The Use of Nanotechnology in Functional Food Product Development
Challenges While Working With Bioactives
- Fig. 1: Phase diagram of formation capsaicin-loaded nanoemulsions, and its samples and transmission electron microscopy.
- Fig. 2: Transmission electron microscopy of capsaicin-loaded nanoemulsions. SN, single-layer nanoemulsion formulated on aqueous phase; CN, double-layers nanoemulsion based on chitosan; AN, double-layer nanoemulsion formulated on alginate; CAN, triple-layers nanoemulsion formulated on chitosan/alginate.
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 .
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 . 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.
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 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 . 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 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 . 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.
Isomerisation reactions of lycopene under the influence of excess heat and light may be beneficial, because the presence of lipids together might have improved lycopene bioavailability and stability . In order to stabilise lycopene and other carotenoids in the products during processing and storage, the development of techniques or suitable processing condition choices are important issues for process optimisation. Recently, with respect to the enhancement of lycopene bioavailability in tomato products, the effects of thermal- and light-irradiation processing on its stability in oil-based food model systems have been reported .
Coenzyme Q10 (CoQ10) is a lipophilic compound, vitamin-like nutrient for every organ in the body. CoQ10 is essential for cell respiration and electron transfer, and it functions as an antioxidant, scavenging free radicals and inhibiting lipid peroxidation . Since CoQ10 is insoluble in water, the efficiency of absorption and bioavailability from food and supplements is poor. Recently, different approaches for the improvement of the solubility, absorption and bioavailability of CoQ10 have been tried, including molecular complexation, emulsification and liposomal systems [9-11].
Astaxanthin is a fat-soluble xanthophyll, containing 13 conjugated double bonds and its two functional groups, ketonic and hydroxilic groups. Astaxanthin is responsible for the red-orange colour of some marine and aquatic animals such as salmon, rainbow trout, shellfish, snow crab and shrimp . Currently, several commercial products of astaxanthin are available in the market, being promoted as anticancer and anti-inflammatory functional foods as well as immunostimulants . It is a highly unsaturated molecule and thus can be degraded by high temperature, light and oxidative conditions during the manufacturing and storage of foods. The bioavailability of astaxanthin is also very poor, because of its physical form as crystals. One way to improve its absorption and stability are to prepare the O/W nanoemulsions containing astaxanthin by using various techniques such as solubilisation, emulsification and encapsulation.
Omega-3 Polyunsaturated Fatty Acids
Omega-3 polyunsaturated fatty acids (ω-3 PUFA) are an essential part of the diet since humans cannot synthesis these fatty acids. The most common ω-3 PUFA are a-linolenic acid, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Health benefits of ω-3 PUFA, particularly EPA and DHA, have been widely attributed to their ability to reduce the risks of cardiovascular disease, diseases affected by immune response disorders, and mental disorders [14-16]. However, ω-3 PUFA are very oxidatively unstable, especially DHA, which has been estimated to be over 50 times more susceptible to oxidation than oleic acid , and they can be detected at very early stages of oxidation. In order to increase the stability of ω-3 PUFA, oxidative protection must be provided by both the wall material and the emulsifiers coating the oil. This can be accomplished by encapsulation.
