Influence of the Acidification Process on the Colloidal Stability of Acidic Milk Drinks Prepared from Reconstituted Nonfat Dry Milk

Influence of the Acidification Process on the Colloidal Stability of Acidic Milk Drinks Prepared from Reconstituted Nonfat Dry Milk N. AMICE-QUEMENEUR,l J.-P. HALUK,1 and J. HARDY2 Ecole Nationale SuMrieure d'Agronomie et des Industries Alimentaires (ENSAIA) 2 Avenue de la forkt de Haye 54500 Vandoeuvre les Nancy, France T. P. KRAVTCHENKO Systems Bio Industries. Research and Development Laboratoire de Baupte 505oO Carentan, France ABSTRACT

producible preparations, can thus be accepted as a good material for studying the stabilization of acidic milk drinks with pectin. (Key words: glucono-A-lactone, acidic milk drink, stability, pectin)

Studies on the stabilization of acidic milk drinks with pectin require the availability of well-defined preparations of acidic milk drink, but traditional acidic milk drinks suffer from problems of reproducibility because of variations of milk quality and bacterial fermentation. The possibility of replacing bacterial fermentation of fresh milk by chemical acidification with glucono-A-lactone of reconstituted nonfat dry milk has been considered. The addition of 3% g1ucono-Alactone and subsequent incubation at 30'C provided acidic milk gels that were similar in texture to those obtained by fermentation with lactic acid bacteria. Despite different setting rates, as evidenced by continuous turbidimetric and rheological measurements, the acidic milk gels set at nearly the same pH for both glucono-A-lactone and microbial acidifications. On the addition of pectin, the corresponding acidic milk drinks exhibited similar rheological changes, indicating that the stabilization mechanism is probably the same for both acidification processes. Reconstituted NDM acidified with glucono-A-lactone, which provides re-

Abbreviation key: GDL = glucono-A-lactone, Rhl = reconstituted milk. INTRODUCTION

Received July 18, 1994. Accepted June 19, 1995. 'Laboratoire de Biochimie Appliquk. hboratoire de Physico-chimie et Wnie Alimentaires 1995 J Dairy Sci 78:2683-2690

Acidic milk drinks are very popular, especially in the Far East and, increasingly, in Europe. Most acidic milk drinks are manufactured by fermentation of milk with lactic acid bacteria, followed by homogenization, and finally dilution with fruit juice. Because of the low pH, the resulting products suffer from sedimentation problems from casein aggregation, which leads to wheying off on storage. The addition of high concentrations of methoxyl pectin prevents the sedimentation of casein particles. However, despite several attempts (7, 8, 9, 23, 24, 29), the fundamental mechanism of pectin and casein interactions is still not completely understood. Studies on the mechanism of such a stabilization require the reproducible preparation of acidic milk drink samples. The texture of the final drink probably depends on the texture of the starting yogurt gel. During yogurt manufacture, a gradual conversion of lactose into lactic acid by suitable microorganisms causes the pH to decrease. As acidity develops, milk proteins aggregate, leading to the formation of homogenous gel. The final texture of such a milk gel depends critically on the conditions of acidification (14), and variations often result from uncontrolled

2683

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AMICE-QUEMENEUR ET AL.

bacterial growth or unexpected contaminations (30). An alternative to achieve milk acidification is the addition of acidogens, such as citric acid, hydrochloric acid, propionic anhydride, or lactides. However, the direct addition of acid to milk does not lead to a gel texture, but rather to an immediate uncontrolled precipitation of the curd and whey separation from the milk. Indeed, only a gradual pH decrease, such as that effected by a bacterial culture, causes casein particles to aggregate into long chains that lead to the formation of a gel (19, 30). Glucono-A-lactone (GDL), which slowly produces gluconic acid, gives satisfactory delicate creamy and jelly-textured yogurts within a relatively short time and is thus often used to simulate acid-set milk gels such as yogurts (1, 3, 4, 9, 10, 1 1 , 12, 13, 14, 17, 18, 30, 31). However, differences still exist between the bacterial and GDL acidifications, leading, for example, to different rheological behavior. In our study, GDL was used to simulate the bacterial fermentation of milk to obtain a reproducible material to study the stability of acidic milk drinks. The results are discussed to determine the limits of such a preparation.

mixed lactic culture (Yalacta. Caen, France) containing Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus in the ratio 1: I . Incubation was at 42°C for 4 h until pH was 4.3, After cooling to 4 ° C further fermentation caused the pH to drop to 4.0.

