Instrumental Texture, Syneresis, and Microstructure of Yoghurts Prepared from Ultrafiltrated Goat Milk: Effect of Degree of Concentration

Abstract

The objective of this study was to determine the effect of concentration degree (1.5-fold, 2-fold, and 2.5-fold [v/v]) of ultrafiltrated goat milk on instrumental texture, syneresis, and microstructure of set-yoghurt. The milk for control yoghurt was non-concentrated. The concentration of goat milk by ultrafiltration caused an increase in the hardness, the adhesiveness, the extrusion force of yoghurt, and a reduction in the syneresis. The microstructure of yoghurts from ultrafiltrated milk showed a compact protein matrix, a small amount of void spaces, and large casein micelles in comparison to the microstructure of yoghurts from non-concentrated milk. The best texture and the smallest syneresis of goat milk yoghurts could be obtained at 1.5- or 2-fold [v/v] goat milk concentration. Yoghurt from 2.5-fold concentrated milk was characterized by a too hard and compact structure with a consistency typical for cream cheese, not yoghurt.

Keywords:

• Goat milk yoghurt
• Microstructure
• Syneresis
• Texture
• Ultrafiltration

INTRODUCTION

Texture of a product comprises its physical properties, such as hardness, adhesiveness, cohesiveness, and springiness derived from the structural elements, and can be perceptible by human senses. Texture is one of the basic quality attributes of fermented dairy products, including yoghurt. Instrumental texture analysis coming from penetrometric methods is one of the main methods for texture determination of set-yoghurts.[1,2] Acoustic/electromyography systems were also recently proposed for the analysis of instrumental texture of food products.[3]

Another important structural property of set-yoghurt is the ability to immobilize water of the coagulum. Total solids content, concentration of Ca²⁺ and fat, pH, temperature and preheat treatment of the milk, and stabilizers addition are the most important variables influencing the degree of water immobilization. Syneresis is the separation of the liquid phase from the gel. It may be spontaneous or may occur only when the gel is mechanically disrupted by cutting, agitating, or freezing. Syneresis is not desirable in yoghurt and can negatively influence consumer acceptance of the product.[4] In order to suppress the occurrence of this detrimental phenomenon, the total solids content and total protein concentration should be enhanced to obtain a rise in gel hardness and water-holding capacity in set-yoghurt. The other factors that can reduce intensity of syneresis are the proper heat treatment, addition of stabilizers, and the type of starter culture.[5,6] 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.

To achieve desirable texture properties of yoghurts without syneresis, the processing milk should be enriched with dry matter components. One of the ways of increasing dry matter content in milk is the ultrafiltration technique.[7–11] Ultrafiltration (UF) is a membrane separation technique used to separate and concentrate substances containing molecules from 500 to 300,000 Da. When milk is ultrafiltrated, the retentate concentration of macromolecules larger than membrane pores, such as casein, whey protein, and fat, increase parallel to the concentration of milk. Small molecules in the soluble milk phase, such as lactose and minerals, are removed via membrane to permeate solution. Concentration of milk by ultrafiltration causes the changes in chemical composition and physicochemical properties of obtained retentate. These changes depend mainly on the degree of concentration and the kind of membranes. The changes in the chemical composition and physical properties of milk concentrated by UF also influence the properties of milk products.[7,12,13]

Although the basic composition of goat milk is similar to the composition of cow milk, the physicochemical properties of both types of milk differed significantly from each other. Such differences come from the distinctive structure, the composition and size of casein micelles, proportion of individual protein fractions, and higher content of mineral components and non-protein nitrogen in goat milk. All these differences influenced the texture properties, syneresis, and microstructure of yoghurts from goat milk. Acid gel from goat milk is softer and more delicate in comparison to gel from cow milk.[14,15] Although the texture, syneresis, and microstructure of yoghurt obtained from cow milk have been extensively documented in the literature,[5,10,16–24] little attention is given to the properties of yoghurt from goat milk. Karademir et al.[9] found that yoghurt from UF goat milk had the higher body and texture scores than yoghurt from evaporated milk and from milk with the addition of goat milk powder. Marshall and El-Bagoury[11] showed that yoghurts from goat milk concentrated by UF had better acidity and aroma in comparison with goat milk yoghurt from milk fortified by reverse osmosis and addition of goat milk powder. Goat milk for yoghurt, especially from the middle lactation period, needs composition and property modification or a change in yoghurt processing, in order to improve the texture and rheological properties of the final product and to reduce syneresis.[25] One of the possible modifications is concentration of milk by UF, but the degree of concentration is a very important factor. The objective of this study was to determine the changes in texture properties, susceptibility to syneresis, and microstructure of yoghurt prepared from ultrafiltrated goat milk, depending on the degree of concentration in order to establish the best concentration of milk for achieving the product of desirable consistency and other quality parameters.

