One of my favorite classes to teach at the University of Georgia was called Flavor Chemistry and Evaluation. We had no textbook and no formal lab, although we usually read a book as supplementary reading such as Taste or Cookwise. The class met twice a week focusing on a specific aspect of flavor for those two sessions. One period featured a class discussion of two to four recent review articles from the scientific literature focused on the topic of the week. The other session would be an open discussion of recent research studies as they related to each student’s personally selected food. Foods that students in the class followed for the entire semester included aromatic rice, blueberries, coffee, chocolate, kimchi, wine and yogurt. Below are some of the key concepts we covered in the class.
As suggested last week in my review of Flavor by Bob Holmes, our perception of flavor is primarily the combination of taste and aromatic components of a food. It is also influenced by other sensory properties such as
- appearance which sets up flavor expectations,
- texture as it is involved in flavor release,
- chemical feeling factors such as the heat of peppers or the cooling effect of menthol, and
- even crunchy and crispy sounds which enhance the overall eating experience.
Taste is perceived by receptors on the tongue and classified primarily into bitter, salty, sour, sweet and umami sensations. Umami, the most unfamiliar taste sensation, can be induced by salts of free amino acids such as monosodium glutamate (MSG) and is highlighted in foods containing soy sauce or parmesan cheese as ingredients. Aroma is the perception of the odor of volatile, chemical compounds by receptors in the nose both before and during the eating process. Orthonasal aroma is perceived through the nose prior to eating the food setting up our flavor expectations. Retronasal aroma results from passage of aromatic molecules through the back of the mouth into the nasal cavity. It combines with taste perception to form the primary flavor experience while the food is in the mouth. Flavor perception is thus very complex changing with time and with mouth action. The wide range of flavor sensations comes from exposure to numerous chemicals.
In Freshman Chemistry class we are taught that a chemical has both chemical properties and physical properties. Chemical properties are those that involve interaction with other molecules. Physical properties are those that we can perceive with our senses such as color, flavor and texture. Food scientists are interested in functional properties of ingredients which relate back to the chemical and physical properties of the chemicals that make up that ingredient. Note that I use the terms chemical, compound and molecule interchangeably throughout this post. Although there are distinctions between the three terms in a chemistry class, in the popular food literature chemical usually has a negative connotation, molecule a positive one and compound more neutral.
All flavor goes back to the chemicals present in the food. Taste is related to compounds that interact with the taste buds in the mouth. Orthonasal aroma results from the evaporation of specific molecules from the surface of the food into the air and into the nose. Retronasal aroma results from the release of volatile chemicals during chewing which seek that back passage from the mouth to the nose. Some molecules can provoke a response at very low levels; others must be present in large amounts to make an impact. For an individual chemical, the level at which it first makes an impact is called a threshold. Thresholds come in two forms—detection and recognition. We may be able to detect an odor but not be able to recognize that odor until it is present in greater amounts. Some individuals can detect and recognize specific odors in very low amounts. Others have higher thresholds. Anosmia is the inability to detect odors; ageusia is the inability to perceive taste.
Flavor is more than the sum of its chemical components. As mentioned in my review of Flavor last week, David Laing performed an elegant series of experiments on mixtures of odors.1 He presented panelists with a single, distinct, odor compound such as eugenol (cloves) or cinnamaldehyde (cinnamon). When two chemicals were presented almost all panelists could distinguish the specific odors. As the mixtures increased in the number of different odor molecules, however, the ability to recognize the specific odors became more difficult to the point that no one could identify the specific odors present. In foods or ingredients which contain numerous volatile compounds, it is difficult for any single chemical to stand out. Rather, the molecules in a food or an ingredient combine to form odor combinations known as notes. When the mixtures combine compounds of distinctive aromas with those connoting distinctive tastes, the problem of distinguishing individual components becomes even more complex. 2 I met briefly with David Laing when I was visiting in Australia and did not appreciate the significance of his work at that time. Oh, how I wish that I had paid more attention as I could have learned so much more from him than I did!
Cooking produces both desirable and undesirable changes in flavor. Many of the desirable sensations we associate with meats develop during various forms of cooking such as grilling, roasting, and frying. Blended flavors from cheeses and casseroles result from chemical changes associated with the transformation of raw ingredients into cooked foods. Heat can also be detrimental leading to undesirable flavors from such processes as canning or boiling. Undesirable flavors can result from the formation of off-flavors or the deterioration of desirable flavors. Many of these heat-generated chemicals represent a breakdown of large, flavorless molecules into smaller, aromatic compounds which contribute to the flavor experience.
Heat also speeds up browning which affects both color and flavor. The golden-brown compounds that we associate with toasting, roasting, frying and other heat processes are due to a series of chemical reactions, known as Maillard browning. One of the early limitations of microwave cooking was a failure to brown foods. Maillard browning begins with a reducing sugar reacting with a free amino group on a protein. A multitude of chemical reactions result in a mixture of compounds that contribute to flavor as well as to color of the food product. Many of the compounds contributing to the flavor of fresh bread flavor are due to molecules generated during this type of browning. Recently, these reactions have come into question as chemical hazards in our foods, suggesting that blackened and brown foods may be hazardous to our health.
The bottom line is that flavor is the result of many complex chemical processes. The simplest sense involved is taste which results from the attachment of a specific chemical compound to a receptor on the tongue called a taste bud. We perceive aroma from a single volatile molecule such as benzaldehyde (cherry or almond) or a complex mixture of numerous chemicals such as those we experience when enjoying coffee or wine. Heating induces profound changes in the flavor profile of a food whether during processing, in the kitchen or on the grill. Flavor changes induced by nature, a combination of ingredients, or cooking can be beneficial or detrimental to the quality of a food. Producing a desirable flavor in a food by any means is both an art and a science as practiced by organisms, product developers or cooks, but it is all about the chemicals.
1 Laing, D.G. and G.W. Francis, 1989. The capacity of humans to identify odors in mixtures. Physiology & Behavior 46:809-814.
2 Marshall, K., D.G. Laing, A.L. Jinks, and I. Hutchinson, 2006. The capacity of humans to identify components in complex odor-taste mixtures. Chemical Senses 31:539-545.