MATERIALS AND METHODS

Yogurt gels (three per condition of acidification) were set in beakers of 6 cm in diameter and 6 cm high. Tests were carried out at 15°C with an Instron Universal Tester (model 1122; Instron, Buc, France), equipped with a 10-mm wide cylindrical probe and operated at a constant speed of 5 m d m in . The structural failure of RM gels was readily recognizable as a discontinuity in the force-penetration chart trace. The force on the probe at this point, the breaking strength, is often taken as the measure of gel strength (17). Each result was the mean of three measurements on three different RN gels.

Materlals

The GDL was from Merck (Darmstadt, Germany). The pectin was an experimental sample of lemon high methoxyl pectin produced by Systems Bio Industries (Boulogne, France). Mllk Preparation

Reconstituted milk (RM)was prepared from low heat NDM W t e r i e Saint Pkre, Saint Pkre en Retz, France), reconstituted as a 14% (wt/ wt) solution in distilled water. A single batch of milk powder, with a moisture content of 6.6% and stored at 4 ° C was used throughout the experiments. The RM was heated under stirring to 90°C in a water bath (heating from room temperature to 90'C took 20 min) and kept at 90°C for 5 min. The RM was then cooled rapidly to the temperature of acidification. Bacterial Acidification

After heat treatment, RM was cooled rapidly to 42°C and inoculated with 2% of a Journal of Dairy Science Vol. 78. No. 12, 1995

Chemlcal Acldlflcatlon

Gelation of RM was achieved by acidification induced by GDL (4, 12); 2.5, 3.0, or 3.5% GDL was added to the RM, and the mixture was incubated at 20, 30, or 42°C. All experiments were performed in triplicate. Gel Transltlon of Mllk

Gel transition of milk was followed simultaneously by measurement of pH, turbidimetry with an Analite turbidimeter model NTM (Novasina, Zurich, Switzerland), and rheology with a Gelograph-M (Gel Instruments, Thalwil, Switzerland), as described by Banon and Hardy (3). The experiments were repeated three times. Gel Penetratlon lest

Preparatlon of Acid Milk Drinks

The gels, prepared as described, were homogenized with an Ultra-turrax homogenizer (model T25 equipped with its S25N-25F adaptator; IKA, Staufen, Germany) at 24,000 rpm for 30 s; 60.7 g of the resulting liquid yogurts were then diluted with 39.3 g of distilled water containing 0, .lo, .15, -20, .30, or .40 g of pectin. Final acidic milk drinks thus contained 8.5% (wt/wt) milk SNF and 0 to .4%

2685

COLLOIDAL STABILITY OF ACIDIC MILK DRINKS

was usually minimized by using milk that had been reconstituted from milk powder. However, reconstituted milks had to be prepared with certain precautions to obtain results that were comparable with those obtained usSedimentation Test ing fresh milk. Samples were placed in a 150-mm test tube Preliminary trials (not shown) showed the to a depth of 100 mm. After storage for 21 d at necessity of heating the milk prepared from 4 ° C the height of the layer of clear supernatant milk powder prior to acidification. Without left at the top was measured. This value was preheating, acidic milk gels were very weak, taken as a measure of the instability of the shrinkable, and showed large syneresis. Kalab acidic milk drink (29). Each result was the et al. (20) and Harwalkar et al. (12) observed a mean of three measurements on three different very similar phenomenon. Kalab et al. (20) acidic milk drinks. showed that, during the acidification of unheated skim milk, casein micelles aggregated Flow Behavlor Measurement as tightly fused clusters. Conversely, in heated The flow behavior of the acidic milk drinks skim milk, casein micelles might be protected was measured at 20'C with a controlled-stress from fusion by the absorption at their surface rheometer (Carri-Med, Dorking, England) us- of whey proteins under the effect of heat ing a cone-plate system (cone: 0 = 40 mm; a denaturation. The presence of more numerous = lo). The shear stress was increased linearly apparent micelles in yogurts prepared from up to 5.968 N/m2 within 1 min, and then was heated skim milk resulted in more chains, immediately decreased to 0 within another more junction points, and a much less open minute. Samples were handled very carefully structure. Heating at 90°C for 5 min increased to minimize induced structural changes prior to gel breaking strength and reduced syneresis measurement. Apparent viscosities were read drastically. All acidic milk gels were thus preon the flow curves at a constant shear rate of pared from heated RM. (wt/wt) pectin. The products were again homogenized for 30 s at 24,000 rpm and stored at 4°C.

