1. Lab of Food Microbiology Jeong, Ji Hee
Rennet coagulation of sheep milk processed by ultrafiltration at low concentration factors
Lab of Food Microbiology
Jeong, Ji Hee

3. The aim of this work
To investigate the rennet properties of the UF sheep milk concentrates in a wide range of low concentration factors
that can be useful for the adoption of correct technological practices in innovative technological procedures in cheese making from sheep milk

4. Introduction
Rennet (Rennin, Chomosin) It is often used in the production of cheese. Rennet contains many enzymes, including a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into solids (curds) and liquid (whey). 
Cheese yield Determined by Protein and fat cheese yield = ( F + C ) N ** F : fat, C : Casein, N : Cheese coefficients

5. Introduction
Rennet coagulation The primary phase of rennet coagulation involves the specific enzymatic modification of casein micelles

6. Introduction
Rennet coagulation

7. Introduction
Rennet coagulation Aggregation of the rennet-altered micelles is the secondary phase of coagulation : paracasein is curd formation in the presence of Ca2+
Casein
Para-casein
Addition of calcium reduces the rennet coagulation time of milk due to the neutralization of negatively charged residues of the casein leading to the increase of the renneted micelles

8. Introduction Why sheep milk?
As sheep milk show special features affecting particularly the coagulation properties
Reported that a decrease in the rate of curd firming (0k20) was observed with increasing concentration of the UF concentrates ( up to x 2.0 fold)

9. Introduction UF(Ultra-filtration)
A variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane
Suspended solid and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane

10. Introduction
UF(Ultra-filtration)

11. Introduction Ultrafiltered milk
Milk protein concentrate that is produced by passing milk under pressure through a thin, porous membrane to separate the components of milk according to their size, permitting greater efficiency in cheese making
Ultrafiltration allows the smaller lactose, water, mineral, and vitamin molecules to pass through the membrane, while the larger protein and fat molecule
Ultrafiltration makes cheese manufacturing more efficient and can benefit consumers if cost savings are passed on

12. Introduction
Ultrafiltered milk

13. Introduction Advantage of ultrafiltered milk
Enables the incorporation of an increased amount of native whey proteins into the cheese curd
It is noteworthy that an increased level of whey proteins has as well a positive effect on the nutritional value of cheese
Lead to physicochemical changes during the renneting, resulting in an increase in curd firmness

14. Introduction
Use of UF for milk concentration in the cheese making industries ☞ at three different ranges of concentration factor : low, medium and high
The medium and high concentration factors → successfully used for the manufacture of cow milk soft cheeses → texture problems encountered in the manufacture of hard and semi-hard types of cheese
The low concentration factors → considered an alternative to overcome the problems of hard and crumbly texture

15. Methods The skim sheep milk Chemical analysis
The Na, K, Mg and Ca contents of skim sheep milk, milk concentrates and permeates were determined by atomic absorption spectroscopy
Skim sheep milk and concentrated milk were analyzed for protein by Kjeldahl method and a nitrogen conversion factor of 6.38 was usd

16. Methods The sheep milk clotting aptitude
Assessed with the Optigraph → non-destructive monitoring of milk clotting
Optigraph
An optoelectronic system based on near infrared signal attenuation measurements
During milk coagulation, the emitted radiation is progressively attenuated due to changes in casein structure
Clot firmness is measured as a function of the volt need to compensate the signal attenuation

17. Methods Diffusing wave spectroscopy(DWS)
Transmission DWS was used to characterize concentrated skim milk for changes in stability and renneting

18. Results
Nomenclature
Rennet clotting time(RCT) : The time for the milk to start clotting (micellar aggregation) after rennet addition
f ( solute apparent rejection coefficient, %) : determined in terms of total organic carbon as in the following equation f = (C feed – C permeate) / C feed
Volumetric concentration factor(VCF) : degree of concentration

19. Results Nomenclature A20 : curd firmness after 20 min, volt
AR : curd firmness after 2 times the RCT, volt
A2R : curd firmness after 3 times the RCT, volt
0k20 : rate of curd firming, min

20. Results Two lots of sheep skimmed milk
uc₁ and uc₂ were concentrated by UF
The composition of the two lost of milk and their concentrates were analyzed

