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- DISCUSSION AND CONCLUSIONS
BACKGROUND: Previous studies showed differences in fatty acid (FA) and antioxidant profiles between organic and conventional milk. However, they did not (a) investigate seasonal differences, (b) include non-organic, low-input systems or (c) compare individual carotenoids, stereoisomers of α-tocopherol or isomers of conjugated linoleic acid. This survey-based study compares milk from three production systems: (i) high-input, conventional (10 farms); (ii) low-input, organic (10 farms); and (iii) low-input non-organic (5 farms). Samples were taken during the outdoor grazing (78 samples) and indoor periods (31 samples).
RESULTS: During the outdoor grazing period, on average, milk from the low-input systems had lower saturated FAs, but higher mono- and polyunsaturated FA concentrations compared with milk from the high-input system. Milk from both the low-input organic and non-organic systems had significantly higher concentrations of nutritionally desirable FAs and antioxidants—conjugated linoleic (60% and 99%, respectively) and α-linolenic (39% and 31%, respectively) acids, α-tocopherol (33% and 50%, respectively) and carotenoids (33% and 80%, respectively)—compared with milk from the high-input system. Milk composition differed significantly between the two low-input systems during the second half of the grazing period only; with milk from non-organic cows being higher in antioxidants, and conjugated linoleic acid, and that from organic cows in α-linolenic acid. In contrast, few significant differences in composition were detected between high-input and low-input organic systems when cows were housed.
CONCLUSIONS: Milk composition is affected by production systems by mechanisms likely to be linked to the stage and length of the grazing period, and diet composition, which will influence subsequent processing, and sensory and potential nutritional qualities of the milk. Copyright © 2008 Society of Chemical Industry
- Top of page
- DISCUSSION AND CONCLUSIONS
The fatty acid (FA) and fat-soluble antioxidant composition in milk fat is known to affect processing and sensory quality of dairy products,1, 2 and may also affect their nutritional value.3–5
The degree of saturation in milk fat has a bearing on the hardness, texture and taste of manufactured dairy products, particularly butter and cheese.6 The presence of longer-chain saturated fatty acids (SFA) increases the hardness of butter, while milk with a high proportion of unsaturated FA content (typical range 275–400 g kg−1 fat) tends to give softer products (e.g., more spreadable butter). Unsaturated (especially polyunsaturated) FAs are also more prone to oxidation, which results in the development of off-flavour and reduced shelf-life in milk and dairy products.6 However, the sensory quality and shelf-life of milk and dairy products is determined by the balance of unsaturated FAs and fat-soluble antioxidants, which protect against oxidation and off-flavour development.6–8
High dietary intakes of SFA (which account for 60–70% of milk fat) is a risk factor for development of obesity, cardiovascular disease (CVD), impaired insulin sensitivity and the ‘metabolic syndrome’.4 In contrast, dietary intake of certain unsaturated fatty acids, in particular conjugated linoleic acid (CLA) and omega-3 fatty acids (n-3 FA), and fat-soluble antioxidants (e.g., α-tocopherol, carotenoids) has been linked to potential health benefits.3, 9, 10 CLA and n-3 FA have been shown to counteract the negative physiological effects of SFA, and CLA has also been linked to anticancer properties, reduced risk of type 2 diabetes, CVD and enhanced immune function.11–13 However, while CLA isomer C18:2 c9 t11 (CLA9) was only linked to beneficial health impacts, another CLA isomer, C18:2 t10 c12 (CLA10), was also associated with some negative health impacts in cell culture and animal models.13 In studies comparing the impact of different (e.g., organic and conventional) production systems on milk fat composition, it is therefore important to compare concentrations of both CLA isomers. Most previous comparative studies14–16 only reported concentrations of individual isomers or total CLA and also did not report concentrations of vaccinic acid (VA), the precursor for CLA. Milk contains significant concentrations of VA and, since a proportion can be readily converted to CLA9 in the human body, the total potential CLA9 supply can only be estimated if both VA and CLA9 levels are known.17
