Global milk production exceeds 600 million tons annually, with cows contributing 81%, buffaloes 15%, and goats, sheep, and camels 4% (FAO, 2020; OECD-FAO, 2019). Cows are the main source of milk production in Iraq, with an annual production of one million liters according to the statistics of the Ministry of Agriculture  [1].

Milk is defined as a white liquid that is nutritionally complete, containing fat, protein, lactose, vitamins, minerals, and hormones. Mammary glands of mature female mammals release it to provide nourishment to their offspring [2]. Milk consists of two main parts: water, the largest proportion of milk components, with some components dissolved and others suspended, and its proportion ranges between 80%-90% [3]. The second part is total solids, which consist of fat, protein, lactose, minerals, and vitamins [4].

Genetic and environmental factors affect milk's chemical and physical components [1]. Direct and indirect factors also affect milk composition [5].

[6] indicated that milk composition is one of the important determinants of processability and nutritional value, while Amenu and Deeth (2007) showed that milk components affect the quality of final products. [7] noted that the fat and protein levels in milk are the primary determinants of milk quality and pricing. 

while density is considered one of the physical properties of milk that is a quality criterion used in the dairy industry [8].

Density is closely related to milk's non-fat dry matter and fat content [9] found a significant and positive link between milk density and protein, as well as a positive correlation with fat content. 

but less pronounced, while density was independent of lactose content. Increasing the non-fat dry matter content increases the density of milk, and the opposite occurs when the fat content increases [10]. The density of milk is also influenced by its protein and fat content 27). There are connections between the density of milk and its components. With knowledge of the fat content and milk density, the total solids content can be calculated according to Fleischmann's equation [11].

Understanding the relationship between milk components and their physical and chemical properties helps determine milk's quality and economic value. Therefore, This study seeks to examine the correlations among milk density and its constituents, as well as establish the associations between the constituents in Friesian cows raised in Shatrah City, Southern Iraq. 

The purpose of this investigation was to examine the correlation between the density of milk and its various components in the Al Sulaiman District of Shatrah, located in the Dhi Qar Governorate. The study was conducted over some time, specifically from 1/10/2022 to 1/8/2023.

The study was conducted on 50 Friesian cows of different ages at the beginning of their production seasons reared in one of the private farms in Al Sulaiman District. 1000 samples of raw milk, 100 ml per sample, were collected. The evening and morning milk samples were mixed to obtain a homogeneous sample, and the samples were taken every two weeks.

Following collection, the samples were placed in a container filled with crushed ice and then transferred to the laboratory for analysis using a German-manufactured EKO Milk instrument. This device was used to assess the proportions of fat, protein, lactose, non-fat dry matter, and density in the samples.

The total solids content was estimated according to [12]. The water (moisture) content and ash (minerals) content were estimated according to [13].

Statistical analysis:

The ANOVA test was used to analyze variance, and Pearson's correlation coefficient was analyzed using the statistical analysis program SPSS (2006).

Table (1) shows the average components of milk (Percentage of fat, percentage of protein, percentage of lactose, percentage of ash, percentage of non-fat solids, percentage of total solids, percentage of water) and density. The averages reached (0.430±3.17, 0.187±3.01, 0.290±4.88, 0.038±0.57, 0.517±8.14, 0.863±11.28, 0.854±88.70, 0.203±25.89), respectively.

These results were within the normal range for the components of fresh cow's milk, and these results agreed with [14,15,16,6]

Table (1) Average (±standard error) milk components and density

Table (2) displays the correlation between the density of milk and its constituents, including the percentage of fat, protein, lactose, ash, non-fat solids, total solids, and water. The results indicate a strong positive connection (P<0.01) between milk density, % protein, % lactose, % ash, % non-fat solids, and % total solids. The link between it and % fat was considerably positive (p<0.05), with a correlation coefficient.

(0.930224, 0.929568, 0.831907, 0.927356, 0.825206, 0.528211), respectively. While its Correlation was negative and highly significant (P<0.01) with % water (humidity), and the correlation coefficient reached (-0.99619).

Milk density and protein:

The results of this study agree with those of [17,18]. The individual identified a robust positive association between the density of milk and its protein content.

They also showed that protein content has a more significant effect on density than fat content since the difference in density between water and fat is much smaller than the difference in density between water and protein. Additionally, the density of each component explains the difference in the extent of its effect on milk density. These findings align with the research conducted  [8,19&20]. 

The findings regarding milk density and lactose

align with the studies conducted [20,21]. This conclusion is logical because lactose is a constituent of both total solids and non-fat dry matter, accounting for 40% of the total solids in milk. Casein is the predominant solid constituent found in milk,[12,21]. 

 

The findings align with the studies conducted by [21-23] regarding milk density and ash content. Ash is a minor milk constituent comprising less than 1% of its composition and consists of milk solids and non-fat dry matter. The protein and fat content of milk can be enhanced to increase the total solids present. However, the ash content has an impact on this increase,asshown [24]. 

