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Colloidal aspects of eating
Current Opinion in Colloid & Interface Science 15 (2010) 84–89
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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s
Colloidal aspects of eating Benjamin J.D. Le Révérend ⁎, Ian T. Norton, Phil W. Cox, Fotios Spyropoulos Centre for Formulation Engineering, School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
a r t i c l e
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Article history: Received 14 September 2009 Accepted 19 November 2009 Available online 1 December 2009 Keywords: Consumer perception Fat replacement Emulsion design Tribology Air-filled emulsions Fluid gels Duplex emulsions
a b s t r a c t This paper is an overview of the work currently carried out on the microstructural approach to reduce unhealthy ingredients in everyday foods, while maintaining the positive eating aspects of the original product. Fat reduction is discussed in detail as an example of how the approach might be used. In particular, we will cover the very new approach using tribology based physical measurements and relate this to oral response as opposed to using rheological measurements. Materials such as low fat, air/oil-in-water and water-in-water emulsions as well as sheared (or fluid) gels will be discussed as this approach has allowed physical, chemical and sensory properties of high fat content foods to be matched by structures containing considerably less fat. This microstructural approach to the engineering challenge of fat replacement has proved very successful in the development of mayonnaise, cream and sauces with good eating properties. If the approach discussed continues to be developed, they promise significant advancement and rewards on the formulation of healthy everyday foods which are perceived by the consumer as indulgent. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Food colloids can represent a significant part of a consumer's everyday diet, with products such as condiments, sauces, dressings and ice creams. In order to address recommendations from nutritionists and various agencies (e.g. FDA, FSA and WHO), the future of food colloids is likely to focus on fat, salt and carbohydrates reduction, as well as on the targeted delivery of nutrients. The use of complex structures that are based on emulsions (single or multiple, fluid gels and air-filled emulsions) seems to offer an attractive range of tools to engineer healthier foods without compromising the organoleptic properties of the product. In order to design the appropriate microstructures and functionality, an understanding of the conditions experienced by foods in the mouth is required. Moreover, an understanding of how oral processing affects or responds to the material properties of food is also needed. This paper presents a short review of the previous and ongoing work on characterising pressure, velocity fields and molecular diffusion rate conditions in the mouth and importantly their relationship to sensory measurements. The application of this understanding for the design of novel structures will also be discussed. 2. Engineering perspective on oral processing and digestion Oral processing defines all the processes occurring within the oral cavity, while digestion can be defined as the processing of foods in the
⁎ Corresponding author. E-mail address: [email protected] (B.J.D. Le Révérend). 1359-0294/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2009.11.009
body from the oesophagus to the rectum. Obviously, this simplified definition ignores the action of oral enzymes for the time being. The tongue, palate (hard and soft), epiglottis and lips delimit the oral cavity (Fig. 1). Although oral cavities from different healthy individuals are similar in their geometry and functions, the individual characteristics of each subject should be taken into account when trying to quantify the behaviour of food in the mouth [1,2]. Typical values for the volume of the human mouth have been reported as 30 ± 10 g for adult males and 25 ± 8 g for adult females [3]. The same authors also measured the average weight of banana to fill the mouth under normal eating conditions as 18 ± 5 g for adult males and 13± 4 g for adult females. Once food has entered the mouth, it is comminuted by mastication and chewing which allows for the release of flavour and the texture to be experienced. As this breakdown proceeds, the coupling of mechanical, thermal and chemical reactions as well as wetting by saliva leads to the formation of a food bolus [4]. The bolus is processed until a threshold of size distribution and lubrication is achieved [5]. This processing not only occurs upon ingestion of solids, where its effect is rather intuitive (reduction of particle size distribution [6], lubrication and hydration [7]), but also occurs upon ingestion of soft solids and liquids, where the effects are more related to the tasting of aroma and the perception of texture [8]. Since many soft solid and liquid foods are colloidal in nature (emulsions, gels, suspensions and foams), they are of particular interest to this review. A typical example of the effect of oral processing on a food colloid can be experienced when eating margarine, i.e. a water-in-oil (w/o) emulsion stabilised by fat crystals (Pickering stabilisation). Fat crystals melt during oral processing, causing the destabilisation and phase inversion of the
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Fig. 1. Human oral organ anatomy [1].
emulsion [9], allowing the consumer to sense the hydrophilic components in the aqueous phase, such as sodium chloride [10]. From an engineering perspective, an increased residence time of solid foods in the mouth, corresponding to more chewing cycles, will produce further breakdown [5] (until the food bolus is completely formed) and will increase the diffusion rate of taste triggering molecules to the receptors. For soft solids, the release and flavour deposition of volatile aroma are associated with the mixing caused by the tongue movements [5] and the residence time in the mouth [11]. To increase the residence time of soft solids and liquids in the mouth and therefore increase aroma or flavour release, another approach is to enhance the interaction between the food and the oral surfaces (mucosa). In this respect, the use of mucoadhesive biopolymers seems an attractive route, a successful demonstration of this has been given by the pharmaceutical industry for controlled drug delivery on the oral mucosa but also on other mucus covered surfaces [12,13]. Some food grade materials such as alginate and pectin have strong mucoadhesive capabilities [12,14–16] and constitute good candidates as taste enhancers through adhesion mechanisms. In order to obtain a better understanding of the behaviour of colloids in the mouth, the action of the mouth on foods from a hydrodynamic point of view has to be considered as well. Data on the shear conditions experienced by a variety of soft solids and liquids in the mouth has been collected, reported and compared with rheological measurements by Shama and Sherman [17,18]. Further literature in this area not only comes from the food science arena but also from dental research. As the shear field in the mouth is very heterogeneous [5], and it has been proposed that the sensations of thickness and creaminess are also “measured” in elongational deformation as well as shear [5]. This idea was further investigated by using a tribometer (see Fig. 2(a)) to measure friction coefficients (torque required to rotate the ball divided by load applied) as a function of the relative rotational speed, also known as a Stribeck experiment. In the case of guar solutions of different concentrations a good agreement with sensory data (slipperiness) was found for rotation speeds between 40 and 250 mm/s [19] (cf. Fig. 2(b)). This range of speeds was used to calculate an average Reynolds number (for a Newtonian fluid) in the mouth considering a gap of 0.25 cm between the tongue and the hard palate [20]. These were then compared with the results from Nicosia and Robbins [20], whose modelling work estimates in mouth Reynolds number during swallowing, as a function of the pressure applied between the tongue
Fig. 2. Schematic of a tribometer (a) and Stribeck curves of o/w emulsions of matched viscosities (at 50 s− 1) [19] (b).
