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Designer colloids—towards healthy everyday foods?

J. E. Norton* and I. T. Norton

Received 15th January 2010, Accepted 24th March 2010

DOI: 10.1039/c001018a

Dietary related diseases are on the increase. Colloids play an important role in food structuring,

stability through the distribution chain, and delivery of in use properties, taste and flavour, and

nutrition. In the future it is imperative that foods are constructed to deliver the desired performance

and sensations, but with reduced calorific content and the ability to modulate food intake. This will

require the design and construction of colloids with increasingly complicated structures and

complex production processes.

1. Introduction

Obesity is increasing throughout the world as a result of chronic

energy imbalance: people intake too many calories and do not

take enough exercise.1 This has many health risks, including

cardiovascular disease, diabetes, stroke, arthritis and some forms

of cancer. Although governments are attempting to educate

people into changing their eating habits, moving towards

a balanced diet, with greater intake of fruit and vegetables and

increased exercise, there is little evidence that this is suitable for

the entire population. As many modern foods are emulsion

based, the challenge for the food scientist is to develop colloidal

foods that are acceptable in terms of their performance, but

contain fewer calories (i.e. with less fat and sugar/digestible

carbohydrate2) and are digested slowly. Specifically dietary fat is

very energy dense, but has a limited effect on suppressing

appetite. However, fat within food is very important for both

texture and flavour release, so cannot be removed entirely from

a product. A number of methods for successful fat reduction will

Department of Chemical Engineering, University of Birmingham,Edgbaston, Birmingham, UK. E-mail: JXN734@bham.ac.uk

J: E: Norton

Jennifer gained a BSc in

Psychology from the University

of Wales, Bangor in 2007. She

began her PhD, entitled Engi-

neering Healthy Indulgence,

within the Department of

Chemical Engineering at the

University of Birmingham in

2007. Her work bridges the

gap between consumer

psychology and microstructural

engineering, focusing on fat

reduction in chocolate, and

consumer perception towards

reduced-fat chocolates. Jennifer

is also completing a Post-

Graduate Certificate in Sensory Science from the University of

Nottingham.

This journal is ª The Royal Society of Chemistry 2010

be discussed here in detail, including the use of emulsions,

Pickering emulsions, double or duplex emulsions, water in water

(W/W) emulsions and gelation. Another significant problem is

that the modern diet contains too much salt, resulting in

increased blood pressure, and an increased risk of stroke. The use

of duplex emulsions for salt reduction will also be discussed.

Convenience foods are often too easily digested. This results in

macronutrients being taken into the body too quickly, which are

stored rather than used as they are released. As a consequence,

the consumer quickly feels hungry again. Structures need to be

developed that retain the convenience and ease of eating, but

slow down digestion once in the stomach. The use of self-struc-

turing systems that respond to environmental changes inside the

body will be discussed. Furthermore, the delivery of micro-

nutrients is not optimal: nutrients are delivered to the wrong part

of the gastrointestinal tract and/or too quickly. The use of

designed colloids to encapsulate and release micronutrients will

be discussed. Finally, it is important to consider both the physical

aspects of food consumption and how psychology plays a role in

food acceptance and liking; consumers are affected by not only

the taste of the food, but also it’s visual appeal, smell and even

I: T: Norton

Ian Norton was educated at the

University of York; firstly

receiving a BA in Chemistry

(1977) and then a DPhil (1980)

for research on Fast Reaction

Kinetics of Conformational

Ordering of Polysaccharides.

He then joined Unilever

Research and progressed to

become Science Director for

Foods and Chief Scientist in

2001. In 2006, he joined the

University of Birmingham as

Professor of Microstructural

Engineering. He has developed

a new research group and in

addition is the Director of Research for the School of Chemical

Engineering. He has published many original papers and has more

than 60 granted patents.

Soft Matter, 2010, 6, 3735–3742 | 3735

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the sound it makes when bitten. This has been touched upon with

the inclusion of a small section on consumer psychology, which

may be used in the future to help the design of colloids for food

use.

Fig. 1 Tribometer curves for o/w emulsions containing different levels of

fat at 35 �C. All emulsions were made to have the same viscosity at a shear

rate of 100 s�1 (Malone et al.5).

Fig. 2 Change in lubricating properties of saliva after addition of water

or test samples. Tests were conducted at 3 N load, 20 mm s�1, and 25 �C.

