Modifying Bitterness

Mechanism, Ingredients, and Applications

Glenn M. Roy

• Publisher: CRC Press
• Year: 2020
• Print ISBN:
  9781566764919
• eBook ISBN:
  9781000160055
• Topic: Technology & Engineering
• Subtopic: Food Science
• Index: Technology & Engineering

Only recently has bitterness control become of commercial importance to a food or pharmaceutical formulation chemist. Over the years, an increasing interest in more palatable food and beverage products with low fat and low sugar content has arisen, thus creating a market need for the control of bitterness perception. This is the first, comprehensive treatment of this subject in book form. Organized primarily by ingredients or processing approaches affecting the bitter taste reduction or inhibition, this thorough review includes an in-depth and thoroughly referenced review of mechanisms, ingredients and applications of bitter taste reduction or inhibition.

SECTION II

A SYMPOSIUM

CHAPTER 6

Interactions between Sweet and Bitter Tastes

D. ERIC. WALTERS¹

INTRODUCTION

THE old adage “a spoonful of sugar helps the medicine go down” reminds us that sweet taste has been used to mask bitter taste for a very long time. Sweet taste is usually considered desirable whereas bitter taste is usually (but not always) considered undesirable, so sweetness is frequently used to cover or balance bitter taste. Sweet taste and bitter taste may interact in several ways. There can be a masking effect, in which sweet taste is used to cover up, balance, or overwhelm bitter taste. There may also be ways in which some sweeteners inhibit perception of some bitter substances.

Taste receptors are proteins on the surface of taste bud cells that recognize certain chemical structures and initiate the signals that the brain recognizes to mean sweet, bitter, salty, or sour. We often think of receptors using a lock-and-key analogy. For instance, sweet-tasting compounds are like keys that can fit into the sweet-receptor locks.

It is likely that there are several different (but related) kinds of sweet receptors and several kinds of bitter receptors. In the case of sweeteners, at least three lines of evidence point to multiple receptor types:

(1) Some sweeteners are perceived nearer the front of the tongue, while others appear to be more intensely sweet near the center of the tongue.

(2) The incredible structural diversity of sweet-tasting compounds argues strongly against a single receptor site, which could recognize all substances known to taste sweet.

(3) Some mixtures of two different sweeteners (e.g., aspartame + ace-sulfame) produce synergy, a higher level of sweetness for the mixture than would be predicted from the sweetness of the individual compounds; other mixtures (e.g., saccharin + acesulfame) are strictly additive and probably compete for a common receptor site (Carr et al., 1993).

All of these phenomena probably apply to bitter compounds as well, although there is far less sensory research on bitter taste. Despite numerous efforts, no one has yet been successful in isolating and characterizing sweet or bitter taste receptors at the molecular level.

In this chapter, we summarize a number of experiments that point to interactions between sweet and bitter taste, propose a likely mechanism by which sweet and bitter tastes may interact, and finally look at the practical implications of such a mechanism for masking or inhibiting bitter taste.

EXPERIMENTAL OBSERVATIONS

Numerous experiments suggest the existence of several related receptor types for sweet and bitter tastes. Also, important experiments indicate a relationship between sweet and bitter taste receptors. And there are experiments that give us some clues about the nature of these taste receptors. The following section summarizes some of the key experiments.

The chemical literature is filled with instances of the relationships between molecular structure and sweet and bitter taste. For example, Janusz has reviewed the more than 1000 analogs of aspartame that have been synthesized and tested in laboratories all over the world (Janusz, 1989). Besides hundreds of sweet-tasting analogs, there are many compounds that are bitter, some are both bitter and sweet, and a few are tasteless. Often, very subtle changes in structure can convert potently sweet compounds into potently bitter compounds. Figure 1 illustrates some examples of this phenomenon.

Aspartame [Figure 1(a)] is formed from the natural amino acids L-aspartate and L-phenylalanine; if D-phenylalanine is substituted [Figure 1(b)], a bitter compound is formed (Mazur et al., 1969). A sweet-tasting oxime [Figure 1(c)] can be converted to a bitter one [Figure 1(d)] just by altering the pattern of double bonds (Acton et al., 1960). Acesulfame [Figure 1(e)] tastes sweet, and to some people, has a bitter aftertaste; addition of an ethoxy group [Figure 1(f)] produces a substance that is only bitter (Clauss and Jensen, 1973; Clauss, 1980). In at least one instance, modification of a bitter substance has produced a sweet compound. Neohesperidin [Figure 1(h)] is a bitter component of citrus peel; breaking a single bond in the molecule produces neohesperidin dihydrochalcone [Figure 1(g)], which is sweet (Horowitz and Gentilli, 1969). In the lock-and-key analogy, these results are an indication that some sweet “locks” are very similar to some bitter “locks.”

