Effects of milk supplementation with skim milk powder, whey protein concentrate and sodium caseinate on acidification kinetics, rheological properties and structure of nonfat stirred yogurt

LWT - Food Science and Technology 42 (2009) 1744–1750

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Effects of milk supplementation with skim milk powder, whey protein concentrate and sodium caseinate on acidification kinetics, rheological properties and structure of nonfat stirred yogurt M.R. Damin a, M.R. Alcaˆntara b, A.P. Nunes b, M.N. Oliveira a, * a b

˜o Paulo University, Av Prof Lineu Prestes, 580. Bl 16, 05508-900 Sa ˜o Paulo, Brazil Faculty of Pharmaceutical Sciences, Sa ˜o Paulo, Chemistry Institute, Av Prof Lineu Prestes, 580. Bl 3, 05508-900 Sa ˜o Paulo, Brazil University of Sa

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 April 2008 Received in revised form 24 March 2009 Accepted 26 March 2009

Milk supplementation with milk proteins in four different levels was used to investigate the effect on acidification and textural properties of yogurt. Commercial skim milk powder was diluted in distilled water, and the supplements were added to give different enriched-milk bases; these were heat treated at 90  C for 5 min. These mixtures were incubated with the bacterial cultures for fermentation in a water bath, at 42  C, until pH 4.50 was reached. Chemical changes during fermentation were followed by measuring the pH. Protein concentration measurements, microbial counts of Lactobacillus bulgaricus and Streptococcus thermophilus, and textural properties (G0 , G00 , yield stress and firmness) were determined after 24 h of storage at 4  C. Yogurt made with milk supplemented with sodium caseinate resulted in significant properties changes, which were decrease in fermentation time, and increase in yield stress, storage modulus, and firmness, with increases in supplement level. Microstructure also differed from that of yogurt produced with milk supplemented with skim milk powder or sodium caseinate. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Casein Whey Texture Yogurt Structure

1. Introduction Yogurt is produced by fermenting milk with Streptococcus salivarius ssp. thermophilus and Lactobacillus delbrueckii ssp. bulgaricus, producing lactic acid. The casein becomes unstable and coagulates to form a firm gel, composed of strands of casein micelles, with whey entrapped within this matrix, which is interlocked via hydrogen bonds, forming a protein matrix. Yogurt structure is the result of disulfide bonding between k-casein and denatured whey proteins and by aggregation of casein as the pH drops to the isoelectric point of the casein proteins during fermentation. One of the most important attributes for yogurt quality is texture, which is mainly affected by milk base heating conditions, starter culture and yogurt shearing after fermentation (Lucey, Munro, & Singh, 1998; Sodini, Remeuf, Haddad, & Corrieu, 2004; Tamime, Robinson, & Latrille, 2001; Walstra, Wouters, & Geurts, 2006). Traditionally, the solids content of milk is increased for yogurt production. The three main systems available nowadays are good

* Corresponding author. Faculty of Pharmaceutical Sciences, Biochemical and Technological Technology, Sao Paulo University, Av Prof Lineu Prestes, 580. Bl 16, 05508-900 Sao Paulo, Brazil. Tel.: þ55 11 3091 3690; fax: þ55 11 3815 6386. E-mail address: [email protected] (M.N. Oliveira). 0023-6438/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2009.03.019

