TRADITIONAL YOGHURT CULTURE VS. SELECTED QUALITY PROPERTIES OF FERMENTED BEVERAGES PRODUCED FROM GOAT‘S MILK

Anna Mituniewicz-Małek¹, Izabela Dmytrów¹, Małgorzata Jasińska², Jerzy Balejko³, Barbara Szymczak^4
1 Department of Dairy Technology and Food Storage, West Pomeranian University of Technology, Szczecin, Poland
² Department of Dairy Technology and Food Storage, Faculty of Food Sciences and Fisheries, West Pomeranian University of Technology, Szczecin, Poland
³ Department of Food Process Engineering, West Pomeranian University of Technology, Szczecin, Poland
⁴ West Pomeranian University of Technology in Szczecin

ABSTRACT

Fermented milk beverages produced from goat's milk are characterized by loose, soft consistency, which may be improved by the selection of appropriate starter cultures. Taking into account that the starters used in cow's milk technology are not always suitable for goat's milk, the aim of this study was to analyze to what extent two commercially-available yoghurt cultures, with traditional composition, enable obtaining a product with desirable quality properties from goat's milk. Two batches of yoghurt were produced during the experiment. Super jogurt starter of the Canadian Institute Rosell Inc. was used to obtain the first batch (J-A) and Yoghurtferment from Lactoferm (Belgium) for the second batch. The analyzed beverages were subjected to organoleptic, physicochemical and rheological evaluations after 1, 3, 7, 14 and 21 days of refrigerated storage (5 ±1°C). The study demonstrated that fermented milk beverages from goat's milk that was produced with both classic yoghurt cultures had desirable qualitative characteristics.
   The experimental yoghurts did not differ in the organoleptic properties during storage.
   The statistically significant differences were noted in the titratable acidity and pH, and not in the viscosity. The produced beverages were characterized by a low content of acetaldehyde, in turn the highest hardness was noted for the J-A beverage produced with the Super jogurt starter. 

Key words: goat’s milk, yoghurt, physicochemical parameters, rheology.

INTRODUCTION

The application of goat's milk for the production of fermented milk products favors the preservation or even enhancement of their nutritional, dietetic and therapeutic value [26, 45]. Fermented milk (yoghurt, kefir etc.) obtained from goat's milk differs in many aspects from cow's milk, which causes that its quality, and especially its peculiar organoleptic properties are not always accepted by a Polish consumer. According to the literature data [20, 42, 45], the quality of fermented milk is-to the significant extent-determined by the selection of appropriate starter culture. A large variety of lyophilized dairy cultures dedicated to the production of fermented beverages, especially from cow's milk, are commercially available throughout the country. Taking into consideration that the starters used in cow's milk technology are not always suitable for goat's milk, the aim of this study was to analyze to what extent two commercial yoghurt starters, with traditional composition, provide products from goat's milk with desirable quality properties. The undertaken aim seemed to be important owing to scarcity of literature data about the usability of commercially-available starter cultures for the production of milk beverages originating from different animal species i.e. goat's milk.

MATERIAL AND METHODS

The experimental material was yoghurt produced in a laboratory scale from goat's milk using the thermostatic method. The raw material was goat's milk acquired at the beginning of March 2009, at the organic farm "Kozi Gródek" in Wołeczkowo near Szczecin. The characteristics of goat's milk was compiled in Table 1.

Table 1. Characteristics of goat's milk

Quality parameters of milk	Mean value
Dry matter content [%]	11.33
Total protein content [%]	3.46
Fat content [%]	3.35
Density [g/cm3]	1.0282
Titratable acidity[OSH]	7.00
pH	6.02

Two batches of yoghurts (J-A and J-B) were produced using a lyophilised yoghurt culture with traditional microflora composition (Streptococcus thermophilus, Lactobacillus delbruecki ssp. bulgaricus). In the case of J-A beverages, processed milk was inoculated with the culture of the Canadian Institute of Rosell Inc. with trade name Super jogurt, and in the case of J-B with the culture Yoghurtferment from Lactoferm company (Belgium).

