SENSORY EVALUATION AND RHEOLOGICAL PROPERTIES OF YOGHURTS PREPARED FROM GOAT, COW AND SHEEP MILK

Jacek Domagała
Department of Animal Products Technology, University of Agriculture, Cracow, Poland

ABSTRACT

Set-yoghurts produced from goat, cow and sheep milk in the middle of the lactation period were examined fresh and after 14 days cold storage for sensory quality, pH and rheological properties. Rheological investigations consisted of the determination of apparent viscosity and drawings of flow curves. They were descripted using three rheological models: Ostwald de Waele, Herschel-Bulkley and Casson. The values of rheological parameters of these models and the area of the hysteresis loop were calculated. In comparison to cow and sheep milk yoghurts, goat milk yoghurt had a looser consistency, higher acidity and was less acceptable sensoricaly. The apparent viscosity of goat milk yoghurt was lower and its flow curve was characterized by a smaller hysteresis loop area and slant than these of yoghurts from cow and sheep milk. Smaller values of the consistency coefficient, yield stress and deviation from the Newtonian flow for goat milk yoghurt were also found in comparison to yoghurts from goat and sheep milk.  To obtain the proper rheological and sensory properties of yoghurt from goat milk, the modification of the composition and properties of milk or modification of processing conditions are necessary.

Key words: apparent viscosity, flow curves, rheological models, sensory evaluation, yoghurt.

INTRODUCTION

From the  point of view of rheology, yoghurt is a non-Newtonian, rheological  unstable, viscoelastic and pseudoplastic fluid. It is also shear thinning, which means that its viscosity decreases as the shear rate increases and depends on the "shear history". In addition, yoghurt behaves as a thixotropic fluid, although its thixotropy is partial, non-reversed or describes as "pseudothixotropy" because the structure of yoghurt cannot be recover completely during the relaxation time, when shear forces are relented [22,23,24]. Yoghurt is a dairy product of high nutritional value and healthful properties. The most important benefits of yoghurt consumption cover the reduction of blood cholesterol level, anti-cancer effects and the improvement of antimicrobial activity and immunity in the human body [8]. Beside the sensory quality, another important factor for the consumer's acceptance of the product are the rheological properties of yoghurt, such as apparent viscosity and flow behaviour. These properties are also of main significance in dairy technology, especially in the manufacturing, storage, process design, product development and establishment of the product's quality [5,17]. Rheological and sensory properties of yoghurt may be influenced by some technological factors, which mainly include the type and amount of dry matter fortification, preheating intensity of the milk and whey protein denaturation, specific properties of starter culture and addition of stabilizers [23,24]. An important role is also played by the composition and physicochemical properties of milk which yoghurt is prepared from. Because of the differences in composition and physicochemical properties of goat, cow and sheep milk differences in the rheological properties and sensory quality of yoghurt from these types of milk can be expected.

In the previous paper [9] a comparison of texture, microstructure and susceptibility to syneresis of goat, sheep and cow milk yoghurts was presented. The aim of  this work was to compare the sensory quality and rheological properties, such as apparent viscosity, flow behaviour and rheological parameters of yoghurt produced from goat, cow and sheep milk.

MATERIALS AND METHODS

Goat, cow and sheep milk for yoghurt preparations were obtained directly from the farm. The investigations were carried out in the middle of the lactation period, when  animals were being fed in the barn. Milk from the morning milking was cooled down and transported to the laboratory. Having performed an initial analysis on milk, yoghurt was prepared. The analysis of processing milk and the procedure of yoghurt preparation were described in the previous paper [9].

Yoghurt samples were examined after 15 h (fresh yoghurt) and after 14 days (stored yoghurts) for sensory evaluation, pH and  rheological properties.

Sensory Evaluation
The sensory evaluation of produced types of yoghurt was done on a 5 point scale (1 – the worst; 5 – the best). The following quality properties were evaluated: colour, taste, smell, consistency and level of syneresis. The proper indexes of importance were ascribed to colour, taste, smell, consistency and syneresis as follows: 0.1, 0.35, 0.15, 0.25 and 0.15, respectively. The study of the parameters allowed to calculate the overall preference. The reference sensory properties of analysed yoghurts are as follows:

• Colour – typical, characteristic, intensive white,

• Taste – sour, characteristic, yoghurt-like,

• Smell – characteristic and intensive,

• Consistency – uniform and compact, creamy not lumpy, without syneresis [21].

Samples of yoghurt for sensory evaluation were presented in glass vessels of a volume of 200 cm³ coded, in the temperature of about 10°C (about half an hour after being taken out of refrigerator). Firstly,  the overall appearance, colour, smell and syneresis in the undisturbed gel  were evaluated. Secondly, the gel was pressed with a spoon so as to assess the hardness and  its springiness. Then the gel was mixed (with a spoon) until an uniform consistency was obtained. After mixing, the oral evaluation was performed on the taste and mouth feel. The consistency of the product was also evaluated by placing a spoon vertically into the product. Smoothness of the product was observed on the convex side of the spoon. A trained panel consisting of 5 persons whose sensory sensibilities were proved, completed the evaluation. The judges were tested for taste and smell daltonism and minimum point of taste and smell sensibility. They were instructed about the process of evaluating the different parameters of sensory quality.