options to achieve desired protein and solids contents: (i) addition of protein powders (skimmed milk, whey protein concentrates, caseinates); (ii) evaporation of water from milk under vacuum; or (iii) removal of water by membrane filtration (Tamime et al., 2001). Several papers report the use of whey protein concentrate and caseinates in milk supplementation for yogurt production (Lucey, Munro, & Singh, 1999; Patocka, Cervenkova, Narine, & Jelen, 2006; Remeuf, Mohammed, Sodini, & Tissier, 2003; Sodini, Montella, & Tong, 2005). Protein fortification and heat treatment are the most important parameters that determine yogurt’s textural properties. Fortification to w5 g/100 g protein with skimmed milk powder improves yogurt’s rheological properties (Fox, 2001; Tamime et al., 2001). The effects of protein and total solids content are difficult to study separately, as the two variables cannot be modified independently. Increasing total solids (TS) improve yogurt texture, based on sensory and instrumental evaluations. The effects of different levels of TS were studied by Tamime and Robinson (1999) and they found that consistency was greatly improved as solids increased from 12 to 20 g/100 g. The greatest change was observed from 12 to 14 g/100 g whereas levels above 16 g/100 g resulted in less pronounced change. According to Prentice (1992), increase in protein levels is the principal factor influencing texture and enrichment of milk with milk powder results in the development of chains and aggregates of casein micelles. The nature and relative

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proportions of the different proteins in the dry matter significantly affect the texture of the final product (Almeida, Tamime, & Oliveira, 2008; Penna, Converti, & Oliveira, 2006; Puvanenthiran, Williams, & Augustin, 2002; Sodini et al., 2004). Penetrometry is used to perform texture profile analyses, which complement instrumental and sensory evaluation of texture. Measure of the force required to push a probe into yogurt to a fixed depth of penetration is called hardness or firmness (Pons & Fiszman, 1996). Heat treatment and high hydrostatic pressure processing denature the whey proteins. The denatured whey protein acts as bridging material which becomes bound to casein micelles, resulting in increases in the storage modulus (G0 ) and yield stress (Harte, Clark, & Barbosa-Ca´novas, 2007; Lucey, Teo, Munro, & Singh, 1997). High-heat treatment causes high degrees of whey protein denaturation (>50%), which is associated with a marked increase in complex viscosity. Yogurt exhibits a variety of nonNewtonian effects, such as shear-thinning, yield stress, viscoelasticity and time-dependency (Benezech & Maingonnat, 1994; Lucey et al., 1997; Sodini et al., 2004; Steffe, 1996). Yield stress, a rheological property that is defined as the minimum shear stress required to initiate flow, characterizes the firmness of yogurt. Several methods have been suggested to measure this property (Steffe, 1996). Rheometers operating in dynamic mode allow the calculation of storage modulus and loss modulus, which describes the elastic and viscous properties. In linear viscoelastic regions, the structure is maintained and gel properties can be characterized (Lucey et al., 1997; Sodini et al., 2004; Steffe, 1996). Scaling models have been used to study several protein and colloidal gel systems to investigate the structure of the gel network and the fractal dimension of protein aggregates. The gel is considered to be a collection of fractal flocks. These models relate the rheological properties of the gel by measuring yield stress (s0), critical strain (g0 limit of linearity), and storage modulus (G0 ). When these properties exhibit power law behavior with respect to particle concentration, it is possible to extract structural information. Two types of behaviors are observed, depending on the strength of the links between the flocks: strong link regime or weak link regime (Bremer, Bijsterbosch, Schrijvers, van Vliet, & Walstra, 1990; Eleya, Ko, & Gunasekaran, 2004; Genovese, Lozano, & Rao, 2007; Shih, Shih, Kim, Liu, & Aksay, 1990). Major gaps exist in our knowledge regarding the texture of stirred yogurt, since the protein network is broken up during cooling and packaging, processes that provoke changes in textural properties. We investigated the influence of milk supplementation with skim milk powder, whey protein concentrate and sodium caseinate, using four different levels of supplements, on maximum acidification rate, time to achieve the maximum acidification rate, time necessary to reach pH 4.5, yield stress, storage and loss modulus, firmness and the structure of nonfat stirred yogurt.