Samples were added at 8 min (dashed line) (Vardhanabhuti et al.7).

1.1 Foods in the mouth

When considering how colloids may be designed for healthy

foods there is a need to consider what happens once they are

consumed. The first stage that requires reconsideration is pro-

cessing of foods in mouth. When food is placed in the mouth it is

mixed with saliva and broken down by both the chewing action

and, if it contains starches, the enzymes that are present in saliva.

In addition, the temperature of the food changes as it starts to

approach body temperature. These effects lead to the release of

flavours and tastes.3 The food will also coat the skin surfaces via

a direct interaction with the bio-surfaces in the mouth or via an

interaction with the mucins. This has to occur in very short

timescales (a few seconds). If the foods contain fat then oil in

water emulsion is formed, even if the food is fat continuous, as it

enters the mouth. However, it is known that the presence of fat

modifies the feel of the food and lubricates mouth/particle

interfaces.4

What does this mean for colloidal foods with a reduced fat

content? A relatively recent development in the understanding of

how colloidal foods modify lubrication involves the use of soft

tribology (thin film rheology). The work, initially developed by

Malone et al.,5 showed that oil in water emulsions reduced the

friction of soft surfaces in a soft tribometer, and that the amount

of fat is critical in determining the friction below a fat content of

around 20%. Researchers also looked for a correlation with

sensory analysis, and at speeds of 20–100 mm s�1 (the tribological

and mixed regimes) a strong correlation (R > 0.95) was observed.

The speeds at which the best correlation was obtained are

reasonable speeds for the movement of surfaces past each other

in the mouth when chewing. As shown in Fig. 1, emulsions with

very low levels of oil (<20%) cannot lubricate the surfaces as well

as the vegetable oil. However, once the emulsion contained 20%

or more oil there was little observable difference between pure oil

and the emulsions. All the emulsions used in the study were iso-

viscous at 100 s�1. To remove any viscosity effects that might

occur the viscosities were matched at 100 s�1 as this shear rate is

often claimed to be most relevant in the mouth.

These data also show that at the higher speeds, where

a correlation was found, there is little difference between the oil

and emulsions studied. These are the speeds where one might

expect the 100 s�1 shear rate to be most appropriate, and this

appears to be the case. The distinction between the emulsions is

greatest at the lower speeds i.e. 20 mm s�1. If mouth coating is

occurring then it might be expected that this is a consequence of

slow moving or stagnant times in the chewing process.

More recently, the use of a tribometer constructed with a pig

tongue6 has been investigated, as has the role of saliva on the

friction of soft surfaces. The work carried out by Vardhanabhuti

et al.7 showed that the addition of saliva to a tribometer running

at 20 mm s�1, with a normal force of 3 N, resulted in an imme-

diate drop in friction (Fig. 2). On addition of water the effect of

the saliva decreased with time slightly, increasing friction due to

a dilution effect. On addition of a number of food polymers the

3736 | Soft Matter, 2010, 6, 3735–3742

friction was found to increase more rapidly than observed for the

addition of water: these materials had a greater effect than

dilution alone. The more astringent molecules (for example

catechin) were shown to have the greatest effect, potentially as

a consequence of binding to the mucins.

This work shows the importance of the mucin layer in

controlling the friction coefficient in the soft tribometer, with

results correlating well with sensory analysis. It clearly demon-

strates that in order to understand the role of foods and food

components in the oral sensation, future work should consider

the role of saliva.

1.2 Foods in the rest of the gastrointestinal tract

What happens after swallowing and how do colloids change this

behaviour? Food that has been masticated in the mouth is then

deposited into the stomach. The enzymes that have been added in

the mouth continue to break down the food structure, and the

This journal is ª The Royal Society of Chemistry 2010

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stomach adds acid to the mix. Stomach acid has two roles: to kill

