This study investigates the effects of raw sheep’s milk composition on the strength and syneresis of obtained gels throughout the lactation. Casein, fat and total solids content, as well as ionic calcium mass fraction significantly (p<0.05) increased during lactation. As lactation progressed, milk formed firmer gel with higher syneresis ability. Increasing casein to fat ratio in sheep’s milk significantly increased (p<0.05) gel strength and syneresis. On the other hand, gel strength and syneresis were significantly reduced as a result of increased fat content in sheep’s milk. Ionic calcium mass fraction affected gel strength but not syneresis. Neither gel strength nor syneresis were affected by casein and urea content or by somatic cell count in sheep’s milk. Correlation coefficients between milk components and gel strength, as well as syneresis, were significant (p<0.01, p<0.05) but never higher than 0.35.
The rennet coagulation of milk involves the conversion of milk from a colloidal dispersion of stable casein micelles to a network of aggregated paracasein micelles in a continuous phase entrapping fat and moisture in its pores (
Numerous factors such as stage of lactation, pretreatment of milk, addition of calcium, rennet concentration and renneting temperature of milk affect the curd firmness to various extents but milk composition is of primary importance (
Gel strength is also greatly affected both by the calcium content and pH value of milk. The removal of caseinomacropeptide by rennet leads to exposure of negatively charged amino acid residues of calcium-sensitive caseins to calcium ions (
Casein micelles are reactive over their whole surface and, after the gel has been formed, they have the ability to form more new bonds among paracasein micelles. This leads to additional reduction of space within the paracasein network, which causes formation of a more compact gel structure and expulsion of the whey from the gel (
As far as the authors know, the combined effect of physicochemical characteristics of sheep’s milk and lactation stage has never been studied in relation to strength and syneresis of rennet-induced sheep’s milk gel. Pellegrini
A total of 42 bulk milk samples were collected and analysed. The study was carried out during two successive years in flocks of Dalmatian Pramenka sheep on three family farms. Two farms were located on the island of Brač, Croatia. The first farm is situated at an altitude of 207 m above sea level in the area of Supetar. The second farm is in the inner part of the island in the area of Pražnice at an altitude of 437 m above sea level. The third farm is in the area of Maovice in inland part of Dalmatia on the north slopes of mountain Svilaja, at an altitude of 640 m above sea level. Lactation period on the first farm was from mid-February until mid-June and lasted on average 120 days. On the second farm lactation started at the beginning of May and ended in mid-July, and lasted on average 75 days. On the third farm, sheep were milked from the first half of April until the end of the first week of August, and during 126 days of lactation the sheep mostly grazed on pastures of mountain Svilaja at an altitude of 1000–1100 m above sea level. The lactation period on each farm was divided into three equally long stages: early, middle and late. The lactation stages on the first farm lasted 40 days each, on the second farm 25 days each and on the third farm 42 days each. Once lambing started on each farm, sheep that were lambed within two weeks were considered. During this research, 100, 50 and 300 sheep were milked twice a day on the first, second and third farm, respectively. Throughout the entire lactation, 1500 mL of raw sheep’s milk samples were collected every 14 days from cooling tanks, two hours after the morning milking. Milk samples represented the mixture of evening and morning milking. Six milk samples from early lactation, six milk samples from middle lactation and four milk samples from late lactation period were taken from the first farm. Since the lactation on the second farm was shorter than on the other two, early and middle lactation included four milk samples each, while the late lactation included two milk samples. The number of milk samples taken from the third farm from each lactation stage was the same as from the first farm. In total, the number of milk samples collected from the first farm was 16, from the second 10 and from the third 16. Milk samples were transported in a portable refrigerator to a laboratory and stored at 4 °C until used in the experiment (within the next 24 h). The content of protein and fat were analysed by infrared spectroscopy using a MilkoScan FT 120 (Foss, Hillerød, Denmark) according to ISO 9622:2013 (
Syneresis and gel strength of the obtained curd were measured at the Institute of Adriatic Crops and Karst Reclamation in Split, Croatia. A laboratory beaker was filled with 1000 mL of milk, left in a water bath at 32 °C and allowed to equilibrate. After the equilibration, rennet (CHY-MAX extra; Chr. Hansen, Hørsholm, Denmark) was added to the milk (0.12 g/kg) following the manufacturer’s recommendation. A volume of 600 mL of milk was removed from the laboratory beaker immediately after rennet addition and evenly distributed into six glass beakers (100 mL each) for the purpose of measuring gel strength. The glass beakers were left in a water bath at 32 °C to coagulate over different coagulation times (45, 60 and 75 min). Two replicates were analysed for each coagulation time. After expiration of coagulation time, gels within the six beakers were subjected to gel strength measurement at room temperature (25 °C) by texture analyser (TA Plus; Lloyd Instruments, Fareham, UK) equipped with a 500 N load cell (XLC -0500-A1; Lloyd Instruments) and cylindrical probe (FG/CY3; Lloyd Instruments). The experimental data were analysed by Nexygen Plus 3 software (
Statistical evaluations of the results were conducted using SPSS v. 21 (
Modulation in milk constituents as well as gel strength and syneresis throughout lactation was extensively studied and well-documented in numerous studies, and therefore it was presented but not discussed in this research. Composition of raw sheep’s milk throughout lactation is shown in
Component | Stage of lactation | p-value | ||
---|---|---|---|---|
Early ( |
Middle ( |
Late ( |
||
(18.6±0.3)a | (19.1±0.3)ab | (19.8±0.3)b | <0.05 | |
(5.7±0.1)a | (5.8±0.1)a | (6.3±0.1)b | <0.01 | |
(4.49±0.08)a | (4.63±0.07)a | (4.89±0.08)b | <0.01 | |
(7.5±0.3)a | (7.8±0.2)ab | (8.3±0.3)b | <0.05 | |
(10.9±0.4)a | (11.4±0.4)a | (12.8±0.5)b | <0.05 |
Data are expressed as mean value±standard error. Mean values within a row with different letter in superscript differ significantly (p<0.05).