MATERIAL AND METHODS

The research was carried out in the middle lactation period. Goat milk for yoghurt preparations was obtained directly from the farm near Kraków in Southern Poland. Milk from a morning milking was cooled down and transported to the laboratory. The raw milk was ultrafiltrated using UF concentrator CH-2A and a Hollow Fiber filter 30 kDa from Amicon (Witter, Switzerland), to obtain the concentration degree of 1.5-fold, 2-fold, and 2.5-fold (v/v). After initial analysis, milk was used for yoghurt preparation. The following analysis was carried out on milk that was to be used for yoghurt preparation: total solids, total protein, casein, non-protein nitrogen, fat, lactose and ash content, density, acidity, and pH.[26] Whey protein content was calculated from the difference between total protein and casein, and non-protein nitrogen content.

Milk for yoghurt was pasteurised in a waterbath at 85°C for 15 min, rapidly cooled down to 44°C, and inoculated with yoghurt culture YC-180 (Chr. Hansen, Horsholm, Denmark) in the amount of 2% of batch starter. Thoroughly mixed milk was poured into containers and incubated at 44°C for 4 to 5 h to obtain a pH of 4.8. When the suitable pH was reached, yoghurt was slowly cooled down to 5°C and stored at the temperature of 5–8°C until the next day (about 15 h). The control yoghurt sample was prepared from non-concentrated milk. One-day old yoghurts were subjected to sensory evaluation and pH measurement, instrumental texture, apparent viscosity, syneresis, and microstructure analyses.

The experiment was carried using five replicates. The data were analyzed statistically using the Statistica v. 7.1 programme[29] and results expressed as mean values ± standard error. One-way ANOVA was performed and the Duncan test was employed to assess the differences between averages.

Sensory Evaluation

The sensory evaluation 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 calculation of the overall preference. Samples of yoghurt for sensory evaluation were presented in glass vessels, with a volume of 200 mL coded, at a temperature of about 10°C (about half an hour after being taken out of refrigerator). First, the overall appearance, colour, smell, and syneresis in the undisturbed gel were evaluated. Second, 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 a uniform consistency was obtained. After mixing, the oral evaluation was performed on the taste and mouthfeel. 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 five persons whose sensory sensitivity were proved, completed the evaluation. The panelists were tested for aguesia and anosmia and taste and smell detection thresholds. They were instructed about the process of evaluating the different sensory attributes.

Instrumental Texture Analysis

Instrumental texture profile analysis (TPA) and extrusion test were carried out using Universal Texture Analyser TA-XT2 (Stable Micro Systems, Surrey, UK) controlled by a 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 a 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). The Fracture TPA algorithm was applied, which allowed assignment of 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 3-mm diameter hole. The mean of extrusion force from the force-time graph was then estimated.

Apparent Viscosity

The measurements of apparent viscosity were performed on a rotary viscometer Rheotest RV2 (VEB MLW, Medingen, Germany), with the shear rate mode controlled in a coaxial cylinder system s/s₂ in measuring range Ia. The proportion of internal to external radius cylinder was 0.94. The temperature of yoghurt samples was 15°C. The apparent viscosity of the yoghurts was counted at the shear rate γ = 3 s⁻¹ by Eq. (1):

(1)

where η is apparent viscosity [Pa · s], τ is shear stress [Pa], and γ is shear rate [s⁻¹].

