To some they are "sweet treats" and to others they are "junk food", but whatever the opinion of their nutritional value, sweet snacks such as candy, soda pop, and gum and rich desserts such as ice cream and cakes are ubiquitous in the American diet and a really big business for the food industry. This leads to a strong interest in the biochemical origins of the sweetness response and in finding new sources of sweet-tasting compounds. Since many consumers would like to give up the calories ordinarily associated with sweet compounds such as sucrose, but not give up the sweet treats themselves, there is also a great deal of interest in finding artificial no-calorie sweeteners. All of this consumer interest makes the topic of sweeteners and sweetness theory that is covered by the symposium in this issue a naturally attractive one for the classroom. Eric Walters, one of the authors in this symposium aptly explains that studying sweeteners is attractive because:

. . . sweeteners provide a link between chemical structure and biological activity to which everyone can easily relate. Receptors (proteins which recognize chemical signals and initiate some biological response) are often just an abstract concept. You are not aware of each neurotransmitter-receptor interaction in your nervous system. But when you put something sweet into your mouth, you can directly feel the stimulation of your receptors. You can experience what is meant by a threshold concentration (the minimum concentration which causes a response). You can sense receptor saturation (at some point, increasing sweetener concentration no longer increases the perceived sweetness). You can feel exactly what adaptation is (the second sip of a soft drink is not as sweet as the first).

The papers in the symposium on Sweeteners and Sweetness Theory cover the major areas of interest to chemists. The first by Ellis (page 671) presents an overview of the various types of sweeteners and serves as an introduction to the material that is covered in the two following papers on more specific topics. He reviews the four fundamental human tastes and how taste quality can be studied and then discusses briefly each class of substance known to produce the sweetness response. Last, he reviews the development of theories and models of how the sweet taste mechanism works in humans, an area of investigation that is still progressing.

While we all associate sweetness with sugars such as sucrose, fructose, or glucose, there are a variety of natural substances that produce the biological response we call "sweet". And the potential commercial value of naturally sweet substances has stimulated a search for new plant sources of sweeteners. Kinghorn and Kennelly (page 676) report on both the strategies and the outcome of this search. They review the currently known potentially sweet plant constituents and then describe the approaches to discovering highly sweet molecules in plants. The search involves ethnobotany to locate the plants as well as a complex strategy to carry out in vivo testing of the sweetness and safety of the extracted compounds. Finally, the authors discuss several intensely sweet naturally occurring compounds they have isolated by this process.

The need for in vivo testing of sweetness is one ramification of the lack so far of a precise identification of the structural factor that makes a compound sweet. Walters (page 680) describes the problems with understanding the connection between chemical structure and sweet taste. Since there is great diversity in the structures of sweet compounds, chemists have developed models as tools to work toward a comprehensive theory. Walters discusses the usefulness of models first in general and then as they are applied to sweetness theory. He reviews the best known and most successful of the models that have been developed and shows how some of them have led to new and improved sweet tasting compounds. The opportunities for future exploration both at the experimental and theoretical level should prove intriguing to students, whether they are future researchers or potential consumers of newly discovered sweeteners.