A review of international standards and the scientific literature on farm milk bulk-tank sampling protocols

Adequate agitation is required to ensure homogeneity before sampling from on-farm bulk tanks, but excessive agitation may cause churning with a resulting loss of milk quality. Homogeneity can be assured by thorough mixing before a sample is taken and can also be combined with intermittent agitation of the bulk tank. There is general but qualified agreement among various countries and agencies, such as the IDF, that 5 min of agitation for small, and 10 min for large, quiescent farm-milk bulk tanks is required to ensure sample homogeneity. However, no empirical studies are cited to support these standards. The few studies that examined bulk-tank mixing estimate required agitation times of 8 to 10 min or longer, depending on the size of the tank. If intermittent agitation is practiced, mixing for 1 to 2 min before sampling is considered acceptable in some jurisdictions but, once again, empirical supporting evidence is absent. Automatic samplers decrease the amount of time needed to obtain a sample from the bulk tank, but both intermittent agitation and agitation during milk transfer are still recommended to minimize fat residue accumulation in the bulk tank. Systematic studies are needed to establish mixing protocols that assure accurate sampling for all tanks in a given jurisdiction.     

Milk quality and automatic milking: fat globule size, natural creaming, and lipolysis

Thirty-eight Italian Friesian first-lactation cows were allocated to 2 groups to evaluate the effect of 1) an automatic milking system (AMS) vs. milking in a milking parlor (MP) on milk fat characteristics; and 2) milking interval (≤480, 481 to 600, 601 to 720, and >720 min) on the same variables. Milk fat was analyzed for content (% vol/vol), natural creaming (% of fat), and free fatty acids (FFA, mEq/100 g of fat). Distribution of milk fat globule size was evaluated to calculate average fat globule diameter (d₁), volume-surface average diameter (d₃₂), specific globule surface area, and mean interglobular distance. Milk yield was recorded to calculate hourly milk and milk fat yield. Milking system had no effect on milk yield, milk fat content, and hourly milk fat yield. Milk from AMS had less natural creaming and more FFA content than milk from MP. Fat globule size, globular surface area, and interglobular distance were not affected by milking system per se. Afternoon MP milkings had more fat content and hourly milk fat yield than AMS milkings when milking interval was >480 min. Milk fat FFA content was greater in AMS milkings when milking interval was ≤480 min than in milkings from MP and from AMS when milking interval was >600 min. Milking interval did not affect fat globule size, expressed as d₃₂. Results from this experiment indicate a limited effect of AMS per se on milk fat quality; a more important factor seems to be the increase in milking frequency, generally associated with AMS.     

Fresh Milk Sampling for Centralized Milk Testing

A 2-yr study of 384 milk producers on nine milk routes was undertaken to investigate fresh milk (unpreserved) sampling for centralized milk testing for payment purposes. Duration of the mixing period in the farm bulk tank was a major variable in obtaining fresh milk samples for analysis. Automatic sampling comparable to sampling from bulk tanks agitated for 10 min. The sampling frequency required for a mean milk fat content representative of each herd was calculated from the standard deviation of the fat test in the herds. The distribution of accuracies among herds varied with frequency of testing. At four samples per month 53.7, 89.8, 97.7, and 98.1% of the herds had accuracies within 1.0, 1.5, 2.0, and 2.5% of the annual mean. If herd standard deviations could be reduced to .2% or less with improved mixing, 59.2, 98.9, and 100% of the herds would have had accuracies within 1.0, 1.5, and 2.0% of the annual mean by four tests per month. Tests on fresh milk gave higher averages than those on composite samples by .035, .037 and .025% for fat, protein, and lactose. Fresh milk sampling is technically feasible by automatic sampling or by manual sampling and improved agitation. Either could improve results of the present composite system.     

Reexamination of Fat Globule Clustering and Creaming in Cow Milk

Fat globule clustering, as characterized by cream volume and cluster time, was studied in raw milk, heated milk, homogenized milk, and in model systems. Immunoglobulin M was confirmed as the heat-labile component in fat globule clustering and was shown to function as a cryoagglutinin rather than as a cryoglobulin as previously indicated. Hapten inhibition studies demonstrated that the antigen is carbohydrate. Skim milk membrane was identified as the homogenization-labile component. Although immunoglobulin M can agglutinate milk fat globules to a limited extent, normal creaming requires both immunoglobulin M and skim milk membrane. Approximately 7% of the immunoglobulin M in normal milk participates in a single creaming. The lower portion of creamed milk (gravity separated skim milk) failed to support creaming on addition of washed fat globules but did so on addition of skim milk membrane. A theory of fat globule clustering consistent with observed experimental results depicts immuno-globulin M interacting in an antigen-antibody mode simultaneously with skim milk membrane and milk fat globules through specific carbohydrate moieties.     

