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Understanding the rheology of yield stress materials

Paredes Rojas, J.F.

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Citation for published version (APA):Paredes Rojas, J. F. (2013). Understanding the rheology of yield stress materials.

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Download date: 15 Apr 2020

8

Hot topics - Recommendations for

future work

In this chapter I present some interesting topics that deserve more research. For

some of them, I already carried out some experiments, but in all cases I give rec-

ommendations that would (hopefully) help improving our current understanding

regarding yield stress materials.

8.1 Behavior of structured materials:

Mayonnaise vs. Margarine

Margarine and mayonnaise are common dietary products. Mayonnaises are oil-

in-water emulsions, in which the dispersed phase consists of a vegetable oil and

the continuous phase of vinegar. Egg yolk is added to stabilize the emulsion, to-

gether with thickening agents and flavoring materials [1–4]. The actual trend is to

substitute regular foods for low-calorie versions, increasing the interest in fat sub-

stitutes and in new formulations with lower oil content; therefore, the production

of dietetic mayonnaise implies the decreasing of the dispersed phase and the use of

additives to stabilize the emulsion and to increase the viscosity of these foodstuffs

[5].

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Margarines are water-in-oil emulsions, in which the aqueous phase consists of wa-

ter, salt and preservatives, while the oil phase is a blend of partially hydrogenated

oil. These water-in-oil emulsions are stabilized by the use of lecithin and mono-

glycerides [6, 7]; however, any substance approved by the FDA can be used as

surfactant [8]. The firmness of margarines is basically given by the crystallization

of the oil phase [8–10], which consists of a hydrogenation process by which liquid

oils are changed into solid fats due to the formation of saturated and trans fatty

acids [11]; however, these acids have numerous negative health effects, such as

increased incidence of heart disease or high cholesterol [12]. Therefore, the trend

is to limit the amount of trans-fatty acids in foods.

The common challenge when developing new mayonnaises and margarines is to

obtain products with a ‘texture’ similar to that to which consumers are used to.

In this context, “... texture encompasses all the rheological and structural (geomet-

rical and surface) attributes of a food product perceptible by means of mechanical,

tactile and, when appropriate, visual and auditory receptors”[13]. Thus, different

formulations can be compared by measuring their rheological properties, which

may give a quantitative contribution to texture characterization and control [5].

8.1.1 Rheological characterization of Mayonnaise

Two commercially-available mayonnaise samples were characterized, one regular

mayonnaise (Calve Mayonaise from Unilever) and one light mayonnaise (Calve

Licht en Romig, from Unilever), whose composition is shown in Table 8.1. To

perform the rheological measurements two rheometers with cone-plate geometries

and roughened surfaces were used: a controlled-shear-stress rheometer (CSS, An-

ton Paar MCR 301) and a controlled-shear-rate rheometer (CSR, Rheometrics

ARES). Measurements consisted in shear stress sweeps and shear rate sweeps;

each shear stress sweep was performed twice using the same sample.

The stress sweep experiments show that the regular mayonnaise exhibits thixotropy

and that the microstructure responsible of this behavior is built—aging—and

destroyed—shear rejuvenation—very quickly, as the different stress sweep cy-

cles superimpose (Figure 8.1 (a)). Different works have shown the existence of

thixotropy in mayonnaises and other foodstuffs, considering only the breaking-up

of the microstructure with shear (see e.g. [3, 14, 15]). Nevertheless, it has been

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Table 8.1: Nutritional values and ingredients of Calve Regular and CalveLight

Calve Regular1 Calve Light2

content pr. 100 ml content pr. 100 mlFat 70 g 27 g

from saturated fat 6 g 2.5 gCarbohydrates 3.5 g 9 g

from sugars 3.5 g 5 gProteins 0.9 g 0.6 gSalt 0.9 g 1.4 g1 Ingredients: Rapeseed oil, water, vinegar, egg yolk 5%, sugar, mustard (water, mustardseed, vinegar, salt, spices, flavor), salt, flavor, thickeners: xanthan gum, antioxidantE385; color: β-carotene.2 Ingredients: Water, rapeseed oil 35%, sugar, modified corn starch, egg yolk 2.8%, mus-tard (water, mustard seed, vinegar, salt, sugar), salt, flavor, citrus fiber, concentratedlemon juice, acidifier: lactic acid, preservative: sorbic acid, stabilizers (xanthan gum,guar gum), antioxidant E385, color :β-carotene.

shown that, under shear, mayonnaise has both a reversible and an irreversible

structural breakdown; the reversible structural breakdown has been associated

with a flocculation-deflocculation process and the irreversible one with the coales-

cence of the oil droplets [16–18]. Considering that by loading our sample in the

measuring geometry, its microstructure could have been partially destroyed [3],

then our measurements confirm that there is a structure, for which the breakdown

is reversible.

Interestingly, Figure 8.1 (b) shows that the light mayonnaise seems to have an

initial structure that is irreversibly destroyed after the first increasing shear stress

sweep is performed. The sample recovers partially, showing that also for light

mayonnaise there seems to be a structure that breaks reversibly. However, more

experiments are needed in order to confirm that the remaining structure after

shearing ages and shear-rejuvenates, in a fashion comparable to the behavior ex-

hibited by the regular mayonnaise.

