Instrumental Texture, Syneresis and Microstructure of Yoghurts Prepared from Goat, Cow and Sheep Milk

Abstract

Set-yoghurts from goat, cow, and sheep milk from middle lactation period were produced. In fresh yoghurts and after 14 days cold storage the following properties were analysed: hardness, adhesiveness, and extrusion force using instrumental texture analyzer, syneresis using drainage and centrifugal methods and microstructure using scanning electron microscope (SEM). Yoghurt from goat milk was characterized by lower hardness, adhesiveness, extrusion forces, and higher susceptibility to syneresis than yoghurts from cow and sheep milk. Microstructure of goat milk yoghurt was more delicate in comparison with microstructure of cow and sheep milk yoghurt. The composition and/or properties of goat milk for yoghurt production, or processing conditions need to be modified to obtain the proper texture and reduced syneresis in final product.

Keywords:

• Microstructure
• Syneresis
• Texture
• Yoghurt

INTRODUCTION

Goat milk is one of the main types of milk produced in the world, accounting for 2% of global milk production. It is destined for direct consumption or manufactured into milk powder, cheese and fermented milks, e.g. yoghurt. Particularly, goat yoghurt manufacture is widely recognized to join high nutritional value, easy assimilation of components, antioxidative, therapeutic and antiallergenic properties of goat milk with the important role of fermented dairy products in human nutrition.[1,2] The average composition of goat milk does not differ markedly from the composition of cow milk. The similarities are found in the contents of total solids, protein, fat, and lactose. However, essential differences occur concerning the structure, composition, and size of casein micelles. Also proportion of individual protein fractions varies according to the type of milk, with higher content of non-protein nitrogen and mineral compounds in goat milk.[3,4]

On the other hand, the composition of sheep milk differs significantly from the composition of cow and goat milk. In the case of former type, a higher amount of total solids, protein, fat, lactose and ash are reported.[5] Hence, the quality of yoghurt described by texture, susceptibility to syneresis and microstructure is highly related to the composition and properties of certain milk type being used as a raw material.[6]

Texture is a collective term and comprises physical properties of the product, such as hardness, adhesiveness, viscosity, and springiness. They are all derived from the structural elements and can be perceptible by human senses. Consequently, texture is one of the basic quality determinator of fermented dairy products, particularly yoghurt. Instrumental texture analysis, coming from penetrometric methods, is one of the main methods for texture determination of set-yoghurts.[7,8] Recently, the acoustic/electromyography systems were also proposed for the analysis of instrumental food texture.[9] The content of total solids and total protein in milk, heat treatment and homogenization of the milk, the type of starter culture and incubation conditions, as well as the addition of stabilizers affect the yoghurt texture to the highest extent.[6,10,11] Acid gel from goat milk is characterized by lower hardness and viscosity compared with gels from cow and sheep milk. The viscosity of gel is thus positively correlated with casein content in milk, especially with casein fraction α_s, which can range in goat milk from 25% to 0%.[3,4]

The ability to immobilize water of the coagulum represents also the important structural properties of set-yoghurt. The degree of immobilization depends on numbers of variables, such as total solids content, concentration of Ca²⁺ and fat, pH of the milk, preheat treatment of the milk, temperature, and stabilizers. Separation of the liquid phase from the gel is called syneresis. It may be spontaneous or may occur only when the gel is mechanically disrupted while cutting, agitating, or freezing. Thereby, syneresis is not desirable in yoghurt and can negatively influence on the consumer acceptance of the food product.[12] In order to suppress the occurrence of this detrimental phenomenon, the total solids content and the protein concentration is increased resulting in a rise in gel hardness and whey-holding capacity in set-yoghurt. Furthermore, the proper heat treatment, addition of stabilizers and the type of applied starter culture are the other factors that reduce intensity of syneresis.[13,14] The texture of yoghurts as well as their susceptibility to syneresis are closely related to the microstructure of the product, in particular the structure of protein matrix. Although the texture, syneresis and microstructure of yoghurt obtained from cow milk have been extensively documented, little attention is given to yoghurt from goat and sheep milk. The aim of the presented work was to compare the texture and susceptibility to syneresis in relation to the microstructure of goat, cow and sheep milk yoghurts.

