1E: NutraSweet: Sweeteners

From breakfast through snacks, most of us experience a variety of flavor sensations during any given day. Whether the foodstuff is a glass of milk, a broccoli floret, or a hamburger, generally we can place the taste with its distinctive source, even blindfolded.

Some have attempted to categorize various taste sensations, common categories being sour or acidic, salty, bitter, and sweet. The latter is likely to be the more pleasing, and such appreciation appears to be universal. Given a taste of sugar water, people in Canada, Brazil, New Zealand, and Mongolia would all classify the taste as sweet and probably joyful. "Sugar" dominates the sweetness that occurs in natural products, such as fruit, or in prepared products, such as brownies. But sugar is not really a single substance and sugars are not the only type of chemical compounds that taste sweet. This subsection takes a greater glimpse at the sugars and at some other chemical that can impart a sweet taste. It also looks at the non-economic rationale for developing synthetic (or artificial) sweeteners.

A trip to the grocery store shelf to find sugar would likely result in the purchase of a 5-pound bag of sucrose, or cane sugar. NutraSweet is described as a dipeptide because its structure links two monomer units called amino acids. Sucrose is a disaccharide because it links two monomer units, fructose and glucose, called sugars (Figure 11). Sucrose occurs naturally in many sources, but is usually obtained from sugar beets or sugar cane. It has a pleasant sweetness that most people enjoy.

< face="Arial">Figure 11

Sugar also has some other very desirable properties that allow its use in a wide variety of products. It is heat and cold resistant, which means it can be used to sweeten baked goods such as cookies, cakes, and ice cream. It is chemically quite stable, so can be stored for long periods of time without appreciable decomposition. It is quite soluble in water and can easily be used to impart sweetness to beverages. To obtain the level of sweetness usually desired, a fair amount of sucrose must be used in any preparation, a mixed blessing as we shall see. But one consequence is the useful addition of bulk or weight and some texture to a product. This would be a problem if sucrose were expensive, but it isn’t. Another desirable trait is that most people metabolize sucrose and other sugars without concerns about toxicity, except for in the case of diabetics, who must restrict sugar intake because of insulin deficiencies.

With so many valued properties as a foodstuff, why might anyone wish to find a substitute?

Three reasons come quickly to the forefront.

• People who need to medically restrict sugar intake would be able to savor the sensation of eating something sweet if the sweetener could come from a source other than sugar.
• Obesity and fitness have become subjects of considerable publicity and interest in the recent past and excess sugar consumption, among many other factors, is linked to obesity and lack of fitness. A reduction in sugar intake, with its 16 kJ per gram food value, is perhaps a laudable goal.
• Sugar is implicated in tooth decay and the serious consequences that can derive from diseased teeth.

For these and other reasons, not the least of which is economic gain, much research over time has been devoted to discovering compounds other than the naturally occurring sugars to impart sweetness to foodstuffs.

Look at Figure 12. These structures represent several different monosaccharides (sugar compounds) that exist naturally. Most of them impart a sweet taste. They are all characterized by several hydroxyl (-OH) groups and a carbonyl (-C=O) grouping, either an aldehyde or ketone.

Figure 12

Many sugars bonded together in a polymeric structure, depending upon the specific type of linkages, are named cellulose or starch. Neither of these structural types is sweet, but both are very important natural products. Cellulose structures are the backbone of plant materials and will pass through our digestive systems unmetabolized. Plant starches, on the other hand, are the vital carbohydrate component of our diet, metabolized by us by hydrolyzing the starch to the smaller sugar units that enter a biochemical cycle to produce energy for our use and chemical degradation products that can be excreted or used to manufacture other biochemicals in our bodies.

The history of artificial sweetener development is an interesting one. The rationale for searching for such material was presented earlier, but the discoveries of the three most well-known compounds apparently were accidental.

• Saccharin’s sweetness was noted in 1879 by a Johns Hopkins worker who inadvertently licked his fingers after spilling the chemical on his hands.
• Similarly, a University of Illinois graduate student detected the sweet taste of cyclamate in 1937 after it made contact with the cigarette he was smoking.
• And James Schlatter’s 1965 discovery of aspartame’s sweetness occurred when he licked his fingers in preparing to pick up a piece of weighing paper. So much for safe laboratory practices!

