Introduction

Milk, due to its nutritional characteristics, is one of the foods of animal origin with the highest demand in the world. In Mexico in 2018, milk production was 12,008,239 thousand liters, of which about 73 % was destined to the production of dairy products and derivatives (8,784,055 thousand liters)¹. The region of the Western Altiplano of the state of San Luis Potosí is characterized by being a milk-producing region, in greater proportion of cattle and in less of goats, in which most of the milk is destined to the production of artisanal fresh cheese. Traditional artisanal cheeses are important, not only for their nutritional and gustatory benefits, but also for their ability to generate and keep rural employment that involves some agents of the agro-industrial milk chain: farmers, cheesemakers and traders². However, organization among producers is required to, for example, create an “artisanal denomination” with a level of regulation and protection that guarantees the preservation of dairy production systems based on traditional practices and their respective socioeconomic benefits³.

On the other hand, in these production units (PUs) of the Western Altiplano of San Luis Potosí, cheese is made from raw milk, with the use of non-standardized traditional methods, scarce technification and in inappropriate facilities (e.g., they use the kitchens of the producers’ homes). In this regard, most producers do not know about the concept of good manufacturing practices and the pasteurization process, which is a thermal process that allows controlling pathogenic bacteria currently found in milk, such as Mycobacterium tuberculosis, Listeria monocytogenes, as well as some species of Campylobacter, Salmonella, Escherichia coli and other fecal coliforms⁴. In addition, cross-contamination of milk after pasteurization must be controlled by applying strict cleaning and disinfection rules⁵. Also, in this type of PU, non-sterile natural rennet is sometimes used in the production of cheese, its production consists of drying extended fragments of abomasum of newborn calves in clotheslines under the sun for up to three days, later they are introduced into a plastic drum with whey, it is covered and left to ferment for at least five days for later use; this practice can be an important point of microbial contamination.

Gastrointestinal diseases due to the consumption of contaminated foods can occur at any time of the year, but the risk increases in the hot season. The most frequent clinical conditions are fever, vomiting, abdominal pain and moderate or severe diarrhea⁶. For health institutions such as the Mexican Institute of Social Security (IMSS, for its acronym in Spanish), these diseases are: brucellosis, cholera, typhoid, gastroenteritis, shigellosis, salmonellosis and diarrhea, which represent a severe public health problem for San Luis Potosi state and the country^7,8. Therefore, dairy products must comply with the sanitary provisions and specifications of NOM 243⁹. Which indicates that Salmonella spp. must be absent in 25 g of cheese or milliliter of milk, a maximum of 100 colony-forming units per gram or milliliter (CFU/g or ml) is allowed for total coliforms and a maximum of 1,000 CFU/g is allowed for Staphylococcus aureus. Therefore, the objective of the present work was to evaluate the microbiological load of total coliforms, S. aureus and Salmonella spp. in milk and cheeses as indicators of the process practices carried out in the cheese dairies of different localities of the region of Salinas, San Luis Potosí, and to be able to recommend good manufacturing practices that help improve the health conditions of production, without altering the characteristics of the dairy product.

Material and methods

Results

Milk composition

Four groups were identified as a result of the cluster analysis (Figure 1), which are detailed in Table 2, resulting in four groups, separating in group I where the PUs that have goats, mainly of the Saanen breed, are, this grouping is based on the similar content of nutrients in milk. In percentage of fat, cluster I showed a statistical difference (P<0.05) in contrast to the other groups (cow’s milk samples), which show some similarity between them. The percentage of protein, lactose, non-fat solids and salts showed no difference (P>0.05) between groups. In the variable total solids, there were significant differences between clusters (P<0.05), the highest value was obtained in cluster I (14.5 %) because it was goat’s milk, and the lowest value was shown by cluster IV (10.8 %). The variable density showed statistical differences, the groups of cow’s milk samples (cluster II, III and IV) obtained the highest value (1,024 kg/m³) and were statistically the same, in contrast, cluster I had an average value of 1,019 kg/m³. Finally, the cryoscopic point in cluster I decreased as a result of a higher content of lactose and salts in milk.

