Seasonal variation in the fatty acid composition of milk available at retail in the United Kingdom and implications for dietary intake

Milk and dairy products are major sources of fat in the human diet, but there are few detailed reports on the fatty acid composition of retail milk, trans fatty acids in particular, and how these change throughout the year. Semi-skimmed milk was collected monthly for one year from five supermarkets and analysed for fatty acid composition. Relative to winter, milk sold in the summer contained lower total saturated fatty acid (SFA; 67 vs 72 g/100 g fatty acids) and higher cis-monounsaturated fatty acid (MUFA; 23 vs 21 g/100 g fatty acids) and total trans fatty acid (6.5 vs 4.5 g/100 g fatty acids) concentrations. Concentrations of most trans-18:1 and -18:2 isomers also exhibited seasonal variation. Results were applied to national dietary intakes, and indicated that monthly variation in the fatty acid composition of milk available at retail has limited influence on total dietary fatty acid consumption by UK adults.     

Validation of a rapid milk fat separation method to determine the fatty acid profile by gas chromatography

An improved rapid method for separating lipids from milk to determine the fatty acid composition using 2 centrifugations at room temperature (20°C) was compared with the ISO-IDF reference procedure based on solvent extraction. The new method is useful for research and routine quality control and has a number of advantages over the reference procedure—mainly no solvents are required and it saves time. Applicability of the rapid separation method was confirmed in fats with different physical characteristics from ewe and goat milk samples. Minor differences were found in the proportions of some fatty acids in the reference and centrifugation methods. Milk fat separated by centrifugation at room temperature did not differ in fatty acid composition from milk centrifuged at 4°C.     

Effect of feeding oilseed supplements to dairy cows on ruminal and milk fatty acid composition

The objective of this study was to compare the effects of oilseed‐based supplements, rapeseed and linseed, against a barley‐based control, on the fatty acid composition, and subsequent solid fat ratio, of the milk fat from dairy cows. In addition, as a means of understanding the digestive processes which influence the milk fat composition, ruminal extracts were collected from the cows and analysed for fatty acid composition. Four lactating dairy cows each fitted with a rumen fistula were provided with silage and one of four concentrate diets. The main constituent of the concentrate supplements was either rapeseed (ground or unground), linseed (unground) or a barley control. The diets were offered in accordance with a 4 × 4 Latin square arrangement. The oilseed‐supplemented concentrates provided the cows with 620–640 g fatty acids day^?1. Experimental treatments were provided to the cows for 2 weeks, after which ruminal extracts were collected over a 24 h period and a milk sample was taken. All extracts were analysed for fatty acid composition. The diets fed influenced the long‐chain fatty acid composition of the ruminal extracts and milk fat. The proportion of C18:1n‐9 in the ruminal extracts increased from 202–224 to 282–321 g kg^?1 of the total fatty acids when the cows were provided with the rapeseed‐based diets. The linseed‐based diet increased the C18:1n‐9 proportion of the ruminal extracts from 164 to 218 g kg^?1 of the total fatty acids. Both rapeseed‐based diets also resulted in a higher proportion of C18:0 in the ruminal extract, possibly owing to biohydrogenation of the dietary fatty acids. This proportion of C18:0 in the ruminal extract was lowest immediately after feeding, increasing to a maximum 4–6 h later. Both rapeseed‐based concentrates increased the proportion of C18:1n‐9 in the milk fat to approximately 300 g kg^?1 of the total fatty acids as compared with 214 g kg^?1 for the control. The proportion of C18:1n‐9 in the milk fat from the cows offered the linseed‐based concentrate was 246 g kg^?1 of the total fatty acids. There were also significant decreases in the proportions of C16:0 in the milk fat from the cows offered all oilseed‐based concentrates. There was no difference between the fatty acid compositions of the milk fats from the cows fed the ground or unground rapeseed‐based supplements. The oilseed‐based supplements also resulted in significant decreases in the solid fat content of the milk fat at temperatures ranging from 0 to 35 °C, which would be indicative of a softer, more spreadable butter. © 2002 Society of Chemical Industry     

