EFFECT OF THICKENERS ON THE TEXTURE OF STIRRED YOGURT

Transcription

1 EFFECT OF THICKENERS ON THE TEXTURE OF STIRRED YOGURT D. GONÇALVEZ 1*, M.C.PÉREZ 1, G. REOLON 1, N. SEGURA 1, P. LEMA, 2, A. GÁMBARO 1, P. VARELA 1, G. ARES 1. 1 Facultad de Química. 2 Facultad de Ingeniería Universidad de la República URUGUAY Abstract. Texture is one of the main characters that define the quality of yogurt. The effect of the addition of gelatin and starch on the rheological properties of sweetened plain stirred yogurt was studied. Six samples were prepared: two with gelatin (3000 and 6000 ppm), three with starch (1000, 5000, ppm) and a sample without thickener. Rheological characteristics of the samples were evaluated by one shearing cycle using a coaxial cylinder Haake VT500 viscometer, shear rate ranging from 11.7 to s -1. Time effect at constant rate was determined shearing the samples at fixed rotation. Yield stress (σ 0 ) and hysteresis were also determined. Syneresis (%) was measured by centrifugation at 1100 rpm during 10 minutes. Sensory characterization was performed with a panel of trained sensory assessors, who evaluated with non structured 10 cm scales the following texture attributes: viscosity, ropyness, creaminess and mouthfeel. All samples showed thixotropic and pseudoplastic behavior. Since the up-ward curve did not fit a unique model, it was divided in two regions. The first one fitted Herschel-Bulkley s model. The addition of gelatin decreased flow behavior index (n). Yield stress significantly increased with the addition of both thickeners. The down-ward flow curve fitted a lineal model, showing disruption of the protein network which forms the yogurt gel. At fixed rotation, apparent viscosity decreased with time, curves fitted Weltman s model. The addition of thickeners increased all texture sensory parameters. Non-oral textural parameters were highly correlated with oral textural parameters. Gelatin was more efficient in reducing syneresis than starch. Non-oral and oral parameters were highly correlated with rheological parameters. The addition of thickeners to yogurt significantly modified all textural parameters (sensory and instrumental), reducing syneresis as well. Keywords: Yogurt, Texture, Thickeners. 1. Introduction Texture is one of the main characters that define the quality of yogurt. The most frequent defects related to yogurt texture, that may lead to consumer rejection, are apparent viscosity variations and the occurrence of syneresis (Kroger, 1975). Thickeners are added to provide an acceptably firm texture and reduce syneresis. Two of the most frequently used thickeners are gelatin and starch (Walstra, 1999) Gelatin improves the texture of yogurt, giving as a result a firmer product with less tendency to syneresis. This effect has been attributed to the interaction of gelatin with the casein matrix of yogurt to develop a stonger three-dimensional network (Fiszman et al., 1999). Starch is used in yogurt to increase its viscosity, improve its mouthfeel, and prevent syneresis. It is one of the most frequently used thickening agents in yogurt production due to its processing ease, and low cost when compared with other hydrocolloids (Foss, 2000). * Address: Cátedra de Ciencia y Tecnología de Alimentos. Facultad de Química. Gral. Flores CP Montevideo, Uruguay. degon@adinet.com.uy 1

