Hannah May-Lee Wong
EFI Taste Modulation Project
Written by: Hannah Wong
Introduction
Taste modification of pharmaceuticals has been a niche area of study as most
drugs that are administered orally are encapsulated (in tablet form) and
swallowed without the need to taste the medication. That being said, particular
attention has been made to prevent the bitter after-taste of many medications
simply because the bitter taste leads to higher chances of non-compliance by the
patients. This is apparent in children and the elderly, as the dosage of medication
to be administered in a tablet form is often too concentrated (Szektli & Szene,
2005).
Generally, the well known taste masking methods that can be applied to bitter or
off-tasting molecules include: numbing the taste buds (creating a “blockade”),
obscuring tastes by adjusting viscosity or adding sweeteners/flavouring agents,
modification of active pharmaceutical ingredient that causes the bitterness,
complexation of bitter ingredient with cyclodextrins and finally, coating bitter
ingredient with lipidic or polymeric coatings (Walsh et al., 2014). The mentioned
methods can be summarized in the table below:
Figure 1. Summary of taste masking methods to block bitterness or off-tastes of
active pharmaceutical ingredients (API) (Walsh et al, 2014).
We will first look into the most well-known method of masking taste, which is
through complexation of the off-tasting molecules with cyclodextrins, which is a
class of cyclic oligosaccharides that are able to entrap small molecules
responsible for undesirable tastes.
1
Hannah May-Lee Wong
Cyclodextrins
Cyclodextrines (CD) can be divided into three types: α, β and γ made out of 6, 7
and 8 glucopyranose units respectively (Szejtli & Szente, 2013).
Below is a structural representation of an α-cyclodextrin molecule:
Figure 1. Drawing of an α-cyclodextrin molecule (Bilensoy, 2011).
As the image shows above, carbons are linked by 1-4 glycosidic bonds. All
secondary hydroxy groups are located on the wider edge of the ring, whereas the
primary hydroxyl groups are located on the narrower edge. The whole structure
has a conical cylindrical shape with a cavity where the cavity is lined with
hydrogen atoms and glycosidic oxygen bridges. The outer surface of the cone is
hydrophilic and the axial cavity is hydrophobic.
The cavity of the CDs are occupied by water molecules. Since these water
molecules are in direct contact with the apolar wall of the CD cavity, the polarapolar interaction is energetically unfavourable. This leads to the water
molecules being readily substituted by ‘guest’ molecules that are less polar than
water provided they fit geometrically into the CD-cavity (Szejtli & Szente, 2013).
Mechanism:
There are two ways that cyclodextrin acts as debittering agents: the first is to
prevent contact between the bitter tasting molecules and the receptors and the
second is to cover the bitter taste by adding intense flavours such as sweeteners.
As for eliminating bad taste, two theoretical possibilities have been suggested.
The method is to enwrap the bad tasting molecules with CD and therefore
preventing interaction with the taste bud, or the CD interacting with the gate
keeper proteins of the taste buds, temporarily paralysing them.
The preparation methods for CD complexes include kneading, co-crystallization
and spray-drying (Szejtli & Szente, 2013).
Previous use of CD in taste modification of food include: masking “grassy” taste
in soybeans, eliminating characteristic odour of vitamin B1 rich rice, removing
excessive bitter taste in grapefruit (limonin and naringin). Fishy odours have
been masked by CDs in canned fish products, where it was found that the sulfide
2
Hannah May-Lee Wong
compounds in the canned fish decreased from 137.7 to 3.3 µg % in the presence
of CD (Sobel, Gundlach & Su, 2014).
Microencapsulation of Omega-3 and -6 Polyunsaturated Fatty
Acids
Microencapsulation on the other hand, is a method of app;ying a physical barrier
over functional ingredients so that factors like bioavailability, timely delivery
and release and even undesirable tastes can be controlled. Here, we focus on the
microencapsulation methods for omega-3 and 6 oils.