Native milk fat globule size and its influence on whipping properties

The objective of this study was to investigate the influence of native milk fat globule size on the aeration of high fat dairy products with regard to maximum firmness time, gas inclusion and foam stability. The results showed that whipping time to maximum firmness was inversely proportional to mean fat globule size for both unhomogenised and slightly homogenised (2 MPa) creams. Additionally, increasing native mean fat globule size of the creams resulted in increased overrun. No significant differences in serum drainage were found between creams with different native milk fat globule size. Furthermore, when creams with native large mean fat globules were homogenised, the results showed that the maximum firmness time was in accordance with the mean fat globule size of non-aggregated creams. In the present study, cream fractionation was achieved by creaming or in a cost effective and fast manner using a modified centrifugal separator.     

Correlation between bovine milk somatic cell count and polymorphonuclear leukocyte level for samples of bulk milk and milk from individual cows

The influence of various factors on the concentrations of polymorphonuclear neutrophilic leukocytes (PMN) in milk samples from bulk tanks and individual cows was investigated. While somatic cell counts (SCC) and PMN level were in both cases significantly correlated, lower correlation coefficients were found between SCC and PMN for samples of bulk tank milks than for milk samples from individual cows. Furthermore, plots of PMN concentrations versus SCC showed great variability in PMN in milk samples of similar total SCC. One factor that may lead to variability in bulk tank PMN levels was shown to be increased proportions of high SCC milk in the bulk tank mixture, which result in relatively high PMN levels without excessive elevation of total SCC. In milk samples from individual cows, it was found that there was also a significant seasonal influence on milk PMN content, with milk from cows calving in the spring having, at SCC > 160,000 cells/ ml, higher proportions of PMN in the total milk SCC than milk from autumn calving cows. The results of this study suggest that the concentration of PMN may be a useful indicator of herd status in bulk tank monitoring schemes.     

Effect of physical properties of milk fat globules on Brucella ring test sensitivity.

Variations in Brucella milk ring test reactions have been attributed to size and disparity in size of fat globules, fat content of milk, inhibitory factors, and cream-rising capacity. Physical properties of milk fat globules were studied in individual milk samples that had diverse sensitivities for detection of Brucella agglutinins. Size or disparity of globule size could not be correlated with the milk ring test sensitivity. Inhibitory and enhancing creaming factors could be transferred from the cream to the skim milk and reacted with other cream to show changes in sensitivity. The proportion of clustered milk fat globules was related directly to the quantity of agglutinins detected by the test. Sensitivity of the test was attributed to a fat globule agglutinin that caused clustering and to an inhibitory factor.     

Processing effects on physicochemical properties of creams formulated with modified milk fat

Type of thermal process [high temperature, short time pasteurization (HTST) or ultra-high temperature pasteurization (UHT)] and homogenization sequence (before or after pasteurization) were examined for influence on the physicochemical properties of natural cream (20% milk fat) and creams formulated with 20% low-melt, fractionated butteroil emulsified with skim milk, or buttermilk and butter-derived aqueous phase. Homogenization sequence influenced physicochemical makeup of the creams. Creams homogenized before pasteurization contained more milk fat surface material, higher phospholipid levels, and less protein at the milk fat interface than creams homogenized after pasteurization. Phosphodiesterase I activity was higher (relative to protein on lipid globule surface) when cream was homogenized before pasteurization. Creams formulated with skim milk and modified milk fat had relatively more phospholipid adsorbed at the milk fat interface. Ultra-high-temperature-pasteurized natural and reformulated creams were higher in viscosity at all shear rates investigated compared with HTST-pasteurized creams. High-temperature, short time-pasteurized natural cream was more viscous than HTST-pasteurized reformulated creams at most shear rates investigated. High-temperature, short time-pasteurized creams had better emulsion stability than UHT-pasteurized creams. Cream formulated with buttermilk had creaming stability most comparable to natural cream, and cream formulated with skim milk and modified butteroil was least stable to creaming. Most creams feathered in a pH range of 5.00 to 5.20, indicating that they were moderately stable to slightly unstable emulsions. All processing sequences yielded creams within sensory specifications with the exception of treatments homogenized before UHT pasteurization and skim milk formulations homogenized after UHT pasteurization.     