2 Yogurt rheological characterization is required for product and process development and to ensure consumer acceptability (Benezech and Maingonnat, 1994). This characterization can be done using either instrumental or sensory measurements. Different sensory texture descriptors have been used to characterize the texture of yogurt. Firmness, creaminess, viscosity, mouthfeel and syneresis are considered the most important descriptors for the textural perception of yogurt (Muir and Hunter, 1992; Gámbaro, 2002). The cost associated with the training and running of sensory panels makes the search for instrumental measurements to characterize yogurt texture, a continuous effort for researchers. The aim of the present work is to study the effect of the addition of two widely used thickeners, gelatin and starch, in rheological and sensory texture properties of plain stirred yogurt, and to study correlations between instrumental and sensory texture parameters. 2. Materials and Methods 2.1. Yogurt manufacture Six samples of yogurt were elaborated: 2 with the addition of gelatin (Leiner Davis, 232º Bloom) at two different concentrations (3000 and 6000 ppm), 3 with modified starch (Purity W E1404: oxidized starch, E1442 hidroxipropilated starch National Starch & Chemical) in three concentrations (1000, 5000 and ppm) and one without thickener (control sample). Samples were elaborated using commercial skim UHT milk (0.1 % milk fat, % lactose, % protein). Total solids of the samples was standardized at 11.1 % (w /w), using 8 % white commercial sugar and dried skimmed milk (1 % fat, 52 % lactose, 36 % protein, 7 % ash and a moisture content of 4%). All the components, including thickeners, were manually mixed. The resulting mixture was heated at 95 ºC for 15 minutes. For those samples containing starch, the thermal treatment was 15 minutes at 90ºC. Finally, the solution was inoculated at 42 ºC with a yogurt starter consisting of Streptococcus salivarius subsp. thermophilus and Lactobacillus bulgaricus (Yo-Mix VM 1-30 Visbyvac Danisco Cultor). Inoculated milk was incubated at 42 C until ph reached 4.5. Then, yogurt was cooled to 5 C and the coagulum was broken using standardized procedure. Samples were kept at refrigeration temperature (5 C) for 3 to 5 days before testing Rheological characterization The rheological characterization of the samples was performed with a Haake VT500 coaxial viscometer equipped with a MV1 measuring head (Haake Mess-Technik, Karlsruhe, Germany). Samples were poured into the temperature controlled cell (7 C) of the viscometer, following the process recommended by the instrument manufacturer. The freshly loaded sample was let to rest for 10 minutes before the flow curves were recorded. The samples underwent one shearing cycle (acceleration and deceleration), operating the viscometer in manual mode, increasing (or decreasing) the speed every 20 seconds, corresponding to shear rates of 11.7 to s -1. Triplicate measures were performed for each sample. 2

3 Shear stress and shear rate were calculated from the measured torque and rotation speed. Yield stress (σ 0 ) was estimated from the intercept of the ascending flow curve with the shear stress axis. Flow curves were fitted to Bingham and Herschel-Bulkley rheological models, and model parameters were calculated. Hysteresis was calculated as the area enclosed by the ascending and descending flow curves during the shearing cycle. Time dependence of apparent viscosity was studied with a constant shear rate (D). The resulting shear stress vs. time curve was fitted to Weltman Model (σ = A B*log(t)), and the parameters of were calculated Syneresis Yogurt (30-40g) was centrifuged at 1100 rpm for 10 minutes. The clear supernatant was poured off, weighed and recorded as syneresis (%). Duplicate measures were performed for each sample Sensory Evaluation The sensory panel consisted of eleven trained sensory assessors, with a minimum of six months of experience in texture evaluation. They evaluated four texture attributes: viscosity, ropyness, creaminess and mouthfeel. A detailed description of texture attributes is shown in Table 1. Attribute Viscosity Ropyness Creaminess Mouthfeel Table 1. Description of texture attributes Description Force necessary to stirred with a spoon. Evaluation of the amount of threads or drops that falls down when the spoon is raised vertically from the sample. Time necessary to dissolve or mix the sample with saliva. Evaluation of the lay that covers the palate and tongue after swallowing the sample. A balanced complete block experimental design was carried out by each panelist for duplicate evaluation of each sample. Non-structured 10 cm long scales anchored with nil and high were used to describe attribute intensity. The test was carried out in a sensory laboratory designed in compliance with ISO 8589: Data analysis Analysis of variance (ANOVA) for all samples was performed. Mean rating and Fisher s Least Significant Difference were calculated. Sensory and instrumental parameter correlation matrix was obtained to identify the relationship between these parameters. Statistical analyses were performed using the Statistica 5.1 package (StatSoft Inc., USA). 3. Results and discussion 3.1. Rheological characterization 3

