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In uence of homogenisation and the degradation of stabilizer on thestability of acidi ed milk drinks stabilized by carboxymethylcellulose

Juan Wu a, Baiqiao Du b , Jing Lic, Hongbin Zhang a , *

a Advanced Rheology Institute, Department of Polymer Science and Engineering, School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, Chinab SGS-CSTC Standards Technical Services Co., Ltd., Shanghai 200233, Chinac Bright Dairy & Food Co., Ltd., Shanghai 201103, China

a r t i c l e i n f o

Article history:Received 4 June 2013Received in revised form10 December 2013Accepted 17 December 2013

Keywords:Carboxymethylcellulose (CMC)HomogenizationCMC degradationAcidi ed milk drinksStability

a b s t r a c t

The present work deals with the in uences of both homogenisation and the degradation of carboxy-methylcellulose (CMC) on the stability of two kinds of acidi ed milk drinks (AMDs), directly acidi edmilk drinks and yoghurt drinks. The effect of homogenisation pressure for direct acidi cation processwas investigated and evaluated. The experimental results showed that homogenisation was required toachieve a signi cantly small particle size (0.7 mm in the present work) and to prevent sedimentation andserum separation. However, homogenisation at too high pressures was not bene cial for the stability of the colloidal systems. The occurrence of degradation of CMC during homogenisation weakened thestabilisation effect of CMC. A quali ed homogenisation pressure of 20 MPa should be chosen to achieve agood stability when a usually practical pressure range of 0 e 30 MPa was applied. In addition, the stabilityof directly acidi ed milk and yoghurt drinks prepared under the same homogenisation pressure was alsoinvestigated. While their stability increased with increasing CMC concentration, the degradation of CMCat low pH during storage gave rise to instability of the nal products.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Acidi ed milk drinks (AMDs) are popular food products foundworldwide. Many types of AMDs are available, including those pre-pared from fermented milk with stabilisers and sugar to those pre-paredby directacidi cationwith fruit juicesand/oracids,suchas fruitmilkdrinks, yoghurt drinks,soy milk, whey drinks,and soon( Laurent& Boulenguer, 2003; Nakamura, Yoshida, Maeda, & Corredig, 2006 ).The nalpH of these products generally ranges from 3.6 to4.6.In rawmilk, at neutral pH, caseins exist in the form of micelles, which arestabilised by steric repulsion due to the extended conformation of k-casein present mainly on their surface ( Tuinier & de Kruif, 2002 ).During acidi cation of milk to a pH close to the isoelectric point (pH4.6) of caseins, casein micelles aggregate mainly because of thecollapse of the extended k-casein ( de Kruif, 1998; Nakamura et al.,2006 ) such that a stabiliser must to be added to avoid protein ag-gregation and subsequent macroscopic whey separation due to theinstabilityof caseinsat their isoelectricpoints.High methoxyl pectins(Laurent & Boulenguer, 2003; Liu, Nakamura, & Corredig, 2006;

Parker, Boulenguer, & Kravtchenko, 1994; Tromp, de Kruif, van Eijk,& Rolin, 2004; Tuinier, Rolin, & de Kruif, 2002 ), soybean solublepolysaccharides ( Asai et al., 1994; Nakamura, Furuta, Kato, Maeda, &Nagamatsu, 2003; Nakamura et al., 2006 ), and carboxymethylcellu-lose (CMC) ( Du et al., 2007, 2009; Wu, Liu, Dai, & Zhang, 2012 ) areoften used to achieve this purpose.

CMC can improve the stability of casein micelles at low pH. Theconcentration, molecular weight ( M w), and the degree of substi-tution (DS) of CMC have different in uences on the stability of AMDs. The system containing 40 g $kg 1 milk solid at pH 4.0 can bestabilised by 4 g $kg 1 CMC, in which particle size of proteins wasabout 0.68 mm, at the moment both the serum and the sedimen-tation fraction were low. Below 4 g $kg 1 CMC, bridging occulationoccurs in the system. While CMC with a high M w (700,000) in-creases the viscosity of AMDs signi cantly thereby contributing tothe stability,CMCwitha high DS(1.2) results in a high z-potential of CMC-coated casein micelles increasing the electrostatic repulsionbetween casein micelles, also bene ting the stability of thecolloidal systems ( Du et al., 2007, 2009 ). Typical electrostatic in-teractions occur between negatively charged CMC and positivelycharged milk proteins at and below pH 5.2 ( Du et al., 2007; Wuet al., 2012 ). The adsorbed CMC layer causes repulsive in-teractions between casein micelles at low pH via a manner similar

* Corresponding author. Tel.: þ 86 21 54745005.E-mail address: hbzhang@sjtu.edu.cn (H. Zhang).