and the palate (squeeze flow). This comparison is shown on Fig. 3, where one can see that similar Reynolds numbers (within one order of magnitude) were obtained over the range of conditions used.
Fig. 3. Comparison of two methods (Re in a squeeze flow as modelled in [20] or standard Re calculated from velocities measured in [19]) to estimate flow fields in the mouth.
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The good agreement between these data is an important finding since slipperiness, correlated to the friction coefficient, is a component of the creaminess perception [8]. 3. Perception of creaminess (fattiness) and astringency during oral processing Kokini and others [8,21–23] developed a creaminess “equation” linking the perception of creaminess (intuitively related to fat content) with other sensory attributes from trained panellists such as thickness (related to the rheological properties), smoothness (related to the particle size in the colloidal system) and slipperiness (related to the tribological properties). This relationship has been formulated as: 0:54
Creaminess∝Thickness
0:82
⋅Smoothness
ð1Þ
or Creaminess = 0:54⋅ logðThicknessÞ + 1:56⋅ logðSmoothnessÞ
ð2Þ
−0:32⋅ logðSlipperinessÞ in two studies [8,23]. The fact that a strongly hedonic sensory attribute such as creaminess can be explained by these attributes suggests that creaminess is not only controlled by the rheology of the product [19,24,25], but also by its lubricating properties. This is consistent with the observations from a study of Lillford [5] in which it is indicated that in the mouth, food is subject to not only a range of shear rates but also extensional flow; for more theory on this matter, the reader is directed to Steffe [26]. On the other hand, astringency seems to be related to an increase in the friction coefficient [19,27]. In recent work [7,28], the injection of a typically astringent compound, epigallocatechin gallate (ECGC), a polyphenol extracted from green tea, was used while measuring the friction coefficient of saliva, was found to increase the friction coefficient from around 0.1 to 20. Under similar conditions, addition of water to the system also increased the friction coefficient, but to a lesser extent. The main difference rising from the introduction of ECGC is the rate at which the friction coefficient increases. Such data seems to confirm the original work from Malone et al. [19] in which astringency is related to the flocculation of the material in contact with the oral mucosa. In conclusion, the various attempts made to formulate low-fat products by only matching their viscosity (to that of the full fat equivalent) have failed because of the lack of understanding oral processing, as well as the microstructure of the product. Indeed it appears that as suggested by Lillford [5] “texture is a consequence of the microstructure”. 4. Strategies for reduction of fat levels in food In order to formulate successful low-fat products, one has to design, construct and control the microstructure of the system so that it resembles to that of the full fat equivalent. This will then be reflected on the macro scale by its rheological and lubricating properties and ensure maximum consumer acceptance. 4.1. Emulsion based products Typical examples of such products are low-fat spreads. It is quite clear that, to match the consumer's reference (butter or cream), a similar microstructure can be achieved by using w/o emulsions of similar droplet sizes [29–31]. In Fig. 2(b), tribological data is used to illustrate this idea. Emulsions containing 20% oil or more respond very similarly to pure vegetable oil, and emulsions with 15% oil or less appear to the pure oil's behaviour. As the rotational speed of the disc
increases (up to ∼100 mm/s− 1) the gap between the ball and plate increases and the flow becomes what is known as a mixed regime (hydrodynamic and tribological). Within this regime the difference between the samples is maintained although the size of the difference is reduced with increasing speed. Finally at higher speeds, where the flow enters the hydrodynamic regime, all the samples behave similarly. This is probably a consequence of the matched viscosities, which dominate in the hydrodynamic regime. This data suggests that there is a lower limit of fat content in emulsion based products required to give acceptable performance upon consumption; that is somewhere between 15 and 20% oil. However, as discussed at the later parts of this review, gel particles or air-filled emulsion droplets can be made to give the same thin film behaviour as the pure oil or high fat emulsions and therefore may well give the sensory properties of fat. A similar approach has been adapted recently to develop a low-fat chocolate. It is well known that chocolate is particularly appreciated for its in mouth melting characteristics and mechanical properties [32]. These are controlled by the polymorphism of cocoa butter and the strength of the fat crystal network. The use of “margarine lines” (scraped surface heat exchanger in series with a pin stirrer) to produce water-in-cocoa butter emulsions to replace the pure fat phase present in chocolate has been identified as a successful opportunity to produce a low-fat alternative without any noticeable impact on the cocoa butter polymorphism [33]. Here the processing conditions are important as the cooling rate and the shear experienced in the margarine lines are similar to that of traditional chocolate tempering processes. However, a future challenge will be to match the microstructure of a fat crystal network using such emulsions, as it controls another important sensory attribute; i.e. hardness [34]. To this end, to keep the macroscopic properties of the final chocolate product similar to that of traditional chocolate, gelatin [35] has been used to set the aqueous phase of the emulsion while keeping melting properties of cocoa butter based chocolate, due to the appropriate melting point of the gelatin. Other products that could benefit from the microstructural approach are sauces and dips. These are typically o/w emulsions and contain a large oil volume fraction (up to 80%). In order to obtain a similar microstructure with a reduced fat content, double emulsions w/o/w could be used [36], using the water droplets as filler particles in