Sauce Structure: Emulsions, Suspensions And Flow

Continuous Phase, Droplets And Particles

In a mixture, the continuous phase is the medium in which other components are distributed. The dispersed phase consists of droplets or particles within that medium. At a smaller scale, a sauce that looks uninterrupted raises a useful question: which component surrounds which?

In an emulsion, one liquid is dispersed as droplets within another. When an aqueous continuous phase surrounds oil droplets, the structure is an oil-in-water emulsion, abbreviated O/W. The name identifies which liquid forms the droplets and which forms the surrounding medium.

In a suspension, solid particles are distributed through a liquid. Vegetable-cell fragments studied in tomato-based systems are one example. A hot sauce with visible pepper particles can also be examined by asking what is dispersed in its liquid. As a product can have several structural features at once, appearance alone is insufficient to describe its whole structure with a single term.

What Happens Around An Oil Droplet?

In mayonnaise, oil droplets sit within an aqueous continuous phase. Despite its high oil content, this is an oil-in-water emulsion. Structural identity depends not only on amounts, but also on which phase surrounds the other.

The interface where a droplet meets the surrounding liquid is an important part of the structure. A review of mayonnaise explains the roles of egg-yolk proteins and phospholipids in its emulsion. The arrangement around droplets and their interactions jointly contribute to the product’s physical character.

A sauce’s separate internal surfaces have not disappeared simply because it looks smooth. An apparently continuous appearance can be built from many droplets and their boundaries. Understanding a texture called silky in the kitchen also involves investigating how this small-scale arrangement is maintained.

How Did Processing Change Tomato-Fibre Structure?

Understanding sauce flow involves asking how processing changes particles as well as which particles are present. In 2018, Wang and colleagues examined suspensions containing 0.1%, 0.25%, 0.5% and 1% tomato fibre by weight before and after high-shear and high-pressure homogenisation. Microscopy showed the appearance of the particles, while a rheometer measured their behaviour under movement. Images of structure and measurements of flow could therefore be compared in the same samples.

After processing, the suspensions studied had a more uniform particle distribution and higher apparent viscosity, describing resistance to flow under the measurement conditions. High-pressure treatment also changed the appearance of cellular fragments and the fibre network. As particle reduction and the arrangement between particles can change together, smaller particles need not produce a more freely flowing result.

In the tomato-fibre samples, processing changed particles and the network between them together. A sauce containing pepper particles invites the same question: how does grinding change the arrangement as well as particle size? Ingredients name the material; microscopy and flow measurements investigate how it is organised within the mixture.

• Explore grinding through the tool and its handstone

How Can Movement Change Consistency?

Viscosity concerns resistance to flow. In some systems, a single number cannot describe this resistance under every movement condition. A decrease in apparent viscosity as shear rate rises is called shear thinning. The same system can therefore show different flow resistance at different shear rates.

Shear rate is the changing quantity in that definition. Observing a consistency change over the duration of stirring does not by itself establish shear thinning: time dependence needs separate investigation. A flow curve carries more information than a single adjective such as thick or runny, because it also identifies the movement conditions of the measurement.

The tomato-fibre samples showed shear thinning. The researchers also measured solid-like and liquid-like response components under small deformations. A sauce’s onset of movement, resistance while flowing and deformation under force are connected aspects of texture. Distinguishing them helps describe both its state in the bottle and its behaviour in motion.

• The Last Drop: Where Contents Meet an Edge

Looking More Closely At Texture

We also move a food when sensing its consistency in the mouth. In 2021, Deblais and colleagues compared flow measurements of fourteen liquid bouillons, spanning viscosities from approximately 1 mPa·s to 1 Pa·s, with a panel’s thickness ratings. Modelling squeezing and spreading between tongue and palate helped explain perceived thickness. Consistency was thus related not only to static appearance, but also to the force required to move food in the mouth and its response to that movement.

Similar resistance to flow can accompany different sensations in the mouth. In Blok and colleagues’ 2021 study, 73 people evaluated xanthan gum and microfibrillated-cellulose samples selected to have similar shear viscosities. The tasting samples contained equal amounts of sugar, strawberry flavouring and colourant. At high thickener concentrations, the xanthan samples were rated stickier and more mouthcoating. Thickener type had no significant overall effect on thickness or creaminess ratings. Following these distinct sensory properties alongside flow measurements gives us a more detailed vocabulary for texture.

Between leaving the bottle and spreading in the mouth, the same sauce undergoes different movements. Following its ingredients, droplet and particle arrangement, flow and sensory texture gives the word consistency greater detail. What the sauce contains, how it moves and how it feels become different aspects of the same whole.

• Describing sauce character through ingredients and sensory words
• Historical distinctions in the culinary language of sauces
• The object around the flow: the design language of bottles