Mouthfeel is a technical term which is used to discuss the chemical and physical interactions of food with the mouth. When professional tasters evaluate food and drinks, mouthfeel is a very important part of their evaluation, as one might imagine, and people can respond very differently to different foods, depending on their mouthfeel. This phenomenon is the result of a very complex series of interactions, and it explains why simple foods can sometimes be so difficult to replicate.
When many people are asked about their response to a food, they are usually asked about how the food tastes. Taste certainly plays a role in mouthfeel, with tasters considering the levels of saltiness, bitterness, sweetness, heat, and umami in their food, but taste is only a facet of mouthfeel. Another consideration is the texture of the food in the mouth, the way it feels to eat, and the linger of the finish after people take a bite. Mouthfeel also considers the physical and chemical changes in the food as it is chewed, ranging from the way some foods dissolve with exposure to saliva to the way some foods taste bitter if they are chewed too long.
Some terms commonly used in reference to texture include: chewiness, hardness, gumminess, moisture, cohesiveness, grainyness, denseness, bounce, juiciness, dryness, release, viscosity, and wetness. The sharp snap of toffee, for example, has a very different mouthfeel than the chewy adhesiveness of caramel, explaining why the two foods taste so different when they are made from very similar ingredients.
When mouthfeel is evaluated, people also pay attention to their emotional reaction to the food. Foods like chocolate bars, with a rich, buttery smoothness, often make people feel fulfilled, happy, or relaxed. Hard candy, on the other hand, can be more agitating, as people suck or chew on the candy, while crisp fruits can be emotionally refreshing.
Companies which specialize in making packaged foods are often forced to consider mouthfeel. Mouthfeel plays a critical role in the reception of fast food, meals ready to eat (MREs), and other packaged foods designed for convenience. If the mouthfeel is “wrong” to consumers, they will usually avoid the product in the future, so companies expend a lot of energy in research to develop food which tastes and feels right, in addition to providing nutritional value.
You don't have to be a trained taster to think about mouthfeel. The next time you have something to eat, think about the texture of the food in your mouth, the way the taste and texture change as you chew, and the way that you feel as you eat. You might also want to think about the way that different foods interact with each other: for example, if you taste chocolate after eating curry, it will taste very different than it does on its own.