Syneresis

Syneresis was determined using the drainage method according to Kessler[27] and centrifugal method according to Pluta et al.[28] Regarding drainage method, hemispherical sample with a volume of 8 mL (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 mL of yoghurt after mixing was placed in a calibrated test-tube and centrifuged for 10 min at 1500×g and than the volume of liberated whey was measured. The syneresis computed in both methods was expressed in percent.

Microstructure

Analysis of yoghurt microstructure was carried out using scanning electron microscopy (SEM) according to Tamime et al.[16] 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. The sections 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 with a Jeol ISM 5410 scanning electron microscope (JEOL Ltd., Akishima, Tokyo, Japan) (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, Poland.

RESULTS AND DISCUSSION

Composition and Properties of Milk

The main composition and physicochemical properties of goat milk concentrated to the degree of 1.5-, 2-, and 2.5-fold (v/v) by UF technique is presented in Table 1. The control sample was the non-concentrated milk. Depending on the concentration degree, ultrafiltration of goat milk caused an increase in total solids content from almost 3 to over 5%, total protein content from 1.4 to over 4%, and fat from 1.3 to 2.8%. Only the content of lactose decreased up to over 2% with the increase of milk concentration by 2.5-fold. Additionally, in comparison to non-concentrated milk, ultrafiltration provoked an increase in non-protein nitrogen and mineral salts content as well as in density and acidity. Significant differences in composition and density were stated not only between non-concentrated and concentrated milk, but also between milks concentrated in a various degree. The acidity of milk increased significantly after 2-fold and 2.5-fold concentration, while the pH was kept fairly constant even after 2.5-fold concentration.

Table 1 Composition and physicochemical properties of raw goat milk with different degrees of concentration

Type of milk	Total solids [%]	Total protein [%]	Casein [%]	Whey proteins [%]	Non protein nitrogen [%]	Fat [%]	Lactose [%]	Ash [%]	Density [g·mL^−3]	Titratable acidity [°SH]	pH
Non-concentrated	11.25^A ± 0.06	2.97^A ± 0.04	2.33^A ± 0.04	0.34^A ± 0.01	0.30^A ± 0.01	3.0^A ± 0.05	4.55^A ± 0.05	0.79^A ± 0.01	1.0281^a,A ± 0.0004	6.6^a,A ± 0.1	6.69 ± 0.02
UF concentrated 1.5×	14.10^B ± 0.17	4.37^B ± 0.09	3.47^B ± 0.08	0.52^A,Ba ± 0.03	0.34^A ± 0.03	4.3^B ± 0.1	4.42^A ± 0.05	0.92^B ± 0.01	1.0304^b,A ± 0.0004	6.9 ± 0.1	6.69 ± 0.01
UF concentrated 2.0×	15.01^C ± 0.12	5.68^C ± 0.08	4.40^C ± 0.07	0.77^B,b ± 0.02	0.49^B ± 0.04	4.9^C ± 0.1	3.35^B ± 0.08	1.02^C ± 0.01	1.0326^B,a ± 0.0007	7.1^b ± 0.1	6.68 ± 0.02
UF concentrated 2.5×	16.73^D ± 0.12	7.35^D ± 0.24	5.17^D ± 0.08	1.64^C ± 0.15	0.55^B ± 0.04	5.8^D ± 0.1	2.31^C ± 0.04	1.12^D ± 0.01	1.0348^B,b ± 0.0008	7.2^B ± 0.1	6.66 ± 0.03
^A–D: Highly significant differences between means (P ≤ 0,01) marked with the different letters in the columns; ^a–b: Significant differences between means (P ≤ 0,05) marked with the different letters in the columns.

Similar changes in the content of main components and physicochemical properties in goat milk after ultrafiltration, as stated in this work, have been observed by Karademir et al.[9] and Domagała and Kupiec,[12] while in cow milk it was observed by Becker and Puhan,[17] Biliaderis et al.,[18] Bird,[7] and Savello and Dargan.[5,19] However, in comparison to results obtained by Becker and Puhan,[17] a lower content of lactose in retentates of goat milk was obtained in the present study. Similarly, after a 6-fold concentration of milk, Bird[7] stated a lower content of lactose, non-protein nitrogen, and mineral components in retentates than in non-concentrated milk.