Omega-3 and -6 Polyunsaturated Fatty Acids (PUFA) are highly unstable in high
temperatures and oxidizing environments. The result of oxidation is unpleasant
odors and off-flavors (Garg et al., 2006), with omega-3 PUFAs are much more
unstable to oxidation than omega-6. To address this issue with omega-3 and 6
PUFAs, the industry has designed protective technologies such as
microencapsulation to entrap the oil.
Spray drying:
Spray drying is the easiest and cheapest method of industrial encapsulation of
omega-3 and 6 PUFAs. The mechanism is such the oils are first atomized to
disperse liquid into hot gas, and as the atomized emulsion droplets enter the
spray dryer chamber where water is quickly driven off in a drying stage. The
resulting biopolymers then form a hardened shall around the omega-3-rich oils.
The initial biopolymer is typically a mixture of biopolymer stabilized in water
(O/W) emulsion, an example of a suitable biopolymer would be proteins that
have an amphiphilic nature (Sobel, Gundlach & Su, 2014). For the purpose of
formulations, functional ingredients such as film formers, fillers, antioxidants
and chelators may be included in the emulsion.
Other processes of microencapsulation include spray cooling, fluidized bed
coating, freeze drying, complex coacervation and extrusion.
The applications of encapsulated omega 3 and 6 PUFAs in fortified food products
include dairy products, baked goods (bread), cereals and juice beverages. An
example list of applications of microencapsulation methods are as followed:
3
Hannah May-Lee Wong
Table 1. Commercial Omega-3 and -6 PUFA rich microencapsulated powders
(Sobel, Gundlach & Su, 2014).
Sporopollenin exines:
Sporopollenin exines are extracted from spores of the plant Lyncopodium
clavatum, where exines are the exoskeletal shells of plant spores and pollen
(composed of sporopollenin). It has been suggested to be a novel taste masking
material for fish oils via encapsulation. In an experiment by (Barrier et al., 2010),
the taste masking effects of sporopollenin exines were investigated on cod liver
oil. The oil was converted into powder form.
Results showed that up to 1.0 g/g cod liver oil per gram of sporopollenin exines,
the 20 volunteers participating in the taste test could not differentiate the taste
of powdered cod liver oil from either water or sunflower oil. Thus, this makes
sporopollenin exines a plausible candidate material for microencapsulating offtasting ingredients.
4
Hannah May-Lee Wong
Miraculin
Miraculin is one of the six sweet tasting small proteins that have been
discovered. The other proteins include thaumatin, monellin, curculin, mabinlin
and brazzein. All these proteins display taste modifying properties (Paladino et
al, 2008). Miraculin is isolated from the fruit Richadella Dulcifica. Although in
itself does not elicit sweetness, but it is able to switch sour tastes into sweet
tastes. For example, when the pulp of the Richadella Dulcifica berries are eaten
with lemons, the sour taste of the lemon sweetens, thus giving these berries the
title “miracle fruit (Hirai et al., 2009). The active component of the berry is
miraculin, a taste modifying protein.
The structure of Miraculin is proposed to be a single polypeptide with the weight
24,600kDa that had covalently linked inter-chain disulphide bonds. Its taste
modifying capabilities range up to a maximum of pH3.0 and is almost inactive at
pH 6.0 (Paladino et al, 2008).
Mechanism:
The mode of action of miraculin in taste modification has been linked to the
human sweet taste receptors (hT1R2-hT1R3). It was recently found that the acid
induced sweetness of miraculin was diminished in the presence of lactisole, a
sweet taste inhibitor. Lactisole inhibits human sweet taste receptors by
interacting with the transmembrane domain of the hT1R3 subunit. This suggests
that miraculin acts in collaboration with the hT1R2-hT1R3 receptor (Misaka,
2013).