The effect of pasteurisation temperature on the CLA content and fatty acid composition of white pickled cheese

In this study, the effect of pasteurisation temperature on fatty acid composition of cheese was investigated. The fatty acid composition of raw and different heat‐treated milk, salt and salt‐free cheese were determined using cheese made from raw milk at temperatures varying between 70 and 90°C for 5 min. Generally, C 16:0 palmitic acid was the major fatty acid present in all milk and cheese samples. C 18:1 t11 vaccenic acid was the major trans fatty acid (TFA) in all samples. C 18:2 cis‐9, trans‐11 (Rumenic acid) was the major CLA isomer in these samples. Pasteurisation temperatures had no effect on TFA, CLA and fatty acid composition of the milk and cheese samples.     

Inclusion of microalgae in the caprine diet improves nutritional profile of milk and its Camembert cheese

The experiment was carried out to investigate the effects of supplementation with microalgae (Schizochytrium sp.) on the nutritional characteristics of milk and Camembert cheese from dairy goats. The yield and composition of milk and cheese and the sensory quality of cheese were not affected by the supplementation. As the level of supplementation increased, the concentration of docosahexaenoic acid and total n-3 fatty acids increased, whereas the ratio of n-6/n-3 and total saturated fatty acids decreased in the milk and cheese. In conclusion, supplemented goats, especially goats that received 35 g/head/day produced milk and cheese with better fatty acid composition for human consumption, without affecting the production and composition.     

Survey of the fatty acid composition of retail milk in the United States including regional and seasonal variations

Consumers are increasingly aware that food components have the potential to influence human health maintenance and disease prevention, and dietary fatty acids (FA) have been of special interest. It has been 25 years since the last survey of US milk FA composition, and during this interval substantial changes in dairy rations have occurred, including increased use of total mixed rations and byproduct feeds as well as the routine use of lipid and FA supplements. Furthermore, analytical procedures have improved allowing greater detail in the routine analysis of FA, especially trans FA. Our objective was to survey US milk fat and determine its FA composition. We obtained samples of fluid milk from 56 milk processing plants across the US every 3 mo for one year to capture seasonal and geographical variations. Processing plants were selected based on the criteria that they represented 50% or more of the fluid milk produced in that area. An overall summary of the milk fat analysis indicated that saturated fatty acids comprised 63.7% of total milk FA with palmitic and stearic acids representing the majority (44.1 and 18.3% of total saturated fatty acids, respectively). Unsaturated fatty acids were 33.2% of total milk FA with oleic acid predominating (71.0% of total unsaturated fatty acids). These values are comparable to those of the previous survey in 1984, considering differences in analytical techniques. Trans FA represented 3.2% of total FA, with vaccenic acid being the major trans isomer (46.5% of total trans FA). Cis-9, trans-11 18:2 conjugated linoleic acid represented 0.55% total milk FA, and the major n-3 FA (linolenic acid, 18:3) composed 0.38%. Analyses for seasonal and regional effects indicated statistical differences for some FA, but these were minor from an overall human nutrition perspective as the FA profile for all samples were numerically similar. Overall, the present study provides a valuable database for current FA composition of US fluid milk, and results demonstrate that the milk fatty acid profile is remarkably consistent across geographic regions and seasons from the perspective of human dietary intake of milk fat.     

