Lennart Björck

Production of conjugated linoleic acid by dairy starter cultures

Nineteen different strains of lactobacilli, lactococci, streptococci and propionibacteria commonly used as dairy starter cultures were tested for their ability to produce conjugated linoleic acid (CLA) from free linoleic acid in vitro. Two strains of Propionibacterium freudenreichii ssp. freudenreichii and one strain of P. freudenreichii ssp. sheramnii were found to be capable of converting free linoleic acid to extracellular CLA. The highest level of CLA formed in the media was 265 μg ml−1. Of the different isomers, cis‐ and trans‐9,11‐octadecadienoic acid represented more than 70% of the total CLA formed. The inhibitory effect of linoleic acid on the growth of the bacteria and its conversion to CLA in different media by propionibacteria are discussed.

Composition of raw milk from sustainable production systems

Abstract Organic milk production has increased rapidly in many European countries during the last decade but the merits of organic dairy products are still disputed. Little unbiased information exists regarding any essential differences in gross composition or other parameters of technological and/or nutritional interest. In order to gather more basic information regarding organic milk, raw milk samples from 31 organic dairy farms in Sweden were collected once a month during 1 year. The samples were analyzed for gross composition, somatic cells, fatty acids, urea, iodine and selenium. As a reference, milk composition data from similar conventional farms was obtained. The results show small or no differences in the investigated parameters between organic milk and the milk from the conventional farms or average values regarding gross composition of Swedish raw milk. The only significant differences found were in urea content and somatic cells, both of which were lower in organic milk. In addition, levels of selenium were lower in organic milk, which is of nutritional importance since dairy products are significant dietary sources of selenium in Scandinavian diets.

Antibacterial effect of the lactoperoxidase system on psychrotrophic bacteria in milk

Activation of the antibacterial lactoperoxidase system in milk, i.e. increasing the thiocyanate concentration to 0.25 mM and adding an equimolar amount of H2O2, results in a substantial reduction of the bacterial flora and prevents the multiplication of psychrotrophic bacteria for up to 5 d. This treatment has no effect on the physico-chemical properties of milk and does not lead to the accumulation of resistant bacteria. The practical application of the lactoperoxidase system in prolonging the storage period of raw milk at low temperatures is discussed.

Lipolysis and the distribution of lipase activity in bovine milk in relation to stage of lactation and time of milking.

The increase in levels of free fatty acids (FFA) in morning and afternoon milk after 48 h cold storage at 5 degrees C was determined at two stages of lactation in milk from 96 cows. Lipolysis was positively correlated to day of lactation (r = 0.6, P less than 0.001), while the correlations between lipolysis and milk yield and fat content respectively were due to these factors being related to day of lactation. In afternoon milk the level of FFA increased almost three times above that in morning milk. Lipolysis was closely related to the lipoprotein lipase (LPL) activity associated with the cream fraction (r = 0.8, P less than 0.001). Afternoon milk had higher LPL activity in the cream fraction than morning milk. Lipase activity in the cream increased with increasing stage of lactation. Cooling of milk increased LPL activity associated with the fat phase. This effect was greater in afternoon milk from late lactation than in morning milk from early lactation. When heparin was added to milk the LPL activity associated with the fat increased substantially; again, this effect was greater in afternoon milk from late lactation than in morning milk from early lactation. Lipolysis was higher in heparinized milk than in normal milk, and there was a close correlation between lipolysis and LPL activity associated with the fat (r = 0.82, P less than 0.001). Characteristics of the milk fat globule influencing the attraction of LPL were found to be fundamentally important to lipolysis.

Biosensor assay for determination of haptoglobin in bovine milk

Despite more than 30 years of research into mastitis diagnostics, there are few alternatives to the somatic cell count (SCC) in practical use for identification of cows with subclinical mastitis. Mastitis is not only an animal welfare problem, but also affects the yield, composition and technological properties of milk. Hence, dairy cooperatives give farmers a premium quality payment to encourage low SCC although there is no clear scientific data defining the level of SCC in bulk tank milk that is associated with additional benefits in terms of milk quality. Recent research on alternative markers for inflammatory reactions in the lactating cow, e.g. in mastitis, includes investigations of the acute phase protein, haptoglobin (Hp). So far, the content of Hp in milk has mainly been studied in relation to mastitis diagnostics, with little attention given to its importance for milk composition and technological properties. At present, Hp in milk is measured using ELISA, but this technique is not suitable for routine large-scale analysis. In recent years, optical biosensor technology has been used for automated and rapid quantitative analysis of different components in milk, but so far not for analysis of acute phase proteins. The aim of the present study was to develop a rapid and sensitive biosensor method to determine Hp in milk. An affinity sensor assay based on the interaction between Hp and haemoglobin was developed using surface plasmon resonance (SPR) biosensor technology. The assay was used to analyse Hp in composite milk samples from cows without any clinical signs of mastitis and quarter milk samples with a weak to strong reaction in the California Mastitis Test (CMT). A commercial ELISA for determination of Hp in milk was used for comparison. The limit of detection (LOD) of the biosensor assay was determined as 1.1 mg/l. Within-assay and between-day variations were determined both with bulk tank milk spiked with human Hp and with composite milk samples containing bovine Hp. Coefficients of variation varied between 3.6 and 8.6% at concentrations between 4.0 and 12 mg/l, respectively. Agreement between the results obtained by the biosensor assay and the ELISA was satisfactory; however, the results obtained by the biosensor were generally lower than the results obtained by the ELISA. Possible explanations for this observation are discussed.

