Assessment of lipoprotein activators of skim milk lipoprotein lipase and the relationship between lipoprotein lipase activity and milk fat synthesis

Bovine plasma and lipoproteins isolated by gel filtration chromatography were examined for their ability to activate skim milk lipoprotein lipase. Addition of equal amounts of protein from either triglyceride-rich lipoprotein, low density lipoprotein, high density lipoprotein or plasma to a lipoprotein lipase assay resulted in 6.0, 2.2, 2.5, or 1.1% hydrolysis of radiolabelled triglyceride emulsion. Lipoprotein lipase activity in skim milk was evaluated as an indicator of mammary lipid secretory capacity. Skim milk lipoprotein lipase activity was significantly lower immediately prepartum as compared with activity immediately postpartum (.2 vs. 5.4% of substrate hydrolyzed). Skim milk lipoprotein lipase was significantly higher during the final 12 d of lactation than in samples obtained 12 d after machine milking was terminated (5.6 vs. less than 1% of substrate hydrolyzed). Although skim milk lipoprotein lipase activity appeared positively related to mammary lipid secretory capacity during the time immediately surrounding initiation and cessation of copious milk production, activity between those periods was not correlated to milk fat percentage, milk fat yield, or stage of lactation.     

Effects of proteose‐peptone fractions from yak milk on lipoprotein lipase lipolysis

The lipoprotein lipase (LPL) was purified almost 3800‐fold from yak skimmilk by Heparin‐Sepharose CL‐6B column chromatography; nonhydrophobic fraction of proteose‐peptone (NHFPP) and hydrophobic fraction of proteose‐peptone (HFPP) from yak milk whey were separated by Phenyl‐Sepharose‐6FF column chromatography. The HFPP was subjected to hydroxyapatite chromatography, and two fractions were collected: one fraction did not absorb onto the calcium phosphate matrix (HA1); the other fraction contained all the protein components of HFPP (HA2). The effects of the proteose‐peptone fractions on lipoprotein lipase lipolysis were studied. The results of experiments showed that NHFPP and HA1 enhanced the LPL lipolysis; in contrast, the HFPP and HA2 inhibited LPL lipolysis.     

Milk lipoprotein lipase distribution in the major fractions of bovine milk

Raw, bovine bulk tank milk and milks from selected cows were separated by ultracentrifugation into four major fractions: casein, sloughed membrane material, serum, and milk fat globule membrane. Milk lipoprotein lipase activity was measured by the pH stat method and protein determinations were made by the Lowry procedure for each of the four fractions in order to calculate specific activity (units per milligram of protein). In six farm-cooled bulk milk samples stored less than or equal to 24 h, casein had a significantly higher milk lipoprotein lipase total activity, 35.66 units/ml of milk, than all of the fractions. Serum had the next highest activity with 11.69 units/ml of milk. Fluff and milk fat globule membrane had activities of .80 and .41 units/ml of milk, respectively. The specific activity of the fluff was 3.3 milk lipoprotein lipase units/mg of protein, which was significantly higher than the casein and serum fractions in pooled milk. Milks from five cows in midlactation were assayed individually for milk lipoprotein lipase activity and protein content immediately after milking and after 12, 24, 48 and 72 h of cold (4 degrees C) storage. Fresh warm milk was characterized by the absence of fluff. Casein had the highest mean activity (29.91 units/ml), followed by serum (10.25 units/ml) and milk fat globule membrane (.26 units/ml) in the warm milk from the individual cows. Upon cooling to 4 degrees C, significant increases in enzyme activity in the fluff and milk fat globule membrane fractions were observed at 12 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dairy cow feeding system alters the characteristics of low-heat skim milk powder and processability of reconstituted skim milk