751s. Yogurt Preparation RESULTS AND DISCUSSION Milk Preparation

The first source of variability of dairy products was milk composition. This variation

In the preparation of acidic milk drinks, the first step was to make a yogurt, the texture of which influenced the texture of the final drink. Experiments were thus conducted to obtain GDL acidic milk gels that were similar in

TABLE 1. Comparison of the texture characteristics of reconstituted nonfat dry milk gels acidified with glucono-Alactone (GDL) under different conditions. GDL

Temperature

(%)

('(3

2.5 3.0 3.5 2.5 3.0 3.5 2.5 3.0 3.5

20 20 20 30 30 30

42 42 42

Gel pH

Gel breaking strength

Visual aspect

(M

-X 4.0 3.9 3.8 4.0 3.9 3.8 4.0 3.9 3.8

,033 ,034 ,036 ,092 ,110

,120 ,227 ,232 ,250

SE ,002 ,004

,003 ,003 .004 ,004 ,005

.006 .008

Smooth gel Smooth gel Smooth gel Smooth gel Smooth gel Smooth gel, slight Lumpy gel. slight Lumpy gel, slight Lumpy gel, slight

syneresis syneresis syneresis syneresis

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AMICE-QUEMENEUR ET AL.

by penetrometry of fermented yogurts and for yogurts prepared with 3% GDL at 30°C. The texture profiles of the yogurts prepared, either by fermentation or by chemical acidification with GDL, were very similar to each other, showing very close similarities in texture. Such textural similarities indicated that the two types of gel might have had very similar structural characteristics. Thus, we decided to prepare yogurts with 3% GDL at 30°C. 0

I

I

1

5

10

15

)

Kinetics of Gelation

Distance (mm)

The aggregation of casein particles with decreasing pH can be considered to be the Figure 1 . Textural profiles of the acidic milk gels result of two major destabilizing actions: the obtained by acidification with lactic acid bacteria (----)and by 3% glucono-A-lactoneat 30'C (-). Arrows indicate 1) elimination of the steric repulsion inherent to the breaking point and 2) the end of probe penetration. the hairy layer and the neutralization of electrostatic repulsions (27). For yogurts prepared with lactic acid bacteria, the pH is decreased by the slow release of lactic acid. For yogurts texture to those of yogurts prepared with lactic prepared by chemical acidification, the pH is acid bacteria. Table 1 shows the influence of decreased by the slow conversion of GDL into GDL content and temperature of acidification gluconic acid. on final pH and final texture of the acidic milk Figure 2 shows that, with GDL, the pH gels prepared with GDL. The final pH (measdecreased very quickly during the initial ured after 14 h) only depended on the amount minutes and then slowly declined; with lactic of GDL added to the milk. The gel breaking acid bacteria, the acidification was more strength mainly depended on the temperature gradual. Indeed, hydrolysis of GDL began as of acidification and was not significantly af- soon as it was brought into solution, but, in fected by the GDL content. These results con- traditional fermentation, bacteria needed a lag firmed those obtained by Banon and Hardy (3). period to adapt to environmental conditions. Desobry-Banon (6) showed that the fractal However, final pH was about 4.0 after 14 h for dimension of casein aggregates was higher both types of acidification. (i.e., lower density) at 20'C than above 30°C. This structural difference was probably due to differences in the mechanism of aggregation (2, 22); at low temperature (below 30°C), casein aggregates grew by the regular addition of monomeric casein particles to existing ag6.5 gregates (floc-particle mechanism), but, at high temperature (above 30"C), casein aggregates grew by the rapid grouping of smaller aggregates (floc-floc mechanism). A penetration test was used to compare the consistency of the yogurts prepared by bacterial fermentation or by GDL acidification. Acidic milk gels prepared with lactic acid bacteria were smooth and f m (breaking strength of .112 f .003 N). Table 1 shows that similar 0 50 100 150 200 250 texture could be obtained by acidification with Time (min) 3% GDL at 30°C. Figure 2. Changes in pH during the acidification of Figure 1 shows the complete texture pro- skim milk by lactic acid bacteria @) and by 3% gluconofiles (i.e., force-deformation curves) obtained A-lactone at 30°C (0).

1

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COLLOIDAL STABILITY OF ACIDIC MILK DRINKS

I

12,

g C c rn

U

z+J--ld,~~~l,,

, , I , , ,

,

I ,

0

0

50

100

150

200

,

.

