FLOW BEHAVIOUR OF GOAT’S MILK YOGHURTS AND BIOYOGHURTS

Jacek Domagała, Lesław Juszczak

ABSTRACT

Flow behaviour of yoghurts and bioyoghurts produced from goat’s unconcentrated milk at using different starter cultures was compared. Four starter cultures were applied to the production of yoghurts and four to the production of bioyoghurts too. Rheological properties were examined using rotary viscometer Rheotest 2 with controlled shear rate in coaxial cylinders system. Flow curves of yoghurts and bioyoghurts were assigned and described by Ostwald de Waele, Herschel-Bulkley and Casson models. An apparent viscosity was counted for shear rate 3 s^-1. Generally, higher viscosity of yoghurts in comparison to bioyoghurts was stated. The yoghurts were generally characterised by higher value of consistency coefficient K and yield stress than bioyoghurts. The yoghurts have shown likewise higher deviation from Newtonian flow (lower values of exponent n) than bioyoghurts.

Key words: yoghurts, bio-yoghurts, goat´s milk, rheology..

INTRODUCTION

Knowledge of rheological properties of food products has got a great importance for the manufacture, the storage, the designing processes, new food products development and creating their quality. Rheological properties can influence significantly on quality of food products. The rheology also gives an objective exploratory implements in the estimation of food texture, what is able to optimise formulas, also designing and the optimisation of technological processes and machines, tools and installations [1, 20].

Yoghurt is one of the food stuffs in which rheological properties influence significantly on its quality. Yoghurt belongs to non-Newtonian, viscoelastic, pseudoplastic fluids showing yield stress. It is rheologically unstable fluid, shear thinning, i.e. which viscosity decreases together with increase in shear rate and depends on “shear history”. Yoghurt behaves as thixotropy fluid, although thixotropy is called here as partial or non-reversed, because its consistency cannot rebuilt quite during the relaxation time, when shear forces relented [8, 9, 10]. Many researchers have investigated the rheological properties of yoghurts from cow’s milk [1, 3, 5, 8, 9, 10, 12, 17], however knowledge of the rheology of goat’s milk yoghurt is significantly less [2, 17, 18]. Although the basic composition of goat’s milk is similar to the composition of cow’s milk, but physicochemical properties both types of milk differed significantly from each other. These differences come from the distinctive struct ure, the composition and size of casein micelles, proportions of individual protein fractions and higher quantity of mineral salts and non-protein nitrogen compounds in goat’s milk. It does not remain without an influence on rheological properties of yoghurts from goat’s milk. Acid gel from goat’s milk is more delicate in comparison to gel from cow’s milk [19]. Additionally, content of individual constituents in goat’s milk submit to big fluctuations during lactation, thus is necessary to use same processes, which create desirable rheological properties of product [2].

The most important factors that are influential in rheological properties of yoghurt are: composition and quality of processing milk, way and level of an enrichment of dry matter components, technological parameters in production, the procedure with end-product during its transport and storage. Very important is also selection of proper starter culture responsible for acidification of milk and giving desirable sensory properties of the product [15]. According to Tamime [16] one of main function of microbiological starter cultures in dairy products is exactly improvement in rheological properties of product.

The aim of this work was to compare rheological properties of yoghurts and bioyoghurts produced from unconcentrated goat’s milk using different starter cultures.

MATERIALS AND METHODS

Milk for production of yoghurts and bioyoghurts came from an improved Polish White breed of 10 goats during the middle of lactation (June – July). Milk for production was pasteurised at 85°C for 15 min, cooled down to 44°C and inoculated with four different starter cultures: V1 and V10 from Visby, YC-180 from Chr. Hansen, and J from Biolacta. Milk for bioyoghurts production was cooled down to 37°C after pasteurisation and was inoculated with four cultures too: MSKB2 from Visby, ABT-1 and ABY-2 from Chr. Hansen, and JM from Biolacta. Most of used cultures were destined for direct milk inoculation (DVS - Direct Vat Set), but two were used to make a batch starter culture. In all cases the culture was added in 2% quantity converted into the batch starter culture. The cultures for yoghurt production included in their composition the traditional yoghurt cultures as: Str. thermophilus and L. delbrueckii subsp. bulgaricus. Only the culture V1 instead of L. d elbrueckii subsp. Bulgaricus included L. delbrueckii subsp. lactis. The cultures for bioyoghurts production included in their composition also L. acidophilus and Bifidobacterium species except traditional yoghurt cultures. The cultures MSK B2 and ABT-1 did not include in their composition L. delbrueckii subsp. bulgaricus. In table 1 is presented the general characteristic of used cultures together with their cultural composition. The yoghurts were incubated at 44°C and bioyoghurts at 37°C until the cut of point of 4.8 pH was reached. Then the products were cooled down to 5°C. At this temperature products were stored for 24 h, next rheological investigations were done. In milk intended for yoghurt production was analysed the content of: dry matter, total protein, non-protein nitrogen, fat, lactose and mineral compounds, density, viscosity and pH [7, 21].