any microbes that have been ingested and to aid break down of

the food structure. This is all carried out under a gentle mixing

process. There have been a number of attempts to build

mechanical models for digestion,8 but thus far they are very

simplistic, based upon the principle of stirred tanks in sequence,

and do not represent the mechanical or biological aspects of

digestion. A better approach has been adopted by Spiller’s

group,9 which attempts to measure inside living humans. This

will improve dramatically when non-invasive measurement

techniques are used (for example MRI). What happens to

colloids in the food? If the colloidal system is an emulsion then it

will often break in the stomach as a consequence of degradation

of the emulsifying agent. However, if the emulsion is stabilised by

emulsifiers that are not influenced by the stomach action they

stay intact, so the consumer is largely unaware of the fat content

of the food consumed. In modern foods this can lead to high fat

products ceasing to give the satisfaction expected, resulting in an

over consumption of fatty products. In addition, the enzymes

and acids attack any protein or polysaccharides that are acces-

sible to the stomach action. This leads to loss of structure for the

proteins or starches, whilst other polysaccharides are left largely

untouched. These processes need to be understood if colloids are

to be designed to impact on food consumption. Examples of how

this might be achieved are given later in the review.

After leaving the stomach the food enters the small intestine

where further digestion and absorption takes place. This is where

bile salts are added which cause emulsification of the oils and fats

(which have been released from the food in the digestion process

within the stomach) into small droplets that are then absorbed by

the body. Again it is important to know how the emulsifiers used,

and the length scale of the structural elements of the food can

impact (both positively and negatively) on this stage of diges-

tion.8 As our ability to design and construct the interfacial layer

of emulsions and make ever smaller food grade emulsion drop-

lets10,11 increases, the manipulation and control of this stage of

digestion will change, which offers great potential for developing

healthier foods.

The last important part of the process (from a colloidal

perspective) is the large intestine. This is where the micro-flora

resides, and where any remaining large molecular weight mate-

rials (for example polysaccharides) can be attacked, and any final

adsorption into the body occurs. Examples of polymers that are

broken down at this stage include crystalline amylose and

amylopectin (both resistant starches), which release essential

materials such as butyric acid. Again colloidal structures can be

designed to carry micronutrients to this part of the digestive

system.

1.3 Foods in the mind

Having understood, to some extent, what happens in the

consumption of foods, it is important to consider how consumers

respond to the food. Understanding the needs of the consumer is

incredibly important as this information can be used to tailor the

reformulation of products. By identifying the sensory cues

(for example visual, tactile or olfactory cues) that influence liking

it may then be possible to create a microstructure that has these

properties but is lower in fat, calories and salt.

This journal is ª The Royal Society of Chemistry 2010

It is difficult to manufacture low fat foods that have the same

depth of flavour as higher fat equivalents, and physically behave

in the mouth in the same way. Therefore, low fat products often

have poorer textures and flavour release. This may have led to

the perception by the consumer that one has to compromise

between either eating healthy, unpleasant foods or indulging and

eating tasty, yet unhealthy foods. What consumers expect from

a product can often impact upon their opinions on tasting the

product. Many studies have investigated how information

influences expectations, or sensory and hedonic ratings of foods.

Such experiments may give insight into how expectations may

affect perceptions and therefore how low fat products may be

perceived. K€ahk€onen et al.12 showed that a high-salt spread was

rated as saltier when information was available, and K€ahk€onen

and Tuorila13 showed that when given information about fat

content participants expected the light version to be less pleasant.

K€ahk€onen et al.14 showed that different types of information

raised different expectations, with reduced-fat information

leading to lower expected melting rate in chocolate. However,

information did not affect the pleasantness ratings of the choc-

olate bars after eating, suggesting that consumers expect sensory

differences between reduced-fat and regular fat products, but

these expectations do not seem to affect hedonic ratings. Wan-

sink et al.15 results suggest that health or diet labels are likely to

influence the subjective taste of unhealthy foods, but not foods

that are already viewed as healthy, where the label has less of an

impact. Wansink and Park16 demonstrated that a label indicating

the presence of a ‘‘phantom ingredient’’ led to health claims

becoming more believable, but negatively influenced taste

perceptions. Levin and Gaeth17 demonstrated that when infor-

mation about fat was framed in a positive way the product was

rated higher than when framed in a negative way. However, this

framing effect is more pronounced in studies where the product is

not consumed, implying that personal experience plays a prom-

inent role in judgments.

Expectations about fat content may also affect the amount of

product that is consumed, with participants eating higher

quantities of a reduced fat product than an equivalent higher fat

product. This may result in increased energy intake, both in

terms of the product itself and also during subsequent meals.18

Thus, in order to design colloids that have a real impact on diet

and health, it is not only important to control the sensory

properties to deliver pleasant tastes and textures, which has been

the focus of research conducted to date, but also consumers need

to be convinced that the product is adding value and that the

technology used is acceptable. This will require consumer cues

(visual, tactile or olfactory) to be designed into the product,

which will often be colloidal in nature.

