Delivery systems for liquid food products
Introduction
Consumers in the industrialised world are becoming increasingly aware of the relationship between diet and health. Thus, the demand for a balanced diet and functional food products that address specific health benefits is growing steadily. Healthy food products, as compared to their standard counterparts, can be characterised by several attributes: containing (i) low to moderate sodium, sugar and trans-fat content, (ii) significantly reduced energy density (iii) an increasing amount of whole grain and dietary fibre, (iv) high quantity of milk and vegetable proteins, or (v) bioactive ingredients, i.e., nutrients which have health sustaining properties [1]. However, many of the nutritionally attractive micronutrients used for fortification cannot just be added to the product, since they are either only hardly soluble in aqueous systems, show a limited stability against chemical or physical degradation, or reveal an uncontrolled release or bioavailability. Moreover, the stability, bioavailability or bioefficacy of active substances strongly depend on the food matrix and the chosen (micro)encapsulation or delivery system [2], [3], [4], [5].
Spray-drying (micro)encapsulation has been used in the food industry since the late 1950s to provide, especially for flavour oils some protection against degradation/oxidation, and to convert liquids to powders [6]. Microencapsulation is defined as a process in which tiny particles or droplets of the active ingredient(s) are surrounded by a coating, or embedded in a homogeneous or heterogeneous matrix, to give small capsules with many useful properties. Microencapsulation can also provide a physical barrier between different active ingredients in the solid product [7]. For example, iron can be isolated from vitamin A. One of the principal goals of microencapsulation nowadays is to protect the active ingredients from both chemical (e.g. oxidation) or physical (e.g. precipitation, crystallisation) degradation induced through exposure to oxygen, light, moisture, temperature or ionic strength changes or to allow controlled or sustained release of active ingredients under desired conditions, i.e., during eating or digestion. Due to the low diffusion coefficient of oxygen in the glassy capsule material and due to the relatively large particle size (usually larger than 200 μm), sensitive oils, such as flavours and essential oils can be stabilized up to several years [8]. Such an impressive delivery performance creates still today an enormous interest by food technologists in using microencapsulation based on spray drying, freeze drying, fluid bed coating or extrusion [6], [9]. In its simplest form, a solid microcapsule is a small sphere with a uniform wall around it. The material inside the microcapsule is referred to as the core, internal phase, or fill, whereas the wall is sometimes called shell, coating, wall material, or membrane [10]. The microcapsule may even have multiple walls. The choice of the wall material is very important for encapsulation efficiency and microcapsule stability. The criteria for selecting a suitable wall material are mainly based on its physico-chemical properties such as solubility, molecular weight, glass/melting transition, crystallinity, diffusibility, film forming and emulsifying properties or costs. Most used wall materials are biopolymers of various sources, such as natural gums (gum Arabic, alginates, carrageenans, etc.), proteins (milk or whey proteins, soy proteins, gelatin, etc.), starches, maltodextrins with different dextrose equivalents, corn syrup, waxes and their derivatives [10].
Fortification of liquid products, such as drinks, juices, etc, is getting more and more fashionable in foods. In this case the (micro)encapsulated active ingredients must be stabilized in a liquid environment, which is considerably different from stabilizing the active ingredients in a solid environment. Since almost all spray-drying processes in the food industry are carried out from aqueous feed formulations, the used wall material must be soluble in water at an acceptable level [6]. Solid microcapsules or powders cannot be simply added to an aqueous food product without losing the barrier and stabilization function of the solid capsule shell material. When adding the solid microcapsules into water, the capsule shell or matrix material is basically dissolved into the aqueous phase releasing the active ingredients into the liquid phase and, in general, protection against degradation is lost. Therefore, delivering active ingredients in a liquid matrix requires the use of different encapsulation and protection strategies.
Obviously, the delivery of active ingredients in a fluid aqueous phase is by far more challenging than the delivery of the active ingredients in a solid phase. Moreover, for liquids, appropriate encapsulation techniques depend critically also on the solubility characteristics of the active ingredients. For hydrophilic components, the suitable delivery strategies and ‘capsules’ of choice differ very much from the approach to take for the delivery of lipophilic micronutrients.