,I 250

Time (min)

0

50

100 150 Time (min)

200

250

Figure 3. Changes in turbidity during the acidification of skim milk by lactic acid bacteria @) and by 3% glucono-A-lactone at 30°C (0).

Figure 4. Changes in gel strength during the acidification of skim milk by lactic acid bacteria @) and by 3% glucono-A-lactone at 30'C (0).

Turbidimetric measurements described the aggregation (i.e., gelation) of casein particles by recording the changes of optical properties: light reflectance increased as aggregate size increased. Turbidity curves (Figure 3) exhibited a sigmoidal profile that could be divided into three steps. During the lag period, turbidity remained constant and low. Modifications that occurred at the surface of casein micelles (4) did not visibly change the organization of milk particles. Then, turbidity increased very quickly as casein particles aggregated and formed a gel. During the last period, no further aggregation was visible, but aggregation corresponded to the stabilization of the gel network (27). Gelograph measurements described the rheological changes by recording the shear stress that was generated when a shear strain was imposed. The low amplitude (am) of the shear strain was assumed not to disturb the gel setting. Moreover, the shear stress values recorded under low shear strain were a good estimation of the elastic modulus. Figure 4 shows that the elastic modulus followed the same development sequences as turbidity. Only the first step was not visible because of the lack of sensitivity of the system used. Gelation occurred earlier with GDL (30 min after the addition of GDL) than with bacteria (90min after the beginning of incubation), but final gel elasticity and final turbidity were very similar for both types of acidification. Figure 5 shows that differences of setting rate were mainly due to differences of acidification rate:

gel strength and turbidity depend primarily on pH. However, coagulation occurred at slightly higher pH with lactic acid bacteria @H 5.3) than with GDL @H 5.0) because of the difference of temperature at which the two kinds of acidification were performed (30°C for GDL and 42'C for fermentation). Indeed, DesobryBanon (6) found that the setting pH decreased as acidification temperature decreased; thermal (Brownian) motion might be a major factor for the aggregation of casein particles because increased temperature increased the frequency of particle encounters (4). The wide similarities in kinetics of gelation between the two modes of acidification provide further evidence for the validity of the acidification of Rh4 with GDL to simulate yogurts fermented with bacteria. Rheological Propertles of Acidic Mllk Drlnks

Acidic milk gels were homogenized and diluted to obtain liquid acidic milk drinks of very low viscosity. In the absence of pectin, the resulting colloidal suspension of casein particles was very unstable, and curd separation occurred within a few hours for both fermented and acidic milk drinks acidified with GDL. Wheying off resulted from the aggregation and subsequent sedimentation of the casein particles. Figure 6 shows the flow curves obtained for milk drinks fermented by bacteria and acidified by GDL in the absence of pectin. For both drinks, the upward flow behavior was highly curved, and a yield stress was present. This Journal of Dairy Science Vol. 78, No. 12. 1995

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AMICE-QUEMENEUR ET AL.

behavior could be described by a HerschelBulkley model (R2 > .W): U

= uo

'4

3%

2%

1%

OS% 0%

+ m(+p

+

where U = shear stress, a0 = yield stress, = shear rate, n = flow behavior index, and m = consistancy coefficients. Shear thinning (n very different from 1) is usually ascribed to the disaggregation of some organized structure of the particles in suspension under the effect of shearing forces. For acidic milk drinks that have not been stabilized, the relatively high viscosity at low shear strain and the apparent yield stress were probably due to the presence of casein aggregates. As shear strain increased, weak casein aggregates were disrupted, and viscosity decreased. The downward shear rate curve (i.e.,

0

300

600

900

1200

1500

Shear rate (Is)

-

7T-

.30/, 2% . I %

0%

.OS%

I

1 2 ,

I

0

300

600

YO0

1200

7

lf;oo

Shear rate (Is) Figure 6. Flow curves as a function of dlfferent pectin concentration (0 to .3%) of the acidic milk drinks obtained by acidification with 3% glucono-A- lactone at 30°C (lop half; b) and with lactic acid bacteria W o r n half; D). 4