Evaluation of Fatty Acid Profile of Wagyu Beef by ATR-FTIR Spectroscopy

Japanese black Wagyu beef has its characteristics of fatty well-marbled texture, flavor, and tenderness which are affected by fatty acid composition. The aim of this study was to develop an analytical method for evaluating the fatty acid profile of Wagyu beef by Fourier transform infrared (FTIR) spectroscopy. In the current study, attenuated total reflection–FTIR (ATR-FTIR) spectroscopy and gas chromatography (GC) were applied to the fat tissues, and the solvent-extracted fats which were sampled from subcutaneous, inter- and intramuscular fat tissues. Results of GC analysis showed that monounsaturated fatty acids (MUFA) content became larger in the order of intramuscular, intermuscular, and subcutaneous fats, and saturated fatty acids (SFA) became smaller in the same order. Subcutaneous fat could be discriminated from inter- and intramuscular fats on the basis of fatty acid composition by principal component analysis. The ATR-FTIR analysis revealed that the shift of the peak positions of alkene C–H stretching vibration at around 3,006 cm⁻¹ occurred depending on the unsaturation degree of fatty acids in beef fat. Partial least squares (PLS) regression analysis with leave-one-out cross-validation was applied to the combined regions of 2,800–3,050 and 1,000–1,500 cm⁻¹ for the fat tissues and the extracted fats. The correlation coefficients of the PLS validation models predicting the content of the MUFA and SFA for solvent-extracted fats were higher than those for fat tissues, and the coefficients (R ²) of determination more than 0.873 were obtained for solvent-extracted fats and 0.522 for fat tissues.     

Stability of fatty acid composition after thermal,high pressure,and microwave processing of cow milk as affected by polyunsaturated fatty acid concentration

Interest has been increasing to enhance the contents of healthy polyunsaturated fatty acid (PUFA) in milk. However, trans fatty acids and conjugated linoleic acid (CLA) can be altered after thermal processing and high pressures disrupt the milk fat globule membrane, exposing the lipid core and helping its oxidation. The objective of the present research was to study whether processing can alter the fatty acid composition of milk and if these changes are affected by PUFA concentration as previous studies suggest. Two cow milk batches (500 L each), one naturally enriched in PUFA, were processed to obtain pasteurized; high temperature, short time; UHT; high pressure; and microwave pasteurized samples. The detailed fatty acid composition was analyzed with special attention to trans fatty acids and CLA isomers. Results showed that after high temperature, short time processing, total CLA content increased in both milk batches, whereas sterilization resulted in a sigmatropic rearrangement of C18:2 cis-9,trans-11 to C18:2 trans-9,trans-11. The extent of these effects was greater in milks naturally enriched in PUFA.     

Association of polymorphisms in solute carrier family 27, isoform A6 (SLC27A6) and fatty acid-binding protein-3 and fatty acid-binding protein-4 (FABP3 and FABP4) with fatty acid composition of bovine milk

The main goal of this study was to develop tools for genetic selection of animals producing milk with a lower concentration of saturated fatty acids (SFA) and a higher concentration of unsaturated fatty acids (UFA). The reasons for changing milk fatty acid (FA) composition were to improve milk technological properties, such as for production of more spreadable butter, and milk nutritional value with respect to the potentially adverse effects of SFA on human health. We hypothesized that genetic polymorphisms in solute carrier family 27, isoform A6 (SLC27A6) fatty acid transport protein gene and fatty acid binding protein (FABP)-3 and FABP-4 (FABP3 and FABP4) would affect the selectivity of FA uptake into, and FA redistribution inside, mammary epithelial cells, resulting in altered FA composition of bovine milk. The objectives of our study were to discover genetic polymorphisms in SLC27A6, FABP3, and FABP4, and to test those polymorphisms for associations with milk FA composition. The results showed that after pairwise comparisons between SLC27A6 haplotypes for significantly associated traits, haplotype H3 was significantly associated with 1.37 weight percentage (wt%) lower SFA concentration, 0.091 lower SFA:UFA ratio, and 0.17 wt% lower lauric acid (12:0) concentration, but 1.37 wt% higher UFA and 1.24 wt% higher monounsaturated fatty acid (MUFA) concentrations compared with haplotype H1 during the first 3 mo of lactation. Pairwise comparisons between FABP4 haplotypes for significantly associated traits showed that haplotype H3 was significantly associated with 1.04 wt% lower SFA concentration, 0.079 lower SFA:UFA ratio, 0.15 wt% lower lauric acid (12:0), and 0.27 wt% lower myristic acid (14:0) concentrations, but 1.04 wt% higher UFA and 0.91 wt% higher MUFA concentrations compared with haplotype H1 during the first 3 mo of lactation. Percentages of genetic variance explained by H3 versus H1 haplotype substitutions for SLC27A6 and FABP4 ranged from 2.50 to 4.86% and from 4.91 to 7.22%, respectively. Tag single nucleotide polymorphisms were identified to distinguish haplotypes H3 of SLC27A6 and FABP4 from others encompassing each gene. We found no significant associations between FABP3 haplotypes and milk FA composition. In conclusion, polymorphisms in FABP4 and SLC27A6 can be used to select for cattle producing milk with lower concentrations of SFA and higher concentrations of UFA.     