Low-heat skim milk powder (LHSMP) was manufactured on 3 separate occasions in mid lactation (ML, July 4–20) and late lactation (LL, September 27 to October 7) from bulk milk of 3 spring-calving dairy herds on different feeding systems: grazing on perennial ryegrass (Lolium perenne L.) pasture (GRO), grazing on perennial ryegrass and white clover (Trifolium repens L.) pasture (GRC), and housed indoors and offered total mixed ration (TMR). The resultant powders (GRO-SMP, GRC-SMP, and TMR-SMP) were evaluated for composition and color and for the compositional, physicochemical, and processing characteristics of the reconstituted skim milk (RSM) prepared by dispersing the powders to 10% (wt/wt) in water. Feeding system significantly affected the contents of protein and lactose, the elemental composition, and the color of the LHSMP, as well as the rennet gelation properties of the RSM. The GRO and GRC powders had a higher protein content; lower levels of lactose, iodine, and selenium; and a more yellow-green color (lower a* and higher b* color coordinates) than TMR powder. On reconstitution, the GRO-RSM had higher concentrations of protein, casein, and ionic calcium, and lower concentrations of lactose and nonprotein nitrogen (% of total N). It also produced rennet gels with a higher storage modulus (G′) than the corresponding TMR-RSM. These effects were observed over the combined ML and LL period but varied somewhat during the separate ML and LL periods. Otherwise, feeding system had little or no effect on proportions of individual caseins, concentration of serum casein, casein micelle size, casein hydration, heat coagulation time, or ethanol stability of the RSM at pH 6.2 to 7.2, or on the water-holding capacity, viscosity, and flow behavior of stirred yogurt prepared by starter-induced acidification of RSM. The differences in the functionality of the LHSMP may be of greater or lesser importance depending on the application and the conditions applied during the processing of the RSM.     

Effect of the lactoperoxidase system on lipoprotein lipase activity and lipolysis in milk

The lactoperoxidase/SCN-/H2O2 system (LPS) was found to inhibit lipoprotein lipase (LPL) in milk. Inhibition was related to the amount of SCN-/H2O2 added to milk. Lipolysis was unaltered until LPL activity was decreased to less than 40% of the original activity. When LPL was inhibited further, decreased lipolysis was observed. Purified LPL was also inhibited by LPS, at similar concentrations of SCN-/H2O2 as in milk. It was shown that non enzymically prepared OSCN- inhibited LPL at concentrations comparable to those produced by the LPS. Cysteine could eliminate the effect of LPS on LPL. The sulphydryl reagents dithiobisnitrobenzoic acid and Na tetrathionate had no effect on LPL activity.     

Cooling causes changes in the distribution of lipoprotein lipase and milk fat globule membrane proteins between the skim milk and cream phase

Lipoprotein lipase (LPL) activity and free fatty acid levels were studied in freshly milked, uncooled milk from individual Danish Holstein or Jersey cows, or after storage for up to 24 h at either a cooling temperature (4°C) or at the milking temperature (31°C). Upon cooling for up to 24 h, LPL activity increased in the cream phase, whereas the activity in the skim milk was steady, as observed for Jersey cows, or increased, as seen for the Holsteins. Storage at 31°C decreased the LPL activity in both the cream phase and the skim milk phase. The increase in free fatty acid levels was found to depend on LPL activity, incubation temperature, substrate availability, and incubation time. Furthermore, the migration of milk proteins between the skim milk phase and the cream phase upon cooling of milk from Jersey cows or from Danish Holstein cows was studied using proteomic methods involving 2-dimensional gel electrophoresis and mass spectrometry. Proteins associated with the milk fat globules were isolated from all milk fractions and analyzed. Major changes in the distributions of proteins between the skim milk phase and the cream phase were observed after cooling at 4°C for 4 h, where a total of 29 proteins between the 2 breeds was found to change their association with the milk fat globule membrane (MFGM) significantly. Among these, the MFGM proteins adipophilin, fatty acid-binding protein, and lactadherin, as well as the non-MFGM proteins β-casein, lactoferrin, and heat shock protein-71, were identified. Adipophilin, lactadherin, and lactoferrin were quantitatively more associated with the MFGM upon cold storage at 4°C, whereas β-casein, fatty acid-binding protein, and heat shock protein-71 were found to be less associated with the MFGM upon cold storage.     

Proteolytic activity of lactic acid bacteria in skim milk with special reference to the biodegradation of casein fractions