Parameter | Stage of lactation | Parameter | Gel strength | Parameter | Syneresis | Parameter | Casein |
---|---|---|---|---|---|---|---|
Gel strength | 0.28** | Casein to fat ratio | 0.33** | Casein to fat ratio | 0.25** | Fat | 0.74** |
Syneresis | 0.24* | Total solids | -0.25* | Fat | -0.32** | ||
Protein | 0.48** | Fat | -0.26** | ||||
Casein | 0.51** | Ionic calcium | 0.25* | ||||
Total solids | 0.43* | Syneresis | 0.22* | ||||
Fat | 0.35* | ||||||
Ionic calcium | 0.46** |
Mean values followed by different letters in superscript differ significantly, *(p<0.05), **(p<0.01)
Both gel strength and syneresis significantly (p<0.05) increased during lactation (
Gel strength and syneresis of rennet-induced milk gel at different stages of lactation
The effects of casein and fat content, as well as casein to fat mass ratio, on syneresis ability and gel strength are presented in
Component | |
Strength/N | Syneresis/% | p-value | |
---|---|---|---|---|---|
Strength | Syneresis | ||||
|
|||||
4.08-4.53 | 13 | 0.23±0.01 | 30.9±3.9 | n.s. | n.s. |
4.54-4.98 | 15 | 0.22±0.01 | 24.4±2.8 | ||
4.99-5.45 | 14 | 0.23±0.02 | 26.4±6.0 | ||
|
|||||
6.48-7.58 | 15 | (0.24±0.01)a | (32.8±3.5)a | <0.01 | <0.05 |
7.59-8.68 | 10 | (0.23±0.02)a | (22.8±2.1)b | ||
8.69-9.78 | 17 | (0.18±0.01)b | (20.8±2.0)b | ||
|
|||||
0.49-0.55 | 12 | (0.18±0.01)a | (25.2±5.0)a | <0.05 | <0.05 |
0.56-0.61 | 16 | (0.21±0.01)ab | (31.6±3.7)ab | ||
0.62-0.67 | 14 | (0.25±0.01)b | (32.9±3.0)b | ||
|
|||||
8.12-10.73 | 13 | (0.21±0.01)a | 31.5±4.0 | <0.05 | n.s. |
10.74-13.34 | 12 | (0.26±0.01)b | 37.1±3.1 | ||
13.35-15.96 | 17 | (0.24±0.02)a | 34.4±6.5 | ||
γ(urea)/(mg/100 mL) | |||||
16.25-24.73 | 14 | 0.22±0.01 | 26.3±3.4 | n.s. | n.s. |
24.74-33.21 | 10 | 0.24±0.01 | 29.1±3.1 | ||
33.22-41.69 | 18 | 0.22±0.01 | 36.4±3.6 | ||
|
|||||
<150 | 12 | 0.22±0.01 | 28.5±3.8 | n.s. | n.s. |
151-600 | 16 | 0.23±0.01 | 25.7±3.6 | ||
601-1.200 | 14 | 0.23±0.01 | 25.7±3.8 |
Data are expressed as mean value±standard error. Mean values within a column with different letter in superscript differ significantly (p<0.05), n.s.=not significant,
The results of this study indicate that the increase in fat content in sheep’s milk significantly (p<0.01) decreased the strength of the obtained gels (
Increasing casein to fat mass ratio in sheep’s milk significantly (p<0.05) enhanced both gel strength and syneresis (
Our results show that urea concentration in sheep’s milk had no effect on gel strength (
The increase in ionic calcium mass fraction in sheep’s milk had a significant (p<0.05) effect on gel strength. As seen in
Gel strength remained unchanged regardless of somatic cell count in sheep’s milk (
At the end of the discussion, it is important to emphasize that although significant correlations (p<0.01; p<0.05) between milk components and gel strength as well as syneresis were observed, these coefficients were low and never higher than 0.35 (
The results of this study show that total solids, casein and fat contents as well as ionic calcium mass fraction of sheep’s milk markedly increased during lactation. However, as lactation progressed, there was an increase in the strength and syneresis of the obtained gels, but correlation between these parameters was weak. Increasing casein to fat mass ratio in sheep’s milk led to the formation of gel with higher strength and syneresis values, while increasing fat content showed the opposite trend. Ionic calcium mass fraction had an effect on gel strength, but not syneresis. Since strength and syneresis of rennet-induced milk gel are important factors in the cheese making, these results could potentially enable the cheese industry to increase cheese production cost-effectiveness. Additionally, the results of this research show that the influence of lactation should not be ignored in the cheesemaking process.
This work was supported by grants from the Split-Dalmatia County and municipalities of Nerežišća, Postira, Pučišća, Sutivan and Supetar, Agriculture Development Programme – A101007, Agreement Number 402-07/11-01/230 and 402–07/12-01/658. The authors would like to thank Božica Kuzmanić, Draga Buvinić and Jagoda Vuletić with special thanks to their families for hospitality and support.