As effective as these "mistakes" were, they are not the way most discoveries were or currently are made. Instead, searching for new and better sweeteners occurs in a much more systematic manner. There are several approaches. A traditional approach is to look at the chemical structures of the many compounds that impart a sweet taste to determine whether some common feature or at least similarities exist among the compounds. The reality of the situation is that there is a wide variety of compounds of quite different chemical structures that taste sweet. For example, the structures of fructose, saccharin, and aspartame are quite different. So what researchers have done is to synthesize scores of compounds whose structures would be similar to, say, aspartame and to screen every compound for relative sweetness. A particular change in structure might lead to a sweeter compound, in which case, that change would be made, and then other structural features would be varied. Of course, just because a compound tastes sweeter doesn’t mean it’s a candidate for the marketplace. Safety, cost, stability, and the like would also need to be evaluated.

A second rational approach looks for sweet compounds from the natural world; there are many natural products other than sucrose and its family of sugars that are sweet. This process is more involved, since it is necessary to isolate and identify the sweet component that is only one of hundreds of chemical compounds that would be present in a bit of, say, some tropical fruit. Once the chemical structure has been identified, then the sweetener might be studied to determine which structural features contribute to sweetness. In the laboratory, similar structures can be prepared and tested. Also, naturally, it is possible that the sweetener’s source could be obtained in quantities sufficient to isolate the material and use the isolate as the sweetener, as in the case of sucrose. The source could become an agricultural commodity like sugar cane or sugar beets.

A third approach to obtaining an artificial sweetener relies on a fundamental understanding of the structural requirements needed to impart sweetness. Such an approach begs to understand the structure of human taste buds and how a chemical that tastes sweet interacts with those sensors. We know enough about structural shapes, bond distances, conformational energies, and the like to know that molecules can be designed and synthesized to fit a particular model. But we don’t yet thoroughly understand what taste receptors look like or, for that matter, how many there are. What can be done, however, is computer modeling of a compound like aspartame to determine its probable shape, intramolecular distances, and relative sites of polarity and nonpolarity. Since we know that aspartame is sweet, other compounds with different atoms and connections can be prepared that approximate the shapes and polarities of aspartame, and these can be tested for efficacy. Such analysis and synthesis can be applied to many different types of molecules, not just aspartame. Study continues apace, however, to understand the structure of our taste buds since, ultimately, such an understanding should lead to the most sophisticated way to design sweetener molecules.

The next generation of artificial sweeteners is likely upon us. Recently, two additional artificial sweeteners have been introduced into the U.S. marketplace and can be found in several products.

• Acesulfame-K (Aa-K) has been combined with NutraSweet in Pepsi One. Acesulfame is approximately 200 times as sweet as sugar, but, like saccharin, is reported to have some bitter aftertaste. Blending it with NutraSweet lessens that adverse effect. Acesulfame-K is produced by Hoechst AG, a giant German chemical and drug company, and has been available since the 1980s. Acesulfame-K is considered noncaloric, for it is neither stored nor metabolized. It is excreted unchanged.
• A check of the ingredient label in a "light" Ocean Spray cranberrry product reveals the presence of sucralose (registered under the brand name Splenda), a chlorine-containing carbohydrate product. Sucralose is reported to be 600 times as sweet as sugar and to possess desirable physical properties, such as tolerance to high temperatures and high and low pH. The FDA approved sucralose for use in the U.S. in April 1998, but it has been used outside of the country since 1991.
• Alitame, similar to aspartame in that it combines two amino acids (alanine and aspartic acid) into a dipeptide, is next in line for FDA approval. Its sweetness has been estimated to be 2,000 times that of sugar. One gram of alitame would produce the same sweetness as ten grams of aspartame, a considerable advantage.

Figure 13

None of these recent introductions have lessened the concerns of those who remain convinced that all of these artificial sweeteners pose health risks to the consumer.