Flavor Analysis in Food

Flavor compounds impart food aroma and taste, which are the major factors influencing consumer's perception of the quality of food. Understanding the nature of flavor compounds and their effects on human organoleptic responses is important and flavor analysis is an inseparable part of the process. Analysis of flavor compounds, however, is complicated by several factors. While taste can basically be categorized as being salty, sour, bitter and sweet, the sensations caused by aroma compounds are much more complex. This results from the presence of numerous volatiles in foods and the ability of humans to differentiate these odorous components. The number of volatile and semivolatile compounds in foods ranges from 50 to 250 compounds in fresh fruits and vegetables to more than double these numbers in thermally processed foods such as roast beef and coffee. The odor-active compounds are usually found in very low concentrations, ranging from a few hundred parts per million (ppm) for strongly flavored food products to less than 10 ppm for weakly flavored foods. In addition, many of these flavor compounds are highly reactive and thermally labile. All of these aspects make flavor analysis a challenging task. Various methods have been developed to cope with the complexity and to resolve practical problems related to food flavors. The first few steps usually involve isolation and concentration of volatiles and semivolatiles from their original food matrix. A number of different techniques, ranging from conventional solvent extraction and distillation to the newly developed direct thermal desorption (DTD) and solid-phase microextraction (SPME) are available for isolating flavors from diverse food systems. Each of these techniques has its own applications with advantages and limitations. No single method will give a flavor isolate truly representative of the flavor profile in the food. Method selection should be based on volatility, thermal stability and reactivity of the flavor compounds and their associated food system. Thereafter separation of the concentrated flavor extract is traditionally conducted by gas chromatography (GC). The combined use of GC with other analytical instruments increases the capacity of basic GC separation. If applicable, pre-GC treatments of flavor extract using liquid chromatography, acid–base separation and chemical trapping of certain volatiles should also be employed to enhance the performance of GC separation. A cyclodextrin-based chiral stationary phase (CSP) is the first choice for separating enantiomers. The follow-up identification of flavor compounds can be carried out by integrating data about retention index (RI) and mass spectrometry (MS) of specific components. However, information obtained by gas chromatography/mass spectrometry (GC/MS) reveals little insight into the actual flavor present in a food system, because many volatiles detected instrumentally contribute little or nothing to the sensory character of a food product. Sensory techniques, coupled with a technique known as gas chromatography/olfactometry (GCO) make it possible to elucidate the relative impacts of various volatiles on the flavor characteristic of a food. CharmAnalysis and aroma extract dilution analysis (AEDA) are two popular methods to measure the potency of flavor components. However, these two methods are inconsistent with psychophysical views. An alternative method named Osme overcomes this problem. It is a quantitative bioassay method used to measure the response to odorants on a time–intensity scale. To quantify volatile compounds, the traditional internal standard method is frequently used, but it is crude and often referred to as “semiquantitative” owing to its inherent limitations. For accurate quantification, a stable isotope dilution assay (IDA) should be used to target specific compounds. This article intends to describe briefly these analytical techniques and discuss their advantages and disadvantages and their applications.