EFFECT OF STARTER CULTURE AND HOMOGENIZATION ON THE RHEOLOGICAL PROPERTIES OF YOGHURTS

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1 Journal of the University V. of Paskov, Chemical M. Karsheva, Technology I. and Pentchev Metallurgy, 45, 1, 21, EFFECT OF STARTER CULTURE AND HOMOGENIZATION ON THE RHEOLOGICAL PROPERTIES OF YOGHURTS V. Paskov, M. Karsheva, I. Pentchev University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria mik@uctm.edu Received 5 December 29 Accepted 12 February 21 ABSTRACT A study of the effect of the homogenization and the starter culture on the final properties of yoghurts was carried out. A comparison between the yoghurts fermented using homogenized, non-homogenized and home milk with the same strains was made. The influence of the strains used on the flow behaviour of the products was also investigated. The rheological properties of yoghurts were studied using a co-axial cylinder viscometer. It was found that all the yoghurts are non-newtonian and their rheological behaviour can be described by the Herchell-Bulkley rheological model. The values of the model parameters and the accuracy of the description were found by standard statistics. It was found that the home milk with wild strain exhibits the least apparent viscosity values compared to other samples studied. It was also proved that the temperature of the inoculation is the parameter with most important effect on the product properties. The rheological behaviour of the yoghurts is affected also by the milk homogenization. The homogenized milk samples fermented with the same strains as the non-homogenized ones are much more consistent. Keywords: yoghurts, homogenization, rheology, strains. INTRODUCTION Yoghurts and related products are very popular in Europe, North America and the Middle Eastern countries, and over the past two decades probiotic fermented milks have gained consumer acceptability in most European countries. Yoghurt is a dairy food with complex rheology that depends on temperature, solids concentration and the physical state of fats and proteins present in the milk. An understanding of the rheological properties of yoghurt is important to texture, stability, and process design. Physical properties of cultured milk are major criteria for quality assessment. These characteristics are affected by many factors, including composition and heat treatment of milk, mechanical handling of the coagulum, the use of stabilizers, and the type of culture [1-3]. Because the type of culture is one of the most critical factors influencing the physical characteristics of yogurt, selection of the appropriate culture is of great importance [4, 14]. Slime-producing lactic cultures are widely used to improve the physical stability of cultured milk [5,6]. These cultures increase viscosity and decrease susceptibility to syneresis [7]. Yoghurt is produced by mixing single strains of Lactobacillus delbueckii subsp. Bulgaricus (Lb) and Streptococcus thermophilus (St). Several studies have reported that the choice and combination of these strains have a significant influence on fermented products [8-1]. The classical technology for the production of the Bulgarian yoghurt is the following: milk qualification; milk purification; cooling and preservation (storage); fat contents standardization; homogenization (t = o C, pressure 18 2 Pa); heat treatment at t = o C for 2-3 min; cooling to inoculation temperature o C; inoculation (2 5 % strain used); packing; holding in thermostat at 4 o C, for 2,5-3 hours 59