Rheological properties of yoghurts and bioyoghurts were examined using a rotary viscometer Rheotest RV2 (VEB MLW Medingen, Germany), with controlled shear rate in coaxial cylinder system S/S₂ in the measuring sphere Ia. Proportion of internal to external radius of cylinder was 0.94. The flow curves for shear rate from 1 to 437.4 s^-1 and from 437.4 to 1 s^-1 were assigned. At the shear rate 3 s^-1 on rising curve, apparent viscosity of yoghurts and bioyoghurts was calculated. The flow curves were described by Ostwald de Waele, Herschel-Bulkley and Casson models. Analyse was done using computer programme US 200 (Physica Messtechnik GmbH Stuttgart, Germany). Experiment was carried out in three independent repeats. Results are described as arithmetic means.

RESULTS AND DISCUSSION

Basic chemical composition and physicochemical properties of processing milk for yoghurt are presented in Table 2. Low content of dry matter (11.10%), total protein (2.92 %) and other main components were obtained in milk for yoghurt production. According to Domagała and Wszołek [2] results of early investigations, goat’s milk during the middle of lactation is characterised by lower content of dry matter and total protein than during the early and final lactation. These authors researching variability of goat’s milk composition during lactation have stated similar content of dry matter and most components and somewhat higher content of total protein and lactose for milk from the middle period of lactation. Guo et al. [4] have obtained the similar composition in the 20-th week of lactation for collective milk from different breeds of goats in the USA. The mean quantities of basic components in goat’s milk cited by Szczepanik and Libudzisz [13] were higher than obtai ned in this work. Also Kudełka [6] has stated higher content of an basic components of goat’s milk in the middle of lactation.

Results of the apparent viscosity of yoghurts and bioyoghurts produced from goat’s milk are presented in Figure 1. Generally the apparent viscosity of yoghurts was higher than the apparent viscosity of bioyoghurts. Yoghurt produced using the culture YC-180 was characterised by the highest apparent viscosity, while using culture J the lowest. Among all bioyoghurts the highest viscosity had bioyoghurt MSK B2, whereas ABT-1 had the lowest.

Figure 2 presents flow curves for investigated yoghurts and bioyoghurts. Yoghurt produced using the culture V10 was characterised by the highest value of shear stress (at maximum shear rate), while the lowest at using the culture J. The highest value of shear stress has achieved yoghurt MSK B2, while the lowest yoghurts ABT-1 and ABY-2 among all bioyoghurts. Flow curves of all yoghurts and bioyoghurts have got shape of hysteresis loop. The hysteresis loop area can be interpreted as measure of yoghurts structure breakdown during shear [8,10], and a slant of flow curve can testify to resistance of yoghurt gel to action of shear forces [5]. Visual evaluation of loop area of obtained graphs allow to determine that yoghurts produced using cultures YC-180 and V10 were characterised by similar size of hysteresis loop area, V1 insignificantly less hysteresis loop area and the least yoghurt J. Among all bioyoghurts, bioyoghurt obtained using the culture ABT-1, was characterised by the least hysteresis loop area. Also for this bioyoghurt was stated the lowest an apparent viscosity and the lowest shear stress at maximum shear rate. Lesser slant of flow curves of bioyoghurts ABT-1 and ABY-2 and yoghurt J show their lower resistance to action of shear forces in comparison to other yoghurts.

Table 3 presents values of rheological parameters obtained in models used for description of flow curves. Generally, yoghurts were characterised by higher values of consistency coefficient K in comparison to bioyoghurts, with the exception of bioyoghurts JM and ABY-2 in Herschel-Bulkley model. Yoghurt V1 in Ostwald de Waele model and yoghurt YC-180 in Herschel-Bulkley model were characterised by the highest consistency coefficient K. Among all bioyoghurts in both models, bioyoghurt JM achieved the highest value of consistency coefficient K. The lowest value of consistency coefficient K were characterised bioyoghurts ABT-1 in Herschel-Bulkley model and MSK B2 in Ostwald de Waele model. The value of exponent n is a measure of deviation from Newtonian flow. For shear thinning fluids n < 1, whereas for Newtonian fluids n = 1 [11]. Generally, yoghurts have shown the highest deviations from Newtonian flow than bioyoghurts. The values of expon ent n were higher for bioyoghurts than for yoghurts with the exception of bioyoghurts ABY-2 and JM in Herschel-Bulkley model. In yoghurts and bioyoghurts determined the yield stress, such values of shear stress below which yoghurt behaves as a solid state [11]. Generally, higher values of yield stress have stated for yoghurts (0.22 – 0.75 Pa). For bioyoghurts the values of yield stress have been in range 0.16 to 0.69 Pa. Yoghurt J has shown the highest value of yield stress among all yoghurts and JM and ABT-1 among all bioyoghurts. The values of Casson viscosity, similar to apparent viscosity, were the lowest for yoghurt J and bioyoghurt ABT-1, whereas for yoghurt V10 and bioyoghurt MSK B2 were the highest. The data obtained from rheological measurements were characterised by good fit both to Ostwald de Waele model (R² in range 0.9862 – 0.9963) and to Herschel-Bulkley model (R² in range 0.9822 – 0.9974). Worse fit was stated to Casson model (R² in range 0.8948 – 0.9771).

