Conformational and physicochemical characteristics of bovine skim milk obtained from cows with different genetic variants of β-casein

Cheddar cheese production, structure and in-vitro semi-dynamic gastric digestion: The role of β-casein phenotype

The objective of this study was to establish the impact of β-casein A1/A1, A1/A2 and A2/A2 phenotypes on the cheese-making process, cheese structure and on the subsequent in vitro gastric digestion properties of the cheese samples. The time required for curd cutting in cheese milk containing β-casein A2/A2 was significantly delayed, compared to milks containing β-caseins A1/A1 and A1/A2. After 180 days of ripening no differences were observed in the level of soluble nitrogen at pH 4.6 between any of the cheese samples, with a decrease in the level of aggregated β-sheets and increase in the level of β-turns and random coils observed in all cheese over the ripening period. During simulated gastric digestion, cheese samples took between 15 and 20 min to form the initial digesta coagulum, which occurred between pH 4.3 and 4.0. The rate of protein breakdown was slower in A2/A2 cheese, with less cheese structure degradation occurring, compared to β-casein A1 containing cheeses. This study has shown that rennet coagulation takes significantly longer in cheese milk containing only β-casein A2/A2, and that there was less protein breakdown during its simulated gastric digestion.

Integrated metagenomics and metabolomics analyses revealed biomarkers in β-casein A2A2-type cows

In Holstein cows, β-casein, one of the most critical proteins in milk, exists in two main genotypes, A1 and A2. Herein, 45 Holstein cows [categorized into three groups based on β-casein A1A1, A1A2, and A2A2 genotypes (N = 15)] with the same feeding management and litter size were enrolled to explore differences in rumen microflora and metabolites across various β-casein genotypes. Rumen fluids were collected for metagenomics and metabolomics analyses. Metabolomics and weighted gene co-expression network analysis (WGCNA) revealed that arachidonic acid (AA), adrenic acid (AdA), glycocholic acid (GCA), and taurocholic acid (TCA) were significantly and positively correlated with milk fat % in dairy cows (p < 0.05). Furthermore, macro-genomics and Spearman's correlation analysis revealed significant positive correlations (p < 0.05) between the characteristic flora (g_Acetobacter, g_Pseudoxanthomonas, g_Streptococcus, and g_Pediococcus) and the five characteristic metabolites in the rumen of A2A2 dairy cows. Moreover, functional enrichment analysis revealed more genes enriched to the TRP channel's inflammatory mediator-regulated pathway and the mTOR signaling pathway in A2A2 genotyped cows. Additionally, the regulatory effects of AA on bovine mammary epithelial cells (BMECs) were examined using CCK-8, EdU, and qRT-PCR assays, revealing that AA promoted triglyceride (TG) synthesis and upregulated the milk fat marker genes including SREBF1, ACSS2, AGPAT6, and FASN. Overall, we identified characteristic microorganisms and metabolites in A2A2 Holstein cows and established that AA could be a biomarker for higher milk fat %.

Effect of UHT Thermal Treatment on the Secondary Structures of Milk Proteins: Insights From FTIR Analysis and Potential Allergenic Activity

Although thermal treatments are beneficial for the preservation and safety of milk, they can also alter its immunogenic activity by affecting its protein components. To achieve precise results, it is essential to identify the specific proteins that cause food allergies. Therefore, investigating the possible alterations of cow's milk proteins (CMPs) resulting from thermal treatments is necessary. In this study, the Fourier transform infrared spectroscopy (FTIR) technique was used to analyze the effect of UHT thermal treatment on the secondary structures of milk casein. Using the second derivative, six characteristic peaks were identified in the Amide I region, ranging from 1700 to 1600 cm-1. It was found that thermal treatments produce shifts in absorption peaks, indicating changes in protein conformation and possibly in allergenic activity. These shifts were clearly identified in the first characteristic peak of samples M8 and M9, from 1621 to 1600 cm-1. The results suggest that thermal treatments may promote protein aggregation by increasing β turns and reducing β sheets and α helices, which could enhance the allergenic potential of the proteins and facilitate the formation of complexes between different milk proteins, such as β-lactoglobulin and κ-casein. Further studies are needed to experimentally validate the allergenic activity of proteins modified by thermal treatments, as only an analytical method (FTIR) was used to evaluate the secondary structures of the proteins.