2 Journal of the University of Chemical Technology and Metallurgy, 45, 1, 21 until reaching an acidity by Torner of o T (ph 5, - 4,5); cooling to 1-4 o C ; storage at t = 1-4 o C. In the manufacture of cultured milk products homogenization is used for a number of purposes. One of these is the prevention of cream separation; another is the enhancement of a full-bodied mouth feel [11]. This processing step also improves the consistency of the yoghurt. The gel produced by unheated milk is lumpy and has an open chain structure. In addition to reducing the size of the fat globules, homogenization promotes the formation of new fat globule membranes which consist, in part, of casein. When the milk is acidified, the fat globules become incorporated into the casein micelle network. The combination of heating, whey protein denaturation and homogenization, was the best method for obtaining high quality yoghurt. A reduction of the fat globules from 1.8 to 1.1 µ m results in doubling of the firmness. In the production of fermented products homogenization not only subdivides the fat globules into smaller globules and thus retards the formation of a cream layer in a low viscous cream, it also increases cream viscosity because the increased number of fat globules formed now occupy a larger volume. The objective of this work is to compare the rheological properties of yoghurt made with different strains of yogurt cultures applied to homogenized and nonhomogenized milk. EXPERIMENTAL Materials and methods Home cow milk and industrial sets of homogenized and non-homogenized milks were used for the experiments. The industrial milk responded to the standard requirements as follows: The raw homogenized and non-homogenized milk is fermented with 2 % of starter culture, combination of industrial starter cultures A and B with ratio 1:1 (the industrial strain combination). The same milk is fermented also with starter culture A and the starter culture B. Every starter culture consists of Lactobacillus delbueckii subsp. Bulgaricus (Lb) and Streptococcus thermophilus (St) strains in different ratios. For comparison an inoculation with a wild starter culture was also done (for raw home milk, industrial homogenized and non homogenized milk). The wild strain was obtained from the industrial one through several fermentations in home conditions. The inoculation temperature is o C. It is well established that the inoculation at an inappropriate temperature can influence the fermentation time and the product quality [14]. After package the milk is introduced in the thermostatic chamber at 4 o C for fermentation. The fermentation time is about 3 hours till the acidity of o T (ph ) is reached. Then it is necessary to cool the product for τ, Pa γ, s -1 tao1, Pa tao2, Pa tao3, Pa Fig. 1. Flow curves for 2 o C: tao1 - A:B = 1:1; tao2 starter culture A; tao3 starter culture B. 6

3 V. Paskov, M. Karsheva, I. Pentchev Fig. 2a. Microphotographs of homogenized milk fermented with starter culture B. Fig. 2b. Microphotographs of homogenized milk fermented with starter culture A. Fig. 2c. Microphotographs of homogenized milk fermented with industrial starter culture (A:B = 1:1). stopping the further development of the fermentation process. The cooling of the yoghurt is a gradual process: for the first two hours, the product is cooled to 2oC, for another two hours - to 1oC. During this period the fermentation continues and the product acidity reaches 9-1oT. This gradual cooling forms the taste, the aroma and the consistency of the final product. The quick cooling can bring to a serum separation discharge (syneresis effect) [13]. After the fermentation, the rheological properties of the final product were studied. The strains were examined microscopically using the Olympus optical microscope with camera and specialized software. The rheological measurements were carried out using co-axial cylinder rheometer RheotestRV2 Germany with cylinder S1. Unique experiments with cylinder S2 did not demonstrate a slip effect on the wall. The yoghurt samples of 25 cm3 were used for the experiments. All samples, after in situ thorough mixing and loading, were presheared at a high shear rate of 132 1/s for 6 s, before rheological examination. Pre-shearing at high shear rate was required to erase the processing history of these semisolid materials. The upward and downward flow curves were obtained for the shear rates range between 1.5 and 132 1/s. All determinations were repeated at least three times. The data were fitted using the HerchellBulkley and the power law rheological models (eqs.1 and 2): (1) τ = τ + Kγ n τ = Kγ n (2) 61

4 Journal of the University of Chemical Technology and Metallurgy, 45, 1, 21 Table 1. Physico-chemical properties of the 1 st quality raw milk [12]. Appearance Homogeneous liquid without sediment Colour White, pale yellow Taste Pleasant slightly sweetish Odour Specific, hardly perceptible Consistency Homogeneous Density at 2 C 1.29 g/cm 3 Fat contents 3.6 Proteins 3.2 Dry solids 8.5 Freezing temperature -.52 C Acidity T Total number of microorganisms (TNM) Up to 1 Total number of somatic cells (TNSC) Up to 4 Here τ is the shear stress, Pa, γ& is the shear rate, 1/s, τ is the yield stress, Pa, a characteristic of the product firmness; K is the consistency index, Pa.s n, and n is the flow index, characterising the degree of the non-newtonian behaviour (more n differs from unity, more non-newtonian is the system). After the pre-shearing the samples did not demonstrated thixotropy. It can be seen that the flow curves are curvilinear and can be described by power-law rheological model or by Herchell-Bulkley one. The curves were fitted by both models. In Fig. 1 the flow curves for the starter culture A, B and their 1:1 ratio (industrially used starter culture) are presented. It is evident that the flow curves are curvilinear with intercept, so they could be described by the Herchell-Bulkley rheological model. The rheological parameters were determined by standard statistic package Excell. The data were fitted also by the power law model. In Table 2 the values of the parameters of both models are presented. From the figure it can be seen that the flow curves for the starter culture A are situated over those with industrial strain (A : B = 1:1). The flow curves for the starter culture B are situated lower. From the values of the rheological parameters presented in Table 2 it can be seen that the highest flow curve has the highest values of the rheological flow index (so it is more Newtonian than the others). On the other hand, the industrial strain (1) is the most consistent and the most non-newtonian. The microscopic photographs of these strains are presented in Fig. 2a,b,c. Taking into account that all samples are homogenized, the only difference in this case is due to the starter culture. It can be seen that the starter culture B is enriched on St, the starter culture A on Lb, Fig.2c is of the industrial culture, where both strains are in 1:1 ratio. So, the higher viscosities can be explained with presence of greater quantity of Lb, giving the net-like structures. In Fig. 3 the comparison between the samples of milk fermented with wild strain (curves 1,3,4) and the industrial one is presented (curve 5) as well as the comparison between the homogenized industrial and raw home cow milk (curves 1 and 2) fermented with the wild strain. In the same figure a comparison between three fermentation sets is given (curves 1, 3, 4). It can be seen a good reproducibility of the data. On the other hand the flow curve which exhibits the lowest values of 62