Effect of solar radiation and flaxseed supplementation on milk production and fatty acid profile of lactating ewes under high ambient temperature

The objectives of this study were to evaluate the effects of protection from solar radiation and whole flaxseed supplementation on milk yield and milk fatty acid profile in lactating ewes exposed to high ambient temperature. The experiment was conducted during summer and involved 40 ewes divided into 4 groups. The ewes were either exposed (not offered shade) or protected from solar radiation (offered shade). For each solar radiation treatment, ewes were supplemented with whole flaxseed or not. Milk samples from each ewe were collected at the morning and afternoon milking every week, and analyzed for pH, total protein, casein, fat, and lactose content, somatic cell count, and renneting parameters (clotting time, rate of clot formation, and clot firmness after 30 min). At the beginning of the experiment, and then at d 23 and 44, milk samples were analyzed for milk fatty acids using gas chromatography. Flaxseed supplementation significantly increased milk yield, fat, protein, and casein yields, and somatic cell count, and increased fat and lactose contents of milk. A decrease of saturated fatty acids from C6:0 to C16:0 and an increase of C18:1 trans-11 and C18:2 cis-9,trans-11 was observed in milk from flaxseed-supplemented ewes. Flaxseed supplementation decreased saturated fatty acids content and increased total monounsaturated fatty acids content, the total content of isomers of conjugated linoleic acid, and polyunsaturated fatty acids content in milk. Flaxseed also increased the α-linolenic acid content of milk. As a result, milk from supplemented groups showed an increase in n-3 fatty acid content. Flaxseed supplementation decreased short-chain and medium-chain fatty acids, and increased long-chain fatty acid content of milk. On average, flaxseed supplementation increased the C18:2 cis-9,trans-11/C18:1 trans-11 Δ⁹-desaturase index starting from d 23 of the experiment, in correspondence with the highest C18:2 cis-9,trans-11 content of milk from flaxseed-supplemented ewes. Flaxseed decreased atherogenic and thrombogenic indices of milk. Protection from solar radiation during summer did not improve yield and composition of ewe milk. Nevertheless, milk from ewes exposed to solar radiation showed decreased long-chain fatty acid and polyunsaturated fatty acids contents, and in particular, decreased vaccenic acid, rumenic acid, and total conjugated linoleic acid contents.     

The effects of sources of supplemental fat on performance,egg quality,and fatty acid composition of egg yolk in laying hens

BACKGROUND: The objective of this study was to determine the effects of sources of supplemental fat on laying performance, egg quality, and fatty acid composition of egg yolk. RESULTS: Two hundreds Isa Brown layers were assigned randomly to be fed ad libitum a standard commercial layer feed (BD), basal diet plus 2% tallow (T), basal diet plus a mixture of 1% tallow and 1% flaxseed oil (MTFO), basal diet plus 2% sunflower oil (SO), or basal diet plus 2% flaxseed oil (FO), which were offered for 8 weeks. Each diet was given to five groups, each containing 10 hens. The feed efficiency and egg production were 2.78 and 53.51% for BD; 2.30 and 63.47% for T; 2.45 and 60.14% for MTFO; 2.29 and 64.30% for SO, and 2.62 and 61.18% for FO groups, respectively. Dietary fat supplementation affected the laying performance but had no significant effects on egg quality parameters. The fatty acid composition of egg yolk lipids were significantly affected by dietary fatty acid composition. The supplemental tallow increased palmitic fatty acid. The proportions of linoleic and arachidonic fatty acids in egg yolks for layers fed the SO diet were higher than in the BD group and those on diets containing other fats. Concentrations of oleic and omega‐3 fatty acids were the highest in layers fed the FO diet during the laying period. CONCLUSION: The results indicated that dietary animal and plant fats changed the fatty acid composition of egg yolk. Copyright © 2008 Society of Chemical Industry     