the oil phase. Fig. 4, shows a typical w/o/w emulsion, stabilised using PGPR (w/o) and NaCas (o/w), and produced in a high shear mixer. Similar systems have been produced using a Pickering stabilised internal phase (w/o), avoiding the use of PGPR [37]. Then a membrane emulsification step can be used to produce the w/o/w emulsion, as the Pickering fat crystals can be damaged by the high shear and temperature involved in high shear mixers. Such emulsions also offer the advantage of allowing entrapment of actives in the internal water phase, which could be delivered in strategic locations in the GI tract or for masking an unaccepted tastes [36] and avoiding its “detection” in the mouth. As already mentioned, successful manufacture of these complex emulsions cannot be easily made with conventional high shear mixers or homogenizers. A more successful, and elegant, solution is the use of membrane emulsification. This technique is usually found in a crossflow mode of operation [38]. Emulsion preparation in such membrane systems is then controlled by trans-membrane pressure, cross-flow velocity and pore size of the membrane [38–40]. However, due to the direction of the flow in a cross-flow setup (parallel to the membrane surface), coalescence can occur between droplets that are newly formed/forming from neighbouring pores. This phenomenon will depend on the density of pores on the membrane surface and if not addressed will lead to an increase of the average droplet size [38]. To avoid such problems, a solution can be found in decreasing the boundary layer and hence the residence time of the droplet close to the membrane by having a rotating membrane. The centrifugal
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In order to reduce the fat content of food colloids but keep a similar microstructure to that of the original full fat system, a phase separating mixture of biopolymers can also be used. One example is the use of gelatin–maltodextrin mixed biopolymer system which under controlled conditions will produce a “water-in-water” emulsion [41]. By studying the gelation kinetics of the individual phases [42] and understanding polymers incompatibility in solution before the sol–gel transition, such structures have been used to produce 0% fat spreads [43]. Although feasibly possible, it should be noted that a small amount of fat may be required for efficient flavour delivery. The gel strength of such structures was also studied and found to be mainly controlled by the properties of the continuous phase [44]. An ingredient that has been used very successfully in this respect is a low dextrose equivalent maltodextrin [3]. The reason for this is that short chain hydrocolloids form crystalline type structures, which, when
used in mixed biopolymer systems, mimic the organoleptic and material properties of fat crystals. Fig. 6 shows some of the data obtained for a gelatin/maltodextrin water-in-water emulsion in which liquid oil was added to produce a low or very low-fat spread with virtually no saturated fatty acids (UoB data). Fig. 6 also shows, for the sake of comparison, data obtained for a commercially available fat continuous low-fat spread containing ∼ 35% fat in which the material properties (spreading, plasticity, etc.) depend upon crystallisation of the saturated fats to form a fat crystal network. Fig. 6 demonstrates that by producing a water-in-water emulsion with similar material properties to the commercial margarine and then by adding either 20% or 40% liquid vegetable oil as a fine emulsion, the flow and fracture properties can be matched. As stated the material properties of the water-in-water emulsion depend upon the continuous phase and as such depend on the bloom strength of the gelatin for this system. As the oil is emulsified into the water-in-water emulsion as fine droplets, the overall product properties have only a weak dependence on the oil content, which appears to be behaving as a soft filler. This is quite encouraging as it suggests that further oil reduction in such a formulation can be made. Another approach to produce particulate structures in water based systems is the use of sheared gels [45–47]. By applying shear during the gelation of polysaccharides (e.g. gelatin, carrageenans, and agar), the gelation process can be controlled to obtain gel spherulites in a continuous phase of water, i.e. cyclising the gel network. It has been found that controlling the formation of such structures during the process can be obtained by modification of the shear and cooling rates during processing. Interestingly it has been shown that increases in the biopolymer concentration do not affect the particles' volume fraction in the fluid gel system [47]. The same study reports on the behaviour of agar fluid gels with concentrations ranging from 0.75% to 5% and concluded that upon formation these fluid gels have the tendency to occupy the maximum volume fraction. However the material properties and architecture of the different particles will change dramatically. These suspensions have a number of interesting properties including the ability to suspend particles or, if constructed in the right way, to produce particulate gels with properties that are very similar to protein gels formed with milk proteins [46]. The viscoelastic properties of fluid gels depend on their microstructure as well as the interactions/bridging between the produced particles. Both can be modified and controlled by typical formulation/ process parameters such as the rate of deformation and the rate of cooling, during the temperature quench applied to induce the conformational ordering and gelation of the biopolymer. In previous
Fig. 5. Droplet size distributions for oil-in-water emulsions produced using the rotating membrane (□) and cross-flow membrane (●) emulsification techniques. Both systems are using a 0.8 μm pore size SPG membrane and both are operating at a 60 kPa transmembrane pressure.
Fig. 6. Stress/strain curves for a low-fat spread (Flora) and for water-in-water emulsions (20%wt/wt maltodextrin/4%wt/wt gelatin containing 0.1 m NaCl) also containing either 20%wt/wt (open symbols) or 40%wt/wt (filled symbols) sunflower oil emulsified with 0.5%wt/wt Tween 80 (oil droplet size ∼5 µm). All curves obtained upon compression of the materials using an Instron material tester.