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0023-6438/$ e see front matter 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.lwt.2013.12.029

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to how k-caseins stabilise casein micelles at neutral pH. In addition,non-adsorbed CMCincreases the viscosity of AMDs, which can slowdown the sedimentation rate, thus also contributing to the stabilityof the colloidal systems ( Du et al., 2007 ).

The stability of AMDs has also been found to depend on the sizeof the protein particles. Generally, larger particle sizes correspondto more unstable dispersions, which are prone to syneresis andwheying off. Usually, homogenisation has to be used in the dairyindustry to reduce the creaming and sedimentation of milk. Sta-bilisation can be achieved by the size reduction of both proteinparticles and fat globules ( McCrae, Hirst, Law, & Muir, 1994; Sandra& Dalgleish, 2005; Sedlmeyer, Brack,Rademacher, & Kulozik, 2004 ).Parker et al. (1994) reported that homogenisation is required toachieve the signi cantly improved stabilisation of casein micellesin acidi ed skim milk systems by pectin. As an important standardoperation during the AMD process, homogenisation has beenshown to enhance the adsorption of the pectin chains onto caseinparticles ( Tromp et al., 2004 ).

When producing directly acidi ed milk drinks (DAMDs), aneutral milk dispersion is directly acidi ed to a certain pH usingacid or fruit juice. During this acidi cation process, there are manychanges in structure and component of casein micelles. Proteins incasein micelles undergo extensive dissociation, reassociation andrearrangement during acidi cation ( Lucey, Tamehana, Singh, &Munro, 1998a; McMahon, Du, McManus, & Larsen, 2009 ). Gastaldet al. observed the pH-induced changes in casein micelles duringdirect acidi cation of reconstituted skim milk (low-heat type) at20 C. They found that a s-, b- and k-casein were dissociated fromthe micelles as pH lowered. For these three kinds of caseins, thedissociation was relatively slow until about pH 6.0, then becamefaster, especially with regard to b-casein, and reached a maximumaround pH 5.4. At pH 5.1 virtually all colloidal calcium phosphate(CCP) was also solubilised and a maximum was also observed withregard to micellar casein dissociation and solvation at about pH5.4 e 5.3. The changes in compositions of casein micelles resulted inthe dissociation and aggregation of proteins observed by SEM

during acidi cation ( Gastaldi, Lagaude, & Tarodo De La Fuente,1996 ). In contrast, when producing yoghurt drinks using a fer-mented base, both whey proteins denatured by pasteurisation andthose denatured whey proteins associated with casein micellesbecome more susceptible to aggregate during acidi cation. Heattreatments of milk at 90 C cause denaturation of whey proteins,which will complex with casein micelles, involving k-casein, viahydrophobic interactions and the formation of intermoleculardisulphide bonds. The presence of b-lactoglobulin was necessaryfor any association of whey protein with casein micelles to occur(Corredig & Dalgleish, 2001 ). Cross-linking or bridging by dena-tured whey proteins produces a rigid gel network by casein mi-celles ( Lucey, 2004; Lucey, Tamehana, Singh, & Munro, 1998b;Lucey, Tet Teo, Munro, & Singh, 1997 ). Yoghurt drinks are usually

manufactured by homogenisation of acid casein gels (i.e., fer-mented bases). This means that they can be considered as sus-pensions of colloidal casein gel particles.

The present work evaluates the in uence of homogenisationand the degradation of CMCduring homogenization on the stabilityof DAMDs. The stabilities of DAMDs and yoghurt drinks induced byCMC via different acidi cation processes of direct acidi cation byan acid and fermentation by a starter culture are also discussed.