Short communication: Composition of milk protein and milk fatty acids is stable for cows differing in genetic merit for milk production

Changing the composition of milk protein and of milk fatty acids alters nutritional and physical properties of dairy products and their consumer appeal. Genetic selection for milk yield decreases concentrations of milk protein and of milk fat. Little is known, however, about how the decrease affects composition of milk protein and milk fatty acids. The objective of this study was to quantify changes in composition of milk protein and of milk fatty acids in cows differing in genetic merit for milk production. Three measures of genetic merit for milk production were used for each cow: genetic line, parent average predicted transmitting ability (PTA) for milk, and cow milk PTA. Composition of milk protein and milk fatty acids were compared in 448 milk samples from 178 cows representing 2 divergent lines of Holsteins that were bred for high or average PTA for milk and combined milk protein and fat yield. High-line cows (n = 97) produced more milk that contained less fat and had higher proportions of α_S1-casein in milk protein than did average-line cows (n = 81). We additionally obtained from 233 cows (178 cows representing the 2 genetic lines and 55 cows with ancestors from both genetic lines) the parent average milk PTA and cow milk PTA and compared composition of milk protein and of milk fatty acids in 592 milk samples. Cows whose parent average milk PTA was above or equal to the median of the 233 cows produced more milk that contained less protein and less fat and that tended to have greater proportions of α_S1-casein in milk protein than cows whose average milk PTA was below the median. Similarly, cows with above or equal median milk PTA of the 233 cows produced more milk that contained less protein and less fat and had greater proportions of α_S1- casein in milk protein than did cows with below-median milk PTA. Milk fatty acid composition was not consistently different between groups. Therefore, selection for milk yield decreased concentrations of milk protein and milk fat but had little effect on composition of milk protein and milk fatty acids.     

Characterisation and comparison of khoa prepared from camel milk with that from cow and buffalo milk

Khoa, a heat‐concentrated milk product, is used as a base material for the manufacture of many popular sweets. The comparison was made between various chemical compositions and characteristics of the khoa prepared from the camel milk with that prepared from the cow and the buffalo milk samples. The khoa prepared from the camel milk had the higher moisture, ash, acidity, soluble nitrogen, free fatty acids and peroxide value, but lower in fat, protein and lactose contents than that prepared from the cow and buffalo milk samples. There was no significant (P > 0.05) difference in 5‐hydroxymethyl furfural between khoa samples prepared from the three milks.     

Factors affecting variations in the detailed fatty acid profile of Mediterranean buffalo milk determined by 2-dimensional gas chromatography