4

5

5

5.5

6.5

6

PH

4

4.5

5.5

5

6

6.5

PI1

Figure 5. Changes in turbidity and gel strength as a function of the pH during the acidification of skim milk by lactic acid bacteria p) and by 3% glucono-A-lactone at 30°C (e). Journal of Dairy Science Vol. 78, No. 12, 1995

after shearing) was essentially linear, showing almost Newtonian flow characteristics. This view was further supported by the time dependency of the viscosity of acidic milk drinks (not shown); the viscosity decreased with time under constant shear strain. Similar behavior occurred for stirred yogurts (5, 15, 16, 25, 26). However, for stirred yogurts, viscosity was generally higher because of a higher casein content. In the ascending phase, the acidx milk drink acidified with GDL presented a lower flow behavior index (.%I for GDL and .62 for fermented milk),higher consistency coefficient (.12 for GDL and .05 for fermented rmlk), higher yield stress (.37 Pa for GDL and .22 Pa for fermented milk),and higher apparent viscosity (.021 Pds for GDL and ,011 Pds for fermented milk) than did the fermented milk

COLLOIDAL STABILITY OF ACIDIC MILK DRINKS

5

25

U

; ;

20

L

+i

f

15

E 2

10

3;

5

0

0

1

1

5

2

3

4

Pectin concentration (9%)

Figure 7. Sedimentahon diagram as a f u n d o n of different pecm concentrations for acid^ mlk dnnks prepared with lacuc acid bactena p)and with 3% glucono-Alactone at 30°C p).

2689

In both cases, on addition of .4% pectin, the acidic milk drinks appeared to remain stable, and did not whey off on storage for several weeks (Figure 7). Figure 6 shows that the addition of pectin reduced the shear thinning and hysteresis drastically but increased the apparent viscosity. The shift toward a Newtonian behavior indicates that the strength of interaction between casein particles decreased as pectin content increased, leading to colloidal stability (23) in the same manner as processed fluid milks (32), which formed stable colloidal suspensions. However, the viscosity of pectin-stabilized acidic milk drinks (025 W S )is much higher than that given by Wayne and Shoemaker (32) for skim milk (.0016 Pds). This increase in viscosity has been explained (21) by the increase in size of the casein particles from the adsorption of a thick pectin layer at their surface. The changes in rheological behavior that occurred as the pectin content increased were very similar for milk drinks fermented or acidified with GDL. In addition, Figure 7 shows that the Same amount of pectin was required to stabilize the two types of acidic milk drinks, and, at this concentration, the flow curves (Figure 6)were almost identical to each other. These results suggested that the mechanism of stabilization by pectin was probably the same for both types of acidic milk

drink. Thus, even after homogenization, the GDL milk drink remained more structured than the fermented milk. Again, the difference of acidification temperature (30°C for GDL and 42°C for fermented milk drinks) might be responsible for these textural differences. Indeed Skriver et al. (28) showed that stirred yogurts exhibited less shear thinning and were more viscous at 32°C than at 43°C. For fermented acidic milk drinks. the presence of bacterial polysaccharides might provide some protection against casein aggregation. Also, ac- drinks. The chemical acidification with GDL incording to Trop (30), and Trop and Kushelevsky (31). GDL has a specific effect in stead of the traditional microbial fermentation interacting with free amino groups of milk can be used as a good means to prepare welldefined acidic milk drinks and to study their proteins by acylation. Flow curves of unstabilized acidic milk stabilization with pectin. drinks also exhibited a large hysteresis loop, REFERENCES indicating that disaggregation was not instantaneously reversible. The area enclosed beI Aguilera, J. M., and H. G. Kessler. 1989. Properties of tween the upward and the downward curves is mixed and filled-type dairy gels. J. Food Sci. 541213. 2 Aubert, C., and D. S. Cannel. 1986. Restructuring of a measure of the extent of structural breakcolloidal silica aggregates. Phys. Rev. Lett. 56:738. down during the shearing cycle. However, vis3 Baoon, S.. and J. Hardy. 1991. Study of acid milk cosity increased again on resting (thixotropic coagulation by an optical method using light reflecbehavior), indicating that casein particles retion. J. Dairy Res. 58:75. 4 Banon, S., and J. Hardy. 1992. A colloidal approach mained sticky. Indeed, because protein-solvent of milk acidification by gluconc-A-lactone. J. Dairy interactions were not changed by the mechaniSci. 75:935. cal treatment, the protein-protein interaction 5 Benezech, T.,and J. F. Maingonnat. 1993. Flow forces that induced gelation remained the properties of stirred yogurt: structural parameter a p proach in describing time-dependency. J. Texture same. These results differed from those of Stud. 24:455. Ramaswamy and Basak (23, who found that 6 Desobry-Banon, S. 1991. Modification de la structure the structural breakdown of stirred yogurts was des micelles de casCine lors de l’acidification du lait almost irreversible. par hydrolyse de glucono-A-Lactone. Ph.D. Diss., Journal of Dairy Science Vol. 78. No. 12, 1995

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