2. Designer colloids

It is obvious that something needs to be changed so that healthier

foods are accepted as part of an everyday diet. How can colloids

be designed to achieve this? A successful strategy is to consider

the microstructure of the product that is to be re-formulated, and

to design the new product with the same microstructural

elements, but with new ingredients used to build the structure

(Fig. 3). This is often referred to as formulation engineering or

indeed microstructure engineering.

Soft Matter, 2010, 6, 3735–3742 | 3737

Fig. 3 Schematic representation of the microstructure approach,

showing typical material properties and consumer aspects that are

impacted by product microstructure along with which parameters can be

used (i.e. process and starting materials/ingredients) to design the product

microstructure.

Fig. 4 Electron microscope picture of a cocoa butter emulsion with 30%

included water, showing the microstructure of a continuous fat phase

incorporating oil and triglyceride crystals and water droplets which have

a fat crystal shell around them (scale bar is 10 mm) (Norton et al.22).

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2.1 Fat reduction

2.1.1 Use of emulsions as partial fat replacement. The first

approach is to replace pure fat with an emulsion, with the

aqueous phase containing as few calories as possible. For fat

continuous systems, for example margarine or bakery fat, the

resulting product will be a fat continuous emulsion with higher

phase volume of the included phase. However, for water

continuous products, with the oil phase as dispersed droplets, the

resulting product can be a duplex or double emulsion of the type

water in oil in water (w/o/w).

The advantage of making emulsions for fat replacement is that

the calorific value of the food will be reduced, while if the

emulsion is designed in the right way, and has the right material

properties, the presence of water is not detected by the consumer.

For example, if water is included in a oil droplet in mayonnaise in

such a way that it is not released until it is inside the stomach, or

intestine, then the consumer only detects the fat droplets, so will

believe it to be a full fat product.

This sound’s too good to be true. What is the penalty for going

down this route? The answer lies in the requirement for long-term

storage stability of many of the products. In order to lower the

fat content of a spread, products that have up to 80% water as an

included phase volume, that are stable for 12 months, and release

salt and flavour in the mouth, need to be produced. This requires

the capability to design droplets that include salt and are detected

on consumption, and other droplets that are not detected, and

contain no salt. As can immediately be seen this is starting to

become a very complex design. How can such complex systems,

with different types of colloidal elements (i.e. that are released in

different ways, with different osmotic pressures), be made at an

economic scale, and with equipment that can be run within

a food factory?

If the product is a water continuous duplex emulsion then the

problem is more challenging, as it contains two water phases,

both with the interface stabilised by emulsifiers, but with

different curvatures. In order to achieve this two different

emulsifiers need to be used. However, if they are partially soluble

in the oil phase (which most are), then due to entropy, they will

move from one interface to the other over time. Consequently,

the distinction between the two interfaces will be lost and the

duplex emulsion will collapse. Until recently this problem has

3738 | Soft Matter, 2010, 6, 3735–3742

only been overcome using PGPR. This is a large molecular

weight emulsifier that has very low solubility in the oil and tends

to aggregate at the interface to give elastic properties. Unfortu-

nately, PGPR is prohibited in many foods. How can colloids be

constructed to deliver the properties necessary to include water in

high quantities that are not detectable?

2.1.2 Pickering. An effective way to produce stable emul-

sions is to use particles at the interface (Pickering emulsions19).

Unfortunately, particles are often not food grade, or are too

large to stabilise emulsions in the required dimensions (microns).

If some water needs to be perceived for flavour release, this

brings a further complication: particulate stabilised emulsions

are very stable due to the high binding energy of the particle in

the interface.20 If a combination of particles and emulsifiers

(Pichot et al.21) is used, this gives greater design freedom. This is

demonstrated to greatest effect if fat crystals are used as the

particulate material. The emulsifier concentration and type are

then selected to position the particles in the interface. This gives

two advantages: the crystals can be produced at the required

point in the production process, and the melting of the crystals

can be used to cause destabilisation of the emulsion when

required, for example in the mouth. By carefully controlling the

process the crystals can be sintered at the interface to produce

shells, which can give perfect coverage to the droplet. By doing

this it is possible to construct a colloidal system in which water

can be added to high levels (50% or more), where it is commonly

regarded as impossible, for example in chocolate. Fig. 4 shows an

electron microscope picture of the cocoa butter emulsion.22 The

shell of fat crystals around the water droplet can clearly be seen.