Buffalo milk is the world's second most widely produced milk, and increasing attention is being paid to its composition, particularly the fatty acid profile. The objectives of the present study were (1) to characterize the fatty acid composition of Mediterranean buffalo milk, and (2) to investigate potential sources of variation in the buffalo milk fatty acid profile. We determined the profile of 69 fatty acid traits in 272 individual samples of Mediterranean buffalo milk using gas chromatography. In total, 51 individual fatty acids were identified: 24 saturated fatty acids, 13 monounsaturated fatty acids, and 14 polyunsaturated fatty acids. The major individual fatty acids in buffalo milk were in the order 16:0, 18:1 cis-9, 14:0, and 18:0. Saturated fatty acids were the predominant fraction in buffalo milk fat (70.49%); monounsaturated and polyunsaturated fatty acids were at 25.95 and 3.54%, respectively. Adopting a classification based on carbon-chain length, we found that medium-chain fatty acids (11–16 carbons) represented the greater part (53.7%) of the fatty acid fraction of buffalo milk, whereas long-chain fatty acids (17–24 carbons) and short-chain fatty acids (4–10 carbons) accounted for 32.73 and 9.72%, respectively. The n-3 and n-6 fatty acids were 0.46 and 1.77%, respectively. The main conjugated linoleic acid, rumenic acid, represented 0.45% of total milk fatty acids. Herd/test date and stage of lactation were confirmed as important sources of variation in the fatty acid profile of buffalo milk. The percentages of short-chain and medium-chain fatty acids in buffalo milk increased in early lactation (+0.6 and +3.5%, respectively), whereas long-chain fatty acids decreased (?4.2%). The only exception to this pattern was butyric acid, which linearly decreased from the beginning of lactation, confirmation that its synthesis is independent of malonyl-CoA. These results seem to suggest that in early lactation the mobilization of energy reserves may have less influence on the fatty acid profile of buffalo milk than that of cow milk, probably due to a shorter and less severe period of negative energy balance. Parity affected the profiles of a few traits and had the most significant effects on branched-chain fatty acids. This work provided a detailed overview of the fatty acid profile in buffalo milk including also those fatty acids present in small concentrations, which may have beneficial effects for human health. Our results contributed also to increase the knowledge about the effects of some of the major factors affecting buffalo production traits and fatty acid concentrations in milk, and consequently its technological and nutritional properties.     

Seasonal variation in the Dutch bovine raw milk composition

In this study, we determined the detailed composition of and seasonal variation in Dutch dairy milk. Raw milk samples representative of the complete Dutch milk supply were collected weekly from February 2005 until February 2006. Large seasonal variation exists in the concentrations of the main components and milk fatty acid composition. Milk lactose concentration was rather constant throughout the season. Milk true protein content was somewhat more responsive to season, with the lowest content in June (3.21 g/100 g) and the highest content in December (3.38 g/100 g). Milk fat concentration increased from a minimum of 4.10 g/100 g in June to a maximum of 4.57 g/100 g in January. The largest (up to 2-fold) seasonal changes in the fatty acid composition were found for trans fatty acids, including conjugated linoleic acid. Milk protein composition was rather constant throughout the season. Milk unsaturation indices, which were used as an indication of desaturase activity, were lowest in spring and highest in autumn. Compared with a previous investigation of Dutch dairy milk in 1992, the fatty acid composition of Dutch raw milk has changed considerably, in particular with a higher content of saturated fatty acids in 2005 milk.     

Effect of redfish meal enriched diets on the taste and n-3 pufa of 42-day-old broiler chickens

Broiler chickens were fed diets supplemented with redfish meal at levels of 4,8 and 12 %. A control group was fed with an all-vegetable protein diet. After 42 days the birds were slaughtered and edible tissues (white, dark and skin) were analysed for lipid content and composition. The white and dark meats were tasted organoleptically for the possible presence of ‘fishy’ off-flavours. There was no change in the content or class composition of the lipids with diet or sex of the bird, but in meats an increase in fish meal in the diet caused an increase in the level of n-3 polyunsaturated fatty acids present, especially 20: 5n-3 eicosapentaenoic acid, 22: 5n-3 docosapentaenoic acid and 22: 6n-3 docosahexaenoic acid at the expense of n-6 fatty acids. The enrichment was most pronounced in white meat of both male and female birds and could make a contribution of n-3 polyunsaturated fatty acids to the human diet quantitatively similar to that of an equal portion of lean white fish. Taste panel results showed that there was no significant difference in flavour or taste among any of the samples. There was a tendency for the 12% dietary group birds to be the least preferred, but no panelist detected ‘fishy’ flavours or objected to the taste of the meat.     

