Multiscale structures of lipids in foods as parameters affecting fatty acid bioavailability and lipid metabolism

4. Supramolecular level: the organization of lipids in food products can modulate their metabolism
TAG, the main dietary lipids, can be consumed as visible or as hidden fats. Visible fats are non-emulsified lipids such as oils, adipose tissues, or water-in-oil emulsions such as butter and spreads. Hidden fats are dispersed in the form of droplets of sub-millimeter sizes surrounded by a liquid or semi-liquid, aqueous phase (oil-in-water emulsions) or inserted in a solid phase (encapsulated lipids) [171, 172]. In oil-in-water emulsions the TAG phase is stabilised by surface-active molecules, namely polar lipids, surfactants or proteins, so-called food-grade emulsifiers.

In raw foods, dietary PL and more generally polar lipids are also present in cell membranes (e.g. meat, fish) or at the TAG/water interface of natural assemblies such as oleosomes, lipoproteins of egg yolk and milk fat globules. In processed foods, lecithins of vegetable origin (e.g. soya, rapeseed, sunflower) or animal origin (e.g. egg yolk), and MAG and DAG, possibly after additional treatments (fractionation, hydrolysis, hydrogenation) are widely used in the food industry as stabilizing agents and as emulsifiers. Lecithins from brain, krill or MFGM are other potential sources of lecithins. Indeed the composition in fatty acid and polar lipid classes of these lecithins varies in large proportions according to their origin and the production process. These polar lipids adsorb at the surface of the TAG droplets, making them less sensitive to destabilisation phenomena [173]. They are also able to interact with othe components of the food matrix shuch as proteins and polysaccharides (i.e. starch). Additionally, amphiphilic lipids such as phospho- and glycolipids, sphingolipids, MAG and DAG and non-esterified FA, organize in various lipid structures such as micelles, vesicles, liposomes, etc when dispersed in an aqueous medium [174-176]. Liposomes of PL are used for vectorisation of therapeutic molecules for oral and parenteral applications [177, 178].

4.1 Native and recomposed supramolecular structures of lipids in raw and processed foods
4.1.1. Organization of milk lipids

The organisation of milk lipids has been widely studied in milk and various dairy products for the last 10 years. Many dairy products are oil-in-water emulsions in which TAG are dispersed in an aqueous liquid phase (milk, cream), in a partially gelled phase (yoghurt, cheeses) or in a dry medium rich in proteins (powders). In milk, the size distribution of the milk fat globules ranges from 0.1 to about 15 µm, with a mean diameter around 4 µm [179-181]. Fractions of milk fat globules with various sizes can be obtained from the native milk with processes such as centrifugation [182] and, more selectively, with cross-flow microfiltration [181]. The milk fat globules are covered by MFGM, a biological membrane, composed by three layers of polar lipids embedding cholesterol, proteins, glycoproteins, enzymes, vitamins and other minor components [183]. In this membrane, sphingomyelin laterally segregates in liquid-ordered domains surrounded by a matrix of glycerophospholipids (PE, PC, PI, PS) in the liquid-disordered phase [146, 184, 185].

During milk processing the structure of milk fat globules is highly altered by mechanical and thermal treatments [186], but also by biochemical (enzymatic) changes as reviewed in [187]. Raw milk is first cooled, then generally partially or totally skimmed, homogenised and then pasteurised or sterilised. Homogenisation of milk induces a huge decrease in the size of milk fat globules from 4 to 0.5 µm or less, depending on the pressure applied (from 50 x 10⁶ to 300 x 10⁶ Pa) [186]. It therefore induces an increase in the surface area of the fat globules. As the excess of water/milk fat interface cannot be covered by the MFGM components, it gets covered by other surface-active molecules present in milk, namely casein micelles and whey proteins. Additionally, some fragments of the MFGM can be scraped from the interface and form vesicles in the aqueous phase, while the smallest milk fat globules may not be affected by homogenisation [83, 188]. During the thermal treatment of milk, whey proteins also interact and aggregate with the MFGM proteins and with the casein micelles adsorbed at the surface of fat globules [189].

Confocal microscopy reveals in-situ the organisation of lipids in milk and dairy products [184, 190]. In cheeses, milk fat is either (i) dispersed as native milk fat globules (soft cheeses), (ii) present as fat globules more or less aggregated or coalesced with reorganisations in the MFGM, (iii) dispersed as small fat globules covered by proteins after high shear stress homogenisation (blue cheeses, some fresh cheeses), or iv) in the form of free fat domains covered by milk polar lipids (hard-type cheeses) [96, 98, 191, 192]. In dairy powders, lipids are dispersed as droplets or present as free fat [193, 194]. In butter, partially crystallised TAG forms the continuous phase, in which water droplets are dispersed, forming a water-in-oil emulsion [195]. In whipped creams and ice-cream, TAG are present at the gas/water interface and participate in foam stabilisation [88].