Аннотация

Introduction. Numerous molecular genetic studies have revealed a correlation between the polymorphism of milk protein genes and the technological properties of milk raw materials. DNA analysis, in particular, initiated research into the influence of allelic variants of κ-casein (CSN3) on thermal stability and cheese suitability of milk. This gives relevance to our study that compares the results of genotypic identification of lactating cows by the κ-casein gene, using raw and processed milk samples.
Study objects and methods. Our study used raw and reconstituted milk samples from first-calf cows of the black motley breed with the AA and BB genotypes of the κ-casein gene. The samples were analyzed by standardized and generally accepted chemical engineering methods, as well as by capillary electrophoresis and PCR-RFLP analysis.
Results and discussion. We compared the results of tests on thermal stability and cheese suitability of raw and reconstituted milk samples from cows with the AA and BB genotypes of the κ-casein gene. We tried out an integrated approach to monitoring milk raw materials based on the most relevant technological criteria and correlating the data with the associated CSN3 gene identification parameters. The PCR-RFLP analysis revealed reproducible results for both raw and dried milk samples in relation to the genotypical identification by the A- and B- allelic variants of the CSN3 gene. The tests showed higher thermal stability in the reconstituted milk from the BB genotype cow and better cheese suitability in the AA genotype sample.
Conclusion. We developed a system for evaluating milk raw materials based on the most important technological parameters in combination with their genotypic characteristics. Our research procedure can unify the accumulation of experimental data and contribute to the formation of bioinformatics algorithms. This approach can be used in mathematical modeling of criteria to evaluate the compliance of the technological properties of milk with the recommended indicators.

Ключевые слова

Casein, genotype, PCR-RFLP, allele, milk, technological properties, whey, thermal stability

ВВЕДЕНИЕ

Each breed of lactating animals has a genetic potential that contributes to certain properties of the resulting milk. Breed characteristics determine not only the physicochemical composition of raw milk, but also its technological parameters and yield. As a result, the quality of dairy products also varies significantly [1–3].

In this regard, scientists are interested in determining genetic markers associated with the qualitative characteristics of milk. DNA technologies for developing technological properties of milk in vivo aim to model those parameters which are significant in certain areas of the dairy industry. Allelic variants of genes of milk proteins, hormones, and enzymes are the most probable genetic markers of milk quality. Numerous studies have analyzed the influence of cow genotypes on the rennet coagulation and thermal stability of milk. Most research has focused on α_S1-casein (CSN1S1), β-casein (CSN2), β-lactoglobulin (BLG), and κ-casein (CSN3) encoded by the genes of the same name [4–6].

Studies have proven that the B allele of the CSN1S1 gene is associated with milk yield, while its C allele affects the protein milk content. Also, the A allele of the CSN2 gene has a positive effect of on the thermal rennet coagulation and is a synergist for the similar effect of the CSN3 B allele. The BLG gene is associated with the biological value of milk, its technological properties, and total protein, namely a high content of casein and fat fractions (B allele) and a high content of whey proteins (A allele).

The κ-casein (CSN3) gene determines both the protein milk content and the technological properties of milk. It is believed that its A allele is associated with thermal stability, while its B allele is responsible for a high protein content and cheese suitability [7].

Testing biomaterial by determining the κ-casein gene locus is currently one of the most widely used methods. This is due to the frequency of occurrence of its prevailing alleles and the evidence of their association with the technological parameters of milk [8–10].

Many years of research have identified more than ten allelic variants of the CSN3 gene in cattle. The A and B alleles are most common and depend on the breed and geographic range. Scientists have also established a significant prevalence of the A over the B variant [7].

Allelic polymorphism of the κ-casein gene includes three main genotypes: AA, AB, and BB. Their variability affects the composition and properties of cow milk. Numerous studies of the κ-casein genotypes in domestic and foreign breeds have revealed a trend of increasing daily milk yield and decreasing protein mass fraction from the B allele to the A allele [7].

Despite some conflicting results [11], most researchers agree that the CSN3^AA genotype has a positive effect on the thermal stability of milk, while the CSN3^BB genotype determines its cheese suitability [12].

The polymorphism of milk proteins was initially studied on lactating cows by means of electrophoresis and isoelectric focusing. Calves and bulls were genotyped by mass testing of milk from their ancestors and offspring through the female line. Such studies took about 5–6 years. Long duration and high costs made it practically impossible to correct the desired alleles in the animal population [13].

New methods of genotyping polymorphic variants of milk proteins at the DNA level opened up wide opportunities for breeding and research. The most widely used method is the polymerase chain reaction (PCR) followed by the analysis of restriction fragment length polymorphism (RFLP) [13].

The PCR-RFLP analysis made it possible to identify the polymorphism of milk proteins using not only the animal’s biomaterial, but also raw milk and processed materials. In particular, these methods enabled scientists to interpret PCR-RFLP fragments of the DNA extracted from a number of dairy products. This opened up new prospects for milk processors in predicting the technological properties of raw milk [14, 15].

We aimed to compare the results of the κ-casein gene genotypic identification of lactating cows, using whole raw and freeze-dried milk samples. After interpreting the data, we assessed the physicochemical parameters and technological properties of the reconstituted product in order to determine the need for further processing of dried milk.

To achieve this aim, we set the following objectives:
– to genotype cows by the PCR-RFLP analysis for allelic variants of the κ-casein gene in dried and raw milk samples;
– to try out a comprehensive approach to monitoring the technological properties of reconstituted milk using multidirectional methods;
– to assess the influence of κ-casein genetic polymorphism on the technological properties of milk.

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