Influence of dietary factors on the macro and micro-composition of bovine milk for use in protein ingredient powder manufacture

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Magan, Jonathan B.
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University College Cork
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The increasing frequency of “grass-fed” labelling claims in dairy product marketing, predicated in part on improved product quality and nutritional benefits associated therewith, has necessitated both rigorous experimental data to determine the relative differences in the composition of milk from grass-fed and non-grass-fed cows and robust methods for verifying these labelling claims. The “Grass-fed Standard” established by Bord Bia in 2020 is the first scientifically-backed standard in use internationally, which is founded on the somewhat unique pasture-based dairy production system practiced in Ireland. The compositional and functional benefits associated with products derived from pasture-fed cows have thus far been focused on business-to-consumer products. The effect of pasture-based production relative to the conventional indoor production system utilised most widely throughout the world on the composition and functional properties of “business-to-business” milk powder products, which represent a major aspect of Irish dairy production, remained to be established. Variation in the composition and functional properties of dairy products arises due to numerous factors, including feed composition, which can differ depending on the type of production system practiced. Therefore, the objective of this research was to investigate the effect of perennial ryegrass only (GRS), perennial ryegrass/white clover pasture (CLV) and indoor total mixed ration (TMR) feeding systems on the composition and functional properties of bovine milk-based protein ingredients. There was a significant effect of feeding system on the concentration of metabolite compounds and B vitamins in skim milk powder (SMP) and whey ingredients, which were distinguishable by liquid chromatography-mass spectrometry. Concentrations of vitamins B1, B2 and B7 were significantly higher in GRS and CLV samples than TMR samples, for which concentrations of vitamin B3 and B3-amide were higher than in CLV and GRS samples, respectively. Gross compositional parameters were similar between the feeding systems; however, the CLV and TMR systems were associated with high concentrations of non-protein nitrogen and high heat stability in whole milk powder and SMP samples. Some differences in total amino acid (AA) composition were also exhibited between WMP and SMP samples from each diet, which suggests that AA profile may be responsive to dietary variation. Yoghurts produced from GRS and CLV-derived WMP were typified by significantly higher gel strength and textural firmness than those derived from TMR, despite significantly higher concentrations of palmitic acid in TMR samples. Fatty acid (FA) profiles of WMP samples from each diet were similar to those in the literature produced from pasture or concentrate-based systems. The level of FA unsaturation and carotenoid content could also distinguish between pasture and TMR samples using Raman spectroscopy. The mineral composition of WMP, SMP and whey protein concentrate (WPC) samples from each diet did not vary significantly, with the exception of selenium and iodine, which were consistently, and significantly, higher in TMR samples than GRS and CLV samples. The viscosity of GRS-derived skim milk concentrate was significantly higher than that for TMR, but the effect of diet on the heat-dependent viscosity of dispersions of skim milk with WPC was not as substantial as the differences between WPC types, with micellar casein whey being most stable on heating, and acid WPC the least stable. Proteomic and peptidomic analysis of WMP and digested WMP samples determined qualitative and quantitative differences in proteins and peptides arising from each diet. In conclusion, this research demonstrated that the influence of bovine diet on milk components is not limited to gross compositional factors, but the micro-composition and functional properties, such as heat stability and acid-induced gelation, of value-added dairy products can also be influenced by feeding practices. This research will be of most interest to the manufacturers of dairy powder ingredients, such as SMP and WPC and premium WMP products in establishing the commercial point-of-difference for pasture or concentrate-derived products along with the unique compositional elements and functional behaviour associated with products from the three feeding systems investigated herein. Furthermore, this research will aid in ingredient selection for manufacturers of value-added dairy commodities, such as infant milk formula. Finally, this research contributes to the list of compositional variables which demonstrate potential for differentiation between feeding systems, providing preliminary information, relating to the establishment of robust analytical methods for verification of “Grass-fed” labelling claims for milk powder products to international entities responsible for policy implementation, such as Bord Bia.
Bovine diet , Pasture , Total mixed ration , Milk composition , Protein , Powder
Magan, J. B. 2021. Influence of dietary factors on the macro and micro-composition of bovine milk for use in protein ingredient powder manufacture. PhD Thesis, University College Cork.