The crystals have sintered together and the structure appears to

be fairly smooth. Of course this requires the production of

a crystalline shell with no defects. An advantage of a crystalline

shell is that it can form part of the crystal network, scattering the

cracks that are formed as a consequence of breaking or spreading

of the product. The crack appears around the droplet, so the

material contained in the droplets is effectively hidden; for

example, water is not detected in the continuous phase of the

This journal is ª The Royal Society of Chemistry 2010

Fig. 6 Photomicrograph of a duplex emulsion with the inner interface

stabilised by crystals (shell).23

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chocolate matrix. More recently, work has been carried out with

gelling of the aqueous phase to address this issue. This gives

further strength to the structure with the advantage that gelatin

melts at the same temperature as the fat phase, so that any

materials contained in the droplets will be released on

consumption.

2.1.3 Duplex emulsions. As mentioned, an alternative

method of fat replacement is to trap water inside the oil/fat

droplet, within a continuous aqueous phase: double (or duplex)

emulsions.4 An example is given in Fig. 5 in which a primary

emulsion of water in oil (w/o) was produced using PGPR as the

emulsifier. Once formed, the primary emulsion is then used as the

included phase in the production of a further oil in water (o/w)

emulsion. This is achieved using a second emulsifier, in this case

Tween 20.

Duplex emulsions can be unstable which has limited their

application in real foods. The major source of instability is

caused by movement of the emulsifiers between the two oil/water

interfaces. The most common way of preventing this is to have

a large emulsifier which aggregates at one of the interfaces to give

gel like structures, hence the use of PGPR. Another major source

of instability is movement of the water between the two phases,

which is enhanced if the interfaced layer of the primary emulsion

is damaged in the emulsification process to form the second

emulsion. This results in release of the contents of the primary

emulsion. The use of PGPR is not ideal as it is prohibited in

foods in general, and the emulsions produced are not stable for

the shelf life of a product. Furthermore, ripening effects occur

when droplets of different sizes are present i.e. Laplace pressure

effects. However, if the primary emulsion can be designed to

include fat crystal shells to deliver long-term stability, then it may

be possible to produce the secondary emulsion by only applying

low levels of shear. Very recently,23 crystallising triglycerides

have been used for Pickering stabilisation of the primary emul-

sion in a duplex emulsion. The crystals were sintered together to

produce an intact structure surrounding the droplet (Fig. 6). The

primary emulsion has been produced to a sub-micron size; and

the secondary emulsion has a droplet size of approximately

10 mm. The crystalline structure, which has been produced using

a combination of crystallising mono- and triglycerides, is visible

within the secondary droplets, and crystals are visible around the

primary droplets.

Fig. 5 A photomicrograph of a w/o/w duplex emulsion stabilised with

PGPR. The inner droplets are approximately 10 mm and the secondary

emulsion has droplets of approximately 100 mm.

This journal is ª The Royal Society of Chemistry 2010

These emulsions are stable and have the ability to segregate

additives, even when there are high osmotic force differences

between the two water phases.

2.1.4 Gels as colloidal particles. An alternative colloidal

approach involves building gel structures with rheological

properties and dimensions similar to the oil droplets being

replaced. Using the microstructure design rules, sheared gels, for

example agar, carrageenan and alginate,24 have been developed.

By applying shear during the gelation process, gel particles are

produced with the same rheological properties as the bulk gel. If

the applied shear is turbulent and high enough the gel particles

are spherical. Thus, particles can be produced with properties

that can be tailored to match the structures they are replacing, for

example oil droplets.

Recent work has shown that the rate of cooling, at a set shear

rate in the process, affects the sheared gel obtained (Fig. 7).

Kappa carrageenan fluid gels produced at a single polymer

concentration, but at different cooling rates, can behave quite

differently in terms of their rheology. Materials produced at the

lower cooling rates having greater inter-particle interactions,

resulting in higher yield stress and higher viscosities once they are

flowing. It is argued that this is a consequence of the particles

being larger and having a more irregular shape when produced at

the higher cooling rates. As the time scales/shear rates in the

process are more closely matched to the kinetics of gelation

(i.e. at the slower cooling rates), the gel particles become more

spherical and have less cross-linking.