Nanoemulsions and Nanoparticles
One of the promising nanotechnologies is the nanometre-sized emulsion delivery system, which offers the potential to improve the solubility and bioavailability of many bioactive ingredients such as carotenoids, polyunsaturated fatty acids, phytosterols, antioxidants, vitamins, minerals and other natural compounds. A few publications explore this technology in the preparation of carotenoid emulsions. Some examples of the nanoemulsions that were prepared by ultrasonication and plotted on the ternary phase diagram are indicated in figure 1 [18, 19]. One phase clear regions are shown by the curved black line. These are in nanometre-sized structures mostly covering the size range 45-600 nm, and these nanoemulsions exhibited a different optical transparency such as red, reddish-yellow and yellow due to their characteristic size. Energy input generated from mechanical devices or chemical potential of the components is basically required in nanoemulsion formation. Therefore, the method of preparation plays an important role. High-energy emulsification methods, such as high-shear stirring, high-pressure homogenisation and ultrasonic homogenisation can be used to prepare nanoemulsions . High-pressure homogenisation has been widely used as an emulsifying apparatus which works in a range of 50-350 MPa depending on the application of emulsion products. Ultrasonic emulsification is an effective tool for making nanoemulsions by using different types of ultrasound devices. Cavitation, which is the formation and collapse of vapour cavities in a flowing liquid due to the change of local velocity, is the main phenomenon responsible for ultrasonically induced effects . Further, given the nature of the nanometre-sized emulsions, low-energy emulsification methods can be used when phase behaviour allows the formation of nanometre-sized droplets. These low energy techniques include self-assembly, coacervation and phase inversion temperature methods [22-25]. Nanoemulsification can be prepared by self-assembly, which can be used to introduce guest molecules of hydrophilic, lipophilic or amphiphilic nature. Very few examples exist of nanoemulsion systems based on food-grade surfactants such as ethoxylated sorbitan esters (Tweens) and sucrose esters. Tweens are especially attractive, commercially inexpensive surfactants used in food, cosmetic and pharmaceutical applications.
Simple solutions or nanoemulsions as food delivery systems may be unstable or degradable due to the change of nature of the emulsion structure and environmental conditions such as temperature, light and pH. It is possible to develop nanoparticles as novel delivery systems using food-grade biopolymers such as polysaccharides (alginate, chitosan, pectin, gum arabic) and proteins (casein, gelatin, whey) by promoting self-association or aggregation of single biopolymers or by inducing phase separation in mixed biopolymer systems . Solid lipid particle (SLP) can be fabricated by the homogenation of food lipids and water phase together with hydrophilic surfactants, followed by controlled, rapid cooling to induce crystallisation . It is possible to create a variety of different structures within SLP (e.g., homogeneous, core-shell or crystal dispersion), and to control the relative location of the different phases within the droplets (e.g., the core could be solid, the shell liquid or vice versa). Transmission electron microscopy (TEM) images of capsaicin-loaded nanoemulsions prepared by self-assembly are shown figure 2 . Various types of nanoemulsions including single-layer, double-layer and triple-layer nanoemulsions could be produced depending on the polyelectrolytes of the biopolymer. The hydrophilic portions of the droplet are stained black while the hydrophobic components are unstained with a size ranging between 25 and 70 nm. Single-layer nanoemulsions (SN) are not uniform in shape and are partially bound between large and small emulsion drops. However, double or triple-layer nanoemulsions (CN, AN, CAN) are trapped with charged polysaccharides as alginate, chitosan and both of them, respectively. CA, AN and CAN showed a more compact and clear particle, which may be due to the flocculation and coalescence caused by phase separation or stabilisation of the nanoemulsion [28, 29]. Nanoparticles can be prepared by the dispersion of preformed polymers in nanoemulsions using techniques such as salting-out, emulsification-diffusion method and nano-precipitation.
Characterisation and Stability of Nanoemulsions
It is important to understand the formation and functional properties of nanoemulsions in order for the nanoemulsion-based food products to be created by the food industry. Generally, the phase diagram of a ternary system comprising surfactant, oil and polar solvent can be used for the determination of nanoemulsion characterisation, with the three components as the apexes as indicated in figure 1. In the case of a four or more components system, a pseudo-ternary phase diagram is used with two surfactants, oil and polar solvent, as the four apexes. The stability of nanoemulsions depends strongly on the environmental stresses that may occur during manufacture, storage, transport and utilisation, for example, chilling, freezing, dehydration, thermal processing and mechanical agitation. Capsaicin-loaded nanoemulsions (CLN) prepared with a complexation of biopolymers such as chitosan and alginate were produced by self-assembly with a particle size of 20 nm or lower. CLN with chitosan/alginate triple-layers exhibited a good thermodynamic stability between -21°C and 25°C. In the re-dispersity test of CLN, the double- and triple-layer nanoparticles formed with alginate and chitosan showed a higher stability than the single-layer nanoemulsion. CLN with alginate or chitosan double-layer exhibited a higher loading efficiency of capsaicinoids than the single nanoemulsion and alginate/chitosan triple-layer CLN. Chitosan double-layer CLN had the highest stability in in vitro release at buffer pH 2 and 7.4 .