Figure 1 Similarities in structure between selected sweet and bitter compounds: (a) aspartame, L-aspartyl-L-phenylalanine methyl ester; (b) l-aspartyl-D-phenylalanine methyl ester; (c) a sweet oxime; (d) a bitter oxime; (e) acesulfame; (f) bitter ethoxy-substituted analog of acesulfame; (g) neohesperidin dihydrochalcone, R = b-neohesperidosyl; (h) neohesperidin, R = b-neo-hesperidosyl.

Extending the lock-and-key analogy, taste inhibitors are compounds that “fit into the lock” but cannot open it. Bitter taste inhibition has been thoroughly reviewed by Roy (1992, 1994). There seem to be no bitterness competitive inhibitors with broad activity against many bitter compounds; instead, each inhibitor blocks some small set of bitter substances. Conversely, the known sweetness inhibitors appear to act against all sweeteners. These include an arylurea that blocks all 10 structural classes of sweeteners against which it was tested (Muller et al., 1992); lactisole, which blocks at least five classes of sweeteners (Lindley, 1991); and gymnemic acid, which blocks 11 different sweeteners (Hellekant and Ninomiya, 1991). Curiously, the arylurea also blocked some (but not all) bitter compounds, with no effect on salty or sour tastes, again indicating a link between sweet and bitter mechanisms.

It is well known that there is mixture suppression when bitter and sweet tastes are combined. That is, in a mixture containing both sweet and bitter substances, the perception of both sweet and bitter tastes is decreased. There is not agreement, however, on whether this suppression occurs at the level of the taste bud (Lawless, 1982) or in the brain (Kroeze and Bartoshuk, 1985).

One very curious aspect of sweetener perception is the phenomenon of temporal profile (Figure 2a)–the time of onset of taste perception and the duration of that perception (Carr et al., 1993). Sucrose, which is generally considered ideal, has a fairly rapid onset and clears fairly quickly. Sweeteners such as saccharin and acesulfame have a more rapid onset than sucrose and have a very short duration. At the other extreme, glycyrrhizin has a very slow onset and lingers for a very long time. Is this an indication that different sweeteners may act by different mechanisms? Again, bitter taste is not nearly so well studied, although qualitatively bitterness seems to be more of a slow onset/lingering taste phenomenon.

A common phenomenon in the study of pharmaceutical receptors is the occurrence of partial agonists. These compounds interact with their receptor and can cause a partial response. They are of interest because they are not able, even at high concentration, to produce a maximal response, and they can block the ability of other drugs to bind to the receptor. The classical example of this is nalorphine. This is an analgesic drug related to morphine. It can be used to treat severe pain, but it is also useful in treating an acute overdose of other narcotic analgesics, since it only partially triggers the receptors and prevents other drugs from triggering a full response. In the lock-and-key analogy, these keys open the lock “part of the way;’ and they prevent other, more active keys from being inserted into the lock. Partial agonists (Figure 2b) have been found among sweeteners (DuBois et al., 1991). These are compounds that have a high potency relative to sucrose at threshold levels, but cannot match the sweetness of a 10% sucrose solution no matter how high their concentration. The phenomenon of partial agonism has not been documented for bitter compounds, but this is very likely because no one has studied the sensory properties of bitter compounds in sufficient detail.

Figure 2a Schematic representation of rapid and slow temporal profiles

Figure 2b Illustration of the “partial agonist” phenomenon

In recent years, it has become clear that sweet and bitter taste receptors are part of a large family of receptors called the G protein-coupled receptors (GPCR) (Akabas, 1990). Members of the GPCR family are utilized in olfaction, vision, neurotransmitters and peptide hormones. The GPCR is a very old family of receptor proteins, since some GPCRs serve as chemosensory receptors in single-celled organisms. As shown in Figure 2c, the receptor is a protein embedded in the cell membrane. The signal molecule on the outside of the cell is recogn...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Acknowledgement
• Introduction
• Section I: Mechanism, Ingredients, Applications
• Section II: A Symposium
• Section III: Applications in Oral Pharmaceuticals
• Index
• Editor’s Biography