MATERIALS AND METHODS

Goat, cow and sheep milk for yoghurt preparations was obtained directly from the farm. The research was carried out in the middle lactation period, when cows, goats and sheep were being fed in the barn. Milk from morning milking was cooled down and transported to the laboratory. Prior to yoghurt preparation, milk was subjected to an initial analysis which included total solids, total protein, casein, non-protein nitrogen, fat, lactose and ash content, density, viscosity, acidity and pH.[15] Whey protein content was calculated from the difference between total protein, casein and non-protein nitrogen content. Milk for yoghurt was pasteurised at 85°C for 15 minutes, cooled down to 44°C and inoculated with yoghurt culture YC-180 (Chr. Hansen, Denmark) in the quantity of 2 kg of batch starter/100 kg milk. The thoroughly mixed milk was poured into containers and incubated at 44°C for 4 to 5 hours until pH value of 4.8 was reached. Thereafter, the yoghurt was cooled down to 5°C and stored in the temperature 5−8°C for 14 days.

Yoghurts were examined after 15 hours (fresh yoghurt) and after 14 days (stored yoghurts) for instrumental texture analysis, syneresis and microstructure. The experiment was carried out in five replicates and the results were described statistically. One- and two-way ANOVA were employed and the differences between averages were assessed with the Duncan test using computer program Statistica v. 6.0.

Instrumental Texture Analysis

Instrumental Texture Profile Analysis (TPA) and extrusion test were carried out using Universal Texture Analyser TA-XT2 (Stable Micro Systems, UK) controlled by PC computer. The temperature of the yoghurt samples was about 10°C. As a tool for the evaluation of textural properties, the penetrometric test was performed using a plastic cylinder type SMS P/20 of 20 mm diameter. The depth of penetration was 25 mm with penetration rate of 1 mm/s. As a result, diagrams of force dependence on time were plotted and thereafter analysed using the computer program Texture Expert for Windows v. 1.05 (Stable Micro Systems, UK). The Fracture TPA algorithm was applied which allowed to determine hardness and adhesiveness of yoghurts. By definition, hardness is a force necessary to attain a given deformation of the probe, whereas adhesiveness describes work for overcoming the force of attraction between the area of foodstuff and other solids coming into contact with each other.[16] Apart from the penetrometric test, the extrusion test was performed where the specimen of yoghurt was squeezed through a hole of 3 mm diameter. The mean of extrusion force from the force-time graph was then estimated.

Syneresis

Syneresis of analysed yoghurts was determined using both drainage method according to Dannenberg and Kessler[17] and centrifugal method as given by Pluta et al.[18] Regarding drainage method, hemispherical sample with a volume of 8 cm³ (obtained with a special spoon) of set-yoghurt was placed with its flat side onto a sieve (mesh width of 260 μm). The amount of drained off whey was measured after 2 h at 10°C. In a case of centrifugal method, 10 cm³ of yoghurt after mixing was placed in calibrated test-tube and centrifuged for 10 minutes by 1500 × g and than the volume of liberated whey was measured. The syneresis computed in both methods was expressed in %.

Analysis of Microstructure in Scanning Electron Microscope (SEM)

For SEM, sections 3 × 3 × 1 mm were excised from the yoghurt, approximately 1 cm below the surface and were fixed in 2.5% glutaraldehyde solution in phosphate buffer at pH of 7.4 for 7 days [19]. The section were then cut into prisms 1 × 1 × 3 mm, dehydrated in a graded ethanol series (20, 40, 60, 80, 96, and 100%), defatted in chloroform and acetone and freeze-fractured in liquid nitrogen. The fragments were melted in absolute alcohol, critical-point-dried from carbon dioxide, mounted on SEM stubs, coated with gold by vacuum evaporation and examined in Jeol ISM 5410 scanning electron microscope (purchased by Foundation for Polish Science) operated at 25 kV. Analyses of yoghurt microstructure were conducted in the Laboratory of Scanning Electron Microscopy of the Jagiellonian University, Institute of Zoology, Krakow.

RESULTS AND DISCUSSION

Composition and Properties of Milk for Yoghurt Preparation

The comparison of the composition and physicochemical properties of goat, cow and sheep milk for yoghurt preparation is presented in Table 1. Significant differences were found in total solids, total protein, whey protein, non-protein nitrogen and fat content and density between these three types of milk. Sheep milk exhibited the highest content of total solids and its components while the lowest content was recorded in goat milk. The concentrations of casein, lactose, mineral compounds, and acidity in the goat and cow milk were similar, but much lower than in sheep milk. Regarding viscosity, goat milk was again characterized by substantially lower values when compared to cow and sheep milk. The pH was similar among investigated types of milk, whereas the titratable acidity was the highest in sheep milk.

