June 2011 Prepared Foods – More than a few perfumes have been sold on the premise they provide a unique aroma on every individual. The explanation often given is “personal chemistry,” where an odor is perceived differently, depending on a person’s own aroma, as well as interactions between the perfume and compounds naturally present on the skin.

An initial first step in evaluating a flavor is to put it in a tasting medium appropriate for the type of flavor. See recommendations
in this article.

A somewhat analogous situation and challenge exists, when product developers work with flavoring compounds. Regardless of whether a flavor falls into the artificial, natural or nature-identical category, any individual flavoring compound will be perceived differently, depending on the type of food matrix into which it is formulated.

There is a Smell in the Air
Although tastes, such as sourness or sweetness, as well as heat (e.g., capsaicin) and cooling (menthol) can influence what a consumer may call a flavor, it is generally understood that olfaction–the sense of smell—provides the most important sensory information that defines a food’s flavor.

Many reasons exist for adding flavoring ingredients. Such compounds can enhance innately present flavoring compounds; replace what has been lost through processing or never developed in the first place, such as grilled aromas; or make products that are healthful or more palatable.
However, as anyone who has ever worked to flavor an application knows, the aroma one detects when first opening a flavor sample and the ability to deliver that exact aroma to a product is virtually impossible. This is due to complex interactions between flavor molecules and other ingredients in the food matrix, as well as the process of eating itself.

In order for an aroma to be perceived before consumption, it must travel through the air in a high enough concentration to be detected by the olfactory epithelium within the nose. When a product is being consumed, volatile flavors are released from the food or beverage matrix within the mouth and pass back up through the nasopharynx, before going on to the olfactory epithelium. If an aroma is not released into the air or headspace, then it will not be detected.

Adding to the complexity is the fact that most all flavoring compounds consist of numerous volatile compounds with a wide range of chemical structures. For example, one study, Siek, TJ. 1969. J Food Sci. 34 (3): 265, investigated the sensory threshold of 31 volatile compounds found in butter oil. Butyric acid, diacetyl, delta-decalactone, 2-nonanone, gamma-undecalactone and n-hexanal were among them. Each of these compounds has differing physical properties, such as solubility, boiling points and tendency to bind with other molecules, which impact the concentration released to the air. Thus, when a food formulation is changed, some compounds may be more easily released than others compared to the original product, altering the perceived overall flavor.

Please Release Me
There are a variety of ways in which flavor compounds associate with other molecules in the food matrix. For example, volatile flavors may react with proteins. Sometimes, the reaction is irreversible, such as flavor aldehydes forming covalent bonds with a protein’s amino or sulfhydryl group. At other times, the reaction may be reversible, such as van der Waals interactions, resulting in hydrophobic bonds between non-polar volatile compounds and proteins.
Flavor binding has been investigated for plant protein—soy being one of the most common—to animal proteins, especially dairy proteins. The process of flavors binding with plant and animal proteins (particularly soy and dairy, respectively) has been widely investigated. Of the dairy proteins, beta-lactoglobulin is the most extensively researched. One recent study confirmed the protein has two different binding sites for flavor compounds and, when in the mouth, both free aroma compounds and those reversibly bound by the protein are released, “pointing out the fact that flavor perception is only affected if strong binding occurs.” (Guichard E. 2006. Flavour Retention and Release from Protein Solutions. Biotechnol Adv. 24(2):226-9.)

These interactions present challenges when flavoring beverages, whether they are milk-, soy- or almond-based. For example, a chia flavoring may work perfectly with milk, but less optimally in one of the newer almond-based beverages. It also provides one explanation of why changing from a fructose to an aspartame-sweetened beverage may subtly affect flavor. Aspartame, a dipeptide sweetener made from aspartic acid and the methyl ester form of phenylalanine, has been found to interact with flavoring aldehydes,  such as those contained in vanilla, benzaldehyde, cinnamaldehyde and citral.

The amount and type of fats and emulsifiers also influence how quickly flavorings are released and detected, if at all. For example, research has shown that in products with a fat phase, whether emulsified or continuous, the general rule is the more solid the fat is (i.e., the higher the solid fat index of the fat or oil), the more slowly a volatile compound will move from the fat into the water phase, and, ultimately, to the headspace, before being perceived by a consumer.
All this has practical applications in food formulations. Efforts to decrease traditional levels of fat in a food have resulted in new foods that are often “good,” but not as great as the original formula. One key reason is provided by professor Gary Reinnecius, writing in Ingredient Interactions: Effects on Food Quality (ed. AG Gaonkar, CRC Press, 1995, p 443). Reinnecius notes that neither carbohydrates nor proteins are lipophilic and, thus, will interact with flavors in a different way than fats do. “The effect of fat on the sensory perception of a flavor is primarily due to its property of affecting the vapor pressure (and, therefore, the concentration in the headspace aroma) of the flavor above the food.” He notes most flavor compounds are hydrophobic in nature and will partition into the fat phase of a food. When fat is reduced or eliminated, “there is no place for the fat to go,” and it is released into the headspace above the food, resulting in “unbalanced” aroma compounds above the headspace of a food. That is, the flavor of a reduced fat food is not the same as what a consumer would associate with a full-fat, traditional product.

Filtering Flavors

So, what does a formulator do first, when faced with a product in need of flavor enhancement on one hand and a choice of dozens of flavor samples on the other? This decision is complicated by knowing that none will smell the same in the product as they do in the container.

One supplier notes that, although trying a flavor in the end application is the best way to check for appropriate levels, overall flavor profile and long-term stability, this often is not practical as a first step. The use of model systems, while not perfect, allows one to narrow the field.

For example, if one wanted to evaluate a sweet, water-soluble flavor, such as almond, banana, butterscotch, vanilla, chocolate, cinnamon or coconut, the flavoring should be tried in an 8% water solution. If the sweet flavor is oil-soluble, it should instead be tried in high-fructose corn syrup. If the flavor is more commonly associated with tartness, such as fruit and berry flavors (e.g., apple, mango, tropical fruits), the tasting medium should then be sweet water, with a lowered pH through addition of 0.1% granulated citric acid. (See chart “Screening Flavors.”) A 0.20% salt solution is recommended for certain water-soluble and savory flavors, such as beef, butter, garlic, mushroom, poultry or nuts (shown as Savory 1 in the chart). Warm, plain water is recommended for water-soluble savory flavors, such as barbecue, bread dough, clove, wine, carrot and herbal, among others (shown as Savory 2 in the chart).

Product formulation is never easy, but understanding why something is “going wrong” is the first step in being able to make corrections. pf

Suggested Further Reading:
Ingredient Interactions: Effects on Food Quality, 2nd Edition. Edited by A. McPherson and A. G . Gaonkar. CRC Press 2006.
Flavor Interactions Were More Extensively Covered in Ingredient Interactions: Effects on Food Quality, Edited by A. G. Gaonkar, Marcel Dekker Inc. 1995. ISBN: 0-8247-9347-1.