Jacob Holm Nielsen

Potentials to differentiate milk composition by different feeding strategies

To investigate the effect of the dietary intake of the cow on milk composition, bulk-tank milk was collected on 5 occasions from conventional (n = 15) and organic (n = 10) farms in Denmark and on 4 occasions from low-input nonorganic farms in the United Kingdom, along with management and production parameters. Production of milk based on feeding a high intake of cereals, pasture, and grass silage resulted in milk with a high concentration of alpha-linolenic acid (9.4 +/- 0.2 mg/kg of fatty acids), polyunsaturated fatty acids (3.66 +/- 0.07 mg/kg of fatty acids), and natural stereoisomer of alpha-tocopherol (RRR-alpha-tocopherol, 18.6 +/- 0.5 mg/kg of milk fat). A milk production system using a high proportion of maize silage, by-products, and commercial concentrate mix was associated with milk with high concentrations of linoleic acid (LA; 19.7 +/- 0.4 g/kg of fatty acids), monounsaturated fatty acids (27.5 +/- 0.3 mg/kg of fatty acids), and a high ratio between LA and alpha-linolenic acid (4.7 +/- 0.2). Comparing these 2 production systems with a very extensive nonorganic milk production system relying on pasture as almost the sole feed (95 +/- 4% dry matter intake), it was found that the concentrations of conjugated LA (cis-9,trans-11; 17.5 +/- 0.7 g/kg of fatty acids), trans-11-vaccenic acid (37 +/- 2 g/kg of fatty acids), and monounsaturated fatty acids (30.4 +/- 0.6 g/kg of fatty acids) were higher in the extensively produced milk together with the concentration of antioxidants; total alpha-tocopherol (32.0 +/- 0.8 mg/kg of milk fat), RRR-alpha-tocopherol (30.2 +/- 0.8 mg/kg of milk fat), and beta-carotene (9.3 +/- 0.5 mg/kg of milk fat) compared with the organic and conventional milk. Moreover, the concentration of LA (9.2 +/- 0.7 g/kg of fatty acids) in milk from the extensive milk production system was found to approach the recommended unity ratio between n-6 and n-3, although extensive milk production also resulted in a lower daily milk yield.

Laccase‐catalyzed polymerization of tyrosine‐containing peptides

Laccase‐catalyzed polymerization of tyrosine and tyrosine‐containing peptides was studied in the presence and absence of ferulic acid (FA). Advanced spectroscopic methods such as MALDI‐TOF MS, EPR, FTIR microscopy and HPLC‐fluorescence, as well as more conventional analytical tools: oxygen consumption measurements and SDS/PAGE were used in the reaction mechanism studies. Laccase was found to oxidize tyrosine and tyrosine‐containing peptides, with consequent polymerization of the compounds. The covalent linkage connecting the compounds was found to be an ether bond. Only small amounts of dityrosine bonds were detected in the polymers. When FA was added to the reaction mixtures, it was found to be incorporated into the polymer structure. Thus, in addition to homopolymers, different heteropolymers containing two or four FA residues were formed in the reactions.

Oxidation in pre-cooked minced pork as influenced by chill storage of raw muscle

During storage for four days at 4 °C of pre-cooked minced pork, lipid oxidation measured as thiobarbituric acid reactive substances (TBARS) and as hexanal by solid phase microextraction was found to depend on the level of lipid hydroperoxides rather than on the TBARS in the raw muscle (M. Longissimus dorsi). Storage for up to six days at 4 °C, prior to mincing and cooking, resulted in a decrease in lipid hydroperoxides measured as conjugated dienes and in increasing TBARS-value in the raw muscle, and in a pre-cooked product with better oxidative stability. The correlation coefficient between accumulation of TBARS and hexanal in the pre-cooked meat was 0.98, indicating that the new technique using solid phase microextraction with subsequent GC-analysis for determination of hexanal can be used as an alternative technique to describe lipid oxidation in pre-cooked meat.