Table 1 Composition and physicochemical properties of goat, cow, and sheep raw milk

Type of milk	Total solids [%]	Total protein [%]	Casein [%]	Whey proteins [%]	Non-protein nitrogen [%]	Fat [%]	Lactose [%]	Ash [%]	Density [g · cm^-3]	Viscosity[mPa · s]	Titratable acidity[°SH]	pH
Goat	10,98^A,B ± 0,07	2,90^A,B ± 0,03	2,26^A ± 0,03	0,31^A,B ± 0,02	0,33^A,B ± 0,01	2,9^A,B ± 0,1	4,49^A ± 0,04	0,72^A ± 0,01	1,0270^a,A ± 0,0006	1,77^A,B ± 0,08	6,7^A ± 0,2	6,66± 0,03
Cow	12,32^A,C ± 0,12	3,16^A,C ± 0,02	2,38^B ± 0,02	0,49^A,C ± 0,03	0,29^A,C ± 0,01	4,0^A,C ± 0,1	4,53^B ± 0,09	0,66^B ± 0,01	1,0290^a,B ± 0,0007	2,36^A ± 0,08	7,1^B ± 0,1	6,69± 0,04
Sheep	15,61^B,C ± 0,12	5,20^B,C ± 0,09	4,25^A,B ± 0,07	0,84^B,C ± 0,02	0,11^B,C ± 0,01	4,7^B,C ± 0,1	4,87^A,B ± 0,05	0,96^A,B ± 0,04	1,0353^A,B ± 0,0003	2,55^B ± 0,05	8,5^A,B ± 0,1	6,63 ± 0,01
A−C: statistically highly significant differences between means (p ≤ 0,01) marked with the same letters in the columns; a - statistically significant difference between means (p ≤ 0,05) marked with the same letter in the column.

The total solids and fat content of analyzed goat milk were lower than the mean content of these components in goat milk as reported by Pełczyńska.[20] Other studies on goat milk by Kudełka[21], Szczepanik and Libudzisz[3] and Ziarno and Truszkowska[22] have revealed higher amounts of total solids, total protein, casein, fat, lactose and ash in goat milk in comparison with results found in this work. Interestingly, the lactation period affects the goat milk composition. Indeed, Domagała and Wszołek[23] have shown that goat milk of Polish White Improved breed from the middle lactation period has a lower content of total solids and total protein than goat milk from the initial and final lactation period. The composition of goat milk investigated in this research was comparable with this reported in the twentieth week of lactation in the US by Guo et al.[24] Also, similar results were presented by Pelczynska[20] and Kudelka[21] for acidity and density of goat milk. In turn, mean density of goat milk of different breeds determined by Guo et al.[24] was lower than density of goat milk analysed in this work. In goat milk from the middle of the lactation period, Domagala and Wszolek[23] found a slightly lower viscosity and similar density and acidity in comparison to the values of these parameters in analysed milk.

The composition of analysed cow milk was similar to mean composition of Holstein cow milk given by Morrissey and Fox.[25] Nonetheless, the contents of total protein, fat and lactose were established as slightly lower compared to the cow milk composition reported by Schlimme and Buchheim.[26] In terms of the composition and acidity, pH and density, the samples of sheep milk exhibited similarities in the composition and physicochemical properties to sheep milk analysed by Bonczar and Reguła.[27] In contrast, the contents of total solids, total protein, fat, lactose, and titratable acidity of sheep milk used for yoghurt production were lower in this study than presented by Bonczar et al.[28], Muir et al.[29] as well as Celik and Ozdemir.[30] In addition, slightly higher viscosity values were obtained in analysed sheep milk than given by Bonczar and Paciorek.[5]

Characteristics of Yoghurts

The characteristics of yoghurts produced from goat, cow and sheep milk, fresh and after 14 days of cold storage are presented in Table 2 . A highly significant dependence was found for all analysed quality parameters of yoghurts prepared from different types of milk. Additionally, the results of hardness of cow and sheep milk yoghurt and syneresis determined by centrifugal method of all assessed yoghurts were markedly influenced by the 14-day storage.

