Authors: Jonathan O’Sullivan, research engineer, Kerry Ingredients and Flavours and Aaron Mitchell Lett, doctoral researcher, University of Birmingham There is an ever-increasing demand from consumers for higher quality products that are more cost effective and possess improved health benefits within emulsion-based food formulations. Thus, the food industry must possess a detailed understanding of the microstructure-function relationship of food emulsions. Emulsions are structures that are commonly found within a myriad of food systems such as mayonnaise, salad dressings and ice cream, and they have been the subject of continual investigation over the past century. The structures of food emulsions play an integral role in their functional attributes. The microstructure of food emulsions can be manipulated by utilisation of the microstructural approach. The microstructural approach for the design of novel food systems relies upon a detailed understanding of the relationship between emulsion formulation and employed processing methodology for the controlled development of specific microstructures for desired function – such as a specific sensory profile, prolonged commercial stability, controlled release of bioactive components and deliverance of nutritional value.

Microstructural approach to emulsion systems

New PictureEmulsions are dispersions of two immiscible phases, typically oil and water, where one phase manifests as droplets (dispersed phase) within the other medium (continuous). Broadly, two different types of emulsion categories exist based on droplet type: oil-in-water (O/W) emulsions with oil droplets in an aqueous phase; and water-in-oil (W/O) emulsions possessing water droplets in an oil phase. Examples of O/W emulsions include yoghurt, milk, cream and cake batter, while examples of W/O emulsions include butter and margarines1. The various ingredients utilised within the formulation of food emulsions determine the bulk physicochemical properties, and consequently the resultant functions, such as the product’s sensory profile. The ingredients employed in these formulations include edible oils, water and emulsifying agents, as well as additional ingredients such as stabilisers, texture enhancing agents or flavours. There is a growing trend within the food industry for the utilisation of proteins as the emulsifying agent of emulsions, as they possess the capacity to stabilise the oil-water interface, are naturally derived components and provide nutritional value to the formulation. This is in comparison to conventionally employed low molecular weight surfactant type emulsifier, which is typically fat derived (2). In addition to the aforementioned ingredients within emulsion formulations, energy is essential for the fabrication of emulsions. This energy is employed in the disruption of larger volumes of oil into discrete emulsion droplets, yielding a homogenous product. Typically, emulsions possessing a droplet size <1μm are resistant to gravitational separation from creaming, due to density differentials between the two aqueous and oil phases for prolonged periods, and are considered ‘stable’. Thus, sufficient energy with the appropriate formulation is necessary for the production of stable emulsion droplets (1). The rationale for the creation of such emulsion systems is known as the microstructural approach for food design, whereby the relationship between the formulation and specific processing conditions dictate the final microstructure of the resultant emulsion (i.e. emulsion droplet size) and its material properties. The desired microstructures of food emulsions aim to possess certain functions, such as long-term stability and ideal sensory attributes for given applications. New Picture The processing required for the fabrication of emulsions is known as emulsification, whereby conventional or novel methods are employed. Conventional methods for emulsification include the application of high shear mixers or homogenisers that utilise high energy to form droplets, whereby the majority of applied energy (in excess of 90%) is dissipated as thermal energy and the remainder for the preparation of emulsion droplets. Novel methods of emulsification include low-energy methods, such as membrane emulsification (crossflow and rotary), for the preservation of friable components within formulations, and forthcoming technologies for the fabrication of nanoemulsions, including power ultrasound and microfluidics. In addition to the specific process employed for emulsification, each process has distinct process parameters that can be varied to alter the resultant microstructure, which can be broadly classified as applied power and processing time. The specific combination of emulsion formulation, processing methodology and associated process parameters dictate the resultant microstructure, the microstructural approach for food design (3).

Microstructure-sensory relationships

A structure-function relationship, which is of considerable interest to food manufacturers, is the relationship between emulsion microstructure and the product’s resulting sensory characteristics. The sensory profile of the food drives the consumer’s enjoyment and therefore largely its commercial success. A consumer’s first sensory assessment will typically be the product’s visual appearance. ‘We eat with our eyes’ is an often-forgotten but important consideration in product development. Food products currently available on the market would typically be further formulated, by addition of water-soluble colourants or other ingredients, to produce a desired visual appearance. However, fundamentally visual properties such as opacity and colour within emulsion systems are determined by the emulsion droplet concentration, droplet size and refractive index differential between the water and oil phases. Emulsions with higher oil contents and/or smaller emulsion droplets have lighter optical characteristics, manifesting as an overall whiter colour. Nano-sized emulsion droplets do not scatter light like conventional macroemulsions and can appear optically transparent. This provides the potential for product novelty whilst allowing for the discrete incorporation of lipophilic components, such as oil-soluble vitamins, within specific applications (4). Based on a product’s appearance and its behaviour when manipulated, such as with cutlery or being poured, consumers will generate an opinion on the food’s sensory properties, based on previous consumption experiences of the same or similar foods. For example, consumers have been shown to visually assume liquids that pour slower are thicker, and products with shiny, even yellowish colouring to have a more creamy profile. Evidently, visual behavioural characteristics are a result of the system’s physical properties, such as viscosity and adhesion, which can be varied by the systems microstructure. Physical properties of which are also primarily responsible for the product’s texture and mouthfeel (4). The presence of ingredients such as biopolymers, particularly those with thickening or gelling properties (for instance high molecular weight polysaccharides), within an aqueous continuous phase will ultimately effect the emulsion’s overall physical properties.