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)     

Using physical processes to improve physicochemical and structural characteristics of fresh and frozen/thawed sheep milk

We assessed the impact of stirring (ST), high shear dispersing (HSD) and low (LPH, 3.5 MPa) and high pressure homogenization (HPH, 50 MPa) on physicochemical and structural characteristics of whole and skimmed sheep milk fresh or previously frozen and thawed (FT). Freezing affected the size of the fat globules, their interaction with caseins, reduced calcium solubility (10%) and buffering capacity (5–11%). Amongst the studied processes, HSD was the only one unable to improve the milk stability. The other ones reduced the size of the fat globules and increased fat and casein interactions, favoring milk stability and reducing the creaming occurrence (>22%). LPH and HPH also reduced the sedimentation in skimmed milk (>37%). Moreover, all processes recovered the buffering capacity of FT samples. The effectiveness of the processes can be ordered as ST < LPH < HPH, but the final choice will depend on the stability improvement required for milk vs. acquisition and operational equipment costs.Practical applicationSheep milk is normally not homogenized because it has a lower fat globule size than cow milk, which reduces the creaming occurrence. However, creaming happens in some instances and it can be intensified if the milk is preserved frozen (to accumulate enough volume) prior to the dairy production, causing defects in the final products (mainly yogurts). The studied physical processes can be strategically used to solve this problem, increasing the milk emulsion stability, reducing the sedimentation occurrence and changing the buffering capacity to reach the same value of fresh milk.     

Occurrence of false-positive results of inhibitor on milk samples using the Delvotest SP assay

Three hundred twenty-one quarter, 207 whole udder, 310 bulk tank, and 93 tank-lorry milk samples were examined for confirmation of the presence of inhibitor by Delvotest SP assay. Four hundred twenty-six Holstein cows of no drug treatment for at least 30 days from January 1998 to September 1999 were used. Reading time was 2.50, 2.75, and 3.00 h, and results of sampling were recorded by four types according to comparison with the color of the well containing the control milk sample. False-positive outcome was identified by Delvotest SP assay in quarter (13 of 321), whole udder (9 of 207), and bulk tank milk samples (4 of 310), but was not shown on tank-lorry milk samples (0 of 93) at the reading time of 2.50 h. All of the 26 false-positive samples were negative from the examination after heat treatment at 82 degrees C for 5 min. But, two bulk tank milk samples that appeared to have positive results in LacTek and Charm II tests were positive from the test following heat treatment. Somatic cell counts (SCC) were related to the probability of a false-positive result. The more SCC increased, the more the occurrence of a false-positive result increased. In our investigations, 4 of 310 bulk tank milk samples at the reading time of 2.50 h produced false-positive results, and no false-positive results were apparent at a reading time of 2.75 h. Also, the occurrence of false-positive results in quarter and whole udder milk samples decreased when agar was cultured for 2.75 to 3.00 h. There were no false-positive results from tank-lorry milk samples. These results indicate that the Delvotest SP assay may provide a suitable means for the detection of drug residues in not only quarter and whole udder milk of cows but also in bulk tank and tank-lorry milk following reading times of 2.75 to 3.00 h.     

Effect of mechanical treatments applied to milk fat on fat retention and lipolysis in minicurds

The aim of this work was to identify a treatment on milk fat, applicable to pretreat cheese milk, which could enhance lipolysis. Three factors were studied in milk fat mixtures: physical treatment (pumping and mechanical agitation), temperature (5, 15, and 45°C) and fat content (5%, 15%, and 30%). Damage to fat globule was estimated by assessing free fat and by phase contrast microscopy. Moderate damage was achieved in a mixture of 30% of fat, treated with mechanical agitation at 2800 rpm/2 min at 5°C. Applying this procedure to prepare minicurds did not modify values of fat content, moisture and lipolysis.     

