molecules
Article
Oleogels—Their Applicability and Methods of Characterization
Eckhard Flöter *, Till Wettlaufer , Valentina Conty and Maria Scharfe
Citation: Flöter, E.; Wettlaufer, T.;
Conty, V.; Scharfe, M.
Oleogels—Their Applicability and
Methods of Characterization.
Molecules 2021,26, 1673. https://
doi.org/10.3390/molecules26061673
Academic Editors: Kiyotaka Sato,
Laura Bayés-García
and Silvana Martini
Received: 1 December 2020
Accepted: 12 March 2021
Published: 17 March 2021
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*Correspondence: eckhar[email protected]
Abstract:
Oleogels or, more precisely, non-triglyceride structured lipid phases have been researched
excessively in the last decade. Yet, no comprehensive knowledge base has emerged, allowing
technology elevation from the laboratory bench into the industrial food application. That is partly due
to insufficient characterization of the structuring systems studied. Examining a single composition
decided upon by arbitrary methods does not stimulate progress in the research and technology area.
A framework that gives much better guidance to product applications can easily be derived. For
example, the incremental structure contribution concept is advocated as a parameter to compare the
potency of structuring systems. These can straightforwardly be determined by combining solubility
data and structural measurements in the recommended manner. The current method to determine
the oil-binding capacity suffers from reproducibility and relevance. A newly developed method
is suggested to overcome these shortcomings. The recommended new characterization of oleogels
should contribute to a more comprehensive knowledge base necessary for product innovations.
Keywords:
non-triacylglyceride structuring; oil binding capacity; incremental structure contribu-
tion; oleogel
1. Introduction
In our diet and the typical habits to prepare food, vegetable fats and oils play a major
role. The properties of different fat phases cover a wide range, satisfying the necessities
of many applications. In a first cut, one could distinguish between the applications of
either liquid or structured lipid phases. It is fair to consider the first as a straightforward
application of seed oils. The criteria to choose raw materials for a specific application are
more than just economical. Obviously, sourcing in terms of price, availability, implying
here volume and flexibility of origin, is essential. The fatty acid composition of the fat phase
relates to chemical stability and nutritional quality. Lastly, somewhat less defined charac-
teristics such as sustainability, carbon footprint, and general image play an increasingly
important role. However, liquid lipid phases shall not be the emphasis of this contribution.
The same criteria also apply to the selection of the raw materials to compose structured
lipid phases. Structured lipid phases are related to either fat originating from a single
source which by themselves already have a semi-solid character at ambient temperature,
e.g., palm oil, coconut oil, or cocoa butter, or are composed of oils that are subject to
structuring or gelation through a structuring agent. Regarding the food market’s current
situation, structured lipid phases are almost exclusively based on fat crystal networks.
To supply a three-dimensional scaffolding that can bind liquid seed oils and generate
macroscopic hardness, the fats preferentially have a limited solubility at ambient and/or
storage temperatures. These high-melting fats, also referred to as hard-stocks, are inevitably
related to high levels of saturated fatty acids. The discussion on fatty acid composition is
deceptive concerning the physical properties of fats and oils. Properties such as melting
point, solubility, and complex crystallization behavior are driven by the triacylglycerols
(TAGs), which are triplets of fatty acid moieties esterified to a glycerol backbone [
1
]. The
design of the functionality of structuring high melting fats can be achieved by generating
Molecules 2021,26, 1673. https://doi.org/10.3390/molecules26061673 https://www.mdpi.com/journal/molecules
Molecules 2021,26, 1673 2 of 19
the desired TAG mixture through the application of oil modification techniques. Details of
the different routes, hydrogenation, interesterification, fractionation, and direct mixing of
hard-stocks can be found elsewhere [
2
,
3
]. In summary, the in-depth understanding of the
fat composition’s interplay and process conditions to generate desired final or intermediate
product properties was accumulated over many decades. For further details on fat crystal
networks, one can be referred to other contributions within this issue or other works [4].