Figure 1 Dendrogram of the physicochemical analysis of cow’s milk and goat’s milk 

Table 2 Nutritional composition of milk samples 

Variable	Cluster I	Cluster II	Cluster III	Cluster IV	P- value
Fat, %	6.3 ± 0.71a	4.3 ± 0.53b	3.3 ± 0.25bc	2.7 ± 0.65c	***
Protein, %	3.2 ± 0.07	3.1 ± 0.12	3.1 ± 0.21	2.9 ± 0.22	N.S.
Lactose, %	5.0 ± 0.07	4.7 ± 0.23	4.5 ± 0.19	4.5 ± 0.30	N.S.
Total solids, %	14.5 ± 0.57a	13.0 ± 0.15b	11.7 ± 0.26c	10.8 ± 0.48d	***
Non-fat solids, %	9.0 ± 0.13	8.7 ± 0.4	8.5 ± 0.12	8.4 ± 0.51	N.S.
Density, kg/m3	1018.8 ± 1.1b	1024.8 ± 1.2a	1024.3 ± 0.2a	1023.5 ± 0.8a	***
Salts, %	0.7 ± 0.0	0.67 ± 0.06	0.69 ± 0.04	0.64 ± 0.05	N.S.
Cryoscopic point, °C	-0.62 ± 0.01b	-0.55 ± 0.03a	-0.53 ± 0.02a	-0.53 ± 0.04a	**

^abc Mean values with different literals are statistically different (P<0.05) by row.

N.S. = not significant; ** = P<0.001; *** = P<0.0001.

Discussion

The results of the physicochemical analysis in cluster I (goat’s milk samples), the percentage values are very similar to those reported for Saanen goats in Mexico¹⁸. In addition, Salinas-González et al¹⁹ point out that these values vary with time, being higher in September. Studies on the factors that affect the stability of the freezing point of milk indicate that the decrease in the cryoscopic point is related to a higher content of lactose and salts (calcium, phosphorus, magnesium), as well as to the passage of time (days) after birth^20,21.

Regarding the physicochemical quality of milk from clusters II, III and IV (milk samples), the three groups are within the reference data of NOM 155²². Álvarez-Fuentes et al²³ mention that small-scale dairy farms located in the south of Mexico City face the challenge of producing milk in quantity and quality; in this regard, the authors point out that the quality of milk is different according to the time of year (dry, rainy and winter periods), the results of milk quality (fat, protein, lactose and total solids) that they recorded in the dry season are very similar to those obtained in this work.

In this study, milk and cheese samples were taken from local cheese dairies to analyze pathogenic microbial loads. In addition, a request and comparison of records of the diseases caused by the consumption of dairy products of 2018 of the Basic Community Hospital of Salinas was made (personal communication, Dr. Sugey Bastidas Gastelum, director), and the response was that the hospital only follows up on cases of brucellosis (4 cases in that year) because it is considered as primordial by Epidemiological Surveillance, other diseases possibly caused by Escherichia coli, Staphylococcus aureus and Salmonella in dairy products and other foods are not analyzed because they do not have their own laboratory for sample processing.

For fluid milk, NOM 243⁹ indicates that the maximum allowable loads for total coliforms and S. aureus must be ≤10 CFU/ml by direct seeding, while Salmonella must be absent. The low counts of microorganisms in PUs 1, 5 and 10 may be due to the fact that these units were the only ones that have exclusive facilities for the handling of milk and cheese making process, and maintain hygiene measures (use of cap, masks, boots and cleaning of equipment, floor and utensils); despite these practices, the facilities do not have hermetic sealing doors that protect from the entry of dust. Table 3 shows that the regulations are not complied with in several PUs, this may be due to the fact that the milk is not pasteurized due to the lack of equipment, knowledge of pasteurization times and temperatures and the influence on the state of health of the animals, cleaning during milking, milking utensils and place of storage of milk. In a study conducted by Fuentes-Coto et al²⁴, the microbial load of organic milk was analyzed, the presence of mesophilic and coliform bacteria was detected in milk and dairy products, where the amounts of CFU/ml were above the allowed limit.

On the other hand, Álvarez-Fuentes et al²³ mention that the presence of mesophilic bacteria and the somatic cell count are related to the type of cleaning performed on the udders (traditional, partial and complete); however, regarding the microbial load, even though the work describes low counts of CFU/ml, the species of microorganisms are not specified.

Although the microbiological analysis in the milk reveals that the microbial loads are low in some farms, in some cases, the samples of fresh cheese had microbial loads higher than allowed by NOM 243⁹, it establishes a maximum of 100 CFU/g for TCs, a maximum allowed of 1,000 CFU/g for S. aureus, and Salmonella spp. must be absent in 25 g of sample.