There are many parameters that can be changed in the

formation of the gel particles, which can affect the shape and size

of the particles, and the extent of bridging between particles. This

affects physical properties, such as yield stress. Other parameters,

which need to be considered include the type and concentration

of the hydrocolloid used. All these factors allow the material

properties to be manipulated to match the oil droplets that are to

be replaced.

Recently it has been shown that agarose sheared gel particles

modify the friction within a tribometer and are predicted to have

the ability to act like fat in the mouth particularly if they are in

the 10 mm size range and reasonably soft.

Do these particles offer a way forward in healthy foods? In

mayonnaise, if the majority of the oil droplets are replaced by

soft elastic spherical gel particles (agarose based), then the

Soft Matter, 2010, 6, 3735–3742 | 3739

Fig. 7 Stress/shear rate curves of 0.5% kappa carrageenan sheared gels

produced at a constant shear rate but with different cooling profiles.30

Fig. 9 Gelatin (LH1e)/maltodextrin (SA2e) phase diagram (squares are

experimental points, circles are fitted binodal: protein is the light phase in

the micrographs) (Amard et al.31).

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overall bulk rheology of the mayonnaise can be matched (Fig. 8).

Tribological data suggest that at 5% oil concentration the thin

film behaviour will be significantly affected, resulting in an

improved lubrication score in sensory analysis, although no such

data have yet been reported in the open literature.

The indications are that gel particles can, to some extent,

replace fat droplets in an o/w emulsion.

2.1.5 Water in water emulsions. If two hydrocolloids are

mixed in solution they are likely to phase separate. The simplest

way to consider this is to regard them as composites where one of

the components forms a continuous network across the entire

system and the other serves as a gel filler. As these systems

contain 70% or more solvent they resemble an oil/water or water/

oil system,25 and behave as such under shear. The phase diagram

(Fig. 9) shows the separation of gelatin and maltodextrin. The

photomicrographs show how the microstructure and phase

continuity depend on the relative phase volumes of the two

phases. As can be seen from this set of photomicrographs, the

Fig. 8 Stress/shear rate curve for: a 5% sheared agar with 3% phase

volume of oil droplets (droplet size z 1 mm) compared to a full fat

Hellman’s mayonnaise using a roughened cone and plate geometry on

a Rheometrics instrument.

3740 | Soft Matter, 2010, 6, 3735–3742

mixed biopolymer system behaves like a w/o mixture in which the

phase with the highest phase volume dominates. When a 50 : 50

mixture is present the structure attempts to be bi-continuous. In

mixed biopolymer systems this bi-continuous region persists over

a far wider phase volume range than is normally observed for oil

and water mixtures: in the absence of emulsifiers, phase inversion

occurs over less than 1% variation between the phases. The

design principles for these types of system have been described

before.25 This figure also clearly shows how the two phases

formed are not pure (as is the case for oil/water mixtures), but

each phase contains a low level of the second component (as is

the case for polymer mixtures). However, in these w/w systems

the solvent (i.e. water) makes up 70% of the total mass. This

water is free to move between the different phases.

It is likely that, in the future, ‘emulsifiers’ will be discovered for

these w/w systems, although what these will be is not, as yet,

clear. If these ‘emulsifiers’ have a similar effect to those observed

for current surfactants (i.e. modification of droplet size, control

of breakup and coalescence of droplets in the process and on

storage) used in oil and water emulsions, then the design of

different structures will be possible.

Can w/w emulsions be used to produce healthy everyday

foods? Fig. 10 shows a gelatin/maltodextrin w/w emulsion that

has been designed to behave like a margarine. Liquid oil has been

added to produce a low or very low fat spread with virtually no

saturated fatty acids (safa). These w/w emulsions have material

properties very similar to the original high fat safa containing

emulsion (e.g. Low Fat Flora, Fig. 10). The major reason that

maltodextrin works in this system is that as an oligosaccharide it

forms aggregates which are crystalline in nature. These crystals

then interact to produce a crystal network. This crystal network

has properties that are similar to the triglyceride crystal structure

and network in the low fat spread.

