Adela Mora-Gutierrez

Review of the chemistry of αS2-casein and the generation of a homologous molecular model to explain its properties

alpha(S2)-Casein (alpha(S2)-CN) comprises up to 10% of the casein fraction in bovine milk. The role of alpha(S2)-CN in casein micelles has not been studied in detail in part because of a lack of structural information on the molecule. Interest in the utilization of this molecule in dairy products and nutrition has been renewed by work in 3 areas: biological activity via potentially biologically active peptides, functionality in cheeses and products, and nutrition in terms of calcium uptake. To help clarify the behavior of alpha(S2)-CN in its structure-function relationships in milk and its possible applications in dairy products, this paper reviews the chemistry of the protein and presents a working 3-dimensional molecular model for this casein. The model was produced by threading the backbone sequence of the protein onto a homologous protein: chloride intracellular channel protein-4. Overall, the model is in good agreement with experimental data for the protein, although the amount of helix may be over-predicted. The model, however, offers a unique view of the highly positive C-terminal portion of the molecule as a surface-accessible area. This region may be the site for interactions with kappa-carrageenan, phosphate, and other anions. In addition, most of the physiologically active peptides isolated from alpha(S2)-CN occur in this region. This structure should be viewed as a working model that can be changed as more precise experimental data are obtained.

Influence of bovine and caprine casein phosphopeptides differing in α s1 -casein content in determining the absorption of calcium from bovine and caprine calcium-fortified milks in rats

Bovine and caprine milks have a similar overall gross composition, but vary considerably in the ratios of their casein components. These differences cause significant changes in the ability of caseins to bind and stabilize calcium (Ca). It might be expected that these in vitro variations, which are thought to be due to differences in casein phosphopeptides (CPP) content, could lead to in vivo differences in the digestion and absorption of Ca. To test this hypothesis three milks with different casein ratios [bovine (B), caprine high in alphas1-casein (CH) and caprine low in alphas1-casein (CL)] were compared with regard to Ca absorption and deposition in growing male rats. For comparison, each milk was Ca-fortified (BCa-milk, CHCa-milk, and CLCa-milk) and CPP, prepared by enzymatic hydrolysis from the respective caseins (extrinsic CPP), were added to both native and Ca-milks. The effects of added CPP (extrinsic) could then be compared with intrinsic CPP released from the gastrointestinal digestion of caseins. Total gastric Ca was sampled at 15, 30 and 60 min after ingestion. No differences were found among the native milks with or without CPP, but the Ca from all Ca-milks (regardless of casein type) appeared to clear the stomach more rapidly and this was enhanced by the extrinsic CPP. The total intestinal Ca was not different among the native milks+/-CPP, however, it rose more rapidly with Ca fortification, and was higher at 30 min for all CPP-Ca-milks. At 60 min the total intestinal Ca level fell for the CPP-Ca-milks while all others continued to rise. These observations suggest that the CPP in Ca-milks enhance gastric clearance and uptake from the intestine. Ca availability from BCa-milk, CHCa-milk, and CLCa-milk with and without CPP was estimated by both plasma and femur uptake of 45Ca. Ca availability was enhanced at 5 h in the plasma in each case by added CPP. In all cases CPP stimulated Ca availability in the femur, but the CL-CPP was higher (P<0.05) than that of either CH-CPP or B-CPP (extrinsic CPP). Based on the results of this study we can conclude that the addition of CPP will have beneficial effect on the absorption of Ca in growing rats from CaCO3 added to bovine and caprine milks.

Short communication: Determination of withdrawal time for oxytetracycline in different types of goats for milk consumption

Antibiotics are widely used in animal husbandry and the presence of antibiotics in milk is a health hazard. The objective of this study was to determine residual amounts of oxytetracycline in fresh, aged, and pasteurized milk of 3 breeds of goats using HPLC analysis. It was also essential to determine the safe withdrawal period of oxytetracycline in lactating goats. The quantitative results obtained using the HPLC system were compared with the tolerance limit of oxytetracycline in milk in the United States. Fifteen milking does, 5 Nubians, 5 Alpines, and 5 LaManchas were randomly selected from the milking herd at the International Goat Research Center at Prairie View A&M University. A simple sample preparation and isocratic HPLC method using ultraviolet detection was used for analysis of milk samples. The HPLC results indicated that the withdrawal period of oxytetracycline in treated Alpine does was 82h (7 milking), whereas for Nubian does the period was 58h (5 milking), and for LaManchas the period was 72h (6 milking) after drug administration. The overall withdrawal period for all the treated goats of 3 breeds was 72h. Although these results indicated that the depletion rate of this antibiotic was faster in goats than the reported data for cows, the 96-h withdrawal period that is currently used for lactating cows is still necessary for these 3 breeds of goats. Additionally, our results indicated that oxytetracycline is not stable in goat milk at refrigeration temperature or during pasteurization and will decrease significantly.

Complexes of lutein with bovine and caprine caseins and their impact on lutein chemical stability in emulsion systems: Effect of arabinogalactan

Lutein is an important xanthophyll carotenoid with many benefits to human health. Factors affecting the application of lutein as a functional ingredient in low-fat dairy-like beverages (pH 6.0-7.0) are not well understood. The interactions of bovine and caprine caseins with hydrophobic lutein were studied using UV/visible spectroscopy as well as fluorescence. Our studies confirmed that the aqueous solubility of lutein is improved after binding with bovine and caprine caseins. The rates of lutein solubilization by the binding to bovine and caprine caseins were as follows: caprine αS1-II-casein 34%, caprine αS1-I-casein 10%, and bovine casein 7% at 100 μM lutein. Fluorescence of the protein was quenched on binding supporting complex formation. The fluorescence experiments showed that the binding involves tryptophan residues and some nonspecific interactions. Scatchard plots of lutein binding to the caseins demonstrated competitive binding between the caseins and their sites of interaction with lutein. Competition experiments suggest that caprine αS1-II casein will bind a larger number of lutein molecules with higher affinity than other caseins. The chemical stability of lutein was largely dependent on casein type and significant increases occurred in the chemical stability of lutein with the following pattern: caprine αS1-II-casein > caprine αS1-I-casein > bovine casein. Addition of arabinogalactan to lutein-enriched emulsions increases the chemical stability of lutein-casein complexes during storage under accelerated photo-oxidation conditions at 25°C. Therefore, caprine αS1-II-casein alone and in combination with arabinogalactan can have important applications in the beverage industry as carrier of this xanthophyll carotenoid (lutein).