2. Materials and methods

2.1. Materials

CMC was provided by DuPont-Danisco Co., Ltd. (Shanghai,

China). The molecular weight and DS of CMC was 5 105

and 0.9,

respectively, and the Brook eld viscosity of 10 mg/ml aqueous so-lution at its neutral pH (7.6) was 352 mPa $s. The Brook eld vis-cosity was measured by a Brook eld DV- _þ instrument (Brook eld,USA) with a No. 3 rotor at 100 rpm and 25 C. Low-heat skim milkpowder was obtained from Fonterra Co., Ltd. (Auckland, New Zea-land). The skim milk powder contains 33.4% protein, 54.1% lactose,7.9% mineral, 3.8% moisture and 0.8% fat. The citric acid and otheranalytical grade chemicals used were purchased from SinopharmChemical Reagent Co., Ltd. (Shanghai, China).

2.2. Preparation of DAMD

A solutionof 80g $kg 1 reconstituted skim milk was preparedbymixing the low-heat skim milk powder with distilled waterat 45 Cfor 30 min. CMC and sucrose were dry mixed and then dissolved indistilled water at 75 C by stirring for 20 min. The stabiliser andskim milk were mixed at a 1:1 ratio to obtain 40 g $kg 1 skim milkpowder containing different CMC concentrations and 80 g $kg 1

sucrose. The pH of this mixture was adjusted to 4.0 using500 g $kg 1 citrate acid at 20 C under continuous stirring at1000 rpm. The nal volume was controlled in 5L. The sample was

homogenised at 25

C with a heat exchanger (TG-UHT-0.2 MJ,Shanghai Nanhua Transducer Manufacture Co., Ltd., Shanghai,China), followed by pasteurisation at 110 C for 30 s with a pas-teuriser (HZ-SJJ, Shanghai Huizhan Technology Co., Ltd., Shanghai,China). The nal product was then stored at 4 C for 24 h.

2.3. Preparation of yoghurt drinks

A solution of 120 g $kg 1 reconstituted skim milk was pas-teurised at 90 C for 5 min and then cooled to 35 C in a water bath(W201-B, Shanghai SENCO Technology Co., Ltd., China). About0.053 g/L of a starter culture (MY-105, Danisco) was added.Fermentation at 42 C was performed for 6 h to achieve a pH of 4.2.To cease fermentation, the yoghurt was rapidly cooled to 4 C. CMCand sucrose were dry mixed, dissolved in distilled waterat 75 C bystirring for 20 min, and then cooled to 15 C. Thereafter, the fer-mented milk was added to the CMC/sugar solution with stirring at1000 rpm. The milk solid non-fat (MSNF) concentration wasadjusted to 40 g $kg 1, and the sucrose concentration was adjustedto 80 g $kg 1 by addition of water. This mixture was acidi ed to pH4.0 by addition of 500 g $kg 1 citrate acid at 20 C with continuousstirring at 1000 rpm. The nal volume was controlled in 5L. Thesample was homogenised at 25 C, followed by pasteurisation at110 C for 30 s with a pasteuriser (HZ-SJJ, Shanghai HuizhanTechnology Co., Ltd., Shanghai, China). The nal product was thenstored at 4 C for 24 h.

2.4. Effect of homogenisation

To study the in uence of homogenisation pressure on the sta-bility of the drinks, a series of DAMDs containing 40 g $kg 1 MSNF,4 g$kg 1 CMC, and 80 g $kg 1 sucrose with a nal pH value of 4.0were prepared at different homogenisation pressures in the rangefrom 0 to 30 MPa. Based on our previous study ( Du et al., 2007 ), theAMDs containing 40 g $kg 1 MSNF can be stabilised by 4 g $kg 1

CMC, so here this concentration of CMC, 4 g $kg 1, was chosen in thepresent work. The batch was pumped at different pressuresthrough a single-step homogenizer (GYB30-6S, Shanghai DonghuaHigh Pressure Homogenizer Company, China) with two passes. Thehomogenisation pressures at the two passes were the same. Theparticle size distribution, Brook eld viscosity and stability of each

DAMD were then measured.

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2.5. Different acidi cation processes

To determine the stability of AMDs induced by CMC via differentacidi cation processes, the particle size, Brook eld viscosity andsedimentation of mixtures containing 40 g $kg 1 MSNF anddifferent CMC concentrations at different pH were measured. Forthe DAMDs, during acidi cation, samples at pH 4.6, 4.4, 4.2, 4.0, 3.8and 3.6 were collected. For the yoghurt drinks, the pH of the fer-mentedbase was adjusted to range from 4.2 to 3.6 using citrate acidunder continuous stirring. Samples were also collected at 0.2 pHincrements in a manner similar to that for the DAMDs. The samplesat various pH were then homogenised using a single-step homog-eniser (GYB30-6S, Shanghai Donghua High Pressure HomogenizerCompany, China) at 20 MPa and pasteurised at 110 Cfor30swithapasteuriser (HZ-SJJ, Shanghai Huizhan Technology Co., Ltd.,Shanghai, China).