4 All samples showed characteristic pseudoplastic behavior flow curves during the first part of the ascending curve (Fig. 1), in agreement with previous work (Benezech and Maingonnat, 1994). 80,0 60,0 σ (Pa) 40,0 20,0 0,0 0,0 200,0 400,0 600,0 800,0 1000,0 1200,0 1400,0 D (1/s) Figure 1. Shear stress (σ) vs. shear rate (D) during the shearing cycle (ascending and descending) for yogurt manufactured with 3000 ppm of gelatin. Yield stress was determined by direct extrapolation of the ascending flow curve. As shown in table 2, the addition of thickeners significantly (p<0.05) increased yield stress; corresponding to firmer yogurts. The higher yield stress values were obtained for samples manufactured with gelatin. For all samples, the up-ward curve did not fit a unique model in the tested shear rate range. This indicates that a major change in the rheological behavior occurred at a certain shear rate, probably due to a disruption in yogurt microstructure (cf. Fig. 1, ascending curve). The shear stress at which this change in the rheological behavior occurred was determined (σ change ). This parameter was significantly (p<0.05) higher in those yogurts manufactured with the addition of thickeners. The highest values of σ change were found in yogurts containing gelatin (ranging from 54.3 to 57.4 Pa), followed by yogurts containing starch (ranging from 45.6 to 56.7 Pa) and control sample (22.1 Pa). These results showed a stronger structure, with a higher resistance to shear rate in yogurts containing thickeners. As a change in the rheological behavior of yogurt occurred, the up-ward curve was divided in two regions. For all the samples, the first region of the up-ward curve fitted Herschel-Bulkey rheological model. As shown in table 2, values for the flow behavior index were always lower than 1, which indicates the pseudoplastic nature of yogurt. The addition of gelatin or starch significantly (p<0.05) decreased the flow behavior index. As this parameter reflects deviation of the fluid from Newtonian properties, the addition of thickeners significantly increases the pseudoplastic properties of yogurt. 4

5 Table 2. Yield stress determined by extrapolation, Herschel Bulkley parameters and model fitting Sample Yield stress (Pa) Flow behavior index Consistenc y coefficient Control 1.7 ª 0.87 d 0.27 a 0.97 Gelatin, 3000 ppm 4.5 d 0.66 b 2.67 d 0.98 Gelatin, 6000 ppm 10.1 f 0.51 a 6.44 e 0.96 Starch, 1000 ppm 2.6 b 0.71 b 1.67 b 0.98 Starch, 5000 ppm 3.6 c 0.70 b 1.55 b 0.97 Starch, ppm 6.3 e 0.66 b 1.97 b 0.99 Means within a column with different superscript are significantly different (p<0.05) r 2 The addition of gelatin or starch significantly (p<0.05) decreased the flow behavior index. As this parameter reflects deviation of the fluid from Newtonian properties, the addition of thickeners significantly increases the pseudoplastic properties of yogurt. Samples manufactured with gelatin or starch showed significant (p<0.05) higher consistency coefficient values than the control sample. On the other hand, the second region of the up-ward curve fitted Bingham or power law model, depending on the sample considered. In particular, those samples manufactured with gelatin showed a Bingham lineal behavior. The down-ward flow curves fitted a lineal model. This behavior shows the disruption of the protein network which forms the yogurt gel. As a result of this disruption, the initial pseudoplastic properties of the product disappear. Hysteresis was determined as the area enclosed by the ascending and descending flow curve. This area is a measure of the extent of the structural breakdown in the sample. The area enclosed by the flow curve was significantly higher for those samples manufactured with the addition of thickeners, increasing an average of 145% with respect to control sample. This indicates that those samples manufactured with gelatin or starch were more susceptible to structural breakdown due to the application of shear stress than the control sample. All samples showed thixotropic behavior, as apparent viscosity decreased gradually when samples where subjected to a constant shear rate, probably due to the disruption of the yogurt structure. Shear stress vs. time curves were fitted to Weltman model and A and B parameters were calculated. This model adequately described the stress decay behavior of the samples. The parameter values and the model fitting are shown in table 3. The Weltman A is a structural parameter that generally has a close correlation with the yield stress and is related to the shear stress at a time equal to 1s. Higher values of this parameter indicate a stronger resistance to start up of shear. The Weltman B value is a measure of the rate of structure breakdown. Higher B values indicate that a sample is more susceptible to structure loss (Basak and Ramaswamy, 1994). As shown in table 3, Weltman model A and B parameters significantly (p<0.05) increased with the addition of thickeners. This indicates that the gelatin or starch contributed to build up the start-up viscosity and also increased their sensitivity to stress 5

