7. Food applications of OBs
OBs as natural emulsions have shown prospects in the preparation of liquid or semi-liquid food products such as milk, yogurt, mayonnaise and salad dressings (Nikiforidis et al., 2012).
Consumer demand for cow’s milk alternatives has increased as a result of lactose intolerance of some people and the demand for vegetarianism and health (Aydar, Tutuncu & Ozcelik, 2020). Plant-based milk substitute (PBMS) is often perceived as healthy possibly due to they contain dietary fiber, vitamins, minerals, and antioxidants (Jeske, Zannini & Arendt, 2018). Generally, PBMS is emulsified systems composed of OBs, solid particles and starch granules all dispersed in an aqueous phase and the presence of water-soluble proteins on the surface of the OBs (e.g. oleosins, caleosins, and steroleosins) improves the texture, stability and nutrition of PBMS (Huang, 2018; Zaaboul, Raza, Cao & Yuanfa, 2019). Ultrafiltration is commonly used to produce an OBs-based emulsion with a lipid content similar to cow milk and rich in polyunsaturated and monounsaturated fatty acids to obtain PBMS and the PBMS can maintain its aroma characteristics and bioactive constituents upon heating and exhibit appreciable microbial, physical and oxidative storage stability (Naziri, Koupantsis, Mantzouridou, Paraskevopoulou, Tsimidou & Kiosseoglou, 2017). Shakerardekani, Karim & Vaseli (2013) prepared pistachio milk by using pistachio OB as a natural pre-emulsified emulsion and adding 5.0% sugar, 0.02% vanilla and 0.0% salt.
Fat globules play an important role in the production of yogurt. The acidification of milk induces the formation of continuous gel networks of aggregated protein molecules where fat globules are incorporated (Horne, 1999). The strength of the gel depends on the size of the fat globules and the extent of their surface interactions with the gel network (Kirimlidou et al., 2017). The physical properties and microstructure of the gel are reflected in the texture and sensory characteristics of yogurt (Öztürk, Aydın, Sözeri, Demirci, Sert & Akın, 2018). Gallier et al. (2012) studied the differences in chemical, physical and structural properties between the almond OBs, and bovine milk fat globules. It was found that almond OBs contained mostly long-chain unsaturated fatty acids, phytosterols and nosphingomyelin. Bovine milk fat globules were rich in saturated fatty acids and cholesterol, larger in size, and richer in sphingomyelin and phosphatidylethanolamine. The monolayer membrane around almond OBs and the trilayer membrane around bovine fat globules might affect the stability of the lipid droplets in a food matrix and the way the lipids were digested. Mantzouridou, Naziri, Kyriakidou, Paraskevopoulou, Tsimidou & Kiosseoglou (2019) replaced the cow milk fat globules in yogurt with maize germ OBs, which had a good acidification kinetic pattern, forming a liquid structure acidic gel and the quality and stability of yogurt improved compared with that prepared with whole cow milk. Romero-Guzmán, Köllmann, Zhang, Boom & Nikiforidis (2020) obtained a plant-based mayonnaise by controlling the conditions of the aqueous extraction process of OBs and co-extracted proteins and soluble fibers.
8. Conclusion and future perspectives
At present, the basic structure of OBs has been determined. The unique conformation of the OBs membranes makes the OBs highly stable. This special structure can be used as reference for the development of synthetic oil droplets such as interface stabilizers for Pickering emulsions. It is necessary to study the interaction and structural dynamics between phospholipid and protein molecules, and investigate the mechanical properties of the OBs membranes to provide the OBs membranes with stability and stretchability. In order to promote large-scale extraction of OBs, it is necessary to design a process with low cost, easy to be sustained, high yield and complete structure of OBs. There has been a tendency to develop alternative extraction processes including enzyme assisted extraction and supercritical fluid extraction. Complete OBs extraction can be realized under the condition of ensuring that the OBs membrane is not destroyed by screening specific enzyme and adjusting operating parameters. At the same time, high pressure, ultrasonic, microwave, steam flash and other auxiliary processing technology can be developed to achieve efficient extraction. The application of OBs dispersion as a naturally pre-emulsified oil-in-water emulsion is a research hotspot. It can be considered as an alternative to animal fat in the development of healthier meat products to reduce fat percentage and improve fatty acid profile. However, its potential allergenic behavior and toxicity cannot be ignored if put into actual production and in vitro and in vivo studies are needed to take care of the associated side effects. It is also worth exploring the important functions of individual OBs components.