Perception of attributes

Concentrating upon the dispersed phase, higher concentrations of oil droplets have been shown to increase perception of attributes such as ‘creaminess’, ‘richness’, ‘thickness’ and ‘fattiness’, a familiar example of this is the comparison in sensory properties between skimmed and full-fat milk. Smaller oil droplet sizes have also been shown to increase perception of sensory attributes including ‘creaminess’, ‘thickness’, ‘smoothness’ and ‘slipperiness’. This has been attributed to smaller oil droplets increasing the system’s viscosity and improving its lubrication capacity. Furthermore, colloidal interactions between droplets also have significant effects on sensory perception, by means of changing the product’s physical properties. Depletion flocculation interactions between emulsion droplets, the aggregation of emulsion droplets, have shown to improve perception of attributes such as ‘thickness’ and ‘fattiness’ through increasing the systems viscosity. However, irreversibly associated emulsion droplets, due to bridging flocculation, increases ‘dry’ and ‘astringent’ sensations through increases in friction, as measured by tribology (5). In addition, droplet coalescence during oral processing has been related to increases in the perception of fat related attributes such as ‘fattiness’ and ‘creamy mouthfeel’ through increasing lipophilic flavour aroma release. Flavour is another key sensory characteristic. The intensity of this is dependent on the distribution of flavour molecules among the emulsion phases and their release profile during oral processing, a result of the flavour molecule’s equilibrium partition coefficients and their mass transport kinetics. Structurally, increasing the oil droplet concentration increases polar flavours, whilst conversely decreases non-polar flavours. In addition, the size of emulsion droplets influence flavour release kinetics, whereby larger droplets produce a more delayed and sustained release of non-polar flavour molecules, ascribed to greater diffusion path length to reach the aqueous phase than the tongue. A delayed diffusion of flavour molecules to the tongue’s taste receptors is also observed if the continuous phase contains biopolymers, which promote thickening or gelling due to altered partitioning and mass transport of the flavour molecules. The larger surface area associated with nano-sized emulsion droplets allows for a more rapid and intense release of flavour in comparison to food systems possessing larger emulsion droplets (4). Understanding of structure-sensory relationships and designing microstructures for the desired sensory profile is further complicated by the fact that the perception of sensory attributes may change throughout oral processing as structure changes due to the mechanical action of the teeth, tongue and palate and the and chemical mixing and dilution of the food by saliva. Research within this area is attempting to understand this complex phenomena via further understanding mechanical and chemical breakdown of food and the application of novel time dependent sensory techniques such as temporal dominance of sensations (6).

Conclusions

In conclusion, the specific microstructure of food emulsions can be designed for numerous desired functions, such as optimising sensory satisfaction utilising the microstructural approach. The microstructural approach relies upon a detailed understanding of an emulsions formulation, employed processing methodology for the fabrication of emulsion, emulsion structure-sensory relationships and the associations between these factors allowing for the development of food microstructures with specific materials properties yielding the desired consumer responses. References:
  1. McClements, DJ (2005). Food Emulsions: Principles, Practices and Techniques. CRC Press.
  2. O’Sullivan, J; Murray, B; Flynn, C; and Norton, IT. (2015), The Effect of Ultrasound Treatment on the Structural, Physical and Emulsifying Properties of Animal and Vegetable Proteins. Food Hydrocoll. (2015).
  3. O’Sullivan, J; Greenwood, R; and Norton, I (2015). 'Applications of ultrasound for the functional modification of proteins and nanoemulsion formation: A review.' Trends Food Sci. Technol, (2015).
  4. Frøst, MB; and Janhøj, T (2007). 'Understanding creaminess.' Int Dairy J, 17,pp. 1298–1311, (2007).
  5. Van Aken, GA; Vingerhoeds, MH; and de Hoog, EHA (2007). 'Food colloids under oral conditions.' Curr Opin. Colloid Interface Sci, 12,pp. 251–262, (2007).
  6. Chen, J (2009). 'Food oral processing—A review.' Food Hydrocoll, 23,pp. 1–25, (2009).
Author biographies [caption id="attachment_19149" align="alignright" width="336"]New Picture Jonathan O’Sullivan[/caption] • Jonathan O’Sullivan: Jonathan O’Sullivan is a research engineer working in collaboration between Kerry Ingredients and Flavours (Listowel, Ireland) and the University of Birmingham (Birmingham, UK). His research interests involve food protein chemistry and hydrocolloid functionality within food formulations (foaming, emulsification and gelation). His work aims to develop novel alternative sources of protein ingredients derived from legume and cereal origins which possess the potential to behave as mimetics for either dairy-/animal-derived proteins or fat. [caption id="attachment_19151" align="alignright" width="336"]New Picture Aaron Mitchell Lett[/caption] • Aaron Mitchell Lett: Aaron Mitchell Lett is a doctoral researcher within the Microstructure Group at the School of Chemical Engineering, University of Birmingham. His current research takes a microstructural approach to engineer emulsion based food systems to control satiation, satiety, hedonic acceptability and sensory quality. His work will ultimately identify microstructures to allow emulsion-based foods to be reformulated to modified food intake behaviour yet maintain or improve the hedonic and sensory qualities of the product.