2.2 Salt reduction

As duplex emulsions have two different water or oil phases,

different ingredients can be trapped in different parts of the

This journal is ª The Royal Society of Chemistry 2010

Fig. 10 Compression stress/strain curves obtained for a low fat spread

(flora) and a w/w emulsions (20% maltodextrin/4% gelatin/0.1 M NaCl)

contained 0 or 10% oil (emulsified with 0.5% (w/w) Tween 80, droplet

size z 5mm) using an Instron material tester.

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structure. If a w/o/w emulsion is constructed, salt can be con-

tained only in the outer water phase, so that it might be possible

to produce a product that is perceived as having a much higher

salt content than is actually present. The extent of salt reduction

possibility would depend on the ratio of the inner and outer

water phases, but potentially an 80% reduction is possible.

Malone et al.5 demonstrated the potential for this approach,

although this work was in controlling acid perception.

As discussed, although duplex emulsions have not found

universal use in foods so far, if the primary emulsion is stabilised

using shells23 this will change. If the shells are controlled care-

fully, to result in an intact structure, these ‘perfect’ shells have

been shown to resist the osmotic pressure differences of up to

a factor of 10, i.e. the difference between one water phase con-

taining 1% salt and the other being distilled water.

2.3 Sugar reduction

The sweetness of sugar has been successfully replaced in many

products using artificial sweeteners. However, in many products

sugar is present to control the structure and moisture content to

give soft/succulent products, for example cakes and biscuits. An

extreme example exists in ice cream, where the sugar controls the

Fig. 11 Effect of pH on the structure of a 2% gellan gel: (a) true stress–true

a function of pH.28

This journal is ª The Royal Society of Chemistry 2010

ice content and hardness of the product. If the sugar content is

reduced the product becomes too hard. As yet there is no way to

significantly reduce the sugar (and as a consequence the caloric)

content of these products. However there are ways that may

prove useful in the future (for example, the use of clays).

However, this will require a radical rethink of how foods are

constructed and what consumers will accept in food products.

This research is yet to be carried out.

2.4 Reduced consumption

An exciting new colloidal approach to help construct healthy

everyday foods is the use of colloids that respond to

environmental conditions, and so self-structure. If a rigid three-

dimensional structure is produced in the human gastrointestinal

tract then food consumption might be impacted. Work by

Rayment et al.26 following initial proposals by Norton et al.27 has

shown that if self-structuring systems are ingested then stomach

emptying is slowed. This was achieved using alginate to form gels

inside the stomach. Initial work was not optimal as only partial

stomach gelation was achieved.

The major problem with alginate is that control of the gelation

kinetics is very difficult, although this is likely to be achieved in

time. As a consequence of this difficulty, investigators have

looked at alternative materials, such as gellan. Recent work28 has

shown that the control of acid gelation of gellan is possible,

which offers real possibilities for self-structuring of the stomach.

An advantage of gellan is that the acid range required for

gelation is very close to that observed in the human stomach.

In addition, and more importantly, the rate of gelation is slower

than that observed for alginate. Maximum gel strength is

observed between pH 3 and 4 (Fig. 11), with the stiffness of the

structures (Young’s Modulus) and the total energy required for

these structures to ‘‘fail’’ (Total Work of Failure), giving

maximum values between pH 3 and pH 4 (Fig. 11b). As pH was

reduced further, extensive aggregation of the gellan chains was

observed, and the gels became turbid. This aggregation resulted

in weaker gels that release water when compressed.

Further work is required, but the early indications suggest that

this method can be used to successfully structure the stomach

contents.

strain curves and (b) Young’s modulus and total work of failure given as

Soft Matter, 2010, 6, 3735–3742 | 3741

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These colloidal structures are likely to become more complex

in the future, as any food has to deliver calories to satisfy the

needs of the consumer. Thus, gels need to be constructed that

release calories over a number of hours following ingestion. This

requires components, such as starch or fats, to be included in the

gel as it forms. The capability to do this is some way off, but it

should be possible in the future.

3. What is the future?

It is highly likely that foods in the future will contain colloids with

complex structures, not only to control the flavour release and taste

of a product, which has been the scenario until now, but also with

increasingly complicated structures designed to impact on human

health. Without the use of new molecules, there is a need for very

clever science: it will require the physical chemist and the engineer

to be highly creative in their design of structures. An example exists

in the emerging area of air filled emulsions.29 If such products are to

be acceptable an understanding of the psychology and physiology

surrounding these colloidal systems needs to be achieved.