Pasteurized goat's milk (85°C/10 min) was used for the production of experimental yoghurt, which after thermal treatment was cooled to a temperature of 40°C, and then standardized using goat's milk powder (Table 2) until its dry matter reached 14%. In a further stage of the production process, the raw material was divided into two equal parts, then each was inoculated with an appropriate, previously activated yoghurt culture as a starter (5%). Thus prepared semi-product was poured in 50 cm³ portions into individual packages of 80 cm³, tightly sealed and incubated at a temperature of 42°C/ 4–5 h until the curd was formed (pH 4,7). When the process of souring had been finished, the experimental yoghurts were placed in the cooling cabinet at 5 ±1°C, where they were stored for a period of three weeks. The samples for analysis were collected after 1, 3, 7, 14 and 21 days of refrigerated storage, 6 cups (300 cm³) from each batch produced.

Table 2. Characteristics of goat's milk powder

Quality parameters of milk powder	Mean value
Total protein content [g]	27,2
Fat content [g]	23,8
Lactose content [g]	38,2
Calcium content [mg]	1105
Phosphor content [mg]	1079

The experimental yoghurt was subjected to oragnoleptic and physicochemical analysis.

The organoleptic analysis was carried out by a properly trained group of 4–5 tasters in an especially-designed room free from extraneous odors, in accordance to the guidelines specified by Baryłko-Pikielna [2] and Kurpisz [22]. The assessment involved the evaluation of the appearance, taste, odor and consistency of the beverages using a 5-point scale expanded with half-points. The physicochemical analysis included the determination of: titratable acidity in °SH [9], active acidity (pH) using Q 150 pH-meter, and content of acetaldehyde [24]. The experimental fermented beverages were also subjected to rheological analysis, which involved the evaluation of their viscosity and texture. The viscosity determination was conducted using a measurement system with coaxial cylinders with a dual slot of AR 2000 rheometer (American Instruments). The apparent viscosity of the analyzed samples was evaluated at a shear rate in the range of 1 to 400 s^-1, at a constant temperature of the samples (Peltier module). In turn, the texture profile analysis (TPA) was conducted using a texture analyzer TA.XT plus of Stable Micro System company. The yoghurt samples were penetrated with an aluminum cylinder 20 mm in diameter at a depth of 25 mm with a rate of 5 mm·s^-1 and force of 1 g [16]. The texture profile analyses included the evaluation of such attributes of yoghurt as: hardness, adhesiveness, cohesiveness, gumminess, springiness. However, guided by the available literature [36], our study focused on hardness analysis as the main texture parameter.

The goat's milk was also subjected to physicochemical analysis: titratable acidity in oSH [9], active acidity (pH), protein content using formol method by Walker [48], fat content using Gerber method [48], and density using aerometric method [48]. The dry matter content was calculated using the Fleischmann formula [48].

All assays of physicochemical and rheological analyses were carried out in three or four parallel replications, and the results obtained were verified statistically using Statistica P 2 7. I software. The normal distribution was tested using K-S and Lilliefors tests, and homogeneity of variance using Levene test. In addition, the analysis of variance with repeated measurements was applied, and the significance of differences between mean values was calculated by Dunkan test. All tests were performed at P = 0.05 and P = 0.01 significance levels.

RESULTS AND DISCUSSION

The characterization of the two batches of fermented milk beverages produced from goat's milk using traditional yoghurt cultures from two different manufacturers, was based on the results of conducted organoleptic evaluations, and the results of titratable acidity, pH, acetaldehyde content as well as viscosity and hardness assays.