Furthermore, the taste-modifying activity of miraculin lasts for more than 1hour
in human sensory tests. This leads to the hypothesis that miraculin directly and
intensely binds to the taste receptors and activates the receptors as the pH
decreases. In cell-based assays investigating cell expression when taste receptors
are incubated with miraculin, it was found that cell expression was pH
dependent, where cells were most responsive as lower pH values, being most
active at pH 4.8 and least responsive at pH 6.5. Studies have also proposed that
miraculin binds to the receptors as an agonist every time sour is being tasted,
however, when pH turns neutral, the miraculin molecule turns into an antagonist
and suppresses the activation of the receptor by other sweeteners (Misaka,
2013).
Adenosine 5’- Monophosphate 5 (AMP) as bitter blockers
Adenosine 5’- monophosphate (AMP) has been used to reduce bitterness in
pharmaceutical drugs (Walsh et al., 2014). It can be also used to improve sweet
taste by eliminating the problem of bitter aftertastes present in certain
sweeteners such as saccharin, acesulfam potassium and stevia. Although AMP
has bitter blocking capabilities in sweeteners, it has a characteristic umami taste.
5
Hannah May-Lee Wong
This limits the use of AMP in sweetener applications and makes it more suitable
for savoury formulations eg. salt reduction where it decreases the bitter taste of
certain salts such as potassium chloride (Mcgregor, 2006).
Mechanism:
In an experiment, AMP was tested on bitter taste receptors. Gustducin is a
transducing-like guanine nucleotide-binding regulatory protein (G-proteins) and
are expressed in taste receptor cells in the presence of bitter tastes. The
activation of G proteins in bovine taste receptors were analyzed in vitro. The
inhibition of the expression of bitter taste receptors were measured in the
presence of bitter taste molecules like denaonium, quinine, strychnine and
atropine. It was found that AMP does decrease the expression of G-protein and
was proposed that AMP binds to bitter-responsive taste receptors or interfere
with receptor-G protein coupling to serve as a bitter blocker (Ming, Ninomiya &
Margolskee, 1999).
Neohesperidin dihydrochalcone
This compound is a glycoside that has been used as a sweetener. It provides a
long lasting sweetness when added; however, has a licorice-like aftertaste
(Gascon, 2007). The exact mechanism of the compound is unknown, however,
through taste tests determined by a panel of trained assessors, it has been
known to provide flavor enhancement in fruity products. Neohespiridon
dihydrochalcone also reduces the intensity of some sharp of spicy flavor
attributes. Overall, it has been considered to bring improvement in product
sensory qualities (Lindley et al., 1993).
Lecithin
Lecithin is a mixture surface-active phospoholipids. Due to the ability of
phospholipids to self-assemble, gelate, form film and biodegrade, they hold great
potential in pharmaceutical applications. The surface-activity combined with the
film forming capabilities make milk proteins good candidates for coating
materials for encapsulation via spray drying. Therefore, calcium caseinate in
combination with lecithin is suitable as material of encapsulation through spray
drying for taste masking purposes. One example of a drug being experimentally
encapsulated to mask its bitter taste is acetaminophen (Thi, Lemdani & Flament,
2013).
Thaumatin
Thaumatin is an intensely sweet-tasting protein isolated by a tropical plant
Thaumatococcus danielli Benth that is native to tropical West Africa. The variant
Thaumatin I, is a single chain protein made up of 207 amino acid residues
connected by eight disulfide bonds. It elicits sweet tastes in humans in
concentrations as low as 50nM (Ohta et al., 2011).
Mechanism:
6
Hannah May-Lee Wong
The exact mechanism of thaumatin is not known. However, experiments have
been done analyzing the response of cells expressing sweet receptors T1R2T1R3 when exposed to thaumatin. It was found that only the T1R3 receptors
were responsive towards thaumatin, and that the cysteine-rich domain (CRD) of
the human T1R3 is important for the interaction with thaumatin (Ohta et al.,
2011). Adding to that, an experiment involving mice with mutated T1R3
receptors had a significant decreased response to thaumatin, indicating the
specificity of interaction between thaumatin and the T1R3 receptors (Masuda et
al., 2013).