It is clear that the future will be very exciting and challenging

for the food colloid scientist, physiologists and food

psychologists!

References

1 I. Norton, S. Moore and P. Fryer, Obes. Rev., 2007, 8, 83.2 I. Norton, P. Fryer and S. Moore, AIChE J., 2006, 52, 1632.3 G. P. Lian, M. E. Malone, J. E. Homan and I. T. Norton,

J. Controlled Release, 2004, 98, 139.4 B. J. D. Le Reverend, I. T. Norton, P. W. Cox and F. Spyropoulos,

Curr. Opin. Colloid Interface Sci., 2010, 15, 84–89.5 M. E. Malone, I. A. M. Appelqvist and I. T. Norton, Food

Hydrocolloids, 2003, 17, 763.6 E. H. A. de Hoog, J. F. Prinz, L. Huntjens, D. M. Dresselhuis and

G. A. van Aken, J. Food Sci., 2006, 71, E337.7 B. Vardhanabhuti, P. W. Cox, I. T. Norton and E. A. Foegeding,

Food Hydrocolloids, submitted.

3742 | Soft Matter, 2010, 6, 3735–3742

8 M. Golding and T. J. Wooster, Curr. Opin. Colloid Interface Sci.,2010, 15, 90.

9 L. Marciani, J. Wright, P. Manoj, R. J. Moore, P. Young, D. Bush,S. Al-Sahab, A. Fillery-Travis, P. A. Gowland and R. C. Spiller,Gastroenterology, 1998, 114, A798.

10 J. V. Henry, W. J. Frith, P. J. Fryer and I. T. Norton, Foods FoodIngredients J. Jpn., 2008, 213, 192.

11 J. V. L. Henry, P. J. Fryer, W. J. Frith and I. T. Norton, FoodHydrocolloids, 2010, 24, 66.

12 P. K€ahk€onen, H. Tuorila and H. Rita, Food Qual. Preference, 1996, 7,87.

13 P. K€ahk€onen and H. Tuorila, Appetite (Amsterdam, Neth.), 1998, 30,13.

14 P. K€ahk€onen, P. Hakanpaa and H. Tuorila, J. Sens. Stud., 1999, 14,35.

15 B. Wansink, K. van Ittersum and J. E. Painter, J. Food Sci., 2004, 69,S340.

16 B. Wansink and S. B. Park, J. Sens. Stud., 2002, 17, 483.17 I. P. Levin and G. J. Gaeth, Journal of Consumer Research, 1988, 15,

374.18 D. J. Shide and B. J. Rolls, J. Am. Diet. Assoc., 1995, 95, 993.19 S. U. Pickering, J. Chem. Soc., 1907, 91, 2001.20 R. Aveyard, J. H. Clint and T. S. Horozov, in Molecular Organisation

on Interfaces, ed. G. Lagaly, Springer-Verlag, Berlin, 2002, vol. 121,p. 11.

21 R. Pichot, F. Spyropoulos and I. T. Norton, J. Colloid Interface Sci.,2009, 329, 284.

22 J. E. Norton, P. J. Fryer, J. Parkinson and P. W. Cox, J. Food Eng.,2009, 95, 172.

23 S. Frasch-Melnik, F. Spyropoulos and I. T. Norton, in preparation.24 W. J. Frith, X. Garijo, T. J. Foster and I. T. Norton, Gums Stab. Food

Ind., 2002, 11, 95.25 I. T. Norton and W. J. Frith, Food Hydrocolloids, 2001, 15,

543.26 C. Hoad, P. Rayment, E. Cox, P. Wright, M. Butler, R. Spiller and

P. Gowland, Food Hydrocolloids, 2009, 23, 833.27 I. T. Norton, W. J. Frith and S. Ablett, Food Hydrocolloids, 2006, 20,

229.28 A. B. Norton, P. W. Cox and F. Spyropoulos, 2010, in press.29 F. L. Tchuenbou-Magaia, I. T. Norton and P. W. Cox, Food

Hydrocolloids, 2009, 23, 1877.30 A. Gabriele, F. Spyropoulos and I. T. Norton, Food Hydrocolloids,

2009, 23, 2054.31 P. Aymard, M. A. K. Williams, A. H. Clark and I. T. Norton,

Langmuir, 2000, 16, 7383.

This journal is ª The Royal Society of Chemistry 2010