solids /100 g. This milk base without supplementation was used as a control. Dairy ingredients that were used included skim milk powder (SMP) (Molico, Nestle, Brazil), concentrated whey powder (WPC) (Dairy Pro 80, ISP Corp., Brazil) and sodium caseinate (SC) (Alanate 180, Fonterra Co-operative Group, New Zealand). The concentration of protein in the supplements was 33.4 (0.7), 71.1 (3.7) and 80.0 (3.5) g/100 g, for SMP, WPC and SC, respectively. These ingredients were added to the milk and dissolved by mechanical agitation for 10 min at 800 rpm (Q-250M1, Quimis, Brazil), using 2 L samples. Twelve milk bases were prepared by supplementing the control milk with four different levels of one of the three dairy ingredients in order to obtain different levels of protein (Table 1). Each milk base was heat treated in a heated boiling water bath until it reached 90  C, stand for 5 min, immediately cooled in an ice bath to 10  C, poured into 250 mL flasks, and stored at 4  C for 24 h before use.

2. Materials and methods

Milk base

2.1. Microbial cultures Commercial yogurt cultures (Danisco, France) were used: S. salivarius ssp. thermophilus, TA040 (ST) and L. delbrueckii ssp. bulgaricus, LB340 (LB). Skim milk was sterilized at 12  C for 10 min, cooled to 42  C and then pre-cultures were prepared individually by adding each pure spray-dried culture to 50 mL of sterilized milk, in amounts sufficient to attain initial counts of 108 cfu/mL. These were maintained at 42  C for 40 min before use. 2.2. Milk base preparation Commercial skim milk powder (Molico, Nestle, Brazil), was diluted in distilled water to obtain approximately 12 g total

2.3. Yogurt manufacture and acidification parameters Milk bases were inoculated at fermentation temperature with 0.4 mL/100 mL of ST and with 0.4 mL/100 mL of LB pre-cultures. They were subsequently incubated at 42  C in a water bath until pH 4.50 was reached. Each trial, performed in two independent replicates, was monitored by using the Cinac (Cine´tique d’acidification) system (Spinnler & Corrieu, 1989), which allows continuous recording of pH and computes acidification rates during fermentation. Chemical changes were followed by measuring the kinetics parameters of the pH–time curves: Vmax (maximum acidification rate that measures the decrease in pH units per minute; values are expressed as 103 pHU/min), tVmax (time to achieve the maximum acidification rate, in h), and tpH4.5 (time necessary to reach pH 4.5, in h). When the fermented milk reached pH 4.5, it was manually stirred with a stainless steel perforated disk with up and down movements for about 1 min. The product was placed in 50 mL plastic cups, which were sealed with a thermal machine Selopar (BrasHolanda, Brazil) and quickly cooled in an ice bath. These yogurt samples were stored at 4  C before evaluation. 2.4. Physicochemical determinations The protein and total solids contents of the heat treated milk were determined with an ultrasonic milk analyzer Ekomilk (Eon Trading, Bullgary), in triplicate before fermentation (Venturoso, Almeida, Rodrigues, Damin, & Oliveira, 2007). The pH of the fermented milk was determined with a pH meter model Q-400M1 Table 1 Addition of supplements to the milk bases and control milk used for nonfat yogurt production, expressed in g/100 g. Supplement (g/100 g) SMP

WPC

SC

Control

12.00

–

–

SMP1 SMP2 SMP3 SMP4

13.00 13.50 14.00 14.50

– – – –

– – – –

WPC1 WPC2 WPC3 WPC4

12.00 12.00 12.00 12.00

0.30 0.60 0.90 1.20

– – – –

SC1 SC2 SC3 SC4

12.00 12.00 12.00 12.00

– – – –

0.25 0.50 0.75 1.00

SMP ¼ skim milk powder; WPC ¼ whey protein concentrate; SC ¼ sodium caseinate.

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(Quimis, Brazil). Analyses were performed in triplicate after 24 h of product storage at 4  C.

Table 2 Concentration of protein (g/100 g) in the milk bases and control milk used for nonfat yogurt production.