In addition, the shear rate sweeps allow us to obtain the steady-state flow curves

shown in Figure 8.1 (c,d), which can be fitted to the classical Herschel-Bulkley

model [19]. It is worth mentioning that by the end of the experiments, changes

in the color of the samples were observed, which can be an evidence of phase

separation.

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Figure 8.1: Flow curves obtained by performing shear stress sweeps (a,b) andsteady-state shear rate sweeps (c,d). (a) and (c) show results corresponding tothe regular mayonnaise. (b) and (d) show results corresponding to the lightmayonnaise. In (a) and (b) empty squares correspond to the first increasingshear stress sweep and the filled squares correspond to the first decreasing shearstress sweeps; empty and filled circles correspond to the second increasing anddecreasing shear stress sweeps, respectively. In (c) and (d) the lines correspondto the fits to the Herschel Bulkley model, which for the regular mayonnaise(c) is σ = 42.4Pa + 26.4Pa.s0.37 · γ0.37, while for the light mayonnaise (d) is

σ = 28.7Pa + 24.1Pa.s0.46 · γ0.46.

8.1.2 Rheological characterization of Margarine

As done with the mayonnaise samples, two margarine samples were studied: a

regular margarine (Becel Gold, from Unilever) and a light margarine (Becel Light,

from Unilever), whose composition is given in Table 8.2. The measurements con-

sisted again in performing shear stress and shear rate sweeps.

Figure 8.2 (a,b) shows that both the regular and the light margarine have an initial

structure that is destroyed after the first increasing shear stress sweep experiment is

performed, i.e. the fat crystal network is destroyed. This structure is not recovered;

nevertheless, the remaining structure seems to behave like a thixotropic material,

which corresponds to the remaining water-in-oil emulsion. New research is needed

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Table 8.2: Nutritional values and ingredients of Becel Gold and Becel Light

Becel Gold 1 Becel Light 2

content pr. 100 g content pr. 100 gProteins < 0.5 g 0.9 gCarbohydrates 0.6 g 3 gSaturated fat 16 g 6 gMono-unsaturated fat 17 g 8 gPoly-unsaturated fat 36 g 16 g

Omega 6 (linoleic acid) 29 g 12 gOmega 3 (α-linolenic acid) 7 g 3.9 g

Trans fat < 1 g < 1 gDietary fiber < 0.5 g < 0.5 gSodium 0.01 g 0.02 gVitamin A 800 µg 800 µgVitamin D 7.5 µg 7.5 µgVitamin E 9 mg 9.4 mg1 Ingredients: Vegetable oils and fats, water, dry milk solids, emulsifiers (mono - anddiglycerides from fatty acids, sunflower lecithin), acidifier (citric acid), flavor, vitaminsA and D, color (β-carotene).2 Ingredients: Vegetable oils and fats, modified starch, gelatin, emulsifiers (lecithin,mono- and diglycerides from fatty acids), preservative (potassium sorbate), acidifier(citric acid), vitamins A and D, flavor, color (β-carotene).

to better understand the interactions between emulsifiers and fat crystals, as well

as, the origin of the thixotropy behavior of the remaining emulsion after breaking

up of the fat crystal network [12].

Conversely, for the light margarine, with the second increasing shear stress sweep,

the sample fractures, making it impossible to obtain valuable rheological data.

Previous works using commercial margarines have also shown instability phenom-

ena and wall slip when performing rheological measurements [12].

Additionally, as done with the mayonnaise samples, shear rate sweeps were carried

out using the margarine samples, which allow us to obtain the steady-state flow

curves shown in Figure 8.2 (c,d). The steady-state flow curves were fitted to the

Herschel-Bulkley model. By comparing the stead-state flow curves, we can observe

that the viscosity values of the regular and the light margarine in the shear rate

range 10 s−1 . γ . 50 s−1 are very different. This shear rate range is the typical

one for spreading butter on bread [20]; if we consider that the consumer expects

that a light and a regular margarine behave in a similar way, then improvements

in the formulation of light margarines need to be made.

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Figure 8.2: Flow curves obtained by performing shear stress sweeps (a,b)and steady-state shear rate sweeps (c,d). (a) and (c) show results correspond-ing to the regular margarine. (b) and (d) show results corresponding to thelight margarine. In (a) and (b) empty squares correspond to the first increas-ing shear stress sweep and the filled squares correspond to the first decreasingshear stress sweeps; empty and filled circles correspond to the second increas-ing and decreasing shear stress sweeps, respectively. In (c) and (d) the linescorrespond to the fits to the Herschel Bulkley model, which for the regular mar-garine (c) is σ = 11.1Pa + 1.4Pa.s0.88 · γ0.88, while for the light margarine (b) is

σ = 40.1Pa + 6.7Pa.s0.73 · γ0.73.