2.6. Measurement of DAMD stability

Sedimentation and serum phases that appeared during storagewere observed using an optical analyser (Turbiscan MA 2000,Formulaction, Ramonville-St-Agne, France). Cylindrical glass tubescontaining 5 mL of the samples were stored at 25 C. Duringobservation, three phases separated out in the tested tubes: sedi-ments at the bottom, an opaque liquid in the middle, and a clearliquid at the top. Changes in the sedimentation and serum fractionsas a function of time were monitored for 15 d after the sample wasprepared and calculated by the ratio of the distance of the corre-sponding transmittance range to the tested tube length.

2.7. Measurement of Brook eld viscosity

The apparent viscosity of the AMDs was measured by a Brook-eld viscometer Model NDJ-79 Brook eld LVDV-I (Brook eld, USA)

witha No.1 rotorand100 rpm at 25 C. The Brook eld viscosities of the unstable AMDs in which phases separation occurs with obvioussedimentation or creaming phenomenon, were not measured.

2.8. Measurement of particle size

The samples were diluted at a ratio of 1:100 with simulated milkultra ltrate containing Na, K, Ca, Mg, phosphate and citrate( Jenness & Koops, 1962 ). The particle size of these samples wasmeasured using a laser diffraction particle size analyser (MalvernMasterSizer 2000, Malvern Instruments, UK).

2.9. Measurement of sedimentation

About 50 g of the AMDs was centrifuged at 3000 g and roomtemperature for 20 min. The supernatant was carefully removedand the centrifugation tube was inverted for 5 min to drain theremaining supernatant. Sedimentation was calculated from theratio of the weight of the sediment to the weight of the sample.

2.10. Determination of serum CMC concentration

The concentration of the serum CMC was determined using themethod of Tromp et al. (2004) . CMC is considered “ adsorbed ” whenit follows the protein phase into the pellet during centrifugation(Beckman Coulter Avanti J-25 centrifuger, Fullerton, USA) at25,000 g for 2 h. The supernatant or serum contains the non-adsorbed CMC, hereafter referred to as serum CMC. The viscosityof the supernatant was measured to determine the serum CMCconcentration. Viscosity measurements of the serum CMC wereperformed on a controlled stress rheometer (Bohlin Instruments,Gemini 200 HR, UK) tted with coaxial cylinders (25 mm and27.5 mm respectively in diameter of the inner and outer cylinder) at

a shear stress of 0.4 Pa.To obtain the CMC concentrations in the serum of the DAMDsand yoghurt drinks from viscosity measurements, two calibrationcurves were made. The CMC was dissolved in solutions centrifugedat 25,000 g for 2 h from the DAMDs and yoghurt drinks withoutCMC addition. The viscosities of these CMC solutions as a functionof CMC concentration were used for the calibration curves, theresults of which are shown in Fig. 1. These curves were used todetermine the serum CMC concentrations in the DAMDs andyoghurt drinks from their viscosity by interpolation. The fraction of adsorbed CMC was calculated by 1 e C serum /C overall , where C serum isthe concentration of serum CMC, C overall is the concentration of overall CMC.

All measurements were done in triplicate. Error bars shown inthe gures indicate the standard deviation.

3. Results and discussion

3.1. In uence of homogenisation on the stability of DAMD

Fig. 2 shows the amounts of sedimentation and serum fractionsmeasured by Turbiscan, as well as the average particle size D[4,3] of DAMDs produced at different homogenization pressures, rangingfrom 0 to 30 MPa. Without homogenisation, the average particlesize of the DAMD was approximately 312 mm, and both sedimen-tation fraction (ca. 40%) and serum fractions were large (ca. 60%),

0.0

0.1

0.2

0.3

0.4

0.5

0.6

C M C c o n c e n

t r a t i o n

/ %

Viscosity/mPa.s

(a)

0 40 80 120 160 200 240 0 40 80 120 160 200

0.0

0.1

0.2

0.3

0.4

0.5

0.6

C M C c o n c e n

t r a t i o n

/ %

Viscosity/mPa.s

(b)

Fig. 1. Calibration curves used for the determination of serum CMC concentrations from viscosity. Curve equation: Y ¼ a þ b*e( x/c ) (Y ¼ serum CMC concentration and

X ¼

viscosity). (a) a ¼

0.505, b ¼

0.585, and c ¼

49.345 in DAMDs. (b) a ¼

0.504, b ¼

0.620, and c ¼

41.237 in yoghurt drinks.