21. Mineral content(mg/kg)
Results
Two lots of sheep skimmed milk
Table 1. Composition of skim sheep milk and variation of the composition of the UF retentates with the volumetric concentration factor(VCF)*
Protein
(g/kg)
Mineral content(mg/kg) Na K Mg Ca
Lot uc₁
VCF 1.0
54.6
444
1197
147
1635
VCF 1.2
65.6
443
1247
171
1994
VCF 1.4
75.3
441
1205
186
2238
VCF 1.6
82.1
435
1257
201
2457
Lot uc₂
56.0
510
1278
165
1962
85.8
612
1398
222
2985
VCF 2.0
106.2
570
1434
249
3552
* Mean values
When the UF concentration factor increased, the concentration of protein and calcium ions increased

22. Results The rejection coefficients
Fig. 1 Variation of the rejection coefficients to proteins and to salts

23. Results The rejection coefficients
The rejection coefficients practically did not vary with VCF
Those relative to the proteins and to the bivalent salt ions, Ca and Mg, yielded the higher values of 97%, 86% and 63%
Ca and Mg losses were probably normal, corresponding to soluble forms of these elements
The small loss of protein in UF process can be attributed to the non-protein nitrogen fraction

24. Results
The effect of protein concentration on the milk rennet coagulation properties
Table 2. Variation of rennet clotting time(RCT), curd firmness(AR, A2R, A20, A40) and rate of curd firming (0k20) with volumetric concentration factor(VCF) of ultrafiltration of skim sheep milk*
RCT
(min) AR
(volt)
A2R
A20
A40
0k20
Lot uc₁
VCF 1.0
16.5(0.33)
8.1(0.93)
11.8(1.45)
2.2(0.03)
10.0(1.01)
8.0(0.79)
VCF 1.2
14.8(0.24)
9.1(0.85)
13.5(1.37)
3.8(0.26)
12.5(1.10)
6.7(0.48)
VCF 1.4
17.1(0.26)
12.8(1.86)
19.6(3.17)
2.7(0.37)
15.7(2.24)
5.8(0.47)
VCF 1.6
16.6(0.31)
13.0(0.72)
21.7(1.23)
2.9(0.18)
17.1(0.58)
5.9(1.14)
Lot uc₂
13.3(0.02)
7.3(0.64)
11.1(0.42)
4.9(0.57)
11.2(0.42)
7.3(0.94)
15.7(0.15)
14.2(1.07)
20.0(1.53)
5.5(0.03)
17.9(1.45)
4.6(0.12)
VCF 2.0
17.8(0.01)
22.5(2.48)
33.8(5.66)
4.1(0.13)
25.7(3.21)
3.8(0.08)
* Mean values (standard deviation)

25. Results
RCT
Fig.2 Variation of RCT with protein content
Overall tendency is for an increase of RCT with the increase of the VCF or protein concentration

26. Results
Curd firmness
Fig. 3Variation of curd firmness parameters A40 as a function of protein content

27. Results
Curd firmness
Fig. 4-5 Variation of curd firmness parameters AR and A2R as a function of protein content
Increase in protein concentration resulted on an increase of the curd firmness

28. Results Curd firmness Mechanism for cow’s milk UF
Due to the increase of the molar concentration of the casein micelles
This is in accordance with a mechanism for cow’s milk UF
Mechanism for cow’s milk UF
The micelle size decreased and possibly there was rearrangement via hydrophobic bonds leading to a more compact structure during UF process
This rearrangement causes some physical changes in the milk protein system during the renneting, resulting in an increase in curd firmness

29. Results The rate of curd firming
Fig. 6 Variation of the rate of curd firming (0k20) with protein content
Protein concentration increases the rate of aggregation of the micelles accelerate, resulting in shorter values of 0k20

30. Conclusion
In the UF of skimmed sheep milk there was decrease of VCF, and there was an increase in the concentration of protein and calcium
The rejections coefficients did not vary with VCF and those relative to the proteins and to the calcium and magnesium yielded the higher values
The curd firmness and the rate of curd firming were dependent of VCF and their linear variation with the PC was quantified
The UF of sheep milk at low VCF, up to the value of 2.0, brought about a clear improvement of the rennet coagulation properties

31. Conclusion
The sheep milk behavior concerning the rate of aggregation of the micelles represents an improvement in milk clotting properties as curd becomes firmer earlier with higher protein content and can reach higher firmness at the same coagulation time and therefore advantageous for cutting and draining

32. Cheese matrix improvement by UF at low concentration factor
Highlights
Curd firmness and curd firming rate increase on UF sheep milk concentrates
Cheese matrix improvement by UF at low concentration factor
Protein concentration leading to earlier curd firmness