Rheological investigations
Rheological investigations consisted of the determination of apparent viscosity, the plotting of flow curves, their description using the rheological models, and the calculation of the rheological parameters of these models. The measurements were performed on a rotary viscometer Rheotest RV2 (VEB MLW, Medingen, Germany), with the shear rate mode controlled in coaxial cylinder system s/s₂ in measuring range Ia. The proportion of internal to external radius cylinder was 0.94. The flow curves were calculated at the shear rate from 1 to 437.4 s^-1 and from 437.4 to 1 s^-1. The temperature of yoghurt probes was 15°C. The flow curves were described by means of rheological models given by Eq. (1), Eq. (2) and Eq. (3):

Ostwald de Waele model:

Herschel-Bulkley model:

Casson model:

where:
τ – shear stress [Pa], γ – shear rate [s^-1], K – consistency coefficient [Pa · sⁿ], n – flow index [–], τ₀ – yield stress [Pa], η_c – Casson viscosity [mPa · s],

For each flow curves the hysteresis loop area S [W · m^-3] was also calculated. These calculations were done using the computer programme Physica US 200 (Physica Meßtechnik GmbH, Stuttgart, Germany). The apparent viscosity of the yoghurts was counted at the shear rate η = 3 s^-1 at rinsing curve by Eq. (4):

where:
η – apparent viscosity [Pa · s], τ – shear stress [Pa], γ – shear rate [s^-1].

RESULTS AND DISCUSSION

The composition and physicochemical properties of goat, cow and sheep milk used for yoghurt preparation and the comparison of texture, microstructure and susceptibility to syneresis of yoghurts produced from these types of milk were presented in the previous paper [9]. This work deals with the comparison of sensory and rheological properties of the yoghurts from goat, cow and sheep milk.

Sensory evaluation
Results of the sensory evaluation, pH and apparent viscosity of fresh and cold stored yoghurts from goat, cow and sheep milk are presented in Fig. 1. Among all analysed types, the yoghurts, from sheep milk revealed the highest sensory quality, while yoghurt from goat milk had the lowest. Yoghurt from goat milk occurred to have a loose and weak consistency, high syneresis and higher acidity than yoghurts from cow and sheep milk. The sensory quality of yoghurts from non-concentrated goat, cow and sheep milk was assessed similarly by Pazáková et al. [20]. In their investigations goat milk yoghurt had a markedly  "goat" flavour, which negatively influenced the sensory quality. Mahdi et al. [18] presented similar opinion about concentrated yoghurt (labneh) from goat milk because of its "goat" flavour and weak consistency (when comparing the quality of labneh from goat, cow and sheep milk). According to Kavas et al. [15], bio-yoghurt from goat milk had lower overall sensory quality than bio-yoghurt from the mix of goat and cow milks. Fresh goat milk yoghurt analysed in this work revealed no "goat" flavour. This flavour was barely perceptible (only) in stored yoghurts. Yoghurt from goat milk had a higher acidity (pH = 4.43) in comparison to the acidity of cow milk yoghurt (pH = 4.58) and sheep milk yoghurt (pH = 4.69). This can be explained by a faster increase of acidity in goat milk due to its lower buffering capacity and higher content of non protein nitrogen and vitamins, which are needed for fast micro-organism development [2]. After 14 days of cold storage, the sensory quality of goat and cow milk yoghurt had deteriorated while the sensory quality of sheep milk yoghurt had improved. Such changes could be attributed probably to the increase in acidity in the case of sheep milk yoghurt and decrease in the case of goat and cow milk yoghurts, increase in syneresis (especially) in goat and cow milk yoghurts, and "goat" flavour. A decrease in the sensory quality of sheep milk yoghurt after cold storage was found by Bonczar et al. [6] while Laye et al. [16] found no difference in the sensory texture of yoghurt from cow milk before and after 12 days of cold storage, but stated however that after storage yoghurt smells and tastes worse. According to Salvador and Fiszman [25] the acidity of cow milk yoghurt increased during cold storage which is natural for a product with living lactic acid bacteria.

Apparent viscosity
Yoghurt from sheep milk was characterized by the highest apparent viscosity, whereas goat milk yoghurt had the lowest value. After cold storage the apparent viscosity of all yoghurts increased. These differences reflected in sensory quality of the yoghurts. Sheep milk yoghurt had the highest sensory quality and goat milk yoghurt – the lowest. Different values of apparent viscosity for the analyzed three types of yoghurt resulted from a different content of total solids and total protein in goat, cow and sheep milk. Karademir et al. [14] have found that goat milk yoghurt had a lower viscosity in comparison to the viscosity of cow milk yoghurt. Abrahamsen and Holmen [1], and Karademir et al. [14] have reported an increase in apparent viscosity after cold storage of goat milk yoghurt while Dankow et al. [7] have found opposite trends in changes. According to Beal et al. [4], apparent viscosity of cow milk yoghurt was increasing until 7th day of storage, thereafter remained constant.