Fig. 4. Typical w/o/w double emulsions.
force applied to the droplet due to the rotation propels it far from the membrane preventing coalescence between the formed droplets. By blocking the back reaction (coalescence), the size of the droplets achievable through rotating membrane emulsification is therefore lower (see Fig. 5) for similar cross-flow process parameters (trans-membrane pressure, membrane pore size, and surfactant concentration). 4.2. Polymer/gel based products
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published work, fluid gels were produced having a broad particle size distribution and in many cases a significant number of irregularly shaped particles, both may produce interesting oral properties. A detailed study on the rheological properties of spherical agar particles has been carried out by Norton et al. [48]. The authors reported a relationship between rheology and morphology of fluid gel particles for a range of agar concentrations. They further showed that the viscoelastic properties (e.g. elastic modulus) increased gradually with polymer concentration, rather than the steep increase predicted from models developed for rigid particle suspensions. From this they concluded that the produced agar particles behaved as highly deformable entities as would fat droplets in an o/w emulsion [48]. Recently a totally new approach for fat replacement in emulsion based foods has been developed [49]. This approach delivers emulsions where a significant proportion of the fat phase is replaced by “emulsified” air droplets. In order for these “air-filled emulsions” to work it needs to be ensured that the air-filled droplets resemble the fat droplets they are designed to replace; this is in terms of their size, shape, rheological properties and interactions with the rest of the structure. The construction of air-filled emulsions uses a novel group of proteins (the hydrophobins), which assemble at the air/water interface and then aggregate to give gel like structures [50]. This interfacial structure then imparts an elastic restoring force and as the air droplets try to ripen it stops any change in droplet size for the months necessary for product stability even with air droplets of only a few microns. The air-filled emulsion structure is shown schematically in Fig. 7(a). In order to give both a rheological match while maintaining the flavour of the product triphasic emulsions can be constructed using a combination of air and oil filled emulsions, as in Fig. 7(b), within a single product structure [27]. Although at first sight it would seem that hydrophobins are unique molecules, they are also a very expensive ingredient, which, in today's fast moving consumer goods industry, can render their use uneconomical. Much cheaper alternative food grade proteins, that can be processed to give all the properties of hydrophobins in foams, air-filled emulsions and triphasic systems have recently been investigated. This advance makes the approach financially viable for foods and will potentially also allow these structures to be used in other non-food products. An important and so far unanswered question is: can air cells give the sensory properties of oil droplets? As previously discussed, tribology is a good way of characterising the “oral” properties of products and correlating them with data from sensory panels. Recent work using tribology has shown that air whipped into a dairy cream alternative reduces the friction between soft surfaces as if the oil content of the product had been increased (Fig. 8). It therefore seems
Fig. 8. Tribometer curves for full fat and low-fat Elmlea (dairy cream alternatives) and aerated (●) low-fat Elmlea.
that not only water could be used to create low-fat alternatives to indulgent creamy foods but that simple air cells (although complex to stabilise) could also be the innovative ingredient required to engineer tomorrow's foods. 5. Conclusion In this review various routes to engineer and manufacture low-fat alternatives to indulgent fatty foods (mayonnaise, cream, sauces and spreads) have been proposed. As such products would have to be accepted by consumers to be commercially successful, an understanding of the in mouth processes from a mechanical point of view is required. A number of studies have been conducted to quantify the mechanics of oral processing and correlations can be found between theoretical and empirical studies. The use of a microstructural approach to build appropriate systems (double and triphasic air-filled emulsions, fluid and mixed gels) that can mimic the properties of fats has been successfully validated using experimental techniques that relate to the oral response, such as rheology and tribology. These have confirmed that the macro scale properties (e.g. texture) derive from the micro scale scaffold. Although this approach has proved successful for fat reduction, there are many more challenges to overcome such as salt reduction as well as the replacement of sugar in product structuring, ice content
Fig. 7. A schematic representation of the hydrophobin stabilised mayonnaise (a) and photomicrograph (b) picture width 100 µm, of an air and oil filled emulsion after 4 days storage. Oil droplets have a mean diameter of ∼ 8 µm and the air-filled droplets have a mean diameter of ∼2 µm.
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control and habit. However the same principles should be applicable to these challenges and it is likely that solutions for such problems will be found in the near future. References [1] Chen J. Food oral processing—a review. Food Hydrocoll 2009;23:1–25. [2] van Aken GA, Vingerhoeds MH, de Hoog EH. Food colloids under oral conditions. Curr Opin Colloid Interface Sci 2007;12:251–62. [3] Medicis SW, Hiiemae KH. Natural bite sizes for common foods. J Dent Res 1998;77:295. [4] Hutchings JB, Lillford PJ. The perception of food texture — the philosophy of the food breakdown path. J Texture Stud 1988;19:103–15. [5] Lillford P. The materials science of eating and food breakdown. MRS Bull 2000;25:38–43. [6] Peyron M, Mishellany A, Woda A. Particle size distribution of food boluses after mastication of six natural foods. J Dent Res 2004;83:578–82. [7] Bongaerts J, Rossetti D, Stokes J. The lubricating properties of human whole saliva. Tribol Lett 2007;27:277–87. [8] Kokini JL, Cussler EL. Predicting the texture of liquid and melting semi-solid foods. J Food Sci 1983;48:1221–5. [9] Norton IT, Spyropoulos F, Cox PW. Effect of emulsifiers and fat crystals on shear induced droplet break-up, coalescence and phase inversion. Food Hydrocoll 2009;23:1521–26. [10] Beauchamp GK, Bartoshuk L. Tasting and smelling. 