Lipophilic bioactives from natural sources, including phytosterols, antimicrobial, antioxidants, ω-3 fatty acids, flavours and numerous other components, are widely utilised as functional ingredients in food industry. However, most of these bioactives are almost insoluble in water and unstable at a specific environmental stimulus such as light, oxygen and temperature during manufacture, storage, transport and utilisation, for example, chilling, freezing, dehydration, thermal processing or mechanical agitation. From this viewpoint, there are technical challenges that need to be overcome to impart functionality for the human body due to the lack of solubilisation, stability and bioavailability of the lipophilic bioactives. This results in unsuccessful quality enhancement and commercialisation [1].

Oil-in-water (O/W) emulsions have been used as vehicles for the delivery of lipophilic bioactives in the food and drug industry. However, food emulsions are thermodynamically unstable systems and will eventually break down due to the increase in interfacial area after emulsification. Consequently, the physical instability of the emulsion occurs, such as creaming, flocculation, coalescence, phase inversion and Ostwald ripening. Recently, the novel fabrication methods for long term stability of O/W emulsions have been investigated at the Korea Food Research Institute. The bioactives are encapsulated in nanometre-sized structures, which are assembled with both core materials and biopolymers, and transparent or translucent systems mostly covering the size range 20-200 nm, called nanoemulsions.

Nanotechnology has been defined as dealing with the materials, systems and processes at a scale of 1-100 nm. Nanomaterials, such as nanoparticles, nanoemulsions, nanocomposites and nanostructured materials, may generally be prepared by using nanotechnology, including nanoemulsification, association colloid and nanostructured multiple or multilayer emulsification [2]. Food nanoemulsions can offer several advantages over traditional delivery systems for nutraceuticals or functional ingredients, which include protection of the bioactive and an increase in its solubility, stability and bioavailability.

Although the interests in food nanotechnology and its research and development are currently at an initial stage, the food industry is slowly embracing it and preparing for a final goal, its commercialisation. It is considered that the field of food nanomaterials is a crucial means for commercialisation of functional foods. In particular, fabrication technologies for nanoemulsions and nanoparticles are considered essential to lift the previously mentioned drawbacks of bioactive substances and produce new functional foods. This article reviews the fabrication methods of food nanoemulsions for food delivering bioactives, their characterisation and their application in functional and nutraceutical foods.

Bioactives

Functional foods are typically rich in phytochemicals, which are derived from plant products, fruits and vegetables. Bioactive phytochemicals, known as functional foods or nutraceuticals, providing health or medical benefits to humans including the prevention and treatment of disease, have gained much attention in the last decade. The most important bioactives that need to be processed and provided are briefly discussed below.

Phytosterols
Phytosterols are 28- and 29-carbon compounds including brassicasterol, campesterol, stigmasterol, β-sisterol, fucosterol, δ-avennasterol, and α-spinnasterol. Since phytosterols are not synthesised in the human body, typical phytosterol consumption is in the range of 200-400 mg/day [1]. Phytosterols are important inhibitors of cholesterol absorption, by interfering with cholesterol synthesis and enhancing cholesterol excretion. Incorporation of phytosterols into foods is difficult due to their high melting point and tendency to form insoluble crystals. For this reason, when phytosterols are to be applied to aqueous-based foods, they need to be either suspended or emulsified [3, 4]. Increasing the solubility of phytosterols in O/W nanoemulsions can enhance their bioavailability and absorption due to the droplet size which is below in the range of several nanometres.

Carotenoids
Carotenoids are mainly produced by photosynthetic plants, algae, bacteria and some fungi, and contribute to the yellow to red colours of many foods. The important biological activities of carotenoids, including the scavengers of active oxygen species and antioxidative activity have been attributed to their ability to decrease the risks of cancer, coronary heart disease, macular degeneration and cataracts [5]. Carotenoids containing oxygen are known as xanthophylls (e.g. lutein, zeaxanthin) while those without oxygen are known as carotenes (e.g. lycopene, β-carotene). However, these have several limitations for use in food systems, such as lipophilicity and the structure of carotenoids, which affects their distribution in the cellular system in vivo, and high melting points, making them crystalline at food storage and body temperatures. Lycopene is an acyclic open-chain unsaturated carotenoid found in red tomatoes, water melon and their processed products. It is fat soluble, so absorption is improved when oil is added to the diet, causing much of the ingested lycopene to pass through the body.