The impact of protein polymorphism, bovine feeding strategy and consumption temperature on the in vitro digestion behaviour of selected dairy products
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Date
2025
Authors
Fitzpatrick, Conor
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Publisher
University College Cork
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Abstract
This work was carried out as part of a wider multidisciplinary research project within the Vistamilk Science Foundation Ireland (SFI) research centre entitled ‘Targeted Project 8 - Digestive characteristics of dairy products. The overall aim of Target project 8 was to use in vitro, ex vivo and animal models to further the understanding of the digestion of dairy products. This research has fulfilled the goal of improving our knowledge of both in vitro digestion and dairy product digestion and has the potential to be of interest and benefit to consumers, industry and future researchers in the field of digestion.
The effect of the genetic profile of milk proteins on dairy product functionality is increasingly being studied due to implications for dairy processing and human health. The work presented in Chapter 2 of this thesis showed a significant effect of β-lactoglobulin genetic polymorphism on both the rheological properties, and subsequent digestion of skim milk yogurt. Milk that contained exclusively β-lactoglobulin B (BB) was found to form a stronger gel with lower levels of syneresis compared to milk with both β-lactoglobulin A and β-lactoglobulin B (AB) and exclusively β-lactoglobulin A (AA). This led to differences in digestion, where AA, with its looser gel compared to BB, showed faster hydrolysis (20% higher proteolysis at 60 min), while BB, with its dense curd was more resistant to gastric digestion than AA.
While protein polymorphisms affected the digestion of milk proteins in yogurt, bovine feeding strategy affected the digestion of lipids in milk. It is well known that cows fed a pasture-based diet (GRS) produce milk with distinct fatty acid profile to cows fed indoors on a total mixed ration diet (TMR), however, the release of these fatty acids during digestion had not been studied. This research showed that GRS milk released higher levels of unsaturated fatty acids (UFAs) during digestion compared to TMR milk. TMR milk released higher levels of saturated fatty acids during digestion with the lipid release profile yielding a higher Artherogenicity index during digestion.
The temperature at which milk is consumed also affects its digestion. By using MRI as a non-invasive technique, this work showed that hot milk coagulates faster in the gastric phase of in vitro digestion compared to cold milk. Cold milk (4°C) delayed coagulation by over 5 min compared to hot milk (60°C) and formed a lipid-rich cream layer at the top of the digesta, which was absent during gastric digestion of hot milk.
Finally, this research addressed a gap in in vitro digestion studies by developing a predictive model of pepsin activity accounting for the effect of both pH (1–6) and temperature (4–60°C) variations. This model was developed for both human pepsin and its porcine counterpart (commonly used as a substitute in vitro) based on measurements of each enzyme’s activity at 37 pH/temperature combinations. It was then integrated into an excel spreadsheet to allow users of in vitro digestion methods to better understand their protein hydrolysis results. This tool also helps to translate in vitro digestion results using porcine pepsin to the in vivo situation with human pepsin.
The protein polymorphism findings (Chapter 2) show the importance of minor changes in amino acid sequence of proteins on both the functionality and digestion of dairy products, while the bovine feeding strategy findings (Chapter 3) reinforce the benefits of high vs low pasture allowance in bovine feeding systems for human health. The temperature-related findings (Chapter 4) present a discussion on what temperature milk should be consumed at to potentially alter the kinetics of nutrients reaching the bloodstream. Finally (Chapter 5), this work has provided a tool for future in vitro digestion research that can be used to predict porcine and human pepsin activity during gastric digestion. This thesis therefore presents several novel research findings that will be of interest to researchers working both in dairy product production and dairy product digestion.
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Keywords
Digestion , In vitro , Milk , Dairy , Protein , Lipid , Pepsin
Citation
Fitzpatrick, C. J. 2025. The impact of protein polymorphism, bovine feeding strategy and consumption temperature on the in vitro digestion behaviour of selected dairy products. PhD Thesis, University College Cork.