2.5. Counts of yogurt cultures

Protein concentration

Protein contenta

Control 1 2 3 4

4.50  0.02 4.71  0.07 4.90  0.11 5.04  0.08 5.14  0.09

2.6. Textural properties 2.6.1. Rheological properties Rheological properties parameters were measured at 10  C with a Physica MCR300 rheometer (Physica, Stutgart, Germany). The rheometer was equipped with parallel plates (50 mm diameter and 1.0 mm gap). Yield stress (s0) was determined in a controlled stress ramp; s0 is the intersection of tangents in the change in slope of the curve log deformation (g) versus log shear stress (s). Stress varied from 1 to 100 Pa, determined at 30 measuring points. Tests were performed with four replications of each of the samples. Strain sweep tests (in duplicate) were performed to measure storage modulus G0 and loss modulus G00 , and to determine the linear viscoelastic region of the fermented milks, with strain between 0.001% and 50% at a frequency of 1 Hz. The limit of linearity as the end point of the linear region was considered the point at which G0 deviated more than 5% from its maximum value. 2.6.2. Firmness Firmness was determined with a puncture test made with a TAXT2 texture analyzer (Stable Micro Systems, Godalming, England) of the samples in plastic cups, at 4–6  C, after one day storage at 4  C. The probe used was a 2.5 cm diameter acrylic cylinder. Pretest and test speed were fixed at 5 mm/s and 10 mm/s, respectively, and penetration depth was 10 mm. Firmness was defined as the force necessary to reach the maximum depth. Each test was replicated four times. 2.6.3. Microstructural analyses After 24 h of storage at 4  C, a sample of yogurt produced with milk supplemented with sodium caseinate and a control yogurt sample were freeze dried using an Edwards model L4KR 118 (BOC Edwards, Brazil). The dried samples were placed on stubs covered with double-face tape for observation in a field emission scanning electron microscope (JEOL model JSM-7401-F, JEOL Ltd, Japan), operating at a voltage of 1.0 kV. The images were registered under magnifications from 2000 to 3000 and six fields were observed. Images were analyzed using the software Image Pro Plus v.4.5.1 (Media Cybernetics, USA) as JPG files. 2.7. Statistical analysis Statistical analysis of data for effects of milk supplementation was performed by ANOVA and Tukey test (P < 0.05) using Statistica 6.0 (Statsoft, USA). 3. Results and discussion

a

Means of triplicate measures.

supplements. The total solids content in the milk control was 12.5 g/100 g, which varied from 13.0 to 14.0 g/100 g in the supplemented milk bases. There was a small decrease in the pH of the yogurts from the time fermentation was stopped till 24 h after storing the product at 4  C (post-acidification). A decrease in pH during storage is expected as result of the activity of LB (Tamime & Robinson, 1999); this has been observed in lactic beverages (Oliveira, Sodini, Remeuf, Tissier, & Corrieu, 2002). Post-acidification occurred independently of the level of supplementation or the ingredient used, and was 0.15 pH units on average. 3.2. Chemical changes during fermentation Maximum acidification rates, Vmax, were 21.0  103 pHU/min for milk base fortified with SC, while low Vmax values were observed for all SMP milk bases. Time to achieve the maximum acidification rate tVmax was not correlated with supplement level. Fermentation time (tpH4.5) decreased with increases in SC concentration. An opposite effect was observed with increases in SMP levels. However the tpH4.5 for WPC supplemented milk increased for 0.6 g/100 g compared to 0.3 g/100 g (supplement levels 2 and 1, respectively), and then remained stable at the higher concentrations (Fig. 1). Similar effects of WPC on increases in time to reach pH 4.6 were observed by Puvanenthiran et al. (2002), with decreases in the ratio of SMP to WPC, although total protein content decreased from 4.25 to 4.14 g/100 g. Lucey et al. (1997) found an increase in pH during cooling, and consequently a reduction in acidification time for milk supplemented with WPC (79.8 g protein/100 g), followed by heat treatment, in a comparison with heat treated milk that had been acidified with glucono-d-lactone (GDL). Islenten and KaragulYuceer (2006) reported that milk supplementation with SMP, SC,

6.0

tpH5.0 tpH4.5 (h)

Bacteria counts were made after the fermented milk products were stored at 4  C for 24 h. Each sample was prepared according to the methods described by the International Dairy Federation (IDF, 1996, 1997, 2003). ST was grown on M17 Agar (Oxoid, England) at 37  C aerobically for 48 h. LB was grown aerobically at 37  C for 72 h on MRS Agar (Oxoid, England) acidified to pH 5.4 with acetic acid. These microbiological analyses were performed in duplicate.