Isolation and quantification of cholesterol oxides in dairy products by selected ion monitoring mass spectrometry.

A method for isolation, detection and quantification of cholesterol oxidation products based on solid phase extraction in combination with preparative HPLC and gas chromatography-mass spectrometry selected ion monitoring has been developed for dairy products. The isolation procedure had a high recovery and artifact formation was minimal, as shown by isotope labelling. The limits of detection ranged from 0.3 to 35 pg/microliters of the isomeric forms of 7-hydroxycholesterol, 20 alpha-hydroxycholesterol, the isomeric forms of cholesterol-5,6-epoxides, cholestanetriol, 25-hydroxycholesterol and 7-ketocholesterol corresponding to a limit of quantification of 2-6 ng oxysterol/g lipid in the dairy product, depending on the nature of the cholesterol oxidation product.

Cross-Linking Proteins by Laccase-Catalyzed Oxidation: Importance Relative to Other Modifications

Laccase-catalyzed oxidation was able to induce intermolecular cross-links in beta-lactoglobulin, and ferulic acid-mediated laccase-catalyzed oxidation was able to induce intermolecular cross-links in alpha-casein, whereas transglutaminase cross-linked only alpha-casein. In addition, different patterns of laccase-induced oxidative modifications were detected, including dityrosine formation, formation of fluorescent tryptophan oxidation products, and carbonyls derived from histidine, tryptophan, and methionine. Laccase-catalyzed oxidation as well as transglutaminase induced only minor changes in surface tension of the proteins, and the changes could not be correlated to protein cross-linking. The presence of ferulic acid was found to influence the effect of laccase, allowing laccase to form irreducible intermolecular cross-links in beta-lactoglobulin and resulting in proteins exercising higher surface tensions due to cross-linking as well as other oxidative modifications. The outcome of using ferulic acid-mediated laccase-catalyzed oxidation to modify the functional properties of proteinaceous food components or other biosystems is expected to be highly dependent on the protein composition, resulting in different changes of the functional properties.

Dityrosine, 3,4-dihydroxyphenylalanine (DOPA), and radical formation from tyrosine residues on milk proteins with globular and flexible structures as a result of riboflavin-mediated photo-oxidation.

Riboflavin-mediated photo-oxidative damage to protein Tyr residues has been examined to determine whether protein structure influences competing protein oxidation pathways in single proteins and protein mixtures. EPR studies resulted in the detection of Tyr-derived o-semiquione radicals, with this species suggested to arise from oxidation of 3,4-dihydroxyphenylalanine (DOPA). The yield of this radical was lower in samples containing β-casein than in samples containing only globular proteins. Consistent with this observation, the yield of DOPA detected on β-casein was lower than that on two globular proteins, BSA and β-lactoglobulin. In contrast, samples with β-casein gave higher yields of dityrosine than samples containing BSA and β-lactoglobulin. These results indicate that the flexible structure of β-casein favors radical-radical termination of tyrosyl radicals to give dityrosine, whereas the less flexible structure of globular proteins decreases the propensity for tyrosyl radicals to dimerize, with this resulting in higher yields of DOPA and its secondary radical.

Cholesterol oxidation in butter and dairy spread during storage

In a dairy spread (800 g lipid/kg, 10 g salt/kg) based on 750 g milk fat/kg and 250 g rapeseed oil/kg fat in 15 g extruded catering packaging, there was a more significant accumulation of cholesterol oxidation products than in butter (minimum 800 g lipid/kg, 12 g salt/kg) in 10 g extruded catering packaging when stored at 4 or at 20 degrees C. There was a lag phase of 7 weeks in cholesterol oxidation in dairy spread stored at 4 degrees C, while no lag phase was observed for storage at 20 degrees C. Total concentrations of oxysterols were, however, very similar for dairy spread stored at 4 and 20 degrees C after 13 weeks storage (approximately 12 micrograms/g milk lipid); storage at -18 degrees C almost prevented cholesterol oxidation (approximately 4 micrograms/g milk lipid). For butter, cholesterol oxidation was less pronounced at 4 degrees C (<3 micrograms/g milk lipid) than at -18 degrees C (approximately 4 micrograms/g milk lipid) and 20 degrees C (approximately 7 micrograms/g milk lipid). 7-Ketocholesterol was the dominant oxidation product, with 1.3 and 5.7 micrograms/g milk lipid in butter and dairy spread respectively after 13 weeks storage at 4 degrees C.