Sensory Evaluation and Acidity of Yoghurt

Table 2 presents the characteristic of yoghurts from goat milk (non-concentrated and concentrated by ultrafiltration). In sensory evaluation, yoghurt from non-concentrated milk obtained a significantly worse score than all yoghurts from UF concentrated milk. Among yoghurts from concentrated milk, the highest score was given to a sample from 1.5-fold concentrated milk, slightly worse—yoghurt from 2-fold concentrated milk, and significantly worse—yoghurt from 2.5-fold concentrated milk. The latter one was characterized by a too hard and compact structure with consistency typical for cream cheese, not yoghurt. The pH of yoghurt from 2.5-fold concentrated milk was significantly higher than the pH of other yoghurts, which were similar. According to Karademir et al.[9] and Marshall and El-Bagoury,[11] ultrafiltration is the best technique to obtain goat milk yoghurt with high sensory quality including proper flavor, texture, and no syneresis. Biliaderis et al.,[18] Domagała and Wszołek,[30] and Domagała and Kupiec[12] have also found a significantly higher sensory quality of goat milk yoghurt from ultrafiltrated milk in comparison to yoghurt from non-concentrated or concentrated milk using other methods. Lower acidity of yoghurts after concentration of milk by UF can be explained by a higher concentration of protein and, thus, higher buffering capacity of concentrates. Karademir et al.[9] confirmed such effect of goat milk concentration by UF technique on the acidity of yoghurt. The sensory quality of yoghurt from non-concentrated goat milk was assessed by Pažáková et al.[31] In their investigations, goat milk yoghurt had a markedly “goat” flavour, which negatively influenced the sensory quality. Mahdi et al.[32] presented a similar opinion about concentrated yoghurt (labneh) from goat milk because of its “goat” flavour and weak consistency (when comparing the quality of goat labneh with that from cow and sheep milk). According to Kavas et al.,[33] bio-yoghurt from goat milk had a lower overall sensory quality than bio-yoghurt from the mix of goat and cow milks. Fresh goat milk yoghurt (even prepared from ultrafiltrated milk) analysed in this work revealed no “goat” flavour. This flavour was barely perceptible only in stored yoghurts (data not presented).

Table 2 Characteristic of yoghurt from goat milk with different degree of concentration

							Syneresis [%]
Type of milk	Sensory evaluation [scores]	pH	Apparent viscosity [Pa · s]	Hardness [N]	Adhesiveness ×10^3 [|N · m|]	Extrusion force [N]	Drainage method	Centrifugal method
Non-concentrated	3.83^A,a ± 0.12	4.57^A ± 0.01	0.48^A ± 0.03	0.15^a,A ± 0.01	0.14^A ± 0.02	1.20^A ± 0.08	32^A ± 2	51^A ± 4
UF concentrated 1.5×	4.54^B,a ± 0.09	4.53^A ± 0.04	1.33^B ± 0.13	0.26^b,A ± 0.02	0.47^B,a ± 0.04	2.54^B ± 0.09	12^B ± 1	16^B ± 2
UF concentrated 2.0×	4.40^B ± 0.08	4.61^a ± 0.02	2.23^C ± 0.11	0.37^a,B ± 0.04	0.73^B,b ± 0.08	3.52^C ± 0.14	14^B ± 1	13^B ± 1
UF concentrated 2.5×	4.18^b ± 0.11	4.71^B,b ± 0.04	2.47^C ± 0.20	0.49^b,B ± 0.04	1.23^C ± 0.09	5.05^D ± 0.16	6^C ± 1	11^B ± 1
^A–D: Highly significant differences between means (P ≤ 0.01) marked with the different letters in the columns; ^a–b: Significant differences between means (P ≤ 0.05) marked with the different letters in the columns.