Film coating
Film coating has been a new concept found to provide foods or pharmaceuticals
moisture protection and taste masking. Film coating can be achieved through the
use of water soluble, cationic, anionic or neutral insoluble polymers (Joshi &
Petereit, 2013). Polymers used in film coating are usually water-soluble
polymers and reliable taste masking requires coatings of at least 10𝜇g in
thickness. The process of film coating involves spray coating the polymers onto
various types of cores from dispersions or solutions in solvents in a drum or a
fluidized bed coater. Suitable solvents include ethanol, isopropanol or acetone
mixtures (Joshi & Petereit, 2013). The examples of polymers used in film coating
are as followed:
Table 2. Water-soluble polymers used for taste masking by film coating(Joshi &
Petereit, 2013).
Table 3. Cationic polymers used for taste masking by film coating(Joshi &
Petereit, 2013).
7
Hannah May-Lee Wong
Table 3. Anionic polymers used for taste masking by film coating(Joshi &
Petereit, 2013).
Ion-exchange resins
Ion exchange resins are water soluble polymers that contain acidic or basic
functional groups in a repeating pattern. They have been widely used to form
weak reversible ionic bonds with an oppositely-charged drug through the
exchange of ions. This leads to the release of the free drug depending on the ionic
environment (Kim et al., 2013). These resins are solid and suitable in solubilized
high molecular weight polyelectrolytes that can exchange their mobile ions of
equal charge with the surrounding medium reversibly. Bitter cationic drugs can
adsorb onto weak cationic exchange resins of carboxylic acid and functionally
form a complex that has a significant decrease in bitter taste. Ion-exchange resins
can be used to formulate lozenges, chewing gum, suspension or dispersible
tablets to mask bitter tastes (Puttewar et al., 2010).
Experiments have been conducted, whereby bitter drugs are taste masked by ion
exchange resins. An example of such application is seen with the combination of
donepezil HCl (that is used to treat Alzheimer’s) and Amberlite (IRP-64) the ion
exchange resin. The complex was prepared by adsorbing the drug onto the ion
exchange resin at different ratios (1:2. 1:1, 2:1) using spray-drying method. The
taste masking capabilities were measured using volunteers and an electronic
tongue and a bitterness scale. Both analyses had a strong correlation with
decreased bitter tastes when the drug was bound with resins. The structures of
the compounds are below:
8
Hannah May-Lee Wong
Figure 2. Compound (A) is the chemical structure of donepezil HCl, compound
(B) is Amberlite IRP-64 (ion-exchange resin).
Conclusion:
Taste masking is a relatively new field that is growing due to its demand in the
pharmaceutical industry. In terms of nutraceuticals, information on taste
masking is still lacking. Furthermore, most of the exact mechanisms in taste
maskings are unconfirmed. However, research is still undergoing and new
technologies are still being produced. For now, the most common means of taste
masking include microencapsulation, complexation, and attempts to mask offtastes by adding sweet compounds like frustose to improve taste.
9
Hannah May-Lee Wong
References:
Albertini, B, Cavallari, C, Passerini, N, Voinovich, D, Gonzalez-Rodriguez, G,
Magaratto, L, Rodriguez, L (2003). Charecterization and taste-maksking
evaluation of acetaminophen granules: comparison between different
preparation methods in high-shear mixer. 21: p295-303.
Barrier, S, Rigby, AS, Diego-Taboada, A, Thomasson, MJ, Mackenzie, G, Atkin, SL.
(2010). Sporopollenin exines: A novel natural taste masking material. LWT- Food
Science and Technology. 43: p73-76
Bilensoy, E (2005). Cyclodextrins in pharmaceutics, cosmetics and biomedicine.