5 V. Paskov, M. Karsheva, I. Pentchev τ, Pa γ, s -1 tao1, Pa tao2,pa tao3, Pa tao4,pa tao5, Pa Fig. 3. Flow curves at 2 C for: 1 - homogenized milk fermented with wild strain; 2 - home raw milk fermented with wild strain; 3 - homogenized milk fermented with wild strain 2; 4 - homogenized milk fermented with wild strain 3; 5 - homogenized milk fermented with industrial strain. Table 2. Values of rheological models parameters for Fig.1 and the accuracy of the description. No, Pa K, Pa. s, τ n n 2 R n, n, 2 R K Pa. s the shear stress is the one of the home milk with wild strain. The microscopic picture of these samples is presented in Figs.4a and 4b. From the microphotographs and Fig.3 it can be seen that in this case the governing is the role of the homogenization. It is explicable, because the homogenization forms a stable emulsion with small fat spheres, which theoretically have to be more viscous than the non-homogenized one. However it can be also seen that the home raw milk fermented with wild strain has shorter Lb rods. This fact together with the role of the homogenization can explain quite lower viscosity values in this case. For the case of homogenized milk fermented with wild strain the Lb rods are longer with tendency to form netlike structures. St globules are in larger quantity, so these facts can explain the higher product viscosities. In Fig. 5 a comparison of the flow behaviour of homogenized milk fermented with industrial culture (curve 1), with wild strain (curve 2) with strain B (curve 3) and with strain A (curve 4) is presented. In Table 3 the values of the rheological parameters of Herchell-Bulkley model and the accuracy are summarized. There is almost no difference in the flow properties of the homogenized milk fermented with industrial and wild starter culture. The highest values of the shear stress are observed for the strain A. Its flow curve is the highest one. But, on the other hand it exhibits the highest flow index n values, being more linear than the others. The lowest values of the shear stress are of the milk fermented with strain B. It can be seen also that there is no great difference between the flow indices of the samples 1 to 3. The difference there is in the consistency index and 63

6 Journal of the University of Chemical Technology and Metallurgy, 45, 1, 21 τ, Pa Fig. 4a. Microphotographs of home raw milk fermented with wild strain. Fig. 4b. Microphotographs of homogenized milk fermented with wild strain tao1,pa tao2, Pa tao3,pa tao4,pa 5 1 γ,s 15-1 Fig. 5. Flow curves at 2C for: 1 - homogenized milk fermented with industrial strain. 2 - homogenized milk fermented with wild strain; 3 - homogenized milk fermented with strain B; 4 - homogenized milk fermented with strain A. in the yield stress values. The industrially fermented milk exhibits the highest values of these parameters, followed by homogenized milk fermented with wild strain and the one fermented with the strain B. All the samples can be characterized as non-ropy (or thick) which replies to the preferences of Bulgarian consumers. A possible explanation of this behaviour is an increased quantity of St in the starter cultures with higher viscosities. This coincides with other authors observations. According to A.Skriver et al. [8] the milk fermented with St strains has a higher viscosity 64 than the milk fermented with Lb strains. The fermentation with St strains and Lb strains in symbiosis not only reduces the fermentation rate and gives the characteristic taste and aroma, but, in addition, the viscosity is increased which is desirable in many yoghurt products. It is proved [8] that milk fermented with a combination of St and Lb has a higher shear stress (and viscosity) level than milk fermented with single strains, indicating that the symbiosis between the single strains produce yoghurt with a significant higher shear stress (and viscosity) than products fermented with single strains.

