果胶对脂类和类胡萝卜素消化利用影响研究进展

果胶已经被证实可以影响脂类的消化,脂溶性的类胡萝卜素在消化阶段需要被脂滴包裹才能进入小肠形成胶束,因此果胶对类胡萝卜素的消化利用也会存在潜在影响。该文综述了近年来果胶对脂类和类胡萝卜素消化利用影响研究进展,主要分为果胶对消化液黏度的影响、对消化酶的影响、与钙离子的相互作用、与胆盐的结合作用以及对脂滴的包裹作用这5个方面。该文为后续分析如何提高果蔬中类胡萝卜素生物利用度提供理论依据。

Review on effects of pectin on digestion of lipid and carotenoids

Pectin is a family of galacturonic acid-rich polysaccharides, which mostly exists in plant cell walls. Pectin structure determines its properties, and the structure characteristics usually refer to the molecular weight, degree of methoxylation and acetylation, galacturonic acid content, neutral sugar composition. The texture and rheological properties of raw fruit and vegetable and their products are dependent on the structure of pectin. Carotenoids are lipophilic pigments responsible for the yellow, orange, and red colors of many fruits and vegetables, which have beneficial health effects. Carotenoid bioavailability is usually considered as the fraction of the ingested carotenoids that are accessible for utilization in normal physiological functions or for storage in the human body. A prerequisite for carotenoid bioavailability is its bioaccessibility in the small intestine. Carotenoid bioaccessibility is defined as the amount of carotenoids that are released from its food matrix during digestion and made available for absorption into intestinal mucosa. A number of studies suggested that certain types of dietary fiber such as pectin could inhibit the digestion and absorption of lipids. Therefore, pectin has a potential impact on the digestion and absorption of carotenoids, since carotenoids might be encapsulated in lipid droplet in stomach phase. The present review summarized recent studies about the effects of pectin on lipid digestion and carotenoid bioavailability in order to provide theoretical basis for further improving the bioavailability of carotenoids in fruit and vegetable-based food products. Additionally, future research challenges in this review are identified. Structure and properties of pectin were summarized at first, and the specific processes of lipid digestion and carotenoid absorption were also elucidated. On one hand, pectin is an important component of plant cell wall, and the presence of cell wall restricts the release of carotenoids from the matrix. On the other hand, pectin may interfere with the the processes of lipid digestion and carotenoid absorption in a variety of different ways, and these could be summarized as 5 aspects. 1) Pectin could change the viscosity of the digestive juice, which would alter the magnitude of the shear forces operating on the chyme, increase the duration in the stomach and small intestinal phase and decrease the transport efficiency. 2) Pectin could act as a physical barrier between substrates and digestive enzymes. Pectin could compete with the substrate for the active site of the enzyme, protonate the enzyme active site through the participation of the carboxylic acid residues and generate direct molecular interactions between pectin and enzymes. 3) Galacturonic and galuronic acid residues of pectin could form gels with calcium ions. These pectin-calcium complexes reduce lipid digestion by reducing the surface area of the lipid droplets where lipase exerts its activity as a consequence of lipid flocculation or microgel formation. Besides gel formation, the binding between calcium ions and pectin might also affect carotenoid absorption in another way. The levels of free calcium in the gastrointestinal medium can be reduced in the presence of pectin, causing accumulation of free fatty acids on the lipid droplet surfaces and reducing lipid droplet digestion. 4) Pectin may bind bile salts in the small intestine, and affect the lipid digestion and the efficiency of carotenoid incorporation into mixed micelles. 5) Pectin may be adsorbed to the surfaces of the emulsified lipids and form a protective coating, which may prevent the lipase from being adsorbed to the droplet surfaces and getting access to the lipids inside the droplets. The effects of pectin on the digestive pathways and digestive efficiency of lipid and carotenoids depend on pectin structure, especially molecular mass, degree of methylesterification and acetylation, and neutral sugars composition. At present, most studies used emulsion system containing carotenoids, and added exogenous pectin to explore the influence of pectin on digestion and absorption of carotenoids. Thus, the results they found in the simulation system might be different in fruit and vegetable-based food products, and the effect of endogenous pectin on the bioavailability of carotenoids in the fruits and vegetables needs further verification. In addition, the effect of pectin on the absorption of lipid and carotenoids is also manifold. Pectin may inhibit digestion and absorption of lipid and carotenoids through the above 5 ways and also form a coating around the carotenoids and thus increase the stability of carotenoids during the processing, storage and further digestion. For fruit and vegetable materials, processing method is the important factor to determine its physicochemical properties, and some crushing and refining treatments, such as colloid mill and high pressure homogenization, can improve the release rate and pathway of carotenoids from the raw materials. Thus, it is worthy to be studied whether these crushing and refining treatments can improve the structural characteristics of pectin and make it play a positive role in carotenoid bioavailability.