Resveratrol nanoemulsions could be obtained with particle sizes of 10-800 nm depending on the emulsification parameters such as the content of the aqueous phase and the ratio of surfactant. The double-layer resveratrol nanoemulsions incorporated with cyclodextrin might be expected to be a good carrier material for bioactive ingredients, providing a good stability such as particle size, z-potential value and oxidation .
Coenzyme Q10 (CoQ10) was solubilised using various oils (soybean, corn, peanut, sunflower and olive), medium chain triglycerides (MCT) and Tween 80 as surfactants in order to prepare the CoQ10 nanoemulsions by self-assembly. MCT provided the highest solubility exhibiting the tendency to form a clear emulsion and spread easily. The solubility of CoQ10 is good with a MCT/Tween 80 ratio of 1:2 and 1:3, and the particle size of CoQ10 nanoemulsions exhibit a range of 16-50 nm. As the CoQ10:Tween 80 ratio increases from a ratio of 1:0 to 1:1.5, the particle sizes of nanoemulsions were all mostly smaller than 100 nm. These results indicated that the concentration of Tween 80 might greatly affect the particle size distribution in the formation of nanoemulsions. The particle size of CoQ10-chitosan stabilised nanoemulsions was maintained below 15-70 nm between pH 3 and 7, indicating that no insoluble aggregates were formed.
Zeta-potential values of nanoemulsions containing CoQ10 were exhibited in the range of -5 to -26 mV, which was governed by the ratio of Tween 80 and CoQ10. Meanwhile, the CoQ10-chitosan stabilised nanoemulsions showed a strong positive charge as 50 to 66 mV between pH 5 and 8, which can be attributed to the fact that chitosan is positively charged at this pH. The loss of positive charge on chitosan at pH 10 is due to the deprotonation of the amino groups .
Applications in the Food Industry
There are technical challenges that need to be overcome to impart functionality to the human body of lipophilic bioactives lacking in solubilisation, stability, and bioavailability. This results in unsuccessful quality enhancement and commercialisation in the food industry. To overcome this problem, it is important to attempt to understand the requirements for the solubilisation of the oil phase and to develop a stable emulsion by the use of food grade components such as emulsifiers and stabilisers for food oil-soluble supplements and nutraceuticals in food products. In addition, it is possible to develop multilayer emulsions providing a wide range of different functionalities by using different combinations of surfactants and biopolymers to produce novel food delivery systems. However, the food industry is not yet utilising nanoemulsions, and no commercial applications of food nanoemulsions are known. Consequently, there are needs for certain applications to have novel and alternative types of food delivery systems that can be produced in the food and beverage industries.
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 emulsion occurs such as creaming, flocculation, coalescence, phase inversion and Ostwald ripening. Recently, it has been suggested that food nanotechnology can help functional foods and nutraceuticals that are recreated as novel foods providing benefits such as nutrition, health, taste and safety. Food nanomaterials, such as nanoemulsions, nanoparticles, nanocomposites and nanostructured ingredients generally may be prepared by using nanotechnology, including nanoemulsification, association colloid and nanostructured multiple or multilayer emulsification. 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. At the same time, although process developments are still required for pilot-scaled versions of nanostructures, the next step toward large-scale production seems to be within reach. In order to also use these technologies for nanometre-sized emulsions, the typical sizes of the structures need to be reduced and the surface properties need to be controlled more tightly.
This study was supported by the Food Nanotechnology Development Project (2008), Ministry of Knowledge Economy, Republic of Korea.
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