2nd ed. Academic Press; 1997. [11] Malone ME, Appelqvist IAM, Norton IT. Oral behaviour of food hydrocolloids and emulsions. Part 2. Taste and aroma release. Food Hydrocoll 2003;17:775–84. [12] Andrews GP, Laverty TP, Jones DS. Mucoadhesive polymeric platforms for controlled drug delivery. Eur J Pharm Biopharm 2009;71:505–18. [13] Smart JD. The basics and underlying mechanisms of mucoadhesion. Adv Drug Deliv Rev 2005;57:1556–68. [14] Sriamornsak P, Wattanakorn N, Nunthanid J, Puttipipatkhachorn S. Mucoadhesion of pectin as evidence by wettability and chain interpenetration. Carbohydr Polym 2008;74:458–67. [15] Thirawong N, Kennedy RA, Sriamornsak P. Viscometric study of pectin–mucin interaction and its mucoadhesive bond strength. Carbohydr Polym 2008;71:170–9. [16] Sigurdsson HH, Loftsson T, Lehr C. Assessment of mucoadhesion by a resonant mirror biosensor. Int J Pharm 2006;325:75–81. [17] Shama F, Sherman P. Identification of stimuli controlling the sensory evaluation of viscosity II. Oral methods. J Texture Stud 1973;4:111–8. [18] Shama F, Parkinson C, Sherman P. Identification of stimuli controlling the sensory evaluation of viscosity I. Non-oral methods. J Texture Stud 1973;4:102–10. [19] Malone ME, Appelqvist IAM, Norton IT. Oral behaviour of food hydrocolloids and emulsions. Part 1. Lubrication and deposition considerations. Food Hydrocoll 2003;17:763–73. [20] Nicosia MA, Robbins J. The fluid mechanics of bolus ejection from the oral cavity. J Biomech 2001;34:1537–44. [21] Kokini JL. The physical basis of liquid food texture and texture–taste interactions. J Food Eng 1987;6:51–81. [22] Kokini J. Predicting the rheology of food biopolymers using constitutive models. Carbohydr Polym 1994;25:319–29. [23] Kokini J. Predicting the rheology of food biopolymers using constitutive models. Carbohydr Polym 1994;25:319–29. [24] Akhtar M, Stenzel J, Murray BS, Dickinson E. Factors affecting the perception of creaminess of oil-in-water emulsions. Food Hydrocoll 2005;19:521–6.
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Current Opinion in Colloid & Interface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o c i s
Colloidal aspects of eating Benjamin J.D. Le Révérend ⁎, Ian T. Norton, Phil W. Cox, Fotios Spyropoulos Centre for Formulation Engineering, School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
a r t i c l e
i n f o
Article history: Received 14 September 2009 Accepted 19 November 2009 Available online 1 December 2009 Keywords: Consumer perception Fat replacement Emulsion design Tribology Air-filled emulsions Fluid gels Duplex emulsions
a b s t r a c t This paper is an overview of the work currently carried out on the microstructural approach to reduce unhealthy ingredients in everyday foods, while maintaining the positive eating aspects of the original product. Fat reduction is discussed in detail as an example of how the approach might be used. In particular, we will cover the very new approach using tribology based physical measurements and relate this to oral response as opposed to using rheological measurements. Materials such as low fat, air/oil-in-water and water-in-water emulsions as well as sheared (or fluid) gels will be discussed as this approach has allowed physical, chemical and sensory properties of high fat content foods to be matched by structures containing considerably less fat. This microstructural approach to the engineering challenge of fat replacement has proved very successful in the development of mayonnaise, cream and sauces with good eating properties. If the approach discussed continues to be developed, they promise significant advancement and rewards on the formulation of healthy everyday foods which are perceived by the consumer as indulgent. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Food colloids can represent a significant part of a consumer's everyday diet, with products such as condiments, sauces, dressings and ice creams. In order to address recommendations from nutritionists and various agencies (e.g. FDA, FSA and WHO), the future of food colloids is likely to focus on fat, salt and carbohydrates reduction, as well as on the targeted delivery of nutrients. The use of complex structures that are based on emulsions (single or multiple, fluid gels and air-filled emulsions) seems to offer an attractive range of tools to engineer healthier foods without compromising the organoleptic properties of the product. In order to design the appropriate microstructures and functionality, an understanding of the conditions experienced by foods in the mouth is required. Moreover, an understanding of how oral processing affects or responds to the material properties of food is also needed. This paper presents a short review of the previous and ongoing work on characterising pressure, velocity fields and molecular diffusion rate conditions in the mouth and importantly their relationship to sensory measurements. The application of this understanding for the design of novel structures will also be discussed. 2. Engineering perspective on oral processing and digestion Oral processing defines all the processes occurring within the oral cavity, while digestion can be defined as the processing of foods in the
⁎ Corresponding author. E-mail address: [email protected] (B.J.D. Le Révérend). 1359-0294/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2009.11.009
body from the oesophagus to the rectum. Obviously, this simplified definition ignores the action of oral enzymes for the time being. The tongue, palate (hard and soft), epiglottis and lips delimit the oral cavity (Fig. 1). Although oral cavities from different healthy individuals are similar in their geometry and functions, the individual characteristics of each subject should be taken into account when trying to quantify the behaviour of food in the mouth [1,2]. Typical values for the volume of the human mouth have been reported as 30 ± 10 g for adult males and 25 ± 8 g for adult females [3]. The same authors also measured the average weight of banana to fill the mouth under normal eating conditions as 18 ± 5 g for adult males and 13± 4 g for adult females. Once food has entered the mouth, it is comminuted by mastication and chewing which allows for the release of flavour and the texture to be experienced. As this breakdown proceeds, the coupling of mechanical, thermal and chemical reactions as well as wetting by saliva leads to the formation of a food bolus [4]. The bolus is processed until a threshold of size distribution and lubrication is achieved [5]. This processing not only occurs upon ingestion of solids, where its effect is rather intuitive (reduction of particle size distribution [6], lubrication and hydration [7]), but also occurs upon ingestion of soft solids and liquids, where the effects are more related to the tasting of aroma and the perception of texture [8]. Since many soft solid and liquid foods are colloidal in nature (emulsions, gels, suspensions and foams), they are of particular interest to this review. A typical example of the effect of oral processing on a food colloid can be experienced when eating margarine, i.e. a water-in-oil (w/o) emulsion stabilised by fat crystals (Pickering stabilisation). Fat crystals melt during oral processing, causing the destabilisation and phase inversion of the
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Fig. 1. Human oral organ anatomy [1].