The experimental yoghurts produced from goat's milk, during the entire period of refrigerated storage, were characterized by homogenous, concise curd and a lack or minimal presence of whey (Fig. 1). On the last day of the study (after 21 days), in both experimental beverages single clumps of mold were noted, which resulted in the lowest scores of the samples (1 point). The taste of experimental products followed the similar pattern. In all beverages more or less intensive goat aftertaste was noticed. The taste of yoghurts deteriorated with time of refrigerated storage, and from the first week (after 7 days) until the end of the study the sour taste was observed to intensify. However, as soon as after 14 days the experimental products were clearly sour. Also Pieczonka and Pasionek [32] noted a sudden decrease of the sensory quality of fermented goat's milk beverages after 9 days of storage. The changes of organoleptic properties of yoghurts from goat's milk during storage were also observed by Borek-Wojciechowska [7]. The odor of yoghurts from both batches examined was assessed similarly. The sample J-A received the scores at the level from 4.25 to 4.37 points, and the sample J-B at the level from 4.0 to 4.37 points. Over the entire period of the study, the products were characterized by homogenous, thick consistency. However, the J-A yoghurt was assessed slightly higher, which was determined by the appearance of a lower amount of lumps than in the J-B yoghurt.

Figure 1. Results of organoleptic test (5-points scale) of yoghurts from goat's milk during refrigerated storage (5 ±1°C)

In the case of the assessed physicochemical parameters, the largest differences between analyzed batches were noted in titratable acidity. During three weeks of storage, the significantly higher acidity was recorded in the J-A beverage (52.04–63.60 °SH) in comparison to the J-B beverage (41.72–48.40 °SH). Despite distinct differences in titratable acidity (Table 3) in both experimental products the acidity was increasing along with prolonging storage period. After 21 days, the greater increase in acidity in relation to initial value, i.e. by 11.56 °SH (18.17%), was observed for J-A beverage, whereas the acidity of J-B beverage increased by 6.68 °SH (13.8%). The statistical analysis confirmed that in both cases the fluctuations noted were significant (Table 5).

Table 3. Characteristics of yoghurts from goat's milk (titratable acidity, pH) during refrigerated storage

Samples	Storage time [days]
	1		3		7		14		21
	x	SD	x	SD	x	SD	x	SD	x	SD
Titratable acidity [°SH]
J-A	52.4	0.24	56.4	0.00	58.8	0.24	62.4	0.48	63.6	0.48
J-B	41.6	0.24	42.4	0.4	44.4	0.48	44.8	0.8	48,4	0.4
SA	A : C**		A : B**		A : B**		A : B**		A : B**
pH
J-A	4.08	0.006	4.05	0.015	4.04	0.015	3.83	0.010	3.84	0.042
J-B	4.35	0.023	4.20	0.021	4.23	0.044	3.78	0.020	3.93	0.010
SA	A : B**		A : B*		A : B*		-		-

Explanatory notes: J-A – yoghurt with Super jogurt culture, J-B – yoghurt with Yoghurtferment culture, x – mean value, SD – standard deviation, SA – results of statistical analysis, * – statistically significant differences at P = 0.05, ** – statistically significant differences at P = 0.01.

Table 4. Characteristics of yoghurts from goat's milk (acetaldehyde, hardness, viscosity) during refrigerated storage

Samples		Storage time [days]
		1		3		7		14			21
		x	SD	x	SD	x	SD	x		SD	x	SD
Acetaldehyde [mg/dm3]
J-A		0.22	0.020	0.40	0.014	0.10	0.008	0.08		0.007	0.07	0.002
J-B		0.14	0.012	0.32	0.007	0.07	0.007	0.06		0.006	0.01	0.001
SA		A : B*		A : B*		-		-			A : B*
Hardness [G]
J-A	32.06		1.537	35.39	1.130	34.60	2.048		35.39	1.187	35.38	1.560
J-B	27.20		0.875	27.96	1.175	28.60	0.763		26.52	2.160	29.18	0.164
WAS	-			A : B**		A : B*			A : B**		A : B*
Viscosity [Pa·s]
J-A		0.5765	0.0924	0.8673	0.0239	0.9091	0.0173	1.4217		0.1427	1.2723	0.0795
J-B		0.6810	0.0212	0.7757	0.0358	0.8866	0.0189	0.9487		0.0168	0.9580	0.0101
SA		-		-		-		-			-

