Bioaccessibility of lipids from dairy products: the cheese matrix

牛奶和乳制品对健康和慢性疾病，特别是心血管疾病（CVD）的危险因素有不利影响的观念正在改变，这主要是由于长期前瞻性队列研究的结果表明，与低乳制品消费者相比，高乳制品消费者的CVD死亡率和风险标志物显著降低。来自随机对照试验的证据表明，不同的乳制品可以对心血管疾病风险产生不同的影响，奶酪形式的乳制品脂肪比黄油形式的乳制品脂肪产生更低的血浆总浓度和低密度脂蛋白胆固醇浓度。这支持了来自动物和人类研究的证据，即奶酪基质的物理化学特性可以最大限度地减少消化系统对奶酪脂质的吸收。据认为，这可能部分与胃后期脂肪中钙皂的形成有关，这些脂肪不容易被吸收。最近的一项体外研究得出结论，固体乳制品基质比液体产品更容易形成钙（Ca）肥皂。然而，目前尚不清楚不同类型的奶酪是否也存在这种情况，因为它们的生产方法各不相同。尽管像切达奶酪这样的硬奶酪仍然受到消费者的欢迎，但近年来，软奶酪的消费量有所增加。此外，奶酪脂肪酸（FA）谱/Ca含量/Ca形态可能对这种基质效应有影响；与不饱和FA相比，饱和FA更容易形成钙皂。这个项目将揭开“奶酪矩阵”效应的奥秘。这将通过使用技术组合（一些新颖的）来实现，以建立奶酪脂质与其他脂质来源相比似乎不容易吸收的机制。首先，该项目将采用萨里大学建立的体外半动态胃/小肠消化模型。然后，雷丁大学的食品加工中心将使用该大学乳制品研究中心的奶牛的牛奶，准备一系列不同类型的奶酪。这些类型在物理（硬度）和化学（钙含量，pH值，FA谱）结构方面会有所不同，以便产生一系列可能影响消化的特性。奶酪将根据不同的奶酪类型进行成熟和储存，并对不同成熟阶段的样品进行分析。不同的显微镜技术（环境扫描电子显微镜，共聚焦显微镜），将被用来评估奶酪的物理结构。在此之后，奶酪将在体外模型中进行消化。来自模型和奶酪的消化样品将在萨里大学进行成分分析（包括Ca皂的存在，脂肪酸谱），并使用标记的脂肪酶来可视化消化过程中酶/底物的接近程度。这个项目为基于“食物链”的博士项目提供了一个独特的机会。它不仅包含了食物链的所有方面（从初级生产，通过加工，到体外消化，然后通过与工业联系的商业方面），而且还包括了以前从未在乳制品中使用过的新颖方法的组合，例如使用标记脂肪酶。该项目将引起乳制品加工者的极大兴趣，并且通过充分了解奶酪基质对脂质生物可及性的影响，它将对所有食物基质中脂质消化的知识做出重大贡献。

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.