6 decay. These results are in agreement with those reported by Basak and Ramaswamy (1994) for the addition of pectin to yogurt. Table 3. Weltman model parameters and model fitting Sample A B r 2 Control 46.7 ª 14.6 a 0.95 Gelatin, 3000 ppm b 35.9 b 0.86 Gelatin, 6000 ppm c 35.8 b 0.79 Starch, 1000 ppm 92.2 b 29.0 b 0.94 Starch, 5000 ppm c 38.3 b 0.97 Starch, ppm c 41.4 b 0.96 Means within a column with different superscript are significantly different (p<0.05) 3.2. Syneresis As shown in table 4, syneresis of yogurts after centrifugation ranged from 0.0 to 59.1%. Samples containing gelatin showed the lowest syneresis values. The addition of thickeners significantly (p<0.001) reduced syneresis. Samples manufactured with the addition of 1000 ppm of starch showed the same syneresis values than the control sample. However, the addition of 5000 or ppm of starch reduced syneresis by 18%. Furthermore, samples manufactured with gelatin showed the lowest syneresis values. Syneresis decreased with increasing levels of gelatin; and samples with 6000 ppm of this thickener did not show syneresis. Table 4. Mean syneresis values Sample Syneresis (%) Control 59.1 d Gelatin, 3000 ppm 30.1 b Gelatin, 6000 ppm 0 a Starch, 1000 ppm 57.1 d Starch, 5000 ppm 47.5 c Starch, ppm 48.6 c Means with different superscript are significantly different (p<0.05) 3.3. Sensory evaluation The addition of thickeners to yogurt significantly (p<0.001) affected all the evaluated texture attributes. As shown in table 5, viscosity significantly increased (p<0.001) with the addition of starch or gelatin, as expected. Samples manufactured with gelatin showed the highest viscosity values. A significant (p<0.001) increase in ropyness was observed when gelatin or starch were added to yogurt. Samples manufactured with gelatin showed significantly (p<0.05) higher ropyness values than those manufactured with starch. Moreover, ropyness significantly (p<0.05) decreased with increasing levels of thickeners. This could be explained considering than yogurts manufactured with higher thickener concentration had a stronger structure and showed a higher solid behavior. 6