Explanatory notes as in Table 3

In general, during 21 days of refrigerated storage slightly higher pH (3.76–4.35) was observed in J-B yoghurt batch, which was produced with "Yoghurtferment" culture from Lactoferm company. When analyzing pH values of the experimental beverages in subsequent periods of storage (after 1, 3, 7, 14 and 21 days), significant differences were observed after 1, 3 and 7 days (Table 3). At the above-mentioned storage periods the value of the analyzed parameter in yoghurt J-B, in comparison to J-A, was higher by 0.27, 0.15 and 0.19 units, respectively. In turn, after 14 days of storage both in J-A and J-B beverage the pH value observed to decrease significantly in comparison to the initial value (Table 5).

The determination of acidity is one of the primary analyses of raw materials and finished dairy products. The degree of acidity of raw milk proves its freshness and determines its suitability for processing, on the other hand the acidity of the finished products affects their organoleptic properties, consistency and shelf life. The titratable acidity of fermented milk produced from goat's milk during refrigerated storage was studied by Pieczonka and Pasionek [32], the authors noted the results in the range from 44.2 °SH to 53 °SH in the case of yoghurt. Similar research was also carried out by Danków et al. [12] as well as by Mituniewicz-Małek and Dmytrów [29]. In the first study the acidity of the yoghurt from goat's milk ranged from 60 to 60.5 oSH, whereas in the second one from 38.4 to 49.5 °SH. Furthermore, Mituniewicz-Małek and Dmytrów [29] analyzed pH values in experimental beverages obtained with a traditional yoghurt culture. The results recorded were similar to those obtained in the present study. Very comparable results of the studies on the changes of titratable acidity and pH of yoghurts during refrigerated storage were also noted by Pourahmad and Mazaheri Assadi [33].

The results obtained of acetaldehyde content measurements (Table 4) revealed that the J-A and J-B beverages were characterized by the low content of the above-mentioned flavor compound (0.088–1.754 mg·dm^-3). However, during the entire period of the study (21 days), the J-A yoghurt contained more acetaldehyde. And so in the quoted beverage the content of the analyzed metabolite ranged from 0.520 mg·dm^-3 to 2.935 mg·dm^-3, and in the J-B beverage from 0.273 mg·dm^-3 to 2.375 mg·dm^-3. Interestingly, in both cases, the highest content of acetaldehyde was observed after 3 days of refrigerated storage. However, after this period (7th day), both in the batch of yogurt J-A and that of yoghurt J-B a substantial reduction in its content appeared, i.e. by 2.198 mg·dm^-3 (75%) and 1.882 mg·dm^-3 (98.83%), respectively, compared to the 3rd day of storage. A gradual decrease in the acetaldehyde content was also observed in the subsequent periods of research and after 21 days of refrigerated storage, both in the J-A and J-B beverages its content was significantly lower in relation to the initial value (Table 5).

Table 5. Results of variance analysis of the examined attributes of yoghurts from goat's milk

EXPERIMENTAL PRODUCT
Analyzed features	Significance of differences during storage [days]
Yoghurt with culture Super jogurt [J-A]
Titratable acidity [°SH]	1 : 3, 7, 14, 21**; 3 : 7, 14, 21**; 7 : 14, 21**
pH	1 : 14, 21**; 3 :  14, 21**; 7 : 14, 21**
Acetaldehyde content [mg/dm3]	1 : 3, 7, 14, 21**; 3 : 7, 14, 21**;
Hardness [G]	no significant differences
Viscosity [Pa·s]	1 :  3, 7, 14, 21**; 3 : 14, 21**; 7 : 14, 21**
Yoghurt with culture Yoghurtferment [J-B]
Titratable acidity [°SH]	1 : 3, 7, 14, 21**; 3 : 7, 14, 21**; 7 : 14, 21**
pH	1 : 3, 7, 14, 21**; 3 : 14, 21**; 7 : 14, 21**; 14 : 21**
Acetaldehyde content [mg/dm3]	1 : 3, 7, 14, 21**; 3 : 7, 14, 21**; 7 : 21**; 14 : 21**
Hardness [G]	no significant differences
Viscosity [Pa·s]	1 :  3, 7, 14, 21**; 3 : 14, 21**;  7 : 14, 21**

Explanatory notes: * – statistically significant differences at P = 0.05, ** – statistically significant differences at P = 0.01.

