Introduction

Agricultural production worldwide requires high quality, safe products for human consumption, a constant search which concerns all the constituents of the dairy chain. This process begins on the farm and must be enforced in order to guarantee the best conditions for obtaining a product of optimum quality¹.

In Colombia, specialized dairy industries are located in high tropical areas such as the Altiplano Cundíboyacense, the Nariño Altiplano and the north and northeast highlands of Antioquia. These systems are characterized by the presence of Bos taurus, intensive use of production factors (land, capital and labor), use of fertilizers, irrigation, rotation of pastures, use of food supplements and two daily milking. The improvement of the hygienic quality of milk is carried out through a simple process, showing rapid results that begin with the improvement of milking practices in order to avoid milk contamination, while maintaining perfect hygienization of the milk canteens or storage tanks².

This type of livestock activity must adhere to Decree 616 of 2006 of Colombian regulations, which outlines the requirements that must be met by bovine, buffalo, and goat milk destined for human consumption, in order to protect life, health, and security and prevent practices that may mislead, confuse, or deceive consumers².

In this regard, Benbrook et al³ defines high-quality milk as having an excellent composition (fat, protein, lactose, vitamins and minerals), low microbial counts (hygienic), is free of pathogens, and has no physical-chemical contaminants. Quality milk is an indispensable requirement for good quality products, and the herd is the first condition in achieving good products.

According to Ministry of Agriculture Resolution 000017 of 2012⁴, the hygienic quality of milk refers to the hygiene level of the process through which milk is obtained and handled. In this order of ideas, the somatic cell count per milliliter (SCC/mL) in most cases can be associated with diseases such as mastitis, an inflammatory reaction of the mammary gland, which produces physical and chemical alterations in milk, an increase in the number of somatic cells due to the presence of pathogenic microorganisms, and changes such as the loss of functionality¹.

For this reason, the somatic cell count (SCC) is one of the most influential parameters in determining the udder health and milk quality. SCC in milk increases in direct proportion to the severity of the infectious disease. In a milk that does not contain subclinical mastitis, the SCC is low (<100,000 SCC/ mL). The increase in SCC depends on the pathogen that causes mastitis⁵. High SCC is associated with inflammation of the udder, which leads to bacteriological problems in milk, an alteration in its composition, and changes in the characteristics of dairy products when compared to normal values⁶. However, in addition to its immune function in the udder and protective functions in milk, it has recently been shown that SCC have a positive influence on the composition and technological properties of dairy products, which influence the final quality of the dairy products through its endogenous enzymes⁷.

In Nariño, as is the case with the rest of Latin America, there is little information on the compositional and sanitary quality of the production and commercialization of raw milk. Along with the above, the lack of responsibility of the producers of milk for the quality of their product (in spite of the Colombian law that establishes health and safety regulations) increases the uncertainty about the quality variables of the products produced in the region. Based on the above, the objective of the present investigation was to evaluate the compositional and hygienic quality of the raw milk received from the different districts evaluated in the Department of Nariño, as well as to observe the relation of the sanitary quality on its compositional profile.

Material and methods

Results

The mean and standard error of the mean of the variables can be seen in Table 1 and the correlation in Tables 2, 3 and 4. The means for the three districts show that the density was the same. Precipitation, total solids, day of lactation, calving interval, age, and number of deliveries showed similar results. On the contrary, the data collected for production, fat, protein and casein varied, with Pupiales showing the highest values, followed by Pasto and finally Guachucal.

The correlation shows that the compositional variables and production are highly related and this set of variables are negatively related to the days of lactation. There is also a highly significant relationship (P<0.01) between day of lactation and calving interval, and also between the number of births and age. Finally, the somatic cell count shows a significant relationship (P<0.05) with production, protein, and casein. The other variables show low correlation.

The results of the analysis of the principal components can be seen in Tables 5 and 6. For the three districts, four components were analyzed, representing 79.97, 77.04, and 75.53 % of the variability explained, for the Guachucal, Pasto and Pupiales respectively. As in the correlation analysis, the results show that production, fat, protein, casein and total solids are highly related and represent the first axis (compositional). Both age and number of parturition constitute the second axis, while the third axis is formed by the variables day of lactation and calving interval. The relationship found for these last two axes, however, may be a consequence of time.

The fourth axis is represented by precipitation. It should be noted that the somatic cell count is not well represented in some of the four components evaluated, however, it is observed that there is a contribution to the components one and four in the three districts, indicating some degree of relationship between these components.

The results of the mixed model analysis can be seen in Table 7. The variables fat and total solids were not affected by season and SCC (P>0.05). In the case of production, protein and casein, the results showed that cell count do have significant influence (P<0.05).

Discussion

The interpretation of the results was made considering current Colombian regulations, which revolves around Decree 616¹² and Resolution 000017⁴, responsible for guaranteeing the safety of human milk consumption and the compositional and sanitary quality of milk. Based on the information provided by the statistical analysis, it can be established that the compositional quality is closely related to milk production. It was also observed that the SCC does not affect the compositional quality of the same, and that there is no evidence to support this premise.

In this regard, various authors found values of 24.28 kg of milk, which is higher than that found in the three districts¹³. Manterola¹⁴ reported an average production of 20 kg/d/cow of milk, and points out that age is a minor factor if the replacement rate is normal, though it does have a greater effect on the volume of production and thereby on the content of total solids. This was proven through the high correlation between production and the compositional parameters of the milk. Various authors also mention that the milk production of a cow is the result of the relationship of the environment and the inheritance¹⁵. Precipitation, however, did not show significant relationships with these variables, as is observed in the diagram of the two components of PCA, the precipitation is very close to the cutoff point of the two coordinate axes.