Identifying cows with subclinical mastitis by bulk single nucleotide polymorphism genotyping of tank milk

Mastitis remains the most important health issue in dairy cattle. Improved methods to identify cows developing subclinical mastitis would benefit farmers. We herein describe a novel method to determine the somatic cell counts (SCC) of individual cows by bulk genotyping a sample of milk from the milk tank with panels of genome-wide single nucleotide polymorphisms (SNP). We developed a simple linear model to estimate the contribution of individual cows to the genomic DNA present in the tank milk from 1) the known genotypes of individual cows for the interrogated SNP and 2) the ratio of SNP alleles in the tank milk. Using simulations, we estimate that 3,000, 50,000, and 700,000 SNP are sufficient to accurately (R(2)>0.98) estimate individual SCC in tanks containing milk from 25, 100, and 500 cows, respectively. Using actual data, we demonstrate that the SCC of 21 cows can be estimated with a coefficient of determination of 0.60 using approximately 9,000 SNP. The proposed method increases the value of the proposition of SNP genotyping individual cows for genomic selection purposes.     

Fat and protein contents, acidity and somatic cell counts in bulk milk of Holstein cows in the Khorasan Razavi Province, Iran

Relationships between total bulk milk somatic cell score (SCS) and milk fat and protein contents and acidity were investigated in the Khorasan Razavi Province, a region that contributes 6.83% of total milk production in Iran. A total of 1476 samples were analysed. Data were obtained by randomly collecting 123 samples of bulk tank milk from 41 dairy farms during April 2006 to March 2007, every month. Milk was analysed for titratable acidity, protein and fat contents and somatic cell counts (direct microscopic cell count and with Somatos, Russia). Microscopic and Somatos somatic cell counts were comparable. Results showed that the season of raw milk production did not have a significant effect on acidity. Milk fat content increased gradually from spring to winter and there were significant differences ( P <  0.05) between spring and other seasons. Higher levels of milk protein fractions were observed during the autumn and winter than in other seasons. The highest total bulk milk somatic cell counts were observed in July. Total bulk milk SCS had significant effects ( P <  0.05) on acidity and fat and protein contents. Moreover, the level of acidity and fat in milk decreased with increasing SCS. A significant positive relationship was observed between total bulk milk SCS and the protein content of milk. Elevated SCS were associated with lowered milk quality in Holsteins in the Khorasan Razavi Province.     

蔗糖酯对酪蛋白乳浊液稳定性的影响

研究蔗糖酯用量对酪蛋白乳浊液的粒度分布、液相蛋白浓度、界面蔗糖酯浓度、脂肪部分聚结率及乳析率等指标的影响,探讨蔗糖酯对酪蛋白乳浊液稳定性影响的机理。实验表明:随着蔗糖酯用量增加,液相蛋白浓度与界面蔗糖酯浓度逐渐增加,脂肪部分聚结率呈下降趋势,乳浊液的粒径和乳析率逐渐减小,乳浊液稳定性逐渐提高。当蔗糖酯用量超过0.60%时,脂肪部分聚结率缓慢下降,而乳析率缓慢上升最后达到动态平衡。综合考虑,当蔗糖酯用量为0.60%时,乳浊液的稳定性最佳。     

Routine testing of producers' milk supplies for compositional quality using randomly selected samples

In a study over a number of months, involving over 500 milk producers in three locations, and representing bulk milk collection from cans as well as mobile and stationary refrigerated bulk milk tanks, the precision of various random sampling and testing frequencies in estimating the fat, protein and lactose content of milk was evaluated. In addition, the traditional procedure of estimating the composition of milk supplies from chemically preserved composites was compared to values obtained by analysis of fresh samples. While sampling and testing every milk collection gave the most precise estimate of milk composition for each producer, it was found that for producers using refrigerated bulk milk tanks with ex-farm collection, four random samples per month gave a precision of ±4% (or ±0.14% fat approx) for monthly fat content and ±l.2% (or ±0.04% fat approx) for annual fat content. Due to lower variability the precision for both protein and lactose estimation was much higher. The sample compositing procedure, while capable of a relatively high level of precision, did not always achieve this in practice and tended to underestimate the constituents in milk, especially protein and lactose.     