Essentially, the triglyceride-based structuring is based on the arrangement of the pri-
mary fat crystals into a space-filling three-dimensional lattice. These lattices are capable of
immobilizing a liquid oil phase. The split between solid and liquid phases can vary greatly
depending on the application at hand. It should be pointed out that the nature of primary
crystals, their mutual interaction, and the structure build-up from crystal agglomerates
can considerably be influenced by the selection of the molecular TAG composition and
processing conditions and the presence of other materials [
3
,
5
–
9
]. Obviously, the variation
of these parameters allows altering the potential of the three-dimensional network to
immobilize oil. Furthermore, fat crystal networks are prone to change over time. Processes
such as Ostwald ripening, relaxation of the network, or simple re-crystallization can result
in significant macroscopic changes such as changes in the macroscopic hardness or the
development of phenomena as chocolate bloom [
1
,
9
]. The particle gel that triglyceride
crystals supply delivers functionalities in food products beyond the immobilization of
liquid oils. These are stabilizing interfaces, bubbles, or droplets, providing macroscopic
hardness, and a specific product disintegration behavior, which possibly accounts for
cooling sensation through melting, flavor release characteristics, and rheological sensations
such as lubrication.
Unfortunately, fat crystals are intrinsically related to increased levels of saturated fatty
acids (SaFA) in a lipid phase. The increased consumption of these fatty acids (SaFA) has
been discouraged for reasons of cardiovascular health. How far the health benefit assigned
to the substitution of saturated fatty acids by polyunsaturated fatty acids (PuFA) is based on
the reduction of the one or increase of the other shall be irrelevant
here [8,10–12]
. Beyond
these nutritional concerns, there is further motivation to disembark from triglyceride crystal
structured lipid phases. Chemical modification (hydrogenation) can be used to convert
unsaturated fatty acids, present in liquid oils, into saturated fatty acids that constitute
consequently high melting fats [
3
]. Since chemically modified food ingredients are less
attractive to consumers and they often link hydrogenation to trans-unsaturated fatty acids
irrespective of the process execution, this route is currently not promising.
Consequently, the past decades have seen a surge in the use of SaFA originating
from tropical oils. That mainly concerns palm oil, which has not only due to food uses
developed into the most produced oil on the globe: more than 70 Mio tons per annum.
Numerous organizations, to name a few, the Round Table for Sustainable Palm Oil (RSPO),
the Malaysian Palm Oil Board (MPOB), Forum Nachhaltiges Palmöl (FONAP), and Non-
governmental organisations such as the World Wildlife Fund (WWF), made efforts to
improve the sustainability of palm oil. For example, the position of consumers in Europe
toward palm oil remains at least controversial [
8
,
13
]. The consumer’s position has resulted
in an increasing demand for healthy, organic, sustainable, and regional food products. That
has led to the desire to offer products comprising alternatives to current structuring routes
based on crystalline triglycerides, mainly originating from palm oil or other tropical oils.
Oleogelation, or more precisely non-triglyceride oil structuring, offers an alternative
route to entrap liquid oils in a macroscopically semi-solid lipid structure. Depending on
the structuring mechanism, this route could offer independence from tropical oils and
the delivery of structured lipid phases with improved nutritional profiles. In essence,
the structuring of liquid edible oils can be achieved through different routes: crystalline
particles, self-assembled networks, polymer networks, and emulsions or foams. The
interest in this field is well documented by the number and diversity of related publications:
more than 3000 on the topic of oleo-/organogelation in the past 20 years. Even though
Molecules 2021,26, 1673 3 of 19
many structuring systems have been reported and many patents have been submitted, the
number of product applications in food remains surprisingly small.
Even though this contribution discusses the applicability of oleogels in foods, the
aim is not to generate another general review of the area. Next to the reviews of the
past years [
14
–
28
], just the year 2020 has seen more than nine reviews discussing oleogels
related to foods [
8
,
29
–
36
]. During this period, also more than 700 original publications
relating to oleogels and food have been published. Consequently, this contribution dis-
cusses the different technical routes of oleogelation only briefly. In contrast, the necessary
non-technical conditions for a successful oil structuring system will be discussed. The
emphasis of this contribution is the characterization of the gels. The methods utilized in
many contributions are reviewed and discussed concerning their relevance for product
applications. For some characteristics, alternative approaches are suggested. Hence, the
manuscript is aiming to at least stimulate discussion on the methods typically applied.
Additionally, this contribution should give some guidance for future characterization to
allow for better comparability and compatibility of the data generated by different labs. A
more standardized characterization relevant for applications will undoubtedly benefit the
process of bringing oleogels towards implementation.
2. Non-Triglyceride Structuring Routes
The field concerned here is often referred to as organo- or oleogelation. The applica-
tion of these terms creates a problem in terminology. The lipid phases fulfilling specific
requirements in food applications that cannot be meet by liquid oils are not necessarily
gelled. Anyhow, the question of whether highly viscous systems should, despite the
absence of a linear visco-elastic region, be considered oleogels is of minor importance.