5.0

4.0

3.0

1

2

3

CS 3.1. Physicochemical determinations Table 2 shows the protein composition in the milk control and milk bases prepared with four different amounts of the three

4

1

2

3

SMP

4

1

2

3

4

WPC

Fig. 1. Time to achieve pH 4.5 (black) and 5.0 (striped) in nonfat yogurt prepared with milk supplemented with four different levels of sodium caseinate, skim milk powder, and whey powder concentrate. Vertical bars denote 95% confidence intervals. Table 1 describes supplement levels 1–4. Means (n ¼ 4).

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whey protein isolate or a texture improver did not affect the time to reach pH 4.7, although it influenced the sensory and physical properties of the yogurt. Sodium caseinate and texture improver additive enhanced yogurt properties.

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1000 800 700 600 500 400 300

3.3. Bacterial counts of yogurt cultures The mean initial microbial counts for the activated cultures used as inocula were wlog 8.3 and 8.9 cfu/mL for L. bulgaricus and S. thermophilus, respectively. The yogurt was cooled in an ice bath immediately after reaching pH 4.5 (time corresponding to fermentation time) and we considered the counts at d1 as the final count when the fermentation was stopped. The mean numbers of viable cells of L. bulgaricus and S. thermophilus found in all yogurt were log 8.60  0.05 and log 9.20  0.06 cfu/mL, respectively, with no significant variability among the three supplements and levels of supplementation. It is normally recommended that yogurt or fermented milk should contain at least one million viable cells per gram at the time of consumption. We found that ST dominated over LB, as has been previously reported (Birollo, Reinheimer, & Vinderola, 2000; Dave & Shah, 1997; Oliveira, Sodini, Remeuf, & Corrieu, 2001). McComas and Gilliland (2003) concluded that whey protein hydrolysate had no effect on the growth of strains of L. bulgaricus, but they reported a strain-dependent effect on S. thermophilus. 3.4. Textural properties 3.4.1. Rheological properties We found significant differences in the rheological properties between yogurts made from milk base fortified with different ingredients and at different concentrations. All samples exhibited typical shear-thinning behavior based on the flow curves. Yield stress was strongly affected by SC supplementation, increasing linearly from 16.3 to 31.4 Pa (192%), with increases in SC concentration from 0.25 to 1.00 g/100 g. When milk was supplemented with SMP s0 values initially increased, with the highest value at 13.5 g SMP/100 g level, but decreased again at 14.0 and 14.5 g SMP/ 100 g. Samples supplemented with WPC had s0 ranging from 15.7 to 16.2 Pa (Fig. 2). Increased protein interactions and protein– protein bonds increases the elastic character of the gel making yogurt less susceptible to rupture (Lankes, Ozer, & Robinson, 1998).

36

32

G' G" (Pa)

200 100 80 70 60 50 40 30 20 10

1

2

3

4

1

CS

2

3

4

1

2

3

4

WPC

SMP

Fig. 3. Storage modulus (G0 ) (black) and loss modulus (G00 ) (striped) as a function of protein concentration for nonfat yogurt supplemented with sodium caseinate, skim milk powder, and whey powder concentrate. Vertical bars denote 95% confidence intervals. Table 1 describes supplement levels 1– 4. Means (n ¼ 8).