 

The findings regarding milk density and non-fat dry matter

 align with the research conducted [21]. The reason for this is that at a temperature of 10°C, the density of non-fat dry matter in milk is approximately 1.6 times higher than the density of water, whereas the density of milk fat is roughly 0.93 times that of water. The density of fat is 0.07 units lower than the density of water. Therefore, the milk density component with the largest impact is the non-fat dry matter [21,25] stated that the density of milk is influenced by the amount of fat and non-fat dry matter it contains at a temperature of 20°C.

Milk density and total solids:

The results are in agreement with those of [21,25,26). Total solids in milk are the main factor significantly affecting milk density [18]. However, milk density depends on the fat and non-fat dry matter content, so variation in total solids affects milk density [27] indicated a direct relationship between fat content, density, and total solids, which can be calculated directly using Fleischmann's equation.

Milk density and water (moisture):

This result is in agreement with those of [21,25,26].

Table (2) relationship between milk's density and its components (% fat, % protein, % lactose, % ash, % non-fat solids, % total solids, % water).

P<0.05)* , **P<0.01), NS: not significant 

The study showed a positive correlation, (p < 0.05) between  milk fat and protein, lactose, ash, and non-fat dry matter (NFDM). The Correlation between milk fat and total solids (TS) was highly significant (p < 0.01). In contrast, the Correlation between milk fat and water was negatively significant (p < 0.01).

Previous Studies Agreement:

The results are consistent with [17,18,6,25,28-30],regarding the Correlation between milk fat and protein.

The findings align with previous studies conducted [6,28,25,29,31,32),regarding the correlation between milk fat and lactose.  The findings are consistent with previous studies conducted [17,6,25,28-30]regarding the correlation between milk fat and ash.  The findings align with previous studies conducted [6,25,28-30] about the correlation between milk fat and non-fat dry matter (NFDM).  Furthermore, the findings align with the research conducted [17,2,33,32] about the correlation between milk fat and total solids (TS).  The results align with the findings of [17,30] about the correlation between milk fat and water.  Additional correlations: The milk protein showed a significant positive correlation (p < 0.01) with lactose, ash, non-fat dry matter (NFDM), and total solids (TS). Nevertheless, there was a significant negative correlation (p < 0.01) between milk protein and water. Lactose was positively correlated (p < 0.01) with ash, NFDM, and TS. Conversely, lactose was negatively correlated (p < 0.01) with water. Ash was positively correlated (p < 0.01) with NFDM and TS. On the other hand, ash was negatively correlated (p < 0.01) with water. NFDM was positively correlated (p < 0.01) with TS. However, both NFDM and TS were negatively correlated (p < 0.01) with water.

The results are consistent with previous research on the relationships between milk components. The positive correlations between milk fat and other milk components can be explained by all these components in milk fat globules.

The negative Correlation between milk fat and water can be attributed to the decrease in water content as the fat content of milk increases.

The positive correlations between other milk components can be explained by the fact that these components are all synthesized in the mammary gland. The negative correlations between water and other milk components can be explained by the fact that water is the major component of milk, and as the concentration of other components increases, the concentration of water decreases.

Table (3) shows the relationship between milk components (%fat,%protein,%lactose,%non-fat solids,%ash,%total solids,%water).

P<0.05)* , **P<0.01), NS: not significant

The authors declare that they have no conflict of interest

No funding sources

The study was approved by the University of Thi-Qar, 64001, Iraq. 

1. Al-Qudsi, N. H., and Elia, J. Victor. Milk Cattle Production. Department of Livestock, College of Agriculture, University of Baghdad, 2010.

2. Jost, R. "Milk and Dairy Products." Ullmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

3. Imran, M., et al. "Physicochemical Characteristics of Various Milk Samples Available in Pakistan." J Zhejiang Univ Sci B, vol. 9, no. 7, 2008, pp. 546-551. https://doi.org/10.1631/jzus.B0820361.

4. Lizzy, A. P. A. "Density and Dynamic Viscosity of Bovine Milk Affected by Temperature and Composition." International Journal of Food Engineering, vol. 8, no. 1, 2012, Article 11. https://doi.org/10.1515/1556-3758.1000061.

5. Khalil, H. M., and Seliem, A. F. "Determination of Heavy Metals (Pb, Cd) and Some Trace Elements in Milk and Milk Products Collected from Najran Region in K.S.A." Life Science Journal, vol. 10, no. 2, 2013.

6. Taher, N., et al. "A Study of the Effect of Age of Dam and Sex of Birth on the Chemical and Physical Composition of Milk in Some Farm Animals." Kufa J Vet Med Sci, vol. 2, no. 2, 2011.

7. Javaid, S., et al. "Physical and Chemical Quality of Market Milk Sold at Tandojam, Pakistan." Pak Vet J, vol. 29, no. 1, 2009, pp. 27-31.

8. SPSS. Statistical Packages of Social Sciences. Version 16.0, 2006, USA.

9. Fox, P. F., et al. Dairy Chemistry and Biochemistry. Springer International Publishing, Basel, Switzerland, 2015. https://doi.org/10.1007/978-3-319-14892-2.