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indicating the instability of the DAMDs. When the samples werehomogenised at 5 MPa, both the average particle size and thesedimentation fraction of the DAMDs decreased signi cantly, sug-gesting that the stability of the sample was strongly improvedcompared to the unhomogenized dispersions. This result also im-plies that homogenisation is required to achieve a signi cantlysmall particle size and prevent sedimentation and serum separa-tion. With increasing homogenisation pressures from 5 MPa to20 MPa, the average particle size further decreased. Above 20 MPa,no signi cant changes in average particle sizes were detected(0.7 mm at20 MPa,0.68 mm at25 MPa,0.6 mm at 30 MPa), indicatingthe limited effect of homogenisation pressure on reducing theaverage particle size. However, at high homogenization pressuresboth the sedimentation and serum fractions increased slightly.

These ndings suggest that intense homogenisation is not bene -cial for the stability of the colloidal systems.

The decrease in the Brook eld viscosity of the DAMDs withincreasing the homogenisation pressure shown in Fig. 3(a) is sup-posed to explain this decrease of stability under intense homoge-nisation. We previously reported that the presence of non-adsorbed, excess CMC in AMDs increased the viscosity of drinksand slowed down the sedimentation of protein particles ( Du et al.,2007 ). Thus, the stability of our colloidal system is susceptible tothe change in the viscosity of the medium.

Fig. 3(b) shows that the Brook eld viscosity of the pure CMCsolution decreased with increasing homogenisation pressure,

strongly indicating a remarkable decrease in the M w of CMC underhomogenisation. Corredig and Wicker (2001) investigated the ef-fect of homogenisation on the M w of pectin and found that theapparent viscosity and M w of pectin decreased at high homogeni-sation pressure. The degradation of CMC shown in Fig. 3(b) isoneof the reasons for the decrease in viscosity of the DAMDs, leading tothe acceleration in sedimentation of the protein particles anddecrease in DAMD stability. More intense homogenisation may alsoresult in an increased number of small protein particles and morebare protein particles, such that relatively more CMC is required tocover all of these particles suf ciently andmaintain a stable system.However, because the M w of CMC decreases with increasing ho-mogenisation pressure, the stabilisation effect of CMC is weakened.At a given CMC concentration, the fraction of non-absorbed CMCbecomes relatively lower and is unable to effectively increase theviscosity of the solution. By combining changes in the particle size,Brook eld viscosity, sedimentation, and serum fractions atdifferent homogenisation pressures, a quali ed homogenisationpressure of 20 MPa was selected and used in the followingmeasurements.

3.2. The stability of DAMD and yoghurt drinks induced by CMC

The evolution of particle size D[4,3] of the DAMDs and yoghurtdrinks as a function of pH are shown in Fig. 4. In the DAMD withover 3 g $kg 1 CMC stabiliser, the particle size was approximately0.4 mm in the pH range from 3.6 to 4.6; the particle size in theyoghurt drinks increased from 4 mm to 6 mm when the pH wasreduced from 4.2 to 3.6. CMC adsorption onto the casein micellesoccurs at and below pH 5.2, just before the aggregation of caseinmicelles ( Du et al., 2007 ). Thus, the protein particles in the DAMDsare small and similar to those in natural casein micelles. The ag-gregation state of caseins is signi cantly different in the yoghurtdrinks. The base of yoghurt drinks is an acid milk gel formed duringfermentation ( Alting, Hamer, de Kruif, & Visschers, 2000; Luceyet al., 1997, 1998b; Lucey, 2004; Vasbinder, Alting, Visschers, & deKruif, 2003 ). The milk used in manufacturing of yoghurt is sub- jected to an extensive heating (90 C for 5 min), which does lead tothe denaturation of whey proteins and their association with caseinmicelles via hydrophobic interactions and the formation of inter-molecular disulphide bonds. This system could aggregate duringacidi cation, wherein denatured whey proteins associated withcasein micelles act as bridging materials by interacting with otherdenatured whey proteins. The cross-linking or bridging by dena-tured whey proteins could result in a rigid gel network by caseinmicelles ( Lucey et al., 1997, 1998b; Lucey, 2004 ). Thus, particlespresent in the yoghurt drink after homogenisation are large-sizedgel particles ( Tromp et al., 2004 ).