Tamime, Kalab, and Davies (1984) and Remeuf et al. (2003) also reported interaction between these properties and heating intensity and the type of protein added. Storage modulus G0 values were higher than G00 for all samples, indicating a typical weak viscoelastic system, with elastic characteristics, resulting in better stability during storage. The rheological results are shown in Fig. 3; the influence of the supplements was similar to that found for s0. The storage (G0 ) and loss modulus (G00 ) were highly influenced by milk base supplementation with SC: G0 increased 277% and G00 increased 284%, with increasing SC levels. Samples supplemented with SMP and WPC resulted in a decrease in G0 with increasing levels of these supplements (23.4 and 11.5% decrease, respectively). Pearson correlation coefficients between protein level and rheological properties are presented in Table 3. However, G00 increased slightly with increasing levels of both of these ingredients. Lucey et al. (1997) studied milk acidified by glucono-d-lactone (GDL); they found an increase in storage modulus for milk supplemented with WPC (which has 79.8 g protein/100 g), followed by heat treatment, when compared with milk that is only heat treated. When milk was heat treated and then WPC added, G0 was lower than for milk without WPC. They concluded that rheological properties at small or large deformation levels are mainly altered by heat treatment. A correlation between protein content and storage modulus was reported by Allmere, Åkerlind, and Andree´n (1999).

τ0 (Pa)

28

24

Table 3 Pearson correlation coefficients between protein level and rheological properties, firmness, and time to reach pH 4.5 of nonfat yogurt made from milk base supplemented with sodium caseinate, whey protein concentrate or skim milk powder.

20

Protein level

16

12

s0 (Pa) 1

2

3

4

Level Fig. 2. Yield stress (s0) of nonfat yogurt prepared with milk supplemented with four different levels of sodium caseinate (black), skim milk powder (gray), and whey powder concentrate (striped). Vertical bars denote 95% confidence intervals. Table 1 describes supplement levels 1–4. Means (n ¼ 8).

G0 (Pa) G00 (Pa) Firmness (N) tpH4.5 (h)

SMP

WPC

SC

0.89*** – – 0.82*** 0.96***

– 0.64* 0.70* 0.65* 0.95***

0.92*** 0.99*** 0.98*** 0.97*** 0.96***

s0 ¼ yield stress; G0 ¼ storage modulus; G00 ¼ loss modulus; tpH4.5 ¼ time to achieve pH 4.5. *P < 0.05; ***P < 0.001.

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increase in exopolysaccharide production by a strain of S. thermophilus when milk was supplemented with WPC, an effect that could result in an increase in firmness and s0; but this WPC effect was not affected by the level of supplementation. When milk was supplemented with SC, a positive correlation was observed between protein levels and s0, G0 , G00 and firmness, with correlation coefficient (r) values of 0.92, 0.99, 0.98 and 0.97, respectively. A negative correlation was observed between protein levels and tpH4.5 (r ¼ 0.96). SC supplementation influences final texture and rheological properties, and it also reduces the time to reach the final pH. Different behaviors were observed in correlations between protein level and yogurt properties when milk was supplemented with SMP or WPC (Table 3).

0.9 0.8

Firmness (N)

0.7 0.6 0.5 0.4 0.3 0.2

1

2

3

4

Level Fig. 4. Firmness of nonfat yogurt prepared with milk supplemented with four different levels of sodium caseinate (black), skim milk powder (gray), and whey powder concentrate (striped). Vertical bars denote 95% confidence intervals. Table 1 describes supplement levels 1– 4. Means (n ¼ 8).