Table 2 Characteristics of yoghurt from goat milk, cow milk and sheep milk, fresh and after 14 days of cold storage

Type of yoghurt	Hardness [N]	Adhesiveness × 10^3 [│N·m│]	Extrusion force [N]	Syneresis[%]
				1)	2)
Fresh yoghurts
Goat	0,19 ± 0,01	0,18 ± 0,01	1,48 ± 0,11	37 ± 2,24	47 ± 2,24
Cow	0,60 ± 0,03	1,47 ± 0,09	2,56 ± 0,19	22 ± 1,79	33 ± 1,34
Sheep	1,23 ± 0,10	6,02 ± 0,29	5,67 ± 0,21	5 ± 0,89	20 ± 1,34
Yoghurts after 14 days of storage
Goat	0,19 ± 0,01	0,16 ± 0,01	1,69 ± 0,06	39 ± 3,13	38 ± 2,24
Cow	0,66 ± 0,02	1,44 ± 0,15	2,73 ± 0,19	25 ± 1,34	24 ± 1,79
Sheep	1,45 ± 0,04	6,48 ± 0,61	6,05 ± 0,22	2 ± 0,89	17 ± 1,79
1): determined using drainage method; 2): determined using centrifugal method.

Instrumental Texture

The values of all measured texture parameters, namely hardness, adhesiveness and extrusion force were significantly different and increased in the following order: goat milk yoghurt, cow milk yoghurt, and sheep milk yoghurt. Yoghurt from goat milk had the lowest hardness, adhesiveness, and extrusion force, while the highest values of these parameters were exhibited by sheep milk yoghurts. Such differences could be attributable to the varying content of total solids and total protein in goat, cow and sheep milk, and also to the type of milk alone. As discussed by Malek et al., [31] even similar content of total solids in goat, cow and sheep milk did result in the lowest hardness of goat milk concentrated yoghurt (labneh). Moreover, further differences were found by Vlahopoulou et al.[32] for yoghurt gels. A looser consistency has been ascribed to the gel from goat milk in comparison to yoghurt gel from cow milk, despite the lower total protein content in the latter milk. Paž˛áková et al.[33] and Karademir et al.[34] have also found that goat milk yoghurt has the poorest consistency, hardness and stability in comparison with cow and sheep milk yoghurt. According to these authors, lower diameter of casein micelles and fat globules, lower casein content and higher non-protein nitrogen content in goat milk than in cow and sheep milk are all responsible for these differences. The texture parameters described for goat milk yoghurt of this work are similar to the data obtained by Domagała and Wszołek [23], who analysed goat milk yoghurt in the middle of lactation period. However, higher hardness was reported by Antunes et al.[35] for cow milk yoghurt and according to Bonczar et al.[28], sheep milk yoghurt was softer than that obtained in this work.

After 14 days of cold storage, the hardness of yoghurts from cow and sheep milk was significantly higher than the hardness of fresh products. Similar results were observed by Salvador and Fiszman [36] and Domagała et al.[37] considering hardness of cow milk yoghurt.

Nonetheless, in the case of goat milk yoghurt, hardness before and after storage remained essentially constant. This is not in conformity with the results of Uysal et al.[38], who reported an increase of hardness in goat milk yoghurt and bio-yoghurt after 14 days of cold storage. Evaluating extrusion force, all yoghurt samples showed a slight increase after storage. Sheep milk yoghurt revealed also elevated adhesiveness after storage, which is not the case of goat and cow milk yoghurt. The latter were similar or slightly lower in stored than in fresh yoghurts. Those findings for sheep milk yoghurt are in accordance with the results of yoghurt investigated by Bonczar et al. [28] In contrast to the results obtained in this study, Salvador and Fiszman [36] and Domagała et al.[37] reported an increase in adhesiveness of cow milk yoghurt.

Syneresis

The syneresis determined by means of both drainage and centrifugal methods varied according to the milk type used for yoghurt production. It was found, that yoghurts from goat milk revealed the highest syneresis, whereas yoghurt from sheep milk the lowest, with the moderate values for cow milk. The differences in syneresis were of statistical significance. Centrifugal method showed that in all types of analysed yoghurt the intensity of syneresis decreased substantially after storage. When yoghurt gel is concerned, syneresis is determined by not only total solids or total protein content, but also by the type of milk. Malek et al.[31] reported that concentrated yoghurt from cow milk showed lower syneresis than yoghurt from goat and sheep milk, despite the total solids of these three types of yoghurts being similar. Conversely, Kavas et al.[39] postulated that the type of milk (goat or cow) had no influence on syneresis of bio-yoghurts.