0 5 10 15 20 25 30

0

10

20

30

40

50

60

70

Pressure / MPa

P a r t i c l e s i z e

D [ 4

, 3 ] / m

% / n o i t c a r f m u r e

S & % / n o i t c a r f n o i t a t n e m i

d e

S 0

20

40

250

300

350

Fig. 2. Effects of homogenisation on the stability and particle size of DAMDs(40 g $kg 1 MSNF, 4 g$kg 1 CMC, and 80 g $kg 1 sucrose; pH ¼ 4.0). Filled square - :particle size; lled circle C : sedimentation fraction; empty circle B : serum fraction.

20

30

40

50

60

70

B r o o

k f i e l d v i s c o s i t y

/ m

P a . s

Pressure / MPa

a

0 5 10 15 20 25 30 35 0 5 10 15 20 25 30100

105

110

115

120

125

130

135

140

b

Pressure / MPa

B r o o

k f i e l d v

i s c o s

i t y / m

P a . s

Fig. 3. Brook eld viscosities of (a) AMDs (40 g $kg 1 MSNF, 4 g$kg 1 CMC and 80 g $kg 1 sucrose; pH ¼ 4.0) and (b) CMC solution ( c ¼ 10 mg/ml) as a function of homogenisation

pressure.

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The Brook eld viscosities of the DAMDs and yoghurt drinksstabilised by different CMC concentrations are shown in Fig. 5(a)and (b), respectively. Large casein aggregates were observed in theunstable samples at low CMC concentrations. The Brook eld vis-cosity cannot be measured in these unstable AMDs. For stablesamples, the Brook eld viscosity increased with increasing con-centration of CMC and decreased with decreasing pH. These resultsmay be attributed to two aspects. Firstly, the net charge of thecasein micelle is less negative during acidi cation. Therefore, moreCMC is adsorbed onto the casein micelles or milk gel particles byelectrosorption. The amount of non-adsorbed CMC that contributes

to the AMD viscosity decreases, leading to a decrease in theBrook eld viscosity. Secondly, the conformation of CMC chainsbecomes compact with lowering pH, thus decreasing the viscosity.The Brook eld viscosity of the yoghurt drinks is experimentallylarger than that of the DAMDs under the same conditions and at thesame amount of milk solids. This is because in the system of yoghurt drinks, there exists gel-like network of proteins cross-linked or bridged by denatured whey proteins, while the proteinparticles in the DAMDs are small and similar to those in naturalcasein micelles.

Sedimentation results obtained by centrifuging the DAMDs andyoghurt drinks are shown in Fig. 6. In the absence and presence of

1 g$kg 1 CMC, both of the sedimentation fractions of the DAMDsand yoghurt drinks are larger than 6%, and these systems are un-stable ( Fig. 6(a)). In the DAMDs, addition of 2 g $kg 1 CMC stabilisedthe milk proteins in the narrow pHrange from4.6 to4.4,addition of 3 g$kg 1 CMC stabilised them in the pH range from 3.8 to 4.6, andaddition of over 4 g $kg 1 CMC stabilised them over the entire pHrange tested. The sedimentation values are almost the same (below1%) when the CMC concentration is above 4 g $kg 1. For the yoghurtdrinks, stability over the whole pH range can only be achieved withaddition of more than 4 g $kg 1 CMC (Fig. 6(b)). The sedimentationof yoghurt drinks decreased with increasing CMC concentration.

3.3. Fraction of CMC adsorption in the DAMDs and yoghurt drinks

To investigate the different stabilising effects of CMC in theDAMDs and yoghurt drinks, the degree of CMC adsorption, i.e., thetotal amount of CMC added and adsorbed onto the casein micelles,was measured. The adsorbed CMC fraction was determined bymeasuring the viscosity of the serum after AMD centrifugation.Using the viscosity calibration shown in Fig. 1, the viscosity wasconverted into a CMC concentration.