Temporal trends in bulk tank somatic cell count and total bacterial count in Irish dairy herds during the past decade

The objective of this study was to document temporal trends in bulk tank somatic cell count (SCC) and total bacterial counts (TBC) in Irish dairy herds during the years 1994 to 2004. Three milk processors participated in the study, providing data on 2,754,270 individual bulk tank SCC and 2,056,992 individual bulk tank TBC records from 9,113 herds. Somatic cell counts decreased during the years 1994 to 2000, followed by an annual increase thereafter of more than 2,000 cells/mL. A tendency existed for TBC to decrease over time. Across all years, bulk tank SCC were the lowest in April and highest in November; TBC were the lowest in May and highest in December. The significant seasonal pattern observed in herd SCC and TBC was an artifact of seasonal calving in Ireland. In general, herds selling more milk had lower bulk tank SCC and TBC. Herds having the highest SCC (i.e., >450,000 cells/mL) and the lowest SCC (i.e., ≤150,000 cells/mL) both contributed substantially to the mean SCC of the milk pool collected by the milk processors. Derived transition matrices showed that between adjacent years, herds had the greatest probability of remaining in the same annual mean SCC or TBC category.     

Impact of cream washing on fat globules and milk fat globule membrane proteins

Milk proteins, contained within the aqueous phase surrounding fat globules, should be removed before analysis of the composition of the native milk fat globule membrane (MFGM). The effect of the conditions applied during washing of cream on MFGM integrity has not been fully studied, and factors potentially effecting a modification of MFGM structure have not been systematically assessed so far. In this study, a cream separator was used to investigate the impact of cream washing on milk fat globule stability and the corresponding loss of MFGM proteins. Flow velocity, fat content, and type of washing solution were varied. Particle size measurements and protein analyses were carried out after each washing step to determine fat globule coalescence, removal of skim milk proteins, and efficiency of MFGM isolation. Significant differences in fat globule stability and protein amount in the MFGM isolates were measured using different washing conditions.     

Effects of Size and Stability of Native Fat Globules on the Formation of Milk Gel Induced by Rennet

Rennet‐induced gelation crucially impacts cheese structure. In this study, effects of the size and stability of native fat globules on the kinetics of rennet‐induced coagulation were revealed by determining the caseinomacropeptide release rate and rheological properties of milk. Moreover, the mobility and stability of fat globules during renneting was revealed using diffusing wave spectroscopy and confocal laser scanning microscopy. By use of a 2‐stage gravity separation combined centrifugation scheme, native fat globules were selectively separated into small (SFG, D_4,3 = 1.87 ± 0.02 μm) and large fat globules (LFG, D_4,3 = 5.65 ± 0.03 μm). The protein and fat content of SFG and LFG milk were then standardized to 3.2 g/100 mL and 1.2 g/100 mL, respectively. The milk containing different sized globules were then subjected to renneting experiments in the laboratory. Reduction of globule size accelerated the aggregation of casein micelles during renneting, giving a shorter gelation time and earlier 1/l^* change. The gel produced from LFG milk was broken due to coalescent fat globules and generated coarser gel strands compared to the finer strands formed with SFG milk. Structural differences were also confirmed with a higher final storage modulus of the curd made from SFG milk than that from the LFG. In conclusion, the size of fat globules affects the aggregation of casein micelles. Moreover, fat globule coalescence and creaming during renneting, also affects the structure of the rennet gel. A better understanding of the size of globules effect on milk gelation could lead to the development of cheese with specific properties.     

Physical,textural, and rheological properties of whipped cream affected by milk fat globule membrane protein

This work aims at improving the textural and whipping properties of whipped cream by the addition of milk fat globule membrane protein. The determination of particle size distribution and average diameter of whipped cream showed that the small particle size was shifted to a larger range after milk fat globule membrane protein was added. The average particle size (d_3,2) of whipped cream reached a maximum value of 5.05 µm at 1% milk fat globule membrane protein, while slowly decreased with increasing milk fat globule membrane protein levels from 2% to 5%. In addition, the partial coalescence of fat increased with the increase of milk fat globule membrane protein levels, and the correlation between the whipping time and the overrun of whipped cream was positive. The addition of milk fat globule membrane protein also altered the rheological behaviour of whipped cream, resulting in the increase of modulus G′ and the loss modulus G″. The results also indicated that higher milk fat globule membrane protein level decreased the serum loss of whipped cream while improved its stability. While milk fat globule membrane protein levels had no significant effect on viscosity, its increasing levels effectively improved the hardness, consistency, and viscosity of whipped cream.