Oxidative stability of buttermilk as influenced by the fatty acid composition of cows' milk manipulated by diet

Milk from cows fed a low-fat diet high in cereals designed to stimulate fat synthesis de novo was lower in unsaturated fatty acids (21.3%) than milk from cows fed a diet high in fat, mainly from roasted soy beans (41.3% unsaturated fatty acids). Buttermilk from the more unsaturated milk was less oxidatively stable during storage (at 4 degrees C, followed for 11 d) than buttermilk from the more saturated milk, as monitored both by primary lipid oxidation products (lipid hydroperoxides) and by the secondary lipid oxidation product, hexanal. Fat-soluble antioxidants, beta-carotene and alpha-tocopherol, analysed by HPLC, were not consumed during storage for either of the two types of buttermilk. In contrast, the antioxidative capacity of the serum phase decreased during storage as evaluated in a radical scavenging assay based on the semi-stable water-soluble radical nitrosodisulphonate (Fremys salt). The time course for the decrease in water-soluble antioxidants was very similar for the two types of buttermilk suggesting that oxidation is initiated in the serum phase independently of fatty acid composition.

Effect of feeding intensity and milking system on nutritionally relevant milk components in dairy farming systems in the North East of England.

There is increasing concern that the intensification of dairy production reduces the concentrations of nutritionally desirable compounds in milk. This study therefore compared important quality parameters (protein and fatty acid profiles; α-tocopherol and carotenoid concentrations) in milk from four dairy systems with contrasting production intensities (in terms of feeding regimens and milking systems). The concentrations of several nutritionally desirable compounds (β-lactoglobulin, omega-3 fatty acids, omega-3/omega-6 ratio, conjugated linoleic acid c9t11, and/or carotenoids) decreased with increasing feeding intensity (organic outdoor ≥ conventional outdoor ≥ conventional indoors). Milking system intensification (use of robotic milking parlors) had a more limited effect on milk composition, but increased mastitis incidence. Multivariate analyses indicated that differences in milk quality were mainly linked to contrasting feeding regimens and that milking system and breed choice also contributed to differences in milk composition between production systems.

Milk quality as affected by feeding regimens in a country with climatic variation

To investigate the influence of climatic conditions and season on milk composition, bulk tank milk was sampled on 5 occasions during a period of 15 mo from 20 Swedish dairy farms. These farms included 5 organic and 5 conventional farms in central Sweden and 7 traditional conventional farms and 3 conventional farms growing maize for silage in southern Sweden. Feed data and milk yield were recorded and milk was analyzed for content of fatty acids, carotenoids, and tocopherol. Differences between milk from the 2 regions and between summer and winter seasons were shown. Milk from central Sweden differed from milk from southern Sweden in that it had a higher content of carotenoids, tocopherol, short-chain fatty acids (C4-C14), C18:0, and C18:3 n-3 and a lower content of C16. Summer milk samples had a lower fat content and contained higher amounts of C18:1 cis-9 and conjugated linoleic acid cis-9,trans-11, and lower amounts of C4 to C16 compared with winter milk. Differences between farm types from central Sweden were lower content of conjugated linoleic acid cis-9,trans-11 and higher content of C18:3 n-3 in organic milk compared with conventional milk. In southern Sweden the use of maize silage caused lower milk content of carotenoids and C18:3 n-3 when compared with traditional feeding. Differences in milk composition could be related to climatic differences because legumes are more dominating in the leys of central Sweden and maize growing is limited to southern Sweden.