10. Vidiyanto, T., et al. "Display of Production, Milk Density, Lactose, and Water Content in Dairy Cow's Milk Due to Different Milking Intervals." Animal Agriculture Journal, vol. 4, no. 2, 2015, pp. 200-203.

11. Prasetya, H. Raising Dairy Cows. Pustaka Baru Press, Yogyakarta, 2012.

12. Suhendra, D., et al. "Protein Contents and Milk Temperatures Correlate with Milk Density of Friesian Holstein (FH) Cow in Ngablak District of Magelang Regency Central Java." Journal of Livestock Science and Production, vol. 4, no. 1, 2020.

13. Saputra, F. T. "Evaluation of the Total Solid Fresh Milk of Tawang Argo Farmers Based on Indonesian National Standards." Journal of Tropical Livestock, vol. 19, no. 1, 2018, pp. 22-26.

14. Abbas, M. R. H., et al. Dairy Cattle (Basics - Applications - Profits). Technical Institutes Authority, Ministry of Higher Education, Iraq, 1990.

15. Rushdi, A. Nutrition and Health. Al-Yazouri Scientific Publishing and Distribution House, Amman, 2004.

16. Abdul Rahim, A. "A Study Comparing Human Milk with the Milk of Some Animals (Camels, Cows, Goats, and Sheep) in the Local Environment." Master's Thesis, Department of Food Science, College of Medical Technology, Misrata, 2007.

17. Watson, P.-D., and Tittsler, R. P. "The Density of Milk at Low Temperatures." J Dairy Sci, vol. 44, 1961, pp. 416-420.

18. Ueda, A. "Relationship Among Milk Density, Composition, and Temperature." Master Thesis, Faculty of Graduate Studies, University of Guelph, 1999.

19. Sugiyono. Qualitative and Quantitative Research Methods R&D. Alfabeta, Bandung, 2006.

20. Food and Agriculture Organization of the United Nations. "FAOSTAT Statistical Database." 2020, http://www.fao.org/faostat/en/#data/QC.

21. Guetouache, M., et al. "Composition and Nutritional Value of Raw Milk: A Review." Issues in Biological Sciences and Pharmaceutical Research, vol. 2, no. 10, 2014, pp. 115-122.

22. Bijl, E., et al. "Protein, Casein, and Micellar Salts in Milk: Current Content and Historical Perspectives." Journal of Dairy Science, vol. 96, 2013, pp. 5455-5464. https://doi.org/10.3168/jds.2013-6707.

23. Costa, A., et al. "Milk Lactose—Current Status and Future Challenges in Dairy Cattle." J Dairy Sci, vol. 102, 2019, pp. 5883-5898. https://doi.org/10.3168/jds.2018-15877.

24. OECD/Food and Agriculture Organization of the United Nations. "Dairy and Dairy Products." OECD-FAO Agricultural Outlook 2019-2028, OECD Publishing, Paris, 2019. https://doi.org/10.1787/0a74713d-en.

25. Sourabh, Y., et al. "Correlation Between Milk Constituents and Somatic Cell Counts in Holstein Friesian Crossbred Cattle." International Journal of Agriculture Sciences, vol. 9, 2017, pp. 3840-3842.

26. Short, A. L. "The Temperature Coefficient of Expansion of Raw Milk." Journal of Dairy Research, vol. 22, 1955, pp. 69-73.

27. FAO. "Technology Unit1-Milk Composition-Part1. Small-Scale Dairy Farming Manual." 2008, vol. 1, pp. 1-13.

28. Dehinenet, G., and Mekonnen, H. "Determinants of Raw Milk Quality Under a Smallholder Production System in Selected Areas of Amhara and Oromia National Regional States, Ethiopia." Agriculture and Biology Journal of North America, vol. 4, 2013, pp. 84-90.

29. Suryam Dora, D., et al. "Relationship Between Different Milk Constituents of GIR Cow." Journal of Entomology and Zoology Studies, vol. 8, 2020, pp. 551-553.

30. Rai, P., and Adhikari, N. "Study of Relationship Among Milk Parameters in Crossbred Dairy Cattle." Research Square, 2022. https://doi.org/10.21203/rs.3.rs-1920222/v1.

31. Televicius, M., et al. "Inline Milk Lactose Concentration as Biomarker of the Health Status and Reproductive Success in Dairy Cows." Agriculture, vol. 11, 2021, p. 38. https://doi.org/10.3390/agriculture11010038.

32. Montero-Prado, P., et al. "Physicochemical Characterization and Correlation of Raw Cow's Milk According to the Classification Assigned in Panama." Agronomía Mesoamericana, vol. 32, no. 3, 2021, pp. 939-948. https://doi.org/10.15517/am.v32i3.41577.

33. Parmar, P., et al. "The Effect of Compositional Changes Due to Seasonal Variation on Milk Density and the Determination of Season-Based Density Conversion Factors for Use in the Dairy Industry." Foods, vol. 9, 2020, p. 1004. https://doi.org/10.3390/foods9081004.