According to the literature data [45, 17], aroma compounds are products of metabolism of carbohydrates, fats, proteins and citrates, whose changes during ripening and storage of fermented products are enabled by proteolytic and lipolytic enzymes released by bacteria. Moreover, according to Dzwolak et al. [17] and Libudzisz [27], the diversity of components affecting the flavor of yoghurt is determined by the properties of strains of bacteria used to produce it. According to references [35, 25], an essential component of yoghurt flavor is acetaldehyde, whose concentration reaches 10–15 mg·dm^-3. However, the cited contents relate to the yoghurts obtained from cow's milk. In the case of fermented goat's milk beverages it is difficult to achieve this level, which is related to the fact that the raw material contains more than twenty times as much glycine inhibiting threonine aldolase which transforms threonine to acetaldehyde and glycine [34, 40]. This also means that the content of acetaldehyde in experimental yoghurts from goat's milk was considerably lower than the content declared in its cow's milk homolog with proper aroma (10 mg·dm^-3), but it was generally consistent with the literature data [21, 39] according to which lactic bacteria can produce from 0.1 mg·dm^-3 to 10.0 mg·dm^-3 of that compound. The low content of acetaldehyde in goat's milk yoghurts was also noted by Cais-Sokolińska et al. [10]. The literature [49, 25] also reported that the content of analyzed flavor compound was strictly determined by the storage period. In fermented milk acetaldehyde is metabolized to ethanol as a consequence of alcohol dehydrogenase activity, which results in the decrease of its content. It finds confirmation in other references [5, 8, 29], and results of our previous studies.

In the case of viscosity there were no significant differences noted between all experimental products (Table 4) during the entire storage period (after 1, 3, 7, 14 and 21 days). The viscosity of the J-A yoghurt ranged from 0.581 to 1.422 Pa·s, whereas that of the J-B yoghurt from 0.681 to 0.958 Pa·s. However, in the course of the study it was concluded that the viscosity of the experimental beverages diverged more after 14 and 21 days than after 1, 3 and 7 days. After the second and third week the yoghurt obtained with the Super jogurt starter (J-A) was characterized by a slightly higher viscosity. On account of the above, the viscosity of J-B yoghurt after 14 and 21 days of refrigerated storage was lower by about 33 and 20%, respectively in comparison to the viscosity of J-A yoghurt batch.

Over the entire period of the study the batch of J-A yoghurt was characterized by much higher hardness (34.595–38.057 G) in comparison to J-B yoghurt (25.693–28.699 G). The statistical evaluation of the results obtained confirmed that the differences noted in hardness between the analyzed products were significant after 3, 7, 14 and 21 days (Table 4). In turn, no significant differences in both yoghurt batches were noted when analyzing the effect of the time of refrigerated storage (Table 5).