One of the factors of the high microbial loads detected in this study is probably due to the fact that, in PU # 15, natural rennet fermented in non-sterile conditions is used as a coagulating agent. In the interviews that were carried out with the producers, they comment that the use of natural rennet is mainly due to the fact that it impregnates flavors and aromas desirable by consumers of fresh cheese. The results of Table 4 are similar to those reported in other cheese-producing areas in Mexico, some authors²⁵ report counts of 9.27 log₁₀ CFU/g for TCs and Salmonella present in fresh cheese. The aro cheese that is marketed in the municipality of Teotitlán de Flores Magón, Oaxaca, Mexico, had counts of 6.94 for TCs, 6.74 for E. coli, 5.76 log₁₀ CFU/g for S. aureus and Salmonella was present, consequently, no sample analyzed complies with the regulations²⁶. Also, artisanal botanero cheese from the northwest of the state of Mexico has serious deficiencies in its microbiological quality, since the counts exceeded the permitted limits of pathogens²⁷. In the findings of this work, only the results of the control samples comply with the regulations. On the other hand, PU #5 partially complies with the counts of TCs and S. aureus. However, there is the presence of Salmonella colonies, which is associated with the use of unpasteurized milk²⁵. In other geographical regions, such as Cajamarca, Peru, industrial fresh cheese from six companies was analyzed, under the guidelines described in the Sanitary Standard of microbiological criteria of sanitary quality and safety for foods and beverages for human consumption in that country. In the results, it is highlighted that all companies comply with what is established by the standard for Salmonella spp., since all the samples showed absence, with respect to other pathogens, only one company presents better microbiological conditions for the production of fresh cheese²⁸. The same happens in Egypt and Middle Eastern countries with soft cheeses (Domiati), where they also have loads of aerobic bacteria S. aureus, TCs, E. coli and yeasts²⁹. In the north of Iran, Kurdish cheese, named after the region where it is produced (Kurdistan, Iran), is a cheese prepared with cow’s or sheep’s raw milk, the loads of pathogenic microorganisms in the first days are very similar to the loads found in this study; the presence of Salmonella, E. coli of 5.27 log₁₀ CFU/g and 8.22 log₁₀ CFU/g for TCs was found. In addition, they point out that the maturation of cheese (60 d), the loads decrease significantly due to the presence of lactic acid bacteria³⁰.

The presence of pathogenic microorganisms in dairy farms is due to several factors, for example, Salmonella is present in other domestic animal species such as pigs and poultry, which can become infected because they are within the production unit, in the environment due to the management of manure and its importance lies in the fact that it can survive for prolonged periods of time in the environment and cause salmonellosis in humans^31,32. On the other hand, Staphylococcus aureus is a very common pathogen that causes mastitis in dairy cows, in this regard, if milk is not pasteurized, this bacterium can be found even in milk storage tanks³³. In the case of coliforms, Van Kessel et al³⁴ point out that these enteropathogenic microorganisms are very persistent in dairy farms; their origin is the intestine of animals, as well as their feces and water contaminated with manure, with a high risk of contamination of milk and storage tanks. The authors described agree in pointing out that the poor sanitary quality of dairy products throughout the milk and cheese production chain is a health problem, since these can be a vehicle for the transmission of food diseases, due to their high content of: E. coli, Listeria monocytogenes, S. aureus, Salmonella and possibly Brucella spp.^26,35,36. Therefore, they recommend improving the quality at the farm level with hygienic milking measures, establishment of a cold chain, adequate transport and good sanitary measures of milk, such as: hygiene of the facilities where the cheeses are made, use of appropriate clothing, use of drinking water, washing of utensils, tables and hand disinfection²⁵.

Conclusions and implications

According to the results of the analysis of milk and cheese samples, most production units do not meet quality standards in dairy products, because they do not have good hygiene practices throughout the entire production process, so it is necessary to implement actions that make producers aware of taking better health measures to reduce possible sources of contamination and that it does not represent a risk to the health of consumers, causing possible gastrointestinal diseases, mainly. It is necessary to conduct more studies on the process of making fresh cheese, such as taking samples of utensils, analysis of natural rennet and carrying out some process of sterilization of natural rennet by physical means. As well as to hold workshops of good practices in the production of dairy products for producers.