Comparison of biosensor, microbiological, immunochemical, and physical methods for detection of sulfamethazine residues in raw milk

A biosensor assay based on biospecific interaction analysis (BIA) was compared with already existing methods for detection of sulphamethazine (SMZ) residues in milk. Microbial inhibitor and receptor assays, an enzyme-linked immunosorbent assay (ELISA), high-pressure liquid chromatography (HPLC), and BIA were used to analyze milk samples from SMZ-treated cows. The results of the commercially available tests (Delvotest SP Special, BR-test Blue Star, Charm II test) were in agreement with the claimed sensitivity of the respective assays. The agreement between the quantitative methods (ELISA, HPLC, BIA) varied. The microbial inhibitor assays and BIA were also used to screen 330 tanker milk samples, All samples were negative in the inhibitor tests, whereas the BIA indicated the occurrence of less than 0.9 μg of SMZ per kg of milk in 5 samples and 1.5 ± 0.6 μg/kg in one sample, HPLC indicated the presence of SMZ in the latter sample, although the concentration was below the detection limit of the method. The advantages offered by the BIA: no sample preparation, high sensitivity, and rapid, fully automated analysis in real time make the technology an interesting alternative to existing screening methods within future food-quality control systems.

Studies on Rheological Properties of Stirred Milk Gels Made from Milk with Defined Genetic Variants of κ-Casein and β-Lactoglobulin

The rheological properties of acidified milk gels made from milk with defined genetic variants of κ-casein and β-lactoglobulin have been investigated. Gels were prepared from skim milk after heating for 4 min at 90°C by acidification with glucono-δ-lactone and gentle stirring after holding for 24 h at 20°C. Stirred acidified milk gels were then kept at 6°C for 96 h and thereafter measured for the rheological properties using a Bohlin VOR Rheometer. The milk gels containing only the B variant of β-lactoglobulin (homozygous cows) were found to have a significantly higher elastic modulus as compared to milk gels containing both β-lactoglobulin A and B or only A. An explanation could be that more β-lactoglobulin B had aggregated with the casein micelles, since a strong correlation was found between the loss of native β-lactoglobulin in the milk serum and the gel firmness. No correlation was found between gel firmness and any of the κ-casein genetic variants, dry matter content and the total protein concentration, respectively. Furthermore, viscous properties did not vary between the acidified milk gels in this experiment.

Survival of orally administered lactobacillus strains in the gut of cannulated pigs

Abstract Two pig strains and one human strain (Lactobacillus acidophilus NCDO 1748) of lactobacilli were fed to pigs with intestinal cannulas. The recovery rate was higher for one of the pig strains than for the two others. No permanent establishment in the gut occurred. Apparently, Lactobacillus strains isolated from an animal species and fed to the same animal species cannot indiscriminately be expected to survive the passage through the stomach and small intestine. The ability of the strains to adhere to squamous cells of the stomach did not ensure establishment in the gut.

Incorporation of dietary 1-O-alkyldiacyl glycerols into tissue lipids of neonatal rats.

3H-[9,10]-1-O-alkyl-2,3-diacyl glycerols were administered per os to suckling rats at 1, 3, 6, 9 and 18 days of age. 12 h after administration the animals were killed and the distribution of the labelled material was analysed by whole-body autoradiography. The brown adipose tissue (BAT), intestine and liver showed the highest levels of label at all ages. The lipid content of BAT, intestine and liver were separated into lipid classes, which were analysed for their 3H levels. About 30% of the recovered label was found in the phospholipid fraction at all ages. Little of the administered dose was recovered as alkyl glycerols: 5% in the 1-day-old rat decreasing to less than 1.5% at 18 days of age, indicating that dietary alkyl glycerols only to a very limited degree are incorporated in tissue ether lipids in the young rat.

The use of different microbial assays in combination with the Charm II test in the detection of antibiotic residues in herd milk

Abstract Three microbial assays, i.e. Delvo test SP, BR-test AS and Valio T101, were used to screen approximately 40 000 milk samples for inhibitory substances at ten different milk grading laboratories. Samples found positive by any of the assays were then analysed by the Charm II test for the presence of β-lactams, tetracyclines, aminoglycosides, macrolides and sulfa drugs. Positive samples were also analysed by the screening assays used routinely at Swedish dairies, i.e. Delvo test P and Arla microtest. The results revealed considerable differences between the methods regarding their sensitivity to various antibiotics. β-Lactams were the prevailing type of antibiotics found, irrespective of the microbial assay method used. A surprisingly large number of samples were positive for tetracyclines and/or macrolides. These substances were best detected by Arla microtest and Valio T101, while β-lactams were somewhat better detected by the Delvo tests. A future control system with an alternative use of more than one microbial screening assay in combination with the Charm II test is suggested.