Streptococci and lactobacilli were assayed for their proteolytic activity in pasteurised (95 degrees C for 30 min) fresh Friesian cows' skim milk incubated at 30 degrees C for 48 h. Lactobacilli were more proteolytic than the streptococci except S. faecalis subsp. liquefaciens. S. faecalis and S. thermophilus followed S. lactis subsp. diacetylactis in the proteolytic activity. Electrophoretic analysis of the precipitated casein revealed K-, pre-beta- and the small slow band of alpha s1-casein to be the most fractions and beta-casein the least fraction susceptible to the biodegradation. S. faecalis subsp. liquefaciens was the only organism able to degrade beta-casein to 3 fractions within 48 h. S. lactis with its subsp. diacetylactis was characterized by its inability to degrade gamma-casein. Addition of 2.0 g glucose, 0.5 g yeast extract, 2.0 mmol Mg2+, 0.5 mmol Mn2+ and 0.1 mmol Fe2+/l of skim milk culture of each L. casei and L. plantarum increased acid formation but decreased proteolysis. 6% NaCl was inhibitory to both. After 60 days at 18 degrees C, a fraction, probably derived from beta-casein, was noticeable. The large intense band of alpha s1-casein appears to be degraded gradually to only small peptides which in turn are transferred immediately into the cells. The hydrolysis of gamma-casein and beta-casein also appears to be inhibited by exogenously supplied NaCl and nutrients, respectively.     

N-acetyl-beta-D-glucosaminidase activity in whole milk and milk fractions

Experiment 1 was conducted to determine NAGase activity in skim, fat, and cell pellet fractions of foremilk and stripping milk from infection-free quarters. Changes in milk NAGase activity during a 12 h in vitro incubation were also determined. Eight cows, two quarters per cow, were used. One quarter of each cow received an intramammary infusion of oyster glycogen. N-Acetyl-beta-D-glucosaminidase activity was highest in stripping milk and in milk from infused quarters. The percentages of NAGase activity in skim, fat, and cell pellet fractions were 62.6, 22.4, and 12.6. The NAGase activity of milk incubated in vitro did not significantly change over time. Experiment 2 was conducted to determine if neutrophils lost NAGase activity during extravasation into milk. Leukocytosis was induced in infection-free quarters of five cows. The NAGase activities of peripheral neutrophils and milk neutrophils were not significantly different. Results from both studies suggest that the major source of milk NAGase is the mammary epithelial cell and that milk somatic cells contribute less than 15% of the total milk NAGase activity.     

Detection of lipase in skim and whole milk powders using triheptanoin as a substrate

As lipase catalysed hydrolysis of milk fat decreases the sensory quality of most dairy products, we developed an assay to measure lipase activity in skim and whole milk powders. Milk powder was incubated with triheptanoin at pH 6.0 for 7 days at 37 °C and the release of heptanoic acid was measured using headspace solid phase microextraction and gas chromatography mass spectrometry. The assay was applied to model samples containing milk powders spiked with commercial lipase preparations, and variables such as substrate amount and pH were optimised. Triheptanoin as a substrate was compared with both tributyrin and trioctanoin. Triheptanoin behaved in a manner similar to that of trioctanoin, but gave the analytical advantage of minimal interference from background levels of free fatty acids (FFAs). The response of the assay was linear over time and with enzyme concentration. The detection limit of the assay was calculated to be 0.05 nmol FFAs released min⁻¹ g⁻¹ powder.     

Effects of different components in skim milk on high-pressure-induced gelatinisation of waxy rice starch and normal rice starch

The gelatinisation of starch in skim milk required higher pressures than did the gelatinisation of starch in water. This study examined the effects of various milk components on the pressure-induced gelatinisation of waxy rice starch and normal rice starch, in order to understand the differences between the gelatinisation characteristics of starch in skim milk and starch in water. Gelatinisation was retarded in skim milk, which was attributed to the effects of soluble milk minerals and lactose. The presence of these components may reduce the plasticising ability of the suspension medium. Direct interactions between the milk components and starch molecules may also contribute to retarded gelatinisation. Milk proteins (casein and whey protein) did not affect the degree of pressure-induced gelatinisation at the concentrations of these components in skim milk, at 10% total solids.     

Enzyme immunoassay for measuring lipoprotein lipase activator in milk.

A method for determining the relative amount of lipoprotein lipase activator in milk was developed. The activator was measured in arbitrary units which were based on a standard curve for high density lipoprotein. Activator varied between cows not only in whole milk but also in skim milk and milk serum. Most of the activator was in the skim milk, and the amount of activator in milk serum varied between 32 and 91% of that in skim milk. In seven of eight cows, free fatty acids of milk increased with increasing units of activator.     