A2 milk: Bioaccessibility of essential minerals and the release of amino groups under static in vitro digestion conditions

Alternative milk products such as A2 milk are gaining popular stand within consumer market, for their healthy profile and expected greater digestibility characteristics. However, total mineral content and its bioaccessible profile have lacked in studies through the years, even more because of their relevance in public health. The present study aimed to evaluate the mineral profile of commercial A2 bovine milk (AT) and estimate the bioaccessibility of calcium, phosphorus and magnesium using the INFOGEST protocol. Non-A2 samples (NAT) were evaluated for comparison purpose. The determination of Ca, Mg, Na and K was performed by FAAS and total P was quantified by colorimetric method. Total protein content was determined by Kjeldahl method. Free amino acids were quantified by OPA method along the in vitro digestion stages. Total content of Ca, Na and P exhibited equivalent results between samples, although A2 milk showed elevated levels of total Mg and K in the analyzed batches. AT showed protein content equivalent to NAT. In addition, levels of free NH2 were observed 2 times higher in AT, during the first hour of pancreatic phase in the intestinal digestion. Bioaccessibility of Ca showed equivalent percentages for AT (12-42 %) and NAT (10-39 %). The observed low values were possibly derived from interferences with saturated fatty acids and standardized electrolytes during digestion. Similar amounts of bioaccessible Mg were found for all milk samples (35-97 %), while A2 samples evidenced percentages of bioaccessible P exceeding 60 % across the three batches. Despite the health benefits associated to A2 milk, the study did not evidence clear distinction from non-A2 milk in terms of enhanced essential mineral solubility in digestive tract simulation, considering the association of greater digestibility expected for A2 milk.

A discussion on A1-free milk: Nuances and comments beyond implications to the health

This chapter provides an overarching view of the multifaceted aspects of milk β-casein, focusing on its genetic variants A1 and A2. The work examines the current landscape of A1-free milk versus regular milk, delving into health considerations, protein detection methods, technological impacts on dairy production, non-bovine protein, and potential avenues for future research. Firstly, it discussed ongoing debates surrounding categorizing milk based on A1 and A2 β-casein variants, highlighting challenges in establishing clear regulatory standards and quality control methods. The chapter also addressed the molecular distinction between A1 and A2 variants at position 67 of the amino acid chain. This trait affects protein conformation, casein micelle properties, and enzymatic susceptibility. Variations in β-casein across animal species are acknowledged, casting doubt on non-bovine claims of "A2-like" milk due to terminology and genetic differences. Lastly, this work explores the burgeoning field of biotechnology in milk production.

Influence of β-casein genotype on physicochemical properties and functionality of bovine milk

Several studies have been focused on the effect of milk protein genetic variants on milk physicochemical properties and functionality in recent years. β-casein, an important protein related to milk processibility, has been reported to have 2 main genetic variants A1 and A2, for which cows may be homozygous or heterozygous. In this study, several physicochemical properties of milk with β-casein variants A1A1, A1A2, and A2A2 from 3 collection occasions were analyzed. Higher manganese content and lower pH were found to be associated with the A1A1 variant compared with the other 2 genotypes. Better rennet and acid coagulation were found in A1A1 milk compared with A1A2 and A2A2 milk (although P > 0.05), whereas A2A2 milk was more stable to creaming compared with the other 2 genotypes, which may be linked to its smaller fat globule size. Thus, milk from cows with A1A1 genotype could be preferable for cheese making, while that with A2A2 variant can be used in formulations requiring good stability against creaming, and for example, yogurt making, where the softer yogurt texture may be easier to digest.

Difficulties in Establishing the Adverse Effects of β-Casomorphin-7 Released from β-Casein Variants-A Review

β-Casomorphin-7 (BCM-7) is a peptide released through the proteolysis of β-casein (β-CN), which is considered a bioactive peptide displaying evidence of promoting the binding and activation of the μ-opioid receptor located in various body parts, such as the gastrointestinal tract, the immune system and potentially the central nervous system. The possible effects of BCM-7 on health are a theme rising in popularity due to evidence found in several studies on the modulation of gastrointestinal proinflammatory responses that can trigger digestive symptoms, such as abdominal discomfort. With the advancement of studies, the hypothesis that there is a correlation of the possible effects of BCM-7 with the microbiota-gut-brain axis has been established. However, some studies have suggested the possibility that these adverse effects are restricted to a portion of the population, and the topic is controversial due to the small number of in vivo studies, which makes it difficult to obtain more conclusive results. In addition, a threshold of exposure to BCM-7 has not yet been established to clarify the potential of this peptide to trigger physiological responses at gastrointestinal and systemic levels. The proportion of the population that can be considered more susceptible to the effects of BCM-7 are evidenced in the literature review. The challenges of establishing the adverse effects of BCM-7 are discussed, including the importance of quantifying the BCM-7 release in the different β-CN genotypes. In summary, the reviewed literature provides plausible indications of the hypothesis of a relationship between β-CN A1/BCM-7 and adverse health effects; however, there is need for further, especially in vivo studies, to better understand and confirm the physiological effects of this peptide.