Regarding the appearance of the samples by the end of the experiments, I ob-

served that while the regular margarine had a more creamy-like aspect, the light

margarine seemed to phase separate as shown in Figure 8.3.

8.1.3 Mayonnaise, Margarine and Future Work

Even when for mayonnaise more rheological studies have been performed than

for margarines (see e.g. [2–5, 16, 21, 22]), a complete rheological characterization

considering the microstrutures of these materials is lacking. This information will

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Figure 8.3: Aspect of the regular margarine (left) and the light margarine(right) after performing the shear rate sweep experiments.

be useful in quality control of commercial production, knowledge and design of

texture, design of unit operations and understanding of the effects of mechanical

processing on the structure of these emulsions [16, 23].

In the spirit of contributing with the improvement of the formulation of mar-

garines and mayonnaises, as well as with the characterization of their rheological

properties, I would like to propose the following:

(i) Margarines and mayonnaises exhibit thixotropy. If we compare these systems

with the castor oil-in-water and silicone oil-in-water emulsions, for which

thixotropy is induced by adding clay to the formulations, then it would be

possible to prepare mayonnaises and margarines using food-grade clays. The

use of clays for the formulation of margarines will be beneficial, as the use of

hydrogenated fatty acids would be reduced or even completely avoided.

(ii) Perform a complete rheological characterization of mayonnaises and mar-

garines, using the experimental techniques described in this thesis: shear

rate and shear stress sweeps, steady-state flow curves, viscosity bifurcation

experiments, oscillatory measurements to determine the storage (G’) and the

loss (G”) moduli, creep tests and stress growth experiments. The complete

rheological characterization of the commercially-available materials will allow

us to compare the mechanical properties and ‘texture’ of new formulations

with the existent products.

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8.2 Clays and Thixotropy

It was shown in this thesis that clay confers thixotropic properties to oil-in-water

emulsions. However, the exact mechanism, due to which this happens, is not

fully understood, Do clay particles gel the continuous aqueous phase, are they a

depletion agent or do they form links between neighboring droplets?

To answer this question, I propose the following:

(i) Following the work of Wang et. al [24] dye hydrophilic clays. The method

consists, basically, in using Auramine O or Rhodamine B to dye the clay.

(ii) After dying the clay, transparent emulsion should be prepared (silicone oil-

in-water) and the died clay should be added to the formulation.

(iii) The clayey emulsion should be diluted to allow the observation of the role of

the clay in the thixotropic character of the system.

(iv) Using the confocal laser scanning microscope couple to the rheometer, the

behavior of the clay in the emulsion can be observed during shearing and

resting.

By following this simple procedure, it would be possible to reveal the mechanism

due to which emulsions become thixotropic when clay is added.

8.3 Stored stress in ‘simple’ yield stress

materials

‘Simple’ yield stress materials, are called ‘simple’ because for them the yield stress

is a well-defined value, considering that measurements are performed in the same

way and that the criterion for defining this value is the same (see Chapter 7).

Nevertheless, if shear rate sweeps are performed by turning the measuring geom-

etry in one direction and then the same experiment is performed in the contrary

direction, results are different.

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Figure 8.4: Flow curves of Carbopol, Hair gel and Silicone oil-in-water emul-sion (a) and silicone oil (b) obtained from shear rate sweep experiments. Theempty symbols correspond to the first shear rate sweeps (clockwise direction)after pre-shearing (counterclockwise). The filled symbols correspond to secondshear rate sweeps (clockwise). The gray line is the fit of the flow curves to the

Herschel-Bulkley model.

I used a controlled-shear-rate rheometer (ARES) for performing rotational exper-

iments, consisting of shear rate sweeps in an emulsion, a carbopol gel and a hair

gel. I used a CP geometry with roughened surfaces and performed experiments

according to the following protocol:

(i) Pre-shearing of the sample at 100 s−1 for 60s. Rotation = counterclockwise.

(ii) Resting for 60 s.

(iii) Shear rate sweep. Rotation = clockwise.

(iv) Shear rate sweep. Rotation = clockwise.

In all cases, flow curves were fitted to the Herschel-Bulkley model and the yield

stress happened to be higher for flow curves obtained just after pre-shearing and

resting, than for flow curves obtained in a second measurement carried out just

after the first shear rate sweep was performed (See Figure 8.4 (a)).

Interestingly, for each system, the only parameter of the Herschel-Bulkley model

that varies is the yield stress; the K and n fitting parameters are approximately

the same.

To be sure that the obtained results are not due to a rheometer’s artifact, the same

experiments were performed in a silicone oil (Rhodorsil R©47 V 500), observing that

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both shear rate sweeps superimpose, indicating the absence of any artifact (Figure

8.4 (b)).

These results suggest that there is a stored stress in ‘simple’ yield stress materials.

More research should be done, involving simulations and experiments, in order to

understand this phenomenon. Questions that need to be answered are: In which

way do ‘simple’ yield stress materials store stress? Is this phenomenon due to the

orientation of the dispersed entities in one direction, and subsequent realignment

in a contrary direction?

If these questions are answered, we will be a step closer to fully understand the

fascinating rheology of yield stress materials.

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