In turn, syneresis evaluated by means of drainage method did not revealed marked changes before and after cold storage in all types of yoghurts. Lower syneresis in stored than in fresh cow milk yoghurt was also found by Barrantes et al.[40] Salvador and Fiszman[36] showed a reverse trend, that is an increase in syneresis determined using drainage method, for stored yoghurts from full-fat and skim cow milk.

Microstructure

In Figure 1 the microstructure of yoghurt from (A) goat, (B) cow, and (C) sheep milk is presented in the form of a micrograph obtained with a scanning electron microscope at a 5000-fold magnification. Each micrograph shows the structure of the yoghurt protein matrix and void spaces filled with whey. Fat globules are not visible on micrographs, because the samples of yoghurts were defatted prior to microscopic analysis. The comparison regarding microstructure of analyzed yoghurts showed some differences, which are dependent on the type of milk used for yoghurt production. When goat milk yoghurt is concerned, the protein matrix consisted of small casein micelles connected in thick chains and large clusters could be observed as well as large void spaces filled with whey or occupied by yoghurt bacteria. This image of the microstructure appears to be of delicate, low compact gel, which is susceptible to faster deformation. Such properties thus affect the results of instrumental analyses of yoghurt texture, namely the low values of texture parameters and high syneresis. The microstructure of yoghurt from cow milk compared to goat milk yoghurt is characterized by lower clusters of casein micelles and thick chains consisting of a few layers of micelles, larger than in goat milk yoghurt. The void spaces occupied by whey or yoghurt bacteria are also visible. The protein matrix of sheep milk yoghurt was composed mainly of clearly formed chains of large, single casein micelles, which were rarely clustered. Moreover, smaller and more regular void spaces were noticed compared with protein matrix of goat and cow milk yoghurt. As a result, sheep milk produced a stronger gel, which was more resistant to deformation. That confirmed the higher values of texture parameters and lower syneresis of sheep milk yoghurt in comparison with goat and cow milk yoghurt.

Figure 1 Microstructure of fresh yoghurt from: A: goat's milk; B: cow's milk; and C: sheep's milk (magnification 5000x).

According to Vlahopuolou et al.[32], goat milk forms a less compact gel structure than cow milk. Acid goat milk gel manufactured by yoghurt culture fermentation examined by Malek et al.[31] had a softer structure than acid gel from other types of milk. Tamime et al.[41] and Malek et al.[31] have demonstrated that concentrated yoghurt (labneh) from cow milk exhibited a more homogeneous and compact microstructure in comparison to the microstructure of products from goat and sheep milk. In the case of latter, the protein matrix had large irregular void spaces and was more susceptible to deformation and gel structure destruction. In turn, Labneh from goat milk was the softest and contained the largest void spaces. The authors have explained the observed differences in microstructure and texture of labneh by the different content of individual casein fractions in these types of milk. Karademir et al.[34] recorded smaller casein micelles and fat globules in goat milk yoghurt than in cow milk yoghurt and thus the goat milk yoghurt gel was less stabile. According to Schkoda et al.[42], the higher protein content in milk before fermentation resulted in a smaller and more compact protein net characterised by lower permeability. The rise in compactness of cow milk yoghurt resulting from the increase of total protein content in milk for yoghurt manufacture was found by Kalab et al.[43] and Pereira et al.[44] who employed a scanning electron microscope and a confocal laser scanning microscope, respectively. The images of the yoghurt microstructure obtained in the presented work are similar to images presented by Tamime et al. [41] and Kalab et al.[43]

CONCLUSIONS

Yoghurt from goat milk was characterized by lower hardness, adhesiveness, extrusion forces and higher susceptibility to syneresis than yoghurts from cow and sheep milk. Such properties are ascribed to the lower content of total solids and total protein in goat milk compared with the content of these components in cow and sheep milk. Texture profile and syneresis were related to the microstructure of yoghurts. The microstructure of goat milk yoghurt was more delicate, less resistant to deformation and more susceptible to syneresis in comparison with the microstructure of cow and sheep milk yoghurt. Consequently, in order to obtain the proper texture and reduced syneresis in final product, the composition and/or properties of goat milk for yoghurt production or processing conditions need to be modified.