Bioaccessibility of lipids from dairy products: the cheese matrix

The perception that milk and dairy products have adverse effects on health and risk factors for chronic diseases, especially cardiovascular diseases (CVD) is changing, mainly due to the outcome of long-term prospective cohort studies that have demonstrated significantly reduced CVD mortality and markers of risk in high, compared with low dairy consumers. Evidence from randomised control trials suggests that different dairy products can exert contrasting effects on CVD risk, with dairy fat in the form of cheese resulting in lower plasma total and LDL-cholesterol concentrations than dairy fat in the form of butter. This supports evidence from animal and human studies that the physico-chemical properties of the cheese matrix may minimise cheese lipid absorption from the digestive tract. It is thought this may partially be related to the formation of calcium soaps of fats in the post-gastric phase which are not as easily absorbed. A recent in vitro study concluded that more solid dairy matrices form calcium (Ca) soaps more readily than more liquid products. However it is not known whether this is observed with different cheese types, which have varying production methods. Although hard cheeses such as Cheddar remain popular with consumers, there has been increased consumption of softer cheeses over recent years. Also, it is possible that the cheese fatty acid (FA) profile/Ca content/Ca form may have an impact upon this matrix effect; Ca soaps are more readily formed with saturated compared with unsaturated FA.

This project will unravel the mystery of the "cheese matrix" effect. This will be achieved by using a combination of techniques (some novel), to establish the mechanism behind why cheese lipid appears to not be as accessible for absorption compared with other lipid sources. Firstly the project will adapt an established in vitro semi-dynamic gastric/small intestinal digestion model at the University of Surrey. Then a range of different cheese types will be prepared at the University of Reading's Food Processing Centre, using milk from cows at the University's Centre for Dairy Research. These types will vary in terms of physical (hardness) and chemical (Ca content, pH, FA profile) structure in order to create a range of properties which might impact upon digestion. Cheese will be ripened and stored as appropriate for cheese type, with samples at different maturity stages also analysed. Different microscopy techniques (environmental scanning electron microscopy, confocal microscopy), will be employed to assess the physical structure of the cheese. Following this the cheeses will be subjected to digestion within the in vitro model. Samples of digesta from the model and cheese will be analysed for composition (including presence of Ca soaps, fatty acid profile) at the University of Surrey, and labelled lipases will be used in order to visualize enzyme/substrate proximity during digestion.

This project offers a unique opportunity for a "food chain"-based PhD project. Not only does it incorporate all aspects of the food chain (from primary production, through processing, to in vitro digestion and then commercial aspects through links with industry) but it includes a combination of methodologies which are novel and have not been used with dairy products before, such as the use of labelled lipases. This project will be of great interest to dairy processors, and by fully understanding the effect of the cheese matrix on lipid bioaccessibility, it will contribute significantly to knowledge of lipid digestion within all food matrices.