Texture and Syneresis of Yoghurt

Hardness is defined as a force that is necessary to obtain the precise deformation of the probe. Adhesiveness is work necessary to overcome the force of attraction between the area of foodstuff and other solids coming into contact with them.[34] The higher the concentration degree of the milk, the higher the values were of the hardness and adhesiveness, as well as extrusion force of the yoghurt samples. The opposite trend was observed when syneresis was determined by both methods. Such differences were statistically significant and they were dependent on the degree of milk concentration. In the case of syneresis, determined by the centrifugal method, significant differences were found only between yoghurt from non-concentrated and UF concentrated milks, but no differences were present between yoghurts from UF milks with different degrees of concentration (Table 2). High effectiveness of ultrafiltration as a way for production of goat milk yoghurt with proper hardness, adhesiveness, and apparent viscosity has also been stated by Domagała and Wszołek,[30] Karademir et al.,[9] and Domagała and Kupiec.[12] Becker and Puhan,[17] Biliaderis et al.,[18] Savello and Dargan,[19] and Lankes et al.[20] have found that yoghurts prepared from concentrated cow milk using the UF method revealed higher hardness, better gel compactness, and higher apparent viscosity after mixing than yoghurts prepared from milk enriched by other methods. Schkoda et al.[35] found that the increase in total protein content in concentrated milk caused higher hardness and higher apparent gel viscosity after mixing in comparison to hardness and apparent gel viscosity from non-concentrated milk. Becker and Puhan[17] and Skriver et al.[21] have stated that an increase in total solids in milk caused an increase in apparent viscosity of the yoghurt samples. Savello and Dargan[5] have found significantly lower syneresis in yoghurts produced from milk concentrated by UF in comparison to yoghurts from non-concentrated milk.

Microstructure of Yoghurt

After milk concentration by ultrafiltration, significant changes were stated also in the microstructure of yoghurts. The microstructure of goat milk yoghurt from non-concentrated milk (a), and from UF concentrated milk 1.5-fold (b), 2-fold (c), and 2.5-fold (d) in magnification 5000-fold is presented in Fig. 1. In comparison to the microstructure of yoghurt from non-concentrated milk, which was more open, consisting of clusters of small casein micelles and large void spaces filled by whey, the concentration of milk by ultrafiltration caused an increase of the size of casein micelles and their clusters in the protein matrix. The protein matrix of yoghurts from concentrated milk was characterized by large casein micelles clusters linked together in bigger agglomerates with a low amount of void spaces. The higher the concentration degree of the milk, the more compact was the protein matrix of the yoghurt and composed with bigger micelles clusters (produced from linking together small micelles in non-concentrated milk). The more compact protein matrix of yoghurt gel obtained in this work resulted in higher hardness and extrusion force of yoghurt and lower syneresis. Using the Cryo-SEM technique, Vargas et al.[36] have found that yoghurt from non-concentrated goat milk was characterized by a more open structure and larger pores than cow milk yoghurt and yoghurt produced from a mixture of goat and cow milks. According to Domagała and Wszołek,[30] the concentration of goat milk by ultrafiltration is responsible for a more compact protein matrix of yoghurt when compared to yoghurt from non-concentrated milk or from milk concentrated by the addition of goat milk powder. Tamime et al.[16] presented an opposite postulate for cow milk yoghurt. They earlier found that yoghurt from ultrafiltrated cow milk was characterized by a more open protein matrix than yoghurt with an addition of skim milk powder.

Figure 1 Microstructure of yoghurt from goat milk: (a) non-concentrated and UF concentrated: (b) 1.5-fold, (c) 2-fold, (d) 2.5-fold (magnification 5000×).

CONCLUSIONS

Concentration of goat milk by ultrafiltration in comparison to non-concentrated milk contributed to an increase in total solids and protein content in milk used for yoghurt production. This, in turn, caused an increase in the hardness, adhesiveness, and extrusion force of yoghurt and a reduction in syneresis. All these changes greatly enhanced the sensory quality of yoghurt. The physicochemical properties of yoghurts were correlated with gel structure. The microstructure of yoghurt from ultrafiltrated milk showed a compact protein matrix, a small amount of void spaces, and large casein micelles. The properties of goat milk yoghurts were significantly dependent on the degree of concentration of the processing milk. The best degree of goat milk concentration, which facilitates the production of yoghurt with the most desired sensory features, the best texture and rheological properties, and the lowest syneresis, is 1.5- or 2-fold (v/v). Yoghurt from 2.5-fold concentrated milk was characterized by a too hard and compact structure with consistency typical for cream cheese rather than for yoghurt.