Flow curves
The flow curves of fresh and stored yoghurts from goat, cow and sheep milk are depicted in Fig. 2. Flow curves have the shape of a hysteresis loop. Characteristics of the hysteresis loop area for obtained flow curves are presented in the Table 1.  According to Schramm [26] the hysteresis loop area is proportional to the energy needed to destroy the thixotropy structure of yoghurt. In this research the hysteresis loop areas of flow curves obtained for different yoghurts were influenced by the type of milk: the smallest value calculated for goat milk yoghurt while the highest for sheep milk yoghurt. After cold storage the hysteresis loop areas were similar to areas of flow curves for fresh yoghurts. Analyzed flow curves revealed in the differences of shear stress  also when  the same values of shear rate was applied. The highest shear stress was found for sheep milk yoghurt with the lowest value for goat milk yoghurt. Similarly, Novakovic et al. [19] stated that acidophilic cow milk had a higher value of shear stress than goat acidophilic milk. In this work, the slant of  flow curves increased in the following order: goat, cow and sheep milk yoghurt. According to Jaros et al. [12] the slant of flow curves can be interpreted as a measure of yoghurt gel resistance to the action of shear forces. The values of all above mentioned characteristics of flow curves were dependent on the type of processed milk, in particular on the content of total solids and total proteins.

Rheological parameters
Obtained flow curves were described by rheological models, such as Ostwald de Waele, Herschel-Bulkley and Casson model. Rheological parameters (calculated from these models) are presented in Table 1.  Concerning the consistency coefficient K of fresh and stored yoghurts, the values obtained  from Ostwald de Waele and Herschel-Bulkley models increased in the following order: goat, cow and sheep milk yoghurt. Same order was observed for  Casson's viscosity similarly to apparent viscosity changes of the yoghurts. After storage a rise in consistency coefficient and slight drop in Casson's viscosity value were found. By definition, the n-value, known as the flow index measures the deviation from the Newtonian flow. For shear thinning fluids n < 1 and for Newtonian fluids n = 1 [24]. Yoghurt from goat  milk had the highest value of flow index when compared to cow's and sheep's milk yoghurts. The values of flow index were lower for stored yoghurts  than for fresh yoghurts. Yield stress, defined as a value of shear stress below which yoghurt behaves as a solid state [26], was found in all analyzed yoghurts. Goat milk yoghurt was characterized by the lowest value of yield stress. The Casson model indicated that yoghurt from sheep milk had the highest yield stress, and Herschel-Bulkley model – yoghurt from cow milk. After storage a slight increase in yield stress values was found. These values were generally higher in the Casson model and lower in the Herschel-Bulkley model. Ostwald de Waele model and the Herschel-Bulkley model fitted the data relatively well with R² in the range 0.9948–0.9996 and 0.9938–0.9997, respectively, while the Casson model did slightly worse (R² in the range 0.9055–0.9507).

According to Benezech and Maingonnat [5], the Ostwald de Waele, Herschel-Bulkley and Casson models are used most often to describe the flow curves of yoghurts. Abu-Jdayil et al. [3] have found the Ostwald model to be the most appropriate for rheological investigations of labneh (concentrated yoghurt). Above mentioned models were employed in rheological investigations of yoghurt and labneh by Rohm [23], Fortuna et al. [11], Jumah et al. [13], Abu-Jdayil et al. [3], and Domagała et al. [10]. Similar shapes of flow curves to the ones obtained in this work were also presented in rheological investigations by Rohm [23], Jaros et al. [12] and Domagała et al. [10]. Lower values of consistency coefficient K, higher values of yield stress, and Casson's viscosity were found in the rheological research of commercial yoghurts by  Fortuna et al. [11]. In turn, the flow index values were comparable with those obtained in this work. In the rhelogical research of cow milk yoghurt when the addition of maltodextrin as a fat replacer was investigated, Domagała et al. [10] found higher values of consistency coefficient K, yield stress and Cassons viscosity and lower values of flow index in comparison to the values of respective parameters from this work. The similarity was noted in goodness of fit.

CONCLUSIONS

Goat milk yoghurt in comparison to cow and sheep milk yoghurts was less acceptable sensorially, because of its looser and weaker consistency. The apparent viscosity of goat milk yoghurt was lower than apparent viscosity of cow and sheep milk yoghurts. The flow curves of goat milk yoghurt were characterized by a smaller hysteresis loop area and slant of the curve than yoghurts from cow and sheep milk. The goat milk yoghurt was also characterized by worse rheological parameters than the cow and sheep milk yoghurts, such as lower values of the consistency coefficient and yield stress and the biggest deviation from the Newtonian flow.  In order to obtain the desired rheological and sensory properties of yoghurt from goat milk, the composition and properties of milk or processing conditions are necessary to be modified.

REFERENCES

Accepted for print: 4.06.2008

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