emulsion [9], allowing the consumer to sense the hydrophilic components in the aqueous phase, such as sodium chloride [10]. From an engineering perspective, an increased residence time of solid foods in the mouth, corresponding to more chewing cycles, will produce further breakdown [5] (until the food bolus is completely formed) and will increase the diffusion rate of taste triggering molecules to the receptors. For soft solids, the release and flavour deposition of volatile aroma are associated with the mixing caused by the tongue movements [5] and the residence time in the mouth [11]. To increase the residence time of soft solids and liquids in the mouth and therefore increase aroma or flavour release, another approach is to enhance the interaction between the food and the oral surfaces (mucosa). In this respect, the use of mucoadhesive biopolymers seems an attractive route, a successful demonstration of this has been given by the pharmaceutical industry for controlled drug delivery on the oral mucosa but also on other mucus covered surfaces [12,13]. Some food grade materials such as alginate and pectin have strong mucoadhesive capabilities [12,14–16] and constitute good candidates as taste enhancers through adhesion mechanisms. In order to obtain a better understanding of the behaviour of colloids in the mouth, the action of the mouth on foods from a hydrodynamic point of view has to be considered as well. Data on the shear conditions experienced by a variety of soft solids and liquids in the mouth has been collected, reported and compared with rheological measurements by Shama and Sherman [17,18]. Further literature in this area not only comes from the food science arena but also from dental research. As the shear field in the mouth is very heterogeneous [5], and it has been proposed that the sensations of thickness and creaminess are also “measured” in elongational deformation as well as shear [5]. This idea was further investigated by using a tribometer (see Fig. 2(a)) to measure friction coefficients (torque required to rotate the ball divided by load applied) as a function of the relative rotational speed, also known as a Stribeck experiment. In the case of guar solutions of different concentrations a good agreement with sensory data (slipperiness) was found for rotation speeds between 40 and 250 mm/s [19] (cf. Fig. 2(b)). This range of speeds was used to calculate an average Reynolds number (for a Newtonian fluid) in the mouth considering a gap of 0.25 cm between the tongue and the hard palate [20]. These were then compared with the results from Nicosia and Robbins [20], whose modelling work estimates in mouth Reynolds number during swallowing, as a function of the pressure applied between the tongue
Fig. 2. Schematic of a tribometer (a) and Stribeck curves of o/w emulsions of matched viscosities (at 50 s− 1) [19] (b).
and the palate (squeeze flow). This comparison is shown on Fig. 3, where one can see that similar Reynolds numbers (within one order of magnitude) were obtained over the range of conditions used.
Fig. 3. Comparison of two methods (Re in a squeeze flow as modelled in [20] or standard Re calculated from velocities measured in [19]) to estimate flow fields in the mouth.
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The good agreement between these data is an important finding since slipperiness, correlated to the friction coefficient, is a component of the creaminess perception [8]. 3. Perception of creaminess (fattiness) and astringency during oral processing Kokini and others [8,21–23] developed a creaminess “equation” linking the perception of creaminess (intuitively related to fat content) with other sensory attributes from trained panellists such as thickness (related to the rheological properties), smoothness (related to the particle size in the colloidal system) and slipperiness (related to the tribological properties). This relationship has been formulated as: 0:54
Creaminess∝Thickness
0:82
⋅Smoothness
ð1Þ
or Creaminess = 0:54⋅ logðThicknessÞ + 1:56⋅ logðSmoothnessÞ
ð2Þ
−0:32⋅ logðSlipperinessÞ in two studies [8,23]. The fact that a strongly hedonic sensory attribute such as creaminess can be explained by these attributes suggests that creaminess is not only controlled by the rheology of the product [19,24,25], but also by its lubricating properties. This is consistent with the observations from a study of Lillford [5] in which it is indicated that in the mouth, food is subject to not only a range of shear rates but also extensional flow; for more theory on this matter, the reader is directed to Steffe [26]. On the other hand, astringency seems to be related to an increase in the friction coefficient [19,27]. In recent work [7,28], the injection of a typically astringent compound, epigallocatechin gallate (ECGC), a polyphenol extracted from green tea, was used while measuring the friction coefficient of saliva, was found to increase the friction coefficient from around 0.1 to 20. Under similar conditions, addition of water to the system also increased the friction coefficient, but to a lesser extent. The main difference rising from the introduction of ECGC is the rate at which the friction coefficient increases. Such data seems to confirm the original work from Malone et al. [19] in which astringency is related to the flocculation of the material in contact with the oral mucosa. In conclusion, the various attempts made to formulate low-fat products by only matching their viscosity (to that of the full fat equivalent) have failed because of the lack of understanding oral processing, as well as the microstructure of the product. Indeed it appears that as suggested by Lillford [5] “texture is a consequence of the microstructure”. 4. Strategies for reduction of fat levels in food In order to formulate successful low-fat products, one has to design, construct and control the microstructure of the system so that it resembles to that of the full fat equivalent. This will then be reflected on the macro scale by its rheological and lubricating properties and ensure maximum consumer acceptance. 4.1. Emulsion based products Typical examples of such products are low-fat spreads. It is quite clear that, to match the consumer's reference (butter or cream), a similar microstructure can be achieved by using w/o emulsions of similar droplet sizes [29–31]. In Fig. 2(b), tribological data is used to illustrate this idea. Emulsions containing 20% oil or more respond very similarly to pure vegetable oil, and emulsions with 15% oil or less appear to the pure oil's behaviour. As the rotational speed of the disc