According to the literature data [48], rheological properties determine the texture of products. Szcześniak [41] specified that fundamental rheological properties include, among others, mechanical properties such as viscosity and hardness of texture. According to the literature [3, 38, 23], the texture of food products, along with the taste and odor, is one of the most important quality attributes of fermented milk products, including yoghurt. Many scientists analyzed the rheological properties of yoghurt, however, the vast majority of the studies concerned products obtained from cow's milk. The knowledge on the rheology of goat's milk yoghurt is scarce and its rheological properties in relation to various factors were studied by Cais-Sokolińska et al. [10], Chojnowski et al. [11], Wszołek [45], Bonczar and Wszołek [6]; Novakovic [31], Danków et al. [12] Domagała and Wszołek [16], Domagała [13], Mituniewicz-Małek and Dmytrów [29], among others. However, from the viewpoint of the present study of major importance are the results of Danków et al. [12]. According to that study, the viscosity of goat's milk yoghurt depended on the composition of the starter culture used in the manufacture of experimental products, as well as on the percentage of individual bacteria. The viscosity of the yoghurt obtained from goat's milk was also analysed by Domagała and Juszczak [14]; the authors concluded that the rheological properties of experimental beverage, more precisely – its viscosity, was dependent on the origin, i.e. the place of purchase of the culture, for all the analyzed yoghurts were manufactured using classical yoghurt starter, but acquired from various companies (Visby, Chr. Hansen, Biolacta). Also, in the presented study the traditional yoghurt cultures used to produce the experimental samples originated from two different sources, which could result in their different viscosity. According to Bonczar and Wszołek [5], the differences in the viscosity of fermented milk products may be due to various abilities of individual bacteria species to form mucilage, and the greater their share in the starter culture, the higher the viscosity of the resulting product will be. Also many authors [16, 5, 4, 15, 36, 13, 28, 19, 29, 46, 18] performed the texture profile analysis (TPA). However, the vast majority of the studies were carried out to compare the values of the individual texture attributes of yoghurt produced from goat's, cow's and ewe's milk, respectively. Based on the results [29, 19], it was concluded that the goat's milk was characterized by the lowest hardness, adhesiveness, gumminess, as well as apparent viscosity in comparison to the yoghurt obtained from cow's milk or ewe's milk. Only in the case of cohesiveness were the relationships different. The hardness of yoghurt was also studied by Torre et al. [43] and Mituniewicz-Małek et al. [30]; according to whom the value of that parameter was affected by the composition of microflora in the starter culture used for its production. From the literature data [13, 1, 19, 44] it may also be concluded that the viscosity and hardness of yoghurt was affected by the time of refrigerated storage. Similarly, it was also observed in the course of the presented study.

To summarize the results obtained it was concluded that the traditional yoghurt cultures with trade names Super jogurt (by Canadian Rosell Institute Inc.) and Yoghurtferment (by Lactoferm company) can be successfully applied to produce goat's milk fermented beverages.

CONCLUSIONS

1. The studies revealed that traditional yoghurt cultures with trade names Super jogurt and Yoghurtferment applied to produce fermented milk beverages from goat's milk enabled obtaining products with desirable quality properties.
2. No visible differences in appearance, taste, odor nor consistency were noted during storage of the experimental yoghurts.
3. The experimental samples significantly differed in titratable acidity.
4. In general, during refrigerated storage significantly higher pH values were noted for the batch of yogurt prepared with the addition of the Yoghurtferment starter (J-B).
5. The experimental beverages were characterized by the low content of acetaldehyde.
6. The highest hardness was observed in the case of yoghurt produced with the Super jogurt culture (J-A).
7. The traditional yoghurt starters did not affect significantly the viscosity of the analyzed products.
8. The time of storage did not affect significantly the hardness of the analyzed yoghurts.

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Accepted for print: 30.09.2011

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Anna Mituniewicz-Małek
Department of Dairy Technology and Food Storage,
West Pomeranian University of Technology, Szczecin, Poland
Papieża Pawła IV/3 71-459 Szczecin, Poland
email: aniamalek4@wp.pl

Izabela Dmytrów
Department of Dairy Technology and Food Storage,
West Pomeranian University of Technology, Szczecin, Poland
Papieża Pawła IV/3, 71-459 Szczecin, Poland

Małgorzata Jasińska
Department of Dairy Technology and Food Storage, Faculty of Food Sciences and Fisheries, West Pomeranian University of Technology, Szczecin, Poland
Papieża Pawła IV/3, 71-459 Szczecin, Poland

Jerzy Balejko
Department of Food Process Engineering,
West Pomeranian University of Technology, Szczecin, Poland


Barbara Szymczak
West Pomeranian University of Technology in Szczecin
al. Piastów 17
70-310 Szczecin, POLAND

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Responses to this article, comments are invited and should be submitted within three months of the publication of the article. If accepted for publication, they will be published in the chapter headed 'Discussions' and hyperlinked to the article.

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