Association between Days Open and Parity, Calving Season or Milk Spectral Data

Milk spectral data on 2118 cows from nine herds located in northern China were used to access the association of days open (DO). Meanwhile, the parity and calving season of dairy cows were also studied to characterize the difference in DO between groups of these two cow-level factors. The result of the linear mixed-effects model revealed that no significant differences were observed between the parity groups. However, a significant difference in DO exists between calving season groups. The interaction between parity and calving season presented that primiparous cows always exhibit lower DO among all calving season groups, and the variation in DO among parity groups was especially clearer in winter. Survival analysis revealed that the difference in DO between calving season groups might be caused by the different P/AI at the first TAI. In addition, the summer group had a higher chance of conception in the subsequent services than other groups, implying that the micro-environment featured by season played a critical role in P/AI. A weak linkage between DO and wavenumbers ranging in the mid-infrared region was detected. In summary, our study revealed that the calving season of dairy cows can be used to optimize the reproduction management. The potential application of mid-infrared spectroscopy in dairy cows needs to be further developed.

Does a Little Difference Make a Big Difference? Bovine β-Casein A1 and A2 Variants and Human Health-An Update

For over 20 years, bovine beta-casein has been a subject of increasing scientific interest because its genetic A1 variant during gastrointestinal digestion releases opioid-like peptide β-casomorphin-7 (β-CM-7). Since β-CM-7 is involved in the dysregulation of many physiological processes, there is a growing discussion of whether the consumption of the β-casein A1 variant has an influence on human health. In the last decade, the number of papers dealing with this problem has substantially increased. The newest clinical studies on humans showed a negative effect of variant A1 on serum glutathione level, digestive well-being, cognitive performance score in children, and mood score in women. Scientific reports in this field can affect the policies of dairy cattle breeders and the milk industry, leading to the elimination of allele A1 in dairy cattle populations and promoting milk products based on milk from cows with the A2A2 genotype. More scientific proof, especially in well-designed clinical studies, is necessary to determine whether a little difference in the β-casein amino acid sequence negatively affects the health of milk consumers.

What is the impact of amino acid mutations in the primary structure of caseins on the composition and functionality of milk and dairy products?

The impact of amino acid mutations within the peptide structure of bovine milk protein is important to understand as it can effect processability and subsequently effect its physiological properties. Genetic polymorphisms of bovine caseins can influence the chemical, structural, and technological properties, including casein micelle morphology, calcium distribution, network creation upon gelation, and surface activity. The A1 and A2 genetic variants of β-casein have recently acquired growing attention from both academia and industry, prompting new developments in the area. The difference between these two genetic variants is the inclusion of either proline in β-casein A2 or histidine in β-casein A1 at position 67 in the peptide chain. The aim of this review was to examine the extent to which milk and ingredient functionality is influenced by β-casein phenotype. One of the main findings of this review was although β-casein A1 was found to be the dominant variant in milks with superior acid gelation and rennet coagulation properties, milks comprised of β-casein A2 possessed greater emulsion and foam formation capabilities. The difference in the casein micelle assembly, hydrophobicity, and chaperone activity of caseins may explain the contrast in the functionality of milks containing β-casein from either A1 or A2 families. This review provides new insights into the subtle variations in the physicochemical properties of bovine milks, which could potentially support dairy producers in the development of new dairy products with different functional properties.

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Impact of β- and other caseins on the casein micelle structure and functionality.

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Proline and histidine in β-caseins play a key role in casein micelle conformation.

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Chaperone activity of β-casein A2 towards heat-induced aggregation of whey protein.

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Gels prepared of milks with β-casein A1 possess a denser and firmer structure.

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Ordered structure of β-casein A2 led to improved emulsion and foam formation.

Structural Changes of β-Casein Induced by Temperature and pH Analysed by Nuclear Magnetic Resonance, Fourier-Transform Infrared Spectroscopy, and Chemometrics

This study investigated structural changes in β-casein as a function of temperature (4 and 20 °C) and pH (5.9 and 7.0). For this purpose, nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopy were used, in conjunction with chemometric analysis. Both temperature and pH had strongly affected the secondary structure of β-casein, with most affected regions involving random coils and α-helical structures. The α-helical structures showed great pH sensitivity by decreasing at 20 °C and diminishing completely at 4 °C when pH was increased from 5.9 to 7.0. The decrease in α-helix was likely related to the greater presence of random coils at pH 7.0, which was not observed at pH 5.9 at either temperature. The changes in secondary structure components were linked to decreased hydrophobic interactions at lower temperature and increasing pH. The most prominent change of the α-helix took place when the pH was adjusted to 7.0 and the temperature set at 4 °C, which confirms the disruption of the hydrogen bonds and weakening of hydrophobic interactions in the system. The findings can assist in establishing the structural behaviour of the β-casein under conditions that apply as important for solubility and production of β-casein.