In the first place, structured lipid phases are concerned that immobilize liquid oil and
can be described as soft solids macroscopically. This feature is essentially based on the
presence of at least two distinct phases. Typically, a dispersed phase is aggregating so that
a three-dimensional scaffolding is established, which, analogous to a sponge, immobilizes
liquid oil. The nature of the aggregating material can be manifold, such as crystalline,
amorphous, self-assembled, or liquid crystalline. Next to the oleogels based on a fragile
structure dispersed in the continuous oil phase, oil-in-water emulsions are considered
oleogels, since these potentially deliver the functionality of structured lipid phases. These
emulsions with preferentially high internal phase volume (HIPE) do not concern structured
oil but can be seen as mayonnaise analogs instead, rendering the oil phase discontinuous
by compartmentalization. Due to their utterly different nature, these oleogels will not be
further considered in this contribution.
The different routes to structure the lipid phases have been discussed in depth else-
where [
8
,
14
–
17
,
20
,
27
–
36
]. There are several attempts to categorize the different structuring
systems. Essentially there is a distinction between oil gelators being either low molecular
weight (LMWOG) or polymeric. A third category covers emulsions and foams. The cate-
gory of LMWOG includes crystalline and fibrous network structures. In both categories,
mixed or single component systems are often distinguished. Unfortunately, these terms
remain ambiguous in this context. The term “single component” suggests an ingredient
with a very high concentration of a single molecule. Regarding this assumption, the inter-
pretation of thermodynamical information is defined by fundamental relations. Most often,
“single component” refers to a class of molecules, if anything. For example, that renders
the term melting point inappropriate for any natural wax because it is a multi-component
mixture. Consequently, these terms should be used cautiously.
Next to the fibrous or crystalline material, the microscopical sponge-like structure
can be based on polymeric networks. Here, typically, a distinction between direct and
indirect dispersion is made. In this case, the use of the terms “pure” and “mixed” should
be considered carefully. Table 1illustrates this categorization and gives some examples
(adapted from [14–17,20,27–36]).
Molecules 2021,26, 1673 4 of 19
Table 1.
Categorization of oil structurants. Low molecular weight oleogelators (LMWOG) appear in either crystalline or
fibrillar structures. Polymeric oleogels can be distinguished based in the dispersion process.
LMWOG Polymeric
Crystalline Fibrillar Directly Disp. Indirectly Dispersed
Single
Component Mixed System Single
Component Mixed System Single
Component Mixed System
monoglycerides
12-hydroxystearic acid ethylcellulose protein-
n-alkanes waxes lecithins phytosterol/oryzanol proteins polysaccharide
fatty acids fatty acid/fatty alcohol MPMC complexes
wax esters sorbitan ester/lecithin
fatty alcohols
When utilizing crystalline material, it is fair to assume that the gelation mechanism
is similar to the reference, fat crystals of high melting triglycerides. The effectiveness
of structuring oil varies significantly for crystals of different TAG compositions. That
variability originates in, among others, crystal size, crystal habit, and crystal–crystal inter-
action. The similarity to the TAG-based approach is apparent when crystals are generated
from materials containing saturated aliphatic chains. For example, these molecules are
monoglycerides, diglycerides and their mixtures [
37
–
40
], fatty acids, fatty alcohols and
their mixtures [
41
], sorbitan esters [
42
], wax esters [
43
], and waxes [
44
]. That implies
that at any given temperature, a distinct solubility of the structurant in the liquid phase
is given [
45
]. In addition to the structuring efficiency—hardness increment per fraction
dispersed phase—the solubility directly relates to the efficiency of the structuring agent’s
dosage. Low solubilities at ambient temperature correspond to the high melting points
of pure components in a first approximation. However, with most structurants being
mixtures, this relation is less clear. Unfortunately, the high melting temperature or better
dissolution temperatures implicitly carry the risk of less desirable organoleptic properties
of the structured lipid phase due to the formation of waxy films. In gels based on at least
two different classes of molecules, the basic structuring mechanism is less obvious. In most
cases, it is not identified how far the synergistic effects found for the interacting compo-
nents are based on co-crystallization, sequential crystallization generating sintering, or
simply crystal habit modification. Examples for these systems are specific combinations of
fatty acids and fatty alcohols [
41
], mixtures of waxes and lecithins [
46
], sorbitan esters and
lecithins [
23
,
47
], and combination sterols and monoglycerides [
48
]. For these systems, the
individual components’ solubility in the oil phase has to be considered carefully because
the relative solubilities might vary on temperature change. Waxes, which were mentioned
in the previous category as a single component, should be considered here because they
contain a significant fraction of different species, such as wax esters, fatty acids, and fatty
alcohols [
49
]. It should be mentioned briefly that for particle gels, the structural elements
are usually subject to crystallization processes. These are generally well understood and
thus influence the structural elements by variation of cooling rate and shear. That can be
used either deliberately to establish the gel itself or be considered during the food product
manufacturing process.