The effect of homogenization of whole milk, skim milk and milk fat on nisin activity against Listeria innocua

Whole milk, skim milk and an emulsion of milk fat in water, inoculated with approx. 10(5) cfu/ml of Listeria innocua, were treated at 30 degrees C with 100 IU/ml of nisin, homogenization at 200 bar or both procedures. Nisin activity and survival of L. innocua after treatments were determined. Recovery of nisin activity from non-homogenized whole milk treated with 100 IU/ml of nisin was complete, whereas a loss of 18 to 28% of activity was detected in non-homogenized fat-in-water emulsion. Loss in nisin activity due to homogenization represented up to 64% in whole milk and 62% in fat-in-water emulsion. Nisin addition by itself achieved a reduction in L. innocua counts of 3.7-3.8 log units in whole milk and 3.6 log units in fat-in-water emulsion compared to numbers in untreated samples. When nisin-containing whole milk and fat-in-water emulsion were homogenized, L. innocua counts were only reduced by 2.6-2.9 log units and 2.5 log units, respectively, compared to numbers in untreated samples. Homogenization of nisin-containing skim milk resulted in a loss of nisin activity of 20% but achieved a reduction of 3.0 log units in L. innocua counts.     

营养强化剂对脱脂乳粉中甲醛产生的影响

目的 研究不同营养强化剂对脱脂乳粉中甲醛生成的影响.方法 脱脂乳粉添加5％有效含量的单一营养强化剂,经热处理后测定甲醛含量,进行单因素实验,考察不同营养强化剂对甲醛产生的影响,选择作用显著的营养强化剂进行双因素组合实验,研究不同营养强化剂之间的相互作用.结果 110℃热处理10 min的情况下,单因素实验结果表明,维生...     

Lipolytic activity with the membrane fraction of bovine skim milk.

Colloidal phosphate-free skim milk was subjected to gel filtration on Sepharose 4B. Lipolytic activity was observed in the membrane material eluted in the void volume fraction and in the protein fraction representing a broad range of molecular weights.     

Effects of tail docking on milk quality and cow cleanliness

The objective of this study was to determine the effect of tail docking on somatic cell count (SCC), intramammary infection (IMI), and udder and leg cleanliness in commercial dairy herds. Lactating dairy cows (n = 1250) from eight Wisconsin farms were blocked by farm and randomly allocated to tail docked (D) or control (C) groups. Milk samples, somatic cell counts, and hygiene scores were collected for 8 to 9 mo. The prevalence of IMI was determined for each of the five occasions when milk samples were obtained. Udder and leg cleanliness were assessed during milk sample collection. Docked and control animals were compared by logSCC, prevalence of IMI, and leg and udder cleanliness score. Variables were analyzed according to all treatment, period, and farm interactions. At the end of the study period 76 (12.2%) and 81 (13%) of cows were culled in the D and C groups, respectively. There were no significant differences in the initial data for parity, daily milk yield, logSCC, or DIM between treatment groups. Effects significant to farms were identified for all variables over all periods. Period was significant for all variables except for the prevalence of environmental pathogens, but no period x treatment interactions were detected. There was no significant difference between treatment groups for somatic cell count. The prevalence of contagious, environmental, or minor pathogens did not differ significantly between treatment groups. This study did not identify any differences in udder or leg hygiene or milk quality that could be attributed to tail docking.     

Immunoreactive properties of peptide fractions of cow whey milk proteins after enzymatic hydrolysis

Commercial whey protein concentrate (WPC) was hydrolysed with either Alcalase 2.4 FG (Novo Nordisk), or papain (Sigma) (in one‐step process) or with two enzymes (in two‐step process) to determine the changes in the immunoreactivity of α‐lactalbumin and β‐lactoglobulin. Enzymatic hydrolysis of WPC was performed by pH‐stat method. Hydrolysates were analysed using sodium dodecyl sulphate‐polyacrylamide gel electrophoresis, immunoblotting and size‐exclusion chromatography (SE‐HPLC). Immunoreactive properties of peptide fractions separated from the hydrolysates by fast protein liquid chromatography (FPLC) were determined using dot‐immunobinding and enzyme‐linked immunosorbent assay (ELISA) methods. Finally the sensory analysis was used to confirm organoleptic changes resulting from the application of different enzymes. The ‘two‐step’ process was observed to be the most effective however allergenic epitopes were still present, as it was found by ELISA with anti‐α‐la and anti‐β‐lg antibodies. The addition of papain as the second enzyme in the hydrolysis process contributed to the improvement of the sensory properties of WPC hydrolysate as compared with the Alcalase hydrolysate. Alcalase‐papain partially hydrolysated WPC can be found a promising base for production of the tolerogenic formula.