increases (up to ∼100 mm/s− 1) the gap between the ball and plate increases and the flow becomes what is known as a mixed regime (hydrodynamic and tribological). Within this regime the difference between the samples is maintained although the size of the difference is reduced with increasing speed. Finally at higher speeds, where the flow enters the hydrodynamic regime, all the samples behave similarly. This is probably a consequence of the matched viscosities, which dominate in the hydrodynamic regime. This data suggests that there is a lower limit of fat content in emulsion based products required to give acceptable performance upon consumption; that is somewhere between 15 and 20% oil. However, as discussed at the later parts of this review, gel particles or air-filled emulsion droplets can be made to give the same thin film behaviour as the pure oil or high fat emulsions and therefore may well give the sensory properties of fat. A similar approach has been adapted recently to develop a low-fat chocolate. It is well known that chocolate is particularly appreciated for its in mouth melting characteristics and mechanical properties [32]. These are controlled by the polymorphism of cocoa butter and the strength of the fat crystal network. The use of “margarine lines” (scraped surface heat exchanger in series with a pin stirrer) to produce water-in-cocoa butter emulsions to replace the pure fat phase present in chocolate has been identified as a successful opportunity to produce a low-fat alternative without any noticeable impact on the cocoa butter polymorphism [33]. Here the processing conditions are important as the cooling rate and the shear experienced in the margarine lines are similar to that of traditional chocolate tempering processes. However, a future challenge will be to match the microstructure of a fat crystal network using such emulsions, as it controls another important sensory attribute; i.e. hardness [34]. To this end, to keep the macroscopic properties of the final chocolate product similar to that of traditional chocolate, gelatin [35] has been used to set the aqueous phase of the emulsion while keeping melting properties of cocoa butter based chocolate, due to the appropriate melting point of the gelatin. Other products that could benefit from the microstructural approach are sauces and dips. These are typically o/w emulsions and contain a large oil volume fraction (up to 80%). In order to obtain a similar microstructure with a reduced fat content, double emulsions w/o/w could be used [36], using the water droplets as filler particles in the oil phase. Fig. 4, shows a typical w/o/w emulsion, stabilised using PGPR (w/o) and NaCas (o/w), and produced in a high shear mixer. Similar systems have been produced using a Pickering stabilised internal phase (w/o), avoiding the use of PGPR [37]. Then a membrane emulsification step can be used to produce the w/o/w emulsion, as the Pickering fat crystals can be damaged by the high shear and temperature involved in high shear mixers. Such emulsions also offer the advantage of allowing entrapment of actives in the internal water phase, which could be delivered in strategic locations in the GI tract or for masking an unaccepted tastes [36] and avoiding its “detection” in the mouth. As already mentioned, successful manufacture of these complex emulsions cannot be easily made with conventional high shear mixers or homogenizers. A more successful, and elegant, solution is the use of membrane emulsification. This technique is usually found in a crossflow mode of operation [38]. Emulsion preparation in such membrane systems is then controlled by trans-membrane pressure, cross-flow velocity and pore size of the membrane [38–40]. However, due to the direction of the flow in a cross-flow setup (parallel to the membrane surface), coalescence can occur between droplets that are newly formed/forming from neighbouring pores. This phenomenon will depend on the density of pores on the membrane surface and if not addressed will lead to an increase of the average droplet size [38]. To avoid such problems, a solution can be found in decreasing the boundary layer and hence the residence time of the droplet close to the membrane by having a rotating membrane. The centrifugal
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In order to reduce the fat content of food colloids but keep a similar microstructure to that of the original full fat system, a phase separating mixture of biopolymers can also be used. One example is the use of gelatin–maltodextrin mixed biopolymer system which under controlled conditions will produce a “water-in-water” emulsion [41]. By studying the gelation kinetics of the individual phases [42] and understanding polymers incompatibility in solution before the sol–gel transition, such structures have been used to produce 0% fat spreads [43]. Although feasibly possible, it should be noted that a small amount of fat may be required for efficient flavour delivery. The gel strength of such structures was also studied and found to be mainly controlled by the properties of the continuous phase [44]. An ingredient that has been used very successfully in this respect is a low dextrose equivalent maltodextrin [3]. The reason for this is that short chain hydrocolloids form crystalline type structures, which, when
used in mixed biopolymer systems, mimic the organoleptic and material properties of fat crystals. Fig. 6 shows some of the data obtained for a gelatin/maltodextrin water-in-water emulsion in which liquid oil was added to produce a low or very low-fat spread with virtually no saturated fatty acids (UoB data). Fig. 6 also shows, for the sake of comparison, data obtained for a commercially available fat continuous low-fat spread containing ∼ 35% fat in which the material properties (spreading, plasticity, etc.) depend upon crystallisation of the saturated fats to form a fat crystal network. Fig. 6 demonstrates that by producing a water-in-water emulsion with similar material properties to the commercial margarine and then by adding either 20% or 40% liquid vegetable oil as a fine emulsion, the flow and fracture properties can be matched. As stated the material properties of the water-in-water emulsion depend upon the continuous phase and as such depend on the bloom strength of the gelatin for this system. As the oil is emulsified into the water-in-water emulsion as fine droplets, the overall product properties have only a weak dependence on the oil content, which appears to be behaving as a soft filler. This is quite encouraging as it suggests that further oil reduction in such a formulation can be made. Another approach to produce particulate structures in water based systems is the use of sheared gels [45–47]. By applying shear during the gelation of polysaccharides (e.g. gelatin, carrageenans, and agar), the gelation process can be controlled to obtain gel spherulites in a continuous phase of water, i.e. cyclising the gel network. It has been found that controlling the formation of such structures during the process can be obtained by modification of the shear and cooling rates during processing. Interestingly it has been shown that increases in the biopolymer concentration do not affect the particles' volume fraction in the fluid gel system [47]. The same study reports on the behaviour of agar fluid gels with concentrations ranging from 0.75% to 5% and concluded that upon formation these fluid gels have the tendency to occupy the maximum volume fraction. However the material properties and architecture of the different particles will change dramatically. These suspensions have a number of interesting properties including the ability to suspend particles or, if constructed in the right way, to produce particulate gels with properties that are very similar to protein gels formed with milk proteins [46]. The viscoelastic properties of fluid gels depend on their microstructure as well as the interactions/bridging between the produced particles. Both can be modified and controlled by typical formulation/ process parameters such as the rate of deformation and the rate of cooling, during the temperature quench applied to induce the conformational ordering and gelation of the biopolymer. In previous
Fig. 5. Droplet size distributions for oil-in-water emulsions produced using the rotating membrane (□) and cross-flow membrane (●) emulsification techniques. Both systems are using a 0.8 μm pore size SPG membrane and both are operating at a 60 kPa transmembrane pressure.