7 V. Paskov, M. Karsheva, I. Pentchev Table 3. Rheological parameters for the fermentations presented in Fig. 5. No,, n τ, Pa K Pa. s n 2 R Mean error, % CONCLUSIONS From the experiments carried out the following more important conclusions can be driven: The yoghurt quality is strongly dependent on the raw milk quality, on the starter culture and the operational conditions. The sensory quality of the final product depends mostly on the starter culture. Some cultures give fermented milk with ropy structure, others with thick one; the exopolysaccharide producing strains influence also the taste of the yoghurts. The wild strain gives yoghurts with strongly different properties of the industrial one due to the microorganisms mutation. The yoghurts fermented with wild strains have also a different expiry terms (quite shorter) then the industrial ones. The homogenization is a process increasing strongly the products consistency. Using of different strain combinations and ratios is an instrument for controlling the yoghurts consistency. Acknowledgements This work was financially supported by Research sector of UCTM-Sofia, Grant No NN REFERENCES 1. A.N. Hassan, J. F. Frank, K. A. Schmidt, S.Shalabi, Rheological Properties of Yogurt Made with Encapsulated Nonropy Lactic Cultures, J. Dairy Sci., 79, 1996, V. Nielsen, Factors which control the body and texture of commercial yoghurts, Am. Dairy Rev., 37,1975, E.Parnell-Clunies, Y. Kakuda, J. Deman, Influence of heat treatment of milk on the flow properties of yogurt, J.Food Sci., 51, 1986, I.Vlahopoulou, A. Bell, Effect of various starter cultures on the viscoelastic properties of bovine and caprine yogurt gels, J. Soc. Dairy Technol., 46, 1993, T.Toba, H. Nakajima, A. Tobitani, S. Adachi, Scanning electron microscopic and texture studies on characteristic consistency of nordic ropy sour milk, Int. J. Food Microbiol., 11, 199, T.Toba, H.Uemura, T. Mukai, T. Fuji, T. Itoh, S. Adachi, A new fermented milk using capsular polysaccharide producing Lactobacillus hefiranofaciens isolated from kefir grains, J. Dairy Res. 58, 1991, S.Schellhaass, H. Morris, Rheological and scanning electron microscopic examinations of skim milk gels obtained by fermenting with ropy and non-ropy strains of lactic acid bacteria, Food Microstruct., 4, 1985, A. Skriver, Chr.B.-Madsen, B. Jelle, Texture characterization of yoghurt fermented with different bacterial cultures, in Texture of fermented milk products and dairy desserts, Proceedings of IDF symposium, Vicenza, Italy, 5-6 May, 1997, p H.Rohm, A.Kovac, W.Kneifel, Effects of starter cultures on sensory properties of set style yoghurt determined by quantitative descriptive analysis, J. Sensory studies, 9, 1989, A.Skriver, H.Romer, K.B.Ovist, Characterization of 65

8 Journal of the University of Chemical Technology and Metallurgy, 45, 1, 21 stirred yoghurt, Viscometry J. Texture studies, 24, 1993, H.G.Kessler, The structure of fermented milk products as influenced by technology and composition, in Texture of fermented milk products and dairy desserts, Proceedings of IDF symposium, Vicenza, Italy, 5-6 May, 1997, p Requirements No 4/ Specific requirements for production, storage and transportation of raw cow milk and trading requirements for milk and dairy products, (in Bulgarian). 13. M.Condratenko, G.Simov, Bulgarian yoghurt, Sofia, 23, (in Bulgarian). 14. Habib Abbasi, M. E. Mousavi, M. R. Ehsani, et al. Influence of starter culture type and incubation temperatures on rheology and microstructure of low fat set yoghurt, International Journal of Dairy Technology, 62, 29,