Fig. 7 shows the degrees of CMC adsorption obtained in thismanner from AMD samples containing 40 g $kg 1 MSNF as afunction of overall CMC concentration. Almost all of the added CMCwas adsorbed onto the protein particles when the CMC concen-tration was between 1 and 2 g $kg 1. However, these AMDs wereunstable. Addition of 1 and 2 g $kg 1 CMCs was not enough to fullycover the protein particles, and the protein particles may aggregateby bridging occulation. These results are consistent with thoseshown in Fig. 6. At a CMC concentration of 3 g $kg 1, the degrees of CMC adsorption were identical in the DAMDs and yoghurt drinks(73%). A considerable amount of the added CMC was adsorbed ontothe casein micelles and milk gel particles. The AMD appeared to bestable when the CMC concentration was above the full coverageconcentration, and some of the CMC was not adsorbed at 3 g $kg 1

CMC. By increasing the CMC concentration to range from 4 g $kg1e

6 g$kg 1, the degree of CMC adsorption decreased in both theDAMDs and yoghurt drinks. However, the fraction of CMC adsorbedby the yoghurt drinks was higher than that adsorbed by theDAMDs. The fraction of CMC adsorbed by the DAMDs decreasedfrom 52% to 38% when the CMC concentration was increased from4 g$kg 1e 6 g$kg 1. However, the amount of adsorbed CMC perkilogram of DAMD did not change so much, only from ca. 2.1 g forthe sample containing 4 g/kg CMC to 2.2 g for the sample con-taining 6 g/kg CMC. We thus conclude that, in 4% MSNF, caseinmicelles in DAMDs are ef ciently covered by the addition of 3 g$kg 1 CMC. Addition of excess CMC (from 4 g $kg 1e 6 g$kg 1)

Fig. 4. Changes in the average particle diameter as a function of pH for DAMDs ( lledsymbols) and yoghurt drinks (open symbols) containing different CMC concentrations:

(A , > ) 3 g/kg1

CMC; (: , 6 ) 4 g/kg1

CMC; (C , B ) 5 g/kg1

CMC and ( - , , ) 6 g/kg 1 CMC.

0

10

20

30

40

50

60

70

80

B r o o

k f i e l d V i s c o s i t y

/ m

P a . s

pH

(a)

3.6 3.8 4.0 4.2 4.4 4.6 3.6 3.8 4.0 4.20

10

20

30

40

50

60

70

80

(b)

pH

B r o o

k f i e l d V i s c o s i t y

/ m

P a . s

Fig. 5. Effects of CMC concentration on the Brook eld viscosity for (a) DAMDs and (b) yoghurt drinks: ( B ) 2 g/kg 1 CMC; (, ) 3 g/kg 1 CMC; (: ) 4 g/kg 1 CMC; (C ) 5 g/kg 1 CMC

and (-

) 6 g/kg1

CMC.

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viscosities of the AMDs decreased with time, with the rate of decrease similar to that observed in the calibration curve. Thedecrease in viscosity of the serum CMC strongly suggests thedegradation of CMC at acidic condition ( Glinel et al., 2000 ), which isprobably the main reason that leads to the instability of AMDduring storage.

4. Conclusions

The stability of AMD induced by CMC depends on several pa-rameters, including the molecular properties of CMC and the pro-cessing conditions. The present work reveals the signi cance of homogenisation and degradation of cellulose gum stabiliser on thestability of AMDs via either directly acidi cation by citric acid orfermentation.

Small protein particles generated by homogenisation arehypothesised to result in stable DAMDs. However, their stabilitydeteriorates when high homogenisation pressure is appliedbecause of the degradation of CMC that results in the decrease inviscosity of the colloidal system and the new-born surface of pro-tein particles to which more CMC is needed to adsorb. Thus, aquali ed choice for homogenisation pressure (20 MPa in the pre-sent work) is required to achieve a good stability when a usuallypractical pressure range, such as 0 e 30 MPa, is applied.

CMC is an effective stabiliser for both DAMDs and yoghurtdrinks, and the stability of these two colloidal systems increaseswith increasing CMC concentration. Given the same amount of milksolids, concentration of CMC and homogenisation pressure, the gelparticle size of yoghurt drinks is larger than that of the casein mi-celles in DAMDs. The amount of CMC adsorbed onto the milk gelparticles in yoghurt drinks is larger than that in casein micelles inDAMDsat the same CMCconcentration and pH. The nal instabilityof the acidi ed milk productsis closely related to the degradation of CMC during storage at low pH.

Acknowledgements

The authors are thankful for the nancial support for this workfrom the Shanghai Leading Academic Discipline Project (No. B202).

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