Fat for the future: designing multifunctional molecular oleogels

Abstract

With the deadline for replacing partially hydrogenated oils less than a year away, there is a compelling need to develop functional, healthy, and nutritious materials that can be used to turn liquid oils into semi-solid fats. Toward that end, our research group has successfully demonstrated the design and synthesis of a new family of naturally derived molecular gelators through a simple GRAS (Generally Recognized as Safe) protocol. These gelators can potentially form stable oleogels with a variety of vegetable oils for food and personal care applications.

SOLID OILS FORTIFIED WITH MULTIFUNCTIONAL GELATORS

The quest for alternative vegetable oil solidifying/thickening or structuring strategies has progressed rapidly since the discovery of low-molecular-weight gelators (LMWGs)—a new class of vegetable oil structuring agents that can be distinguished from existing polymeric gelators/waxes. While the majority of research on molecular gelators has focused on the gelation of organic solvents (organogels), few research groups are working on structuring vegetable oils using small molecules. The design, synthesis, and application of a carbohydrate derivative in edible oil structuring was reported by our research group in 2006. Since then, we have been actively involved in designing new amphiphilic molecules and developing technologies for structuring different edible oils.

Based on our expertise in small molecular gelators, we recently aimed to design oil structuring agents with multifunctional properties (eg., low in calories, low glycemic index, antioxidant). Naturally available sugars (e.g., trehalose, sucrose, mannose, amygdalin), sugar alcohols (e.g., sorbitol, mannitol, xylitol), and glycosides (e.g., raspberry ketone glycoside, salicin) were investigated as precursors for the design of multifunctional amphiphilic molecular gelators. The carbohydrate group acts as the hydrophilic head, and the fatty acyl chains act as hydrophobic tail. The fatty acids provide the necessary lipophilic nature to balance the hydrophilicity associated with the carbohydrates. Hence, care has to be taken to achieve appropriate hydrophilic-lipophilic balance (HLB) while designing molecular gelators for targeted applications.

SUGAR ALCOHOL DERIVATIVES

Sugar alcohols were esterified with fatty acids having different chain lengths ranging from butyric acid (C4), caprylic acid (C8), capric acid (C10), lauric acid (C12), myristic acid (C14), palmitic acid (C16), and stearic acid (C18). By following a simple one-step enzymatic reaction (regiospecific esterification), high yields (> 90 %) of molecular gelators were synthesized. We synthesized these molecular gelators in kilogram scale as well. Based on the simplicity, yield, and cost effectiveness, synthesis of sugar alcohol derivatives using regiospecific esterification seems to be plausible at the industrial level. The sugar alcohol derivatives have shown excellent gelation ability at exceedingly low amounts (Fig. 2), often less than 5%, in canola oil, grape seed oil, olive oil, sesame oil, soybean oil, and sunflower oil [1]. Sorbitol and mannitol dialkanoates have proven to be efficient in the molecular gelation of vegetable oils. The amphiphiles self-assemble to form supramolecular hierarchical structures, notably called self-assembled fibrillar networks (SAFINs). During gelation, sugar alcohol derivatives self-assemble as inverse bilayers which extend unidirectionally as fibrils, and further aggregate to form supramolecular structures such as fibers and microcrystals.

FIG. 2.

Structured vegetable oils (oleogels) using sorbitol and mannitol dialkanoates

The three-dimensional entanglement of supramolecular structures arrests the free flow of vegetable oil and results in a semisolid oleogel. Sorbitol and mannitol derivatives respectively form microcrystalline and fibrous micro-architectures in vegetable oils (Fig. 3). The molecular level stereochemical differences in the chemical structure of sugar alcohols are responsible for the microscopic morphological orientation of the gelators in the oleogels. These microstructural differences have yielded translucent and opaque oleogels when sorbitol and mannitol derivatives were used respectively with medium-chain vegetable oils. Transparent oleogels were obtained using raspberry ketone glucoside-based oleogels (Fig. 3). The microstructural differences have, in turn, affected the textural and rheological properties of oleogels. Thus, the aesthetics, texture, rheology, and solid phase content (generally referred to as the solid fat content) of the oleogels can be fine-tuned by formulating with appropriate sugar alcohol dialkanoates.

FIG. 3.

Stable oleogels and their microstructure (from left to right): Oleogels showed increase in opacity when prepared using RKG, sorbitol, and mannitol dialkanoates.