The Ministry of Social Protection, through decree 616, has established that the density of raw milk at 15 ºC ranges between 1.030 and 1.033 g/cm³. In this sense, the milk density of the samples evaluated fall within the regulatory framework. Other authors found an average value of 1.032 g/cm³ in the milk samples evaluated, and concluded that milk from healthy animals compared to that of animals with subclinical mastitis do not show variation in the density value². In animals with mastitis, however, the reflected density is affected by values lower than 1.029 g/ml.

A study indicates that the composition of milk determines nutritional and industrial quality, which directly affects the profitability and competitiveness of milk production systems¹⁶. Composition depends on the availability of blood precursors that reach the mammary gland, which can be manipulated through nutrition to vary milk components, though this factor was not evaluated in the present investigation.

It was found that the raw milk received from the three districts complies with the parameters established by Decree 616 of 2006 regarding fat. The average value of the districts surpasses that reported by other authors with an average of 0.577 kg fat, implying an optimal value of milk fat for Holstein dairy¹⁷. On the other hand, Gallego-Castro et al¹³ reported values of 0.84 kg of milk protein for Holstein and Manterola¹⁴ reported 0.90 kg of milk per cow per day. A study’s suggest that variations in the production of milk fat within a group of cows fed in similar conditions, depends on the individual metabolic capacity of each animal¹⁸. It should, however, be taken into account that the values observed in the districts of Pasto and Pupiales demonstrate higher levels of fat per day cow compared to the district of Guachucal. These differences may be the result of the handling of the herds in the area, though the present study cannot corroborate this hypothesis, as management variables could not be included in the analysis. On the other hand, biochemical adaptations of lipid metabolism depended directly on the stage of lactation of the cows. High values in milk fat during early lactation (5.49 %) suggest a lipid mobilization from body fat deposits, a factor that is not observed in the present investigation.

Protein values comply with the parameters established by Decree 616 of 2006. Other authors report lower protein content per day cow, with an average value of 0.451 kg of protein¹⁷. In this regard, other articles found values of 0.67 and 0.7 kg of protein^13,14, values close to those found in the present investigation. Various authors state that the protein concentration of milk does not present outstanding changes with nutritional manipulation¹⁹. However, the effect of soybean meal on nitrogen use and protein production in Holstein cows has been evaluated, reaching conclusions that milk and protein yield do not show an increase with a supplementation level of soybean meal higher than 16.5 %.

As an alternative to nutritional manipulation, the effect of genetic variants and haplotypes on the protein composition of milk has been studied. In a study conducted with 1,912 Holstein cows, the authors indicated that the genotypes β-CN and κ-CN haplotype A²B, were associated with protein yield and protein/ L of milk concentration respectively²⁰. The authors mentioned that selection of these genotypes and haplotypes would result in cows that produce milk better suited for cheese production. In a separate investigation, the author suggested that knowledge of genetic variability could be useful when altering the composition of milk protein, since the estimation of the genetic parameters of the six main milk proteins determined by capillary electrophoresis in zone are highly related²¹. According to García et al¹⁹, this information suggests the possibility of modifying the protein composition of cow's milk through selective breeding, which in turn offers the opportunity to satisfy the new consumer demands.

Casein results reported an average value of 0.454 kg, which in contrast to that found by other authors with values of 2.4 %, presents a desirable and superior casein value of within milk production²². Recent research claims that casein constitutes about 78 % of milk proteins, and precipitate when the milk is acidified to a pH of 4.6¹⁹. They also state that casein is mainly linked to calcium phosphate Ca₃(PO₄)₂ in a solid and spongy structure called casein micelle, an important component for cheese making. The treatment of milk with the chymosin enzyme of the rennet of suckling calves produces the destabilization of micelle, as the κ-casein (κ-CN) loses its hydrophilic region by proteolysis in the caseinomacropeptide segment, facilitating the addition of the para-κ-CN fragment²³. As this protein component is fundamentally hydrophobic, the casein content directly influences the coagulation time of all cheeses, and therefore quality and yield²⁴.

For total solids, it was found that the raw milk received from the three districts meets the parameters established by Decree 616 of 2006, indicating excellent milk quality^4,12. Similarly, other authors reported values of 1.351 kg of total solids, a value that is lower than that found in this study²⁵.

A national somatic cell standard was not adopted, as it does not exist in Colombian legislation. For Decree 616 of 2006 and Resolution 00017 of 2012 of the MADR, the SCC benefits are voluntary and discretionary for the companies that wish to improve this aspect of the quality of the milk. Even so, companies like Colanta report that values below 400,000 SCC/ mL and up to 200,000 SCC/mL are rewarded with $USD 0.007 per liter. Additionally, if the values are below 200,000 SCC/mL the incentive increases to $0.01. If they are above 1,000,000 SCC/ mL, the milk is not received, and a deduction is made²⁴.

Currently, one quarter of the mammary gland is considered healthy; that which does not show any external pathological changes, when the milk is free of pathogenic microorganisms, and has a somatic cell level of <100,000 CFU/mL²⁶. The results of the mixed model indicate that counts higher than 500,000 CFU/mL affect the compositional quality of milk, decreasing production as well as protein and casein contents the milk. In this regard, other studies found similar results in Canadian Holstein cows, demonstrating that subclinical mastitis affects the compositional quality of milk²⁷.

Conclusions and implications

Somatic cell count affects protein, casein, and production variables in the specialized milk systems of Guachucal, Pasto and Pupiales.