Oil Content and Fatty Acid Composition of Seeds of Guizotia Cass (Compositae)

Seeds of Guizotia abyssinica and wild Guizotia were analysed for oil content and fatty acid composition by multisequential and gas chromatographic methods, respectively. The oil content of G abyssinica ranged from 416 to 436 g kg^-1 (weight per kg dry matter of seeds) and that of the wild Guizotia taxa from 214 to 328 g kg^-1. Two unsaturated fatty acids (linoleic (54·3–72·8, weight percent of total oil) and oleic (5·4–26·8% of oil by wt) and two saturated fatty acids (palmitic (7·8–10%) and stearic (5·5–8·1%)) were about 91–97% of the fatty acids present. Palmitoleic, linolenic, arachidic, eicosenoic, behenic, erucic and lignoceric acids constituted about 2–3%. An unidentified fatty acid, probably an epoxy form of C: 20 or C: 22, has been found in all the materials and it was 1·1–6·6%. Total saturated and unsaturated fatty acids were about 74–84% and 15–20%, respectively. Differentiation in fatty acid composition between the taxa is too small to be of taxonomic use. It is inferred that when gene transfer is desired hybridisation between the wild and cultivated taxa may not affect the oil quality of the latter and the oils of the wild taxa are possibly safe for human consumption. © 1997 SCI.     

Fatty acid composition of milk from multiparous Holstein cows treated with bovine somatotropin and fed n-3 fatty acids in early lactation

Multiparous cows (n = 59) were blocked by expected calving date and previous milk yield and assigned randomly to treatments to determine effects of bovine somatotropin (bST; Posilac, Monsanto Animal Agricultural Group, St. Louis, MO) and source of dietary fat on milk fatty acid composition during the first 140 d in milk. Diets were provided from calving and included whole, high-oil sunflower seeds (SS; 10% of dietary dry matter; n-6/n-3 ratio of 4.6) as a source of linoleic acid or a mixture of Alifet-High Energy and Alifet-Repro (AF; Alifet USA, Cincinnati, OH; 3.5 and 1.5% of dietary dry matter, respectively; n-6/n-3 ratio of 2.6) as a source of protected n-3 fatty acids (15.7% 18:3, 1.3% 20:5, and 1.3% 22:6). Treatments were derived from a 2 × 2 combination of supplemental fat source (SS, AF) and with 0 (SSN, AFN) or 500 (SSY, AFY) mg of bST administered every 10 d from 12 to 70 d in milk and at 14-d intervals thereafter. Milk fatty acid composition was determined in samples collected from 32 cows (8 complete blocks) during wk 2, 8, and 20 of lactation. Data were analyzed as repeated measures using mixed model procedures to determine the effects of diet, bST, week of lactation, and their interactions. Proportions of 18:3 (4.02 vs. 3.59 ± 0.16%), 20:5 (0.52 vs. 0.41 ± 0.02%), and 22:6 (0.11 vs. 0.02 ± 0.02%) were greater and the n-6/n-3 fatty acid ratio (7.40 vs. 8.80 ± 0.30) was reduced in milk from cows fed AF compared with SS. Proportions of de novo-synthesized fatty acids increased and preformed fatty acids decreased as lactation progressed, but bST administration delayed this shift in origin of milk fatty acids. Transfer efficiency of 18:3, 20:5, and 22:6 from AF to milk fat averaged 36.2, 4.9, and 5.2%, respectively. These efficiencies increased as lactation progressed, but were delayed by bST. Apparent mammary Δ⁹-desaturase activity and milk conjugated linoleic acid (cis-9, trans-11 conjugated linoleic acid) content increased through the first 8 wk of lactation. Based on the product-to-substrate ratio of 14:1/14:0 fatty acids in milk, there was an interaction of diet and bST because bST decreased apparent Δ⁹-desaturase activity in SSY cows but increased it in AFY cows (0.10, 0.09, 0.08, and 0.09 ± 0.01 for SSN, SSY, AFN, and AFY, respectively). Feeding Alifet-Repro increased n-3 fatty acids in milk and bST prolonged the partitioning of dietary fatty acids into milk fat.