According to Tamime and Marshall [14] fast development of acidity in yoghurt is necessary for making of the stable product with desirable rheological parameters. Although, development of acidity in biocultures (especially for Bifidobacterium bifidum strain) is worse in comparison to traditional yoghurt strains. However L. acidophilus is able to good milk fermentation, whereas Bifidobacterium strains grows weakly in milk. Among typically yoghurt strains Str. thermophilus has lesser acidic abilities than L. delbrueckii subsp. bulgaricus. This difference in acidic abilities of bacteria, which are included in cultures used for yoghurts and bioyoghurts production was probably main reason for differences in rheological properties of yoghurts and bioyoghurts from goat's milk. Quantity of added culture is one of significant factors which have an influence on milk fermentation [15]. In this work was applied 2% addition of batch starter culture. Vlahoupol ou et al. [18] suggest lesser than 2% addition of culture and slower acidification to improve of the yoghurt gel properties. Vlahopolou and Bell [17] during their investigations into the influence of different starter cultures on viscoelastic properties of yoghurt from goat's milk and cow's milk stated that yoghurts from goat's milk in a very little degree were susceptible to an influence of culture type. Skriver et all. [12] have stated differences in rheological properties of yoghurt from cow's milk inoculated with commercial multi-strains culture and yoghurt obtained using single strains of yoghurt bacteria. Rheological investigations were carried out at the shear rate from 0 to 238 s^-1. Yoghurt produced using the culture YC-190 were characterised by higher shear stress and viscosity but lower values of yield stress than yoghurts obtained at use of single strains cultures. Yoghurts produced Str. thermophilus strain had higher viscosity and yield stress than yoghurts produced using only L. delbrueckii subsp. bulgaricus, with the exception of case when strongly ropy cultures were applied.

According to Benezech and Maingonat [1] Ostwald de Waele, Herschel-Bulkley and Casson rheological models are mostly applied to the description of flow curves of yoghurts. Fortuna et all. [3] applied these three models to the description of flow curves of commercial yoghurts and bioyoghurts from cow's milk. They have obtained lower values of consistency coefficient, exponent n and significantly higher values of yield stress, especially in Casson model and Casson viscosity than obtained in this work. They did not determine significant differences in rheological parameters for traditional yoghurts and bioyoghurts from cow's milk. Rohm and Schmidt [9] and Rohm [10] produced yoghurt and bioyoghurt from cow's milk using two yoghurt cultures and one culture for bioyoghurt production. Flow curves assigned by them in range 0-100 s^-1 were described with Herschel-Bulkley model. Bioyoghurt obtained by them was characterised by higher value of yield stress, consistency coefficient K and hig her deviation from Newtonian flow (lower value of n) in comparison to traditional yoghurts. The reason for this was probably presence of ropy bio-strain in the culture. The values of rheological parameters of traditional yoghurts except for consistency coefficient and hysteresis loop shape, were similar to obtained in this work. Shear stress for bioyoghurt was lower than in this work, whereas for traditional yoghurts was similar or slightly higher. These scientists in their investigations stated that type and properties of starter cultures significantly influence on flow curves shape and size of hysteresis loop area.

CONCLUSIONS

1. The differences in rheological properties of yoghurts and bioyoghurts produced from goat's milk at use of different cultures were ascertained.

2. The yoghurts were characterised by higher an apparent viscosity than bioyoghurts.

3. Most yoghurts were characterised by higher values of consistency coefficient K and yield stress in comparison to bioyoghurts.

4. Most yoghurts have shown higher deviation from Newtonian flow (lower values of exponent n) than bioyoghurts.

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Lesław Juszczak
Department of Analysis and Evaluation of Food Quality
University of Agriculture Cracow
Ul. Balicka 122, 30-149 Cracow, Poland
Ph. (+4812) 662 47 78
E-mail: rrjuszcz@cyf-kr.edu.pl

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