firmness determination on

yogurt,Journau

of Food Engineering, Vol 80, Issue 3, pp 990-995

Haquea A, Richardson R. K, Morris R, 2001, Effect of fermentation temperature on the rheology of

set and stirred yogurt, Food Hydrocolloids, vol 15, pp 593-602

Herh P.K.W, Colo S .M, Roye N, Hedman K, 2000, Rheology of foods: New Techniques capabilities,

Journal of Food Engineering, Vol 74, pp 16- 20

Jaros D, Rohm H, McKenna .B .M, (ed.) 2000, Texture in food: Semi solid foods, Woodhead Publishing

Limited, Cambridge, England, Vol 1, pp321-342

Lee .W.J, Lucey J.A, 2004, Structure and Physical properties of Yoghurt Gels : Effect of Inoculation

rate and incubation temperature,Journal of Dairy Science, vol 87, p3153

Montella J, Sodini I, Tong P.S, 2005, Physical properties of yogurt fortified with various commercial

whey protein concentrates Journal of the Science of Foodand Agriculture J Sci Food Agric,vol 85, pp

853859

Mullineux G, Simmons, M.J.H,2007, Effects of processing on shear rate of yoghurt, Journal of Food

Engineering, vol 79, Issue 3, pp850-857

Lorenzi L.D., Pricl .S, Torriano .G, 1995, Rheological behaviour of low-fat and full-fat stirred yoghurt,

International Dairy Journal, vol 5, no 7, pp 661-671

Peng, Y; Horne, DS; Lucey, JA, 2009 Impact of preacidification of milk and fermentation time on the

properties of yogurt ,Journal of Dairy Science, vol. 92 Issue: 7, pp 2977-2990

Raphaelides S. T, Gioldasi A, 2005, Elongational flow studies of set yogurt, Journal of Food

Engineering, vol 70, no 4, pp 538-545.

Remeuf .F, Mohammed .S, Sodini .I, Tissier .J.P, 2003, Preliminary observations on the effects of milk

fortification and heating on microstructure and physical properties of stirred yogurt, International

Dairy Journal, vol 13, Issue 9, pp773-782

Robinson R.K, 1995, The potential of inulin as a functional ingredient, British Food Journal, Vol 97, No4, pp 30-32

Robinson R.K., Tamime A.Y., Wszolek M., 2002, Dairy Microbiology Microbiology of Milk Products,

Elsevier Applied Science Publishers, London, Vol 2, 2nd

Edition, pp 291-343

Snoeren, T.H.M., Damman, A.J., Klok, H.J, 1982, The viscosity of skim milk concentrates, Netherlands

Milkand Dairy Journal, vol 36, pp 305-316


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Steffe JF, 1992, Rheological methods in food process engineering, Freeman press, East Lansing, USA,

pp 27-29

Suwonsichon, T; Peleg, M,1999, Rheological characterisation of almost intact and stirred yogurt by

imperfect squeezing flow viscometry,Journal of the Science of Foodand Agriculture, vol 79, No 6,

pp911-921

Tamime A.Y., Robinson R.K.,2007, Tamime andRobinsons Yoghurt: Science andTechnology,

Woodhead Publishing Limited, Cambridge, England,3rd

Edition, pp1-20

Velez-Ruiz .J.F; Canovas .G.V.B, 1997, Rheological properties of selected dairy products, Critical

reviews in food Science and Nutrition, vol 37, Issue 4, pp311-359

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