Fig. 6. Stress/strain curves for a low-fat spread (Flora) and for water-in-water emulsions (20%wt/wt maltodextrin/4%wt/wt gelatin containing 0.1 m NaCl) also containing either 20%wt/wt (open symbols) or 40%wt/wt (filled symbols) sunflower oil emulsified with 0.5%wt/wt Tween 80 (oil droplet size ∼5 µm). All curves obtained upon compression of the materials using an Instron material tester.
Fig. 4. Typical w/o/w double emulsions.
force applied to the droplet due to the rotation propels it far from the membrane preventing coalescence between the formed droplets. By blocking the back reaction (coalescence), the size of the droplets achievable through rotating membrane emulsification is therefore lower (see Fig. 5) for similar cross-flow process parameters (trans-membrane pressure, membrane pore size, and surfactant concentration). 4.2. Polymer/gel based products
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published work, fluid gels were produced having a broad particle size distribution and in many cases a significant number of irregularly shaped particles, both may produce interesting oral properties. A detailed study on the rheological properties of spherical agar particles has been carried out by Norton et al. [48]. The authors reported a relationship between rheology and morphology of fluid gel particles for a range of agar concentrations. They further showed that the viscoelastic properties (e.g. elastic modulus) increased gradually with polymer concentration, rather than the steep increase predicted from models developed for rigid particle suspensions. From this they concluded that the produced agar particles behaved as highly deformable entities as would fat droplets in an o/w emulsion [48]. Recently a totally new approach for fat replacement in emulsion based foods has been developed [49]. This approach delivers emulsions where a significant proportion of the fat phase is replaced by “emulsified” air droplets. In order for these “air-filled emulsions” to work it needs to be ensured that the air-filled droplets resemble the fat droplets they are designed to replace; this is in terms of their size, shape, rheological properties and interactions with the rest of the structure. The construction of air-filled emulsions uses a novel group of proteins (the hydrophobins), which assemble at the air/water interface and then aggregate to give gel like structures [50]. This interfacial structure then imparts an elastic restoring force and as the air droplets try to ripen it stops any change in droplet size for the months necessary for product stability even with air droplets of only a few microns. The air-filled emulsion structure is shown schematically in Fig. 7(a). In order to give both a rheological match while maintaining the flavour of the product triphasic emulsions can be constructed using a combination of air and oil filled emulsions, as in Fig. 7(b), within a single product structure [27]. Although at first sight it would seem that hydrophobins are unique molecules, they are also a very expensive ingredient, which, in today's fast moving consumer goods industry, can render their use uneconomical. Much cheaper alternative food grade proteins, that can be processed to give all the properties of hydrophobins in foams, air-filled emulsions and triphasic systems have recently been investigated. This advance makes the approach financially viable for foods and will potentially also allow these structures to be used in other non-food products. An important and so far unanswered question is: can air cells give the sensory properties of oil droplets? As previously discussed, tribology is a good way of characterising the “oral” properties of products and correlating them with data from sensory panels. Recent work using tribology has shown that air whipped into a dairy cream alternative reduces the friction between soft surfaces as if the oil content of the product had been increased (Fig. 8). It therefore seems
Fig. 8. Tribometer curves for full fat and low-fat Elmlea (dairy cream alternatives) and aerated (●) low-fat Elmlea.
that not only water could be used to create low-fat alternatives to indulgent creamy foods but that simple air cells (although complex to stabilise) could also be the innovative ingredient required to engineer tomorrow's foods. 5. Conclusion In this review various routes to engineer and manufacture low-fat alternatives to indulgent fatty foods (mayonnaise, cream, sauces and spreads) have been proposed. As such products would have to be accepted by consumers to be commercially successful, an understanding of the in mouth processes from a mechanical point of view is required. A number of studies have been conducted to quantify the mechanics of oral processing and correlations can be found between theoretical and empirical studies. The use of a microstructural approach to build appropriate systems (double and triphasic air-filled emulsions, fluid and mixed gels) that can mimic the properties of fats has been successfully validated using experimental techniques that relate to the oral response, such as rheology and tribology. These have confirmed that the macro scale properties (e.g. texture) derive from the micro scale scaffold. Although this approach has proved successful for fat reduction, there are many more challenges to overcome such as salt reduction as well as the replacement of sugar in product structuring, ice content
Fig. 7. A schematic representation of the hydrophobin stabilised mayonnaise (a) and photomicrograph (b) picture width 100 µm, of an air and oil filled emulsion after 4 days storage. Oil droplets have a mean diameter of ∼ 8 µm and the air-filled droplets have a mean diameter of ∼2 µm.
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