The multifunctionality of sugar alcohol derivatives can also be measured by their lower glycemic index, in addition to their structuring ability. Sugar alcohols such as sorbitol and mannitol possess low glycemic indices compared to sugars. Sorbitol and mannitol are currently used as sugar substitutes, as their calorific values are 2.6 and 1.6 cal/g, respectively, lower than that of glucose (3.35 cal/g). The calorific value of medium-chain fatty acids are reported to be in the range of ~7.95–9.5 cal/g, thus the glycemic index of sugar alcohol-dialkanoates are expected to be minimal and within the proposed limit (~10 cal/g). The non-hypercholesterolemic nature of medium-chain fatty acids also may add value to the multifunctionality of the structuring agents. For example, caprylic acid has been used as food contact surface sanitizer in dairy, food, and beverage processing plants; the use of caprylic acid does not illicit immunological response when ingested. High cell viability of human liver carcinoma cells (HepG2 cell line) was observed during cytotoxicity tests of molecular gelators, supporting the suitability of the molecules toward food applications.

SUGAR AND GLYCOSIDE DERIVATIVES

The design of multifunctional oleogelators is also possible with trehalose and raspberry ketone glucoside (RKG). Trehalsoe, a disaccharide, has demonstrated anti-aging, neuroprotective, cardiovascular, and gastrointestinal benefits. RKG is a sugar glucoside primarily used in cosmetics that is also used in weight management and therapy. Trehalose and RKG diesters are synthesized by enzymatic catalysis in a similar fashion as with sugar alcohol dialkanoates. Trehalose and RKG diesters have demonstrated excellent gelation ability in olive oil, and particularly with medium-chain triglycerides such as coconut oil, respectively [2]. The inherent nutritional qualities of precursors complement the structuring ability of these derivatives for use as multifunctional oleogelators.

WHAT WE LEARNED

Low- or non-calorific sugar/sugar alcohol/glucoside derivatives have shown excellent results and promise in structuring vegetable oils. The functionality of such bioderived gelators can be tuned by employing the appropriate structure and stereo-chemistry of carbohydrates and fatty acid components. The acquired microstructural and physico-chemical differences facilitate the use of structured oils (oleogels) in food materials, personal care, cosmetics, and medicinal applications. We envision that the design, synthesis and self-assembly of bioderived, multifunctional small molecules can be tailored to meet future niche market needs.

FIG. 1.

One-step synthesis of amphiphilc molecular gelators from natural sources (SCFs, MCFs, and LCFs: Short-, medium-, and long-chain fatty acids)

• Oleogels from low-molecular-weight gelators are a potential alternative to traditional vegetable oil structuring methods.

• Such gelators are synthesized from bioderived raw materials including sugars, sugar alcohols, glycosides, and fatty acids using a one-step GRAS (Generally Recognized as Safe) protocol.

• The thermal, rheological, textural, and aesthetic properties of vegetable oils structured with these gelators can be fine-tuned using functional molecular gelators, opening the door to future fat substitutes.

Biography

Sai Sateesh Sagiri is a postdoctoral fellow in the Department of Chemistry and Biochemistry at The City College of New York, New York, USA. He earned his Ph.D. from the Department of Biotechnology and Medical Engineering, National Institute of Technology, Rourkela, India in 2015. His research expertise is on the self-assembly of small molecular gelators, formulation of oleogels, and their applications in food and pharmaceutics

Malick Samateh is a candidate in the Ph.D. Program in Chemistry at the Graduate Center of the City University of New York, where he works on the design and synthesis of amphiphilic molecules from sugar-based precursors. His research includes studying the assembly of small molecular gelators, mainly in vegetable oils, and analyzing the resulting structured oils for properties such as gelation efficiency, thermal stability, and mechanical strength using different physicochemical techniques.

George John is recognized for his active research in the field of functional molecular materials from renewable resources and their potential utility in food materials. After receiving his Ph.D. from NIIST-CSIR (India) in Chemistry in 1993, he held research positions in the Netherlands, Japan, and the United States before joining the City College of New York (CCNY). Currently he is a Professor of Chemistry and Biochemistry at CCNY. The research in John’s laboratory is highly interdisciplinary, and is focused on molecular design of synthetic lipids (biobased), membrane mimics, soft materials, trans fat alternatives, food materials and food chemistry, and organic materials chemistry. His group has successfully developed environmentally benign antibacterial paints, oil spill recovery materials, molecular gel technologies, vegetable oil thickening agents, and trans-fat alternatives. He is a Fellow of the Royal Society of Chemistry, a senior Fulbright Scholar to India, and was a recipient of the Tokyo University of Sciences (TUS) President Award, the Pfizer Visiting Professorship at (IISc) Bangalore, and the Kerala Center Award. He can be contacted at gjohn@ccny.cuny.edu.