Canada: John Wiley and Sons. p3.
Hirai, T, Mayuko Sato, M, Toyooka, K, Sun, HJ, Yano, M, Ezura, H. (2009).
Miraculin, a taste-modifying protein is secreted into intercellular spaces in plant
cells. Journal of Plant Physiology. 167: p209-215.
Garg, ML, Wood, LG, Singh, H, Moughan, PJ. (2006). Means of delivering
recommended levels of long chain n-3 polyunsaturated fatty acids in human
diets. Journal of Food Science. 71: p66-71.
Gascon, M. (2007). Modifying flavour in food. Woodhead Publishing Series in
Food Science, Technology and Nutrition. p232-242.
Joshi, S, Petereit, HU. (2013). Film coatings for taste masking and moisture
protection. International Journal of Pharmaceutics. 457: p395-406.
Kim, JI, Cho, SM, Cui, JH, Cao, QR, Oh, E, Lee, BJ. (2013). In vitro and in vivo
correlation of disintegration and bitter tastemasking using orally disintegrating
tablet containing ion exchangeresin-drug complex. International Journal of
Pharmaceutics. 455: p31-39.
Lindley, MG, Beyts, PK, Canales, I, Borrego, F. Flavor modifying characteristics of
the intense sweetener neohespiridin dihydrochalcone. Journal of Food Science.
58: p592-594.
Masuda, T, Taguchi, W, Sano, A, Ohta, K, Kitabatake, N, Tani, F. (2013). Five amino
acid residues in cysteine-rich domain of human T1R3 were involved in the
response for sweet-tasting protein, thaumatin. Biochimie. 95:1502-1505.
McGregor, R. (2006). Future directions: using biotechnology to. In: Spillane, WJ
Optimizing sweet taste in food. USA: CRC Press. p410.
Ming, D, Ninomiya, Y, Margolskee, RF. Blocking taste receptor activation of
gustducin inhibits gustatory responses to bitter compounds. 96: p9903-9908.
Misaka, T. (2013). Molecular mechanisms of the action of miraculin, a tastemodifying protein. Seminars in Cell & Developmental Biology. 24: p222-225.
10
Hannah May-Lee Wong
Ohta, K, Masuda, T, Tani, F, Kitabatake, N. (2011). The cysteine-rich domain of
human T1R3 is necessary for the interaction between human T1R2–T1R3 sweet
receptors and a sweet-tasting protein, thaumatin. Biochemical and Biophysical
Research Communications. 406: p435-438.
Palladino, A, Costantine, S, Colonna, G, Facchiano, A (2008). Molecular modelling
of miraculin: Structural analyses and functional hypotheses. Biochemical and
Biophysical Research Communications. 367: p26-32.
Putterwar, TY, Kshirsagar, MD, Chandewar, AV, Chikhale, RV. (2010). Formulation and
evaluation of orodispersible tablet of taste masked doxylamine succinate using ion
exchange resin. Journal of King Saud University (Science). 22: p229-240.
Sobel, R, Gundlach, M, Su, CP (2014). Microencapulation in the food industry.
USA: Elsevier. p438, 447.
Szejtli, J, Szene, L. (2005). Elimination of bitter, disgusting tastes of drugs and
foods by cyclodextrins. European Journal of Pharmaceutics and
Biopharmaceutics. 61: p115-125.
Thi, TH, Lemdani, M, Flament, MP. (2013). Use of calcium caseinate in association
with lecithin for masking the bitterness of acetaminophen—Comparative study
with sodium caseinate. International Journal of Pharmaceutics. 456: p382-389.
Walsh, J, Cram, A, Woertz, K, Beitkreutz, J, Winzenburg, G, Turner, R, Tuleu, C.
(2014). Playing hide and seek with poorly tasting paediatric medicines: Do not
forget the excipients. Advanced Drug Delivery Reviews. 73: p14-33.
11