7 Table 5. Mean values for the sensory texture attributes Sample Viscosity Ropyness Creaminess Mouthfeel Control 1.9 ª 3.2 a 2.6 a 3.2 a Gelatin, 3000 ppm 6.1 c 8.0 e 6.3 c 4.9 c, d Gelatin, 6000 ppm 7.4 d 6.9 d 7.1 d 5.3 d, e Starch, 1000 ppm 4.3 b 6.5 d 5.4 b 3.9 e Starch, 5000 ppm 3.9 b 4.8 c 5.3 b 3.6 a, b Starch, ppm 6.4 c 3.8 b 6.3 c 4.5 b, c Means within a column with different superscript are significantly different (p<0.05) Regarding creaminess and mouthfeel, the addition of gelatin or starch resulted in a significant (p<0.001) increase in the intensity of these parameters. Viscosity was highly correlated (p<0.001) to creaminess and mouthfeel. Due to this high correlation values, the number of attributes to evaluate sensory yogurt texture could be reduced, selecting only manual texture attributes (viscosity and ropyness) that are easier to evaluate by assessors Correlation matrix A sensory and instrumental parameter correlation matrix was obtained to identify the relationship between these parameters. As shown in table 6, non-oral and oral texture parameters were highly correlated to instrumental texture parameters such as yield stress, consistency index, hysteresis and Weltman model parameters. These results show that the characterization of yogurt texture could be done using these instrumental parameters. Table 6. Correlation matrix between sensory and instrumental parameters (r) Viscosity Ropyness Creaminess Mouthfeel Yield stress *** * * σ change *** ** *** *** Flow behavior index Consistency coefficient ** * ** ** Weltman model A parameter *** * 0.881*** ** Weltman model B parameter *** * *** ** Hysteresis *** ** *** *** *, **, *** significant at p<0.05, p<0.01 and p<0.001 respectively 4. Conclusions The addition of gelatin and starch to yogurt significantly affected sensory and instrumental texture parameters and had a significant effect on the occurrence of syneresis. Furthermore, the addition of these thickeners to yogurt resulted in a significant increase in viscosity, ropyness, mouthfeel and creaminess. 7

8 Gelatin was more effective in preventing syneresis than starch: while the addition of 6000 ppm of gelatin prevent the occurrence of syneresis, the addition of starch only reduced it. From all the instrumental parameters that significantly correlated to sensory texture attributes, yield stress was found to be the more appropriate to characterize the texture of yogurt in quality control or product development, since several methods that allow a quick and direct determination of yield stress are available. References Basak, S. and Ramaswamy, R.S. (1994). Simultaneous Evaluation of Shear Rate and Time Dependency of Stirred Yogurt Rheology as Influenced by Added Pectin and Strawberry Concentrate. J. Food Engng., 1994, Benezech, T. and Maingonnat, J.F. (1994). Characterization of the Rheological Properties of Yogurt-A Review. J. Food Engng., 21, Fiszman, S.M., Lluch, M.A. and Salvador, A. (1999). Effect of Addition of Gelatin on Microstructure of Acidic Milk Gels and Yogurt and their Rheological Properties.Int Dairy J., 9, Fiszman, S.M. & Salvador, A. (1999). Effect of Gelatine on the Texture of Yogurt and of Acid-heat-induced Milk Gels. Z Lebensm Unters Forsch A 208: Foss, J.W. (2000) How Processing Affects Starch Selection For Yogurt. National Starch & Chemical Company, Bridgewater, N.J. Gámbaro, A. (2002). Aplicación de Nuevas Metodologías Sensoriales a un Estudio Integral de Yogur. Tesis Doctoral. Facultad de Química, Universidad de la República. Uruguay ISO 8589 (1988) Sensory Analysis. General guidance for the desing of test rooms. International Standar Organization (ISO). Switzerland Kroger, M. (1975). Quality of yogurt. Journal of Dairy Science 59(2): Muir, D.D.; Hunter, E.A. (1992). Sensory evaluation of fermented milks: vocabulary development and the relations between sensory properties and between acceptability and sensory properties. Journal of the Society of Dairy Technology 45(3): Walstra, P., Geurts, T.J., Noomen, A., Jellema, A., van Boekel, M.A.J.S. (1999). Dairy Technology. New York, Marcel Dekker, Inc. Acknowledgments The authors would like to thank the companies that provided the ingredients to elaborate the samples used in this study: CONAPROLE, NIPAX S.A., ABASTECIMIETOS S.A., FELIZATTI S.A. We also want to thank all the sensory assessors for their patient effort and helpful suggestions. 8