The list of micronutrients and bioactive substances, which are interesting to be added to food products is quite wide. Also their physico-chemical properties are very different. Most of them are extracted from plants, fungi, micro algae or marine biomass. Polyphenols, a class of dietary antioxidants, (primarily consisting of flavonoids such as catechins, flavones, isoflavones, flavonols, flavonones and anthocyanins) capture free radicals, prevent lipids from oxidation and often exhibit antimicrobial and antiviral properties [11], [12]. They, therefore, protect human cells and help reducing the risk of chronic diseases. Carotenoids, another class of important antioxidants, (e.g. lycopene, luteine, zeaxanthin, astaxanthin and β-carotene) also protect human cells from oxidative stress [13]. Essential poly-unsaturated fatty acids and their derivatives (e.g. docosahexaenoic acid, arachidonic acid) are another class of important nutrients, since they are required for the development of the human brain and play an important role in immunity [14]. Last but not least, various amino acids, peptides (e.g. glyco-macro peptides from milk) or proteins, vitamins, (CADEK), phytochemicals (e.g. phytosterols), minerals (calcium, magnesium, iron, zinc, selenium, chromium), and probiotic bacteria, are often added to food products to induce a range of different health-promoting and sustaining effects [15].
The successful development of bioactive containing delivery systems for liquid products depends on several factors: (i) the ability to disperse active ingredients into an aqueous phase, in case the actives are water insoluble, (ii) the stability of the ‘capsule’ structure, preventing effects like creaming or sedimentation, (iii) minimising the impact on the textural, rheological or optical properties of the final food product, (iv) protection of the encapsulated active molecules against degradation during processing and storage, and (v) controlled release during consumption, either in the mouth or during digestion in the GI tract. Most challenging in practical applications seems to be the sufficient stabilization of oxidation sensitive active molecules, such as vitamins or polyphenols. Moreover, masking possible off-taste effects or increasing (or controlling) the bioavailability or bioefficacy of the active ingredients during digestion are also quite demanding tasks to achieve in practical situations.
In this review, we will describe the different types of delivery systems which can be used for the fortification of liquid food products, and their interactions with the active molecules. We will mention the functionalities focussing on advantages and limitations of the available delivery systems. Finally, we will discuss some remaining challenges and speculate on what kind of research will be necessary in the future to develop further the science area of liquid delivery systems.
Section snippets
Systems available for the delivery in liquid food products
In general, a wide selection of delivery systems is available for the use in food systems. Ultimately, one would like to relate the characteristics of the delivery systems to the functional attributes of the final product, such as sensory, physico-chemical and biological/nutritional impact [15], [16]. Fig. 1 summarizes the various types of systems which can be used for the delivery of active ingredients in aqueous liquid products. Although solid microcapsules represent the large majorities of
Oil-in-water emulsions
Lipophilic active ingredients can be delivered in different forms. The best described and researched ‘encapsulation system’ for lipidic materials in aqueous products are oil-in-water emulsions. Emulsions, such as, milk, yogurt drinks, dressings, sauces or mayonnaise, are ubiquitous in food. Their oil droplets can easily be used for the delivery of lipophilic active ingredients. For example, the delivery of the antioxidant vitamin E (tocopherol) is very important in food. Vitamin E is the major
Solid lipid nanoparticles
Solid lipid nanoparticles carriers (SLNs) have some similarities with nanoemulsion systems. The diameter of such lipid particles can be also quite small, i.e. in the range between 50 nm and 1 µm. The main difference is, however, that the SLNs consist of a solid or semi-solid lipidic core containing lipophilic active ingredients. Active ingredients can be solubilized homogeneously either in the core of the SLNs or in the outside part [37]. The advantage of SLNs as delivery system for lipophilic
Molecular complexes
Another strategy to deliver active ingredients in aqueous foods is by physically complexing them with other molecules, hoping that in this way a better solubilization and/or an increase in the chemical stability of the complexed bioactive can be achieved. In this context a molecular complex is referring to the physical association between a host and a guest (active ingredient) molecule. The most studied host molecules are the cyclodextrins. However, molecular association with other
Self-assembly delivery systems
Self-assembly structures, such as micelles, microemulsions, and liquid crystalline phases, are formed by the spontaneous association of surfactants in aqueous (or oil) phases. Surfactants are used in many food applications, such as in bread and cake production for the improvement of shelf-life (prevention of starch retrogradation due to formation of monoglyceride–amylose complexes) and flavour retention [58]. Another important application of surfactants deals with the control of emulsion or
Conclusion and outlook
In this work we presented and described various types of delivery systems which are used for the fortification of liquid food products. They all have advantages and limitations as depicted and summarized in Fig. 1. Most delivery systems are able to solubilize lipophilic nutrients into aqueous products. Concerning the appearance of the final food product, only nanoemulsions, microemulsions and some complexes are able to keep the product transparent, if desired especially for some beverage
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