Alternatively, self-assembled fibrils have been found to structure liquid oil. In this case,
the fibrillar structures have to assemble such that a 3-dimensional network evolves [
50
–
53
].
On the one hand, it can include fibrils of single components such as 12-hydroxystearic acid
(HSA) [
54
,
55
] or ricinelaidic acid [
56
]. On the other hand, binary fibrils such as those based
on
γ
-oryzanol and
β
-sitosterol [
52
,
53
,
57
] or those based on tocopherol and lecithin [
50
]
have been discovered. Lastly, the structuring of oil can also be achieved by liquid crystalline
elements, as exemplified by the combination of lecithin/water or monoglyceride/water.
Next to the particle gels based on crystals, polymers such as ethylcellulose, which
are dissolvable in oil, reportedly generate oleogels [
58
–
61
]. It is much less clear for these
systems than for crystal-based gels how the establishment of the structure can be influenced
Molecules 2021,26, 1673 5 of 19
and possibly controlled by processing. Systems based on structurants that are practically
not soluble in oil represent a very different category of structured lipid phases. In these
cases, the three-dimensional scaffolding or building blocks that host the liquid oil are
mostly fabricated in the absence of the oil phase. Examples of this approach are protein-
based oleogels [
62
,
63
] and oil structuring based on
κ
-carrageenan [
64
]. For these types of
oleogels, the structure is typically fixed in a water-continuous system. Subsequently, the
continuous water phase is substituted with the oil phase. This substitution has been demon-
strated successfully by applying various techniques based on either solvent exchange or an
intermediate aerogel production through drying [62,64].
The examples given illustrate that there are several routes to structure liquid oil.
However, from the brief discussion, it is apparent that the identification of successful
substitutes for high melting triglycerides is anything but trivial. The requirements for
structurants used in industrial food applications have been formulated previously by Co
and Marangoni [
26
] and Bot and Flöter [
19
,
65
]. Materials considered for oil structuring
purposes should preferentially:
•Be food grade
•Provide a functional structure to edible oil
•Be affordable so as not to excessively increase the price per volume lipid phase
•Not interfere with other ingredients in a product
•
Be versatile, having the flexibility to manipulate product properties such as disinte-
gration temperature
•Ensure similar taste and mouthfeel to the reference product.
This list of criteria is a formulated multidimensional ambition rather than a tick-
off list since it is practically impossible to meet all criteria. Anyhow, the list is missing
another crucial parameter for any successful product introduction: consumer acceptance.
Consumer acceptance remains crucial and cannot be achieved if the substitute does not
deliver a similar sensory sensation or other exceptional benefits. Considering that most
oleogel systems have an entirely different disintegration characteristic than traditionally
structured lipid phases, matching organoleptic perception is challenging. Additionally,
the substitution of TAG-structured lipid phases driven by consumer concerns regarding
chemical processes, sustainability, and regionality should satisfy consumers’ expectations
concerning these aspects.
The vast body of research touched upon is practically insurmountable. Despite the
large number of contributions to the field, the comprehensive knowledge generated remains
fragmented. Results from different labs and on different structuring systems do not give
much guidance for food developers interested in applying oleogels. The first step toward
an improved comparability of results would be a more standardized characterization of
oleogels. In doing so, it would be desirable to verify the relevance of oleogel characteristics
determined for product applications.
3. Characterization of Oleogels
The characterization of structured lipid phases could aim for either an improved
understanding of the oleogel itself or the determination of relevant properties for product
applications. Since the field of non-triglyceride structuring in foods can still be considered
in its infancy, no substantial application knowledge in food products exists. That leaves the
question of how far a particular characteristic is relevant for application open for debate.
Still, a specific set of characteristic properties that distinguish different structured liquid
phases from each other and appear meaningful has emerged. These can consequently
be found in most publications concerning oleogelation. Even though not well defined,
the critical concentration indicates the lowest concentration of the structurant yielding a
non-leaking, non-flowing soft solid. Secondly, a measure for the structure’s ability to hold
liquid oil is expressed as oil-binding capacity (OBC). It is mostly expressed as the fraction
of oil remaining in the structure after applying external forces, mostly centrifugal force.
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