Previous studies showed that the feeding regime has a major effect on the FA profiles of milk, but that other factors (including breed/genotype, stage and number of lactations) may also influence milk composition.17–19 Dietary unsaturated fatty acids are likely to undergo hydrogenation by rumen microorganisms and long-chain fatty acids may be subjected to desaturase activity in the mammary gland.17–20 The FA profile of milk, therefore, is primarily determined by: (i) the balance of fatty acids in the diet; (ii) the extent of rumen hydrogenation; and (iii) mammary desaturase activity. CLA levels are linked to dietary supply of α-linolenic acid (αLA) and linoleic acid.17 However, while 70–90% of CLA9 (which constitutes > 70% of total CLA in milk) is generated from desaturation of VA in the mammary gland, all other CLA isomers (including CLA10) are generated as intermediates of rumen biohydrogenation and are therefore found at much lower concentrations than CLA9 in milk.17
Fat-soluble antioxidants/vitamins present in milk are derived from dietary sources, either from (i) natural constituents in feedstuff (especially the forage component of the diet)21 or (ii) synthetic compounds added as supplements to the diet of lactating cows.22 Carotenoids derived from fresh forage are dominated by β-carotene, but also include lutein, zeaxantin, cryptoxanthin, lycopene and α-carotene.23 The main vitamin E activity in fresh forage is associated with the RRR isomer of α-tocopherol (the only isomer synthesized by plants), with some activity being associated with β-, γ- and δ -tocopherol and α-, β,- γ- and δ -tocotrianol.24
Most high-input conventional dairy production systems supplement diets with proprietary mineral and/or vitamin products containing A vitamins, vitamin D3 and E vitamins (in particular α-tocopherol); such supplements are prohibited in organic production.25
The naturally occurring RRR isomer of α-tocopherol has a higher vitamin E activity (1.49 IU mg−1) than synthetic vitamin E (1.0 IU mg−1), which contains equal proportions of the eight different stereoisomers of α-tocopherol.24 Synthetic α-tocopherol products are referred to as ‘all rac’ α-tocopherol and consist mainly of 2R stereoisomers. Synthetic α-tocopherol is absorbed with the same efficiency as the RRR stereoisomer of α-tocopherol, but levels of uptake into key tissues (e.g. the brain) are lower.24 Also, a recent study with dairy cows found higher α-tocopherol concentrations in blood and milk following supplementation of RRR compared with ‘all-rac’ α-tocopherol and reported preferential transfer of RRR isomers into milk by cows receiving the synthetic isomer mix.22
Milk and dairy products from certified organic dairy production systems have been reported to contain higher concentrations of polyunsaturated fatty acids (PUFA), αLA (the main n-3 FA in milk), and/or CLA, and fat-soluble antioxidants than those from high-input conventional production.14–16 These studies did not include non-organic, low-input systems in comparisons. However, an increasing number of dairy farms in Europe, New Zealand/Australia and North America are adapting ‘lower-input’ production methods similar to those used in organic farming, but do not comply with all input restrictions prescribed by organic farming standards.26 Most importantly, these systems use mineral NPK fertilizers, but often at reduced levels compared with conventional high-input systems. It is unclear whether such non-organic, low-input systems can provide similar benefits in milk composition to certified, organic dairy production systems.
Milk composition is known to change when switching from outdoor grazing to indoor forage-based diets in winter;6, 12, 20, 27 however, little is known about whether this dietary change affects the differential in milk quality between organic and conventional systems reported previously.14–16 There is also limited information on differences in the composition of fat-soluble antioxidants in milk from high- and low-input dairy systems and the few studies available show contradictory results.14, 28, 29 Such information would, however, be essential to assess (i) the overall nutritional value of milk from low-input systems and (ii) whether the higher unsaturated fat content of organic milk (and associated risk of oxidation and off-flavour development) is compensated for by higher concentrations of antioxidants.
The objectives this study were therefore to: (i) compare the fatty acid and fat-soluble antioxidant composition of milk from three UK production systems—certified-organic ‘low-input’ (O-LI), non-organic certified ‘low-input’ (NO-LI) and standard ‘high-input’ (HI) conventional production systems, during the outdoor grazing period; (ii) quantify differences in fatty acid and fat-soluble antioxidant content of milk between O-LI and HI systems, during the winter indoor (conserved forage-based) feeding period; and (iii) identify whether there are differences in milk composition between certified-organic ‘low-input’ (O-LI) and non-certified ‘low-input’ (NO-LI) systems that use spring block calving systems and graze cows outdoors throughout lactation.