The tendency that we observed of increasing s0 and G0 for milk supplemented with SC indicates that the system tends to be in strong linkage regime; i.e. inter-floc links become stronger than intra-floc links (Shih et al., 1990). Yogurt is suggested to have weak bonding; but SC supplementation tends to change the character of the gel. The protein concentration in the yogurt formulations varied little; this variation was not sufficient to allow us to use our rheological results to obtain fractal dimensions (data not shown). Our observations on gel firmness are consistent with those observed for s0 and G0 . Fig. 4 shows that firmness values increased linearly with SC level, as observed in s0 and G0 for this supplement. When milk was supplemented with SMP or WPC, there were only small no significant differences in firmness. Tamime et al. (1984) also found that for a protein/total solid ratio of 0.–0.42 the firmness of yogurt made with SC was 30% higher than that of yogurt made with SMP-fortified milk base, although the total solid content was lower (12.8 g/100 g). The effect of milk fortification with milk protein or hydrolysate on increase in firmness and complex viscosity has been demonstrated in several studies (Damin, Minowa, Alcaˆntara, & Oliveira, 2008; Dave & Shah, 1998; Oliveira et al., 2001; Sodini et al., 2004). Zizu and Shah (2003) reported an

3.4.2. Microstructure We examined microscope images of samples SMP3 and SC3 (Fig. 5a and b, respectively), because yogurt prepared with SC differed significantly in texture and rheological properties. Digital images were flattened, and brightness and contrast were inverted. Aggregates could be seen in a network form, and pores appeared as dark spots. These pores or void spaces were places where the aqueous phases were confined; the diameters of these pores vary from 1 to 30 mm (Lucey & Singh, 1998). The mean diameter of the pores was determined, and analyzed in distribution plots. Yogurt made from milk made only with SMP (Fig. 6a) had a large number of pores; 49.6% of the pores were less than 1 mm; they had a mean diameter of 3.39 mm. The mean diameter of the pores in yogurt made from milk supplemented with SC was 4.81 mm, with fewer pores; 36.9% of the pores were less than 1 mm (Fig. 6b). The influence of heating milk has been well studied by Lee and Lucey (2006), and Lucey et al. (1998, 1999). The effects of milk supplementation were studied by Puvanenthiran et al. (2002) and Remeuf et al. (2003); they concluded that while whey powder concentrate and caseinate cause small changes in structure, they provoke greater changes in physical properties. 4. Conclusions Supplementation of milk with skim milk powder, whey protein concentrate and sodium caseinate at different levels caused a number of changes in the acidification kinetics and rheological properties of nonfat stirred yogurt. Fermentation time decreased with increases in sodium caseinate supplementation 1evels. An

Fig. 5. Microstructure of nonfat yogurt prepared with milk supplemented with skim milk powder (a ¼ SMP3) and sodium caseinate (b ¼ SC3).

M.R. Damin et al. / LWT - Food Science and Technology 42 (2009) 1744–1750

a

SMP 1500 1350

Number of observations

1200 1050 900 750 600 450 300 150 0

0

1

2

3

4

5

6

7

8

9

10

7

8

9

10

Mean diameter (μm) SC

b 1500 1350

Number of observations

1200 1050 900 750 600 450 300 150 0

0

1

2

3

4

5

6

Mean diameter (μm) Fig. 6. Distribution of pore diameters in nonfat yogurt prepared with milk supplemented with skim milk powder (a ¼ SMP3) and sodium caseinate (b ¼ SC3). Means (n ¼ 6).

opposite effect was observed when skim milk powder supplementation increased, and no changes were found when whey protein concentrate was added. Bacterial counts were similar for all supplements and supplement levels. Yogurt from milk supplemented with sodium caseinate resulted in a more elastic product, with greater firmness; these are important characteristics for commercial products. Microstructures of nonfat stirred yogurt made from milk supplemented with sodium caseinate differed from those found in yogurt made with skim milk powder in size, number and diameter. The structure of nonfat yogurt prepared with milk supplemented with skim milk powder was less firm than that made with sodium caseinate, based on texture measurements (lower yield stress and lower elasticity). Acknowledgement The authors thank FAPESP (the state of Sa˜o Paulo research foundation), CAPES and CNPq for financial support and DANISCO for furnishing the cultures. References Allmere, T., Åkerlind, M., & Andree´n, A. (1999). Rheological properties of acidified gels of skim milk from cows selected for high or low milk fat concentration. International Dairy Journal, 9, 703–707.

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